COMPLETE DENTURES

COMPLETE DENTURE IMPRESSIONS

HENRY A. COLLETT, D.D.S. Jacksonville, Flu.

T HE PURPOSE of this article is to discuss some of the theories and philosophies of complete denture impressions and to correlate those theories and philosophies

with some of the recommended techniques. Impression techniques seem to be interwoven with fad, fallacy, phantasy, phi-

losophy, and fact, and our heads whirl in an effort to choose between the alternatives Techniques are often presented with fervent feelings akin to patriotism for one’s country when it is in eopardy. This might be considered a heroic infirmity, which we as dentists cannot afford to possess. Occasionally, when we question a technique, we find the clinician taking the stand of a general besieged by guerrilla troops who fought their way to his secured city, threatening his peaceful and constructive en- deavors. Still, this type of stand, taken for what it is worth, often engenders thought- provoking reaction which prevents us from taking a ride on the pseudoscientific roller coaster such as we have occasionally done in the past.

By analyzing techniques scientifically, and breaking them down into elements, we establish a pattern where some of the elements might be used by others to synthesize new and better techniques. This approach might help to medicate us

with the tranquilizer of curiosity, and stimulate us to ask why, when we are told to do something in a specific manner and expect a scientific answer, rather than a superlative promise of success.

We often find what we thought was fact, was instead fallacy and often we find ourselves going along with a fad, established as the result of a supersalesman’s fantasy. Was this not true of the “mucostatic” theory of some years ago? Still, we learned from the elements of this theory.

From recent studies we might see that much of what we do is based on our philosophy, and that many basic procedures are based on the professional judg- ment of the individual. There is much that has not been entirely resolved to the satisfaction of all of us.

Let us look at some of the ideas that have been presented to us over the years in relation to just one consideration: pressure. There are methods that tell us to place the tissue with pressure; these are sometimes called pressure techniques. Then there are the nonpressure or sometimes called “mucostatic” techniques. If we

Read before the Academy of Denture Prosthetics in Milwaukee, Wis. 603

604 COLLETT J. Pros. Den. July-August, 1965

add negative pressure and selected pressure techniques, we have run the gamut of all but one element-pressure.l

Then we might consider techniques that are allegedly based on the anatomy of the mouth, others that are based on arbitrary landmarks,l and techniques that overextend the borders of the denture and those that underextend them. Multiply all of these elements by the fact that impressions can be made with the mouth open or closed. Some techniques functionally record the tissues of the basal seat with varying degrees of tissue detail. There are many hundreds of possibilities. At one time or another we have been told that each was the only method to use.

BASIC PRINCIPLES

Basic principles are not like fashions that need restyling from year to year. A grand old house, even though well constructed on a sound foundation, if abandoned, will deteriorate until it is useless. The same is true of basic principles, but the question is “which principles are basic?”

All techniques have advantages and disadvantages. When an advantage is introduced, often a disadvantage is introduced at the same time. To some extent every technique is a compromise. Our objective is maximum retention and stability with no damage to the supporting tissues. This is impossible to accomplish because the resorption that occurs under dentures is to some extent accelerated by them.

FORCES OF RETENTION

While the literature is replete with possible explanations, the forces retaining dentures have not been entirely explained. Many authors say that adhesion and co- hesion are the primary retaining forces and that atmospheric pressure is a secondary rescue retaining force.le3 Some say interfacial surface tension is a factor,4 others mention capillary attraction. 3 Although there is some technical difference, I wonder if the last two do not mean the same as adhesion and cohesion when the terms are applied to complete denture impressions. We also find van der Waals forces and optical contact mentioned as primary retaining f0rces.j

CLOSE CONTACT OF INVOLVED SURFACES

One factor on which all authors agree is that all of these forces, except atmospheric pressure, require close contact of the involved surfaces. This factor might be considered as a basic principle.

It is generally believed that a thin fluid film of low surface tension is ideal for retention. However, Campbell” administered relatively large doses of atropine to volunteer patients, and with a dry mouth and dry denture bases demonstrated greater retention without the apparent fluid film than with it. This experiment does not mean that all who tell us that a thin fluid film of low surface tension is necessary for retention are wrong. Campbell demonstrated only that with no apparent fluid film there was greater retention. Perhaps there was a thinner fluid film, with lower surface tension, invisible to the eye, in which case he would be in agreement with the other investigators. Still, can we say that a thin fluid film of

low surface tension which enhances retention is a basic principle?

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PRIMARY RETENTION

Primary retention depends upon close adaptation to the tissues and is pro- portionate to the area covered.i A posterior palatal seal and a border seal add to retention by acting as a rescue force that takes over when a dislodging force is exerted.7-‘1 A thick fluid film that would result from a lack of close aclapta- tion to the tissues detracts from retention.”

If we believe that primary retention depends up011 close adaptation to the tissues and is proportionate to the area covered, and if we believe that retention is one of our primary objectives, dentures should cover as much area of the mouth as the anatomy will permit without damage. The relief should be used only to compen- sate for variations in the thickness of the supporting tissues that might interfere with the stability of the denture and to prevent interference with the nerve and blood supply. In the relief area (at least until the tissue has had an opportunity to prolif- erate and fill the space left by the relief), there will be a thicker fluid film. This thick film will detract from retention and not add to it as the so-called vacuum chamber was supposed to do. l1 If we follow these principles, the snowshoe effect of a large denture base will give better support, the flanges will give better stability, and the borders located in soft tissue will provide rescue retention.

IGONPRESSURE IMPRESSIONS

Those that adhere to the mucostatic (nonpressure) principlell or a modifica- tion of this theory that would seem to make it more scientifically acceptable believe that, since the denture will be at rest most of the time, the ridge and vault areas should be recorded at rest. “At rest” does not mean the same thing to everyone. Many now believe that it is desirable to have a recovery period for the tissues.]’ This is accomplished either by leaving the old dentures out of the mouth for several days, or by using a soft temporary reline material that is subject to flow for a specified period of time. I3 The material is alleged to massage the tissue, allo\\- ing it to recover functionally. Some believe this method to be superior to mere1.y allowing the patient to remain without his dentures for several days. It is quite probable that the same result is not obtained with both of these methods. This being so, it is not difficult to see that “tissue at rest” (even before the impression is made) does not mean the same to each of us. Both of these methods seem to have scientific merit. It seems desirable to do either one or the other rather than leave the tissues in a distorted condition from the current dentures when the final impression is made.

Tissues that are displaced with impression materials tend to displace the denture in their attempt to return to their original form. It is somewhat like press- ing the finger into a rubber ball. The ball exerts an equal and opposite pressure in its attempt to displace the finger. This is not an exact analogy because both temporary and, later, permanent changes will take place in the tissue, preventing it from exerting an equal pressure. The pressure of the denture will cause reactive and resorptive changes, some of which are brought about by interference with the local circulation. Sature will not attempt to relieve a constant pressure, with permanent resorptive changes in the underlying bone, if the pressure has not been introt1uced.l’ On the other hand, it is recommended by the proponents of some

COLLETT J. Pros. Den. July-August, 1965

pressure and selected pressure techniques that tissue be placed or displaced in certain areas to gain specific advantages .2,3 For example, it is recommended by some that the tissues on the buccal shelf of the mandible be recorded with some- what greater pressure than those on the remainder of the mandible because it is the primary denture-bearing area. Research in the partial denture field has shown that there is less vertical movement of the denture base under function if this area is recorded under pressure. I4 The clinical significance of this observation perhaps needs further study.

TISSUES AT REST

A close analysis will show that it is not possible to record tissues at com- plete rest for several reasons. The mere introduction of a tray into the mouth will cause some displacement of tissues. The direction and amount of force we apply while making the impression will displace tissue. The moisture on the tissue and the secretions of the mucous glands will add to the inaccuracies of impressions. The fluid content of the tissue is not constant. In fact, it can be intentionally changed with the topical use of vasoconstrictors such as Neo-synephrine hydrochloride.* The permeability of the ca$laries and the vasomotor changes cause constant varia- tions. Even if we were to record the tissues with complete accuracy, we would not have an entirely accurate record after several days.

MATERIALS

We are also confronted with the limiting factors of available materials. Slight changes take place in all impression materials, plasters, stones, and in the denture base materials.15 Because we are faced with inaccuracies over which we have no control, it is extremely important that we do not add to them.

The issue then arises as to whether or not we should record the details of the surface of the tissue with meticulous accuracy. Each little irregularity of the tissue structure will not be identical with its form several days later. The changes in the dental materials will slightly move the tiny elevations and depressions on the sur- face of the tissue. Still, if we could make use of the additional area resulting from these elevations and depressions of the tissue surface,ll theoretically, we would have greater primary retention. Primary retention is proportional to the area of the surface covered.

SURFACE DETAIL

Some impression materials such as the Justi Mucosealt material,la inten- tionally did not reproduce the tissue surface with intimate detail. Modeling plastic (modeling compound) impressions without a wash record very little surface detail. This does not necessarily make them unsatisfactory materials. Swedged metal denture bases, constructed on zinc and Babbitt metal dies did not reproduce minute surface detail, yet these resulted in very satisfactory bases.

Woelfel17 pointed out the differences in accuracy of tissue surface reproduction

*Winthrop Laboratories, New York, N.Y. tH. D. Justi & Son, Inc., Philadelphia, Pa.

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of several different impression materials. How accurately we should attempt to reproduce this detail must be left to the professional judgment of the dentist until further knowledge is available. Two impressions made by the same dentist, one immediately after the other, would not be identical in surface detail.

PRESSURE DISTRIBUTION

Many clinicians believe that there should be a slight controlled pressure in the border region based on the anatomy of the mouth to take advantage of the additional retention made possible by a border seal. Woelfel, Hickey, and BerglO pointed out that there was a marked difference in the borders of the impressions, even though all the dentists were experienced teachers and all used the same im- pression materials. Apparently no two dentists will exert the same amount of pressure at the borders of an impression. The same dentist probably cannot, with precision, duplicate his own results. Even a scientific approach is influenced by professional judgment.

The pressure on impressions is a compromise. When pressure is exerted on the borders of the denture it will be a displacing force. In the beginning, the dis- placing force will be equal to the amount of force used to displace the tissue. As permanent tissue changes occur, the amount of the displacing force will be reduced.

Fig. l.-(1) The buccal space is filled by the border of the denture to obtain an adequate border seal. (2) An underextended border does not complete the border seal and results in a loss of retention. (3) The zygomatic bone is close to the mucobuccal fold. Overextension in this region will interfere with the seating of the maxillary denture. (4) The labial flange should be in close contact with the mucolabial fold. The extension of the border in this region depends to some extent on the judgment of the dentist. (5’) The labial notch should be narrow because of the limited movement of the labial frenum. (6) The notch for the buccal frenum should be somewhat wider than that for the labial frenum because there is more horizontal movement of this structure which results from the movement of the muscles of facial expression. (7) The indentation of the denture border to provide for the zygomatic bone.

608 COLLETT J. Pros. Den. July-August, 1965

To establish the desired border seal, the border of the denture must be in soft tissue with a slight positive pressure.

MAXILLARY BORDER ANATOMY AND FLANGE DEVELOPMENT

A narrow opening is allowed in the labial flange for the labial frenum (Fig. 1) . The function of the labial frenum is similar to that of a ligament in that it limits the movement of the lip ; therefore, it is not necessary to allow much space on either side of it.

The border of the labial flange extending from the labial frenum to the buccal frenum on each side is developed with a positive pressure on the mucobuccal fold. The thickness of the labial flange is determined by the amount of tissue lost through resorption. The buccal frenum is in close relationship with the caninus muscle. Since this is a muscle of facial expression, some of its movement is controlled by other muscles of facial expression and this results in some horizontal movement of the buccal frenum. Therefore, more anteroposterior freedom should be allowed for the buccal frenum to move than was allowed for the labial frenum. In some mouths, the buccal frenum is smaller and requires no special consideration. The buccal flange of the impression extends posteriorly from the buccal notch provided for the buccal frenum. The mucobuccal fold in this region overlies part of the buc- cinator muscle, and this muscle controls the fold. The thickness of the border of the impression in this region is determined by the amount of ridge tissue lost through resorption, the size of the space between the alveolar ridge and the cheek before the teeth were removed, and by the movements of the ramus of the mandible at the distal end of the buccal vestibule. The patient should move the mandible laterally and protrusively to make sure that the impression does not interfere with these functions.

To determine the correct position of the posterior border of the maxillary den- ture, the hamular notches are located by placing a mouth mirror behind the maxil- lary tuberosity, with a slight pressure. The posterior border of the denture runs through these notches, displacing the soft tissue to help perfect the border seal. The vibrating line is located by having the patient say, “Ah.” The posterior border of the denture is located on this line, displacing the soft tissue slightly. The ex- tent of tissue displacement is determined by a combination of its resilience and the professional judgment of the dentist. Some latitude is permitted in the position of the posterior border of the maxillary denture when the pa1at.e is wide and flat with the soft palate extending horizontally backward from the hard palate. When the palate is high and the soft palate drops vertically downward from the vibrating line, the position of the posterior border is critical.

MANDIBULAR BORDER ANATOMY AND THE FLANGE DEVELOPMENT

The labial frenum, labial arch, and buccal frenums of the mandible are very similar to those of the maxillae. The border of the labial flange of the lower denture should be developed in a manner similar to that of the flange of the upper denture. There is no muscle in the lower labial frenum and the triangularis muscle (a muscle of facial expression) is in close contact with the buccal frenum on each side.

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Fig. Z.-(l) The buccinator muscle. The denture rests on some of the fibers of this muscle but, since they run anteroposteriorly in a horizontal direction, they are not dislodging factors. (2) The mylohyoid ridge. The body of the mandible (3) shown in cross-section extends laterally from the mylohyoid ridge while the mylohyoid muscle (4) extends vertically downward when it is relaxed or medially when it is contracted. Since the denture should be extended below the attachment of this muscle (according to many techniques), there is danger of forcing this muscle laterally under the mylohyoid ridge. The determination of the extension and position of the denture flange in this region is most difficult.

The buccal flange extends distally in the buccal vestibule from the buccal notch for the buccal frenum (Fig. 2). The external oblique line may be palpated on the lateral aspect of the buccal shelf of the mandible and can be used as a landmark for determining the extent of the impression in this region. It is not always a limiting factor. It is often advisable to extend the impression laterally beyond the external oblique line. It is seldom advisable to terminate the border of the mandibular impres- sion short of this anatomic landmark. The buccal shelf is the primary denture- bearing area. The buccal flange of the denture usually covers 5 mm. or more of the buccinator muscle in this region. The fibers of this muscle run horizontally in an anteroposterior direction so the buccinator muscle is not a dislodging factor.

When the masseter muscle is in function, its anterior fibers push against the distal part of the buccinator muscle. It may be necessary to reduce the lateral ex- tent of the buccal flange sharply in this region. If this is not done, the mandibular denture will cause soreness on these tissues when heavy pressure is applied. The retromolar pad should be covered to complete the border seal.

LINGUAL FLANGE

The anterior part of the border of the lingual flange approximates the lingual frenum. Because this is part of the floor of the mouth and is attached to the tongue, it must be given more consideration than the labial frenum. The sublingual crescent is the part of the floor of the mouth covering the sublingual gland. Whether or not this structure should be covered is a controversial questi0n.l” If it is covered, much greater retention is obtained for the denture. The tongue is guided over the lingual surface of the lingual flanges of the denture and becomes a retaining factor. The tissue in this region is quite moveable and is controlled by the movements of the tongue. If the impression is overextended here, it will limit the movements of the tongue. Also, it might cause soreness and exert a displacing influence on the den- ture. This border should be molded to allow for nornlal movements of the tongue,

610 COLLETT J. Pros. Den. July-August, 1965

but not for excessive tongue movements. For maximum retention, the sublingual crescent should be covered almost to the submaxillary caruncles (the openings of the submaxillary ducts). Several disadvantages have been mentioned for this procedure. The submaxillary caruncles may be inadvertently covered. This might block the secretions from the sublingual glands. I have attempted, without success, to do this experimentally when the submaxillary ducts were normal. Blockage of the submaxillary caruncles resulted in swelling in the submaxillary region in one patient who had a stone in the submaxillary duct. In a number of patients, the sub- maxillary caruncles were accidentally covered without ill effect. Where the ducts are normal, it is doubtful that the salivary secretions could be blocked because of the range of movement of the structures in this area. Another disadvantage is that calculus is more likely to be deposited under the denture if the caruncles are covered. This may be true but, if a patient is prone to deposit calculus under a den- ture, it is likely that some will be deposited regardless of the technique used. It seems desirable for these patients to have their dentures serviced more often.

The distolingual aspect of the lower impression has had various interpretations because of the anatomy of the mandible in this region. Some authors recommend cutting the lingual flange of the denture short, with the use of the mylohyoid ridge as a guide.l* If this is done, the borders of the denture will not be in soft tissue and the border seal will not be complete. This will eliminate the rescue retention for the denture. This region has been given different names by various authors. Neil* called it the “lateral throat form.” Edwards and Boucher” called it the retromylo- hyoid eminence because it extends beyond the distal end of the mylohyoid ridge. From an anatomic standpoint, the latter term seems to be more descriptive. If the impression is extended so that the border contacts soft tissue in this region, several advantages will be gained. Retention by atmospheric pressure will be enhanced, and the tongue will be guided on top of the lingual flange of the dentur’e and help retain it mechanically. This section of the border on one side is molded by having the patient extend the tongue to the opposite corner of the mouth when the im- pression material is in a moldable state. This is the normal extent of tongue move- ment.

OPEN OR CLOSED MOUTH TECHNIQUE

Assuming that the maximum area permitted by the anatomy of the mouth is to be covered, the choice of an open or closed mouth technique should be determined by an evaluation of the advantages and disadvantages of each method. With the closed mouth method, the tissues of the mucobuccal fold are more relaxed and the extension of the borders is likely to be greater. The lingual borders of the man- dibular impression present a greater problem. The tongue is confined and cannot be moved functionally to form the lingual border. The tongue is in a more retracted position so that the impression material will have a greater tendency to flow under the mylohyoid ridge where the mylohyoid muscle is not supported by bone. The finished denture is likely to cause irritation in this undercut region. On the other hand, certain materials, such as Iowa wax, + lend themselves better to a closed

*Neil, E.: Personal communication, 1944. +Kerr Manufacturing Company, Detroit, Mich.

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mouth technique. Closed mouth impressions made of materials, such as hard-setting zinc oxide and eugenol impression paste, can be used as baseplates for recording maxillomandibular relationships.

PRESSURE AS A DETERMINING FACTOR

The amount of pressure used, the area where pressure is exerted, and how well it is controlled depends on the dentist’s skill in the use of the available materials. Waxes and modeling compound, when confined in a tray, will produce pressure impressions. Zinc oxide and eugenol paste with a heavy consistency will do the same. Excellent retention can be obtained with a complete mandibular impression made in modeling compound alone in a specially prepared acrylic resin tray. If this technique is used for a complete maxillary impression, however, the modeling compound is likely to exert its greatest pressure in the center of the palate. This is because the material is more confined in this part of the tray, and it has less chance to escape over the borders of the tray. The result is undesirable since the ridge, and not the center of the palate, is the primary denture-bearing area. This same dis- advantage seems to be present when other materials of heavy consistency, such as Iowa wax, are used to make a maxillary impression.

A “nonpressure impression” implies that the impression does not displace even the soft tissue at its borders .ll This, of course, would eliminate any possibility of a border seal and result in the loss of secondary retention. This method was not found to be very satisfactory by the average dentist, but a modification, with the use of a border seal, can be accomplished by using a high fusing black modeling compound tray. The borders are molded with a low fusing stick modeling com- pound. The entire basal seat area is relieved by removing one or more millimeters of compound and the impression is made in a very soft material, such as a thin mix of alginate (irreversible hydrocolloid) or plaster. Large holes are made in the palate of the maxillary tray to allow the excess impression material to escape, thus preventing hydraulic pressure. lg This method records the contours of the denture- bearing (basal seat) area at rest. This seems to be desirable because the tissues in this area will be at rest most of the time. It is of the utmost importance to preserve the tissue that remains. Unfortunately, the method that will do this best is not known ; on the one hand, even if the best method were known, there is a question as to whether or not it would be better to preserve everything that remains at all costs. The patient’s life expectancy might be taken into consideration. Might we not risk the loss of a little supporting tissue to give the patient a better-functioning denture? The risk from the introduction of pressure by an impression may be slight when It comes to the loss of supporting tissue.

The followers of the “mucostatic” school of thought used Pascal’s law to argue their method. They pointed out that fluids are not compressible and that ;a confined fluid exerts an equal pressure on all parts of its container.ll This was a rather poor application of the law. Tissue fluids are not completely confined be- tween the bone and denture base. They are contained in the tissues and in vessels from which they can be partially removed by various conditions.

Some “selective pressure” methods are designed to exert greater pressure on the primary denture-bearing areas than in other areas. This is done effectively with

612 COLLETT 3. Pros. Den. July-August, 1965

mandibular impressions by a method recommended by Boucher.3 He prepared a vulcanite tray from an accurate preliminary impression. To this very accurate tray he added modeling compound on the primary stress-bearing areas. This procedure caused a greater pressure in these areas. Later, he accomplished similar results by relieving the parts of the tray that cover the unfavorable parts of the basal seat (the secondary stress-bearing areas) and making the final impression in a zinc oxide and eugenol impression paste.*O

If the tray is constructed for making the maxillary impression (with the relief of all the basal seat except the crest of the ridge) and a wash impression is made with a soft material such as impression plaster, the impression being seated with considerable pressure, a result similar to the selective pressure mandibular im- pression will be obtained. Pressure would be exerted on the primary stress-bearing area, causing some displacement of the soft tissues. These techniques are likely to produce negative pressure in the areas that are relieved. This negative pressure, caused by the resilience of the displaced tissues, will be in areas where the tissues have not been displaced. The result may be a proliferation of the tissues in these negative pressure areas. This would cause an eventual equalization of pressure under the denture. The equalization might also be brought about by the loss of tissue on the pressure-bearing areas.

In patients who have had a breakdown of the denture-bearing area and in whom surgical correction is not desirable, special techniques must be used to record the shape of the soft pendulous (hyperplastic) parts of the ridge. In these techniques, the part of the impression covering the pendulous ridge is removed. Then the im- pression is placed in the mouth, and the exposed soft tissue is painted with soft impression plaster. This will record the shape of the hyperplastic tissue with the least amount of displacement.

REPRODUCTION OF SURFACE DETAIL

The accurate reproduction of surface detail seems to result more from the selection of the impression material than from the dentist’s skill. Certain materials, such as Thiokol and silicone rubber base and alginate (irreversible hydrocolloid) impression materials, reproduce tissue surfaces with fine detail. Some zinc oxide and eugenol impression pastes record surface detail better than others. Materials such as impression waxes and modeling compounds give a rather poor reproduction of surface detail when they are used for final impressions. Theoretically, a meticulous reproduction of surface detail, with no pressure to displace or distort the surface tissue, should give better retentionll

RELIEF AREAS

If we were to follow the theories to the letter, we would obtain greater reten- tion if we used no relief areas. However, there are factors other than retention to consider. The denture-bearing areas (basal seats) are covered with mucosa that is not of equal thickness. For instance, the median suture between the maxillae or a torus palatinus is covered with relatively thin mucosa. If this area is not relieved, it can act as a fulcrum and result in an unstable base. The instability might in-

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directly interfere with retention. These areas require relief in most instances, but the space under the denture will result in a greater thickness of the fluid film and detract from retention. For this reason, the minimum thickness of relief should he used. This can be done by functionally relieving the complete denture with the aid of a disclosing wax. The displacement of the disclosing wax resulting from func- tional movements of the denture will be a guide to the necessary depth of the re- lief.* This method cannot he used in metal denture bases. Parts of denture bases that might interfere with the nerve and blood supply should be relieved, for ex- ample at the incisive papilla.

SUMMARY AND CONCLUSIONS

The theories attempting to explain the retention of dentures seem to agree that close adaptation of denture bases to the tissues is desirable, and that retention is proportionate to the area covered. Experiments show that a border seal and a posterior palatal seal add much to the retention of dentures.

Displaced tissues have a tendency to return to a position and form of equilib- rium. Because of this, it seems to be desirable to have the patient either remove his old dentures for a day or two or to have the tissues conditioned in some other manner before making an impression.

Tissues that we displace while making impressions have a tendency to return to their original form and position because of their resilience. Therefore, they have a tendency to displace the denture. A positive pressure should be exerted on the border tissues by the denture. The amount of this pressure must be based on our professional judgment. This will result in a force that will have a tendency to dis- place the denture, but it will result also in added retention from the border seal. A compromise must be made to gain this added retention.

The selection of an impression material or combination of materials should be determined by the objective of the dentist and the skill with which he can manip- ulate the materials he chooses. The same material and technique cannot be used for all patients with equal success. The dimensions of some materials, such as irrever- sible hydrocolloid, change rapidly. Therefore, impressions made of these materials must have casts made in them immediately. If the impression is to be sent to a laboratory where some time may pass before the cast is poured, these materials should not be used.

As we analyze our work and the work of others, we break it down into basic elements. We select the elements we desire, and fit them into our fads, fallacies, fantasies, philosophies, and what we like to call facts, and put them together like a jigsaw puzzle into a method for making impressions which coincides with what we each call “basic principles,” but, more accurately, with what suits or harmonizes with our own personalities.

The problem would be much simplified, if we could find “principles” that would transcend individuals, but our objectivity is limited by our personalities. Perhaps this is good because true objectivity would make us too cautious. The criticism of our peers is the check and balance.

*Hardy, I. R.: Personal communication.

614 COLLETT J. Pros. Den. July-August, 1965

When we examine impression techniques, we might find that each is at least partly right. With this in mind, we should not defend a technique with a feeling akin to patriotism for one’s country when it is in jeopardy. We should analyze the available techniques, try them, and consider some of the ideas back of them to synthesize new and better techniques.

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Boucher, C. 0.: Impressions for Complete Dentures, J.A.D.A. 30:14, 1943. Stanitz, J. D.: An Analysis of the Part Played by the Fluid Film m Denture Retention,

J.A.D.A. 37:168, 1948. Moses, C. H.: Physical Considerations in Impression Making, J. PROS. DEN. 3:449, 1953. Campbell, R. L.: Some Clinical Observations Regarding the Role of the Fluid Film in the

Retention of Dentures, J.A.D.A. rE8:58, 1954. Skinner, E. W., Campbell, R. L., and Chung, P.: A Clinical Study of the Forces Required

to Dislodge Maxillary Denture Bases of Various Designs, J.A.D.A. 47:671, 1953. Tilton, G. E. : The Denture Periphery, J. PROS. DEN. 2:290, 1952. Edwards, L. F., and Boucher, C. 0.: Anatomy of the Mouth in Relation to Complete Den-

tures, J.A.D.A. 29:331, 1942. Woelfel, J. B., Hickey, J. C., and Berg, T., Jr.: Contour Variations in One Patient’s Im-

pressions Made by Seven Dentists, 67:1, 1963. LeVan, M. M.: Basic Principles of Impression Making, J. PROS. DEN. 2:26, 1952. Lytle, R. B.: The management of Abused Oral Tissues in Complete Denture Construction,

J. PROS. DEN. 7:27, 1957. Chase, W. W.: Tissue Conditioning Utilizing Dynamic Adaptive Stress, J. PROS. DEN.

11:804, 1961. Holmes, J. B.: Influence of Impression Procedures and Occlusal Loading on Partial Den-

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