F.A. Peyton

Photoelasticity as a Research Technique for Analyzing Stresses in Dental Structures

THE primary purpose of the masticatory process is to apply forces to the nutritional substances which are taken into the mouth, thereby causing a reduction in their size. The mechanism for this action includes the medium of tooth structure which not only applies these forces directly to the food masses, but is itself subject to the effects of these forces. In general, the effects of forces applied to a body are manifest in the development of internal stresses which are distributed in accordance with the direction of the applied forces, the manner in which the body is supported, and the shape of the body. Furthermore, these stresses are accompanied by internal deformations or strains within the body, and if their magnitudes are sufficiently large, permanent deformation and, ultimately, failure will ensue. Tooth structure and dental restorations are not exceptions to these rules. The structural design of normal tooth structures is such that they withstand the effects of masticatory forces. However, when tooth structure is removed and replaced by a dental restoration, the normal internal stress distribution is altered and the new stress situation will depend on the design of the cavity preparation. Furthermore, the shape of the restoration, which is also determined by the cavity preparation, is of considerable importance as to whether the restorative material will undergo permanent deformation or fracture. In general, then, an evaluation of the internal stresses in both dental restorations and tooth structures should yield valuable criteria for successful design. The consideration of stresses in dental structures is not a new concept. There has been considerable reflection concerning the design of dental restorations so as to avoid failure caused by internal stresses. Most of these efforts have resulted in generalities deduced from the theory of elasticity as applied to uniformly shaped structures and, therefore, leave much to be desired when confronted by the irregular shapes of dental structures. Empirical rules or criteria have developed over the years on the basis of clinical evidence and these have served to avoid the deleterious effects of internal stress magnitudes. However, since both the design and size of dental restorations are extremely limited by the biologic aspects of tooth structure, an exacting analysis is the only solution to the determination of an optimum structural configuration.

Friction and Wear of Restorative Dental Materials

Friction coefficients for a variety of material couples are reported. The relative abrasive wear of materials followed the order of their hardness, with one exception.

Experimental stress analysis of dental restorations

Abstract The general stress distribution in full crown restorations was investigated using a two-dimensional photoelastic stress analysis method. Crowns on tipped molar abutments as well as on normally aligned molars were analyzed. The maximum compressive stresses on the interior of crowns were generally found to be on the reduced cusp surface of the restorations. Maximum tensile stresses were observed in the central fossa and along the axis of symmetry of the crown, when the crowns were loaded bilaterally to simulate oral conditions. Stresses were also investigated at the free boundaries of the crown, and tensile stresses were observed at the cervical margins of the restoration with compressive loads of 133 pounds and at a cusp angle of 39 degrees.

Experimental stress analysis of dental restorations. Part VII. Structural design and stress analysis of fixed partial dentures

lh e principles of engineering design should be used in the structural design of fixed partial dentures as well as the biologic, esthetic, and mechanical restrictions of the oral cavity.’ The conventional structural design is primarily concerned with the analysis of given structures, using the conventional equations of strength of materials.2z 3 Bending moments in models representing fixed partial dentures were studied,’ and it was found that bending moments of semi-fixed partial dentures were higher than those estimated for fixed partial dentures attached at both ends. Load carrying capacities of dental beams were theoretically determined by Brumfield”g 6 who asserted that the most important factor was depth. The relationship of design to restorative materials was generally discussed in several papers.‘-g The first report concerning the measurement of stresses in fixed partial dentures using a brittle coating technique was published in 1965. lo Strain gages were also used to study the stress distribution on gold and chromium alloy bridges.ll

Properties of resilient denture liners in simulated mouth conditions

Abstract Tests were performed on nine resilient denture-lining materials using distilled water at 37° C. to simulate mouth conditions. The data obtained were compared to those results obtained previously at room temperature. The consistency of the self-curing resilient liners ranged from 41 to 48 mm. These values, on the average, were about 4 mm. larger than those with direct, hard, self-curing denture liners, indicating that the resilient materials flow to a greater extent during their processing. The highest temperature rise observed during the processing of these materials was 2.1° C., and the values ranged from zero to 2.1° C. This indicates that there should be no irritating effect to the oral tissues from temperature rise. The weight increases after 1 month in water at 37° C. were generally a little larger than they had been in water at 25° C. The weight increases after 6 months in water at 37° C. varied from 2 to 22.1 per cent. Of the total material leached out during 1 month in water at 37° C., the percentage of nonvolatile material varied from zero to 27 per cent, with most of the materials losing under 10 per cent nonvolatile material. Therefore, the major portion of the weight loss was due to the leaching out of volatile plasticizers and organic solvents. The Shore A hardness in water at 37° C. was much less than the hardness values in water at 25° C., except for the two silicone products. A temperature rise from 25° C. to 37° C. in air had little effect on the two silicone materials. A temperature rise alone from 25° C. to 37° C. had a greater effect on the hardness than did the effect of transferring the liners from air at 37° C. to water at 37° C. Thus, the major softening was due to an increase in temperature. The original tear resistance was slightly less at 37° C. than at 25° C. in water for all the materials except the two silicone products. After storage for 6 months in water at 37° C., the tear resistance decreased slightly for four materials, and increased extensively for one material. One liner had no harmful effect on the transverse strength of an acrylic resin base, and four materials reduced the transverse strength by about 10 per cent. The remaining four materials, which were self-curing, powder-liquid types containing monomers and organic solvents in the liquids, reduced the transverse strength of the acrylic resin base by approximately 25 per cent. A measurement of the adhesion of the resilient liners to a smooth acrylic resin base after storage for 1 month in water at 37° C. showed that six of the materials possessed enough adhesion to tear within themselves instead of stripping from the base material. One material showed a low value of 7 pounds per inch, and the two silicone products showed the poorest adhesion, 2 and zero pounds per inch. A silicone adhesive was used with the latter silicone product and improved the adhesion to 1 pound per inch. The recovery from 20 per cent compression, placed on specimens of the nine liners for 24 hours at 37° C., was measured after 1 minute and after 1 hour. On the basis of these measurements, the nine materials could be classified into four general groups. One material showed no recovery after 1 minute or 1 hour, three materials showed no recovery after 1 minute and slight recovery after 1 hour, three materials showed some recovery after 1 minute and a larger amount after 1 hour, and the two silicone products showed the greatest total recovery, which took place within the first minute.

Surface hardness, compressive strength, and abrasion resistance of indirect die stones

HE common use of the indirect method in the preparation of inlays, threequarter and full crowns, various bridge forms, and other dental castings, together with the appearance on the market of newer type gypsum products for use as dies, indicates a need for more knowledge of some of the physical properties of the quick-setting artificial stones. In addition, an understanding of the best method of die preparation is necessary in order that the stone may be used to its fullest advantage. Every dentist who includes in his practice gold castings, or other precision techniques requiring dies, should ‘have a thorough knowledge of the factors involved in the fabrication of the die which lead to its improvement. Such a study should enable him, with a minimum of extraneous effort, to produce the most satisfactory stone die obtainable by practical methods. Recently, much has been written about the hydrocolloid technique for use in crown and bridge procedures. Special equipment for taking agar impressions has received widespread recognition, and the accuracy of the material is acclaimed by many dentists as superior to that of other impression materials. This method of impression making, however, requires that the operator use artificial dental stone as a die material inasmuch as no other well-accepted method appears practicable. Amalgam cannot be packed readily into the hydrocolloid impression because of the frailty of the impression material. The hydrocolloid impression cannot be copperplated unless special preparations and extremely careful methods are employed, although some operators have succeeded in accomplishing this. The technique is not a simple one, however, nor can it be done with the equipment generally found in the average dental office. The regular and improved dental stones have been found to be reasonably satisfactory as a die material to be used in conjunction with the hydrocolloid impression materials. In the past, objections have been made to the use of dental

Evaluating dimensional accuracy of denture bases with a modified comparator

Abstract A method of comparing the contours of the inner surface of dentures with the contours of a master impression has heen developed in conjunction with certain modifications of the comparator. These contours are reproduced in graph form, and the accuracy of fit of the denture for any point which has been recorded may be determined by measuring the shortest distance between the contour lines. A series of heat-cured dentures were processed under nearly identical conditions, and from their contours, a representative median contour line was drawn. This contour line has been compared with the contour of the master impression and will be used for future comparisons with dentures processed from other materials and by other methods.

Differential Thermal Analysis of Commercial and Dental Waxes

Since phase changes generally result in dimensional changes, the temperatures at which the phase changes occur in waxes and the influence of adding one wax to another are important in understanding the behavior of waxes. Differential thermal analysis (DTA) offers a useful tool in the study of phase transitions. This study examines the temperature of phase transitions for various commercial, dental, and combined waxes. This investigation was supported by USPHS Research Grant DE-01234 from the National Institute of Dental Research, National Institutes of Health, Bethesda, Md. Presented at the 45th General Meeting of the IADR, Washington, D.C., March 18, 1967.

Hygroscopic technique for inlay casting using controlled water additions

Abstract In summary, a hygroscopic technique for dental gold inlay casting has been described which is based on the results of a fundamental study 6 of the nature of hygroscopic expansion. It was found in this study that the amount of hygroscopic expansion which occurs when a specific amount of water is brought into intimate contact with a dental casting investment during setting is directly related to or defined by this amount of water. In addition, a critical point was found to exist which represents the maximum capacity of the investment for water pick-up and subsequent expansion. Water added below this point was found to define precisely the concomitant expansion. A hygroscopic technique based on the results of this study was designed. It features a flexible inlay ring in order to eliminate the asbestos liner, yet not restrict the expansion, as well as a calibrated syringe for adding controlled amounts of water to the investment during the setting process, The advantages of this technique which were established in the measurements made on investment in the previous study 6 and corroborated by the results of practical castings as reported in this study, are stated as follows: 1.1. The effects of influencing factors such as water/powder ratio, spatulation conditions, age and batch number of investment are eliminated. 2.2. A convenient and accurate means of changing the expansion is provided by the addition of different amounts of water, if such a prodecure is desired by the operator. 3.3. More than adequate reproducibility of the investment expansion, and subsequent casting fit is achieved by adding a controlled amount of water.

Capillary penetration between dissimilar solids

Abstract The major purpose of this study was to develop and evaluate a mathematical model for the capillary penetration of a liquid between two dissimilar plates. Attempts have been made in the past to apply the Young-Laplace equation to this situation, but without success because of the complex curvature of the meniscus. Instead, free energy considerations were used to derive the proposed equation. The equation obtained for the capillary rise h for a liquid of surface tension γLV into a space b between two plates whose contact angles with the liquid are θ1 and θ2 was found to be: h= γLV(cos θ 1 + cos θ 2 ) b dg , where d is the liquid density and g is the gravitational constant. Experimental values of capillary penetration between combinations of three liquids and several solids were obtained with the use of the hyperbola method. A factorial design was employed. These data were compared with those predicted by means of the proposed equation. Regression, correlation, and analysis of variance were used to test for the degree and significance of the association between the predicted and observed values. These results and a dimensional analysis indicate that the proposed equation is satisfactory.