Thermal incompatibility analysis of metal-ceramic systems based on flexural displacement data.

The feasibility of simple tests or analytical methods for prediction of residual stress states in metal-ceramic (MC) prostheses has not been demonstrated. Biomaterial metal-ceramic strips have been proposed to provide sensitive measures of transient and residual stress states through the measurement of midpoint deflection after cooling from the ceramic sintering temperature. The objective of this study was to apply the elastic-viscoelastic analogy to calculate transient and residual midpoint deflections in MC biomaterial strips and to compare these values with deflections measured with a beam-bending viscometer (BBV). Calculations and measurements were made for five MC systems that were found from a clinical study to be "thermally compatible" systems. Metal strips, 64 mm in length, 3 mm wide, and either 0.5 mm, 1.0 mm, or 2.0 mm in thickness, were veneered with four 0.25-mm thick layers of opaque porcelain. Midpoint deflection of the MC strips (ceramic oriented in the posterior position) was measured during cooling from an initial temperature of 700 degrees C. In general, the directions of the measured residual deflections did not agree with the "textbook" convention that negative deflections are associated with positive thermal contraction mismatch (alpha(M) - alpha(c) > 0) regardless of metal thickness. For a metal thickness of 0.5 mm, the residual midpoint deflection for all thermal contraction mismatch cases, except one, was positive (upward deflection) whereas the residual midpoint deflections were all negative when the metal thickness was increased to 1 or 2 mm, independent of the thermal contraction mismatch. The best agreement between calculated and measured values of residual midpoint deflection (+16 microns vs. +14 +/- 2.3 microns, respectively was obtained for MC biomaterial strips with a Ni-Cr alloy (0.5 mm thick) while the largest difference (+346 microns vs. +61 +/- 43.8 microns) was obtained for MC bimaterial strips with a Au-Pd allow (0.5 mm thick). In all but one case, changes in deflection direction as a function of metal thickness were correctly predicted by the viscoelastic analysis. The results of this study indicate that a viscoelastic model is useful for estimating thermal compatibility conditions of MC systems.

Effect of grinding and fluoride-gel exposure on strength of ion-exchanged porcelain.

Strengthening of dental porcelain through a diffusion heat treatment at 450 degrees C of a potassium-enriched, ion-exchange surface coating has been demonstrated in several recent studies. However, little attention has been focused on the potential strength reduction of these materials when the treated surfaces are ground or etched under clinically simulated conditions. The objective of this study was to test the hypothesis that partial removal of the surface layers of ion-exchanged porcelains by grinding or exposure to acidulated fluoride gel will significantly reduce their flexure strength. Nine groups of body porcelain disks were ion-exchanged at 450 degrees C for 30 min. One of these groups was subjected to ion exchange and no further surface treatment. Eight specimen groups were subjected to the following procedures after ion exchange: grinding to depths of 50 microns, 100 microns, 150 microns, 200 microns, and 250 microns, and exposure to acidulated fluoride for 30 min, 60 min, and 300 min. A tenth group (FC) was fired at 960 degrees C and fast-cooled in air, but the disks were not subjected to the ion-exchange treatment. Surface stress was calculated from measured values of cracks induced in the treated surfaces. Fluoride exposure for up to 60 min resulted in a significant decrease in surface compression (P < or = 0.05), although this treatment had no effect on strength. Grinding to a depth of from 100 microns to 250 microns caused a significant decrease in strength, while removal of a 50-microns layer caused no significant change (P > 0.05).

Effects of initial temperature and tempering medium on thermal tempering of dental porcelains.

The objective of this study was to test the hypothesis that quenching of porcelain in silicone oil rather than in compressed air will significantly increase the flexure strength by reducing the potential for crack formation during transient cooling. A secondary hypothesis to be tested is that the initial tempering temperature can be reduced significantly below the porcelain maturing temperature of 982 degrees C but well above Tg without a decrease in strength. Opaque-body porcelain disks, 16 mm in diameter and 2 mm in thickness, with a thermal contraction mismatch (delta alpha) of -1.5, 0, and +3.2 ppm/degrees C were tempered from initial temperatures of 650, 750, 850, and 982 degrees C in silicone oil with kinematic viscosities of 50, 1000, and 5000 centistokes. Porcelain disks were also subjected to three cooling procedures in air: slow cooling in a furnace (SC), free convective cooling in a laboratory bench (FC), and tempering (T) by blasting the surface of body porcelain with air. The crack size induced by a Vickers microhardness indenter was measured within one minute after crack development. For determination of the influence of initial cooling temperature on biaxial flexure strength, six body porcelain disks (delta alpha = 0) were tempered in air from initial temperatures of 650, 750, 850, and 982 degrees C. The mean crack size of specimens tempered in oil was significantly smaller (p < or = 0.001) than that of specimens that were slowly-cooled or fast-cooled in air for all thermal contraction mismatch cases.(ABSTRACT TRUNCATED AT 250 WORDS)

Tensile stress in glass-ceramic crowns: effect of flaws and cement voids.

The objective of this study was to analyze the relative effect of loading site, occlusal thickness, ceramic flaws, elastic modulus of the cement, and voids in the cement layer on tensile stress that develops in molar glass-ceramic crowns under applied loads. Finite-element stress analyses were performed. Resin cement with a thickness of 0.05 mm was used. A central conical flaw (0.05 mm [diameter] x 0.05 mm) and a circular grooved flaw located under the cusp tips were included in all flaw cases. A void space confined within the occlusal region of the cement layer was also included in selected cases. For a ceramic thickness of 0.5 mm and a vertical distributed load applied at a distance of 1.3 mm from the vertical axis, the maximum tensile stresses were 100 MPa for a crown with flaws and a void, 87 MPa for a crown with no flaws and a void, and 75 MPa for a crown with flaws and no void. For a 1.5-mm-thick crown with flaws and a void, the tensile stress decreased to 22 MPa. When the load of 600 N was concentrated at the central point of the occlusal region, the peak tensile stress in a crown with flaws and no void was increased to 325 MPa. For the conditions analyzed in this study a large void in a flawed occlusal region of a thin molar crown (0.5 mm) is proposed as a mechanism of crown failure.

Effect of thermal tempering on strength and crack propagation behavior of feldspathic porcelains.

The objective of this study was to test the hypothesis that tempering stress can retard the growth of surface cracks in layered porcelain discs with variable levels of contraction mismatch. Porcelain discs, 16 mm in diameter and 2 mm thick, were prepared with a 0.5-mm-thick layer of opaque porcelain (O) and a 1.5-mm-thick layer of body porcelain (B). The materials were selected to produce contraction coefficient differences, alpha O-alpha B, of +3.2, +0.7, -0.9, and -1.5 ppm/degrees C. Body porcelain discs with a thickness of 2 mm were used as the thermally compatible control specimens (delta alpha = 0). The discs were fired to the maturing temperature of body porcelain (982 degrees C) and were then subjected to three cooling procedures: slow cooling (SC) in a furnace, fast cooling (FC) in air, and tempering (T) by blasting the surface of the body porcelain with compressed and dried air for 90 s. The dimensions of cracks induced by a Vickers microhardness indenter under a load of 4.9 N were measured at baseline and six months after indentation at 80 points along diametral lines within the surface of body porcelain. In addition, biaxial flexure tests were performed to determine the influence of mismatch and tempering on flexure strength. The results of ANOVA indicate that crack dimensions were influenced significantly by the interaction of cooling rate and contraction mismatch (p less than 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)

Three-dimensional finite element analysis of glass-ceramic dental crowns.

Because of the improved esthetic potential of glass-ceramic crowns as dental restorations, they are sometimes preferred over metal-ceramic crowns for restoration of anterior teeth. Because of their relatively high strength, these ceramic crowns are also frequently used for restoration of posterior teeth. However, due to the larger magnitude of biting forces on posterior teeth, intraoral fracture of all-ceramic crowns tends to occur more frequently in posterior crowns (Moffa, 1988). The objective of this study was to determine the relative influence of load orientation and the occlusal thickness of posterior ceramic crowns on the stress distribution which develops under these loading and design conditions. Three-dimensional finite element models for a molar crown were developed to determine the stress distribution under simulated applied loads. Glass-ceramic crowns with occlusal thicknesses of 0.5, 1.5, and 3.0 mm were considered. The largest principal tensile stresses induced in ceramic due to a distributed load of 600 N applied in a cuspal region were approximately 12 and 182 MPa for vertical and horizontal loading orientations, respectively. Stresses which developed in the facial and lingual marginal regions were primarily compressive under vertical loads. However, tensile stresses developed when the load was applied horizontally. Differences in stress distribution within crowns with the three occlusal thicknesses occurred only near the site of loading. Because of the relatively large failure rates of ceramic crowns in the posterior regions, these restorations should be strengthened by improvement in design, composition, and thermal processing conditions. Before any significant progress is made in these areas, these restorations should be used for the anterior teeth. The results of this study suggest that orientation of the applied load has a more important effect on development of large tensile stresses than the occlusal thickness of ceramic.

Influence of tempering and contraction mismatch on crack development in ceramic surfaces.

Tempering of glass produces a state of compressive stress in surface regions which can enhance the resistance to crack initiation and growth. The objective of this study was to determine the influence of tempering on the sizes of surface cracks induced within the tempered surfaces of opaque porcelain-body porcelain discs, with contraction coefficient differences (alpha O-alpha B) of +3.2, +0.7, 0.0, -0.9, and -1.5 ppm/degrees C. We fired the discs to the maturing temperature (982 degrees C) of body porcelain and then subjected them to three cooling procedures: slow cooling in a furnace (SC), fast cooling in air (FC), and tempering (T) by blasting the body porcelain surface with compressed air for 90 s. We used body porcelain discs as the thermally compatible (delta alpha = 0) control specimens. We measured the diameters of cracks induced by a microhardness indenter at an applied load of 4.9 N at 80 points along diametral lines within the surface of body porcelain. The mean values of the crack diameters varied from 75.9 microns (delta alpha = -1.5 ppm/degrees C) to 103.3 microns (delta alpha = +3.2 ppm/degrees C). The results of ANOVA indicate that significant differences in crack dimensions were controlled by cooling rate, contraction mismatch, and their combined effect (p less than 0.0001). Multiple contrast analysis (Tukey's HSD Test) revealed significantly lower (p less than 0.05) crack sizes for tempered specimens compared with those of fast-cooled and slow-cooled specimens.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of incisal length of ceramic and loading orientation on stress distribution in ceramic crowns.

For ceramic crowns, the recommended depth of tooth reduction from the incisal edge of anterior teeth is 1.5 mm to 2.0 mm. Although some prosthodontists have suggested that incisal heights of ceramic which exceed 2 mm are associated with dangerously high intra-oral stresses, this theory has not been verified. The objective of this study was to test the hypothesis that the stress distribution in ceramic crowns designed for a prepared maxillary central incisor which are subjected to applied loading is relatively insensitive to the incisal length of ceramic. Finite element stress analyses were performed on three crown designs loaded with a horizontal or vertical force of 200 N along the lingual surface near the incisal edge. Ceramic crowns for maxillary central incisors were modeled with incisal lengths of 1.0 mm (Case I), 1.9 mm (Case II), and 4.0 mm (Case III). Zinc phosphate cement with a film thickness of 30 micron was included for each case. Plane-stress finite element analyses indicated that tensile and compressive stresses which were induced in cement and ceramic due to a vertically applied load of 200 N were comparable in magnitude for all three cases within the gingival third of the crowns. For Cases I, II, and III, tensile stresses at the facial region were 6.7, 5.4, and 6.5 MPa, respectively, in cement and 46.2, 48.6, and 49.2 MPa, respectively, in the ceramic. The results of this study tend to support the hypothesis that the amount of tooth reduction (1 to 4 mm) in an incisogingival direction does not significantly influence the stress distribution in the crown or cement layer.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of contraction mismatch and cooling rate on flexural failure of PFM systems.

The interactive influence of cooling rate and the sign and magnitude of thermal contraction difference between metals and ceramic veneers on bond strength have not been extensively analyzed, although numerous bond-test studies have been reported during the past two decades. A previous analytical study of residual incompatibility stress in bond-test specimens indicated that bond strength values may be of relatively little value if the residual stress state of the metal-ceramic specimens is not considered. The objective of this study was to determine the influence of cooling rate and contraction mismatch on the flexural failure resistance of metal opaque-porcelain strips. Specimens were subjected to four-point loading in an Instron testing machine until crack initiation occurred at the metal-ceramic interface. The residual stress states in the ceramic region were estimated from finite element stress analyses of the bond-test specimens by use of dilatometry data obtained at the cooling rate of 3 degrees C/min. The total stress induced from the residual stress and the applied flexural load was also determined for these specimens. Statistical analyses of the experimental data revealed that the slowly cooled specimens exhibited a significantly lower (p < 0.05) flexural strength compared with rapidly cooled specimens. Regardless of the cooling technique, metal-ceramic specimens with a negative thermal contraction difference (alpha m - alpha p < 0) failed at significantly lower (p < 0.05) flexural loads than did specimens with a positive thermal contraction difference.

Stress distribution in metal-ceramic crowns with a facial porcelain margin.

The use of metal-ceramic restorations with porcelain butt-joint facial margins has increased in the past several years. Although these crowns exhibit improved esthetics compared with metal-ceramic crowns which display a metal gingival collar or metal knife-edge margin, the effectiveness of this design in resisting intra-oral forces is not known. The objective of this study was to analyze the stress distribution induced by simulated intra-oral loads on crowns with variable coping configurations. The copings, with a thickness of either 0.1 or 0.3 mm, were modeled with a facial termination of metal at three locations: at the gingival floor, 0.9 mm above the gingival floor, and 4.2 mm above the gingival floor. The coping and crown dimensions were based on a prepared maxillary central incisor with a facial shoulder and a lingual chamfer. Both Ni-Cr and Au-Pd alloy copings were employed in the crown models. Finite element stress and analyses were performed on crowns which were subjected to several loading conditions. A cement film thickness of 0.030 mm was assumed. For all cases, the stresses which developed in porcelain and cement near the facial and lingual margins due to a vertical load of 200 N were predominantly compressive in nature. For the crowns with Ni-Cr copings, the tensile stress in porcelain ranged from 11.0 MPa (for crowns with a facial metal thickness of 0.3 mm) to 12.5 MPa (for a metal thickness of 0.1 mm). The corresponding stresses for crowns with Au-Pd alloy copings were 8.3 MPa and 8.6 MPa, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of metal thickness on stress distribution in metal-ceramic crowns.

The objective of this study was to calculate the stress distribution induced in anterior metal-ceramic crowns fabricated with either gold-alloy or nickel-alloy copings of reduced thickness using plane stress analyses. Two-dimensional finite element models of three crown designs were subjected to a simulated biting force of 200 N which was distributed over porcelain near the lingual metal-ceramic junction. Based on plane stress analyses, the maximum tensile and compressive stresses in porcelain for the three cases were 29.5 MPa and 123.1 MPa, respectively. The highest tensile strains in porcelain for veneered Ni-Cr and Au-Pd copings with conventional dimensions were 0.016% and 0.014%, respectively. The maximum stresses and strains in porcelain for the crowns with a conventional coping thickness (0.3 mm) and a reduced coping thickness (0.1 mm) were not significantly different. All values were below the critical failure values of porcelain.