Thermal cycling distortion of metal ceramics: Part I--Metal collar width

Evaluation of marginal adaptation of Co-Cr-Mo metal crowns fabricated by traditional method and computer-aided technologies

Background/purpose

The purpose of this study was to evaluate the marginal gaps of dental restorations manufactured using conventional loss wax and casting, computer-aided design/computer-aided manufacturing (CAD/CAM), and 3D printing methods.

Materials and methods

A zirconia master die model with an upper right first molar resin crown was prepared as a standardized model. A total of 30 resin master die models were duplicated from this standard model. Simultaneously, 10 Co—Cr—Mo metal crowns were individually obtained using the conventional loss wax and casting method (Group A), selective laser sintering (Group B), and CAD/CAM (Group C), respectively. The marginal gaps between the crowns fabricated conventional and digital methods with master die models were calculated using a 3D replica and mapping technique.

Results

Statistical analyses revealed there were significant differences in the marginal gaps in the group A with group B and C (p < 0.05). The mean marginal gaps between dental crowns with die models were 76 ± 61 μm, 116 ± 92 μm, and 121 ± 98 μm for groups A, B, and C, respectively.

Conclusion

Within the limitations of this study, the marginal gaps were clinical acceptable in conventional and digital techniques.

Evaluation of the marginal and internal gap of metal-ceramic crown fabricated with a selective laser sintering technology: two- and three-dimensional replica techniques

PURPOSE

One of the most important factors in evaluating the quality of fixed dental prostheses (FDPs) is their gap. The purpose of this study was to compare the marginal and internal gap of two different metal-ceramic crowns, casting and selective laser sintering (SLS), before and after porcelain firing. Furthermore, this study evaluated whether metal-ceramic crowns made using the SLS have the same clinical acceptability as crowns made by the traditional casting.

MATERIALS AND METHODS

The 10 study models were produced using stone. The 20 specimens were produced using the casting and the SLS methods; 10 samples were made in each group. After the core gap measurements, 10 metal-ceramic crowns in each group were finished using the conventional technique of firing porcelain. The gap of the metal-ceramic crowns was measured. The marginal and internal gaps were measured by two-dimensional and three-dimensional replica techniques, respectively. The Wilcoxon signed-rank test, the Wilcoxon rank-sum test and nonparametric ANCOVA were used for statistical analysis (α=.05).

RESULTS

In both groups, the gap increased after completion of the metal-ceramic crown compared to the core. In all measured areas, the gap of the metal cores and metal-ceramic crowns produced by the SLS was greater than that of the metal cores and metal-ceramic crowns produced using the casting. Statistically significant differences were found between cast and SLS (metal cores and metal-ceramic crown).

CONCLUSION

Although the gap of the FDPs produced by the SLS was greater than that of the FDPs produced by the conventional casting in all measured areas, none exceeded the clinically acceptable range.

Prosthetic misfit of implant-supported prosthesis obtained by an alternative section method

PURPOSE

Adequate passive-fitting of one-piece cast 3-element implant-supported frameworks is hard to achieve. This short communication aims to present an alternative method for section of one-piece cast frameworks and for casting implant-supported frameworks.

MATERIALS AND METHODS

Three-unit implant-supported nickel-chromium (Ni-Cr) frameworks were tested for vertical misfit (n = 6). The frameworks were cast as one-piece (Group A) and later transversally sectioned through a diagonal axis (Group B) and compared to frameworks that were cast diagonally separated (Group C). All separated frameworks were laser welded. Only one side of the frameworks was screwed.

RESULTS

The results on the tightened side were significantly lower in Group C (6.43 ± 3.24 µm) when compared to Groups A (16.50 ± 7.55 µm) and B (16.27 ± 1.71 µm) (P<.05). On the opposite side, the diagonal section of the one-piece castings for laser welding showed significant improvement in the levels of misfit of the frameworks (Group A, 58.66±14.30 µm; Group B, 39.48±12.03 µm; Group C, 23.13±8.24 µm) (P<.05).

CONCLUSION

Casting diagonally sectioned frameworks lowers the misfit levels. Lower misfit levels for the frameworks can be achieved by diagonally sectioning one-piece frameworks.

Evaluation and comparison of the effect of different surface preparations on bond strength of glass ionomer cement with nickel-chrome metal-ceramic alloy: a laboratory study

Retention of fixed partial dentures is mostly dependent upon the bond between metal and cement as well as cement and tooth structure. However, most of the time clinical failure of bond has been observed at metal and cement interface. The treatment of metal surface, prior to luting, plays a crucial role in bonding cement with the metal. This study is conducted to evaluate and compare the effect of different surface preparations on the bond strength of resin-modified glass ionomer cement with nickel-chromium metal ceramic alloy. Fifty caries-free extracted molar teeth were made flat until the dentin of the occlusal surface was exposed. After fabrication of the wax patterns and subsequent castings, the castings were subjected to porcelain firing cycles. The nickel-chromium metal ceramic alloy discs were also divided into five groups and subjected to various surface treatments: (1) Unsandblasted (U), (2) sandblasted (S), (3) sandblasted and treated with 10% aqueous solution of KMnO4 (SK), (4) unsandblasted and roughened with diamond abrasive points (UD) and (5) unsandblasted and roughened with diamond abrasive points and treated with 10% aqueous solution of KMnO(4) (UDK). After surface treatments, the castings were cemented using Fuji PLUS encapsulated resin-modified glass ionomer cement. The obtained values of all the groups were subjected to statistical analysis for Tensile and Shear bond strength. Different surface treatments of the metal affects the bond strength values of resin-modified glass ionomer cement when used as luting agent.

Influence of framework design on the cervical color of metal ceramic crowns

STATEMENT OF PROBLEM

The replication of natural teeth, especially for single tooth restorations in patients with high esthetic needs, represents a challenge.

PURPOSE

The purpose of the study was to analyze the color of the cervical portion of single metal ceramic crowns fabricated with different metal framework designs.

MATERIAL AND METHODS

The color, as measured on the CIELAB color scale, of 3 different groups of restorations (n=10) fabricated with a high noble metal alloy (V-Deltaloy) and feldspathic porcelain (Noritake Super Porcelain) was analyzed with a colorimeter. Conventional metal ceramic crowns with metal facial margins were compared to metal ceramic crowns with porcelain facial margins and a horizontal reduction of the metal framework (1.0 mm reduction) or an additional vertical reduction (1.0 mm reduction). In all specimens, the finish line was positioned at a subgingival or equigingival level. The 6 groups obtained by the combination of the levels of the 2 factors (framework extension and finish line location) were examined with an ANOVA Fisher's F-test and a post hoc Tukey's HSD test (α=.05).

RESULTS

The mean color difference for all the groups was clinically acceptable (ΔE<3.7). Conventional metal ceramic crowns showed higher differences in relation to finish line location (ΔE=2.34), while a vertical reduction of the framework was related to lower values (ΔE=0.96). Mean Lab* values were reported for all the groups of crowns. Statistically significant differences were present for L*, a*, and b* values when related to framework extension. Considering each value in relation to the interaction between framework extension and finish line location factors, significant differences were present only for L* and a* values.

CONCLUSIONS

No significant differences in base shade were present among the investigated crowns. Nevertheless metal ceramic crowns with vertical cut-back and porcelain facial margins presented more consistent results that tended to be closer to those of natural teeth.

Alternate metal framework designs for the metal ceramic prosthesis to enhance the esthetics

PURPOSE

The objective of the present study was to evaluate the effect of five different metal framework designs on the fracture resistance of the metal-ceramic restorations.

MATERIALS AND METHODS

For the purpose of this study, the central incisor tooth was prepared, and the metal analogue of it and a master die were fabricated. The counter die with the 0.5 mm clearance was used for fabricating the wax patterns for the metal copings. The metal copings with five different metal framework designs were designed from Group 1 to 5. Group 1 with the metal collar, Group 2, 3, 4 and 5 with 0 mm, 0.5 mm, 1 mm and 1.5 mm cervical metal reduction respectively were fabricated. Total of fifty metal ceramic crown samples were fabricated. The fracture resistance was evaluated with the Universal Testing Machine (Instron model No 1011, UK). The basic data was subjected to statistical analysis by ANOVA and Student's t-test.

RESULTS

Results revealed that the fracture resistance ranged from 651.2 to 993.6 N/m². Group 1 showed the maximum and Group 5 showed the least value.

CONCLUSION

The maximum load required to fracture the test specimens even in the groups without the metal collar was found to be exceeding the occlusal forces. Therefore, the metal frameworks with 0.5 mm and 1 mm short of the finish line are recommended for anterior metal ceramic restoration having adequate fracture resistance.

Effect of cast rectifiers on the marginal fit of UCLA abutments

Objectives:

This study assessed the effect of cast rectifiers on the marginal misfit of cast UCLA abutments compared to premachined UCLA abutments. The influence of casting and porcelain baking on the marginal misfit of these components was also investigated.

Methods:

Two groups were analyzed: test group – 10 cast UCLA abutments, finished with cast rectifier and submitted to ceramic application; control group – 10 premachined UCLA abutments, cast with noble metal alloy and submitted to ceramic application. Vertical misfit measurements were performed under light microscopy. In the test group, measurements were performed before and after the use of cast rectifiers, and after ceramic application. In the control group, measurements were performed before and after casting, and after ceramic application. Data were submitted to statistical analysis by ANOVA and Tukey's test (α= 5%).

Results:

The use of cast rectifiers significantly reduced the marginal misfit of cast UCLA abutments (from 25.68μm to 14.83μm; p<0.05). After ceramic application, the rectified cylinders presented misfit values (16.18μm) similar to those of premachined components (14.3 μm). Casting of the premachined UCLA abutments altered the marginal misfit of these components (from 9.63 μm to 14.6 μm; p<0.05). There were no significant changes after porcelain baking, in both groups.

Conclusion:

The use of cast rectifiers reduced the vertical misfit of cast UCLA abutments. Even with carefully performed laboratory steps, changes at the implant interface of premachined UCLA abutments occurred. Ceramic application did not alter the marginal misfit values of UCLA abutments.

Precision of fit of two margin designs for metal-ceramic crowns

STATEMENT OF PROBLEM

Although metal-ceramic restorations are widely used, there is a lack of information about how the fit is affected by margin designs.

PURPOSE

This study measured and compared the precision of fit of metal-ceramic crowns with two margin designs.

MATERIAL AND METHODS

Thirty-two extracted human premolar teeth were prepared for complete-coverage restorations with an internally rounded shoulder preparation. Impressions were made from all teeth, and master dies were poured with improved stone type V. MC crowns were fabricated with a porcelain-butt margin on the buccal aspect (n = 32) and a feather-edge metal margin on the lingual aspect (n = 32), which served as a control group. Precision of fit was measured three times at the mid-buccal and mid-lingual margins of each crown with a profilometer. The data were statistically analyzed with the paired t-test (alpha < .05).

RESULTS

Mean marginal gap size of porcelain-butt margins was 27.93 microm (+/-15.84) and of feather-edge metal margins 42.43 microm (+/-24.12). The marginal gap size of feather-edge metal margins was statistically significantly greater than that of porcelain-butt margins (p = 0.0045).

CONCLUSION

The marginal fit of porcelain-butt margins was significantly better than that of feather-edge metal margins.

CLINICAL IMPLICATIONS

Porcelain-butt margins in this study had a better marginal fit than feather-edge metal margins, and thus have given clinicians evidence to prefer the use of porcelain-butt margins.

Influence of finish line design on marginal adaptation of electroformed metal-ceramic crowns

STATEMENT OF PROBLEM

Gold electroformed metal-ceramic restorations have been promoted as alternatives to conventional metal-ceramic restorations. However, little is known about the relationship between tooth preparation design and marginal adaptation for this type of crown.

PURPOSE

This study evaluated the influence of 3 different finish line designs on the marginal adaptation of electroformed metal copings and metal-ceramic crowns.

MATERIAL AND METHODS

Three steel dies were prepared for maxillary central incisor crowns with 3 finish line designs: shoulder, rounded shoulder, and deep chamfer preparations. Eight standardized electroformed metal-ceramic crowns were fabricated for each group. Marginal discrepancies were measured at 60 points for each specimen along the circumferential margin at 4 sites (labial, mesial, lingual, and distal surfaces, with 15 points for each site) before and after firing procedures using a laser microscope. Data among the 3 different groups were statistically analyzed using the Kruskal-Wallis test and the Mann-Whitney U test with the Bonferroni correction. Marginal discrepancies between prefiring and postfiring procedures were evaluated using the Wilcoxon signed-ranks test (alpha = .05).

RESULTS

Significant differences in the marginal discrepancies of electroformed metal copings without porcelain and metal-ceramic crowns were found among all groups. The lowest range of median marginal discrepancy values (P < .05) at 4 sites, both before and after firing, occurred with the deep chamfer preparation (17.64-21.78 microm and 23.96-25.72 microm, respectively). The highest range values were observed in the shoulder preparation (38.13-49.89 microm and 73.87-89.44 microm, respectively). In all situations, the marginal discrepancies of the postfiring procedures were significantly greater (P = .02 or less) than those of the prefiring procedures.

CONCLUSION

Within the limitations of this study, the marginal adaptation of electroformed metal copings or metal-ceramic crowns is affected by finish line design and sequentially diminished by porcelain firing procedures.

The influence of different screw tightening forces on the vertical misfit of implant-supported frameworks

OBJECTIVES

The present in vitro study was designed to compare the differences in the vertical misfit of implant-supported frameworks using three different forces for tightening the bridge locking screws: fastening by hand until first resistance, and using torque drivers with 10 and 20Ncm.

METHODS

The investigation was conducted based on the results given by 9 six-unit nickel-chromium (2 abutments/ 4 pontics) screw-retained implant-supported frameworks. The structures were exposed to simulated porcelain firings. The marginal misfit measurements were made using a traveling measuring microscope at selected screw tightening forces: fastening by hand until first resistance, and using torque drivers with 10 and 20Ncm. The results were submitted to one-way ANOVA with repeated measures on one factor, and post hoc pairwise comparisons using Tukey test (5%).

RESULTS

The mean marginal misfit of the frameworks, fastening the screws by hand until first resistance, was 41.56µm (SD±12.45µm). The use of torque driver devices caused a significant reduction in marginal opening (p<0.05). With the lowest torque available (10Ncm), the mean marginal discrepancy at the abutment-framework interface was reduced an average of 52% to a mean marginal opening of 19.71µm (SD±2.97µm). After the use of the 20Ncm torque driver, the mean marginal discrepancy of the frameworks was reduced an average of 69% to a mean marginal opening of 12.82µm (SD±4.0µm). Comparing the use of torque drivers with 10 and 20 Ncm torque, the means are not significantly different from one another.

CONCLUSION

The seating force has an important effect on the vertical misfit measurements, once it may considerably narrow the vertical misfit gaps at the abutment-framework interface, thus leading to a misjudgment of the real marginal situation.

Stress-relieving and porcelain firing cycle influence on marginal fit of commercially pure titanium and titanium-aluminum-vanadium copings

OBJECTIVES

This study evaluated: (1) the marginal fit of copings fabricated from commercially pure titanium (cp Ti) and titanium-aluminum-vanadium alloy (Ti-6Al-4V) after each firing cycle for a recommended porcelain; and (2) the effectiveness of a stress-relieving treatment on the fit. The results were compared with copings made with Pd-Ag alloy submitted to the recommended porcelain firing protocol.

METHODS

Bovine teeth were machined for metal-ceramic crowns and die casts were obtained for waxing patterns. The firing cycle of titanium porcelain was performed on cp Ti and Ti-6Al-4V copings obtained using the Rematitan System. A stress-relieving treatment (cp Ti treated and Ti-6Al-4V treated groups) was performed on original invested castings at the first porcelain firing temperature (800 degrees C). Fit (microm) was measured in the 'as cast' condition and after each porcelain firing cycle with a traveling microscope. The values were compared using the analysis of variance and Tukey's test (P<0.05), the Pd-Ag group was compared to titanium groups using Dunnett's test (P<0.05).

RESULTS

The marginal fit values in the as-cast condition (initial) were 84.43 microm for Ti-6Al-4V not treated, 76.35 microm for Ti-6Al-4V treated, 86.09 microm for cp Ti not treated, 97.66 microm for cp Ti treated and 58.11 microm for Pd-Ag group. The Ti-6Al-4V, cp Ti, and Pd-Ag groups exhibit significant differences in marginal fit values when the as-cast condition (initial) was compared with those after the recommended porcelain firing protocol (by Tukey's test at 5% probability level). No significant differences were observed between stress-relieving treated and not treated groups.Significance. The firing cycles used for porcelain build-up may worsen the fit of titanium copings to values that suggest no inferences to the detriment of clinical applications.

Influence of cold working and thermal treatment on the fit of implant-supported metal-ceramic fixed partial dentures

STATEMENT OF PROBLEM

The application of porcelain may alter the fit of metal-ceramic frameworks. In fixed partial denture frameworks, this altered fit may cause marginal opening and require sectioning and soldering of the prosthesis.

PURPOSE

The purpose of this study was to compare the fit of implant-supported metal-ceramic fixed partial dentures when different thermal treatments were performed.

MATERIAL AND METHODS

Fifteen implant-supported metal-ceramic fixed partial denture frameworks were fabricated and divided into 3 treatment groups: Group 1 = cold working and thermal oxidation; Group 2 = unrestrained thermal treatment, cold working, and oxidation; and Group 3 = investment-restrained thermal treatment, cold working, and oxidation. After the appropriate treatment procedures, porcelain was applied. Measurements were made with a digital caliper, with the help of fiduciary marks, after each step in 3 dimensions and at 2 locations. These measurements were compared with those of the as-cast framework (baseline) to determine framework distortion. Distortion data among groups were compared with repeated-measures analysis of variance and Tukey's Studentized range test (P<.05).

RESULTS

Absolute values of distortion (regardless of direction) after thermal treatment and after porcelain application were not significantly different among groups. However, total raw values of distortion, which describe the specific direction of distortion, were significantly different for 4 of the 6 recorded measurements between Groups 1 and 3 (P<.05). Although the absolute distortion values did not differ among groups, heat treatment and finishing of the alloy affected the timing and direction of distortion.

CONCLUSION

Within the limitations of this study, heat treatment of implant-supported metal-ceramic fixed partial denture frameworks before cold working did not decrease the total absolute distortion. Definite patterns of distortion were, however, found in Groups 1 and 3, suggesting that specific manipulations and heat treatments of the alloy affected the direction of distortion.

Evaluation of high-temperature distortion of high-palladium metal-ceramic crowns

STATEMENT OF PROBLEM

Crown fit is a prerequisite for long-term clinical success; however, crown distortion may occur during porcelain firing. The dimensional stability of some high-palladium alloys at high temperatures has been questioned.

PURPOSE

The purpose of this study was to use a new method to measure the distortion of copings for metal-ceramic single units of selected high-palladium alloys with compositions representative of commercial alloys.

MATERIAL AND METHODS

Four high-palladium alloys containing copper and 3 containing no copper were tested. A palladium-silver alloy was included for comparison, and a gold-palladium alloy served as the control. By using reference points scribed on the margin, the mesiodistal and buccolingual margin diameters of identical copings were measured with a traveling microscope at 4 stages: as-cast, oxidized, after 2 simulated opaque porcelain firings, and after 2 simulated dentin porcelain firings. The margin distortions for the various specimen groups representing combinations of alloys, stages, and measurement diameters were compared with the use of 1-way analysis of variance and a multiple range test.

RESULTS

Most of the high-palladium alloys had high-temperature distortions that were not significantly different from those of the control alloy. The distortions occurred principally during the oxidation cycle. The effect of mesiodistal groove reinforcement on preventing distortion was not the same for all alloys.

CONCLUSION

The results suggest that small observed distortions of these alloys will not produce clinical problems. Several laboratory techniques are available to counteract the distortions.

Distortion of three-unit implant frameworks during casting, soldering, and simulated porcelain firings

PURPOSE

The aim of this study was to assess distortion inherent in casting, soldering, and simulated porcelain firings of screw-retained, implant-supported three-unit fixed partial dentures (FPDs).

MATERIALS AND METHODS

Ten wax patterns were fabricated on a die-stone cast containing two implants, 20 mm apart from center to center. Five specimens were cast in a high-palladium alloy, exposed to simulated porcelain firings, sectioned, and then soldered with low-fusing solder. Five specimens were cast, sectioned, soldered with high-fusing solder, and then exposed to simulated porcelain firings. For each specimen, two horizontal and six vertical distances between appropriately scribed reference points were measured with a traveling microscope. Comparisons were made among the various measurements taken after wax-pattern fabrication, casting, high- and low-fusing soldering, and each porcelain firing. Data were analyzed using a repeated-measures factorial ANOVA (alpha = 0.05).

RESULTS

Significant difference was detected in the amount of horizontal distortion during casting (53 +/- 24 microns) and high-fusing soldering (-49 +/- 50 microns), as well as in the amount of horizontal distortion during high-fusing soldering (-49 +/- 50 microns) and low-fusing soldering (17 +/- 26 microns). However, no clinically significant difference was found in the amount of horizontal distortion during casting, low-fusing, and high-fusing soldering. The greatest amount of distortion during the simulated porcelain firings took place during the oxidizing cycle.

CONCLUSIONS

Soldering did not improve the casting misfit of a three-unit implant-retained FPD model. Metal-ceramic implant frameworks should be oxidized before intraoral fit evaluation.

Thermal cycling distortion of porcelain fused to metal fixed partial dentures

STATEMENT OF PROBLEM

The initial fit of porcelain fused to metal restorations deteriorates during the firing cycle of porcelain.

PURPOSE

This study evaluated thermal cycling distortion of 3-unit porcelain fused to metal frameworks at different firing stages.

MATERIAL AND METHODS

A master model was designed to represent the 2 abutments of a 3-unit fixed partial denture replacing a missing mandibular molar. Standard techniques were used to fabricate 10 castings. Half of the copings were cast in a Ni-Cr alloy and the other half in a Pd-Cu alloy. Framework distortion was measured by means of inner fit changes, horizontal linear measurements of the framework length, and vertical fit changes of each retainer. Measurements were made (1) initially, (2) after degassing firing, and (3) after glaze firing. Differences between the firing cycles created distortion values of the retainers in 3 dimensions. Repeated measures ANOVA was used to analyze data statistically.

RESULTS

Measured differences between the 2 firing stages ranged from -47 to 81.7 micrometer. For both alloy groups, retainers showed increase in vertical gap that implied poorer vertical fit after porcelain application. Mean values of inner fit change recorded for porcelain application firing were higher in magnitude than the values of metal-conditioning firing. In addition, no statistically significant differences were found among alloy types.

CONCLUSIONS

A 3-dimensional distortion was observed both in Pd-Cu and Ni-Cr frameworks during porcelain firing cycle. The distortion seen after porcelain application firing was significantly greater than that seen after metal-conditioning firing. This result can be attributed to these factors, contamination of porcelain to the inner surface of metal coping and reduction in resilience of metal.

Ceramics in dentistry: historical roots and current perspectives

This article presents a brief history of dental ceramics and offers perspectives on recent research aimed at the further development of ceramics for clinical use, at their evaluation and selection, and very importantly, their clinical performance. Innovative ceramic materials and ceramics processing strategies that were introduced to restorative dentistry since the early 1980s are discussed. Notable research is highlighted regarding (1) wear of ceramics and opposing enamel, (2) polishability of porcelains, (3) influence of firing history on the thermal expansion of porcelains for metal ceramics, (4) machining and CAD/CAM as fabrication methods for clinical restorations, (5) fit of ceramic restorations, (6) clinical failure mechanisms of all-ceramic prostheses, (7) chemical and thermal strengthening of dental ceramics, (8) intraoral porcelain repair, and (9) criteria for selection of the various ceramics available. It is found that strong scientific and collaborative foundations exist for the continued understanding and improvement of dental ceramic systems.

Effects of firing cycle and surface finishing on distortion of metal ceramic castings

The "as-cast" fit of metal ceramic restorations has been reported to deteriorate during the high-temperature firing cycles used for application of porcelain veneer. In this study, thermocycling and surface finishing or cold working were examined for their effects on marginal adaptation of metal ceramic castings. Methods for minimizing the loss of marginal adaptation were evaluated, and casting variables were eliminated by construction of acrylic resin measuring dies directly in the restorations. Thermocycling of metal ceramic restorations resulted in increased marginal openings, and all of the loss of marginal fit occurred during the first thermocycling of the alloy. The restorations that were cold worked and then oxidized by conventional manipulation had substantially more marginal opening than any other group. A fourfold, statistically significant improvement (p < 0.001) in the marginal adaptation of a metal ceramic restoration was observed when the initial thermal cycle was completed before the specimens were finished.

Marginal fit changes during porcelain firing cycles

The marginal fit changes that occur during the procelain firing cycles of palladium-copper and nickel-chromium copings both with shoulder and chamfer finish lines were investigated with scanning electron microscopy. Three copings from each alloy test group were used as nonporcelainized controls. Comparisons of the firing cycles revealed a greater change during the degassing stage, and the opaque firing caused a decrease in marginal gap. There was a small increase in gap size after firing body porcelain. The marginal fit change for the palladium-copper copings (19.39 microns) during degassing was significantly greater than that for the nickel-chromium copings (8.65 microns). However, no significant differences were found when the effects of margin design and porcelain proximity to fit of metal-ceramic crowns were compared.

Annual review of selected dental literature: report of the Committee on Scientific Investigation of the American Academy of Restorative Dentistry

The annual review of selected dental literature this year cites 384 published papers and reports. This year's review contains more editorial comment than reviews of years passed. New data on the biological responses to materials is emphasized in several sections. Observations on new compounds able to prevent plaque formation are presented. Clinically relevant advances in knowledge concerning the etching of different tooth structures are reported along with the effect of etching procedures on the dental pulp. Evaluation of periodontal diseases in all age groups is a topic. Limitations of current diagnostic techniques in periodontal disease, temporomandibular disorders, and implant therapy are included. There are new views on the use of dental amalgam. The future use of dental mercury is predicted. Interest in new ceramic systems is indicated as the demand for esthetics continues. Clinical information is emphasized over scientific information throughout this year's review.