Porcelain-fused-to-metal Restorations Research Articles

The Effects of Different Restoration Materials on the Trueness of Intraoral Scanners

Aim: This study aimed to assess how different restoration materials affect the trueness of intraoral scanners. Materials and Methods: Artificial teeth on a typodont model were prepared for crowns and fixed partial dentures (FPDs) using full metal, monolithic zirconia, and porcelain-fused-to-metal (PFM) materials. Each group underwent 10 scans with a Trios intraoral scanner, generating 60 STL files. A reference scanner created a master model using scanning spray to reduce reflection errors. The STL files were aligned with reverse engineering software for comparison and were analyzed in micrometers (μm) using Root Mean Square (RMS) and Mean Distance measurements. The Levene test and two-way ANOVA with Post Hoc analysis were used for statistical evaluation. Results: The RMS deviations for the FPDs were 77.9 ± 15.2 μm (full metal), 84.6 ± 6.9 μm (monolithic zirconia), and 130 ± 19.7 μm (PFM). For the crowns, the RMS values were 76.9 ± 6.5 μm (metal), 71 ± 8.2 μm (monolithic zirconia), and 153 ± 22.4 μm (PFM). The mean distance deviations for the FPDs were 11.4 ± 4.8 μm (metal), 11.2 ± 3.4 μm (monolithic zirconia), and 18.3 ± 2.6 μm (PFM). For the crowns, the mean distances were 8.6 ± 3.4 μm (metal), 10.2 ± 3 μm (monolithic zirconia), and 24.7 ± 3.3 μm (PFM). Significant differences were noted in the PFM groups. Conclusion: Restoration materials notably affected intraoral scanner trueness, especially PFM restorations. The restoration length did not significantly affect the accuracy.

Survival, technical and biological outcome of fixed tooth- and implant-supported restorations: A retrospective analysis of a patient cohort treated in an undergraduate dental education program

ObjectivesThere is a paucity of data on the outcome of fixed tooth- and implant-supported porcelain-fused-to-metal (PFM) and ceramic-based (CB) restorations fabricated during undergraduate dental education. Therefore, this retrospective study examined the outcome of fixed restorations placed in patients participating in an undergraduate program. MethodsPatient records were searched for data on fixed PFM and CB restorations. For these restorations, the functional period in situ and technical and biological complications, namely restoration fracture, decementation, screw loosening, secondary caries, and loss of vitality, were recorded. Periodontal parameters were also documented. Kaplan-Meier survival and complication estimates after 1 and 5 years were calculated for restoration types, materials, designs (veneered/non-veneered), cementation/retention modes and localizations with group sizes ≥ 15. Regression models were used to analyze the influence of demographic data and abovementioned restoration characteristics on restoration survival, technical and biological outcome. Results1196 restorations with a mean observation time of 5 years were included in the analysis. The different restoration types exhibited survival rates of > 90.8% after 5 years, accompanied by a favorable biological outcome. However, monolithic resin-matrix or feldspathic ceramic restorations were found to have a higher fracture rate than lithium disilicate or zirconium dioxide ceramic restorations resulting in lower survival rates for these restorations. Additionally, resin-matrix ceramic restorations showed a higher decementation rate than lithium disilicate restorations. ConclusionsFixed tooth- and implant-supported CB restorations fabricated from lithium disilicate and zirconium dioxide ceramics demonstrated better survival and lower complication rates than restorations fabricated from resin-matrix or feldspathic ceramics in an undergraduate dental education program. Clinical significanceDue to the more favorable outcome of CB restorations fabricated from lithium disilicate or zirconium dioxide ceramics, undergraduate dental education should focus on the application of these ceramics to increase restoration survival by reducing technical complications.

Clinical evaluation of posterior zirconia-based and porcelain-fused-to-metal crowns with a vertical preparation technique: an up to 5-year retrospective cohort study

ObjectiveTo evaluate the clinical conditions of single-unit posterior restorations on teeth prepared without finishing line, after 5.6 years of clinical service. Materials and methods50 crowns (25 zirconia-based (Zr) and 25 porcelain-fused-to-metal (PFM)) were selected from 34 patients. The restorations were evaluated according to the California Dental Association (CDA) Quality Criteria, and periodontal variables were studied in the abutment teeth compared with the unrestored contralateral teeth. Variables were examined using Mann-Whitney and Pearson´s Chi-Square tests (α = 0.05). The success of the prosthesis was calculated using Kaplan-Meier test. ResultsCDA Quality Criteria was considered satisfactory-excellent in all restorations except for one of them, due to chipping on a PFM crown. At, 66 months, the success rates for PFM and Zr crowns were 85.7 % and 100 %, respectively. The plaque index (PI) showed that the restored abutment teeth accumulated significantly less plaque than the control teeth, but the gingival index (GI) was statistically higher in the abutment teeth. In 80 % of cases the probing depth (PB) was ≤3 mm. In addition, in 21 % of the cases, gingival recession was less than 2 mm. The restoration material had a statistically significant effect on GI and PB, with Zr crowns showing less inflammation and less deep pockets than PFM restorations. On the contrary, greater gingival recession was found at the margins of the Zr crowns when compared to the PFM. No statistical differences were found between the two materials in the GI. ConclusionsCemented crowns on vertical preparations show good clinical behavior after 5 years. The periodontal parameters (PI, GI, PD) of the Zr restorations are significantly better than those of PFM, with the exception of gingival recession. Clinical relevanceThe use of restorations on vertically prepared teeth is a suitable alternative to classical horizontal preparations.

A Scanning Electron Microscopy and Energy Diffraction X-Ray Spectroscopy Study to Evaluate the Effect of Firing Temperature at the Ceramic-Noble Metal Alloy Interface in Porcelain Fused to Metal Restorations

Introduction: Porcelain Fused to Metal (PFM) restorations offer aesthetics of ceramics and the strength of metal hence are the gold standards of fixed partial dentures. However, the fracture of porcelain-metal interface still remains a matter of concern. Aim: To study the porcelain-noble metal alloy interface of PFM restorations at different firing temperatures, using Scanning Electron Microscopy (SEM) and Energy Diffraction X-ray Spectroscopy (EDS). Materials and Methods: This in-vitro study was conducted between November 2018 to October 2019 at Prosthodontics Department of Saraswati Dental College and Hospital and King George’s Medical University Lucknow, Uttar Pradesh, India. A total of 75 strips of noble metal alloy were prepared and layered with 1 mm of porcelain on one surface, conforming to American National Standard/American Dental Association Specifications (ANSI/ADA) specification no. 38 for Metal-Ceramic Dental Restorative Systems: 2015. These were randomly divided equally into three groups. Specimens of each group (n=25) were fired at different temperatures that is 850oC, 900oC and 960oC, respectively. Scanning electron microscopy and Energy diffraction X-ray spectroscopy were performed at the noble metal alloy and ceramic interface of all specimens. Data was recorded and statistically analysed using one-way Analysis of Variance (ANOVA) and post-hoc Tukey HSD test. Results: Irregularities/Coarseness (50 μm) was seen on the noble metal alloy surface adjacent to ceramic layer in SEM images at all the three chosen temperatures. The EDS study revealed intermingled zones of partial oxygen depletion at the interface region with formation of intermetallic compound, lead zirconate titanate and complete oxygen depletion zone near metal alloy end of interface. The mean value of Shear Bond Strength (SBS) was 16.31 MPa at 850oC, 24.33 MPa at 900oC and 19.41 MPa at 960oC (p-value <0.05). Conclusion: Difference in properties and location of the intermetallic compound formed at the interface, as well as the location of the oxygen depletion zone could account for the weaker bond formed between noble alloy-porcelain interface compared to base metal-porcelain interface.

Evaluation of maxillary anterior endodontically treated teeth restored with different types of crowns

Statement of the problem: restoring endodontically treated teeth (ETT) is one of the major treatments provided by a dentist. Glass fiber posts (GFP) showed good clinical performance during last few years.Aim of the study; to assess and compare the clinical as well as the radiographic performance of different types of ceramic crown systems used in restoration of maxillary anterior teeth over a cemented GFP and composite resin core.Materials and methods: 50 ETT with GFP were included in this study. These teeth were divided into four gropes (composite resin. Porcelain fused to metal (PFM), e. max and zirconia restorations). Both the clinical and radiographic assessments were done for the restoration at a period of one week, 3, 6, 9, and 12 months after composite build up and crown's cementations. All data were registered and analyzed by SPSS program using percentages and Kaplan-Meyer analysis. Fisher’s exact test was used for categorical values while log-rank test was used for descriptive statistical analysis.Results: the clinical assessment showed no changes in the one week, 3 and 6 months in the four groups. While during the 9 and 12 months, a movement of the crown margin under finger pressure was present in one case, loss or retention in 2 cases of zirconia, the periodontal status with violation of biological width was present in one case of PFM and finally the color changes were obvious in one case of PFM and 2 cases of composite restoration. All the restorations in the four groups had no radiographic changes in the one week and three-month assessments. While during six-month follow-up, a loosed of retention in one case of the zirconia crown was detected. At the 9 and 12 months, two cases showed recurrent caries at the cervical margin of the composite restoration, cases with periapical infection and other with loss of retention of the post were recorded in the PFM restoration.Conclusion: e. Max and zirconia all ceramic crowns showed better clinical and radiographic performance than the PFM and composite restorations over 12 months recall.

Clinical Characteristics of Abutment Teeth with Gingival Discoloration.

The grey-bluish discoloration of gingiva (known as "amalgam tattoo") does not appear only in the presence of amalgam restorations. It may also be seen in cases of teeth restored with cast dowels and porcelain-fused-to-metal (PFM) restorations. The aim of this article was to determine the clinical characteristics of abutment teeth with gingival discoloration. This research was conducted on 25 patients referred for cast dowel and PFM restorations. These restorations were manufactured from Ni-Cr alloys. Ninety days after cementing the fixed prosthodontic restorations, the abutment teeth (n = 61) were divided into a group with gingival discoloration (GD) (n = 25) and without gingival discoloration (NGD) (n = 36). The control group (CG) comprised the contralateral teeth (n = 61). Plaque index, gingival index, clinical attachment level, and probing depth were assessed before fabrication and also 90 days after cementation of the PFM restorations. The gingival index, clinical attachment level, and probing depths of the abutment teeth that had GD were statistically higher before restoration, in comparison with the abutment teeth in the NGD and control groups. Ninety days after cementation, the abutment teeth with GD had significantly lower gingival indexes and probing depths, compared to the abutment teeth in the NGD group. Both abutment teeth groups (GD and NGD) had significantly higher values of clinical attachment levels when compared to the control group. There were no statistically significant differences in plaque index values between the study groups. The results of this study indicated that impairment of periodontal status of abutment teeth seemed to be related to the presence of gingival discolorations. Therefore, fabrication of fixed prosthodontic restorations requires careful planning and abutment teeth preparation to minimize the occurrence of gingival discolorations. With careful preparation of abutment teeth for cast dowels and crown restorations it may be possible to decrease the frequency of gingival discolorations adjacent to abutment teeth.

Hardware complications and failure of three‐unit zirconia‐based and porcelain‐fused‐metal implant‐supported fixed dental prostheses: a retrospective cohort study with up to 8 years

The aim of the present study was to assess the hardware complications and survival of three-unit implant-supported zirconia-based fixed dental prostheses (IZ) and implant-supported porcelain-fused-metal fixed dental prostheses (IP). The study is a retrospective cohort study with up to 8years (mean 4.8years) follow-up. Patients with conventional three-unit implant-supported fixed dental prostheses (without cantilever) in posterior area were reviewed. Hardware complications and survival rate were evaluated. Chi-Square test was used to test the difference between IZ and IP groups. Two hundred and thirty-seven patents (IZ:112, IP:125) with 279 three-unit restorations (IZ:127, IP:152) participated in the study. The overall survival rate was 95.3% in IZ group and 94.7% in IP group at implant-level and 94.6% in IZ group and 94.4% in IP group at subject-level. Veneer chipping was the most frequently seen complication (20.1%). Significant higher minor veneer chipping rate (Grade 1) was found in IZ group (P=0.04). No significant difference of veneer chipping rate (Grade 2 and Grade 3) was found between the two groups. The overall hardware complication rates of IZ were significantly higher than IP (33.07% and 18.42%, P=0.01). High survival rate of zirconia-base and Porcelain-fused-to-metal (PFM) restorations can be achieved with up to 8years follow-up. The zirconia-based restorations need more polishing procedures, maintenance, and professional care than PFM restorations. The number of implants supporting a fixed dental prosthesis did not influence the implant survival and hardware complications. Well-designed studies with high evidence level are still needed to further explore the hardware complications and clinical survival of IZ and IP.

Ceramic Defects in Metal-Ceramic Fixed Dental Prostheses Made from Co-Cr and Au-Pt Alloys: A Retrospective Study.

Ceramic defects in porcelain-fused-to-metal (PFM) restorations may depend on framework alloy type. This study assessed ceramic defects on cobalt-chromium- (Co-Cr-) and gold-platinum- (Au-Pt-) based PFM restorations. In this study, 147 Co-Cr-based and 168 Au-Pt-based PFM restorations inserted between 1998 and 2010 (139 patients) were examined for ceramic defects. Detected defects were assigned to three groups according to clinical defect relevance. Ceramic defect rates (Co-Cr-based: 12.9%; Au-Pt-based: 7.2%) revealed no significant difference but a strong statistical trend (U test, P = .082). Most defects were of little clinical relevance. Co-Cr PFM restorations may be at higher risk for ceramic defects compared to Au-Pt-based restorations.

Metal-ceramic bond mechanism of the Co-Cr alloy denture with original rough surface produced by selective laser melting

The porcelain fracture caused by low metal-ceramic bond strength is a critical issue in porcelain fused to metal(PFM) restorations. Surface roughening methods, such as sand blasting, acid etching and alkaline degreasing for the metal matrix are used to increase bond strength. However, the metal matrix of PFM processed by selective laser melting(SLM) has natural rough surface. To explore the effect of the original roughness on metal-ceramic bond strength, two groups of specimen are fabricated by SLM. One group of specimen surface is polished smooth while another group remains the original rough surface. The dental porcelain is fused to the specimens’ surfaces according to the ISO 9693:1999 standard. To gain the bond strength, a three-point bending test is carried out and X ray energy spectrum analysis(EDS), scanning electron microscope(SEM) are used to show fracture mode. The results show that the mean bond strength is 116.5 ± 16 MPa of the group with rough surface(R a=17.2), and the fracture mode is cohesive. However, when the surface is smooth (R a=3.8), the mean bond strength is 74.5 MPa ± 5 MPa and the fracture mode is mixed. The original surface with prominent structures formed by the partly melted powder particles, not only increases surface roughness but also significantly improves the bond strength by forming strong mechanical lock effect. Statistical analysis (Student’s t-test) demonstrates a significant difference (p<0.05) of the mean value of bond strength between the two groups. The experiments indicate the natural rough surface can enhance the metal-ceramic bond strength to over four times the minimum value (25 MPa) of the ISO 9693:1999 standard. It is found that the natural rough surface of SLM-made PFM can eliminate the porcelain collapse defect produced by traditional casting method in PFM restorations.

Fatigue Behavior of Zirconia‐Ceramic, Galvano‐Ceramic, and Porcelain‐Fused‐to‐Metal Fixed Partial Dentures

This study evaluated the fatigue behavior of three fixed partial dentures (FPDs) before and after artificial fatigue testing. Sixty, three-unit zirconia-ceramic (ZC), galvano-ceramic (GC), and porcelain-fused-to-metal (PFM) FPDs (N = 20) were fabricated. Ten specimens from each group were exposed to fatigue testing by being thermocycled (5 to 55°C, 10,000 cycles) and loaded (100,000 cycles, 50 N, 0.5 Hz). All specimens were then subjected to occlusal loading in a universal testing machine until fracture. The fractures were characterized using scanning electron microscopy. Data were analyzed using one-way ANOVA followed by Tukey's significant difference post hoc test and the paired t-test. The chi-squared test was used to evaluate the type of fracture (α = 0.05). The mean fracture loads of non-fatigued and fatigued specimens for ZC were 2434.9 ± 154.3 and 2333.1 ± 183.0 N, respectively; for GC were 1678.1 ± 211.6 and 1475.8 ± 227.9 N, respectively; and 1878.5 ± 176.5 and 1687.8 ± 162.2 N, respectively, for PFM restorations. Significant differences were observed between fatigued and non-fatigued specimens of both the GC group and PFM group (p < 0.05), but not between fatigued and non-fatigued ZC specimens (p > 0.05). Differences between the PFM and GC specimens were not statistically significant for fatigued or non-fatigued specimens (p > 0.05). Fracture types did not differ significantly among groups. ZC specimens were not significantly affected by fatigue, whereas GC and PFM specimens were affected. All tested restorations have the potential to withstand occlusal forces applied in the posterior region.

Comparison of edge chipping resistance of PFM and veneered zirconia specimens

To investigate the chipping resistance of veneered zirconia specimens and compare it to the chipping resistance of porcelain fused to metal (PFM) specimens. Veneered zirconia and PFM bar specimens were prepared in clinically relevant thicknesses. The specimen edges were chipped with different magnitude forces, producing chips of various sizes. The range of sizes included small chips that did not penetrate all the way through the veneers to the substrates, and also chips that were very large and reached the zirconia or metal substrates. The relationship between force magnitude and chip size (edge distance) was graphed. The resulting curves were compared for the veneered zirconia and PFM specimens. Knoop hardness vs. force graphs for the veneers and substrates were also obtained. The zirconia and PFM veneer chipping data followed a power law (coefficient of determination, R(2)>0.93) as expected from the literature. The curves overlapped within the combined data scatter, indicating similar resistance to chipping. The chips made in both types of specimens detached and did not penetrate into the substrate when they reached the veneer/substrate intersections. The hardness-load curves for the veneers and substrates all exhibited an indentation size effect (ISE) at low loads. The Knoop hardness values with uncertainties of +/-one standard deviation at 4N loads for the metal, zirconia, and the metal and zirconia veneers are: (2.02+/-0.08, 12.01+/-0.39, 4.24+/-0.16 and 4.36+/-0.02GPa), respectively, with no statistically significant difference between the veneers (Tukey pairwise comparison at 0.95 family confidence). This work indicates that a similar resistance to chipping might be expected for veneered zirconia and PFM restorations, in spite of the large difference in substrate hardness. Differences in susceptibility to chip spalling were not detected, but the chips in both specimen types detached off the sides in a similar manner instead of extending into the substrates.

Porcelain-Fused-to-Metal Versus Zirconia-Based Ceramic Restorations, 2009

P orcelain-fused-to-metal (PFM) restorations have been the dominant option in esthetic crown and fixedprosthesis restorations for about 50 years. When these restorations were introduced, there was pessimism about their value and their potential for long-term service. However, both research and clinical observation since have shown that they serve well. Research regarding the longevity of service of PFM crowns and fixed prostheses shows variable but relatively impressive results, depending on the number of units replaced, period of observation and location of the study. The method of PFM failure is apparent to clinical practicing dentists. The most destructive type of failure is breakage of a large piece of ceramic from the metal substructure. If the piece is small, the restoration can be smoothed and continue to serve. If it is a larger piece of ceramic, the restoration must be replaced. Another challenge arises when, across time, the gingival tissues recede, the crown margins become visible and the restoration is easily identified by a lay observer as a “false tooth.” Additionally, superficial color placed on crowns and fixed prostheses may abrade or dissolve from the restorations, leaving them chalky and often a lighter color than when they were placed. Again, they appear to be unnatural. Nevertheless, from a purely functional standpoint, the PFM restorations continue to serve long after they have become relatively unesthetic, and if a chip breaks from the restoration, the underlying metal substructure protects the tooth structure until some other restoration can be placed. Dentists trust PFM restorations because of their clinically observable service longevity when compared with several previous generations of all-ceramic crowns. The current generation of zirconia-based all-ceramic crowns and fixed prostheses is becoming popular. I have observed in the continuing education (CE) courses I teach that many general dentists and prosthodontists have used these restorations to a significant degree. Jim Schuck, vice president of sales and marketing of Glidewell Laboratories, Newport Beach, Calif. (the largest laboratory in the United States), estimates that use of all-ceramic restorations is increasing (J. Schuck, oral communication, April 2009). He also estimated Porcelain-fused-to-metal versus zirconia-based ceramic restorations, 2009

The Role of Saliva in Dental Erosion and a Prosthetic Approach to Treatment: A Case Report

The aim of this report is to describe the relationship of some salivary parameters to dental erosion resulting from excessive citric acid consumption and present a description of a prosthetic approach used to restore the damaged dentition of a patient with severe erosion. The high consumption of dietary sources of acids can lead to erosion or the excessive wear of dental hard tissues. Erosion may be modified by salivary parameters such as flow rate, pH, and buffering capacity. Porcelain-fused-to-metal (PFM) restorations and composite resin veneers can be used successfully to restore impaired esthetics and eliminate tooth hypersensitivity in such cases. A 37-year-old woman with a history of excessive lemon consumption presented with a complaint of tooth hypersensitivity and the poor appearance of her dentition due to erosion. Stimulated and unstimulated salivary samples of the patient were evaluated for flow rate, pH, and buffering capacity before and after treatment. The pre-treatment values were found to be higher than post-treatment values. Stimulated samples showed an increase of salivary flow rate, pH, and buffering capacity. The measured parameters put forth the defensive potential of saliva against the acidic diet, and the salivary flow rate and buffering capacity decreased after reducing acidic consumption. The excessively eroded teeth were restored using PFM restorations whereas the superficially eroded teeth were restored with composite resins. The introduction of acidic foods, beverages, or other agents can exceed the natural buffering capacity of saliva. The result is a lowering of the pH of the oral environment which can lead to erosion of enamel and dentin. Loss of tooth structure due to erosion can compromise the esthetics of the dentition and lead to hypersensitivity of the teeth. Teeth damaged by erosion can be successfully restored by composite resin or porcelain restorations and esthetics and function of dentition can be improved. This report is a profound example of how the over consumption of acidic agents affect not only dental tissues but also the chemical balance of the oral environment as well as the oral habitat.

Cold Testing Through Full-Coverage Restorations

Endodontic diagnosis often requires thermal testing through porcelain fused-to-metal (PFM) and all-ceramic restorations. The purpose of this study was to measure and compare the temperature change during thermal testing by three commonly used methods occurring at the pulp-dentin junction (PDJ) of nonrestored teeth and teeth restored with full coverage restorations made of PFM, all-porcelain, or gold. The methods used to produce a thermal change were (a) an ice stick, (b) 1,1,1,2-tetrafluoroethane (TFE), and (c) carbon dioxide snow. A thermocouple measured temperature changes occurring at the PDJ in 10 extracted premolars when thermal tested by each method over a period of 30 seconds. Temperature reduction was also measured for the same samples restored with full gold crowns, PFM, and Empress crowns. Results showed intact premolars and those restored with PFM or all-ceramic restorations to respond similarly to thermal testing. In these teeth, TFE produced a significantly greater temperature decrease than carbon dioxide snow between 10 and 25 seconds (p < 0.05). In conclusion, application of TFE on a saturated #2 cotton pellet was the most effective method for producing a temperature reduction at the PDJ of intact teeth and those restored with gold, PFM, and all-porcelain when testing for less than 15 seconds.

Patient perception of shade match of porcelain fused to metal (PFM) restorations shows a greater satisfaction than that perceived by a prosthodontist. This patient satisfaction input should be taken into account when fabricating a PFM restoration

Original Article Al-Wahadni A, Ajlouni R, Al-Omari Q, Cobb D, Dawson D, Shade-match perception of porcelain-fused-to-metal restorations: a comparison between dentist and patient. J Am Dent Assoc 2002;133(9):1220-5; quiz 1260-1. Level of Evidence 2c Purpose To compare patients' and a prosthodontist's satisfaction with the shade of existing PFM restorations Source of Funding Not reported Type of Study/Design Retrospective cohort study

Margin design for porcelain fused to metal restorations which extend onto the root.

A 90 degree shoulder is probably the most commonly used margin design for porcelain fused to metal (PFM) restorations. However, this design requires the removal of a significant amount of tooth tissue to provide a predictable restoration. This increases the risk of irreversible damage to the pulp, particularly if little tooth tissue is present, for example on exposed roots. In these situations more conservative designs, such as the 135 degree shoulder or deep chamfer, should be considered. Ideally these designs should be used with an all-metal margin since this is associated with the most predictable marginal seal. Where an all metal margin is aesthetically unacceptable a porcelain butt or subgingival margin can be considered. Newer techniques, such as resin bonded crowns, which minimise tooth destruction and provide acceptable aesthetics may be a viable alternative to traditional PFM restorations. Unfortunately, these newer techniques have not been evaluated long-term.

The histopathology of oral mucosal lesions associated with amalgam or porcelain-fused-to-metal restorations.

To analyse the interface stomatitis patterns of oral lichenoid lesions in contact with amalgam and to compare these with the histologic changes in oral lesions clinically associated with porcelain-fused-to-metal (PFM) restorations. To relate these features to the presence of tissue-bound mercury (Hg). A retrospective analysis of tissue biopsies, with clinical data collected via a complementary questionnaire. 479 biopsies diagnosed in 1987 as 'lichenoid reactions'. From these, we retrieved all with amalgam contact and without candida or medication. From 1990-91, all mucosal lesions stated to be associated with PFM restorations were then retrieved for comparative analysis. The biopsies were examined with routine histologic and autometallographic methods. 77 amalgam-associated lesions were found and could be subdivided into five pre-defined interface stomatitis types. We found 22 lesions associated with PFM and 20 showed histopathologic features similar to those associated with amalgam. Hg accumulations were detected in the majority of amalgam-associated but only in part of the PFM-associated lesions. Amalgam-associated lichenoid lesions present a wide spectrum of histopathologic patterns, corresponding to similar patterns in dermatopathology but with no evidence of association with specific disease. PFM-associated lesions tend to display similar lichenoid features, suggestive of common pathogenetic mechanisms. Hg accumulations may play a role to maintain the chronicity of such lichenoid lesions.

Transient and Residual Stress in a Porcelain-Metal Strip

Porcelain-fused-to-metal (PFM) restorations may develop cracks during processing or in-mouth service if the relative physico-mechanical properties of the porcelain and metal are highly mismatched. Precise conditions when this might occur are not known. Many processing and property variations can affect the stresses developed throughout a porcelain-metal system. To understand this, we conducted a computer simulation of stress developed in a PFM beam. The simulation considers cooling from temperatures higher than the porcelain sagpoint. The following temperature-dependent factors were incorporated: the elastic modulus, shear viscosity (porcelain), and coefficients of thermal expansion. The cooling rate dependencies of the glass transition temperature, (Tg), and the temperature distribution during cooling were also included. The results suggest that transient tensile stress at the porcelain alloy interface may result in cracks in the porcelain during cooling. Occlusal forces may set up stresses to cause cracking at the surface of the porcelain if the compressive residual stress is not high enough. PFM restorations with an alloy of high thermal expansion coefficient require rapid cooling; on the contrary, PFM restorations with the alloys of lower coefficients require slow cooling. A high cooling rate can make up for thermal expansion mismatches between the alloy and the porcelain up to 2 x 10(-6)/degrees C. Finally, the results indicated that curvature was not a sensitive indication of stress for a multimaterial beam when visco-elastic relaxation and high cooling rates are involved. For the case modeled here, curvature varied inversely with a 1/2 to 1/7th power of the stress.