Position of the lingual nerve.

An Anatomical Study of the Relationships of the Lingual Nerve, Inferior Alveoloar Nerve and Mandibular Foramen, including Clinical Implications.

Zac MORSE

Department of Dentistry (Anthropology & Genetics) The University of Adelaide

CONTENTS

PREFACE PART A INTRODUCTION a n d REVIEW of t h e LITERATURE Classification of Nerve Injuries Pattern a n d Rate of Recovery Assessing a n d Documenting Nerve Injury MATERIALS a n d METHODS RESULTS Assessment of Errors DISCUSSION CONCLUSION

PART B INTRODUCTION a n d REVIEW of t h e LITERATURE ANATOMICAL FEATURES O F THE MANDIBLE EMPIRICAL LOCAL ANAESTHETIC LANDMARKS MATERIALS a n d METHODS RESULTS DISCUSSION CONCLUSION REFERENCES ACKNOWLEDGEMENTS APPENDIX I APPENDIX I1

PREFACE I t could be said t h a t the literature adequately describes human anatomy and there are no new discoveries to be made. This is far from the t r u t h as most anatomic textbook descriptions are of a qualitative nature and very few are quantitative in description, particularly in terms of the amount of normal variability that exists. The clinician, however, m u s t be aware of, and deal with, these variations. Hence there is a justifiable need to conduct more clinicaly orientated research.

Lack of anatomic understanding will have a n array of effects, some of which may include lowered quality of patient care with possible legal implications.

Basically the main aims of this s t u d y were to quantitatively determine the position of the lingual nerve in the associated regions of the mandibular third molar and retromolar area in cadaveric dissections; to determine the position of the mandibular foramen and lingula on dried human mandibles; and finally it h a s been the author's intention to clinically apply the findings uncovered.

Two differing groups of materials were studied; one directed at the dry bony component the other at soft tissue relationships.

Many authors of textbooks describe the mandibular foramen only in terms of the entry point of the inferior alveolar nerve into the the mandible and the lingula only when referring to the attachment of the sphenomandibular ligament. The mandibular foramen is labelled as being in a position directly posterior to the lingula (Romanes 1987). Others describe the medial surface of the mandible as "being characterized by the mandibular foramen "which" is limited medially by a projection termed the lingula" (O'Rahilly 1986). In Hollinshead's Anatomy for Surgeons (Head a n d Neck) (1982) description of the mandible is as follows; "On the inner surface of the ramus, about its middle, is the mandibular canal, and overhanging it above and anteriorly is a large sharp ledge of bone, the lingula".

Recent qualitative descriptions of the lingual nerve have been substantially superior to those of the mandibular foramen, sadly though any complete descriptions are lacking. The above authors who describe the mandibular foramen and lingula describe the path of the lingual nerve i n t h e following manner, "The lingual nerve curves anteroinferiorly on the medial pterygoid muscle to reach the medial surface of the mandible in front of the inferior alveolar nerve. Below the inferior edge of the superior constrictor muscle of the pharnyx is where the nerve enters the mouth. Here it passes between the mandible and the

mucous membrane covering it just inferior to the last molar tooth and above the posterior fibres of the mylohyoid muscle. The lingual nerve hooks round the inferior surface of the duct, and r u n s upwards into the tongue medial to the duct" (Romanes 1987). OIRahilly (1986) adds that as the nerve lies under the mucous membrane of the mouth it is palpable against the mandible about 1 cm below and behind the third molar tooth.

The distance quoted is probably a gross estimate, as no reference as to the source of this value is made. Hollinshead (1982) acknowledges that the lingual nerve "sends off a number of small branches to this gland and across it to the gingival mucosa a n d the floor of the mouth". Other authors neglect to describe the supply to the lingual gingivae altogether.

PART A

3

INTRODUCTION and REVIEW of the LITERATURE There are many procedures where dentists and specialist dentists such a s periodontists, endodontists and particularly oral surgeons perform procedures where the peripheral nerves in the mandibular third molar region are susceptible to injury in several ways. There is pressure from infection, inflammation, scarring and haematoma. There is also the quite real potential of needle perforation and surgical damage from bruising, laceration or transection.

Dysaesthesia is defined as the impairment of any sense, particularly that of touch. Paraesthesia is a morbid or abnormal sensation, such a s burning, tingling, prickling, hypersensitivity, or pain. Anaesthesia refers to the total loss of feeling or sensation (Koerner 1986). The term formication is sometimes used to describe abnormal sensation with pain (Ishikawa 1986).

Paraesthesia of the inferior alveolar nerve or the lingual nerve can be extremely frustrating and embarrassing to both the patient and practitioner (Robinson and Williams 1986). The psychological consequences are important a s the patient inay complain of unnoticed drooling of food on the chin, biting and burning of the tongue or lip with hot fluids, abnormal chewing, or irritating sensations of pain. It is fortunate though that the majority of patients who have sensory nerve dysaesthesia following t h e third molar surgery make practical accommodations (Alling 1986). Nerve damage can impair physiologic function by interrupting a n important neural loop which leads to the loss of protective inhibition of bite forces (Merrill 1979).

Different studies examining nerve trauma may use different terms to describe different conditions and hence make comparisons difficult.

Other times different terms are used to describe the same condition synonymously.

When encroaching upon the lingual aspect the third molar region, either knowingly or accidentally there is the hazard of involving the lingual nerve.

It is recognized that standard textbooks of oral surgery, particularly the older ones contain much information on the possibility of damage of the inferior alveolar during third molar surgery but few recognize the effects of such surgery on the lingual nerve, apart from the fact that it may be a risk in some cases. When damage does occur though it is generally considered to be due to surgical carelessness. Some textbooks, including those recommended to undergraduates do not consider these complications or their causes at all (Mason 1988).

The most common oral surgical procedure is t h e removal of mandibular third molars as was shown in a Swedish study (Nordenram 1987). The number of surgical extractions in England in the period from 1974 to 1984 increased by 113% while during the same period routine extractions of permanent a n d deciduous teeth decreased by 40% (Shepherd and Jones 1987).

Complications will invariably occur with any surgical procedure, a n d t h e frequency of complications can be expected to parallel the frequency of surgery (Goldberg et a1 1985).

Many will argue that all impacted third molars should be removed regardless of the absence of symptoms and others will argue t h a t the validity of prophylactic removal of impacted third molars is in doubt particularly in the light of our present lack of knowledge. Furthermore the link between lower incisor crowding and erupting third molars is equally in doubt.

More t h a n half of removed mandibular third molars present no subjective symptoms (Lysell and Rohlin 1988). 33% of mandibular third molars were extracted because they were non functional teeth (Osborn et a1 1985). I t is claimed by some t h a t the removal of these lower third molars is justified in order to prevent pathological conditions s u c h as follicular cyst, tumours, root resorption of the second molar and periodontal problems. The actual frequency of s u c h pathological conditions as the cause of removal of lower third molars is very low in comparison to the frequency of the removal of asymptomatic lower third molars. The fact though t h a t the removal of asymptomatic lower third molars is carried out with such a high frequency may result in the low incidence of the above named pathological conditions. Such a hypothesis h a s not yet been scientifically proven. Goldberg and workers (1985) found almost half as many complications in asymptomatic extracted mandibular third molars when compared to symptomatic extracted third molars. When symptoms are present they are more frequently associated in elderly individuals and associated with fully or partially erupted molars t h a n molars completely covered by soft or bone tissue. In a study by Osborn a n d workers (1985) where over 1 6 , 0 0 0 extractions were examined; 17% were soft tissue impactions, 73% were partial bony impactions and 10% were complete bony impactions. Deeply impacted third molars without evident pathology are probably best left until they cause symptoms (Shepherd et a1 1987; Lysell et a1 1988).

The surgeon m u s t be able to inform the patient preoperatively of the statistical likelihood of complications, so that .the patient may make a

reasonable judgement a s to whether to undergo surgery, especially for elective procedures (Goldberg et a1 1985).

Treatment options for impacted third molars include enucleation, lateral trepanation, surgical removal, exposure and transplantation.

The following account of the treatment of the lower third molar by surgical extraction is based on the descriptions of Phillips (1987).

Kalamchi et a1 (1987) and Merrill (1979).

The safest flap for the surgical removal of lower third molars is considered to be the "envelope flap". The reason being that there is less risk of the scalpel slipping and cutting the lingual nerve or the facial artery. The mucoperiosteal flap is raised by initiating the cut distally and slightly buccally to the second molar. It is sometimes stated injury may occur when the incision is placed too far lingually although a quantified definition of what is "too far" lingually, is never given. The cut is continued distobuccally almost to the buccal edge of the bone [ie external oblique ridge). The distal incision should be angulated buccally and incising with the blade lingually inclined can lead to nerve damage (Koerner 1986). Those who wish to carry the incision further distally up the ramus and are aware of the dangers of the scalpel slipping into the lingual tissues will start raising a full thickness flap and then proceed to cut their way up the ramus with small surgical scissors and not with the scalpel [Hunt 1976).

The knife is then turned to face anteriorly and is inserted distal to the second molar and cut forwards along the gingival crevice to about halfway along the buccal surface of the first molar. The buccal mucoperiosteal flap is elevated then retracted. Subsequently a Howarth's periosteal elevator is usually inserted into the lingual sulcus under the periosteum to protect and maintain the safety of the lingual nerve. Overzealous elevation and manipulation should be avoided. The use of a Howarth's periosteal elevator is said to, "cause minimal postoperative swelling and discomfort and is much prefered to having the lingual nerve damaged". Prior to this stage some advocate carrying out lingual "dissection" which allows greater access lingually. The tip of the elevator which is inserted under the lingual periosteum should always be kept on the bone. It is worked down and around the lingual side of the mandible.

The difficulty of this stage is that the mandible sharply curves inwards and it is often here where inexperienced operators slip with their instruments into the floor of the mouth.

When the flap is raised intact there is usually very little bleeding but if the periosteum is perforated there is much greater potential for problems.

Once the area is fully exposed, sufficient bone can be removed to expose the tooth, provide a point of application for elevators, and to allow a n adequate path of delivery. This can be done using either chisels and/or burrs. The choice mainly depends on the age of the patient, on the type of anaesthetic being used, the orientation of the tooth, the thickness of the mandible and on which method is taught and encouraged. Some authorities indicate that there is no absolute contraindication for the use of either chisel or bur. A tungsten carbide straight fissured bur should create a gutter around the tooth. A coolant should be used when drilling near a nerve to prevent overheating and hence damage. Injury can also be prevented by adequate retraction and careful drilling. If not adequately conducted the bur may slip and become embedded in the lingual tissues. It is most likely to happen in the most vulnerable area, about the distolingual aspect of the lower third molar (Fig 1).

Depending on the type of impaction the tooth may be elevated and removed in its entirety or it may require sectioning with a bur. When removing a vertically impacted mandibular molar the tooth is sectioned in a buccal to lingual direction. The bur should not be allowed to cut all the way through the tooth, instead it should be sectioned two thirds of the way through which will permit the insertion of a straight displacement elevator which can severe the two portions of the tooth without endangering the lingual nerve, one must be aware of the angulation and path of the bur, a s the bur may enter the midpoint of the occlusal surface but the tip could be through the lingual surface of the tooth and even through the lingual plate of bone (Swanson 1989).

Drilling too far apically through the tooth or through the buccal plate may involve the inferior alveolar canal. This is usually indicated by active bleeding. Haemostasis by packing or clamping should be conducted with extreme care to avoid any additional nerve damage. Electrocoagulation should not be conducted. Schwartz's study (1973) indicates that 2% of lingual nerve anaesthesias following third molar extractions are due to pinched or cauterized lingual nerves during the process of obtaining haemostasis of bleeding lingual vessels. Similarly, as described previously damage to the lingual nerve can be achieved by perforating the lingual plate. Burs passing through the lingual plate when the tooth was sectioned accounted for 3% of lingual nerve anaesthesias following third

Fig l . nerve injury.

. l ' l ~ edistolingual area with a thin lingual plate is a h i g t ~risk area toi- lir~gilal

molar extractions (Schwartz 1973). Primary displacement and secondary retrieval attempts of a broken bur, a bone, or a tooth or part of it either into or near the mandibular canal or lingual space may also cause nerve damage. To prevent lingual displacement a finger may be placed lightly over the lingual aspect of the lower third molar. Increased exposure should not be achieved by removing the overlying pad of soft tissue. When inadequate bone h a s been removed and excessive force is used in elevation, a fracture of the mandible can be produced with simultaneous nerve injury. When inadequate bone h a s been removed in a mesioangular impaction with roots in close proximity to the inferior alveolar canal may force the roots or bone into the canal which may tear, lacerate or crush its contents because upward elevation of the crown produces downward rotation of the roots.

Lingual plate fracture with t h e conventional technique is uncommon, although, when it does happen, its vascularity may be compromised and may necrose. I t is at this stage that the surgeon must decide whether to remove the lingual plate or leave it.

Some oral surgeons, primarily in Europe, commonly work to the lingual side of the mandible to remove third molars. The reason being that the lingual plate is much thinner than the buccal (Hunt 1976). This fact highlights the need to practice caution when instrumenting near the lingual plate of bone.

The lingual split technique utilizes chisels to remove bone once the soft tissues have been retracted. A vertical stopcut is made in the buccal bone a t the junction of the lower second and third molars with the bevel of the chisel facing posteriorly. Another stopcut is made approximately 4 mm distobuccal to the third molar with a horizontal cut joining the two stop cuts the buccal plate is gently split off. A 5 mm chisel is then placed distal to the crown of the third molar and near to the lingual plate. Its cutting edge should be parallel to the external oblique ridge, with the bevel facing lingually and the shaft of the instrument held at a n angle of 45" to the horizontal. A light tap on the chisel will drive its tip towards the lingual plate causing the lingual plate to fracture at the interdental space between lower second a n d third molars. A hazard at this stage is that control of depth into the lingual tissues can not be guaranteed. Once the lingual plate is separated it can be removed with toothed forceps while the lingual tissues are being protected with a periosteal elevator. Koerner (1986) apparently indicates that the same frequency of complications is observed if the lingual plate is left in situ or removed. If the lingual plate is still attached to the soft

tissues then it is certainly best left there. Resistance to withdrawal may be generated from the deep level of attachment of the mylohyoid muscle which may involve the mylohyoid nerve which runs in the mylohyoid groove below. Similarly a deeply placed Howarth's periosteal elevator could result in paraesthesia of the mylohyoid nerve (Stacy 1977). A Ward periosteal elevator is recommended by some to be inserted into the lingual space to separate the bone from the mylohyoid's attachment. The same author continues to recommend that the bone fragment should be removed carefully, "as it may have sharp edges and damage adjacent structures, such as the lingual nerve ". No mention is made though of damage to the mylohyoid nerve. Attention has been drawn to the potential involvement of the mylohyoid nerve [Hunt 1976). Stacy appreciates that most oral surgeons who have used the lingual split technique will recall examples of the lingual plate that have included landmarks a s high up a s the lingula and a s low a s displaying the mylohyoid groove, while coming very close to the inferior border of the mandible (Fig 2). In such cases one might expect submental analgesia as a sequel.

Blackburn and Bramley (1989) show that the lingual split is not associated with increased lingual nerve damage, given that the mylohyoid groove is not present on the bone fragment (implying a large bone fragment). Such cases (78%) resulted in temporary lingual dysaesthesia.

Filing of the sharp bony lingual plate following removal of the tooth should be conducted with great care when the margin lies close to the level of the mandibular canal because the mylohyoid nerve is nearby on the lingual periosteum. When the mylohyoid nerve is traumatized it may cause sensory disturbance in the skin over the mental prominence, but this is said to be rarely confused with paraesthesia of the lip (Rood 1983). The incidence of such sensory disturbance was stated to be unknown by Merrill in 1979 and is still unknown today.

The advantage of the lingual split technique is that it leaves little or no socket and it is common to deliver the tooth intact whereas with the use of a b u r the tooth is often sectioned. Many proponents of the technique especially "believe" it to be faster and less destructive of mandibular bone (Hunt 1976). Recently Middlehurst and workers (1988) found that the lingual split technique produced less pain and swelling in comparison to the bur technique. They also noted a strong correlation between pain and swelling.

Fig 2.

I t appears that the inferior border of the lingual plate was fractured below the mylohyoid groove. '

M - Mylohyoid groove

Fig 2. t appears that the inferior border of the lingual plate was fractured ~elowthe mylohyoid groove.

VI - Mvlohvoid rrroove

As will be shown soon and described further later the lingual split technique r u n s the risk of lingual nerve damage (Figs 3 & 4).

The degree of postoperative pain and swelling is more severe if large amounts of periosteum have been elevated from bone, if bone removal was extensive or if the surgical technique was clumsy.

The tooth follicle must then be removed b u t the lack of presence of the lingual plate due to resorption may result in the adherence of the follicle to the lingual periosteum. It is also not uncommon for the lingual plate of bone to be extremely thin, especially in the apical region of the third molar. The bone may exhibit fenestration and one study cited by Koerner (1986) reported a n incidence of 2.5% of dry mandible specimens showing third molar roots perforating the lingual nerve, although dislodgement of root tips or even entire teeth can present a difficult challenge for their removal and may secondarily involve the lingual nerve (Fig 5).

Schwartz (1973) indicated t h a t the absence of the lingual plate secondary to long standing infection, cyst formation or lingually inclined third molars was the cause of lingual nerve anaesthesia in 16% of cases of anaesthesias following mandibular third molar extractions. He also stated that extension of the root through the lingual plate was the cause of lingual anaesthesia in 3% of cases. Lingual exploration in a n effort to retrieve t h e displaced root accounted for 4% of lingual nerve anaesthesias.

A disadvantage of bone removal with burs, is that when bone is removed with a bur, the flap is sutured against a largely intact lingual plate which would not accommodate much swelling.

The lingual nerve can be easily damaged if care is not executed.

The follicle must be removed though in its entirety with much caution.

The wound should be closed with only one suture if possible. The first suture is placed 2-3 mm distal to the second molar. The real danger here of which we do not know the frequency of, is placing the lingual "bite" very deep which could pierce the lingual nerve or inadvertently encircle the nerve and c r u s h it. The respondents of Schwartzfs questionnaire (1973) suggest t h a t the s u t u r e needle passing deeply through t h e lingual flap may account for 5% of the lingual nerve anaesthesia following mandibular third molar extractions.

Significant oedema around the mandibular canal can lead to injury of the inferior alveolar nerve. Not only can oedema cause inferior alveolar nerve injury due to the bony confines of the mandible, but it can similarly cause nerve injury in the lingual tissues. A case was reported by

THIRD

MOLAR

REMOVAL

Fig 3.

From: Hunt (1976) Diagramatic display of mandibular third molar removal utilizing the lingual split technique.

Fig 4.

A- Lingual split simulated on a cadaver.

B- Nerve in contact with perfosteum behind the posterior border of the mvlohvoib muscle fie distolin~ualarea of the third molar).

..

-.

--

. -

-

..

---*=**. .--.p* - - .,*-& .-L

-

-1-

.'

I

-

, l+-

.,

C- The distal aspect of the fractured lingual plate has been forcec

against the nerve. The inferior border of the fractured lingual plate k below the attachment of the mylohyoid muscle which may generatt resistance to the removal of the lingual plate. Conventional -tea elevators are not wide enough to adequately protect the h j g d nerve.

-p----

.

-

---- *

:-

'..L

.F-

. h

:

.

_ ;>-? .,.. :x:2a -.: S:.

I.

.- . . ..*-*---'L

,

"

T,,<

, .i w : .

,

.-

>

-L

A

- .. .

,

.

C

. .>.,-&..-

D- Lingual plate and tooth removed leaving empty socket39

......... ......... [giji;: 1%::

E%::.

:

. .

..... p": C;;;m: :n '

I . . . . . '

.__ *-.

. . . . . _.

a

~xarnilesof .. . .--. ical fenestratio -. ~. .. .L:-. - 3 .

- . , . .

P - Pathological f e n e ~ t r a t i o ~ - ~ ..+, ~'. - . ! *

'

-d.

'

-

P

-S---

:

Stankiewicz and Pazevic (1988) where following mandibular third molar surgery, oedema probably led to the development of temporary neurapraxia of the hypoglossal nerve.

Prophylactic antibiotics may be helpful in avoiding oedema a s a result of infection. Goldberg and workers (1985) described a case where labial dysaesthesia was coincident with the onset of infection. Sensation in the patient returned immediately after the resolution of the infection.

Schwartz (1973) noted that postoperative oedema was responsible for 3% of lingual nerve anaesthesias following mandibular third molar extractions.

Kalamchi and Henser (1987) list the pre-, intra- and postoperative complications of mandibular third molar surgery. Dysaesthesia a s a complication of third molar removal ranks third, below alveolar osteitis and secondary infection (Sisk et a1 1986 and Osborn et al 1985).

Lower third molar surgery is definitely not the only situation where lingual tissues may be involved and Hunt (1976) describes some surgical procedures conducted on the lingual aspect of the mandible which may violate the lingual tissues. Modern periodontal techniques have led to a n increased use of flap surgery as it is really only by retraction of a flap that a full evaluation of the osseous defect is possible. The potential hazards to be encountered in lingual surgery are probably greater than for any other anatomical region in which the periodontist might work. The major periodontal texts do not cover in detail these potential risks and how they can be minimized and managed. Periodontists or anyone else using sharp instruments on the lingual side of the mandibular third molar region runs the risk of slipping with a sharp instrument such as a scalpel or otherwise into the lingual floor. All main nerves and vessels are lingual to the circum-mandibular periosteum hence when raising the lingual flap the lingual periosteum should be reflected and maintained intact.

Retractors should be designed to retract the lingual tissues but also preserve them.

There are situations where the prosthodontist may wish to have surgery performed on the lingual aspect of the mandible. In the edentulous patient where there is a prominent mylohyoid ridge, the denture may compress the tissues underneath it against the mylohyoid ridge, causing pain. Mylohyoid ridge reduction does not impair denture function, it usually increases denture stability. The incision is carried along the crest of the edentulous ridge. Those who do not wish to make the incision on the crest in order to prevent scarring and alteration of the crest should carry the incision buccal to the ridge crest and NEVER

should it be placed lingual to the ridge crest. Sometimes surgeons continue to carry incisions in the lingual sulcus. Under NO conditions should a vertical relieving incision be made on the lingual aspect of the posterior part of the body of the mandible. A full thickness flap is raised and retracted to expose the ridge and the attached muscle. The periosteal elevator continues to severe the mylohyoid ridge. Some surgeons expose the ridge and its muscular attachment, fracture the ridge and subsequently cut the bony attachment away with either surgical scissors or scalpel. This latter method posses increased risks with no advantages. The wound is then closed with a n optional drain being inserted to prevent haematoma formation.

When the alveolar ridge has atrophied and the tissues of the floor of the mouth abut directly onto the ridge, the whole floor of the mouth may need to be lowered and maintained to increase denture stability.The incision and flap retraction is essentially the same, with the difference being that the flap is larger. Many surgeons will carry the incision directly into the floor of the mouth. Of these the more conservative will make the primary incision "superficially" with subsequent deepening being carried out by blunt dissection (Fig 6). The only means of reducing the risks with this method would be to limit the initial incision anteriorly and then with blunt dissection visualize the lingual nerve. Once the nerve was visualized the incision could be carried posteriorly staying as far away from the lingual nerve a s possible. Mucosal grafts can then be placed to cover the exposed mandible. The floor of the mouth is maintained inferiorly by suturing the floor of the mouth to the outside of the face.

Injury to the lingual nerve can be sustained whenever any type of lingual mandibular surgery. A common reason for seeking and exposing the lingual nerve is in the treatment of lingual neuralgia where the nerve is cryofrozen.

Iatrogenic injury to the lingual nerve can occur with the surgical removal of the closely related submandibular gland. Dentures can impinge on the lingual nerve and alter its functioning. Dentures can also impinge on the mental nerve which can lead to altered labial sensation. If the provision of dentures coincides with third molar surgery then alteered nerve functioning can be misinterpreted as being caused by the surgery. A case has been reported where pressure from a complete lower denture was transmitted to a residual root fragment and produced numbness and tingling in the lower lip and cheek.

Fig 6.

9 female patient aged 63 yrs was to have a lingual swelling removed. The ~ncisionwas carried lingual to the alveolar ridge and into the lingual ~ulcus.

1 A - Alveolar ridge B - Buccal sulcus T - Tongue :Continued over page.)

.A

2. Lingual retracbon dispTa5ng the rich vascularity of the regim __ /

-

-

Root canal fillings that extrude pass the apex of the tooth may traumatize the inferior alveolar nerve. Paraesthesia of the inferior alveolar nerve by compression has also been reported to have been caused by periodontal-endodontic pathosis (Lambrianidis and Molyvdas 1987).

Noniatrogenic injury of the lingual nerve may result from an automobile accident, knife wound, or gunshot wound. Pathological conditions which may effect the lingual nerve are neoplasms, trigeminal sensory neuropathy and other conditions such a s sialothiasis. Sadler and colleagues (1985) reported a case of intermittent episodes of hemilingual numbness. The attacks occurred simultaneously with submandibular swelling. Surgical exploration confirmed the presence of a sialolith in the submandibular duct. Relief of the obstruction gave complete relief of the symptoms.

It is important that clinicians inform their patients of the degree of probability of risks involved with a proposed operation. Statistical data is now available which must be recognized by the surgeon and passed on to the patient in a form which is understandable by the patient so that they can make their decision. Swanson (1989) cites The National Institute of Health's Consensus Development Conference on The Removal of Third Molars (1979). in the U.S.A. I t recommended in consideration of the morbidity in third molar removal that patients should be informed of potential surgical risks, including any permanent condition that has an incidence of greater than 0.5% or any transitory condition that occurs with a n incidence of 5% or more. They concluded that even for routine cases patients should be informed about nerve injury.

INCIDENCE OF NERVE DAMAGE ACCORDING TO VARIOUS STUDIES Inferior Alveolar Nerve Lingual Nerve 6% -Temporary Woffo@&mkr (1987)

sthesia

(1986)

L%-Lingual Split

l %-Lingual Split 0%-Buccal Approach Overall for both nerves 2.6% - operations = 2.4% - patients I

Anaesthesia

Middlehurst et a1

Mason (1988)

Lingual

Rood (1983) MacGregor (1976)

5.5% -0.4% at 6 months 1%

Goldberg et a1 (1985)

0.6% -0% at 6 months

Memll(1979) Osborn et a1 (1985)

0.41% ,+O.O l %lasted more than 1 year 3%-(estimate) 0.57% 1*0.07% at 6 months.

sthetic.

46.7% (7115patients) -Buccal Approachs+O% by 3 months 0% -Lingual Split 11.5%-overall 8.3%-No Lingual Split 19.8%-Lingual Split -5% failed to make full recovery at 6 months -0.6% operations=l% patients had no recovery 6.6% -0% at 6 months

0.6% 4*0.2% at 6 months (permanent) 0.06%- patients -<0.01% lasted more than l year

Limitations of studies and difficulties in comparisonThe definition of nerve damage varies amongst authors. Some examine "nerve damage", others dysaesthesias or paraesthesias. Authors such a s Blackburn and Bramley (1989) who cite results in terms of "nerve damage" include all cases of sensory disturbances, including short term transient ones such a s tingling of the lip or side of tongue, without appreciable numbness. This may tend to overestimate the incidence of significant long term nerve damage, a s in the case of Mason's study (1988) who reports a very high initial incidence. Other studies do not include temporary injuries (functional neurapraxias) which may exclude those cases with delayed onset of symptoms and hence may underestimate nerve damage. Different studies report values of nerve damage in terms of the number of teeth operated on or the number of patients affected and does not allow the figures to be directly comparable.

Type of technique and conditions under which the damaged occurred vary between studies. Different studies include different types of patients and operators. Middlehurst's study tried to standardize procedures in order to compare techniques where patients acted a s their own internal controls and a single operator conducted one technique on one side of symmetrically impacted teeth, and the other technique on the otherside of the patient. The shortcomings of this and other studies is their limited sample size.

Different studies use different methods of detecting affected individuals and hence the reported frequency of those affected will vary.

For example Wofford and Miller (1987) objectively examined their patients only after the patient subjectively responded to alteration or loss in sensation to areas innervated by the inferior alveolar and lingula nerves.

Middlehurst and workers (1988) report the highest frequency of 46.7% of patients exhibiting temporary lingual nerve damage. This was conducted under general anaesthesia using the buccal approach and their sample size was extremely small (15 patients). Contrastingly Blackburn and Bramley (1989) report frequency of 0.5% of patients exhibiting permanent lingual nerve damage. From the table it can be seen that some frequencies fall below the temporary 5% level and 0.5% permanent level of disclosure a s recommended by the National Institute of Health (1979) where a condition should be disclosed to the patient.

Only the study by Wofford and Miller (1987) reports a temporary value of

0% lingual nerve damage which was conducted under local anaesthesia from the buccal approach.

The highest reported frequency of lingual nerve damage per se occurred when the buccal approach was utilized (Middlehurst et a1 1988). Mason's English study (1988) however compared dysaesthesias following the two techniques in 1,040 operations in 602 patients, in contrast to the mere 30 patients examined by Middlehurst and workers (1988). He found that the lingual plate splitting technique was associated with a higher incidence of lingual dysaesthesia. An interesting finding and one requiring further examination is the fact that retention of the lingual plate did not have a significant effect on the incidence of lingual dysaesthesia compared with the removal of the plate, either before or after elevation of the tooth.

At this stage we should consider, in a broader sense the factors influencing lingual nerve damage. It appears that lingual and labial dysaesthesias do not differ aetiologically, apart from the closeness of the roots to the inferior alveolar nerve which is age dependant. Nerve damage is not related to sex, race, side of operation or medical history of the patient (Sisk 1986, Mason 1988 and Wofford and Miller 1987).

These authors also noted that age is not a contributing factor to the lingual nerve damage. It is interesting that Koerner (1986) suggest that females have (report ?) a higher incidence of dysaesthesias than males.

The type of anaesthetic used appears to be significantly related with the level of nerve damage. Blackburn and Bramley (1989) found lingual nerve damage to be more likely when teeth were removed under general anaesthesia t h a n under local anaesthesia (P<O.005). 6% occurring under local anaesthesia compared with 22% under general anaesthesia. I t could be considered that the choice of general anaesthesia would signify procedures were more difficult to perform and there may be a bias towards more difficult impactions when general anaesthesia is used (Mason 1988). However,difficulty of procedure was not found to be related to the lingual nerve damage when consideration was given to the degree of difficulty declared by the operator, the time taken (even with grade or experience of the operator taken into account), or other aspect which might indicate a complex procedure, such a s the need to section the crown or the roots.

Some oral surgeons indicated in a survey conducted by Schwartz (1973) that they believed that postoperative anaesthesia was related in some way to the fact that the surgery was done in hospital. One oral surgeon had no experience with lingual anaesthesias except in two

hospitalized patients treated the same day with the same surgical assistant and anaesthesiologist as usual. Several surgeons "believed" the anaesthesia was secondary to intubation.

A quite possible and yet ungrasped explanation for increased nerve damage with general anaesthesia is t h a t t h e general anaesthetic procedure itself may be the causative factor. Loughman (1983) reported unilateral numbness of the tongue in a patient who underwent breast surgery and no oral surgery whatsoever. He also noted that difficulty was experienced with laryngoscopy and that the most likely cause for the patient's symptoms was damage of the lingual nerve by the laryngoscope blade.

There is some evidence which may not support the above though.

In the study of dysaesthesia following odontectomy of impacted third molars by Wofford and Miller (1987), all of the affected patients had unilateral dysaesthesia and none had bilateral dysaesthesia. This is the general finding, although Fielding and Reck (1986) describe a case of bilateral lingual anaesthesia following bilateral mandibular exodontia.

Most importantly though is the fact that in none of the studies cited was there any nerve damage on the non-operated side (Mason 1988) when a single lower third molar had been removed suggesting that in the vast number of treated patients under general anaesthesia, one would expect to find lingual nerve damage in the non-operated side if damage was a result of the anaesthetic procedure.

The traditional view is that the lingual mucoperiosteal flap should be retracted so as to protect the lingual nerve, particularly when bone is removed distally. Blackburn and Bramley (1989) found quite the opposite; where lingual nerve damage was found to be the most likely to occur when a Howarth's periosteal elevator was used to retract the lingual flap (p<0.005):18% with, 4% without. Interestingly enough it was also found t h a t lingual nerve damage was most likely to occur when lingual flap retraction was performed under general anaesthesia. There is no relationship between the likelihood of nerve damage and the length of the procedure. Middlehurst and workers (1988) noted that lingual nerve sensory impairment occurred more frequently when burrs where used rather t h a n chisels which could possibly be related to the need for greater duration of lingual flap retraction. The length of the procedure as such did not affect the time of recovery and no relationship was found between the grade of assistant and incidence of nerve damage although it may be assumed that the inexperienced assistant may use excessive force on the retractor. If difficulty is encountered during elevation of the

lingual flap dysaesthesia occurred in 20% of cases compared with an incidence of 17% when no complications were encountered (Mason 1988).

The main purpose in using a lingual flap retractor is to offer protection to the lingual nerve. If it were not utilized and recorded in the notes then it would be more difficult to defend against legal action if the nerve was damaged. It appears though, that the "protective device" appears to be the wounding agent itself and may also provide inadequate protection against lingual nerve damage. It is unfortunate that surgeons do not have many alternatives. When using a (Howarth's) periosteal elevator then its usage should be done extremely cautiously. Stacy (1977) demonstrates how easy it is to engage and traumatize the lingual nerve against the lingual plate with the lingual flap retractor (Fig 7). For this reason Stacy (1977) designed a stable, self-retaining lingual flap retractor which also acts dually to depress and control the tongue and to reflect light directly into the wound to aid illumination, yet this and other designs have not been widely adopted.

Tongue depressors can with equal ease crush the lingual nerve against the body of the mandible if used incorrectly with excess force. It is the authors opinion that the frequency of nerve damage by tongue depressors, even if temporary is severely underestimated. Schwartz (1973) suggests that in 5.5% of cases with lingual nerve damage are caused by retraction of the tongue with stretching of the lingual nerve.

The type of tongue retractor used by Mason (1988); hand-held or selfretaining had no significant effect on the incidence of lingual nerve damage.

Another possible explanation of increased nerve damage under general anaesthesia is the state of eruption of the teeth to be removed.

The incidence of nerve damage is associated primarily with the removal of complete bony or partial bony impacted mandibular third molars (Sisk et a1 1986). Dysaesthesia has been reported to vary from 4% when the tooth was a t the occlusal level to 24% when the mesial marginal ridge was more than 1.5 cms below the occlusal level (Mason 1988). Blackburn and Bramley (1989) found lingual nerve damage to occur with 11% of partially erupted teeth compared with 21% of teeth t h a t were unerupted. However, it may be considered that unerupted teeth are more likely to be removed under general anaesthesia. In the same author's sample 64% of the teeth removed under general anaesthesia were unerupted, a s opposed to 27% of unerupted teeth which were removed under local anaesthesia. When examined individually the state of

Fig 7.

FROM: STACY G.C. (1977) Int. J. Oral Surg.

Corona1 sections through the mandibular third molar region during lingual flap dissection.

A- First stage - corret position for instrument blade as it lips the lingual plate.

B- Second stage of correct lingual flap dissection.

C- First stage - instrument blade incorrectly placed too far medially.

D- Second stage - lingual nerve engaged and traumatized following incorrect use of periosteal elevator a s in C.

eruption did not appear to effect the outcome under local anaesthesia, whereas under general anaesthesia lingual nerve damage was much more likely to occur when the tooth was unerupted.

In one study (Mason 1988) involving 1,040 operations, vertical impactions were followed by lingual dysaesthesias in 4.4% of cases. The incidence was higher after disto-angular (10.2%). mesio-angular (13.7%) and horizontal impactions (15.2%). Aberrant impactions s u c h a s buccolingual impaction were followed by a n incidence of 15%. In another study (Blackburn and Bramley 1988) involving 1.1 17 operations, those cases which failed to make a full recovery involved teeth that were either vertical or disto-angular impactions. In these cases bone was removed with a bur. Contrastingly the majority of cases which presented with temporary dysaesthesias involved mesio-angular impactions.

Mason (1989) discovered t h a t the presence of distal bone requiring removal to disimpact the tooth was associated with a n incidence of 21%. compared with 3% when no bone was removed.

Before continuing any further it must be emphasized that generally no single factor can be identified except in cases where the nerve is directly visualized and severed. This statement is highlighted by the fact that lingual dysaesthesias will present themselves in 3% of cases where no bone at all was removed (Mason 1988).

There is the suggestion that distal bone is often removed blindly, because the view may be obscured by the crown and faith is then placed in the retractor to protect the nerve. Bone should only be removed where it can be seen and the bur advanced towards the operator, never blindly around the distal aspect of the tooth. If removal of bone that can be visualized is not sufficient to enable the tooth to be elevated then the tooth should be sectioned but should not be allowed to be extended past the tooth and through the lingual plate (Blackburn and Bramley 1989).

As no studies show a significant difference in incidence of nerve damage observed between left and right sides, accessibility by the operator usually working on the right side of the patient does not appear to be important.

The respondents of Schwartz's questionnaire (1973) suggested that 4% of lingual nerve damage was due to the bur catching the lingual nerve during removal of bone over the distal segment of the tooth crown.

The problem with any retrospective study is that such causative factors a s the above named factor as well a s others, such a s the position of the incision are more speculative in nature and it is difficult to substantiate

a t the time of operation let alone by another examiner some time in the future.

When removing bone with a bur the nerve must be protected lingually by the careful subperiosteal positioning of the retractor on the lingual aspect of the mandible particularly if the "collar technique" is employed (Rood 1983).

Operating time was shown by Mason (1988) to be associated with lingual dysaesthesia. When the procedure was completed in less than 5 minutes the incidence of lingual dysaesthesia was 5%. This increased to 14% for operations lasting 5 to 10 minutes, and 23% for operations 11 to 15 min. There was an insignificant fall to 18% for operations lasting over 15 minutes. This may represent firstly the degree of difficulty of removal which may secondly be associated with increased lingual nerve damage following prolonged and vigorous stretching of the lingual flap.

When examining operator identity and incidence of lingual nerve damage, available data is quite conflicting. Blackburn and Bramely's (1989) study found students as operators to be associated with the lowest incidence of lingual nerve damage (6%). whereas the highest incidence was found with house officers (38%) and lecturers/senior registrars (25%). More senior operators do however treat a significantly greater proportion of more difficult extractions (Sisk et a1 1986). Other explanations could be found in terms of other factors such as; students operated almost exclusively under local anaesthesia and most frequently on partially erupted teeth, while the reverse was found with the senior operator group, which was associated with a high incidence of nerve damage.

The most significant finding, when operator identity was considered was that there was a variable incidence between individual operators of same grade and similar experience. The conclusions reached by the above authors was that the individual operators with higher incidence of lingual nerve damage used a similar (deleterious) technique and operated on patients of greater complexity. It should be noted though that no operator escaped nerve damage (Mason 1988) on the other hand he found no significant difference between individual operators. When Sisk and workers (1986) compared lingual dysaesthesia between residents and faculty, they found no instances of lingual dysaesthesia in the faculty group but found 3 of 359 (0.84%) mandibular extractions treated by the resident group ended in lingual dysaesthesia. A similar relationship was ascertained in terms of labial dysaesthesias. They

concluded that less experienced surgeons as a group have a significantly higher incidence of complications.

Often perceived as not being possible is nerve damage by local anaesthetic needles. In Schwartz's questionnaire [1973), one responding oral surgeon indicated that a patient of his felt a sharp pain in the tongue when a local anaesthetic was injected for a mandibular block. The postoperative anaesthesia t h a t ensued was permanent. I t h a s been estimated t h a t the greatest contributing factor (44%) of lingual nerve anaesthesia following mandibular third molar extractions is the local anaesthetic needle passing through the nerve trunk (Fig 8). The needle can severe perineural blood vessels (especially above the lingula during mandibular block) which can lead to haematoma formation. The nerve usually heals in several weeks upon resorption of the haematoma. If the haematoma matures and forms fibrous scar tissue, it can cause external and internal compression of the nerve and impair nerve conduction [Koerner 1986).

Lingual nerve injury iatrogenically induced during sagittal ramus osteotomy is rare. Hetvedt a n d Zungia (1990) describe a case of unilateral lingual nerve anaesthesia a s a complication of rigid fixation of the sagittal r a m u s osteotomy. B u r holes and screws were placed transorally in a bicortical fashion with the lingual soft tissues unprotected. As the neurosensory deficit did not improve, the bone screws were removed on t h a t side. Sensation in half of the patients tongue still did not return. Surgical exploration revealed a neuroma around the region of the b u r holes. J u s t as the inferior alveolar nerve should be carefully visualized and protected, care should be undertaken to minimize penetration into the lingual soft tissues when placing bur holes and bone screws.

Classification of Nerve Injuries Nerve damage h a s been classified in various ways (Merrill 1979 and Rood 1983). Classification can aid in the understanding of nerve degeneration a n d regeneration, clinical symptoms, diagnosis and prognosis for recovery after nerve injury and ultimately the approach to treatment.

The most simple a n d traditional classification separates nerve damage into three classes: Neurapraxia (Class I). This represents the least severe type. I t can result from blunt trauma, tissue retraction, inflammation or transient local ischaemia [Koerner 1986). There is a block in electroconductivity

3

Fig 7.

The lingual and inferior alveolar nerves being simultaneously pierced b: a 27 guage, long needle.

Note the corda tympani nerve entering the posterior surface of thc lingual nerve and the connection between the inferior alveolar an( lingual nerves.

L - Lingual nerve I - Inferior alveolar nerve M - Maxillary artery A - Inferior alveolar artery 6 ; - Lingula '-L&: - Submandibulk ganglion 1 \

-

h#J

b u t the anatomic damage is minimal and axonal degeneration does not occur. Recovery tends to be rapid and is complete within a few days to three weeks.

Axonotmesis (Class 11). I t is a more significant form of nerve injury and can result from contusions, crushes or extreme stretching. There is degeneration and necrosis with loss of nerve axons distal to the point of injury, b u t t h e integrity of the nerve s h e a t h is maintained. After degeneration of the nerve axon there must be regeneration, hence this is a somewhat slower process.

The initial signs of returning sensation a r e i n the form of paraesthesia which may not occur until six to eight weeks after nerve injury. Considerable improvement occurs by six months. The eventual recovery is variable a n d may be less t h a n normal as the internal orientation of the crushed nerve fascicles may be disorientated.

Neurotmesis (Class 111) represents physical division as may occur by iatrogenic transection or the separation may be d u e to internal disruption and scarring. Only part of the nerve may be sectioned which will alter the rate and quality of recovery. The prognosis for regeneration may be better if the transection is clean a n d there is good fascicular orientation.

The above classification may be a little too simple a n d does not cover other possibilities of nerve damage and their associated clinical implications. Rood (1983) presents a more thorough classification of which six types of nerve injury are recognized.

Neurapraxia (Grade 1). Functional conduction block, caused by ischaemia and/or oedema with no anatomical changes - Paraesthesia resolves within one or two post-operative days. If oedema is present it may take 1 week for resolution to occur.

(Grade 2). Demyelination, which is minimal structural change with no axonal loss. - Paraesthesia may progress to anaesthesia over the first few post-operative days. Recovery usually occurs by 4 weeks.

I t is useful to inform the patient about the likely duration of their symptoms (Rood 1989). In cases of neurapraxia, the nerve can be described as being "bruised" which would indicate to the patient that there is little physical damage a n d satisfactory recovery c a n be anticipated.

Axonotmesis (Grade l ) . Axon degeneration - If anaesthesia is profound after surgery a n d takes 3 to 4 months to recover then degeneration followed by regeneration h a s occurred.

(Grade 2). Axon degeneration with disruption of the endoneural sheaths. -These may take up to 6 months to recover and if "cross-shunting" has occurred then the quality of recovery may be less than normal.

In cases of axonotmesis the patient can be informed that their nerve has been "contused" or "compressed" with some loss of nerve fibres which must regenerate. The patient can be reassured that the nerve has not been divided.

Neurotmesis (Grade 1 ). Destruction of axons with perineural disruption but epineurium remains. -Permanent loss may occur with altered sensation due to nerve disorganization and scarring.

(Grade 2 ). Nerve division -If permanent anaesthesia occurs then the nerve has totally been severed in two, or scar tissues may prevent axon regeneration.

Pattern and Rate of Recovery -Lingual Nerve When trauma involves nerve degeneration, tissues once supplied by the nerve become unreactive. Sweating and salivation disappear and a zone of hypoaesthesia exists where neighbouring nerves overlap (Merrill 1979). In the absence of nerve regeneration, significant reduction in area of anaesthesia may be achieved by a n ingrowth of collateral nerves. This form of sensory compensation has its limitations a s it can not be totally reinervated by the collateral (for example mental) nerve and it does not increase after 6 months post injury and it does not increase.

Furthermore the sensation evoked by stimulation of tissues receiving a collateral innervation is abnormal (Robinson 1988).

When regeneration of peripheral nerves occurs it begins within 24 hours of the trauma. If newly sprouting fibres are able to make contact with vacant Schwann cell tubes of the degenerated peripheral nerve, growth advances approximately 1.5 mm a day. Merrill (1979) suggests a regeneration rate of 2.5 mm a day after a three week latent period following a crushing injury and 2 mm a day after a five week latent period following a severing injury. An 8 0 mm distance between the third molar and the lip would require about one and one-half to two months after anxonotmesis and two and one-half months following neurotmesis with suture. In this case the assumption is that the inferior alveolar nerve regenerates a t the same rate a s other nerves b u t it is known that peripheral nerves differ from each other in speed of recovery (Rood 1983). The arrival of the tips at a sensory organ does not necessarily

correspond with return of nerve function. Proprioception and deep pressure-pain sensations return first. This gives the itching and burning feelings (paraesthesia) and may persist for weeks. Maturation must occur with fine tactile and pinprick sensation returning subsequently.

As mentioned before a n y operation t h a t is associated with a "permanent" complication that h a s a n incidence greater t h a n 0.5 per cent, then the patient should be informed accordingly. The question is though what is the legal and clinical definition of "permanent"? Those unlikely to make a full recovery should be detected and managed as soon as possible. Presently lingual nerve repair is undertaken on the basis of a failure of normal recovery, which may eventually be "acknowledged" 2 years postoperatively (Blackburn 1990).

The patient who suffered nerve damage as a result of orolaryngoscopy, fully recovered this transient form of damage six weeks after the intubation. In these situations complete recovery in several months is most likely (Loughman 1983).

In Middlehurst and workers' (1988) small sample, all their cases of lingual nerve paraesthesia resolved by 3 months.

13% of Alling's (1986) dysaesthesias were persistent, that is they lasted for more t h a n a year. Wofford and Miller (1987) who examined both lingual and inferior alveolar nerves found all but one dysaesthesia to resolve within 6 months and it was deemed to be "permanent".

In the 120 cases of lingual dysaesthesia examined by Mason (1988), complete recovery of sensation took place within 6 months and in all cases in which sensory deficit affected less t h a n the whole of the lingual distribution. The type of nerve damage would be likely to have been neurapraxia, especially in those cases that resolved within 1 month.

When partial lingual sensory function remains it may be assumed that part of t h e nerve s h e a t h and t r u n k remains undamaged and the prognosis is good. When lingual sensation is lost over the whole of the nerve distribution, degree of damage may still be slight, and most cases resolve in 3 months. In these patients it is difficult to predict early if they will recover, b u t if partial recovery occurs by one month they generally recovered totally. Hence even a total loss of sensation is not an indication for surgery unless it is known that the nerve is severed (Merrill 1979).

In general, the less severe alterations in sensation, including p a r a e s t h e s i a s , settled relatively quickly whereas t h e profound anaesthesias tend to be more persistent and often recover inadequately.

In the study by Blackburn and Bramley (1989). no patient made a full spontaneous recovery later than 36 weeks following injury. They perceived anticipation of any recovery later than this a s "optimistic".

Schwartz (1973) indicates t h a t the majority of temporary anaesthesias lasted from 6 to 9 months, although temporary anaesthesia was recorded for up to 10 years! The difficulty is the lack of description of what methods of assessment were used. It is quite likely that the situation that is being reported is that of patients becoming accustomed to their sensory deficit.

To be able to thoroughly understand lingual nerve injury and its prognosis one must understand the neuroanatomical distribution of the lingual nerve. Lack of proper understanding can lead to incorrect diagnosis and treatment.

In Mason's study (1988). he includes sensory deficit to the "tip only" a s part of lingual nerve damage. It is taken that only part of the nerve has been traumatized and a s such the prognosis of recovery is good. It is possible that these symptoms may not involve the lingual nerve at all.

Bilateral inferior alveolar and lingual nerve blocks conducted by Tier and workers (1984) consistently failed to anaesthetize a n area approximately one centimetre in diameter involving the tip of the tongue and part of the inferior surface of the tongue. It also does not effect the sense of position of the tongue. This can imply two things. Firstly that sensory supply to the tip of the tongue is not supplied exclusively by the lingual nerve and secondly proprioceptive afferents may travel by an alternative route to the lingual nerve.

Two point discrimination may be registered in patients who are otherwise totally anaesthetized on the affected side. The author (Blackburn 1990) continues to state that this provides reassurance to patients who considered that the nerve may have been cut but was more likely to have been compressed. The nerve may have been in fact severed even though two point discrimination was being registered, which can be explained by the following account.

It is known [Tier et al) that many connections exist between the hypoglossal and the lingual nerve which may further connect the lingual nerve with the ventral rami of C2 and C3. In the Neck-Tongue syndrome, head turning is the necessary and adequate precipitant of symptoms by subluxating C2/C 1 articulatory processes and effecting the lingual nerve via the above described connections.

Embryologists on the other would argue that when considering the embryology of the region it is unlikely that the cervical nerves can directly innervate eg. teeth (Heasman and Beynon 1986).

Loughman (1983) who presented the case of lingual nerve damage following laryngoscopy and tracheal intubation which eventually left an area of hypoaesthesia of the tip of the tongue but which did not reveal any loss of taste appreciation or motor function, would further support the previously described nerve connections.

Blackburn (1990) noticed that in the majority of those affected there was a wedge-shaped area a t the tip of the tongue which was partially or fully sensitive to touch stimuli and was present in patients where the lingual nerve was known to be divided. Unfortunately no detailed explanation was given for the account apart from the fact that different types of nerve fibres had different regenerative properties. This is somewhat misleading a s it is likely that the fibres travelling to the sensory receptive area had not been severed at all and no regeneration would have needed to have taken place.

-Inferior Alveolar Nerve.

4% of Alling's (1986) dysaesthesias were persistent, that is they lasted for more than one year.

As with lingual nerve recovery, if no changes of sensation suggesting a return of function are evident after 6 months, then it is "unlikely" that sensation will be returned. Very few cases continue to improve for up to 1 year after which little alteration is detected. These patients had a permanent sensory deficit, which was often an inadequate quality of sensation rather than loss in total area of sensation (Rood 1983).

Upton and workers (1987) who observed 15 traumatized nerves, 40% remained abnormal over a period of 6 to 55 months. The nerves that returned to normal did so by six months. A higher incidence of 64% remained abnormal following sagittal osteotomy. Regeneration usually occurs by 8 months after sagittal osteotomy. This greater regeneration time is probably related to the larger amount of nerve that must regenerate, a s the site of nerve injury may be proximal to the third molar area. Those who returned to normal and those that remained abnormal, both indicated that the descriptive term "pain" appeared progressively less as time passed. There is a gradual recovery in the whole of affected skin and mucosa of patients who had short lasting sensory disturbances.

In patients who recovered at 3 to 6 months the intensity and area of anaesthesia followed by paraesthesia gradually decreased. The margin of

normal sensation advances toward the mid-line. In patients with prolonged impairment, the affected area tended to shrink towards the vermillion border of the lip.

Although dysaesthesia of the lingual nerve is less common than that of the inferior alveolar nerve it is accompanied by a higher persistence rate (Alling 1986). The two ends of the lingual nerve may be displaced in the lingual soft tissues whereas the bony mandibular canal may decrease the amount of separation between the two severed ends and may act a s a guiding tube for the repair of the inferior alveolar nerve. In addition it must be considered that regenerating axons enter the endoneural tubes of the distal stump apparently a t random and may be guided to a n inappropriate receptor or sense organ and a new location. This inappropriate reinnervation is likely to be less disruptive in the inferior alveolar nerve which contains only somatic afferent and sympathetic fibres than in the lingual nerve which contains several functionally distinct fibre types and will be further discussed later.

Sympathetic fibres do appear to travel within the inferior alveolar nerve a s was exemplified by a patient whose lip became blue on the side that was affected by the injured mental nerve (Robinson 1988).

When inferior alveolar nerve regeneration is related to patient age a direct relationship is noted between increasing incidence of persistent altered nerve sensation and an increase in patient age. Upton (1987) noted 78% of traumatized inferior alveolar nerves returned to normal in patients aged under 21 years. In patients aged between 21 and 3 0 years, 47% returned to normal, while in the group aged between 31 and 40 years, 33% returned to normal. This may indicate the decreased healing capacity and prognosis with age (Merrill 1979) but might also indicate the severity of the nerve damage.

With increased age, and root development, roots become fully formed, and more closely approximate the mandibular canal. Roots are often dilacerated or may be affected with hypercementosis and ankylosis.

Bone is deposited around the tooth which becomes more mineralized, giving u p its elasticity and resilience. These factors tend to increase the elevator forces required (Koerner 1986). Additionally the likelihood of roots fracturing and requiring bur usage increases. This point is verified by a study conducted by Osborn and workers (1985) who found patients aged between 12-24 years displayed dysaesthesia in 0.5% cases, patients aged between 25-34 years displayed and incidence of 2.1% which is more than four times greater when compared to patients under 24 years.

I t is therefore very important to establish the proximity and

relationship of the roots of the mandibular third molar to the inferior alveolar canal. Panoramic radiographs should be supplemented with periapical radiographs which should provide much better detail. I t is generally agreed that if superimposition occurs near the upper or middle third of the root, it is buccal to it. However when the nerve is close to the apex of the tooth one can not be certain the nerve lies directly beneath the root or buccal or lingual to it (Merrill 1979). Not as often commented upon in the literature is the possibility of the nerve being entrapped by the roots of a n impacted mandibular third molar. Mishra (1987) describes of s u c h a case which produced a preoperative complaint of "shooting pain" in the affected side of the jaw. Patients should be informed of the possibility of nerve damage in advance b u t particularly so in cases where the nerve is closely involved with the roots of the mandibular third molar. Apart from radiographic evidence clinical evidence can also aid in determining s u c h a condition. If after being elevated slightly from the socket, the tooth springs back as if being pulled down by a rubber band this is often indicative of neurovascular tissue involvement with the root (Koerner 1986). If, after delivery of the tooth, there is profuse bleeding then it is probably a good indication that the vasculature h a s been interfered with.

Probably the best way of avoiding injury to the inferior alveolar nerve is the prophylactic removal of the mandibular third molar before the roots are completely formed. To reduce the possibility of inferior alveolar nerve damage this is probably best undertaken in patients under the age of 18 (Merrill 1979). In older patients Mishra (1987) suggests that asymptomatic mandibular third molar teeth closely associated with the nerve should not be removed.

Assessing and Documenting Nerve Injury.

In the literature many articles deal with reviewing patients with postoperative nerve trauma b u t very few deal with methods of rapid clinical assessment and documentation of nerve injury.

Mapping of the area of nerve injury and the rate of improvement or lack of it can aid in determining the type and onset of treatment.

Various avenues exist. Firstly palpation of the lingual tissues for a bulbous neuroma (which represents a n unsuccessful repair) may elicit pain.

Patients' subjective reports are not a very reliable indicator of recovery. Upton (1987) found no correlation between the terms used by the patient to describe the altered sensation a n d prognosis of nerve

regeneration. Furthermore reports of paraesthesia are not associated with a n increased likelihood of recovery which was demonstrated in a study which compared reports of paraesthesia in two groups of patients; one that recovered and one that did not.

Mapping of the affected area is usually determined by painful and non-painful stimuli. Photographs can be taken of the affected area.

Robinson and Williams (1986) devised a standardized method of charting the patient's problem. A schematic anatomical drawing outlines the sensory distribution of both lingual and inferior alveolar nerves which is not easily achieved photographically. I t is a very simple and quick method which acts a s a very nice medico-legal record, with a criticism of being very course in its measurement of paraesthesia.

Two-point discrimination h a s also been employed, although recently Blackburn (1990) recommends t h a t static two point discrimination may underestimate a patient's actual and potential recovery and that moving two-point discrimination may evaluate a greater proportion of touch fibres.

It has been shown that probably the best methods of assessing lingual and inferior alveolar nerve injuries comprise sensory testing, using touch, pin-prick threshold, localization and moving two-point discrimination. However, Upton a n d workers (1987) noted no correlation between the topographical distribution of altered sensation and the prognosis or residual deficit. Obviously different types of injury of the same severity can affect different areas.

Most cases of lingual nerve damage are unilateral, hence it is possible to compare the affected side with the unaffected side. It is good practice to test the unaffected side to determine the level of sensitivity which may be decreased a s is found in people who ingest spicy foods. An interesting finding was that when testing left and right sides of normal tongues, there is a n asymmetry in sensory thresholds ie. "sensory sidedness". Although the asymmetry is definitely there, clinically it may not be considered to be of great importance, though it should be borne in mind.

Interruptions and distractions will alter results. Given the reality that the tongue is a relatively inaccessible area prolonged mouth opening and tongue protrusion may bias results.

Confusing enough, some patients with normal results (eg. pain threshold) continue to complain of altered sensation (Blackburn 1990).

Treatment.

I t is indicated that to decrease the likelihood of nerve damage and

to increase the chances of healing, mandibular third molars should be removed during the teenage years. In most individuals mandibular third molar calcification is completed by about age 16. If radiographs reveal there is not sufficient space for the tooth to fully erupt, then the tooth should be considered for removal. I t is optimal to conduct this procedure before the roots are two thirds formed (Koerner 1986).

In the literature more has been written about the repair of the inferior alveolar nerve and less on the lingual nerve. Only 3 (2.7%) oral and maxillofacial practices in America in 1982 out of 105 responded to Alling's survey that the standard of practice included rnicrosurgical or other types of surgical intervention for dysaesthesias of the inferior alveolar and lingual nerves following third molar surgery.

Most repairs are undertaken if the patients subjective symptoms are seemed to be permanent. Many repairs are undertaken more than a year after the onset of dysfunction (Alling 1986). One can easily understand that delay in repair of a severed nerve can lead to reduced efficiency of regeneration. End organs in the skin, mucosa, teeth and periodontal structures atrophy, and reestablishment of the appropriate connections may never occur (Merrill 1979). The longer microsurgical repair is delayed the greater the likelihood of no or incomplete return of sensation. Although the quality of returned sensation diminishes progressively a s the time between injury and repair increases, it never falls to zero. It is worthwhile trying to re-establish some degree of sensation if only protective, which can be achieved even years after the original injury (Merrill 1979).

Somewhat of a dilemma is generated, a s rnicrosurgical repair is not 100% successful. Its success rate is closer to 90%. hence premature intervention and higher rates of normal healing have been reported earlier. Most cases of damage to nerves are probably due to crush injuries and nearly all recover (Rood 1983). Surgical treatment is usually not indicated for lesions of neurapraxia or axonotmesis but is indicated for lesions of neurotmesis particularly so if visualized during operation.

Some patients may be content to tolerate anaesthesia but others may not, particularly if the patient is eager to attempt correction. There can be much suffering with permanent paraesthesia or anaesthesia and to be able to relate to someone in such a condition all one has to do is recall the feeling (or lack of it) while having their last mandibular block during dental treatment.

It would be wise that if there is no or inadequate recovery in 6 months after nerve injury and certainly by 9 months then microsurgical decompression, anastomosis or grafting should be considered by a surgeon qualified in these practices.

During surgical exploration of the nerve, the nerve must be located and exposed without further injury. The nerve is examined, which in addition to being diagnostic may serve to decompress the nerve. Any impinging scars, bony fragments and foreign material is removed. The lingual nerve is accessed by raising a safe mucoperiosteal flap from the coronoid notch through the lingual gingival crevice of the molar and premolar teeth. The inferior alveolar nerve is usually approached intra orally by raising a buccal mucoperiosteal flap and removing a window of buccal bone over the area of injury (Merrill 1979).

If a nerve is seen to be severed, then it must be immediately anastomosed, particularly the lingual nerve a s its ends may be separated within the soft tissues. It should be noted that experiment microsurgical repair is 100% successful if repair is performed immediately following severance (Mozsary et a1 1982). The torn ends of the nerve should be aligned as correctly a s possible which may be difficult with the bleeding that usually obscures the field. Sutures should only be placed in the perineural sheath in order not to destroy the nerve fibres. Sufficient sutures should be placed in the sheath to prevent escape of axons through the suture. Repair of a specific nerve only differs in the surgical approach dictated by its location.

Injury to the lingual nerve may involve the loss of nerve length because of trauma, retraction of the severed ends, or both. Neuroma, a s well a s any other damage portion of the nerve has to be excised. Length of the remaining nerve would be critical. If tension is present, the repair is most likely to fail. Up to 1.5 cm of nerve length may be compensated for by repositioning the lingual nerve above the submandibular duct and hence by-passing the loop which the lingual nerve takes under the submandibular duct (Mozsary et a1 1982).

The lingual nerve in the region of the third molar effectively contains nerve fibres from two nerves. The chorda tympani nerve carrying special fibres of taste and parasympathetic secretomotor information and the proximal trunk of the lingual nerve which carries sensory information (touch, pressure, pain and temperature) to be distributed through the whole of the lingual nerve's distribution.

Girod (1989) who studied the fascicular structure of the lingual nerve and the chorda tympani, found it impossible to identify a chorda 4

tympani fascicle in the lingual nerve in the third molar region, where most damage occurs.

Because of the distribution of the gustatory fibres of the chorda tympani nerve, the chance of a proximal gustatory segment meeting and regenerating in the correct distal gustatory segment, is very low. This would explain the clinical phenomenon of a lack in the restoration of gustatory function after microsurgical repair of the lingual nerve in the third molar region.

I t is important to know the size of the nerve a s a whole and also the size and number of fascicles in a nerve with similar characteristics.

In nerve grafting procedures Heasman and Beynon (1986) showed that the number of lingual nerve fibres varies enormously between individuals.

They found a ratio of lingual nerve fibres to inferior alveolar nerve fibres, of 1 to 1.1.

Svane and workers (1986) noticed a significant decrease in fascicle number as the distal portion of the nerve was approached. Grafts that are larger than the nerve to be repaired would have the advantage of being better able to collect the sprouting axons of the proximal stump. Smaller donor grafts, would lead to reduced numbers of axons a t the distal stump.

Haschemi (1981) describes a method of restoration of sensibility in the mental area after injury to the mandibular nerve by partial anastomosis between the lingual and mandibular nerves. A partial anastomosis between the distal mandibular nerve stump and the lateral third of the lingual nerve recreated normal sensibility in the mental nerve area without leaving behind a sensibility defect in the tongue.

There are a few advantages offered by this technique, one of them being that it can be conducted in cases of unexpected nerve injury during operations under local anaesthesia.

There are certain pathological conditions where part of the mandible must be resected often sacrificing the neurovascular bundle in the resected segment of the mandible. Ishikawa (1986) describes a method of conserving the inferior alveolar nerve during resection of the mandible. After the inferior alveolar nerve is sectioned, traction on the proximal segment before it enters the mandibular canal delivers the nerve from the mandibular canal. Traction is used to deliver the distal segment through the mental foramen. The mandible is then resected and reconstructed with bone grafts for example. The ends of the sectioned nerve are excised to "freshen" the ends which are then anastomosed. When axoplasmic fluid oozes from the nerve this is an

indication that healthy tissue is reached. Lack of bleeding is also a clue which indicates further sectioning although haemostasis must be achieved before suturing is started, which can be achieved by waiting or placing mild pressure on the nerve (Merrill 1979). Any unnecessary loss of nerve length increases the difficulty of correct matching because the fascicular pattern within the nerve changes rapidly (Rood 1983). The greater part of the nerve now comes to lie in the buccal soft tissues, but posteriorly it crosses over the bone graft to ascend on the lingual aspect of the mandible.

Nerve sharing and nerve transfer are techniques for restoring sensory innervation to a denervated area by supplying axons from a distant dermatome or muscle. La Bane and workers (1987) have found the outer diameter and number of fascicles of the sural nerve in the middle calf region to be most compatible with the inferior alveolar nerve.

A suitable length of sural nerve can be harvested with minimal sensory loss to the lateral side of the foot and is interposed between one division of the greater auricular nerve and the distal segment of the severed inferior alveolar nerve.

MATERIALS and METHODS In attempting to determine the position of the lingual nerve in the mandibular third molar and retromolar regions, 3 6 Caucasian cadavers became available courtesy of the Department of Anatomy. No cadavers had any post-mortem examinations, all deaths being natural and atraumatic. All were embalmed via perfusion with a formalin mixture solution. The study was conducted early in the year soon after placement for use by undergraduate students. 3 cadavers had a full complement of teeth, 3 were partially dentate and the remaining 3 0 were edentulous.

The edentulous cadavers studied had a mean age 79 years, the youngest being 61 and the oldest 96 years. The dentate cadavers had a mean age of 79 years, ranging from 47 to 84 years. Exactly half of the edentulous cadavers were male and half were female. In the dentate and partially dentate 4 were male, 1 female and one was of unknown age and gender (Table I).

The cadaver heads were initially sectioned mid-sagittally to increase intra-oral access a s the jaws and perioral tissues were rigid.

This was achieved by incising through the skin down to the bone.

Anteriorly the incision was carried from the thyroid cartilage around the head past the external occipital protuberance. A circular power saw then followed the soft tissue incision and cut through the midline of the mandible and then the rest of the head. This cut was then completed down the neck to the level of the thyroid or a s far down a s the sternum by sawing through with a hand saw. The two halves were then separated a s far apart a s possible by placing a wooden wedge in the neck region.

Sometimes it was possible to detach one hemi-head. Dissection was then carried out to expose the contents of the paralingual sulcus.

Manipulation of the cadavers was under some restraint. More information could have been obtained if part of the nerve in the area of interest could have been resected. Bilateral access was not always possible a s some of the half heads were not available due to usage by undergraduate students. Bilateral dissections and measurements were conducted on 16 out of the 30 edentulous cadavers ie. a total of 46 sides.

Bilateral dissections were also carried out on 3 out of the 6 fully and partially dentate cadavers ie. total of 9 sides. Dissection involved a n extremely superficial incision through the mucosa and underlying tela submucosa which was carried along the anterior border of the ramus of the mandible from below the coronoid notch downwards and forwards, slightly medial of the midline of the retromolar pad and papilla and along the attached lingual gingiva in the dentate whilst just medial to the

alveolar ridge in the edentulous cases. The extension of the incision anteriorly varied in its terminating point. This was established until enough access was obtained.

The advantage of examining cadaveric lingual nerves is that they are stable in their position due to lack of mobility and most importantly, a s was discovered, is that the nerve position is maintained by the significant surrounding fascia. With the tongue slightly retracted medially, tissue forceps aided sharp dissection of the fascia and other tissues medial (lingual) to the lingual sulcus. Sharp dissection rather than sharp and blunt dissection was carried out in order to minimize the displacement of the tissues around the lingual nerve.

Black and white and colour film was then used to photograph the dissections which were recorded in a photographic log book. These acted a s permanent records of the dissections. Two black and white prints were made; one with a millimetre scale inserted and one without.

The equivalent colour record was achieved by using Kodak 35 mm EKTAR 100 ASA daylight slide film. The camera used was fitted with a 105 mm lens mounted on 9 cm bellows.

A schematic diagram was made of the retromolar pad, alveolar ridge, lingual nerve and any other interesting feature such as retromolar glands, submandibular duct, arterioles, veins, mylohyoid muscle etc. for each specimen. The shape of the lingual nerve a t both the level of the crest and papilla of the retromolar pad was estimated and recorded according to the classification used by Kisselbach and Chamberlain (1984). All teeth and root fragments were drawn and labelled. The table number also acted a s the identification number and examination of left or right side was noted.

Initially 5 measurements were taken on each side using either a thick needle with a rubber endodontic stopper and/or a bent probe with a rubber endodontic stopper. All measurements from the tip of the instrument to the rubber stopper were measured with a digital Mitutoyo hand held sliding calliper. Measurements were taken to the nearest millimetre although they were displayed to hundredths of a millimetre (Fig 9).

Fig 10 displays graphically all the measurements in a n edentulous specimen. The first measurement was from the crest (highest point) of the retromolar pad to the highest point on the lingual nerve (DC). This was taken a t a plane perpendicular to the mandibular basal plane which was approximately parallel to the occlusal plane of the lower molars. The next measurement measured the length of the retromolar pad from the

Fig 9.

Instruments utilized. k o m top to bottom: Digital ~ l l u l ~ callipers lg Bent probe Needle with endodontic stopper Scalpel Fine forcers

retromolar papilla to the crest of the pad and is perpendicular to the previous mentioned measurement (LP). Next was measured the depth of the superior surface of the lingual nerve from the mucosal surface of the retromolar papilla parallel to the depth measurement from the crest of the pad (DP). Subsequently the distance from the surface of papilla to the immediate bone underneath was taken to try to obtain some indication of the depth of the papilla (DPB) and hence the depth of the lingual nerve from bone under the retromolar papilla. Finally the distance from the superior surface of the lingual nerve away from the lingual surface of the body of mandible in the edentulous and the corresponding lingual plate in the dentate was taken (DLP).

Somewhat into the investigation it was thought that another measurement may have been useful in determining the relationship of the retromolar papilla to the lingual nerve, so a s well a s measuring the vertical distance of the nerve from the pad the shortest distance (SD) from the nerve to the pad was also determined for comparison.

All data were entered into a n Apple Macintosh plus computer and all statistical analysis and graphics were done utilizing the Statview programme.

RESULTS Displayed in Table I are the raw data from left and right sides of all cadaveric measurements, a s well a s the mean between left and right sides. Included is also the difference of mean distances between the depth of the lingual nerve from the mucosal surface a t the level of the retromolar papilla and the depth from the mucosal surface to bone a t the same level (ie depth of lingual nerve from bone a t the level of the retromolar papilla), as well a s the difference between of the means between the depth of the nerve from the mucosal surface a t the level of the crest and papilla of the retromolar pad.

The shapes of the lingual nerves a t both the level of the crest and papilla of the retromolar pad in the edentulous and dentate are summarized below.

Shape of the lingual nerve a t the level of the crest of the retromolar padEdentulous Dentate Round Ovoid Flat Round Ovoid Flat 40-Total 3 5 3 1 12 25 9-Total 33.3% 55.6%11.1% 62.5% 30% 7.5% Shape of lingual nerves at the level of the retromolar papillaRound Ovoid Flat Round Ovoid 4 1 40-Total 20 16 5 9-Total 10% 50% 40% 11.1% 55.6% Appendix I contains scattergrams of all the values observed for each variable, summary of the means and standard deviations of the variables and tables for more comprehensive descriptive statistics of all the variables, firstly for edentulous cadavers and secondly for dentate cadavers. No variable in the edentulous group was excessively kurtotic or skewed.

Paired t-tests revealed no significant difference for any variable between left and right sides. Similarly no differences were noted between males and females. All statistical tests utilized the 5% probability level to determine statistical significance.

In the edentulous group, the superior surface of the lingual nerve, was 12.4 mm on average from the crest of the retromolar papilla, ranging from 4 to 23.5 mm and 10.4 mm from the retromolar papilla, ranging from 4 to 17 mm. The shortest (perpendicular) distance from the retromolar papilla to the superior surface of the nerve on the other hand was 8.3 mm with a shorter maximum distance of 12 mm. This distance is from the mucosal surface and obviously would be less from

bone. (The mean thickness of the retromolar papilla (ie from mucosal surface to bone) is 1.9 mm. Hence when considering the mean vertical distance of the lingual nerve from the ridge of bone a t the level of the retromolar papilla (ie MDPL-MDPB) it was 8.5 mm with a mere 2 mm minimum.

Correlating age against the variables produced no obvious age related trends. A weak negative correlation of -0.231 and -0.312 appeared between age and the vertical and shortest distance from the retromolar papilla respectively.

The superior surface of the lingual nerve was on average 4.8 mm from the medial (lingual) surface of the body of the mandible but ranged from a minimum of 2 mm to a maximum of 9 mm. A moderate positive correlation of 0.415 was noted between the depth of the lingual nerve from the mucosal surface and the depth of the lingual nerve from the medial surface of the body of the mandible.

The depth of the lingual nerve from the crest of the retromolar pad was on average only 2 mm deeper than at the level of the retromolar papilla (ie MDC-MDPL). This relationship a s with the other variables showed a large amount of variability.

Of importance was that statistically the variables examined in the dentate group were not significantly different to those in the edentulous group apart from one variable; the length of the retromolar papilla which was 7.1 mm, in comparison to the longer edentulous retromolar pad with length 9.4 mm (P=0.037). A moderate negative correlation of -0.475 was computed between the depth of the lingual nerve from the retromolar papilla and the length of the retromolar pad in the edentulous.

Although statistically no differences were observed between the two groups some interesting findings were noted in the dentate group.

The minimum values for the depth of the lingual nerve from both the crest and papilla of the retromolar pad were considerably larger in the dentate group.

Min. Depth a t the Crest- In dentate 11 mm. In edentulous 4 mm.

Min. Depth a t the Papilla- In dentate 9.5 mm. In edentulous 4 mm.

The thickness of the retromolar pad was more variable in the dentate group with a maximum mean thickness of 7 mm in subject No 18 who had a third molar present. A maximum value of 7 mm was larger than the corresponding 4 mm in the edentulous.

Assessment of Errors.

Inevitably errors in the observations will occur mainly due to limitations in the technique employed, skill and care of the researcher, condition of the materials used and the general conditions under which the research was conducted.

Two types of errors exist (Townsend 1985): 1) Systematic errors which tend to bias the results by either consistently over or underestimating the results. In the present cadaveric study the results could have been biased due to the nature of the materials being investigated. Fixation of tissues tends to cause them to contract, subsequent exposure to a dry environment can further compound the contraction due to dehydration of the tissues. Post-mortem alterations must occur and the relationships of tissues vary a s the cadavers are laid in the supine position and the tongue collapses towards the pharynx.

This posterior collapse may have caused a n over estimation of the depth of the lingual nerve.

Deficiencies in the instruments utilized can further bias results.

2) Random or Accidental errors add to the natural variability of the measurements and may distort the interpretation of results. They can occur for example when it is difficult to obtain access to the structures being measured or when it is difficult to accurately identify landmarks.

This was particularly so in measurements taken from the crest of the retromolar pad which was sometimes difficult to define. The very steep path of the nerve through this area caused large differences in the length of measurements if the angulation of the measuring needle was only slightly varied. This was the most difficult aspect to try and standardize.

So instead of using vertical measurements, it was later decided to measure the shortest perpendicular distance from the retromolar papilla to the lingual nerve and a s will be shortly shown this was a more reliable measurement.

It is hoped that the magnitude of errors is small in comparison to the variability of the measurements obtained.

Assessment of reproducibility (precision) of results was determined by performing double determinations for the measurements taken (ie remeasuring the same subject). This was conducted in 14 cases (Table 11). One subject was excluded because of quite frank errors in measurement. This procedure prior to assessing the magnitude of error is called "cleaning" the data.

No variable was excessively kurtotic or skewed, which not only means that data were distributed normally but it also indicates that there were no unusually aberrant values which distorted the distribution.

Three methods were used to determine the reproducibility of the measurements in the edentulous cadavers.

1) Paired t-tests between the two determinations showed no significant differences between the two measurements.

2) Dahlberg statistic or technical error of the measurement.

The following values were obtained for the Dahlberg statistic for the variablesLength of papilla 1.0 mm Depth from crest 1.3 mm Depth from papilla 0.87 Depth of papilla 0.4 mm Shortest distance from papilla 0.56 Depth to medial surface of bone 0.6 mm The interpretation can be demonstrated by chosing the depth of the lingual nerve a t the level of the retromolar papilla. From the results observed the mean is 10.4 mm but the true mean would 10.4k0.87 mm (68% of the time).

3) Reliability The Observed Variance = True Variance + Variance due to Error therefore The True Variance = Observed Variance - Variance of Errors Reliability can be expressed as a percentage: Reliability of variance-Depth from crest 93% Length of papilla 82% Depth of papilla 67% Depth from papilla 94% Shortest distance from papilla 94% Depth to medial surface of bone 87%

DISCUSSION In agreement with Kisselbach and Chamberlain (1984) the literature certainly lacks detailed descriptions and measurements of the exact location of the lingual nerve relative to the third molar region of the mandible. Those who do make some attempt to qualitatively describe the nerve do not discuss the variability of its position. In the study of Kisselbach and Chamberlain (1984). vertical measurement of the lingual nerve was taken from the alveolar crest and horizontal distances from the lingual plate. As far a s the position of the lingual nerve in the edentulous is concerned these landmarks become irrelevant. The present study is the first study where the position of the lingual nerve has quantitatively been described in edentulous humans.

Currently the majority of cadavers in Adelaide are edentulous which is probably a good reflection of the population at hand. Although initially it was hoped that the lingual nerve could be measured predominately in the third molar region of dentate specimens, it was a blessing in disguise that the specimens were mainly edentulous for this allowed data to be generated relating to the lingual nerve in the retromolar region of the edentulous.

In Kisselbach and Chamberlain's dentate study (1984). hard tissue landmarks were used. It may be that these hard tissue landmarks may be more unreliable than the soft tissue landmarks used in the present study.

The alveolar crest and lingual plate will vary according to disease present and the state of eruption of the tooth, which may have caused dehiscence and resorption of the lingual plate, particularly in aberrant bucco-lingual impactions. It could be counter argued that the soft tissue landmarks such as the retromolar papilla may also be distorted, for example by being compressed by dentures which usually extend a t least over the retromolar papilla. If this were the case in the cadavers studied then it would be realistic of the surgical scene where dentures are usually removed only shortly prior to the operation.

Up high in the pterygomandibular space some 5-10 mm below the foramen ovale the lingual nerve branches off the mandibular trunk where it soon receives into its posterior surface the chorda tympani nerve. It is then almost always described as travelling anteroinferiorly to sweep under the superior constrictor where it continues its passage in the paralingual (sublingual) space/sulcus/pouch. Although this is true enough, the nerve does not travel in a straight path downwards and forwards.

Generally round or ovoid in shape the nerve travels in a sigmoid shaped course; sigmoid shaped when viewed from both the sagittal and corona1

planes (Fig 8). In the pterygomandibular space the lingual nerve may communicate with the inferior alveolar nerve of which the significance is probably unknown (Fig 11). Such connecting bridges between the inferior alveolar and lingual nerves were found in 25% of cases examined

by RAcz and Maros (1981). The path of the nerve beyond the anterior edge of the medial pterygoid muscle and hence the pterygomandibular space is usually oversimplified and is superficially ascertained by both the majority of anatomists and surgeons.

The posterior boundary of the paralingual sulcus can be defined by placing a finger into the floor of the mouth and anterior to the palatoglossal arch. By advancing the finger posteriorly into the reflection of mucous membrane from the tongue to the mandible a resistance will be encountered against the anterior border of the medial pterygoid muscle (Barker 1979). It is said that it is possible to palpate the lingual nerve in this posterior part of the lingual sulcus by rolling the nerve and tissues under the finger at this posterior point in the paralingual sulcus.

Heulke (1973) describes the pterygomandibular raphe a s extending from the pterygoid hamulus to the apex of the retromolar triangle. The buccinator muscle lies lateral to the raphe and the superior constrictor medially. By carefully stripping away only the mucosa the three parts of the superior constrictor can be visualized. The bulky, traditionally described part of the superior constrictor can be more accurately described a s the buccopharyngeal part of the superior constrictor. From the inferior border of the bulk of the superior constrictor run two slips of muscular fibres (Figs 12 & 13). The plossopharyn~ealpart runs medially towards the superior aspect of the root of the tongue. The mylopharyngeus' fibres named by some as the lingual fascicle (Barker 1969, 1971, 1979) run laterally to insert into the region of bone above the mylohyoid muscle and anterior to its posterior border. The attachment of the mylopharyngeus is along a horizontal elevation of bone a few millimetres below the crown of the third molar and just under the retromolar mucosa and is best termed the mylopharyngeal line which extends a variable distance in front of the third molar tooth.

The glossopharyngeus is considered by some (Barker 197 1, 1979) to be continuous with the mylopharyngeus muscle, where by it intermingles and becomes indistinguishable from the styloglossus.

Effectively this creates a sickle shaped fold of muscular fibres spanning between t h e mandible and the tongue. Spread between the mylopharyngeus and glossopharyngeus and forming the posterior

-. - -..

L

.

.

Fig 11.

Note the connection between the inferior alveolar and lingual nerves. .* L - Lingual nerve [ - Inferior alveolar nerve M - Mvlohvoid nerve -c

a

INJECTION

TRIANGLE

UCCINATOR M.

YLOPHARYNGEUS

Fig 12.

M.

,:..

l

l

Fig 13. .

Above: ~ y l o- p--h a p g e u sM and ~ l o s s o ~ f i a r ~ n g e(G); u s parts of thc superior constrictor -- --which span across the paralingual space. -Below: Lingual nerve exposed by removing the overlying muscle fibres.8 L

&

.,.

A

boundary of the paralingual space is a fascia1 sheet called the quadrangular fascia which maintains the lingual nerves position whilst allowing some mobility, but increases the difficulty in finding the nerve (Fig 14).

The lingual nerve leaves the pterygomandibular space by sweeping under the mylopharyngeal portion of the superior constrictor muscle and enters the paralingual space by penetrating the lateral aspects of the quadrangular fascia. In order to expose the lingual nerve, the attachment of the mylopharyngeus muscle to the mylopharyngeal line has to be removed. The mylopharyngeal line which purely represents a muscular attachment may be confused by those whom have not examined the relationship of the nerve to bone a s being caused by grooving below the elevation by the lingual nerve which is not the case. This elevation may have a beneficial role of protecting the lingual nerve from surgical trauma a s it lies some distance above the nerve.

As the lingual nerve passes medially to eventually enter the tongue it does come very close to the lingual surface of the mandible, more so at the level of the crest of the retromolar papilla, where no prominent feature separates it from bone and may in fact groove the periosteum of the mandible (Fig 15). This close relationship to the medial surface of bone is as mentioned closest a t the level of the crest of the retromolar papilla and not at more inferior levels such as the apex of the third molar a s others have stated (Tier et a1 1984). The area where it comes closest to bone is of course the equivalent to the hazardous distolingual area where in mandibular third molar surgery elevators, burs, chisels or bone fragments can impinge on the nerve just where the nerve is a t greatest risk for injury (Figs 16 & 17).

At the level of the crest of the pad the mandible and hence the course of the nerve flares laterally. In cases where the nerve deviates excessively in a mediolateral direction to follow the corresponding lateral flaring of the mandible, the nerve will commonly a t a position between the crest and papilla of the retromolar pad actually run through the retromolar pad (Figs 18 & 19).

Average values may not be a s useful a s examining the range of values. The minimum distance of the lingual nerve from the crest of the retromolar pad was only 4 mm and if the soft tissue thickness is removed then there would not be much separating the nerve from the crest of bone. This, in addition to the fact that the nerve may run through the retromolar pad must dictate that an incision must always be carried lateral to the retromolar pad. The incision may be carried to the

Fig 14.

Above: R - Retromolar pad ,. --..-..

M - Mylopharyngeus .. n t i G - Retromolar gland Q - Quadrangular fascia ~eloW:Parts of the quadrangular fascia removed to expose the lingua ' . & l 4 -

-C,

9

Fig 15. . -Above the level dA-&itF;e'o&&% % f the mylohyoid muscle lies the lingua nerve very close to the medial surface of the mandible.

R - Retromolar pad L - Lingual nerve with gingival branch

k',

Fig 16.

Fully and possimy over erupted lower third molar present. The lingua nerve lies very close to the whole of the lingual aspect of the lower thin $"-C

rnnlr~r

3.

--.&

Fig 17.

Impacted lower - t h i r m r not visible intra-orally. Retromolar pal remains unaltered whilst the contents of the paralingual space ar exposed. Note gingival branch of the lingual nerve.

- -f(?nntinil~d RvPr nacre

l

Above: Retromolar pad and other soft tissues overlying the crown of tht impacted tooth is removed. Scale indicates that the nerve --- - -is less -- -than - -- ---l mm plate. .I - -- below the crest of the lingual ---Below: The lingual plate is removed and the lingual nerve is positionec *3:directlv over the crown of the lower third molar. .

-----v---

p

U.

*" A

Fig 18.

Above: Frontal view. In its ascent from tne tongue the lingual nerve run mediolaterally and may run through the -- retromolar pad. ---a.&L - - --Below: Medial view. Blue line indicates alveolar ridge to the point of th retromolar papilla and the black line the the periosteum of th r-, * r

---J*Ll-

$ c

l,,...*

.f

S

.

f

Fig 19.

The blue line indicates the alveolar ridge to the level of the retromola papilla. Note that the lingual nerve runs through the substance of thc retromolar ~ a ~ i l l a . p -

point of the retromolar papilla where the nerve lies medially and a t least a very close 4 mm from the mucosal surface of the papilla which is effectively 2 mm from the ridge of bone but the incision against bone should never be extended medial to the retromolar papilla.

It was noticed a s with Kisselbach and Chamberlains' study (1984) that a s the nerve passes anteriorly from the crest to papilla, it changes shape from being predominately round to mainly avoid and flat. The trend in shape change is in agreement with Kisselbach and Chamberlain (1984), although the percentage frequency figures are not, this may not be a s important. The differences were most likely to be due to the difficulty in standardizing the procedure which was very subjective in its nature and crude in its categorisation. The size and shape of the nerve presented can be quite different from the actual size and shape of the nerve due to varying amounts of surrounding connective tissue and fat which may make the gross calculation of size and shapes of nerves invalid (Fig 20). The nerve in some cases became extremely thin and this was not only true for flat nerves a s some round nerves were very thin. To be able to accurately measure the diameter of a nerve would have required resection of the nerve which would have had to be done at the time of exposure in order to prevent further distortion. This unfortunately was not possible in this study due to the various restraints mentioned earlier. Merrill (1979) suggests that the lingual nerve in the region of the third molar is in the range of 3 mm in diameter. Kisselbach and Chamberlain (1984) reported that some of their flat nerves (8.8%) in the lower third molar region were 0.5 mm thick (Fig 21).

If a n exploring 27 gauge long needle with a 0.4 mm diameter encounters a similar sized nerve then it should be of no surprise that damage can be created by local anaesthetic needles (Fig 8).

Apart from Schwartz's questionnaire (1973) which indicated that 44% of lingual nerve anaesthesias were a result of local anaesthetic needles passing through the nerve trunk. No other information could be found a s to this possibly very serious and most likely underestimated problem.

One of the most interesting findings to come from this study was that the position of the lingual nerve does not vary statistically between dentate and edentulous individuals and no age related associations were noted. Obviously resorption and loss of bone occurs with loss of teeth, yet no significant difference in position from the mucosal surface to the nerve was noted. This became obvious to the author prior to the statistical analysis of the results a s the nerve usually ran across the

Fig 20.

Two examples of the shape of the lingual nerve being distorted by , large amount of surrounding connective tissue. In the figure below the probe indicates the position of the retromolar pad and also note that th nerve runs through the substance of the ad.

Fig 21.

The lingual nerve becomes more rounded further back and some fll nerves m a v he extremelv thin.

I. h

C'-

lingual plate in the dentate specimens and whilst maintaining a similar span, ran across the mylohyoid muscle in most of the edentulous cases particularly where resorption had caused the mylohyoid line to be very close to the mucosal surface.

The lack of difference between the two groups plus the fact that the depth from the mucosal surface at both the level of the papilla and crest of the retromolar pad are similar, tends to indicate that the lingual nerve maintains a stable relationship with the mucosal surface, migrating inferiorly with bone loss.

The surgeon should not be mislead into believing that because the retromolar pad is thicker at the level of the crest than a t the level of the papilla that the lingual nerve should lie deeper at the level of the crest.

The only weak indication of the position of the nerve in the edentulous can be obtained by calculating the length of the pad. The shorter the retromolar pad the steeper the course of the lingual nerve through the retromolar region. In those with longer pads the lingual nerve runs a more horizontal course which places the nerve closer to the mucosal surface. It is suggested by some though, that the nerve may actually be visualized through the mucosa or be located by palpation (Merrill 1979).

Even given that it is sometimes possible to see in a cadaver a bulging of the mucosa produced by the lingual nerve it is the authors opinion that this would be very difficult and unreliable if not totally impossible in the living situation (Fig 22).

At first glance the average depth of 10.4 mm from the surface of the retromolar papilla to the superior surface of the lingual nerve appears to be significantly larger than 2.3 mm from the alveolar crest as reported by Kisselbach and Chamberlain (1984). This is somewhat misleading because if we examine specimen NO 18 which was fully dentate we notice that the mean depth of the lingual nerve from the retromolar papilla is 8 mm which is similar to the overall mean of 10.4 mm. If though, the soft tissue thickness (6 mm) of the retromolar papilla is subtracted from the depth of the nerve, one will notice that it is actually only 2 mm from bone which compares favourably to the distance of 2.3 mm from the alveolar crest of Kisselbach and Chamberlain's (1984) results.

There can be no doubt that some degree of distortion occurs in any cadaveric study. One trusts that the amount of distortion is minimal in relation to the natural variability that exists. Cadavers post -mortem are usually in a supine position and remain that way well after the onset of rigor mortis. I t is more than likely that there is a posterior collapse of

Fig 22.

M - Mandibular symphysis A - Alveolar ridge R - Retromolar pad - . -- - .

.

I

a--

.

- 7

I

..,F.

-

-

~) I - ..::-, :;',,.

- ..

-,

-

.. .

-'..;

3

. a1 -a-

*.

the oral tissues including lingual and paralingual tissues. It is hence quite possible that with posterior collapse of the root of the tongue that the measurements of the depth of the lingual nerve at the level of the retromolar papilla may be severely overestimated and the depth of the lingual nerve in the living may be quite shallower. Another cause of overestimation of the depth of the lingual nerve is due to the relationship of the nerve to the relative positioning of the jaws. When the jaws are open the nerve becomes more taut and runs a steeper course which increases the distance between mucosal surface and the nerve at the level of the retromolar papilla (Fig 23). Clinically though these two causes of overestimation of the depth of the lingual nerve may be irrelevant because most procedures, particularly conducted under general anaesthesia are undertaken in the supine position with the mouth widely opened.

Kisselbach and Chamberlain (1984) were able to include clinical observations in their study of the relationship of the lingual nerve to the mandibular third molar region. In the course of 256 consecutive mandibular third molar extractions performed by the buccal approach, they "attempted to carefully expose" and selectively photograph the lingual nerve. It is the present authors opinion and experience that it is relatively easy to violate the nerve whilst seeking it. During exposure of the cadaveric nerves in the present study the nerve was sometimes partially or in one case totally severed (Fig 24) and it must be borne in mind that the procedures were conducted with great care under more favourable conditions with much greater control being available. In the cadaver there is no haemorrhage and access is markedly better. The ethical viability of such clinical studies may be questionable particularly if the subject is undergoing routine buccally approached extractions and is not aware that the lingual nerve is to be exposed with no treatment related benefits to the patient.

The large amount of haemorrhage that occurs during surgical procedures in this region whereby the surgeon may be working in a pool of blood indicates the difficulty of operating in the area, further compounding the inherent risks of the procedure. Access is already restricted and awkward, haemorrhage only makes visualization worse.

It is clear that the floor of the mouth is highly vascular, a feature which was evident during dissection (Figs 25 & 26 ). Accessory arteries may be emitted from the bone and haemorrhage from these vessels can usually be brought under control by compressing these vessels against the bone. Branches of the sublingual branch of the lingual artery can be found throughout the paralingual space. The lingual artery is at first deep

Fig 24.

Above: Muscular fibres of the superior constrictor obscuring the locatiol of the lingual nerve$.- . -.I,' -%'--Relnw- T irtmlnl nerve was severed wmst seeking it. -p

;

-a

5 .-

?ig 25. rhe lingual nerve [wlul gUi@vzd branch) hooks uilder the submandibular iuct. Note the rich vascularitv of the reeion.

Fig 26.

---A---* l * During dissyction three cord like structures were uncovered and --- . confusion a s to which is the lingual nerve can be generated. +f 2* Further dissection revealed the highest structure to be the lingual nerve; the middle structure the submandibular duct and the lowest an artery (sublingual). (Continued over page.) . , : ,L;>-=%- ?

,--I#

-

3* Lateral disdeetf6n af the infratemporal fossa verified that the mghc structure was in fact the lingual newe.

to the hyoglossus muscle but subsequently gives off many branches. The lingual vein is separated from the lingual artery by the hyoglossus and tributaries of the lingual vein will be found in close association with the nerve (Fig 27). Branches of artery branch of facial artery) can penetrate through the mylohyoid muscle to also supply the floor of the mouth (Castelli et a1 1969).

Commonly coming off the inferior alveolar artery or directly off the maxillary artery in the pterygomandibular space is a small artery which runs adjacent to the lingual nerve. Extending into the paralingual space with the lingual nerve it is assumed that it supplies the nerve given its close proximity.

The path of the lingual nerve past the retromolar papilla is also important and should not be neglected. As the lingual nerve courses towards the tongue it comes into intimate contact with the submandibular duct and this relationship is of much surgical importance.

The deep portion of the submandibular hooks around the posterior, free margin of the mylohyoid muscle and gives of the submandibular duct which corses obliquely forwards and medially. Initially lying lateral to the submandibular duct the lingual nerve which is also travelling forwards, descends under the submandibular duct, to ascend medial to the submandibular duct and into the tongue muscles. Castelli and colleagues (1969) determined that this cross-over of the lingual nerve and duct was variable and extended from the distal part of the second premolar to the retromolar triangle. 55% of the cross-overs occurred at the level of the third molar tooth or behind it. As mentioned before if it is desirable to gain length of nerve, the repositioning of the nerve above the submandibular duct can achieve a gain of 1.5 cms in length. I t is possible and not so uncommon for the nerve to run parallel to the submandibular duct instead of crossing under it (Mozsary et a1 1982) which, during operations, may prevent a n unfavourable or confusing relationship.

Much variation exists between specimens and individuals but also between left and right sides. The sublingual gland when well developed can extend a s far a s the second molar tooth and more often than not the submandibular glands would appear a s one great mass, with the lingual nerve travelling through the salivary tissues (Figs 28 & 29).

Before entering the muscles of the tongue, the lingual nerve gives off its parasympathetic nerve fibres to the submandibular ganglion. The submandibular ganglion can only be visualized through extremely delicate dissection and can be seen more easily from a n extra oral approach a s it may only exist a s a n irregular swelling of the nerve trunk (Barker 197 1).

Fig 27.

L-

nerve 1ch 43&./

v - Lingual vein

H - ~yoglossusmuscle

2

qa;ab

Fig 28.

Above: Prominent sublingual gland (SG) clearly visible in the sublingua sulcus and may reduce the stability of the denture.

Below; Dissection revealing the extent of the sublingual gland medial c t the alveolar ridge and lateral to the inserted in.

Fig 29.

Above: The deep portion of the submandibular gland (G) is large and superficially located, which obscures the view of the lingual nerve (L).

Below: Further dissection exposing the lingual nerve. Red pin separating the lingual nerve laterally and the deep portion of the submandibular 4;-{L:, F gland medially.

R..crg .--' -- c , dt Y -.

P -

,

~

Lr

L

Sometimes the lingual nerve while travelling along the mylohyoid muscle and usually before looping under the submandibular duct would give off a significantly sized branch to the mylohyoid muscle, usually a t a right angle to the nerve. At first uncomprehensable these abutments were followed through the mylohyoid muscle a n d were seen to be communicating with the mylohyoid nerve (Fig 30). Further investigation of the literature revealed t h a t Racz and Maros (1981) documented this phenomenon of which they termed the mylohyoid or sublingual curl.

They found in 33% of cases, one of the terminal branches of the mylohyoid nerve would perforate the homonymous muscle to anastomose with the lingual nerve.

Another interesting finding to be encountered during this study was the relatively constant appearance of a branch coming off the lingual nerve usually before crossing under the submandibular duct to supply the area of the lingual gingivae. Most anatomical descriptions will indicate that the lingual nerve supplies the lingual mucosa and lingual gingivae (Heulke 1973) b u t n o n e describe the distinct a n d often very large gingival branch. This relatively constant branch is present in both the dentate and obviously the edentulous (Fig 31). Lacking description, the clinical significance of this branch is not understood and as s u c h may lead to misdiagnosis and mismanagment of patients with lesions of this nerve. The following example demonstrates the situation. A young female patient h a d recently h a d their third molars surgically extracted.

Following surgery the patient complained of paraesthesia along the lingual gingival mucosa of one side b u t not along the distribution of the tongue, and taste sensation had not been impaired. It is known that fine mucosal nerves cut during incision will regrow without difficulty (Merrill 1979) b u t it is likely though that the above described patient suffered i n j u r y to t h e gingival b r a n c h a n d n o t to t h e m a i n nerve trunk.Consequently the patient had multiple lingual nerve explorations under general anaesthesia. During the procedures the main trunk was cryofrozen which obviously hampers the physiological functioning of the nerve even though the main trunk was probably originally unharmed.

It is the author's future intention to document the anatomical description and incidence of these gingival branches and to investigate the clinical effects and treatment of injury to these gingival branches as information in this area is lacking. It is already known t h a t testing for nerve injury in the floor of the mouth and adjacent areas is difficult due to the problems with access and the need to use a tongue retractor which distracts patients from the test stimulus (Blackburn 1990).

Fig 30.

Above: Lingual nerve with mylohyoid or sublingual curve piercing the mylohyoid muscle.

Below: Extra-oral view of the i n f e m u r f a c e of the mylohyoid muscle.

Piercing the mylohyoid muscle is the anastornosis between the lingual and mylohyoid (M) nerves.

Fig 31.

Above: Dentate specunen.

G - Gingival branch S - Sublingual branch ..

Below: Edentulous specimen Black line indicates alveolar ridge and terminates posteriorly at thc retromolar papilla.

b

Lingual nerve damage can occur even when meticulous care is taken (Fielding and Reck 1986). Cases have been documented, and also noted by the author, where the lingual nerve has not necessarily been severed but has been affected by scarring which constricts the nerve.

The formation of scar tissue may cause neurapraxia but may also compromise other neuronal lesions such a s axonotmesis and present the false impression that the nerve may actually be severed due to lack of progress in recovery. Such sensory impairment can be misdiagnosed a s other forms of neuralgia. With the advent of cryofreezing, surgeons may be less hesitant in surgically exposing the lingual nerve and cryofreezing it. Not only is this a risky procedure but it may be inappropriate if injury is due to scar formation of which further inappropriate surgical intervention may add to the problem (Fig 32).

Rood (1983) describes a case of paraesthesia which was present for 2 years and following the removal of constrictive bands of fibrous tissue around the nerve there was total recovery in 60 days. One oral surgeon, respondind to Schwart's (1973) questionnaire, described a case of postsurgical lingual anaesthesia where the lingual nerve was exposed and it was found to be intact but was freed from scar tissue. The patient had feeling in her tongue the next day and was elated. The latent problem with this and other patients is that after decompression, constriction may gradually recur with the reformation of scar tissue.

Probably the single most common reason of lingual nerve damage is a lack of knowledge of the anatomical position of the nerve damage, for example there can be no exuse for clamping and tying the nerve thinking it to be a vessel (Merrill 1979). Robinson and Williams (1986) state that, "Although iatrogenic in origin, paraesthesia does not necessarily suggest surgical error or incompetence". Their statement is probably correct given the limitations of present surgical techniques and the accompanying instruments used. It has been known for quite a while that nerve injury may arise as a result of mucoperiosteal flap elevation alone (Merrill 1979) yet no major alteration in technique or instruments used has ensued. Traditionally once the elevator is down the lingual side of the mandible, more likely than not in the region where the nerve is closest to bone (ie retromolar region), the elevator is worked forward exposing the site of operation (Merrill 1979). It may in fact be wiser to start elevating the mucoperiosteal flap anteriorly where the nerve is deeper to the mucosal surface and subsequently the dissection should be conducted posteriorly.

pig 32.

I13a;al.e patient

-.

'. -,. -.--

55

- 4il;ud

.

with recurrent h p d nerve neuralgia and a

us treammt Mstoq @f L * Preapesative. fb t i n w d ) k , ? ' ;

af the lingual nerve. .

P$

..,

2. Postincision. rool of blood makes surgery increasingly difficult.

fr\nntiniied\

l@ Exposure

#carring.

'

..

ol-&&Tingudnerve (L) reveals signiflcanf-gm&nts 6

As in all cases, the informed patient is the best patient who is lest

apt to be a problem (Robinson and Williams 1986). The patient should be informed of the possibility of nerve injury during surgery which includes a variety of procedures other than mandibular third molar surgery.

Swanson (1989) realizes that changing social mores,more sophisticated consumerism and a more informed public has lead to a n exponential increase in malpractice suits related to neurosensory dysfunction. If not already so, it probably won't be long before verbal consent is no longer adequate a s the surgeon does not have to be negligent to be liable. Nor will it be sufficient to briefly note on the patient's records that they were informed of the possible risks. As far a s the legal community is concerned, written informed consent is the only acceptable standard. In the U.S.A. some insurance companies will not insure if written consent is not practised. Fielding and Reek (1986) and Swanson (1989) provide some sample consent forms (Figs 33 & 34).

The litigation trend is not only increasing in the U.S.A. but is also increasing internationally. In the U.S.A., in a two-year period between 1984 and 1986 there was a 1552% increase in claims reported and during the same period there was a n 8390% increase in dollars awarded.

Apart from a comprehensive understanding of the anatomy of the region high quality records of surgical procedure should be kept which should document any unusual happenings or unusual anatomical variations. It is good records which can prompt one's memory if a malpractice case is heard some time after the actual event. Those practioners lacking the anatomical knowledge and other necessary skills should realize their limitations and make the appropriate referral.

TEMPLE UNIVERSITY DEYTAL SCHOOL DEPARTMENT OF ORAL AND M A X I L M F A C I A L SURGERY CHECK LIST FOR THIRD MOLARS

-

Informed Consent 1.

'

Nunbnees

A.

B.

Complications lingual nerve - Nerve to the tongue resulting in numbnesa of the tongue decreased o r loss of taste. and decreased o r 100s of salivary f l w resulting in a d r y mouth. inferior Alveolar nerve - n e w e t o the teeth and lips. May have nunbnesa of the teeth and lips cm the side of the surg .ry.

2.

Trismus. or difficulty in opening or closing your mouth.

3.

Swelling

L.

Pain

5.

Sinusitis

6.

Excessive henorrhage or bleeding

7.

Fracture of mandible (Broken jaw)

8.

Damage (fracture of fillings) or death of adjacent teeth.

9.

Inflammation of. or opening into. the maxillary sinus that is located above your upper teeth. and may require additional treatment or surgcry to correct.

10.

A small piece of root being left in the jaw when its removal would mquire extensive surgery o r result in further complications.

11.

Injury to or bruising or scarring of the corners of the m t h of skln o f the face; retraction o r bur abrasion.

12.

Reaction to the anesthetic agent or antibiotic

The frequency of occurrence of the above complications are different for each item. but infrequent for any of them. Please complete fora. Feel free to consult with thc surgeon about any of the above at any time.

I certify that these complications ndvc teen discussed with me and I the posslbillty of their orcurrrnce.

unders'and

DATE

51 GIJEU

Temple University School of Dentistry informed consent form for removal of impacted or unerupted teeth.

FG I 33.

CONSENT TO ORAL AND MAXILLOFACIAL SURGERY S A M P L E 1. I, the undersigned, hereby authorize and request Dr. (S) r.H . C ~ ~ ~ C C ~ L and/or , to perform the following oral and maxillofacial procedure (S) upon me:

And, I consent to the performance of the above procedure(s1, as well as to the performance of such additional/alternative procedures as in the judgment of the above doctor(s) may be necessary to restore and/or preserve my overall dental health, as well as to treat the particular dental disorder described to me as:

2.

I also authorize and request the administration of local or general anesthesia as may be deemed advisable by the above named doctor(s); and I hereby consent to the administration of said anesthesia as the said doctor(s) deem(s) advisable.

3. It has been explained to me, and I understand that success of the aforesaid surgical procedure(s) and treatment is not guaranteed or warranteed and, in addition, I understand that such success cannot be guaranteed or warranteed.

4. Such alternative treatment methods to the surgical procedure(s) above described as are available to treat the aforesaid dental disorder in my case were fully described to me prior to the time I executed this consent to oral and maxillofacial surgery as witnessed by my signature and the date below.

5. Finally, such risks and complications as may ultimately develop and/or immediately follow upon the above mentioned procedure(s) and administration of anesthetics have been fully explained to me, including, but not limited to:

bC

. L 7

DATED :

SIGNED: WITNESSED:

Sample o f consent form.

f

CONCLUSION Lingual nerve damage can lead to severe and serious impairment of one's quality of life. Problems with speech can affect communication and can be most detrimental to those dependent on public speaking. For example Blackburn (1990) reports that loss of taste threatened the livelihood of a chef and also resulted in family complaints about cooking by patients who were responsible for home cooking. He also notes that several patients who developed a tendency to lisp were accused of drunkenness a t work.

The incidence of iatrogenic injury of the lingual nerve associated with the most common oral surgical procedure, removal of impacted third molars, is usually estimated on the basis of clinical impression and leads to a n underestimation of the risk involved with this complication due to a lack of statistical information. Statistical data provided by Schwartz's (1973) questionnaire points out how common this problem may be, with 66% of responding oral surgeons indicating that their patients had experienced lingual nerve damage following mandibular odontectomy.

Underestimation of the risk involved in violating the lingual nerve during surgical procedures in dentate and edentulous individuals is partly due to a lack of thorough understanding of the anatomical variability in the area of operation. This is most likely due to the small amount, or even the total absence, of quantitative data describing the relationships of the lingual nerve. In order to address the problem a t its roots, undergraduate dental students should conduct dissections of the infratemporal fossa and paralingual space from the medial approach during applied gross anatomy practicals. Surgeons may also benefit from regularly perform intra-oral dissecting exercises of the paralingual space in cadavers.

A careful understanding of procedures which may involve injury to the lingual nerve must be acquired, even for unusual and less reported complications related to rigid fixation during orthognathic surgery or local anaesthesia eg. piercing local anaesthetic needles, both of which may be worthwhile additions to the informed consent practice.

With the development of studies such a s the present one, the quality of present day techniques and their associated instruments should improve to a point whereby prevention of nerve injury will be accomplished and there will come a time where there will be no exuse for cutting or ligating the lingual nerve.

Finally then, a s the late and great Australian anatomist, Dr. Barker (1969) suggested, there is a continuing need for investigation into the diverse variation in anatomic form which can be observed a t a macroscopic level.