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Use of elastic impression materials in operative dentistry
Use of elastic impression materials in operative dentistry
George M. Hollenback,
B efore the developm ent o f elastic impres sion materials, restorations made by the indirect technic were not com pletely satis factory. T h e inaccuracies o f the restora tions resulted from the inferior physical properties o f the impression materials involved. T oday, gypsum reproductions are dimensionally almost com pletely ac curate. D uring this century, impression materials have advanced from modeling com pound to wax to amalgam alloy m odel material to hydrocolloid to poly sulfide and silicone impression materials. From his personal experiences, the author recounts a brief history of most o f the recent developments.
In the early years of this century, vari ous technics and impression materials were used in the hope and perhaps belief that accurate duplications of prepared teeth could be made that would permit the fabrication of single-tooth restora tions, such as, porcelain jacket crowns, porcelain inlays and the various types of cast restorations, such as, full coverage, partial coverage and intracoronal res torations. Modeling compound was used as an impression material in the construction of these various types of restorations. In
D D S , M S D , D S c, E ncino, Calif
restorations for full coverage, either of porcelain or gold, results were fairly satis factory because the contours and irregu larities of a given tooth had been oblit erated. A hard, brittle and comparatively unyielding material such as modeling compound could be used for taking im pressions of a tooth so prepared and achieve a considerable degree of ac curacy. With intracoronal and partial veneer coverage, however, the contours of a tooth cannot be obliterated. When a material such as modeling compound is used in taking an impression under these conditions, it becomes practically impos sible, from a mechanical standpoint, to take an impression with complete accu racy. In making a model from such an impression, any inaccuracy that exists in the impression itself will exist to the same degree in the model. Although modeling compound was extensively used for many years for this purpose, it is evident that, except for the foregoing conditions, a satisfactory degree of accuracy was seldom if ever obtained. WAX USED FOR IMPRESSIONS Weston A. Price ,1 about 1912 to 1915, introduced a technic that used a wax for taking such impressions. He con tended that, since the wax was much more yielding than modeling compound,
Hollenback: ELASTICIMPRESSION MATERIALS•641 the resulting cast therefore would be more accurate. It is evident, however, that his concept was wrong. It is true that an impression made with wax of an ir regular shape can be removed more easily than an impression made with modeling compound. If there is great irregularity when modeling compound is used, the impression may not yield enough so that it can be removed without fracture. When there is a lesser degree of irregu larity, a modeling compound impression can be removed without fracture but, since this material is hard and quite unyielding with no elastic memory, it must necessarily distort in being re moved from such an object. Again, the distortion produced in the modeling com pound will be directly reflected in the resultant model or die. W hen wax is used according to the Price technic, fracture never results, but waxes, on the whole, have extremely lim ited elastic properties. Impressions made with a wax can be easily removed be cause the wax yields and becomes per manently distorted in the process of re moval. Again, the result is an inaccurate and distorted model. In the next decade, an attem pt was made to substitute a material that had an elastic memory for modeling com pounds and waxes. Several of these sub stitutes were placed on the market. (Lit tle is known of the composition of these materials; however, from their physical appearance and working qualities it can be assumed that these materials, as far as their constituents are concerned, were somewhat similar. The shelf life of these materials was short. Soon after com pounding they all became completely unusable.) Impressions made with them usually showed beautiful reproductions as far as minute detail was concerned, but the overall dimension of such repro ductions was inaccurate because these materials did not have sufficient elastic memory to permit them to be distorted momentarily and then to return to their normal dimension.
During this period, almost every den tist was looking forward to the develop ment of materials that would permit the accurate reproduction of prepared teeth for all types of restorations. This problem is twofold. Not only must the material used have sufficient elastic memory to undergo momentary distortion with complete return to di mension, but also the material from which the duplication is made must have a high degree of accuracy. In the days when modeling compound was used extensively, it was the almost universal practice to make all models of prepared teeth from silver amalgam. Lit tle attention was paid to the fact that many silver amalgams are highly inaccu rate because of the volume change that occurs during the setting reaction. AMALGAM ALLOY MODEL MATERIAL During this period, a manufacturer of dental amalgam alloys placed on the market an alloy that was compounded especially for use as a model material in the indirect method, with modeling com pound as the impression media. At that time, I was extensively engaged in the study of the physical properties of the various types of dental amalgams. The main consideration in studying the prop erties of an amalgam then was the meas urement of the amalgam’s volume change during the setting reaction. In thoroughly testing the aforementioned model amal gam alloy, I found that this material had an average shrinkage of from 50 to 75 ¡x per centimeter of unit length during set ting. To illustrate the general inaccuracy of indirect technics at that time, I wish to point out that this particular amalgam alloy was purchased in large quantities by many operators and used exclusively in the construction of indirect restora tions of various types. So here, wrapped up in a single technic, we have the tak ing of an impression with a material that, by no stretch of the imagination, can be
642-J. AMER. DENT. ASSN.: Vol. 71, Sept. 1965 considered as accurate, and then the forming of a die from an amalgam with a high-shrinkage coefficient during the setting process. DEVELOPMENT OF HYDROCOLLOID In the late 1920’s, an entirely new type of elastic impression material—the re versible agar base material known as hydrocolloid—was developed in Switzer land. The L. D. Caulk Company of Mil ford, Del, obtained a license to manufac ture this type of material in the United States. Hydrocolloid immediately became popular in the construction of orthodon tic and study casts. A few dentists soon began to experiment with this material by taking impressions of prepared teeth and running them with a hard gypsum. It seems probable that J. D. H art, of Wewoka, Okla, was the first to use and demonstrate this method of indirect pro cedure. I have in my possession docu mentary evidence that Doctor H art pre sented his method to the profession in August, 1931, at Wewoka, Okla. Many others soon began using reversible hydro colloid for all indirect procedures, ap parently obtaining results considered to be satisfactory. In time, methods were developed for measuring the accuracy and elastic limit of these impression materials. As soon as means were developed whereby their ac curacy could be measured, most m anu facturers of these materials adopted such methods. It then became possible for every manufacturer, if he so desired, to test the accuracy of every batch of m ate rial that was produced. T he result has been that for a number of years, the great majority of elastic im pression materials of all types have be come fairly dimensionally accurate. In 1957, my associate was a technician fairly familiar with dental technics and more especially with the behavior of elas tic impression materials of all types, that is, reversible and irreversible hydrocol loids, polysulfide and silicone materials.
For several months, we carefully inves tigated all of the then available elastic impression materials. O ur results were anything but reassuring. Most of the ma terials of all types proved to be highly inaccurate. Soon after this experience, I headed a small, independent research group. It is and has been the policy of this group to test dental materials with -adequate equipment and with better-than-average technical skill. We have maintained a friendly relationship with most of the manufacturers of these materials and, on the whole, these manufacturers generally have put forth their best efforts to im prove the quality of their materials in every way, particularly in regard to ac curacy. From time to time since 1957, we have evaluated all of the elastic impression materials available. O n the whole, the degree of excellence progressed rapidly and, by 1959 to 1960, the great majority of elastic impression materials were fairly accurate and possessed satisfactory work ing qualities. RUBBER BASE MATERIALS The rubber base materials—polysulfide and silicone—have proved more accurate than the hydrocolloids. Whereas the shelf life of the polysulfide materials has been satisfactory, the shelf life of the silicone materials first placed on the m arket was not satisfactory. Almost all of them showed continuing deterioration with time. It is becoming evident that this difficulty has now been greatly removed. A thorough investigation was made of these materials in our laboratory about 17 months ago .2'5 We again found the silicones and polysulfide materials to be quite accurate and also to have satis factory working qualities; however, with both types of these materials, we experi enced one difficulty. W hen impressions were taken at room temperature and maintained at that temperature until they were poured and the gypsum had
Hollenback: ELASTICIMPRESSIONMATERIALS• 643
Fig. I • a: One-and-a-half dimension ivorine Fig. 3 • Same as Figure 2 except inlay has been tooth with large MOD preparation, in which inlay placed in reproduction c. Careful examination of is seated, b, c: Gypsum duplicates fit of inlay in original and two reproductions shows that it fits equally well in all of them. Accuracy of this inlay, which was made from wax pattern swaged directly on ivorine tooth, was ascertained mercury-entrapment method; 24.1 mg. of mer set, both materials showed a tendency to by cury was entrapped between inlay and ivorine die. expand. This expansion resulted in un Duplication b\ when evaluated by mercury, showed of 30.8 mg. Duplication c showed en dersize casts that would not be satisfac entrapment trapment of 31.1 mg. Indicated difference between tory in clinical dentistry. When the test original tooth and gypsum reproductions is exceed ingly small. Globule of mercury in each instance ing equipment was kept at 95 °F. and was approximately 0.47 mm. (0.19 inch) in diameter impressions were taken and run at room temperature, however, the resulting casts were accurate. These temperature con ditions (95°F.) are similar to those ex isting in the clinical use of these m ate can be swaged on the gypsum die under rials. heavy pressure assuring a well-adapted T he general excellence of rubber base pattern. The pattern is then invested, the elastic impression materials has continu resultant mold processed and a casting ally improved. It is now possible to use made therein. In placing the casting on silicone impression materials to make al the original and on the die, it is usually most completely accurate gypsum repro impossible to tell on which one the wax ductions of a 1.5-dimension tooth with a pattern was made (Fig. 1-3). large M OD cavity. As a model material It seems at last the dream dentists had a densite gypsum was used, its setting so long hoped for is now a reality: Indi expansion being approximately 0.1 per rect restorations of all types can be made cent. with almost complete accuracy. Several gypsums that meet this re Involved in this procedure is nothing quirement are available. A wax pattern that requires much greater than average skill and care. Everything that occurs in this entire procedure has been accurately measured as follows: The impression m a terial has been measured with several de vices to ascertain its accuracy. The physi cal properties of gypsums have been given due consideration ,6'9 particularly in regard to their dimensional change during the chemical reaction that occurs in the setting process. The better quality gypsums have a normal setting expansion of about 0.1 per cent, in some instances Fig. 2 • Original tooth and two gypsum reproduc even less than that. This means that a tions. Inlay has been placed in b (reproduction) test block of gypsum expands only about
644-J. AMER. DENT. ASSN.: Vol. 71, Sept. 1965
Fig. 4 • Equipment used in making wax patterns and investing. It should be located in a draft-free environment insofar as is possible. Wax patterns are made and invested only when two thermometers register same temperature. Equipment includes the following: a, investment; b, vacuum mixer; c, special swaging device for cold swaging wax patterns; d, water container; e, pipet; f, dial ther mometer indicating water temperature; g, dial thermometer indicating air temperature; h, small bunsen burner 0.001 inch per unit inch. This is an ex ceedingly small inaccuracy and, clinically perhaps, of little consequence. A method has been developed to en able a dentist to manipulate wax and to invest patterns in an environment that permits an accuracy of procedure almost equal to that obtained in a constanttemperature room 10’11 (Fig. 4). SUMMARY AND CONCLUSIONS The attempts made to produce satisfac tory indirect restorations before the ad vent of elastic impression materials did not achieve particularly impressive re sults because of the inferior physical properties of the impression materials in volved. With the advent of elastic im pression materials, the indirect technic immediately achieved a much higher de
gree of excellence. From that day to this, a period of little more than 30 years, the indirect method, because of the general excellence of elastic impression materials and their continued improvement and a thorough understanding of the physical properties of dental gypsums, has resulted in a technic in which gypsum reproduc tions dimensionally are almost completely accurate. It can, therefore, be assumed that these reproductions meet all clinical requirements extremely well. The cast restoration itself has shown continuing improvement. At this time, it is possible to make cast restorations of all types that leave little, if anything, to be desired from the standpoint of accuracy. Thus, it will be appreciated that a technic that, a comparatively short time ago, existed only as a dream on the part of the den tal profession has now become a reality.
Hollenback: ELASTICIMPRESSIONMATERIALS• 645 Materials and methods are now avail able, which if adequately applied, com pletely meet the requirements of the in direct technics .12 17000 Ventura Boulevard
Presented at the one hundred and fifth annual session of the Am erican Dental Association, November II , 1964, San Francisco. 1. Price, Weston A . The laws determining the behavior of gold in fusing and casting. D. Cosmos 53:265, 1911. 2. H o lle n b a c k .G . M . Study o f the physical properties of elastic impression materials. J. South. C alifornia D. A. 31:204 June 1963. 3. Hollenback, G. M. Study of the physical properties of elastic impression materials. J. South. C alifornia D. A. 31:227 July 1963.
4. H ollenback, G. M . Study o f the physical properties of elastic impression materials. J. South. C alifornia D. A. 31:369 Nov. 1963. 5. H ollenback, G . M . Study of the physical properties of elastic impression materials. J. South. C alifornia D. A. 31:403 Dec. 1963. 6. H ollenback, G. M . Physical properties of gypsum plasters. J. South. C alifornia D. A . 30:400 Dec. 1962. 7. H ollenback, G . M . Physical properties of gypsum plasters. (Instruments and technics.) J. South. C alifornia D. A. 31:14 Jan. 1963. 8. H ollenback, G. M . Physical properties of gypsum plasters. J. South. C alifornia D. A . 31:47 Feb. 1963. 9. H ollenback, G. M . Physical properties of gypsum plasters. J. Soutn. C alifornia D. A. 31:130 A p ril 1963. 10. H ollenback, G. M . C ontrol o f erratic behaviors of casting investments. J. South. C alifornia D. A. 30:159 May 1962. 11. Hollenback, G. M. Further lig h t on dimensional casting. J. South. C alifornia D. A. 30:341 Oct. 1962. 12. H ollenback, G. M., and others. Science and tech nic o f the cast restoration. St. Louis, C. V. Mosby Co., 1964.
Are you going to the annual session in Las Vegas this November? If you are, you will find winter snows and pine forests only 35 miles away. The Nevada National Forest is famous for its scenic beauty and easy accessibility. There you can enjoy all manner of cold weather sports in the shadow of Mt. Charleston which towers 9,000 feet above the desert.
George M. Hollenback,
B efore the developm ent o f elastic impres sion materials, restorations made by the indirect technic were not com pletely satis factory. T h e inaccuracies o f the restora tions resulted from the inferior physical properties o f the impression materials involved. T oday, gypsum reproductions are dimensionally almost com pletely ac curate. D uring this century, impression materials have advanced from modeling com pound to wax to amalgam alloy m odel material to hydrocolloid to poly sulfide and silicone impression materials. From his personal experiences, the author recounts a brief history of most o f the recent developments.
In the early years of this century, vari ous technics and impression materials were used in the hope and perhaps belief that accurate duplications of prepared teeth could be made that would permit the fabrication of single-tooth restora tions, such as, porcelain jacket crowns, porcelain inlays and the various types of cast restorations, such as, full coverage, partial coverage and intracoronal res torations. Modeling compound was used as an impression material in the construction of these various types of restorations. In
D D S , M S D , D S c, E ncino, Calif
restorations for full coverage, either of porcelain or gold, results were fairly satis factory because the contours and irregu larities of a given tooth had been oblit erated. A hard, brittle and comparatively unyielding material such as modeling compound could be used for taking im pressions of a tooth so prepared and achieve a considerable degree of ac curacy. With intracoronal and partial veneer coverage, however, the contours of a tooth cannot be obliterated. When a material such as modeling compound is used in taking an impression under these conditions, it becomes practically impos sible, from a mechanical standpoint, to take an impression with complete accu racy. In making a model from such an impression, any inaccuracy that exists in the impression itself will exist to the same degree in the model. Although modeling compound was extensively used for many years for this purpose, it is evident that, except for the foregoing conditions, a satisfactory degree of accuracy was seldom if ever obtained. WAX USED FOR IMPRESSIONS Weston A. Price ,1 about 1912 to 1915, introduced a technic that used a wax for taking such impressions. He con tended that, since the wax was much more yielding than modeling compound,
Hollenback: ELASTICIMPRESSION MATERIALS•641 the resulting cast therefore would be more accurate. It is evident, however, that his concept was wrong. It is true that an impression made with wax of an ir regular shape can be removed more easily than an impression made with modeling compound. If there is great irregularity when modeling compound is used, the impression may not yield enough so that it can be removed without fracture. When there is a lesser degree of irregu larity, a modeling compound impression can be removed without fracture but, since this material is hard and quite unyielding with no elastic memory, it must necessarily distort in being re moved from such an object. Again, the distortion produced in the modeling com pound will be directly reflected in the resultant model or die. W hen wax is used according to the Price technic, fracture never results, but waxes, on the whole, have extremely lim ited elastic properties. Impressions made with a wax can be easily removed be cause the wax yields and becomes per manently distorted in the process of re moval. Again, the result is an inaccurate and distorted model. In the next decade, an attem pt was made to substitute a material that had an elastic memory for modeling com pounds and waxes. Several of these sub stitutes were placed on the market. (Lit tle is known of the composition of these materials; however, from their physical appearance and working qualities it can be assumed that these materials, as far as their constituents are concerned, were somewhat similar. The shelf life of these materials was short. Soon after com pounding they all became completely unusable.) Impressions made with them usually showed beautiful reproductions as far as minute detail was concerned, but the overall dimension of such repro ductions was inaccurate because these materials did not have sufficient elastic memory to permit them to be distorted momentarily and then to return to their normal dimension.
During this period, almost every den tist was looking forward to the develop ment of materials that would permit the accurate reproduction of prepared teeth for all types of restorations. This problem is twofold. Not only must the material used have sufficient elastic memory to undergo momentary distortion with complete return to di mension, but also the material from which the duplication is made must have a high degree of accuracy. In the days when modeling compound was used extensively, it was the almost universal practice to make all models of prepared teeth from silver amalgam. Lit tle attention was paid to the fact that many silver amalgams are highly inaccu rate because of the volume change that occurs during the setting reaction. AMALGAM ALLOY MODEL MATERIAL During this period, a manufacturer of dental amalgam alloys placed on the market an alloy that was compounded especially for use as a model material in the indirect method, with modeling com pound as the impression media. At that time, I was extensively engaged in the study of the physical properties of the various types of dental amalgams. The main consideration in studying the prop erties of an amalgam then was the meas urement of the amalgam’s volume change during the setting reaction. In thoroughly testing the aforementioned model amal gam alloy, I found that this material had an average shrinkage of from 50 to 75 ¡x per centimeter of unit length during set ting. To illustrate the general inaccuracy of indirect technics at that time, I wish to point out that this particular amalgam alloy was purchased in large quantities by many operators and used exclusively in the construction of indirect restora tions of various types. So here, wrapped up in a single technic, we have the tak ing of an impression with a material that, by no stretch of the imagination, can be
642-J. AMER. DENT. ASSN.: Vol. 71, Sept. 1965 considered as accurate, and then the forming of a die from an amalgam with a high-shrinkage coefficient during the setting process. DEVELOPMENT OF HYDROCOLLOID In the late 1920’s, an entirely new type of elastic impression material—the re versible agar base material known as hydrocolloid—was developed in Switzer land. The L. D. Caulk Company of Mil ford, Del, obtained a license to manufac ture this type of material in the United States. Hydrocolloid immediately became popular in the construction of orthodon tic and study casts. A few dentists soon began to experiment with this material by taking impressions of prepared teeth and running them with a hard gypsum. It seems probable that J. D. H art, of Wewoka, Okla, was the first to use and demonstrate this method of indirect pro cedure. I have in my possession docu mentary evidence that Doctor H art pre sented his method to the profession in August, 1931, at Wewoka, Okla. Many others soon began using reversible hydro colloid for all indirect procedures, ap parently obtaining results considered to be satisfactory. In time, methods were developed for measuring the accuracy and elastic limit of these impression materials. As soon as means were developed whereby their ac curacy could be measured, most m anu facturers of these materials adopted such methods. It then became possible for every manufacturer, if he so desired, to test the accuracy of every batch of m ate rial that was produced. T he result has been that for a number of years, the great majority of elastic im pression materials of all types have be come fairly dimensionally accurate. In 1957, my associate was a technician fairly familiar with dental technics and more especially with the behavior of elas tic impression materials of all types, that is, reversible and irreversible hydrocol loids, polysulfide and silicone materials.
For several months, we carefully inves tigated all of the then available elastic impression materials. O ur results were anything but reassuring. Most of the ma terials of all types proved to be highly inaccurate. Soon after this experience, I headed a small, independent research group. It is and has been the policy of this group to test dental materials with -adequate equipment and with better-than-average technical skill. We have maintained a friendly relationship with most of the manufacturers of these materials and, on the whole, these manufacturers generally have put forth their best efforts to im prove the quality of their materials in every way, particularly in regard to ac curacy. From time to time since 1957, we have evaluated all of the elastic impression materials available. O n the whole, the degree of excellence progressed rapidly and, by 1959 to 1960, the great majority of elastic impression materials were fairly accurate and possessed satisfactory work ing qualities. RUBBER BASE MATERIALS The rubber base materials—polysulfide and silicone—have proved more accurate than the hydrocolloids. Whereas the shelf life of the polysulfide materials has been satisfactory, the shelf life of the silicone materials first placed on the m arket was not satisfactory. Almost all of them showed continuing deterioration with time. It is becoming evident that this difficulty has now been greatly removed. A thorough investigation was made of these materials in our laboratory about 17 months ago .2'5 We again found the silicones and polysulfide materials to be quite accurate and also to have satis factory working qualities; however, with both types of these materials, we experi enced one difficulty. W hen impressions were taken at room temperature and maintained at that temperature until they were poured and the gypsum had
Hollenback: ELASTICIMPRESSIONMATERIALS• 643
Fig. I • a: One-and-a-half dimension ivorine Fig. 3 • Same as Figure 2 except inlay has been tooth with large MOD preparation, in which inlay placed in reproduction c. Careful examination of is seated, b, c: Gypsum duplicates fit of inlay in original and two reproductions shows that it fits equally well in all of them. Accuracy of this inlay, which was made from wax pattern swaged directly on ivorine tooth, was ascertained mercury-entrapment method; 24.1 mg. of mer set, both materials showed a tendency to by cury was entrapped between inlay and ivorine die. expand. This expansion resulted in un Duplication b\ when evaluated by mercury, showed of 30.8 mg. Duplication c showed en dersize casts that would not be satisfac entrapment trapment of 31.1 mg. Indicated difference between tory in clinical dentistry. When the test original tooth and gypsum reproductions is exceed ingly small. Globule of mercury in each instance ing equipment was kept at 95 °F. and was approximately 0.47 mm. (0.19 inch) in diameter impressions were taken and run at room temperature, however, the resulting casts were accurate. These temperature con ditions (95°F.) are similar to those ex isting in the clinical use of these m ate can be swaged on the gypsum die under rials. heavy pressure assuring a well-adapted T he general excellence of rubber base pattern. The pattern is then invested, the elastic impression materials has continu resultant mold processed and a casting ally improved. It is now possible to use made therein. In placing the casting on silicone impression materials to make al the original and on the die, it is usually most completely accurate gypsum repro impossible to tell on which one the wax ductions of a 1.5-dimension tooth with a pattern was made (Fig. 1-3). large M OD cavity. As a model material It seems at last the dream dentists had a densite gypsum was used, its setting so long hoped for is now a reality: Indi expansion being approximately 0.1 per rect restorations of all types can be made cent. with almost complete accuracy. Several gypsums that meet this re Involved in this procedure is nothing quirement are available. A wax pattern that requires much greater than average skill and care. Everything that occurs in this entire procedure has been accurately measured as follows: The impression m a terial has been measured with several de vices to ascertain its accuracy. The physi cal properties of gypsums have been given due consideration ,6'9 particularly in regard to their dimensional change during the chemical reaction that occurs in the setting process. The better quality gypsums have a normal setting expansion of about 0.1 per cent, in some instances Fig. 2 • Original tooth and two gypsum reproduc even less than that. This means that a tions. Inlay has been placed in b (reproduction) test block of gypsum expands only about
644-J. AMER. DENT. ASSN.: Vol. 71, Sept. 1965
Fig. 4 • Equipment used in making wax patterns and investing. It should be located in a draft-free environment insofar as is possible. Wax patterns are made and invested only when two thermometers register same temperature. Equipment includes the following: a, investment; b, vacuum mixer; c, special swaging device for cold swaging wax patterns; d, water container; e, pipet; f, dial ther mometer indicating water temperature; g, dial thermometer indicating air temperature; h, small bunsen burner 0.001 inch per unit inch. This is an ex ceedingly small inaccuracy and, clinically perhaps, of little consequence. A method has been developed to en able a dentist to manipulate wax and to invest patterns in an environment that permits an accuracy of procedure almost equal to that obtained in a constanttemperature room 10’11 (Fig. 4). SUMMARY AND CONCLUSIONS The attempts made to produce satisfac tory indirect restorations before the ad vent of elastic impression materials did not achieve particularly impressive re sults because of the inferior physical properties of the impression materials in volved. With the advent of elastic im pression materials, the indirect technic immediately achieved a much higher de
gree of excellence. From that day to this, a period of little more than 30 years, the indirect method, because of the general excellence of elastic impression materials and their continued improvement and a thorough understanding of the physical properties of dental gypsums, has resulted in a technic in which gypsum reproduc tions dimensionally are almost completely accurate. It can, therefore, be assumed that these reproductions meet all clinical requirements extremely well. The cast restoration itself has shown continuing improvement. At this time, it is possible to make cast restorations of all types that leave little, if anything, to be desired from the standpoint of accuracy. Thus, it will be appreciated that a technic that, a comparatively short time ago, existed only as a dream on the part of the den tal profession has now become a reality.
Hollenback: ELASTICIMPRESSIONMATERIALS• 645 Materials and methods are now avail able, which if adequately applied, com pletely meet the requirements of the in direct technics .12 17000 Ventura Boulevard
Presented at the one hundred and fifth annual session of the Am erican Dental Association, November II , 1964, San Francisco. 1. Price, Weston A . The laws determining the behavior of gold in fusing and casting. D. Cosmos 53:265, 1911. 2. H o lle n b a c k .G . M . Study o f the physical properties of elastic impression materials. J. South. C alifornia D. A. 31:204 June 1963. 3. Hollenback, G. M. Study of the physical properties of elastic impression materials. J. South. C alifornia D. A. 31:227 July 1963.
4. H ollenback, G. M . Study o f the physical properties of elastic impression materials. J. South. C alifornia D. A. 31:369 Nov. 1963. 5. H ollenback, G . M . Study of the physical properties of elastic impression materials. J. South. C alifornia D. A. 31:403 Dec. 1963. 6. H ollenback, G. M . Physical properties of gypsum plasters. J. South. C alifornia D. A . 30:400 Dec. 1962. 7. H ollenback, G . M . Physical properties of gypsum plasters. (Instruments and technics.) J. South. C alifornia D. A. 31:14 Jan. 1963. 8. H ollenback, G. M . Physical properties of gypsum plasters. J. South. C alifornia D. A . 31:47 Feb. 1963. 9. H ollenback, G. M . Physical properties of gypsum plasters. J. Soutn. C alifornia D. A. 31:130 A p ril 1963. 10. H ollenback, G. M . C ontrol o f erratic behaviors of casting investments. J. South. C alifornia D. A. 30:159 May 1962. 11. Hollenback, G. M. Further lig h t on dimensional casting. J. South. C alifornia D. A. 30:341 Oct. 1962. 12. H ollenback, G. M., and others. Science and tech nic o f the cast restoration. St. Louis, C. V. Mosby Co., 1964.
Are you going to the annual session in Las Vegas this November? If you are, you will find winter snows and pine forests only 35 miles away. The Nevada National Forest is famous for its scenic beauty and easy accessibility. There you can enjoy all manner of cold weather sports in the shadow of Mt. Charleston which towers 9,000 feet above the desert.