THEORIES OF IMPRESSION MAKING IN COMPLETE DENTURE TREATMENT

DIFFERENT THEORIES OF IMPRESSION MAKING AND RATIONALE FOR THE DIFFERENT TECHNIQUES IN COMPLETE DENTURE TREATMENT

Introduction

Theory means observation based on principles and concept is the application of these theories. Impression forms a important virtue for the success of compete denture treatment and hence the concepts of impression should be properly understood. From time immemorable there have been different theories that had been advocated. Green Brothers were the first to introduce the principle of muco compression during impression technique. The shortcomings of this principle gave rise to the mucostatic technique by Hary L. Page with high regard for tissue health. But again due to the disadvantage of this technique, there was an impetus for the introduction  of the selective pressure technique which combined the concepts of both the previous techniques. There are various techniques adopted by different practitioners and there may be as many techniques as the number of dentists regarding impression which in general means negative likeness but in prosthodontics it is the negative registration of the denture bearing denture stabilizing, denture bracing and peripheral limiting structures obtained in one of the plastic / semiplastic materials which is registered at the moment of crystallization of the impression material.

At the moment of crystallization means that the tissues are registered at a particular moment. Since the denture bearing tissues are always in a state of flux with new cells being generated and cells being shed of at different moment of time, the tissues at the time of impression making will differ from that at the time of denture insertion.

It is not feasible to group all the techniques into rigid compartments but a broad classification is possible. They may be classified as scientific / empheric depending on whether they are based on knowledge of anatomy.

b.      They may be classified as open / closed mouth impressions depending on the condition of the mouth at the time of impression making.

c.      They may be classified as either pressure, nonpressure / minimal pressure, and selective pressure depending on the amount of pressure applied at the time of making impressions.

Prior to 1600 complete denture replacement were not made due to lack of understanding of retention.

Ø  In 1711, Mathian Gottfried Purman recorded the use of wax.

Ø  In 1728 Pieree Fauchard made dentures measuring the mouth with compases and cutting bone into an approximate shape for the space to be filled.

Ø  In 1736, Phillip Pfaff of Germany made impressions in wax sections of half of the mouth at a time.

1845-1899

Ø  In 1886 Richardson mentioned about making plaster impressions of tissues at rest and achieving adhesion by contact.

·        Concepts of atmospheric pressure, maximum extension of the denture bearing area, equal distribution of pressure and close adaptation of the denture bearing tissues were stressed.

·        Many changes in impression making became evident during this era. A single impression formely deemed sufficient, advanced to a method using priliminary impression of guttapercha, beeswax or modelling compound followed by secondary wash impression made of plastic within preliminary impression.

1900 – 1929

Ø  A concentrated effort was directed towards accuracy.

           Most impression were of compressive type and by the closed mouth technique. To prevent buildup of excessive pressures vents were made.

Closed mouth technique

Ø  In this technique the supporting tissues recorded in a functional relationship.

Ø  The movement of all related tissues were in normal functional movements such as swallowing, talking, sucking and occlusal contacts.

Ø  A pressure similar to that of mastication was developed through the occlusal rins.

This according to Stanley P Freeman-amount of tissue compression is like that in function.

Ø  Selective pressure technique.

Ø  The disadvantage of closed mouth technique is the tendency of overextention or underextention.

Ø  Release of pressure of occlusion may permit a rebound of denture.

Ø  It is contraindicated in the presence of considerable amount of movable tissue.

The open mouth technique is preferred because the operator can see whether the border molding is done properly.

The functional manipulation cannot be used routinely not all patients can truly move the impression materials as needed, some may use extreme movements and others use.

Ø  Two techniques were developed for the management of flabby ridge.

1^st technique – it was of muco compressive type compound impression which displaced the flabby tissue paratally.

2^nd technique – it was advocated by Greene Brothers, which captured the tissue in its passive form.

Ø  Concepts of posterior palatal seal were developed by Liberthal and Greene.

Ø  For the first time there were references to movement of tissues and the mandible during impression making.

Ø  Border molding was done against the direction of muscle fibres as advocated by Wilson.

Ø  There were others like Nichais, Neil Fish, Swenson et al who advocated manipulation in border molding in the direction of its fibres.

Ø  It was during this era that the concept of esthetics in impression making was introduced.

MUCOCOMPRESSIVE TECHNIQUE

Ø  The muco compressive technique was initiated by Greene Brothers. They introduced a modeling  plastic, a method for manipulating it and a technique that is said to have been the first to utilize all the denture bearing area for denture retention.

Ø  They were the first to teach the closed mouth all modeling plastic technique called the Greene Brothers all compound impression.

Ø  The main objective of this technique was to attain better retention of the dentures.

The typical technique by Greene brothers was as follows.

·        A preliminary impression was made in impression compound and a custom tray was constructed using baseplate with its periphery 1/8^th inch shorter than the denture outline.

·        With this tray another impression with compound was taken.

·        Well fitting rinse with uniform occlusal surface were made and the height of the bite adjusted against a similar bite rim on the mandibular ridge.

·        Areas to be relieved like median raphe was softened on the impression and was again inserted in the mouth and was held under biting pressure for one / two minutes.

·        The peripheral margins of the impression was then softened and border molding was done by asking the patient to give various cheek and lip movement as in whistling and smiling.

·        The posterior palatal seal was obtained by swallowing movements by the patient under biting pressure.

·        The claims made by the advocates of this technique was that since border molding was done in their functional positions, the final dentures would retain well and cannot be dislodged during functional movements of the jaw.

Variations in this technique

·        Some used the preliminary impression itself as the tray and impression to be improved by border molding.

·        Some preferred to make custom trays in a more stable and stronger material than compound for better results.

·        Relief in hard areas was obtained in number of ways. Some custom trays were made with escape holes in areas overlying the hard tissues and close adaptation provided in those areas covering the soft tissues.

·        Some use low fusing compound by softening and adapting it to the soft tissues.

·        Some advocate unnatural movement of the mouth along with massaging of the cheeks and lips from outside during border molding.

·        Post dam is obtained in number of ways.

·        The addition of soft wax like carding wax or low fusing compound for this purpose is common.

·        Scraping of cast is also used.

The amount of pressure applied to the tissues in the muco compressive technique was not only great but was applied to the centre of the palate and the peripheral tissues which were not well suited to receive the maximum biting load this interferes with normal blood supply of the tissues resulting in their breakdown.

As soon as this change took place both the peripheral seal and excellent retention were lost. Hence the retention achieved by these means was transient and harmful to the health of tissues.

Dentures made by this technique would fit well during mastication i.e. only a short period each day, but would not be closely adapted to the tissue when the patient was at rest. This is because of the rebounding of tissues.

These disadvantages indicated a need for spacer in the custom tray fabrication.

1930-1948

·        Concept of mucostatics was introduced by Harry L. Page in 1938.

·        Addison in 1944 also mentioned the same principle of making impressions of displaceable tissue in its passive state and considered interfacial surface tension as one of the main factors of retention.

·        With new materials like zincoxide eugenol, waxes, elastomers, individual tray construction was emphasized.

Minimal pressure technique based on mucostatic principle

·        In a Brochure published by Hary L. Page in 1946 he stated that all soft tissues were cheifly fluid and 80% or more of the tissues are composed of water. According to pascal’s law which states that any pressure applied to a confined fluid is transmitted undiminished and equally in all directions. Page contended that since the soft tissues are confined under a denture, any pressure applied will be transmitted in all directions.

·        The advocates of this principle considered interfacial surface tension as the only important retentive mechanism in complete dentures. Therefore they did not resist vertical displacement, which was the only movement capable of interrupting surface tension. However, Dykins recommended a short lingual flange to resist lateral displacement.

·        According to the principle of mucostatics the impression material had to record without distortion, every detail of the mucosa so that a completed denture would fit all minute elevations and depressions. So much emphasis was placed on recording details that separating substances could not be used at any point in the procedure.

·        Mucostatics further demanded a metal base. Gold, one of the most accurate metals was bypassed in favour of chrome alloy which are not considered to be quite so accurate as gold.

A typical impression method representing this technique was as follows.

·        A compound impression was made in a suitable tray and a cast was made.

·        On this base plate wax was adapted which acted as a spacer according to denture outline.

·        Custom tray was fabricated over this spacer.

·        A soft ribbon of carding wax was applied at the posterior margin of the maxillary tray and it was placed in the mouth under light pressure and patient was asked to do swallowing movements inorder to obtain a posterior palatal seal.

·        A small amount of impression plaster mixed into a smooth consistently was placed in the tray, introduced in the mouth and was slowly raised to position and held with as little pressure as possible.

·        No border molding was advocated but the soft plaster was expected to mold itself to the relaxed vestibular tissues.

·        The impression was held till the impression hardened and was then removed.

Variations in the technique

·        Some techniques use compound instead of wax for obtaining post dam.

·        Some techniques advocate post dam over the final impression.

·        Zinc oxide eugenol and alginate had also been used for similar results.

·        Page’s application of Pascal’s law to the field of denture impressions is only partly correct because the tissues involved are not wholly incompressible and fluids may escape through the borders of the denture.

·        Page’s claim that retention is a function of surface tension alone is also objectionable because this tensile force itself is dependent upon adhesion and cohesion.

·        The elimination of use of separating media results in distortion of the cast.

·        The use of chrome cobalt as denture bases results in failure of accurate detail reproduction.

·        The mucostatic principle ignores the value of dissipating masticatory forces over as largest possible basal seat area. Further the mucostatic denture minimized the retentive role of the musculature as described by Fish in 1948.

The merit of this technique was its high regard for health and preservation of tissue.

1948 – 1964

·        There was an increased emphasis on biologic factors of complete denture impression making.

·        Selective pressure concept by Boucher became popular.

·        Craddock, Landa et al advocated use of escape vents.

·        More attention was given to esthetics in the impression techniques used greater emphasis was on flanges, border molding, posterior palatal seal and denture extension.

·        In 1948, the mucoseal technique – a variation of the mucostatic technique was introduced.

·        Vacustatics concept was developed by Milo V. Kubalib and C. Buffington to eliminate the functional limitations of impressions.

Selective pressure technique based on selective pressure theory

·        Advocated by Boucher in 1950 it combines the principles of both pressure and minimal pressure techniques.

·        The philosophy of the selective pressure technique is that certain areas of the maxilla and mandible are by nature better adapted for withstanding extra loads from the forces of mastication. These tissues are recorded under slight placement of pressure while other tissues are recorded at rest or relieved with minimal pressure in a position that will offer maximum coverage with the least possible interference with the health of surrounding tissues.

·        Here an equillibrium between the resilient and the non resilient tissues is created.

Primary stress bearing areas of maxilla are crest of alveolar ridge and the horizontal plate of palatine bone and in the mandible it is the buccal shelf area.

Secondary stress bearing areas of the maxillary foundation are rughae area and the slopes of the ridge in the mandibular foundation.

Areas requiring minimum pressure are incisive papilla, midpalatine suture, tori in the maxilla and crest of mandibular residual ridge.

In the maxilla, the tissue underlying the region of posterior palatal seal has glandular and soft tissue between the mucous membrane lining and the periosteum covering the bone. This tissue can be more readily displaced for the maintenance of peripheral seal of the maxillary denture.

An earlier technique representing this group consisted of the following steps:

Ø  A well fitting tray with a uniform clearance of about 5mm was selected and a compound impression was obtained with little border molding done on the peripheries.

Ø  This compound impression was separated from the metal tray and its peripheral borders were trimmed 1 – 2 mm short.

Ø  The base portion of the impression was then scrapped evenly to a depth of about 2mm except in the posterior seal area where no scraping was done.

Ø  A sufficient amount of creamy mix of plaster was spread over this impression and was placed in the mouth with little pressure.

The cheeks and lips were lightly patted from outside while the plaster was still soft. This procedure gave sufficient value like seal without exaggerated pressure on soft tissues.

Variations in the technique

Ø  Most of the techniques prefer taking a preliminary impression and using a custom tray rather than use the initial compound impression for further improvement.

Ø  The preliminary impressions are usually taken in compound but materials like alginate, elastomeric impression materials are also used.

Ø  Certain methods advocate the use of three small compound stops in the base area of special tray before doing border molding. This prevents the periphery of the tray from impinging on the tissues and it standardizes the relation of the tray to limiting tissues for every insertion of tray.

Ø  The amount of material, consistency of material, use of space or escape vents and the manual pressure with which the impression is made are all possible variable which have been used to advantage by different techniques.

The mucoseal technique was stated by Pryor in 1948 which was introduced as a variation to the mucostatic technique.

·        The anterior lingual border is molded by the floor of the mouth with the tongue in repose.

·        The tray is extended horizontally backward, over the sublingual glands towards the tongue to effect a border seal.

·        Thus this technique utilizes the benefit of minimal pressure and also provides maximum extension of denture borders and maximum coverage of denture bearing area.

Sub-atmospheric pressure technique based on the concept of mucostatics

Ø  Milo V. Kubalik and Bert C. Buffington developed this technique the objective of which was to reduce the stress on any given tissue by increasing load bearing area. the form of the tissue is recorded vertically and laterally, when a controlled partial vacuum is established in impression tray specially built for the patient. It is maintained in the mouth without direct mechanical support of any kind. The difference between the subatmospheric pressure within the tray and the atmospheric pressure outside the tray is all that is needed to centre the tray over the ridges in a static position. A vacuum is developed between the soft tissues and the tray. A recording material in a fluid state flows from the border region into the evacuated space and develops the basal tissues. Border seal is determined by the readings remaining constant.

Materials used

1.       Alginate, modeling plastic or a reversible hydrocolloid for preliminary impression.

2.       Clear acrylic resin for making the final impression.

3.       An adequate sealing agent for use around special fittings in the tray.

4.       Thermoplastic border recording impression material.

5.       A fluid (low viscosity) impression material that seats firmly enough to avoid distortion.

6.       A periphery wax to be used as a flexible material between impression and the boxing wax.

Molding Exercises

For the maxillary impression the patient is told

1.      To suck on the tube (this pulls the cheeks in a starts border molding).

2.      To say “00000” and EEEE alternately (This refines the border molding of the buccal and labial flanges and provides space for the frenum.

3.      To blow against closed nostril (This flexes the soft palate and molds the posterior palatal seal area. Wipes of any excess adapted extending beyond the border of the tray.

4.      To move the mandible from side to side (This molds the flanges lateral to the tuberosities.

5.      To swallow warm water (This allows for swallowing movements in the shape of the posterior palatal seal.

6.      To open and close the mouth (This rec­ords the shape and action of the paramusculature used in extreme opening and closing movements.

For mandibular impression the patient is instructed

1.      To suck on the tube (This flexes the labial, buccal and lingual vestibular structures and mold the flanges in these regions).

2.      To force the tip of the tongue against the palate (This forcibly molds the flange in the sublingual space with the paralingual musculature.

3.      To say “0000” and “EEEE” alternatively (This further molds the buccal and labial flanges)

4.      To lick the upper and lower lip (This molds the flanges in the lingual space in the region of Wharton’s ducts and genioglossus muscle.

5.      To place the tongue in the right cheek and left cheek (This further molds the flange in the sublingual fold space).

6.      To swallow warm water (This molds the posterolingual flange in relation to the palatoglossus and mylohyoid musculature).

7.      To tense and flex the lower jaw as if clenching one’s teeth (This molds the buccal flange from the external oblique ridge to the retromolar pad.

1965 – 1982

Ø  New techniques had been developed to manage compromised conditions.

For poor mandibular ridges – Sublingual flange technique by Tyrde and Robert Flange technique by Lott and Levin.

For hyperplastic alveolar ridges by Zafarulla Khan, William H. Filler.

Impression techniques for severely resorbed foundation

Flange technique by Lott and Levin introduced in 1966 involves making impressions of soft tissues of mouth adjacent to the buccal, lingual, labial, palatal surface and incorporating the resulting extensions or flange in the denture. Flange wax was rolled from the retromolar pad area to the sublingual region, large enough to restore the diameter of estimated resorption and patient is asked to forcefully perform functions of swallowing etc to give border extensions which covers maximum surface area (genial tubercles and sublingual gland).

Tyrde in 1965 used the dynamic impression method on the same principle to obtain sublingual flange.

Roberto Von Krameck et al in 1982 used modeling compound to record the extensions. This sublingual flange extension increases the tissue surface without interfering the functions of mastication, deglutition and phonation. The active incorporation of tongue activity also stabilizes the denture.

Impression technique for patients with unsupported movable tissue (Hyperplastic or flabby tissue):

William H. Filler described a technique using two trays.

a.      Preliminary maxillary and mandibular impressions were made in stock trays with alginate impression method and casts were poured.

b.      The maxillary and mandibular casts were placed on the surveyor and all the tissue undercuts were blocked out with utility wax.

c.      A single thickness of baseplate wax was formed over the casts to form a spacer. The spacer is terminated short of the posterior palatal seal area so that the tray material would contact the tissue in this area.

d.      A tinfoil sustitute was applied to the casts and the first of the two trays was made in autopolymerizing acrylic resin. Most of the basal surface of the tray was removed except for the lattice work of acrylic resin which strengthens the trays. 

e.      The maxillary and mandibular trays are then keyed to orientate the second tray in atleast three places. These keyed positions correspond with an extension of the second tray and will insure proper seating of the second tray over the first tray.

f.       The entire first tray was covered with a single thickness of baseplate wax, ensuring that the keyed positions here kept free of wax. Both the first resin tray and the casts were painted with tin foil substitute.

g.      The second trays were made in the same manner as the first and extend past the relieved area of maxillary and mandibular trays and fit into keyed positions.

h.      With round bur, numerous holes were made in the second tray.

i.        The deepest portion of the vault of maxillary tray was removed to create a stop when the final impression was made. The initial tray was sealed with minimum pressure and autopolymerizing resin on a tongue depressor was gently placed in the opening in the vault. When the resin had set a stop was created on the firm and stable palatal tissue.

Clinical impression procedure

Ø  The borders of the maxillary tray are formed by adding low fusing compound and border molding it. A finger placed over the resin stop will ensure a stable tray. The basal plate was removed and the flanges reduced 1-2mm with the exception of the part over the tuberosites and posterior palatal seal area of the maxillary tray.

Ø  The mandibular tray was stabilized by the addition of modeling plastic on the buccal flanges in the region of first and second molars and in the anterior part of the tray in the incisor area. The mandibular tray was border molded and baseplate wax was removed from the mandibular tray every where except at the three points used for stabilization.

Ø  Both the trays were painted with permlastic adhesive. Light body permlastic was used in initial tray as a corrective wash impression material. After it set the tray was removed from the mouth and all excess material was trimmed from the borders and from the area where the second tray would come into contact with the first tray to key themselves.

Ø  The second impression was made with plastogum used in corrective wash impression and plastogum was painted over the entire vault and all available tissue surface not included in the first impression. The second tray was filled with plastogum and gently vibrated into place until keyed parts of the tray were in contact. The two trays were held lightly together until the impression material set and then the impression was removed as a unit and the two trays were sealed together with sticky wax.

Zafarulla Khan described a technique where a window was cut in the custom tray where the unsupported area was present. The unsupported area was recorded with impression plaster and the remaining area was recorded with perrmlastic impression material.

Other techniques used in case of flabby tissues

a.      Hobrick described a technique where only a single custom tray was used. Border molding was done in the usual manner and impression was made with heavy bodied addition silicone. The area of movable tissue was cut out and relief holes were made and wash impression was made with light bodied impression material.

b.      Joh D. Watter recorded the healthy denture bearing tissue with ZnoE and the displaced tissue with impression plaster.

c.      Split method by Allan Mack is useful if tissues are exceptionally flabby. A loosely fitting tray made with heavy relief over the flabby areas was taken. Plaster was mixed and applied over the flabby area to a thickness of about 3mm and was allowed to set tray was filled with 2^nd mix of plaster and the impression was made with the initial coating of flabby areas thus acting as a splint while the impression was made and being removed.

Other techniques used for poor foundation

a.      Modified Fournet Tuller  technique by Allan Mack also utilizes the principle of achieving maximum peripheral seal together with minimal pressure on the crest of the ridge to obtain retention and stability.

b.      Winkler described a technique which used tissue conditions and over extended primary impression of alginate was made. Occlusal wax rims were constructed and the borders were adjusted so that the lingual flange and sublingual crescent area were in harmony with the resting and active phases of the floor of the mouth by as open and closed mouth technique 3 applications of conditioning material were used – each application for approximately 8-10 minutes. The third and final wash was made with light bodied material. The technique resulted in an impression that had tissue placing effect with relatively thick buccal lingual and sublingual crescent area.

Miller used mouth temperature waxes instead of tissue conditioners.  Klein proposed the development of impression without a tray, as a stock tray may cause some distortion of the tissue and may result in a over extended impression. He used a moldable material (putty silicone) reinforced by an internal metallic core which was placed over the residual ridge and the borders molded by speech exercises. A low viscosity material was placed on the impression surface of this tray and functional impression was made.

Impression technique for restricted access to the mouth

Walter described a technique with the use of sectional stock trays. Impressions of each side of the jaw was made on at a time and two holes were joined and cast was poured.

The recording of denture borders may be done by either hand manipulation and functional movement.

Hand manipulation

The contour of the denture borders may be obtained by the dentist with the use of manipulation of lips and cheeks within functional limits. Patients tongue movements record the lingual borders.

Functional movements

The denture borders are also formed by having the patient make functional or physiological movement such as swallowing sucking, grinning, licking etc.

Tench’s neuromuscular concept values the functions of sucking and swallowing while making the impression to bring the denture base into harmony with the physiological behaviour of the muscles. Forming an impression by neuromuscular concept develops a completely passive contact of all impression borders to the basal seat tissues, passively fills all marginal spaces and develops basal seat area coverage that is compatible with function.

Barone states that normal or natural movements will provide better borders than by manipulation.

The only truly functional or physiological method of making impressions is the so called dynamic impression. In this technique the basal seat and borders are obtained with the use of impression materials that continue to flow over an extended period of time such as tissue conditioning materials or wax. This material is placed in the patients transitional denture and the patients normal activities mold the borders over a period of time.

Functional reline rebase technique is based on the same principle.

Discussion

In the mucostatic principle

Clinical procedure in selective pressure technique:

1.      Preliminary examination and conditioning of the patient.

2.      Seating the patient:

i.        Patient should be in a upright position and relaxed.

ii.     The jaw should be at the level of the operator’s elbow for maxillary and at the level of operator’s shoulder for mandibular impression.

3.      The hands should be washed in the view field of the patient even though they may have been previously washed.

4.      The tray should be selected from the stock trays which should be kept ready sterilized while inserting the tray in the patient’s mouth using a rotatory movement. There should be an equal clearance of 6-8 mms. Between the tray and the tissues all round.

5.      Operator’s position:

i.        Right back side of the patient for upper impression.

ii.     Right side front of the patient for lower impression.

The selected tray should cover the entire denture bearing area. Check the tuberosity area in the maxillary and lingual pouch in the mandibular foundation.

6.      Compound is softened in chotwater.

i.        A large bowl should be used.

The compound is kneaded thoroughly to soften it uniformly. In case maxillary impression the compound is molded to a rounded form, placed in the centre of the tray and thoroughly spread over the surface of the tray.

In case of mandibular impression the compound is formed into a rope form and spread over the surface of the tray.

In case of maxillary impression, the tray is centered slightly anterior to the final position assumed by the tray when it is correctly seated. It is then moved upward and backward direction. The compound is manipulated by index finger into the deep buccal sulcus area. In case of mandibular impression the tray is centered exactly over the ridge and seated straight down. With the index finger the compound should be manipulated into the deep lingual pouch.

Ø  Simulation of the tissue should then be done.

Ø  The compound is allowed to harden and withdrawn from the mouth.

Ø  The impression is chilled in cold water and examined thoroughly. It is examined for completeness border tissue functions, distortion and gross physical defects.

Materials used:

a.      Low fusing impression compound sticks – Advocated by Boucher.

b.      Autopolymerizing acrylic resins

Advocated by Jones – not used due to the heat of polymerization and monomer irritant.

c.      Tissue conditioning materials (modified resins)

Chare has described the use of one such tissue conditioning material. They are effective when used correctly. They set slowly and continue to flow under pressure at a rate inversely proportional to time becoming stiffer but never losing resiliency.

d.      Metallic pastes and elastomeric materials.

Ideally body elastomeric impression material is used. Smith Dale E has advocated one technique where the border molding is done in one step with polyether impression material.

e.      Impression waxes

Use of impression wax adapted for border molding was reported by Knapl. But these waxes distort easily.

f.       Perio pack : Kerk and Idolt has described one step border molding with the use of periopack.

The diagnostic cast is made of dental plaster. The form of the custom tray helps us to make impression based on specified theory. The areas to be retrieved on the casts and undercut areas are marked and blocked with wax. Care must be taken while providing relief, as excessive relief causes flabby tissue formation. The custom tray must be 2mm less than the denture outline except in the posterior palatal seal and retromolar pad area. The peripheries of the tray should not be sharp / rough.

The custom trays are checked in the mouth. The tray should cover the entire denture bearing area. If the tray is underextended, compound should be added wherever necessary. If the tray is overextended the tray should be trimmed where required. The tray is also checked for retention and stability.

Border molding is done quadrant by quadrant (By hand manipulation) within the functional limits of tolerance.

Border molding:

The shaping of the border areas of an impression tray by functional or manual manipulation of the tissue adjacent to the borders to duplicate the contour and size of the vestibule.

Glossary of prosthodontic lesions 7^th edition.

Ø  The anterior limit of posterior palatine seal area is marked using T – burnisher. The line of minimal function is marked by asking the patient to tell Ah.

Ø  The low fusing impression compound is softened and placed in this area the tray is seated in the mouth to obtain posterior palatine seal.

Ø  The tray is then checked for completed border molding. It should have same appearance as the finished denture. The tray is reinserted and border seal and retention and stability are checked.

The different material used for final impression are

a.      Impression plaster (Rarely used).

b.      Zinc oxide eugenol paste – 2mm.

c.      Irreversible and reversible hydrocolloids – 6mm

d.      Elastomeric impression materials – 4mm, 2mm.

e.      Mouth temperature waxes –

f.       Soft acrylic resins (functional impression) – 1-2mm

The relief wax spacer is removed. If zinc oxide eugenol paste is used, it should be mixed fairly stiff and a ribbon of even thickness of paste should be applied to the tray. The tray is quickly inserted and sealed in the correct position and border molding is carried out by gently simulating tissue function in those areas.

Conclusion

Although there are many techniques with varied logic, the success of the prosthodontics treatment depends on the clinical diagnostic alumen, understanding of the theories of impression making and its application by the operator.

References

1.      Boucher : Prosthodontic treatment for edentulous patients.

2.      Boucher C.O. : A critical analysis of mid century impression technique for full dentures. J. Prosthet. Dent., 1 : 472-491.

3.      Ellinger Charles W. “Synopsis of complete denture.

4.      Edgar N. Starke : Historical review of complete denture impression materials. JADA, 91 : 1037-1041.

5.      Filler W. H. : Modified impression technique for hyperplastic alveolar ridges. J. Prosthet. Dent., 25 : 609-612, 1971.

6.      Glossary of Prosthodontics. J. Prosthet. Dent., Edition 7^th, 81 : 48-110, 1999.

7.      Heartwell Charles M. : Syllabus of complete dentures.

8.      Luin Bernard : Impressions for complete dentures.

9.      Lott F. and Luin B. : Flange technique : An anatomic and physiologic approach to increase retention, function, comfort and appearance of dentures”. J. Prosthet. Dent., 13 : 394-413, 1966.

10. Milo V. Kubalek and Bert C. Bufington : Impressions by the use of substathmospheric pressure. J. Prosthet. Dent., 16 : 213-223, 1966.

11. Page H.H. : Mucostatics, A principle not a technique by Harry L. Page, Chicago, 1946.

12. Portar C.G. : Mucostatics – A panaua or propagan. J. Prosthet. Dent., 3 : 464-466.

13. Sharry J.J. :Complete denture prosthodontics.

14. Tyrde G.K. : Dynamic impression method. J. Prosthet. Dent., 15 : 1023-1034, 1965.

15. Udani T.M. : Critical analysis of complete denture impression procedures (unpublished article).

16. Victor O. Lucia : Mucostatics, text book of treatment of edentulous patients. 17-21.