Impressions In Removable Partial Dentures
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Introduction ď Ź
The expression making the impression rather than' taking the impression is used to refer to the impression phase of partial denture construction to indicate that this procedure is not a passive activity in which the impression material accomplishes the task and the operator is merely an observer.
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The operator must be in complete control of all aspects of the impression procedure: the position and intra-oral condition of the patient, the size and position of the tray, and the selection of www.indiandentalacademy.com material and technique.
IMPRESSION - DEFINITION
Impression : a negative likeness or copy in reverse of the surface of an object; an imprint of the teeth and adjacent structures for use in dentistry
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Impression Trays Glossary of Prosthodontic Terms defines an Impression tray as 1)A receptacle into which suitable impression material is placed to make a negative likeness 2)A device that is used to carry,confine and control impression material while making an impression Impression trays can be classified broadly into 1)stock trays and 2)custom trays. www.indiandentalacademy.com
Stock trays 1)are made by the denial manufacturers, most. commonly of metal, 2)variety of sizes are there to fit large, medium, and small mouths. www.indiandentalacademy.com 3)Stock trays may be dentulous or edentulous.
4)Stock
trays for partial dentures may be perforated to retain the impression material in place better
5)Stock
trays may be constructed with rimlock for the purpose of retaining the material in position in the tray by means of an overlapping edge that wedges it in place
6)Another
stock tray designed for use with reversible hydrocolloid is the water cooled tray www.indiandentalacademy.com
Custom trays
1)Custom trays are sometimes needed for mouths abnormally large or small or of unusual configuration. 2)Another indication is when all peripheral borders must be delineated precisely in the impression. 3) One major advantage of is that , thickness of the impression material can be precisely controlled. This is important when using rubber-base type of materials that should not exceed a thickness of 2 to 4 mm, b’coz thicker sections are more subject to distortion as the material polymerizes www.indiandentalacademy.com
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4)Another advantage of the custom tray is the fact that a well-fitted tray will better support the impression in the palate, thus avoiding the everpresent danger of the material slumping in this vital area.
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5)The custom tray is recommended especially for the impression desired to establish precise borders, such as in Kennedy class I maxillary partial denture, when an accurate post dam is an important requirement. The peripheries, including the post dam, can be established with a high degree of accuracy with the customized www.indiandentalacademy.com tray.
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The custom tray may be made of acrylic resin or of either guttapercha or shellac baseplate material(Acrylic resin trays are more stable than the others)
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Modified Stock Tray (Individualized. Tray).
The stock tray can be modified with modeling composition or with wax to create a very accurately fitting tray which may be called the "individualized tray." (To differentiate it from custom tray)
The technique has an important advantage over a customized tray in that an alginate impression, in a stock tray, need not precede custom tray fabrication
The prime indication for this tray is the mouth with edentulous spaces that are not tooth-bounded, the www.indiandentalacademy.com Kennedy classes I and II in particular
Technique. ďƒ˜ Softened
modeling composition is placed in the stock tray inorder to capture the edentulous areas of the mouth and include one or two teeth adjacent to the space. The tray is positioned in the mouth and and the compound allowed lo cool, but not permitted to harden completely. Thus it is prevented from becoming' locked around the adjacent teeth.
ďƒ˜ When
it has hardened sufficiently to maintain its contour, it is removed from the month and www.indiandentalacademy.com chilled thoroughly
The compound is trimmed so that it does not come into contact with the adjacent teeth, and the surface of the compound in the edentulous areas is scraped to a depth of 2 to 4 mm to provide space for a uniform layer of impression material. ď Ź Before placing the corrective material into the tray, the compound must be made adhesive so that the elastic material will adhere to it ď Ź Wax can be used instead of modeling compound to create a tray,but if used it should contain perforations to help retain the elastic material, preventing its lifting or shifting ď Ź
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Advantages of Modified stock tray 1)Useful especially for exceptionally large / small mouth or for a mouth with an anomalous contour for which a stock tray will not fit 2)Impression can be accomplished in one appointment compared to the custom tray 3)Posterior border can be established with precise accuracy compared to a conventional impression
Disadvantages of Modified stock tray 1)Peripheral borders with the exception of the post dam area cannot be established accurately www.indiandentalacademy.com 2)Tray is more bulky than a custom tray
Impression materials - Classification Rigid Materials POP Metallic Oxide Pastes Thermoplastic Materials Modeling plastic Impresion waxes and natural resins www.indiandentalacademy.com
Elastic Materials Reversible Hydrocolloids(Agar-Agar) Irreversible Hydrocolloids(Alginate) Rubber Base Impression Materials Polysulphides Condensation Silicones Addition Silicones Polyethers www.indiandentalacademy.com
Reversible hydrocolloid Agar-Agar
1)First successful elastic material to be used in dentistry
2)It is ones of the most accurate and cleanest elastic materials.
3)Basic tray material is supplied as a gel in a collapsible tube.The lighter inject able material is supplied in a small collapsible tube for injection into a disposable syringe www.indiandentalacademy.com
4)Tray material consists of 15% agar,0.2% Borax for strength,0.1% Benzoates as preservative and 1 to 2% potassium sulfate to obtain surface hardness of cast.water content is 80 to 85%.syringe material has same combination but slightly less agar in it 9 6 to 8% 5)Agar gel is composed of a matrix of agar molecules.on heat application ,they disperse in water and form a sol. Gel to Sol â&#x20AC;&#x201C; ( 100 C or 212 F ) Sol to Gel www.indiandentalacademy.com â&#x20AC;&#x201C; (< 43 C )
6)Agar Impressions must be poured immediately.if storing is needed it must be done in 100% humidity and not more than 10 mins 7)Another insidious weakness of agar is its tendency to sag as the gel state is approached.sagging is significant for impressions of maxillary arch especially for RPDâ&#x20AC;&#x2122;s with palatal major connector (Sagging-incorrect cast- major connector will not adapt to palate) www.indiandentalacademy.com
Clinical Use. 1)Conditioning Units are must if this material is to be used as a primary impression.Units contain 3 controlled temperature water baths 1st bath â&#x20AC;&#x201C; Liquefies gel 2nd bath-Stores agar at a liquid phase 3rd bath-tempering bath used to reduce heat of the Sol to a temperature tolerated by tissues 2)Heat of agar material is dissipated by the www.indiandentalacademy.com use of water cooled trays
3) Selection of tray size for agar is same as that of alginate ,but one draw back is that the tray cannot be modified safely. 4)Chair side impression technique remains same except that the mucosa need not be dried 5)After tray is seated,30 seconds pause is allowed before water is used to cool.this pause permits the material to flow.once cooling started,it is done for 3 minutes. 6)After cooling tray is held for 30 seconds and then removed with a sudden jerk
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IRREVERSIBLE HYDROCOLLOIDS 1)Alginate is the most widely used and the most versatile of the impressions materials available for the practice of RPD. 2)The Reasons for its popularity are 1)Ease of handling 2)Relative inexpensiveness 3)Dimensional accuracy 4) Lack of need of additional items of equipment www.indiandentalacademy.com 5)Cleanliness of material
Alginate Composition
1)Sodium or Potassium alginate(soluble alginate) - 15% 2)Calcium sulphate (reactor) - 16% 3)Zinc oxide (filler particle) - 4% 4)Potassium titanium fluoride (accelerator) - 3% 5)Diatomaceous earth (filler particle) - 60% 6)Sodium phosphate (retarder) - 2% www.indiandentalacademy.com
Alginate impression is supplied in powdered form.Water is mixed with the powder to form a viscous sol,which forms an elastic gel through a series of chemical reactions after placed in mouth The water: powder rations is 40 ml of water for 15 gm of powder. The soluble Sodium alginate reacts with Calcium sulphate in an aqueous solution to form insoluble Calcium alginate gel. Gelation time of commercial alginate is about 3 to 4 minutes. www.indiandentalacademy.com
PROPERTIES AND WORKING CHARACTERISTICS
Shelf Life: Irreversible hydrocolloid does not have a good shelf life (deteriorates rapidly in elevated temperatures and high humidity). Water/powder ratio: Accuracy of the material is not affected by changes in W/P ratio. However W/P ratio changes affect consistency and setting time of the mixed material and the strength and quality of impression www.indiandentalacademy.com
Thick Mix-insufficient flow to record details Thin Mix-Tends to flow out of tray and tends to tear easily on removal from mouth Setting Time Both fast and regular set alginates are available By varying temperatures of water used setting time can be controlled (Recommended-22 C) (cooler water-provide more working time/warmer water-hastens set of impression material) www.indiandentalacademy.com
Storage of Alginate Impressions ď&#x192;&#x2DC; A definite disadvantage it cannot be stored for long and has to be poured within 12 mins ď&#x192;&#x2DC; Alginate expands when it contacts liquids as it imbibes water(Hence wrapping in wet paper towels or soaking impressions in water is contraindicated) ď&#x192;&#x2DC; Evaporation and shrinkage of alginate occurs when exposed to atmosphere.Dimensional changes results from release of strains within the material as moisture content changes.Even in completely humid environment the alginate shrinks thru a process called syneresis,in which a a fluid exudate forms on www.indiandentalacademy.com the surface of the impression
Sticking of Alginate
This being a potential problem , usually occurs on the flat labial and buccal tooth surfaces and occasionally on the tips of the cusps.
Sticking is caused by a chemical bond between alginate radical and hydoxyapatite crystal of surface enamel
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If alginate sticks to teeth,impression and cast will be incorrect
Sticking of alginate occurs,if tooth are too dry/too polished/Repeated impressions of a arch are made
Treatment is to use a prophylactic paste to which silicone ointment is added-silicone film prevents sticking www.indiandentalacademy.com
Polysulfide Rubber The first rubber impression material introduced to dentistry was known as Thiokol Later the material became known as mercaptan rubber, because the un-reacted base contained the -SH, or mercaptan, group. The group reacts during the setting process to form a rubber containing polysulfide group. For this reason the rubber material in general is referred to as poly-sulfide rubber impression material. www.indiandentalacademy.com
The impression material is supplied as two tubes of paste . One tube contains the catalyst, or accelerator, and the other the base.
The base material consists of 80% low-molecularweight organic polymer containing the reactive mercaptan groups and 20% reinforcing agents.
The catalyst tube contains the agent that activates the mercaptan groups to react and form the polysulfide rubber. Common catalyst used are lead dioxide. (Other catalysts example - tert-butylhydroperoxide) www.indiandentalacademy.com
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Based on viscosity and flow characteristics. it is classified as being light, regular, or heavybodied.
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For removable partial prosthodontics the regular-bodied or a mix of regular with lightbodied is used for the impression.
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Clinical Use ď Ź
Equal lengths of the base and catalyst are extruded on a paper mixing pad and mixed with a stiff spatula. If an injection with light-bodied material is to be made, the two mixes should be started simultaneously,. absence of streaks and the uniformity of the color indicates complete mix. This take about 45 seconds. The injection material is loaded into the syringe and the tray material into the custom tray immediately.
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The injection material is deposited into rest seat preparations and other critical areas;as carefully www.indiandentalacademy.com with the syringe to avoid trapping
The
loaded tray is seated & held steady until the final set of the material has taken place, usually about 8 minutes
After
removal from the mouth the impression is inspected for completeness and accuracy and cleansed by rinsing with cool tap water. The moisture should be removed by shaking the impression or by a gentle blast of air and poured as soon as possible
Recovery
of cast from impression should be delayed longer www.indiandentalacademy.com bcoz allowing extra time will produce a harder cast
Drawbacks of polysulphides Offensive
odor and color,
The
staining of clothing caused by lead dioxide
Effort
required to mix the material
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Silicone Rubber The material is supplied as a base and a catalyst, or accelerator. The base paste contains a low-molecularweight silicone liquid, dimethylsiloxane, which has the reactive OH groups . Silica or other agents are added to give the paste the proper consistency and provide stiffness to the set rubber. The catalyst, a tin octoate suspension and an alkyl silicate, is supplied as a liquid usually, but may be provided as a paste by the addition of thickening agents. www.indiandentalacademy.com
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The silicone pastes are supplied in the same consistency as the polysulfide rubber(light, regular, heavy-bodied,with the new addition of a very heavy bodied paste called a putty)
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Silicone rubber impression materials usually are supplied as a white base material in a tube or occasionally in a jar. The catalyst normally comes as a liquid in a small bottle with a dropper. Less frequently the catalyst is provided as a paste in a collapsible tube. The color of the mixed www.indiandentalacademy.com silicone is controlled by dyes
Clinical Use The mixing technique is carried out on a paper pad & continued until the material is free of streaks, normally 45 seconds. The syringe or tray is loaded, and the impression procedure followed as described for the polysulfide rubber material ď Ź The use of the putty is seldom indicated for a RPD. The putty is used to make a loose fitting impression tray generally of a segment of an arch. The light-bodied, or injection, material is then used as a wash within the putty to refine the impression, www.indiandentalacademy.com usually of a crown or fixed partial denture. ď Ź
Impressions in RPD Diagnostic Preliminary Working
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Diagnostic Impressions
Diagnostic impression is a negative likeness made for the purpose of diagnosis,treatment planning or the fabrication of a tray
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Technique for Making Diagnostic Impressions 1)Position of patient and Dentist 2)Impression Trays a)selection b)Checking trays for Correct size c)Extending impression trays 3)Control of Gagging 4)Control of saliva www.indiandentalacademy.com
Technique for Making Diagnostic Impressions Contdâ&#x20AC;Śâ&#x20AC;Ś 5)Manipulation of the impression material 6)Making the Impressions 7)Removal of Impression from the mouth 8)Inspecting the Impression 9)Cleaning the impression 10)Disinfecting the impression 11)Poring the castwww.indiandentalacademy.com
POSITION OF PATIENT AND DENTIST The position of the patient and the dentist can facilitate or complicate the making of impressions. The procedure is most convenient, with greatest comfort for the patient if the dentist is standing and the patient is seated upright. The occlusal plane of the arch for which the impression is being made should be parallel to the floor when the patient mouth is open . The height of the chair should be adjusted so that the patient's mouth is at the same level as the dentists elbow. www.indiandentalacademy.com
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When making the maxillary impression, the operator should stand at the right rear of the patient.This permits the dentist's left arm and hand to encircle the patient's head and manipulate the left corner of the mouth and lips
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For the mandibular impression the dentist stands at the right front of the patient. With the impression tray held in the right hand the dentist can manipulate the right corner of the patient's mouth with the left thumb and index finger www.indiandentalacademy.com
Tray Selection for Diagnostic Impressions Tray
of choice - Rim lock tray because it is rigid it confines the impression material, helping to force it into all the areas to be included in the impression. Perforated trays are rigid, but do not confine the material as well as the rim lock tray . Disposable plastic trays - too flexible to ensure the accuracy of impression and cast that is needed for removable partial dentures www.indiandentalacademy.com
Checking Maxillary tray for correct size The width of the tray is the determining factor in the selection of tray size. There should be a buccal clearance of 5 to 7 mm between the inner flanges of tray and facial surfaces of the remaining teeth and the edentulous ridge ď Ź The 5 to 7 mm space is necessary so that the impression material will be thick enough to spring over the undercuts.Too large a tray may be difficult to insert, and interference with the coronoid processes of the mandible may be en-countered in seating the tray www.indiandentalacademy.com ď Ź
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Frequently a tray with proper width is not long enough to cover the entire desired impression area, or a greater space is present between tray and the palatal tissues than the desired 5 to 7 mm. If an impression is attempted with such a tray, a great bulk of alginate tends to sag before gelation is complete, and an inaccurate impression of the palatal vault results
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The length of the tray and excessive palatal space can be corrected by the use of modeling plastic. Modification of tray is more important in the case of a master impression than a diagnostic www.indiandentalacademy.com impression
Checking mandibular tray for correct size ď Ź
A mandibular tray should provide 5 to 7 mm of space both buccal and lingual to the remaining teeth and the residual ridge.
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If the tray extends too far from the teeth in a lingual direction, there is a tendency to trap the tongue or the floor of the mouth. The problem is corrected by selecting a tray of a different size or by bending the lingual flanges of the tray to provide the 5 to 7 mm of clearance. www.indiandentalacademy.com
Extending an Impression Tray
Frequently
an impression tray that has a correct width is too short to cover the entire desired impression area. The impression tray can be lengthened by the use of modeling plastic .
The
modeling plastic is softened in a 60° C (140° water bath, kneaded, and adapted to the tray partially removed several times to prevent the locking of the tray into the undercuts.
After
the modeling plastic has been chilled, it is relieved to provide approximately 5 to 7 mm of clearance & thenwww.indiandentalacademy.com coated with alginate adhesive
Extending an Impression Tray
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Control of Gagging
Procedures That Will Help Prevent Gagging
1. Seating the patient in an upright position with the occlusal plane parallel to the floor. 2. Correcting maxillary tray with modeling plastic and leaving sufficient unrelieved modeling plastic at the posterior border so that positive contact can be maintained against the posterior palate during the setting of the alginate. 3. Not overfilling the tray with alginate. www.indiandentalacademy.com
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Seating posterior part of tray first and then rotating the tray into position, thereby forcing excess alginate in an anterior direction rather than out the posterior border of the tray.
5.Asking
the patient to keep the eyes open during the impression procedure (reduces the patient's tension).
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Asking the patient to breathe through the nose.
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Asking the patient to keep the eyes focused on some small object. www.indiandentalacademy.com
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8.Giving all instructions to the patient in a firm, controlled manner.
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9. Having the patient use astringent mouth rinse and cold water rinses before the impression is made. The use of an anesthetic spray is usually contraindicated because it will cause numbness of the tongue and palate and may contribute to the urge to gag www.indiandentalacademy.com
Excessive
Control of Saliva
amounts of saliva, will displace the alginate impression material and will contribute to an inaccurate impression Some of the methods employed for salivary control are 1)Astringent Mouth wash usage followed by a rinse of cold water 2)Guaze trips can be packed in vestibule areas & removed just before impression making 3)Usage of antisialogogues (eg 15mg ProBanthine tablet 1/2hr before impression procedure in combination with above methods can be used for patients with www.indiandentalacademy.com copious saliva secretion
Mixing the impression Material 1)Alginate is mixed by hand spatulation,or mechanical spatulation under vacuum. 2)Objective is a smooth,bubble free mix 3)Strength of gel could be reduced by 50% if mixing is incomplete
Loading Impression tray 1)Small increments should be placed in the tray and forced under the rim lock.placing large increments might entrap air www.indiandentalacademy.com
Making Impressions 1)Mandibular
impression is usually made first 2)Guaze packs placed for isolation are removed,syringe material of thickness 3-4mm is applied over occlusal surfaces of teeth and in vestibular /alveololigual sulcus areas. 3)The tray is then rotated into the mouth and correctly lined up over teeth and the patient is made to protrude tongue. 4)Tray is carefully seated into flanges and is not over seated to avoid cusps contacting tray www.indiandentalacademy.com
ď&#x192;&#x2DC; 4)Tray
is carefully seated into flanges and is not over seated to avoid cusps contacting tray ď&#x192;&#x2DC; 5)Tip of tongue is placed in contact with upper surface of tray during gelation and operator should stabilize the tray during entire gelation time which usually sets within 3 to 4 minutes www.indiandentalacademy.com
Making Impressions - maxillary ď&#x192;&#x2DC; 1)Loaded
tray is supported with left hand and guaze packs are removed first
ď&#x192;&#x2DC; 2)Alginate
is injected onto occlusal surfaces,vestibular areas and large amount is wiped onto the palate.Tray is then introduced into the mouth ,centered ,aligned & and checked for proper relation between labial flange of tray and anterior teeth or residual ridge. after ensuring correct anteroposterior position the tray is seated in the mouth by using fingers of both hands over premolar www.indiandentalacademy.com areas
3)Buccal tissues and lip are lifted outward during tray seating to prevent their entrapment under tray flanges
4)Over seating of tray is not done to avoid tray contact with teeth cusps.
5)tray is stabilized throughout the gelation time by operator www.indiandentalacademy.com
Removal of Impression from mouth. 1)Set alginate loses surface tackiness. 2)Leaving impression for additional 2 â&#x20AC;&#x201C; 3 mins in mouth gives additional strength.Gel strength doubles in first 4 mins after gelation 3)Impressions removed too early after gelation produce rough surface of pored cast.hence desirable it is to leave set alginate in mouth for at least 2-3 mins 4)At the time of removal,lips and cheeks are retracted to release some of seal and www.indiandentalacademy.com impression is removed with a sudden jerk
Inspecting the Impressions Reasons for rejecting an Impression 1)Alginate sticking to teeth 2)Alginate pullin away from any tray area 3)Voids in critical areas. 4)Layered impressions 5)Granular Impressions 6)Trapping of lips,cheeks,tongue,floor of mouth 7)Inadequate extension to soft tissue areas 8)tearing of important areas of impression www.indiandentalacademy.com
Cleaning/Disinfecting
Impressions
1)Failure to remove saliva from impressioncast becomes rough 2)Removal of saliva – running water/camel hair brush & soap suds/sprinkling stone powder as disclosing agent 3)Disinfecting – impression surface is sprayed with an appropriate disinfectant and left undisturbed for 10 mins
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Diagnostic cast uses Diagnostic cast which in turn permits 1)analysis of the contour of both the hard and soft tissues of the mouth: 2)determination of the types of restorations to be placed on the abutment teeth; 3)and determination of the need for surgical correction of exostoses, frena, tuberosities, and undercuts. www.indiandentalacademy.com
4)Surveyed diagnostic casts serve as a blueprint for the placement of restorations, the recon touring of teeth, and the preparation of rest seats. 5) The designed casts aid in presentation of the proposed treatment plan to patient. 6)Diagnostic casts that are accurately mounted on a suitable articulator permit analysis of the patients occlusion, of the adequacy of interarch space and of the presence of overerupted or malposed teeth and tuberosity interferences www.indiandentalacademy.com
Primary Impression and Cast ď&#x192;&#x2DC; After
completing all the preprosthetic procedures, the RPD is designed.
ď&#x192;&#x2DC;A ď&#x192;&#x2DC;
primary cast is required to design the RPD.
If the patient does not require any preprosthetic procedures/ the diagnostic cast is directly used as the primary cast to design the RPD. www.indiandentalacademy.com
ď&#x192;&#x2DC; The
primary cast is poured from a primary impression. Primary impression is made using alginate and the primary cast is poured using dental plaster.
ď&#x192;&#x2DC; The
primary impression should be made atleast six weeks after the completion of any surgical preprosthetic procedure. This time period is to ensure complete healing of the surgical -wounds
ď&#x192;&#x2DC; The
procedure & technique used to make and pour an impression are similar to that of a diagnostic impression www.indiandentalacademy.com
Working Impression or the Master Impression & the cast Glossary of Prosthodontic terms defines a master impression as â&#x20AC;&#x153;The negative likeness made for the purpose of fabricating a prosthesisâ&#x20AC;?
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Working Impressions ď&#x192;&#x2DC; Impression
for Kennedys Tooth supported partial Denture(Class III and IV arches)
ď&#x192;&#x2DC; Impression
making for Tooth-Tissue supported partial denture (Distal Extension Denture Bases)
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Making Master Impressions (Tooth supported partial denture) ď&#x192;&#x2DC;
In removable partial dentures that are completely tooth supported ( Class III and many IV arches ), occlusal forces-transmitted to abutment teeth are directed vertically down the long axes of teeth through occlusal, incisal, or lingual rests .
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Edentulous ridges will not contribute to support of partial denture, because teeth absorb these forces before the forces are transmitted to the residual ridge www.indiandentalacademy.com
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Since the denture base does not contribute to support of the partial denture & the underlying mucosa and bone are not subjected to functional forces, a tooth-supported removable partial denture can be constructed on a master cast made from a single, pressure-free impression that records the teeth & the residual ridge in their anatomic form
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For a accurate master cast the impression technique far outweighs the selection of the impression material www.indiandentalacademy.com
ď&#x192;&#x2DC; The
procedure for impression making is similar to that of a diagnostic impression making ď&#x192;&#x2DC; Additional care is given to accurately record rest seats, guide planes, dimples, etc. prepared during prosthetic mouth preparation.
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Tray Selection for Master impression of Tooth supported RPD ď&#x192;&#x2DC;
The production of a working cast sufficiently accurate for the construction of a partial denture frequently necessitates the use of a properly selected tray .
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The tray selection is done mostly on the basis of the type of material used and anatomical considerations www.indiandentalacademy.com
Tray selection for alginate material 1)Alginate usually requires only a stock tray. 2)When a standard rim lock or metal perforated tray is used for a RPD ,it must be modified to reduce the bulk of the alginate ď Ź Accurate Modification is achieved by using modeling plastic,which is softened in a water bath at 57C and molded over the area that requires modification (Usually palate and distal extension ridge in maxilla or distal extension edentulous ridge inwww.indiandentalacademy.com the mandibular arch
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Modifications in the maxillary tray is to prevent slumping of the material in the palatal region which can cause an inaccurate impression.
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Modifications in the mandibular arch is usually to displace the tissues of the floor of the mouth so that an extension of the lingual slope of the edentulous ridge can be obtained
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Alginate requires a custom tray only when the size of the arch will not accommodate a standard tray.the presence of exostoses or tori may interfere with tray seating and can hence require a custom tray
Tray selection for agar-agar material Special Water cooled trays are necessary when using agar â&#x20AC;&#x201C;agar to dissipate the heat ,in order that the hard or soft tissues are not burnt due to the heat involved in this techniquewww.indiandentalacademy.com
Tray selection for Elastomeric material ď&#x192;&#x2DC;A
individual tray enables an accurate impression to be made of the functional depth and width of the sulci in those areas which will be related to the denture border and to the denture components
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For accurate impression , constructed tray is uniformly spaced from the teeth and tissues which provides for even layer of impression material of adequate thickness for optimum elastic recovery on removal from mouth www.indiandentalacademy.com
Tray Selection (Contdâ&#x20AC;Ś..) Individual trays are usually made from cold cure acrylic resin with high filler content Wax spacer is given Borders are 2mm short of depth of sulci,uniformly Resorbed edentulous areas where the U cross sectional shape is lacking,tray rigidity should be increased by adding a fin of acrylic tray material to www.indiandentalacademy.com its external surface
Working impression materials for partial dentures When choosing an impression material for a partial denture working impression, the properties to be considered are as follows:ď Ź 1) The packaged material should have an adequate shelf-life ď Ź 2) It should be compatible with the oral tissues, for although it is only in contact with these tissues for a short period of time, hypersensitivity reactions can occur. A pleasant taste and odour are also desirable www.indiandentalacademy.com ď Ź
3. The working time should be long enough for mixing, loading in tray & placing in mouth , while short setting time is desirable to limit the total time taken for, the procedure & time the patient needs to have the impression in the mouth. ď Ź
4. It must be elastic when set in order to record undercut areas, reasonably compliant so that removal from the mouth and the subsequent casts will not stress mobile teeth in the mouth or fracture teeth from the cast, and sufficiently stiff when adequately supported by the tray to avoid distortion on casting the dental stone www.indiandentalacademy.com
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5. It must have adequate tear strength so as not to tear when removed from important undercut areas, although it should ideally tear from between the teeth without excessive force needed orelse the impression may be distorted, the natural teeth unduly stressed or the cast fractured.There must also be adequate adhesion to tray so that removal fro'n mouth does not cause separation of impression material from the tray.
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6. It must be dimensionally stable. Changes in dimension can occur on setting, on removal from the mouth temperature to room temperature and on www.indiandentalacademy.com storage.
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7. It should not be significantly affected by the moisture with which it inevitably comes in contact although a satisfactory impression of the oral tissues can rarely be obtained if they are covered in a layer of saliva. For this reason the oral cavity should always be dried prior to impression taking.
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The impression material must also be compatible with (the dental Stone with which it is to be cast.) www.indiandentalacademy.com
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8. Impressions will be rinsed free of blood and saliva and should be capable of being disinfected prior to transfer to the laboratory. The WHO recommends a 1-hour immersion time when using 2% glutaraldehyde solution or hypochlorite solution with 10000 p.p.m. of available chlorine. This will only be acceptable if no change in dimensions or surface detail occurs during the disinfection process.
ď Ź
9. Finally, the cost of the material should be considered. www.indiandentalacademy.com
Impression Material Selection All major classes of impression materials are capable of producing accurate results if their properties are shown due respect.However ,no single material can be used for all impressions
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Factors determining selection of impression material 1)Factors which will influence the operator's choice are once again likely to be : ď Ź cost, convenience, experience, the laboratory's preference and the clinical circumstances, including the type of denture to be provided
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Impression Material Selection (Clinical considerations) ď&#x192;&#x2DC;
. Where a few teeth are to be replaced with a
simple, low-cost, acrylic denture, an alginate impression in a stock tray is often used, although the difficulty of disinfection is a real disadvantage. ď&#x192;&#x2DC;
The replacement of a greater number of teeth increases the indications for the use of a special tray and a mucodisplacive impression technique, thus shifting the balance in favour of an www.indiandentalacademy.com elastomeric impression material.
Impression Material Selection (Clinical considerations) ď Ź
In the production of a complex, high-cost, cast cobalt-chromium denture the use of an accurate, dimensionally stable elastomeric impression material should be mandatory.
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In addition, the presence of significant undercut areas in the mouth is a strong indication for the use of the elastomeric impression materials. www.indiandentalacademy.com
Impression Material Selection (Clinical considerations) ď&#x192;&#x2DC;
Selection of polysulfide rubber for impression of an arch where remaining teeth have long clinical crowns and weakened periodontal support would be poor clinical judgment because the impression material would lock around long clinical crown, making it difficult to remove the impression and tray from the mouth without endangering the remaining teeth. Hence alginate is preferred in such cases www.indiandentalacademy.com
Impression Material Selection (Clinical considerations) ď&#x192;&#x2DC; The
same situation could occur if polysulfide rubber were used for an impression of a partially edentulous arch with splinted teeth or a FPD, in which case the impression would lock under the solder joint of the splinted crowns or under the pontic of the fixed partial denture. It is often necessary to section the tray to remove the impression. The impression, of course,is destroyed. Hence alginate is preferred in www.indiandentalacademy.com such cases
It is possible to "block out" or obliterate these undercuts in the mouth by pressing soft utility wax under the solder joint or under the pontic to prevent the impression material from becoming locked around these surfaces, but critical areas of tooth or soft tissue surfaces may be lost. ď Ź In the circumstances just described, an impression material such as irreversible or reversible hydrocolloid, which is as accurate as polysulfide rubber but fragile, allowing the material to release from undercuts, is indicated. ď Ź
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Master Impressions-Procedure (Tooth Supported RPD) 1)The procedure employed for making a master impression is the same as that of the diagnostic impression with the exception that the requirements o an acceptable impression are more demanding. ď Ź 2)Special care must be shown in recording critical areas like the rest preparations,hard palate and peripheral extensions ď Ź
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Reasons for rejecting Impression
1. Bubbles or voids in and around rest preparations
2. Contact of cusps with the tray, especially when the teeth are involved in the framework design
3. Show through between teeth and modeling plastic or modeling plastic and hard palate if the tray has been modified for an alginate impression www.indiandentalacademy.com
4. Voids or bubbles in palatal vault when palatal major connectors are to be constructed 5. Peripheral under extension when a denture base has been designed and a corrected cast impression is not planned 6. Inter proximal tearing of the impression material when coverage of those teeth has been designed 7. Lack of detail on the impression surface 8. Any doubt as to the accuracy of the impression
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Tooth-Tissue-Supported Removable partial dentures Introduction ď Ź A tooth-tissue-supported removable partialdenture constructed on a cast made from a pressure free impression, will exert excess pressure on the teeth that help support the denture as the soft tissue under the denture base compresses ď Ź
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Dual impression technique This technique is used to equalize as much as possible the support derived from the edentulous ridge and that received from the abutment teeth. The impression of the teeth should be made with a material that captures the teeth in anatomic form, because normally the teeth do not change position under function to any measurable degree. The impression of the soft tissue, on the other hand, must be made in such a manner as to record the tissues in a functioning form
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Requisites of a Dual Impression Technique The impression must: 1)record and relate the tissues under the same loading, 2)Distriute the load over as large a area as possible 3)Delineate accurately the peripheral extent of the denture base www.indiandentalacademy.com
INDICATIONS ď Ź
Decision to use a dual impression may be determined by the following test Acrylic Resin bases are added to frame work, the frame work is placed in the mouth and finger pressure is applied to the base. If the base can be depressed enough that the indirect retainers or lingual plating lifts away from the teeth, a dual impression should be made www.indiandentalacademy.com
INDICATIONS The dual impression technique is most often indicated for mandibular distal extension ridge, because only a limited ridge area can be used as a stress baring site. In addition, obtaining proper peripheral extension for denture base is more complicated in mandibular than maxillary arch. www.indiandentalacademy.com
INDICATIONS ď Ź
The other indication for dual impression is long span anterior edentulous ridge, where the ridge must supply some support for the partial denture. Improving the accuracy of that cast with a secondary impression and defining the peripheral extension of the anterior flange can be helpful in distributing the forces that will take place against the normally weaker portion of dental arch. www.indiandentalacademy.com
Dual Impression Methods There are basically two dual impression techniques: ď Ź 1)The physiologic, or functional, impression technique ď Ź 2)The Selected pressure impression technique
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Physiologic, or functional, impression technique ď Ź
This records the ridge portion by placing an occlusal load on the impression tray as the impression is being made.
ď&#x192;&#x2DC;
The underlying supporting tissues will be displaced because displacement will normally occur under function. www.indiandentalacademy.com
Physiologic impression techniques ď&#x192;&#x2DC; Three
physiologic impression techniques that are practiced include: McLean's and Hindels' methods, Functional relining method, Ftuid wax method. www.indiandentalacademy.com
McLeanâ&#x20AC;&#x2122;s Physiologic Impression Principle ď&#x192;&#x2DC;
Two impressions are made in this procedure: First a functional impression of the edentulous ridge is made. Second impression is made over the functional impression and it records the structures in their anatomic form www.indiandentalacademy.com
PROCEDURE
1)
A custom-made impression tray is fabricated over the edentulous areas of the preliminary cast. A spacer is not adapted bâ&#x20AC;&#x2122;coz - intend to record only the supporting www.indiandentalacademy.com tissues (not record any relieving tissues )
2)
Occlusal rims are made on the custom tray. Occlusal rims are required for the patient to close (bite) on while making the impression.
3) Impression material loaded tray is inserted into the patient's mouth & patient is asked to close on the occlusal rims. When the patient closes on the occlusal rim, the tissues under the tray are compressed and the impression is recorded in this relation www.indiandentalacademy.com
4) After making the impression/ the custom tray should not be removed from the mouth. 5) An alginate over-impression (this impression is made over the existing impression) is made using a large stock tray www.indiandentalacademy.com
6)
7)
When over impression is removed, functional master impression comes with it. alginate over impression carries the functional impression along with it (pick up impression) While making the overimpression, finger pressure should be applied on the stock tray so that the custom tray is pushed towards the tissues while making the www.indiandentalacademy.com over impression.
Disadvantages of McLeanâ&#x20AC;&#x2122;s technique ď Ź Finger
pressure used to settle the functional impression while making the over impression is not equal to the biting force used while making the functional impression. Hence, supporting tissues may not be as compressed as they were while making the functional impression. This can lead to errors. www.indiandentalacademy.com
Disadvantages of McLean’s technique(Contd…) Secondly,
there will be a small quantity of alginate between the occlusal rim of the custom tray and the over-impression stock tray .This alginate may act like a buffer and prevent the transfer of the entire load(finger pressure) applied on the stock tray to the special tray www.indiandentalacademy.com
Hindles modification of Mcleans technique ď Ź
1)Main change was that the impression of the edentulous ridge was not made under pressure but was n anatomic impression of the ridge at rest made with a free flowing zinc oxide eugenol paste
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2)As the hydrocolloid second impression was being made ,however finger pressure was applied through holes in the tray to the anatomic impression www.indiandentalacademy.com
Iâ&#x20AC;&#x2122;ll-effects Produced by McLean and Hindle's Methods â&#x20AC;˘
The denture is fabricated using an impression made over compressed tissues. But tissues in oral cavity are not always in a state of compression. The retentive components will try to retain the denture as it was fabricated. This gives the denture an inherent property to constantly compress the tissues even when there is no occlusal load www.indiandentalacademy.com
• Since the tissues are constantly compressed there will be excessive bone resorption. Bone resorption occurs due to two reasons: •
1)Constant pressure stimulates the osteopro-genitor cells to form osteoclasts. Osteoclasts resorb bone.
•
2)Constant pressure decreases the blood supply to the bone which again through a series of chemical mediators stimulate osteoclasts www.indiandentalacademy.com
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If retentive clasps do not hold the denture base properly, the partial denture will be slightly occlusal to the normal position (pushed away due to tissue rebounce).
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Since the dentures are occlusally displaced ,they will be the first to contact the opposite teeth during occlusion.This will produce premature contacts
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FUNCTIONAL RELINING METHOD Most methods of obtaining a physiologic impression for support of a distal extension denture base accomplish impression procedure before denture completion , usually after framework construction. ď Ź The same results can be obtained after the partial denture has been completed. The technique is referred to as a functional reline. It consists of adding a new surface, to the inner, or tissue, side of the denture base. ď Ź
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This technique is done to a completed partial denture before insertion for the perfecting the fit of the denture base to the residual ridge The main problems that arise with this technique are caused by failure to maintain correct relationship between framework and abutment teeth and failure to maintain accurate occlusal contact following the reline
The partial denture is constructed on a cast made from a single impression, usually alginate.This is an anatomic impression, and no attempt is made to alter it or www.indiandentalacademy.com produce a functional impression of the edentulous ridge
ď&#x192;&#x2DC; To
allow room for impression material between denture base & ridge, space must be provided & is got by adapting a soft metal spacer(Ash's No. 7metal),over the ridge on the cast before processing denture base.
ď&#x192;&#x2DC; After
processing, the metal is removed, leaving an even space between the base and the edentulous ridge www.indiandentalacademy.com
The portion of the technique that introduces the greatest hazard is the reline impression procedure. ď Ź The patient must keep the mouth partially open throughÂout the procedure to permit appropriate tissue control and the required www.indiandentalacademy.com visual assessment. ď Ź
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The actual impression for functional reline consists of flowing low fusing modeling plastic over the tissue surface of denture base,heating,tempering and reseating several times until accurate impression is got.
Modeling plastic over ridge is relieved by scraping to 1 mm or entire thickness of modeling plastic from the crest of ridge can be www.indiandentalacademy.com removed ď Ź
ď&#x192;&#x2DC; Making
the final impression 1)Usually free flowing ZnO eugenol paste is used 2)If undercuts are present on the ridge,light bodied polysulphide or silicone rubber may be used
ď&#x192;&#x2DC; In
functional reline procedures as in all other reline procedures occlusal discrepancies must be corrected
ď&#x192;&#x2DC; Functional
reline method has the advantage that the amount of soft tissue displacement can be controlled by amount of relief given to modeling plastic before making final impression.greater the relief,less will be the tissue displacement www.indiandentalacademy.com
FREE END SADDLE MADE FROM A MUCOSTATIC IMPRESSION ď Ź Differential
in the support offered by the abutment tooth in its relatively incompresible periodontal ligament and the more compressible denture bearing mucosa is greatest. www.indiandentalacademy.com
FREE END SADDLE MADE FROM A MUCOSTATIC IMPRESSION Contd…… Tendency
for free end saddle to sink under occlusal load and the pivot about the rest on the abutment tooth is increased
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ALTERED CAST TECHNIQUE ď Ź
In both the fluid wax functional impression technique and the selected pressure technique an impression of the displaced edentulous ridge is made by using an impression tray attached to the framework, and the master cast is altered to accommodate the new ridge impression . For this reason the technique is often referred to as the altered cast impression www.indiandentalacademy.com
OBJECTIVE OF ALTERED CAST TECHNIQUE ď Ź To
reduce the support differential for a free end saddle by obtaining a compressive impression of the edentulous area under conditions which mimic functional loading www.indiandentalacademy.com
OBJECTIVE OF ALTERED CAST TECHNIQUE Contd….. Thus
by obtaining a compressive impression of edentulous area ,distribution of load from the denture to the residual ridge is improved and the denture is more stable www.indiandentalacademy.com
1)Acrylic resin tray material is added to framework to form a base covering the edentulous area. ď Ź 2)Surface is dried and zinc oxide impression paste or medium viscosity silicone impression material is applied www.indiandentalacademy.com ď Ź
Frame
work - placed in the mouth and care is taken to seat it on teeth by pressure on the occlusal rests and indirect retainers only No finger pressure is applied to the base area and teeth are not occluded After seating framework fully,border moulding is carried out www.indiandentalacademy.com
ď Ź Set
impression is removed from the mouth and inspected ď Ź Errors are corrected by appropriate modification or impression is retaken www.indiandentalacademy.com
ORIGINAL APPROACH TO ALTERED CAST TECHNIQUE ď Ź In
the original approach to the altered cast technique the impression is developed using a specially formulated wax which flows readily at mouth temperature.
ď Ź This
technique has the advantage of allowing progressive modification of the impression until an ideal result is achieved. www.indiandentalacademy.com
ORIGINAL APPROACH TO ALTERED CAST TECHNIQUE ď Ź However,
it requires significantly more chair side time than the technique described and employs a commercial wax (Korrecta Wax No. 4 (Kerr)) that is not readily obtainable, or a mixture of waxes (75 per cent paraffin wax, 25 per cent yellow beeswax) that needs to be specifically prepared. www.indiandentalacademy.com
ď Ź Once
a satisfactory impression is obtained need for an extra clinical stage to record record jaw relation can be avoided by adding wax rims to the frame work at the chairside and proceeding with the recording at the same appointment www.indiandentalacademy.com
ď Ź Relevant
edentulous areas are cut from the original master cast ď Ź Frame work is carefully and accurately seated on the teeth www.indiandentalacademy.com
A new cast is produced by pouring stone into the saddle impression
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Fluid Wax Functional Impression ď Ź
The fluid wax impression may be used to make a reline impression for an existing partial denture or to correct the distal extension edentulous ridge portion of the original master cast.
ď Ź
The objectives of the technique are to obtain maxnum extension of the peripheral borders of the denture base while not interfering with the function of movable border tissues, to record the stressbearing areas of the ridge in their functional form and to record the non-pressure-bearing areas in www.indiandentalacademy.com their anatomic form
The fluid wax impression is made with the open mouth technique so that there is less danger of overdisplacement of ridge tissue by occlusal or vertical forces
The term fluid wax is denotes waxes that are firm at room temperature and have the ability to flow at mouth temperature. The most frequently used fluid waxes are Iowa Wax, and Korrecta Wax No. 4 Korrecta Wax No. 4 is slightly more fluid than Iowa
The borders of all movable rissues are short but not more than 2mm short because the wax does not have sufficient stregth to support itself beyond this www.indiandentalacademy.com
The clinical technique for the use of fluid wax calls for a water bath maintained at 51 to 54C.At this temperature the wax becomes fluid..At this stage,the tray is coated with the wax on the tissue side.
Tray is seated in the mouth.Patients must remain with a half open mouth for about 5 minutes
Then tray is removed and wax examined for tissue contact evidence.Where tissue contact is www.indiandentalacademy.com present,wax surface will be glossy and if there
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When the impression evidences complete tissue contact and when anatomy of the limitin border structures is evident,impression is replaced in mouth for a final time and is left in mouth for 12 mins,just to ensure that the wax has completely flowed and released any pressre that might be present
ď Ź
The new cast has to be poured immediately. www.indiandentalacademy.com
SELECTED PRESSURE IMPRESSION ď Ź
The physiologic impressions produced a generalized displacement of the mucosa to a greater or lesser degree.This displacement was intended to record the tissue in the configuration it would assume when occlusal loading was applied to partial denture in function
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The selected pressure impression technique not only equalizes the support between the abutment teeth and the soft tissue, but has the added advantage of directing the force to the portions of the ridge that are most capable of withstanding the www.indiandentalacademy.com force.
ď Ź
The equalization of support between tissues and teeth in the selected pressure impression technique is accomplished by providing relief in the impression tray in selected areas and permitting the impression tray to contact the ridge in other areas .
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Areas where relief is provided will be least displaced as the impression is recorded. In those areas of the tray where relief is provided greater displacement of the underlying mucosa will occur www.indiandentalacademy.com
This sequence must be repeated until an accurate impression of the edentulous ridge has been accomplished (Fig 1215). The border extensions are determined by heating to the borders and guiding the placement of the cheek and tongue. To provide space for the impression material, 1 mm of modeling plastic is removed from the intaglio surface (Fig 1216). An impression is made with a free www.indiandentalacademy.com
flowing zinc oxideeugenol paste or a light bodied polysulfide rubber base. In the functional reline procedure, as in all reline procedures, occlusal discrepancies must be corrected after the new denture base has been processed. Since the openmouth impression technique must be used, it is impossible to maintain previously established occlusal contacts. www.indiandentalacademy.com
If errors in occlusion are slight, the correction may be accomplished in the mouth. How ever, in a majority of cases, it will be necessary to remount the partial denture on an articulator to correct the occlusion. (The technique for remounting a partial denture to correct occlusal discrepancies is described in chapter 16.) Although fraught with danger, the functional reline method can be used successfully www.indiandentalacademy.com
Adding an impression tray Preparation for the corrected cast impression appointment requires the addition of one or more impression trays. This should be accomplished only after the framework has been fitted to the teeth and soft tissues. A tray may then be added using a chemically activated or lightactivated resin. www.indiandentalacademy.com
ď Ź
trimmed using a laboratory knife (Fig 12Â21). Folk ing polymerization, the denture base resin is trimn and smoothed using laboratory burs and arbor ba (Fig 12Â22). Care is taken to ensure that all bon are gently rounded. The tissue surface of the iin( sion tray is not relieved at this time. www.indiandentalacademy.com
Correcting peripheral extensions The frameworktray assembly is fully seated ii mouth. It is important to note that from this forward, each time the framework is seated c cast or in the mouth, visual verification of cor seating must be made. This description is primarily concerned with ing the extensions for a mandibular distal exi impression, but manipulating border tissues ti at the proper tray contour applies to anysituati www.indiandentalacademy.com
ď Ź
ď Ź
The buccal extension of the tray is observe! cheek is moved downward, outward, and upv this movement is taking place, the edge of should be 2 to 3 mm from the depth ofth' vestibule. Posteriorly, the mandibular tray should n thirds the height of the retromolar pad.t must be determined by direct observation www.indiandentalacademy.com
ď Ź
ď Ź
The distolingual length of the tray is asses recting the patient to place the tip ofth against the upper lip. The operator's fing( rest lightly on the tray during this process. lifts, even slightly, during this movement, d gual length should be shortened. The remainder of the lingual flange ise\ having the patient place the tip oftheti each cheek. This movement need notbefi only enough to distend the cheek slightly.
ď Ź www.indiandentalacademy.com
Before adding the impression tray material, the master case is carefully examined. Undercuts chat would interfere with removal of the tray are blocked out using baseplate wax (Fig 1217). An appropriate separating medium is then placed on the corresponding area of the cast (Fig 1218), and the framework is completely seated (Fig 1219). A chemicallyactivated or lightactivated resin is adapted to the struts of the minor connector (Fig 1220). The borders are www.indiandentalacademy.com
The viscosity of unset materials is also relevant - a more viscous material is desirable where a muco-displacive impression is required.
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ď Ź
Deteriorated alginate become thin during mixing,exhibit erratic setting times,have reduced strength and high degree of permanent deformation of impression
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Impression Trays â&#x20AC;&#x201C; Diagnostic Impressions
Stock impression trays for dentulous and partially edentulous dental arches are of three basic types 1) rim lock trays, 2) perforated metal trays 3) plastic disposable trays. www.indiandentalacademy.com
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Thank you www.indiandentalacademy.com Leader in continuing dental education
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