Mandibular Foramen

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

INTRODUCTION and REVIEW of the LITERATURE Knowledge of the position of the mandibular foramen and related nearby structures is important for those in medical and paramedical sciences who wish to anaesthetize the inferior alveolar nerve and its area of sensory supply.

Failures in mandibular (inferior alveolar nerve) anaesthesia continue to be a significant problem especially when considering how often the clinician practices this anaesthesia technique a n d the long term (psychological) effects of failures in anaesthesia.

Failure rates of 20% have been described for mandibular block anaesthesia (Rood 1976).

Lack of understanding a n d familiarisation of t h e empirical landmarks is probably the most likely reason for failures in achieving anaesthesia, particularly in the inexperienced. The truly disturbing problem is that even the experienced clinicians fail to produce adequate anaesthesia (Harvey and Robb 1967). The likely explanation here is that there is a lack of understanding i n the accuracy of the empirical anatomical landmarks as well as a lack in thorough understanding of the amount of anatomical variability t h a t exists in the structures associated with mandibular anaesthesia.

It h a s been known for a long time t h a t the teeth may receive accessory nerve innervation. Sicher (1946) reported t h a t the mylohyoid nerve sends a branch to the mandible in 10% of the cases he examined.

Sillanpaa and workers (1988) as well as Heasman and Beynon (1986) have shown t h a t the mylohyoid nerve contributes to the innervation of mandibular teeth and may be a cause of anaesthetic failure. The question which h a s not been adequately answered is, whether it is possible to anaesthetize the inferior alveolar nerve without anaesthetizing the mylohyoid nerve. Barker and Davies (1972) believe t h a t the mylohyoid nerve is commonly anaesthetized with the parent trunk as it is usually given off j u s t before the inferior alveolar nerve enters the mandibular foramen. Only when the mylohyoid nerve is given off high and enters a bony canal could it possibly escape being anaesthetized.

Barker and Lockett (1972) found multiple canals in the ramus of the mandible which lead to the posterior teeth, particularly the third molar. Later Ossenberg (1986) showed that these canals may in fact carry sensory fibres of the buccal nerve.

ANATOMICAL FEATURES OF THE MANDIBLE (Fig 35).

The medial surface of the mandible is not smooth and is quite irregular. Not being vertical the ramus of the mandible flares laterally.

From the tip of the coronoid process, the sharp temporal crest (internal oblique line) r u n s down the coronoid process behind the anterior border of the ramus of the mandible, becoming more prominent inferiorly. Behind the region of the last molar tooth, the temporal crest bends horizontally and divides to lie medial and later distal to the last tooth. Continuing with the buccal and lingual alveolar crests it forms the retromolar triangle. The temporal crest is formed and hence named after the attachment of the deep tendon of the temporalis muscle. On the other hand the superficial tendon of the temporalis muscle attaches along the anterior border of the ramus of the mandible. The bony depression between the two eminences is termed the retromolar fossa which continues forward lateral to the last molar. The anterior border of the mandible has a depression in it, the coronoid notch. Continuing forwards, the anterior border of the ramus of the mandible becomes the external oblique line.

The mandibular notch on the superior surface of the ramus is formed between the coronoid and condylar processes.

Running from the medial pole of the condyle downwards and forwards over the mandibular neck is the ridge of the mandibular neck.

Running forwards anterior to the mandibular foramen it blends with the lingula and the temporal crest.

Lying behind and below the ridge of the mandibular neck is the mandibular sulcus which funnels into the mandibular foramen. I t is only present because of the anteriorly lying elevation, the ridge of the mandibular neck and the posteriorly lying elevation the subfossal crest.

Superiorly, towards the condyle the mandibular sulcus becomes shallower. Continuing forwards and downwards from the mandibular foramen is the mylohyoid groove which in the living is bridged over by the continuation of the sphenomandibular ligament. The groove may at various parts ossify to form mylohyoid bridges (Ossenberg 1974). These ossifications may become extremely large and extend from the lingula obliterating the lateral part of the mandibular foramen. This phenomenon will alter, (ie raise) the position of the mandibular foramen.

It is important to target the needle point a s close a s possible to the mandibular foramen but ideally above the lingula a s well. I t is considered by some (Barker and Davies 1972) that the most suitable level for needle insertion is a t the level of the lingular apex. Not often considered,

Mandibular Nutrient foramen Temporal crest,

\

notch

Med. cond y lar tubercle (Med. Condylion)

\

Pterygoid foss0 X - Insertion

pteryqomondibular raphe

Mandibular

\

l \ Coronoid notch,

Mylophorynqeo ltne

.

Mvlohvoid line ,

,

r

.

Dnrnl~nn~ ~nl v.-. n l n.n~ U, U " , ,yvv.

Sublingual

'

. l

.',,L,

fossa

I

,

Subfossol crest

%hlandtbulor

foramen

Subcondylor in~isure Mylohyoid N. sulcus Pteryqoid tubercles l . L t M ) u a l tuberos~t y 'Gonion)

Mental spines

\

Otqastr~c fossa

,

Fig 35.

FROM: HUELKE D.F. (1973) Selected Dissections of the Facial Regions for Advanced Dental Students.

Shields (1977) describes the lingula a s a possible obstruction in the plane of injection. I t is possible that decreased success rates with the standard technique may be due to the lingula presenting itself as a n obstacle and the needle tip is therefore not negotiated around it. The standard technique may predispose to this problem because it stipulates that "as soon" as bone is struck the needle tip should be slightly withdrawn several millimetres and hence the lingula is not by-passed by palpating around it. The reason for withdrawal to avoid intravascular injection into the blood vessels that have been punctured against bone h a s been well documented by Shields (1977) and in doing so this also frees the needle point from the sensitive periosteum (Shields 1970).

As the mandibular trunk divides to give off the inferior alveolar nerve it runs through the pterygomandibular space, a well defined space between the slanting medial pterygoid muscle and the upright medial surface of the ramus of the mandible. Viewed in a corona1 section the nerve runs a recognizable sigmoid course. Contact with bone is only present in the close vicinity of the mandibular forarnen, while above the foramen the nerve h a s no contact with the bone, and the distance between the bone and nerve increases rapidly (Sicher 1946).

As the jaw opens forwards and downwards the mandibular foramen is displaced downwards without deviating anteriorly or posteriorly. This in effect tenses the inferior alveolar nerve bringing it to lie closer to the medial surface of the mandible and within the mandibular sulcus. The mandibular sulcus was once known as the groove of the mandibular nerve. The sulcus obtained its name from the mandibular (ie inferior alveolar) nerve which was believed to groove the bone in this area but with the mouth closed, this is not the case. The anaesthetic should be deposited in the lowest part of this sulcus (Sicher 1946). There are therefore two reasons why the patient should be asked to open and maintain their mouth a s wide a s possible: Firstly, the nerve is brought to lie closer to the inner surface of the ramus and secondly it lies in the sarne plane as the mandibular sulcus which can be used as a landmark.

Hence it is ideal that the needle tip be located above the obstructing lingula but not too high as it will lie lateral to the nerve and may involve the maxillary artery or lead to injecting the lateral pterygoid muscle. "High" block techniques inject into the condylar neck at a level just inferior to the insertion of the lateral pterygoid muscle and rely mainly on diffusion to the inferior alveolar nerve. The possible anatomical hazards that can be encountered can lead to serious consequences. If the anaesthetic is deposited too far medially it will enter the medial

pterygoid muscle which can be both painful and result in trismus. Too deep an injection into the parotid gland can anaesthetize the facial nerve, resulting in a temporary hemi-facial paralysis (Gray 1978). An excessively inferiorly placed needle can enter the attachment of the medial pterygoid muscle but also lie below the sphenomandibular ligament which may prevent the anaesthetic solution from reaching the inferior alveolar nerve. When the anaesthetic is deposited above the attachment of the sphenomandibular ligament the solution tends to be funneled down into the mandibular foramen (Ossenberg 1974). Galbreath and Eklund (1970) showed that the anaesthetic solution diffuses posteriorly and somewhat superiorly. Hence there is no problem in lying anteriorly, in fact it may be desirable to be slightly anteromedially given the path of diffusion of the anaesthetic solution. If placed too far anteriorly though, the needle tip may lie in front of the temporopterygoid membrane (fascia) which forms the anterior boundary of the pterygomandibular space. The anaesthetic solution in this case may be prevented from travelling posteriorly (Barker and Davies 1972).

EMPIRICAL LOCAL ANAESTHETIC LANDMARKS A few mandibular landmarks are utilized in the direct thrust technique for anaesthetizing the inferior alveolar nerve. The anterior aspect of the mandible can be determined by palpating the external oblique ridge which continues superiorly along the anterior surface of the ramus of the mandible. The ball of the palpating finger usually falls into a concavity area along this anterior surface, the coronoid notch.

Bone is not actually being palpated, it is the overlying superficial tendon of the temporalis muscle that is being felt. More medially the deep tendon of the temporalis, overlying the temporal crest can be palpated well, just behind the third molar where the temporal crest is most prominent. The other intra-oral landmark commonly used is the lower occlusal plane.

Shields (1977) recommends t h a t more "reliable" extra-oral landmarks be made use of in the mandibular anaesthesia to supplement intra-oral landmarks. There is no doubt that many of the landmarks and ultimately the position of the needle tip is not visible to the naked eye.

Hence it involves projecting the position of the needle tip, unless the needle tip is used to explore the medial surface of the ramus of the mandible a s is suggested by some (Sicher 1946 and Barker and Davies 1972), but runs the increased hazard of tearing vessels which can lead to haematoma formation.

When a n extra-oral technique for mandibular anaesthesia is required then it totally relies on extra-oral landmarks. Extra-oral landmarks are therefore similarly useful for both extra-oral and intra-oral techniques. The condyle is easily palpated with the index finger, the anterior border of the ramus can be verified with the thumb, while the posterior border of the ramus and the angle of the mandible can be palpated with the third and fourth fingers. This helps determine the dimensions of the ramus and the amount of variability that exists. The index finger can then be used to point to where the needle tip should be aimed at.

MATERIALS and METHODS Dry bony mandibular specimens were basically of three types. The first group (Grp. 1) was designated the "Hindi" group consisting of Indian mandibles possessed by students and the Department of Anatomy. 31 specimens were complete mandibles and 1 was unilateral which made a total of 6 3 hemimandibles. No information was available in relation to the age or gender of these specimens.

The second group (Grp. 2) was designated the "Forensic" group and these specimens were made available by the Forensic Odontology Unit, Department of Dentistry. 13 were complete a n d 4 were hemimandibles. 30 sides were effectively examined. Both age and/or gender of the specimens were known. The mean age of the subjects examined was 33 years (s.d.=12 yrs). In 2 out of the 17 specimens, age was unknown. The majority of the specimens were male (12). 4 female specimens were present and 1 was of unknown gender.

The specimens of the first two groups were selected on the basis that they were dentate with a t least the first and second molars being present.

The third group (Grp. 3) was designated the "Edentulous" group which, as its name suggests, consisted of totally edentulous mandibles. 6 of these were completely edentulous mandibles and 1 specimen was a hemimandible. Hence 13 sides were effectively examined. As in the Hindi group they were not aged or gender typed.

The examination involved recording all non-metric information concerning the specimen such a s ownership, age, gender etc. which was entered on a data sheet designed for this purpose (Fig 36). In addition three extra categories, unrelated to the present study were included.

With the aim of utilizing the present material for future study: any apical fenestrations, multiple mental foramina and Stafney's idiopathic bone cavities were noted, photographed and commented upon.

Previous studies involving any metric analysis on dry mandibles were predominately conducted utilizing direct measuring methods.

Nicholson (1985) used lateral photographs to determine measurements for 2 out of his 12 variables. Hetson and workers (1988) took photographs of mandibles in a plane perpendicular to the ramus. This was possible in his sample which was comprised of hemisected mandibles, however it is not possible to perpendicularly photograph rami in complete mandibles a s the opposite ramus to that being photographed obstructs the view. With foreign ownership of the skeletal material it was not possible to hemisect the mandibles. Even if this was possible it was

Fig 36.

Data Sheet.

WENT FOR ~ S T A F N E Y

i

iac

thought to be undesirable as the hemi-mandibles would not be able to sit on the standard basal plane and remain upright ie. the relationship to the standard basal plane would be lost.

A non-intrusive, indirect method of measurement needed to be devised. A device was designed by the author using plastic sheets to fulfil these requirements. It allowed a mandible be rest on its lower border while a mirror was placed at 45' to the ramus (Fig 37). The ramus was positioned against a lined sheet which acted as a scale. Photographs were processed from 35 mm black and white film, utilizing a 200 mm lens with a 27.5 mm extension tube. The camera was set an arbitrary distance which filled the field of view. Prior to each photographic session the camera was set a t 0' to the vertical and horizontal planes in order to maintain a 45' relationship to the ramus of the mandible. Any landmarks that were not clearly visible through the camera were highlighted on the mandible with pencil.

Direct measurement of the distance between the tip of the lingula and the interproximal contacts of the opposite premolars was conducted on 74 sides of 34 dentate subjects from both the Hindi and Forensic groups.

Using (SilasconTM), a silicone impression material, impressions were taken of the medial surface of the ramus of the mandible in the region of the lingula and mandibular foramen of all specimens examined.

Hard stone Welmix) records were produced from the impressions which acted a s permanent three-dimensional duplicates of the specimens examined. These were not only taken to act as records of the material at hand but it is also the intention of the author to examine the morphology of the lingula and the mandibular foramen of the specimens examined in the near future.

Computer analysis was conducted in two phases. The first phase to calculate linear and angular measurements utilized a n Apple I1 GS computer which acted as a controller for a Hewlett Packard digitizer and plotter, each with a resolution of 0.025 mm (Fig 38).The software which was created and provided by Professor T. Brown made use of the Cephs.

Sys tern@package (Fig 39). No previous study analysing mandibular dimensions has ever employed such an indirect method of measurement.

Fig 37.

Non-intrusive device contstructed and used to photograph tht medial surface of the ramus of the mandible. (Mirror at 4 5 O to the ramus

Fig 38.

Fig 39.

From left to rightHewlett Packard digitizer, plotter and Apple I1 G S .

Professor T. BROWN Department of Dentistry, T h e University of ~ a e l a i d e (Anthropolom and Genetics)

2 5 reference points were marked on the photographs which were printed as close as possible to life-size (Fig 40).

The 2 5 reference points were1- Tip (apex) of lingula.

2- Tubercle a t apex of retromolar triangle - taken to be the insertion of the pterygomandibular raphe.

3- Point on anterior border of ramus going through occlusal plane.

4- Point on anterior border through narrowest antero-posterior plane (ie. coronoid notch).

5- Point on anterior border parallel to occlusal plane and passing through lingula.

6- Point on anterior border perpendicular to rameal plane and passing through lingula.

7- Point on anterior border parallel to narrowest anteroposterior plane and passing through lingula.

8- Point on temporal crest parallel to narrowest anteroposterior plane and passing through lingula.

9- Point on mandibular notch perpendicular to narrowest anteroposterior plane and passing through lingula.

10- Point on mandibular notch parallel to rameal plane and passing through lingula.

1 1- Medial pole of condyle.

12- Point on superior maximum convexity on posterior surface.

13- Point on posterior surface perpendicular to the rarneal plane and passing through lingula.

14- Point on posterior border parallel to occlusal plane and passing through coronoid notch.

15- Point on posterior border passing through occlusal plane.

16- Point on posterior border passing through narrowest anteroposterior plane.

17- Point on basal plane representing the point of intersection of the basal plane and a line perpendicular to basal plane passing through lingula.

18- Point of intersection of standard basal plane and rarneal plane (ie tangent gonion) (Origin).

19- Point of anterior maximum convexity on inferior surface. (Forms anterior point of standard basal plane).(The standard basal plane is the horizontal plane which makes tangential contact with the two most prominent convexities of the inferior border of the mandible).

Fig 40.

The 25 mandibular reference points:

Dentate specimen.

Edentulous sDecimen.

3

A .

-

20- Point on basal plane perpendicular to occlusal plane passing through lingula.

21- Point on inferior border perpendicular to narrowest anteroposterior plane and passing through lingula.

22- Point on inferior border parallel to rameal plane and passing through lingula.

23- Point on mandibular foramen (ie lowest point on the lower border of the foramen where it merges with the lingula.

24- Point on intersection of plane parallel to the basal plane while passing through the point of the mandibular foramen and the plane perpendicular to the basal plane while passing through the lingula.

25- Point on probe which enters and is continuous with the mandibular canal.

The points were then digitized and a computer plot was generated (Figs 41 & 42). Reference points are used to generate linear and angular variables. Values for these variables are determined (Fig 43). The first twenty variables were linear distances between two points. Variables 2224 measured the perpendicular (shortest) distance between a point and a line. Variables 25-31 are the ratios (%) of two distances. Variable 32 represents a n angle defined by 3 points. The last two variables 33-34 represent the angles between two lines. Below is a table that provides the definitions of all the variables which are graphically displayed in Fig 44.

Variable 1 (17-1)- represents the perpendicular height of the lingula from the standard basal plane.

Variable 2 (1-7)- represents the narrowest anteroposterior depth of the lingula from the anterior border.

Variable 3 (1-5)- represents the depth of the lingula from the anterior border parallel to the occlusal plane.

Variable 4 (1-8)- represents the depth of the lingula from the temporal crest parallel to the narrowest anteroposterior plane.

Variable 5 (1-6)- represents the depth of the lingula from the anterior border, perpendicular to the rameal plane.

Variable 6 (21-1)- represents the height of the lingula from the inferior border of the mandible, perpendicular to the narrowest anteroposterior plane.

Variable 7 (20-1)- represents the height of the lingula from the inferior border of the mandible, perpendicular to the occlusal plane.

Variable 8 (1-4)- represents the depth of the lingula from the coronoid notch (ie anterior point of the narrowest anteroposterior plane).

Fig 41.

Digitized raw data and transformed data after correction for magnification.

Ident L32 M Age 9

Date 12-MAY -1990

Origlnal data on file /ZAC/HINDI .RAW Transformed data on file /ZAC/HINDI.TRN Order on file is - SUBJECT 1 1 First record no: 568 Last record no: 621 Index on file /ZAC/HINDI.SUB is in record 23 Scale points Y Scal e distance 59.455 Magnification -0.910 % Reduction factor 1.00Q Registration on line 18(18) to 19(19) - Origin at 18

F'OINT

RAW DATA X Y

TRANSFORMED X Y

--------------- .---------------2965 2427 2274 2292 1964 2330 2099 2507 2152 2595 2268 . 2978 2231 2847 2585 2652 3263 2968 3387 2932 4005 2960 4189 2852 3405 2063 3518 2202 3378 2033 3281 1851 2920 1130 2790 1133 869 1155 2686 1137 2270 1159 1979 1195 2767 2064 2962 2061 3520 2416

-

4.04 -13.35 -21 .l8 -17.83 -16.52 -13.70 -14.60 -5.61 11.40 14.54 30.12 34.80 15.25 18.06 14.57 12.18 3.28 0.00 -48.47 -2.62 -13.13 -20.48

-0.85 4.07 18.05

32.70 29.09 29.96 34.46 36.70 46.39 43.08 38.26 .46.43 45.56 46.44 43.77 23.64 27.18 22.87 18.26 -0.04 0.00 0.00 0.07 0.51 1.33 23.48 23.46 32.58

SUBJECT L 3 2 M A g a

Oat-

Fig 42.

Q

19-JAN-1915

Computer plot of the digitized reference points.

VAR i ABLE NAME 1 17-1 2 1-7 3 1-5 4 1-8 5 1-6 6 21-1 7 20-1 8 1-4 9 15-3 1C 23-i

Description

N

l1 12

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

3 Distance between 2 points 3 Distance between 2 points 3 Distance between 2 pcints 3 Distance between 2 points 3 Distance between 2 points 3 Distance between 2 points 3 Distance between 2 points 3 Distance between 2 points 3 Distance between 2 points 3 Distance between 2 points 3 Distance between 2 points z<-: 3 Distance between 2 points 23-24 3 Cistance between 2 points Zi-C 3 Distance between 2 points 22-13 . 3 Distance between 2 points 18-1 3 Distance between 2 points 1-2 3 Distance between 2 points 16-4 3 Distance between 2 points 1-6 3 Distance between 2 points 13-1 3 Distance between 2 points 18-11 3 Distance between 2 points 14-4 -4 Distance from point to line 1116-4 -4 Distance from point to line 1115-3 -4 Distance from point to l ine 1114-4 8-7/16-4 5 Ratio ( % l of 2 distances 21-1/21-9 5 Ratio ( % l of 2 distances 22-1/2210 5 Ratio ( % l of 2 distances 1-7116-4 5 Ratio ( % l of 2 distances 5 Ratio ( % l of 2 distances 1-7/14-4 1-5115-3 5 Ratio ( % l of 2 distances 1-5/14-4 5 Ratio ( % l of 2 distances <12-18-19, 1 Angle defined by 3 points 16-4515-3' 2 Angle between 2 lines 2325<18191 2 Angle between 2 lines

Oata input file is /'ZAC/'HINDI.VAR STATISTICAL ANALYSIS - Using Program CEPHS-STATS - gate: 24-RUG-l??2 Data input fiie is /'ZAC/HINDI.VAR Kissing vaiue designated by 999.9 SUBJECTS SELECTED AND VARIABLE SCORES -

Subject L32 M Age (1) 32.750 (6) 36.480 (11) 9.240 (16) 17.760 (21) 36.620 (26) 70.080 (31) 57.200

9 (2) (7) (12) (17) (22) (27) (32)

21.340 33.300 4.920 34.100 8.840 70.580 128.490

(3) ( 8) (13) (18) (23) (28) (33)

20.950 21.940 52.060 22.410 7.590 62.560 17.140

(4) ( 9) (14) (19) (24) (29) (34)

11.140 36.450 56.420 14.410 2.620 58.260 154.290

5) (10) (15) (201 (25) (30) (

22.410 10.440 32.950 55.350 29.910 57.470

V=. 1 17-1 Var. 6 21-1 Var. 7 20-1 Var. 15 18- 1

Var. 10 23-1 Var. 11 24-1

Var. 12 23-24

Var. 13 21-9 Var. 14 22-10 Var. 20 18-11

Var. 2

I-'/

Var. 3

1-5

v r s

Var. 9 15-3 Var. 17 16-4

-6 .-g,!!-

c;': . . -;"

f'i ..

*-

var.3 l yar. 8 1-4 Var. 16 1-2 yar. 19. 13-1

V s 22 l\ 16-4 V=. 23 1 \ 15-3 Var. 24 1 \ 14-4

V=. 25 V=. 28 Var. 29 V=. 30 Var. 3 1

8-7/ 16-4 H / 16-4 k7/14-4 1-5/15-3 1-51' 14-4

Variable 9 (15-3)- represents the width of the ramus of the mandible at the occlusal plane. Distribution of values are significantly positively skewed.

Variable 10 (23-1)- represents the (shortest) height of the lingula its self ie the height of the lingula from the mandibular foramen.

Variable 11 (24-1)- represents the vertical height of the lingula from the mandibular foramen, perpendicular to the basal plane.

Variable 12 (23-24)- represents the width of the lingula ie the horizontal width of the lingula from the mandibular foramen.

Variable 13 (21-9)- represents the height of the ramus of the mandible perpendicular to the narrowest anteroposterior plane.

Variable 14 (22-10)- represents the height of the ramus of the mandible parallel to the rameal plane.

Variable 15 (18-1)- represents the (shortest) height of the lingula from the tangent gonion.

Variable 16 (1-2)- represents the depth of the lingula from the apex of the retromolar triangle (ie the perceived insertion of the pterygomandibular raphe).

Variable 17 (16-4)- represents the narrowest anteroposterior width of the ramus of the mandible.

Variable 18 (1-6) is the same as variable 5.

Variable 19 (13-1)- represents the distance from the lingula to the point on the posterior surface of the ramus, perpendicular to the rameal plane and passing through the lingula.

Variable 20 (18-11)- represents the height (shortest distance) of the ramus of the mandible from the medial pole of the condyle to the tangent gonion.

Variable 21 (14-4)- represents the width of the ramus of the mandible at the level of the coronoid notch parallel to the occlusal plane.

Variable 22 (1\ 16-4)- represents the height (shortest distance) from the lingula to the narrowest anteroposterior plane.

Variable 2 3 (1\15-3)- represents the height (shortest distance) from the lingula to the occlusal plane.

Variable 24 (1\ 14-4)- represents the height ( shortest distance) from the lingula to the plane parallel to the occlusal plane and passing through the coronoid notch.

Variable 2 5 (8-7/16-4)- represents the ratio (%) of the depth of the temporal crest from the anterior border of the mandible to the narrowest anteroposterior width of the ramus.

Variable 26 (21-1/21-9)- represents the ratio(%) between the 1. the lingula from the inferior border of the mandible perpenc to the narrowest anteroposterior plane.

Variable 27 (22-1/22-10)- represents the ratio (?h)between the he. of the lingula from the inferior border of the mandible parallel to the rameal plane and the overall height of the ramus parallel to th, rameal plane.

Variable 28 (1-7/16-4)- represents the ratio(%) between the narrowest anteroposterior depth of the lingula from the anterior border of the ramus and the narrowest anterior posterior width of the ramus.

Variable 29 (1-7/14-4)- represents the ratio (%) between the narrowest anteroposterior depth of the lingula from the anterior border of the ramus and the width of the ramus at the level of the coronoid notch parallel to the occlusal plane.

Variable 30 (1-5/15-3)- represents the ratio (%) between the depth of the lingula from the anterior border parallel to the occlusal plane and the width of the ramus at the occlusal plane.

Variable 31 (1-5/14-4)- represents the ratio (?h)between the depth of the lingula from the anterior border parallel to the occlusal plane and the width of the rarnus at the level of the coronoid notch parallel to the occlusal plane.

Variable 32 (<12-18-19)-represents the gonial angle.

Variable 33 (16-4<15-3)-represents the angle between the narrowest anteroposterior plane and the occlusal plane.

Variable 34 (23-25<18-19)-represents the angle between the mandibular canal and the standard basal plane.

The second phase of the analysis was conducted by transfering the data to The University of Adelaide's Vax Computer. The transfer to the more powerful main frame computer was conducted with the aid of a Macintosh Plus computer.

The data were reformated and cleaned of any frank errors, for example any wrongly placed decimal points were corrected. The data are presented in Appendix I1 for reference. The first row indicates to which group the specimen belonged. The subject identification number and whether the side was left or right follows. Next the subject's age is specified, if unknown a missing value of 0 is present. Finally the subject's gender was recorded, M for male, F for female and X for unknown.

Descriptive statistics for all variables of the various groups were calculated. The second row contains variables 1-9, the third row 10-18, fourth row 19-27, 28-34. Paired t-tests between left and right sides of

the various groups were undertaken to determine if any differences existed between left and right sides. Unpaired t-tests were undertaken in the Forensic group to determine if any differences existed between males and females. Finally Pearson correlations were calculated between age, which was known in the Forensic group, and all variables. A s well a complete correlation matrix of all variables was computed in each group.

RESULTS Pooled r e s u l t s from all three groups a r e presented for t h e 33 variables unless a significant difference was detected by t h e Analysis of Variance (ANOVA) including t h e Scheffe procedure. Paired t-tests displayed a n y significant differences between left a n d right sides.

X S.D. C.V. Min. - Max. * 13.7 21 - 42 30 4.1 Var. 1 17.6 10 - 23 17 Var. 2 3.0 17.1 1 1 - 2 3 2.9 17 Var. 3 var. 4 Groups 1 a n d 2 a n d groups 2 a n d 3. * 6 - 17 10 30 Grp.1 3 2 12 16.7 9 - 16 Grp.2 2 22.2 5 - 13 9 Grp. 3 * 11 - 24 20 3.2 Var. 5 16 11.2 2 5 - 4 3 3.8 Var. 6 34 4.1 13.7 21 - 42 Var. 7 30 18.1 11 - 23 16 2.9 Var. 8 + 10.9 27 - 45 Var. 9 3.6 33 1.8 Var. 10 8 22.5 3 - 13 var. 11 Groups 1 a n d 2 vary.

22.5 3 - 12 1.8 8 Grp.1 15.7 5 - 9 1.1 7 Grp.2 15 1.2 5-9 8 Grp.3 var. 12 Denate a n d edentulous groups vary.

Dent.

3 1.6 53.3 0.03 - 7 120 0 . 2 5 - 4 1 1.2 Edent.

40 - 60 5 10 Var. 13 5 0 Var. 14 Groups 1 a n d 2 vary.

43 - 61 9.2 4.8 Grp. 1 52 47 - 58 8.1 4.5 55 Grp. 2 * 10 21 - 43 Var. 15 3 0 4.3 Var. 16 Grp. l varies from groups 2 a n d 3. * Grp. 1 14 3.3 23.6 7 - 23 17 14.1 13 - 21 Grp. 2 2.4 17 18.2 12 - 22 3.1 Grp. 3

1

(

1

1

1

Var. 17 Var. 18 Var. 19

3.5

3

var. 20 var. 21 var. 22 Var. 23 Var. 24

1

1

11.7 as Var. 5 2 14.3 ll 5.7 10.3 3.4 3.3 82.5 97.5 3.9 -320 3.2

1 24 - 41 8-20 41 - 66 27 - 44 -4 - 15 -8 - 13 -1 1 - 5

Above values are expressed in mm.

I

I

I

1

K

I

Var. 25 Group 2 varies from groups 1 and 3.

Left a n d right sides varied in group 3.

7 - 41 20 Grp. 1 5.9 . l - 22 5.4 Grp. 2 .l2 12 - 33 6.4 Grp. 3 21 Left 24 6.5 14 - 33 Right 19 5.1 12 - 26 4 V a r . 2 6 68 56 - 79 57 - 79 Var. 27 68 3.9 A Var. 28 5 3 6.5 39 - 80 33 - 70 Var. 29 49 6.8 37 - 64 5.3 Var. 30 49 k 38 - 67 V a r . 3 1 51 5.3 Above values are emressed as %.

Var. 32 Dentate and edentulous groups vary.

Dent.

126 6.5 109 - 142 # 5.2 101 - 129 = 7 120 8.4 Edent.

A 41.7 2 - 4 2 7.5 Var.33 18 132 - 164 * 4 V a r . 3 4 151 6 Above values are expressed i n O .

Distribution of values * ,Slightly positively skewed + ,Significantly positively skewed ,Significantly positively skewed and slightly positively kurtotic # ,Slightly positively kurtotic - , Slightly negatively skewed A ,Significantly positively skewed and kurtotic f , Slightly positively skewed and kurtotic = , Significantly negatively skewed

1

I

- -

-

I

Gender Differences.

Unpaired t-tests were applied to the Forensic group, where specimens were gender typed to determine if there were any statistically significant (P<0.05) differences between males and females.

The majority of variables were not significantly different apart from variables 6, 9, 13 & 14.

Variable 6 (21-1) - Height of the lingula from the inferior border of the mandible, perpendicular to the narrowest anteroposterior plane.

Males - Mean height = 36 mm (s.d.=2.7 mm) Females - Mean height = 32 mm (s.d.=2.5 mm).

Variable 9 (15-3) - Width of the rarnus at the occlusal plane.

Males - Mean width = 36 mm (S.d.=4.5 mm) Females - Mean width = 32 mm (s.d.=1.9 mm) Variable 13 (21-9) - Height of the ramus of the mandible perpendicular to the narrowest anteroposterior plane.

Males- Mean height = 53 mm (s.d.=4 mm) Females- Mean height = 45 mm (s.d.=3.9 mm) Variable 14 (22-10) - Height of the ramus of the mandible parallel to the rarneal plane.

Males- Mean height = 57 mm (s.d.=3.8 mm) Females- Mean height = 50 mm (s.d.=2.6 mm) Age Associations Again the Forensic group was utilized to determine if there were any associations between age and any of the variables. Pearson correlations displayed that only variables 6 and 7 showed a moderate positive correlation with age.

Variable 6 (21-1)- Height of the lingula from the inferior border of the mandible, perpendicular to the narrowest anteroposterior plane.

Pearson correlation = 0.409, P=0.02.

Variable 7 (20-1)- Height of lingula from the inferior border of a mandible, perpendicular to the occlusal plane.

Pearson correlation = 0.345, P=0.04.

A Pearson correlation matrix was carried out on all variables against

all other variables and some of those that were deemed to be of interest are included below-

Correlations with variables associated with vertical (height) measurements.

21 20 17 32 13 8 9 3 -0.735 0.819 1 0.543 0.869 6 0.629 7 0.469 0.5 0.391 0.478 0.288 13 0.363 0.444 14 -0.695 0.8 18 15 -0.914* 0.496 20 0.495 0.465 0.536 0.188 0.141 22 0.248 0.489 -0.062* -0.252 23 -0.034 0.282 -0.07 : -0.043 24.0.048 .0.432 .0.001, Sample size=93 *-Edentulous Sample size=13 Variable 6 (21-1) * Variable 2 1 (14-4) 0.543 ie. the height of the lingula from the inferior border of the mandible, perpendicular to the narrowest anteroposterior plane is only moderately correlated to the narrowest anteroposterior width of the ramus.

Variable 22 ( l / 16-4) * Variable 8 (1-4) 0.489 Variable 23 ( l / 15-3) * Variable 3 (1-5) -0.034 ie the height of the lingula from the narrowest anteroposterior or occlusal planes is poorly correlated to the width of the ramus a t the level of the above named planes.

Variable 1 (17-1) * Variable 20 (18-11) 0.819 Variable 15 (18-1) * Variable 20 (18-11) 0.818 ie the height of the lingula from the inferior border is highly correlated to the height of the ramus of the mandible as they cover a similar anatomical area.

Variable 13 (21-9) * Variable 17 (16-4) 0.288 Variable 20 (18-11) * Variable 17 (16-4) 0.476 Variable (18-11) * Variable 9 (15-3) 0.536 ie the height of the ramus is poorly correlated with the width of the rarnus.

Correlations with variables associated with depth.

2 11.829 I

1.4961

I .l881-.252 I

1 -.414

Sample size=93 *-Edentulous Sample size=13 Variable 8 (1-4) * Variable 15 (18-1) 0.529 Variable 8 (1-4) * Variable 20 (18-11) 0.465 Variable 8 (1-4) * Variable 7 (20-1) 0.5 Variable 3 (1-5) * Variable 7 (20-1) 0.469 ie the depth of the lingula is only moderately correlated to the height of the lingula and the height of the ramus of the mandible.

Variable 3 (1-5) * Variable 9 (15-3) 0.807 Variable 3 (1-5) * Variable 21(14-4) 0.829 ie the depth of the lingula is highly correlated to the width of the mandible as they cover a similar anatomical area.

Variable 8 (1-4) * Variable 17 (16-4) 0.759 ie a s with the others above, the two variables are highly correlated as they cover a similar anatomical region and also share a common reference point.

Correlations with variables associated with angular variables.

1

32 -0.715

2

5

21 33 34 9 8 15 -0.331 -0.452 -0.695 -0.434 0.451 0.516

-0.829 -0.803 -0.847 0.115 -0.914 33

-0.04

DISCUSSION In this study the position of the lingula in particular was determined in three different sample groups, of different racial origin and dental status.

For the majority of the variables examined there were not any major racial differences which was the finding of Hayward and colleagues (1977) but was dissimilar to the finding of Harvey and Robb (1967) who stated that differences existed between English and Indian mandibles.

The depth of the temporal crest form the anteroposterior plane is less in the Caucasian/Forensic group. As a consequence the lingula is located further from the temporal crest. The apex of the retromolar triangle which is stated by Heulke (1973) as being the attachment of the pterygomandibular raphe [which is doubtful) was found to be closer to the lingula in the Hindi group.

The small racial differences that seem to exist may in fact be nonexistent because of the difficulty in defining some landmarks, such as the two mentioned above ie. the temporal crest and the apex of the retromolar triangle.

The present study is the only study to consider if the position of the lingula and many other related variables vary between dentate and edentulous mandibles. Some studies (Hetson et a1 1988) provide no information as to the origin or dental status of the material examined.

Other studies have provided data utilizing a mixed collection of bones including juvenile and senile, dentate and edentulous (Gabriel 1958 and Bremer 1952). Such studies do not consider any differences that may occur with altered dental status whereby the information provided may not be applicable, to specific groups, for example data collected from dentate specimens may not be applicable to edentulous specimens.

For the majority of the variables examined in this study, no such differences existed between dentate and edentulous mandibles. The gonial angle though was, as expected, different in the edentulous group.

Unexpected was the finding that the Edentulous group had a smaller (120") gonial angle when compared to the dentate group (126").

Although both gonial angles from the two groups lie within the reported normal range of 120"-130" (Heulke 1973). The reason that the dentate group had a larger gonial angle may be due to the fact that the distribution of values in the sampled dentate mandibles were kurtotic, particularly in the Hindi group. This meant that only a few large values in the Dentate group were sufficient to increase the mean gonial angle. I t is unlikely that the methodology could be responsible for the observed

differences a s similar methods were applied to both the dentate and edentulous groups.

Age associations were considered b u t t h e youngest known specimen, was 16 years of age. All dentate specimens had at least the second molar present. Nicholson (1985) who examined only eight mandibles, only included specimens when both the third molars had erupted into occlusion. In the present study, mandibles without third molars were included for practical reasons. The occlusal plane was taken as a straight line passing through the buccal cusps of the premolars and molars. The occlusal plane did not take into consideration the third molar which is less reliable in its position and is considered to be a most unreliable guide (Harvey and Robb 1967). Studies such as Bremer's (1952) include the third molar in establishing the occlusal plane and may not allow variables which include this landmark in other studies to be directly comparable.

Harvey and Robb (1967) are amongst the few workers who have examined gender differences. In the present study it was noticed that the overall width and height of the ramus of the mandible is significantly greater in males. The height of the lingula from the inferior border of the mandible, perpendicular to the narrowest anteroposterior plane was also larger in males whereas no other variables displayed any significant differences to exist in nine of their measurements between males and females in their English specimens.

A consistency of average values between left and right sides was found which is in agreement with Bremer (1952) and Hayward and workers (1979). One would expect no differences contralaterally and if s u c h differences existed the methodology could require reviewing.

Although on the whole average values did not differ between sides individual bilateral differences can exist (Bremer 1952).

As described before there are various reasons why the lingula is a more appropriate target point for mandibular anaesthesia. The needle tip should preferably be located just above the lingula and into the mandibular sulcus .

Even though the morphology of the lingula has not been described in detail it is known that considerable variation efists in the shape of the lingula and the mandibular forarnen (Harvey and Robb 1967) which are considered to be a probable cause of failure in anaesthesia.

The tip of the lingula was found to be 8 mm away from the mandibular foramen which is similar to that found by Bremer (1952) and Nicholson (1985). The tip of the lingula does not cover the mandibular

foramen or lie just anterior to the mandibular foramen (Hayward 1977).

These notions have developed because of the dissimilarity in defining the location of the mandibular foramen. Hayward (1977) defines the position of the mandibular foramen a s being at the midpoint of the foramen a t a higher point and not at the base of the foramen where it merges with the lingula and may in fact artificially displace the mandibular foramen posteriorly. The tip of the lingula is always above and slightly behind the mandibular foramen.

Various measurements were determined to gauge the vertical position of the lingula. The lingula was 30 mm above the standard basal plane and the tangent gonion. If the occlusal plane is a preferable guide, then the distance of the lingula from the inferior border of the mandible, perpendicular to the occlusal plane is also 3 0 mm which is close to Miller's (1953) and Kay's (1974) findings.

On average the lingula is very close to two thirds of the height of the ramus between the inferior border of the mandible and the mandibular notch of which both landmarks can be palpated extra-orally.

This finding compares most favourably to Miller's (1953) results where it was also found that the lingula was on average 34% of the distance between the mandibular notch to the notch in front of the gonion. A useful rule of thumb for the clinician in determining the position of the lingula would be to palpate the gonion and condyle and visualize the half and two-third points. The estimated midpoint between the two points is approximately where the lingula lies. From these calculations the lingula lies above the middle of the ramus (ie 58% of the rameal height between the gonion and condyle).

Nicholson (1985) places the mandibular foramen approximately at the junction of the lower one-third with the upper two-thirds of a line joining the tip of the coronoid process and the angle of the mandible.

Because the former of the two measurements can not be palpated such information is clinically useless.

I t appears that the age of a n individual is not a good predictor of the position of the lingula. However there is a weak to moderate correlation between age and the height df the lingula from the inferior border. Hence a n elderly individual with greater rameal growth would require the needle to be inserted higher above the inferior border and vice-versa for younger individuals.

The height of the lingula, perpendicular to the basal plane is highly correlated (0.819) with the height of the medial pole of the condyle from the tangent gonion. The position of the medial pole can be roughly

approximated to the lateral pole of the condyle which can be palpated extra-orally. A very good evaluation of the height of the ramus of a patient can be achieved by simultaneously palpating the gonion and the condyle the higher the ramus or, as seen previously, the older a n individual then a higher point of needle insertion will be selected.

The height of the lingula from the tangent gonion is similarly highly correlated (0.818) to the height of the condyle from the gonion.

One would expect a high correlation to exist as they share common reference points and such correlations have been termed "topographical" correlations (Brown 1972). Although displaying a high statistical correlation, caution must be taken in the biological relationships of such values.

I t can be seen that the height of the lingula from the inferior border is only moderately correlated with the width of the ramus.

The height of the lingula was on average 4 mm above the projected occlusal plane and hence the mandibular foramen is almost always below the occlusal plane. Even though a different definition of how the occlusal plane is established, astonishingly similar results to Bremer's (1952) study of 400 specimens was achieved. The distribution of values was not skewed or kurtotic and there was a standard deviation of 3.9 mm therefore in 98% of cases the lingula will lie under a point of 11.8 mm above the occlusal plane. A point of insertion 12 mm above the occlusal plane will be above most lingulae but will be 20 mm above one extreme specimen whose lingula was already 8 mm below the occlusal plane. I t may be better to be much higher, above the lingula than slightly below as t h e needle tip may lie below the line of attachment of the sphenomandibular ligament where injection may occur within the substance of the medial pterygoid muscle which can be both painful to the patient and may also prevent the anaesthetic from diffusing to the inferior alveolar nerve.

Although the mean distance of the lingula is approximately 4 mm above the occlusal plane, the dictum that the mandibular block injection should be made 3-4 mm above the occlusal plane (Kay 1974) does not seem to be valid as it will below the lingula in many cases. If one accepts that the mandibular foramen and not the lingula is the target point then the above dictum would place the needle tip closer to the mandibular foramen.

A practical guide could involve measuring the height of one's fingers and seeing which one was greater than or equal to 12 mm (eg the index finger). The pad of this finger could then be placed over the

opposite occlusal plane to that being injected and the needle should be guided just above the superior point of the index finger.

If the index finger is being placed over the occlusal plane then the middle finger might be used to palpate t h e coronoid notch simultaneously. A point of insertion 10.6 mm above the point indicating the coronoid notch and on a plane parallel to the occlusal plane will correspond to the apex of the lingula in 98% of the time. The coronoid notch is another landmark which can supplement the occlusal plane as a n indication of the height of needle insertion. Because what we are feeling when we palpate the coronoid notch is the overlying superficial tendon of the temporalis muscle the actual point of the coronoid notch may be a few millimetres above the palpable point of the coronoid notch.

The narrowest anteroposterior width of the ramus of the mandible can be determined extra-orally or partially extra-orally with the index finger palpating the posterior border and the thumb palpating the coronoid notch intra-orally. On average 1 mm below this narrowest anteroposterior plane should correspond to two-thirds of the height of the ramus from the inferior border to the mandibular notch. A point 5 mm above this plane will be above the lingula at least 98% of the time.

Hetson and workers (1988) developed a linear equation whereby t h e distance to t h e mandibular foramen from t h e narrowest anteroposterior plane could be predicted if t h e narrowest anteroposterior width and gonial angle were known.

The equation they derived is presented below.

Distance to foramen from the narrowest anteroposterior width = 4.7 mm + (122'- gonial angle) X 2.5 + (rameal width-30.8 mm) X 2.43.

Substituting the mean values established in this study the distance to the lingula from the narrowest anteroposterior width extrapolated from the relationship between the lingula and the mandibular foramen is overestimated by 3 mm in dentate specimens and underestimated by 4 mm in the edentulous specimens.

I t is hoped that in the near future a n appropriately precise linear equation will be formulated using a step-wise regression technique. Such a mathematical formula may be of limited clinical benefit in comparison to other information provided in this report.

The clinical operator generally knows when their needle insertion is high enough because of the ease of passage of the needle through the pterygomandibular space which lies above the insertion of the medial pterygoid muscle.

\

Now that the vertical position of the lingula and hence the point of insertion for the needle h a s been established, we should consider the horizontal depth of the lingula.

The depth of the lingula from the anterior border of the mandible in a plane parallel to the occlusal plane was 17 mm. The depth of the lingula directly from the coronoid notch is a similar 16 mm. If the narrowest anteroposterior width (30 mm) or the width of the ramus at the occlusal level (33 mm) is calculated then the lingula would tend to be located almost in the middle of the ramus. It should be kept in mind though that the position of the lingula varied dramatically from one-third to two-thirds of the width of the ramus which is of particular interest in relation to oral surgical procedures which will be discussed later.

Miller (1953) has placed the mandibular foramen in the horizontal dimension behind the half way mark and similarly Kay (1974) placed the mandibular foramen a t a point 58% of the width of the ramus from the anterior border.

If one wishes to use the narrowest anteroposterior plane a s a guide then the lingula lies 16 mm from the anterior border of the mandible on a plane parallel to that of the narrowest anteroposterior plane. Once again it shows that horizontally the lingula lies in the middle of the rarnus.

One can use other landmarks apart from the anterior border of the ramus such a s the temporal crest or the apex of the retromolar triangle (ie insertion of the pterygomandibular raphe (Heulke 1973). These are landmarks that are harder to define and are less reliable. It should be emphasized that the temporal crest should not be confused with the true anterior border of the ramus of the mandible.

High positive correlations (>0.8) are noticed between the depth of the lingula from the anterior border and the width of the ramus eg narrowest anteroposterior width. Hence the wider the ramus of the mandible the deeper one would expect to find the lingula from the anterior border although it still should be located near the centre of the ramus. Caution must be exercised when trying to interpret the biological significance of s u c h highly correlated relationships. Because the measurements of depth of the lingula from the anterior border of the ramus and the width of the ramus span the same anatomical region one would expect a high correlation. It would be indeed hard to imagine one of the measurements large and the other smaller. Such correlations have been termed "Spurious" (Brown 1972). These types of correlations may provide less biological information in comparison with smaller

correlations existing between unrelated variables that do not span the same anatomical region. Only moderate correlations (>0.5) are noted between the height of the lingula from the inferior border of the mandible and the depth of the lingula from the anterior border of the rarnus.

Correlations with the gonial angle tend to indicate that a s the gonial angle increases the injection should be made lower and closer to the anterior border of the ramus and vice-versa which suggests that the height of the injection should be made lower in cases where the gonial angle is wider, contradictory to the suggestions of Hetson and workers (1988). It should not be forgotten though that the older the individual and the hpigher their ramus, the higher is the point of injection.

The extra-oral location of the described landmarks may be more beneficial. Shields (1977) describes a technique which may assist in estimating the height and depth of needle penetration. His description unfortunately does not include any quantitative measurements of guides and presumption of the position of the mandibular foramen is made.

Given that the height of the ramus is poorly correlated with the width of the ramus, the anteroposterior and superoinferior position of the lingula must be determined separately.

The patient is asked to open and maintain their mouth open for many of the reasons previously described. The condylar head is palpated with the middle finger and the mandibular angle is palpated with the third finger. The anterior border and particularly the coronoid notch is palpated with the thumb. The position of the coronoid notch can be verified by the intra-oral palpation with the index finger of the opposite hand and the thumb is directed into the correct position. Finally the extra-orally located index finger can now point to the projected position of the lingula by moving up two thirds of the height of the ramus and half-way between the anterior and posterior borders of the ramus of the mandible.

The direct method of mandibular block involves the needle passing into the pterygomandibular depression a n d through the pterygomandibular space to the mandibular depression and through the pterygomandibular space to the mandibular sulcus which lies posterior to the lingula.

Bremer (1952) measured the depth of the mandibular sulcus to the mucosa in the pterygotemporal depression at a level of about 10 mm above the occlusal plane. Measurements were conducted on 215 patients where the mandibular sulcus could clearly be palpated with the needle

tip. Bremer (1952) obtained a mean distance of 24 mm from the mucosa to sulcus. Given the limitations of the bony materials utilized in this study such methods of determining the oblique depth of needle insertion were not possible. The direct measurements taken between the lingula and the interproximal contacts between the premolars of the contralateral side indicated that on average this distance was approximately 7 5 mm, with a standard deviation of 4mm. Hence 95% of the time the lingula would lie between 68 mm and 84 mm from the interproximal contacts of the opposite premolars. These boundaries could be marked on the barrel of a syringe and could indicate the region in which one would expect to strike the lingula (Fig 45). Such a guide would be of greater practical use because it is closer to the orifice of t h e mouth rather t h a n trying to assess how far past the mucosal surface the needle tip h a s extended which may be obscured by trylng to make judgements within the depths of the mouth.

If bone (eg the lingula) is hit slightly prematurely the needle tip may lie slightly anterior to the desired position. Being slightly anterior to the mandibular sulcus may not be so deleterious as it h a s been shown t h a t the local anaesthetic solution diffuses backwards (Galbreath and Eklund 1970) b u t if all the indications are that the position of the lingula h a s been properly evaluated and the needle tip h a s been struck far too prematurely then the barrel is redirected more medially and another attempt should be made. If on the other hand bone is struck after the needle h a s been inserted past the upper boundary guide on the syringe or if bone is not struck a t all this would tend to indicate that the ramus of the mandible flares more laterally and the needle tip may be too far medially or posteriorly (Fig 46). Injection of t h e local anaesthetic solution should not be performed at this incorrect position. Some of the side effects include anaesthetic blocking of t h e facial nerve or intravascular injection in one of the larger vessels that occupies this region, such as the retromandibular or carotid vessels which are within reach of a 4 7 mm (long) needle (Murphy and Grundy 1969). At this stage the barrel of the syringe should be realigned more laterally and another attempt should be made.

Menke and Gowgiel (1979) took 7 0 measurements of the distance from t h e oral mucosa i n t h e pterygotemporal depression to the mandibular foramen with the height of penetration at the narrowest anteroposterior width of the mandibular ramus. They found this distance to be on average 16 mm and this measurement was half the narrowest anteroposterior width of the r a m u s which is confirmed by the present

Fig 45.

Interproximal contacts between premolars are within the "safety" zone and the needle tip is on target (ie within the area of the lingula).

?ig 46.

Barrel of the syringe has been inserted past the upper boundary and hence the needle tip is approximating the posterior border

study. It is therefore recommended that the depth of oblique penetration be half the estimated anteroposterior width of the ramus.

The lingual nerve can be anaesthetized if required in the same path a s that for mandibular block anaesthesia,with the difference that it is delivered much closer to the mucosal surface. It has been suggested that the lingual nerve can be anaesthetized 5-10 mm after mucosal penetration.

Knowledge of the position of the lingula and the mandibular foramen, as well as the dimensions of the mandible, is not only relevant to those who wish to anaesthetize the inferior alveolar nerve but is also of interest to surgeons who must avoid the inferior alveolar neurovascular bundle during mandibular ramus osteotomy procedures.

The lateral ramus prominence, otherwise known as the antilingula is a prominence located on the lateral surface of the ramus of the mandible which is used as a n indirect guide to aid in determining where the lingula or mandibular foramen is located. The antilingula, though, is a n inconsistent anatomical landmark. For example Yates and colleagues (1976) demonstrated that the lateral ramus prominence was a definite landmark in less than half of the mandibles examined.

The position of a n osteotomy cut can be determined from the information provided in this study. Mean values are not a s clinically important a s the range of variability of structures. We note that the maximum depth of the lingula from the coronoid notch on the anterior border is 23 mm. Similarly the lingula is located at a maximum distance of 24 mm from the anterior border or a minimum of 8 mm from the posterior border on a line perpendicular to t h e rameal plane.

Consequently a safe cut can be placed anterior or posterior to these measurements depending on whether the anterior or posterior borders act as references.

If sagittal splitting is the choice of osteotomy then it is known from the study by Tamas (1987) that the narrower the width of the ramus, the closer the mandibular canal is to the lateral surface of the ramus. The narrowest anteroposterior width can quickly estimated by palpating the r a m u s of more accurately from standardized radiographs. The information uncovered by Tamas is very useful, however he does not provide any standards a s to what is indicative of the range of widths of rami which is currently provided in this study.

REFERENCES Alling CC I11 (1986)Dysaesthesia of the lingual and inferior alveolar nerves following third molar surgery. J. Oral Maxillofac. Surg.

44(6):454-457.

Barker BCW (1969) Anatomy of the hard and soft tissues of the oral cavity. Ann. Aust. Coll. Dent. Surg. 2:22-38.

Barker BCW (1971) Dissection of regions of interest to the dentist from a medial approach. Aust. Dent. J. 163-171.

Barker BCW (1979) Anatomical considerations in full denture construction. Aust. Soc. Prosthetic. Bull. 3-34.

Barker BCW, and Davies PL (1972) The applied anatomy of the pterygomandibular space. Br. J. Oral Surg. 10:43-55.

Barker BCW, and Lockett BC (1972) Multiple canals in the rami of a mandible. Oral Surg. Oral Med. Oral Pathol. 34(3):384-389.

Blackburn CW (1990)A method of assessment in cases of lingual nerve injury. Br. J. Oral Maxillofac. Surg. 28:238-245.

Blackburn CW, and Bramley PA (1989) Lingual nerve damage associated with the removal of lower third molars. Br. Dent. J. 167(3):103107.

Bremer G (1952) Measurements of special significance in connection with anaesthesia of the inferior alveolar nerve. Oral Surg. Oral Med. Oral Pathol. 5:966-988.

Brown T (1973) Morphology of the Australian Skull. Canberra:Australian Institute of Aboriginal Studies.

Castelli WA, Heulke DF, and Celis A (1969) Some basic anatomic features in paralingual surgery. Oral Surg. Oral Med. Oral Pathol. 27(5):613621.

Fielding AF,and Reck SF (1986)Bilateral lingual nerve anaesthesia following mandibular third molar extractions. Oral Surg. Oral Med. Oral Pathol. 62(1):13-16.

Gabriel AC (1958)Some anatomical features of the mandible. J. Anat.

92:580-586.

Galbreath JC, and Eklund MK (1970) Tracing the course of the mandibular block injection. Oral Surg. Oral Med. Oral Pathol.

30(40):571-582.

Giglio J A (1988)Postoperative morbidity with mandibular third molar surgery: A comparison of two techniques. J. Oral Maxillofac. Surg.

46(6):476.

Girod SC, Neukam FW, Girod B, Reumann K, and Semrau H (1989) The fascicular structure of the lingual nerve and the chorda tympani: An anatomic study. J. Oral Maxillofac. Surg. 47:607-609.

Goldberg M, Nemarich AN, and Marco WP I1 (1985) Complications after mandibular third molar surgery: a statistical analysis of 500 consecutive procedures in private practice. J. Amer. Dent. Assoc.

111:277-279.

Gray RLM (1978) Peripheral facial nerve paralysis of dental origin. Br. J. Oral Surg. 16(2):143-150.

Haschemi A (1981) Partial anastomosis between the lingual and mandibular nerves for restoration of sensibility in the mental nerve area after injury to the mandibular nerve. J. Maxillofac. Surg. 9:225227.

Hayward J, Richardson, and Malhortra (1977) The mandibular foramen: Its anteroposterior position. Oral Surg. 44(6):837-843.

Harvey W, and Robb RA (1967) Lingula and mandibular foramen in relation to anaesthesia of the inferior alveolar nerve. J. Dent. Res. 46(6):1289.

Heasman PA, and Beynon ADG (1986) The role of the mylohyoid nerve in mandibular tooth innervation. J. Dent. 14(2):80-81.

Heasman PA, and Beynon ADG (1986) Quantitative diameter analysis of lingual nerve axons in man. J. Dent. Res. 65(7):1016-1019.

Hetson G, Share J, Frommer J, and Kronman JH (1988) Statistical evaluation of the position of the mandibular foramen. Oral Surg.

Oral Med. Oral Pathol. 65:32-34.

Hetvedt AK, and Zungia J R (1990) Lingual nerve injury as a complication of rigid fixation of the sagittal ramus osteotomy: Report of a case. J. Oral Maxillofac. Surg. 48:647-650.

Heulke DF (1973) Selected Dissections of the Facial Regions for Advanced Dental Students. 6th Ed. Ann Arbor: The Overbeck Co. Hollinshead HW (1982) Anatomy for Surgeons: Vol. 1 The Head and Neck. Philadelphia: Harper and Row. Hunt PR (1976) Safetey aspects of mandibular lingual surgery. J. Periodontol. 47(4):224-229.

Ishikawa T, Nomura M, Nagahata H, Tani N, Yasui R, and Shimosato T (1986) A new method of conserving the inferior alveolar nerve during resection of the mandible. Brit. J. Oral Maxillofac. Surg.

24(2):107-113.

Kalamchi S, and Hensher R (1987) The managment of impacted mandibular third molars: 2. treatment. Dent. Update 14:437-440.

Kaufman K, Weinstein P, and Milgrom P (1984) Difficulties in achieving local anaesthesia. 108(2) 205-207.

Kay LW (1974) Some anthropologic investigations of interest to oral surgeons. Int. J. Oral Surg. 3:363-379.

Kazanjian VH (1939) Congenital absence of the ramus of the mandible. J. Bone Joint Surg. 37(3):761-772.

Kisselbach J E , and Chamberlain J G (1984) Clinical and anatomic observations on the relationship of the lingual nerve to the mandibular third molar region. J. Oral Maxillofac. Surg. 42:565567.

Koerner KR (1986) Clinical Procedures for Third Molar Surgery. Penwell Books.

LaBanc JP, Epker BN, Jones DL, and Milam S (1987) Nerve sharing by a n interpositional sural nerve graft between the greater auricular nerve and inferior alveolar nerve to restore lower lip sensation. J. Oral Maxillofac. Surg. 95(7):621-627.

Lambrianidis T, and Molyvdas J (1987) Paraesthesia of the inferior alveolar nerve caused by periodontal-endodontic pathosis. Oral Surg. Oral Med. Oral Path. 63(1):90-92.

Langston JR, and Tebo HG (1977) The incidence and relationship of the lateral ramus prominence to the mandibular foramen. Oral Surg.

Oral Med. Oral Pathol. 44(2):190- 196.

Loughman E (1983) Lingual nerve injury following tracheal intubation.

Anaesth. Intens. Care. 11:171.

Lysell L, and Rohlin M (1988) A study of indications used for removal of the mandibular third molar. Int. J. Oral Maxillofac. Surg. 17:16 1164.

Mason DA (1988) Lingual nerve damage following lower third molar surgery. Int. J. Oral Maxillofac. Surg. 17(5):290-294.

MacGregor (1976) (cited by Rood J P 1983) The radiological assessment of ectopic power third molars. DDS thesis, University of Leeds.

Menke RA, and Gowgiel JM (1979) Short-needle block anaesthesia a t the mandibular foramen. J. Amer. Dent. Assoc. 99(1):27-30.

Merrill RG (1979) Prevention, treatment, and prognosis for nerve injury related to the difficult impaction. Dent. Clin. N. Amer. 23(3):471488.

Middlehurst RJ, Barker GR, and Rood J P (1988) Postoperative morbidity with mandibular third molar surgery: A comparison of two techniques. J. Oral Maxillofac. Surg. 46(6):474-475.

Miller J A J r (1953) Studies on the location of the lingula, mandibular foramen and mental foramen. h a t . Rec. 115:349.

Mishra YC (1987) Entrapment of the neurovascular bundle by the roots of an impacted mandibular third molar-A case report. Br. J. Oral Maxillofac. Surg. 25(3):261-264.

Mozsary PG (1987) Inferior alveolar and lingual nerve injuries: regeneration or operation ? J. Oral Maxillofac. Surg. 45(3):204,286.

Mozsary PG, Middleton RA, Szabo 2, and Machado L (1982) Experimental evaluation of microsurgical repair of the lingual nerve. J. Oral Maxillofac. Surg. 40:329-331.

Murphy TR, and Grundy EM (1969) The inferior alveolar neurovascular bundle a t the mandibular foramen. Dent. Pract. 20(2):41-48.

Nicholson ML (1985) A study of the position of the mandibular foramen in the adult human mandible. h a t . Rec. 2 12(1):110-112.

HIH Consensus Development Conference Summary. Vol 2 (cited by Swanson AE 1989) Removal of third molars.

Nordenram A, Hultin M, Kjellman, and Ramstrom (1987) Indications for surgical removal of the mandibular third molar. Swed. Dent. J. 1l(1-2):23-29.

Osborn TP, Frederickson G,Small IA,and Torgerson TS (1985) J. Oral.

Maxillofac. Surg. 43(10):767-769.

Ossenberg NS (1974) The mylohyoid bridge: h anomalous derivative of Meckel's cartilage. J. Dent. Res. 53(1-3):77-82.

Ossenberg NS (1986) Temporal crest canal: Case report and statistics on a rare mandibular variant. Oral Surg. Oral Med. Oral Pathol.

62(1):10- 12.

Phillips J G (1987) The difficult extraction: 3(i) Lower wisdom teeth.

Dent. Update. 14(3):111- 118.

M c z VL, and Maros T (1981) The anatomic variants of the lingual nerve in human. h a t . Anz. 149:64-71.

Robinson PP (1988) Observations on the recovery of sensation following inferior alveolar nerve injuries. Br. J. Oral Maxillofac. Surg.

26(3):177- 189.

Robinson RC, and Williams CW (1986) Documentation method for inferior alveolar and lingual nerve paraesthesias. Oral Surg. Oral Med. Oral Pathol. 62(2):128- 131.

Rood J P (1977) Some anatomic and physiological causes of failure to achieve mandibular anaesthesia. Br. Dent. J. Oral Surg. 15:75-82.

Rood J P (1983) Degrees of injury to the inferior alveolar nerve sustained during the removal of impacted mandibular third molars by the lingual split technique. Br. J. Oral Surg. 21(2):103-116.

Rood J P (1983) Lingual split technique: Damage to inferior alveolar and lingual nerves during removal of impacted mandibular third molars.

Br. Dent. J. 154:402.

Sadler RM, Curran T, and Pryse-Pillips WEM (1986) Numbness of half of the tongue. Can. J. Neurol. Sci. 13(2):107-108.

Schwartz W (1973) Lingual anaesthesia following mandibular odontectomy. J. Oral Surg. 31:9 18-920.

Shepherd J, and Jones GM (1987) Trends in oral surgery practice. Br. Dent. J. 10:237240.

Shields PW (1970) Mandibular anaesthesia. Aust. Dent. J. 15(5):428-432.

Shields PW (1977) Further observations on mandibular anaesthesia. Aust.

Dent. J. 22(5):334-337.

Shields PW (1986) Local anaesthesia and applied anatomy. Aust. Dent. J. 31(5):319-325.

Sicher H (1946)The anatomy of mandibular anaesthesia. J. Arner. Dent.

Assoc. 33: 1541-1557.

Sillanpaa M, Vuori V, and Lehtinen R (1988) The mylohyoid nerve and mandibular anaesthesia. Int. J. Oral Maxillofac. Surg. 17:206-207.

Sisk AL, Hammer WB, Shelton, and Joy ED (1986) Complications following removal of impacted third molars: The role of the experienced surgeon. J. Oral Maxillofac. Surg. 44(11):855-859.

Stacy GC (1977) Lingual exposure during mandibular third molar surgery.

Int. J. Oral Surg. 6(6):334-339.

Stankiewicz JA, and Pazevic J P (1988) Hypoglossal nerve palsy after tooth extraction. J. Oral Maxillofac. Surg. 46(1-4):148-149.

Svane TJ, Wolford LM, Milam SB, and Bass RK (1986) Fascicular characteristics of the human inferior alveolar nerve. J. Oral Maxillofac. Surg. 44(1-6):431-434.

Swanson AE (1989) Removing the mandibular third molar: Neurosensory deficits and consequent litigation. Can. Dent. J. 55(5):383387.

Tamas F (1987) Position of the mandibular canal. Int. J. Oral Maxillofac.

Surg. 16:65-69.

Tier GA, Rees RT, and Rood J P (1984) The sensory nerve supply to the tongue: A clinical reappraisal. Br. Dent. J. 157:354-357.

Townsend GCT (1985) Errors of measurement and their analysis.

Anthropology and Genetics Research Group Seminar Series, Department of Dentistry The university of Adelaide.

Upton LG, Rajvanakam, and Hayward J R (1987) Evaluation of the regenerative capacity of the inferior alveolar nerve following surgical trauma. J. Oral Maxillofac. Surg. 45(3):2 12-2 16.

Walter JM, and Gregg J (1979) Analysis of post surgical neuralgic alteration in the trigeminal nerve. J. Oral Surg. 37:4 10.

Wofford DT, and Miller RI (1987) Prospective study of dysaesthesia f ollowing odontectomy of impacted mandibular third molars. J. Oral Maxillofac. Surg. 45(1):15-19.

Yates G, Olson D, and Guralnick W (1976) The antilingula as a n anatomic landmark in oral surgery. Oral Surg. Oral Med. Oral Pathol.

41 (6):705-708.

ACKNOWLEDGEMENTS I would initially like to recognise the willingness of the Department of Anatomy to make the Dissectorium available to myself; the first and hopefully not the last dental student to undertake a degree which was predominately in the field of gross anatomy. More specifically I would to thank Mr. Jim Johnson, the curator of of the dissectorium for the numerous occasions in which without obligation he went out of his way to personally assist me.

It would be unworthy if I did not acknowledge the goodwill of the most dignified Professor T. Brown for again without any obligation did not hesitate to assist me whenever possible; a truly sincere and lighthearted gentleman.

Finally I would like to declare that unquestionably the major reason these studies were attempted where and when they did, was due to the inspirational Associated Professor G.C. Townsend. I consider myself fortunate to have met the Professor but to have been his student was a frank honour. I only wish I possessed half of his impeccable qualities.

The only regret I carry is that I could not spend more time with him.

Being somewhat less ignorant than a t the beginning of the corse and unmistakably richer, if ever asked if I have a role model and who they may be, the spontaneous answer would be Associate Professor Grant Townsend; an intellectual and a friend.

CONCLUSION Every mandibular block injection and ramus osteotomy procedure requires a detailed knowledge of applied anatomy. A quantitative scientific approach rather than a n intuitive approach is less apt to fail or cause complications and is also preferable in the teaching of techniques in mandibular anaesthesia.

Although some studies have attempted to quantify the position of the mandibular foramen, this study has emphasised that the apex of the lingula is a more suitable level for needle insertion. As such, the needle tip should be directed toward a position slightly higher than that generally recommended by other studies. Because of the great degree of variability in mandibular dimensions, absolute distances for the placement of the needle tip may not be a s useful as information which relates to the variability of distances. Although absolute dimensions may vary according to age, gender and dentate state the position of the lingula in relation to other proposed landmarks remains reasonably constant. Therefore the recommended method is based on establishing the spatial relationships of structures to each other, given their inherent variability, rather than using fixed and often misleading, average values.

Extra-oral landmarks which aid in determining the path of the needle can supplement intra-oral landmarks and the following steps are recommended when attempting to estimate the position of the lingula during mandibular block anaesthesia.

The pad of the index finger is placed over the opposite occlusal plane and the needle should clear the superior surface of the finger. This level of insertion should correspond to the coronoid notch which can be confirmed by intra-oral palpation. The oblique depth of needle penetration from the interproximal contacts between the opposite premolars can be determined by two markings on the barrel of a syringe; 68 mm indicating the lower boundary and 84 mm indicating the upper boundary.

Extra-oral landmarks supplement intra-oral landmarks in helping to hone in on the position of the lingula. The tip of the lingula should be located midway between the upper third and midpoint of the ascending ramus. This should be on average 30 mm from the inferior border of the mandible. The depth of needle insertion from the anterior border to approximate the lingula is about half of the width of the ramus which on average is at least 15 mm. Halving the narrowest anteroposterior width of the ramus is also equivalent to the oblique depth of the lingula from

the point of penetration of the mucosal surface in the pterygotemporal depression.

Each guide available in determining the location of the lingula can supplement the ones already considered.

I t must be recognised that this study was based on normal adult specimens and may not be applicable to young children although this point needs to be verified. It also is not applicable to cases with gross mandibular deformities and is certainly not relevant in cases where there is congenital absence of the ramus of the mandible (Kazanjian 1939).

Murphy and Grundy (1969) correctly indicate t h a t the threedimensional shape of the pterygomandibular space, the space where the local anaesthetic solution is deposited is prismatic: the three surfaces of the prism are formed by the lateral pterygoid above, medial pterygoid medially and the ramus of the mandible laterally. They have determined the volume of the space as being about 2 C.C. which is also equivalent to the contents of a local anaesthetic carpule. I t may therefore not be so critical in practice to exactly locate the needle tip at the ideal position.

Depositing t h e local anaesthetic solution anywhere within the pterygomandibular space may lead to delayed anaesthesia but should nevertheless be sufficient to achieve the desired anaesthesia.

The exact difference in success between the techniques suggested here and other more conventional ones now needs to be tested clinically.