The influence of veneering porcelain thickness of all-ceramic and metal ceramic crowns on failure resistance after cyclic loading

https://doi.org/10.1016/S0022-3913(09)60006-8Get rights and content

Statement of problem

In some clinical situations, the length of either a prepared tooth or an implant abutment is shorter than ideal, and the thickness of a porcelain crown must be increased. Thickness of the coping and the veneering porcelain should be considered to prevent mechanical failure of the crown.

Purpose

The purpose of this study was to investigate the influence of veneering porcelain thickness for all-ceramic and metal ceramic crowns on failure resistance after cyclic loading.

Material and methods

All-ceramic and metal ceramic crowns (n=20) were fabricated on an implant abutment (RN Solid Abutment) for the study. Two different framework designs with 2 different incisal thicknesses of veneering porcelain (2 mm and 4 mm) were used for each all-ceramic and metal ceramic crown system, resulting in 4 experimental groups (n=10) with identically shaped crowns. The all-ceramic crown consisted of alumina (Procera AllCeram) frameworks and veneering porcelain (Cerabien), while metal ceramic crowns were made of high noble metal (Leo) frameworks and veneering porcelain (IPS Classic). All crowns were cemented on the corresponding abutments using a resin cement (Panavia 21). They were subjected to 1000 cycles of thermal cycling (5°C and 55°C; 5-second dwell time). The crowns were tested with a custom-designed cyclic loading apparatus which delivered simultaneous unidirectional cyclic loading at 135 degrees, vertically, at an rpm of 250, with a load of 49 N. Each specimen was loaded for 1.2 × 106 cycles or until it failed. The specimens were thoroughly evaluated for cracks and/or bulk fracture with an optical stereomicroscope (x10) and assigned a score of success, survival, or failure. The specimens without bulk fracture after cyclic loading were loaded along the long axis of the tooth, on the incisal edge, in a universal testing machine at a crosshead speed of 1.5 mm/min, until fracture. Fisher's exact test was used to compare the success and survival rate between the 2 different materials (α=.05). Two-way ANOVA was used to analyze data in terms of material, porcelain thickness, and interaction effect. Also, a 2-sample t test was performed to compare between 2 thicknesses within the same material (α=.05).

Results

According to the Fisher's exact test, the all-ceramic group showed significantly higher success (P=.003) and survival rates (P=.001) than the metal ceramic group. For the failure load, the 2-way ANOVA showed significant effects for material (P<.001) and porcelain thickness (P=.004), but not a significant interaction effect (P=.198). For the metal ceramic groups, crowns with a 2-mm porcelain thickness showed a significantly greater failure load than crowns with a 4-mm porcelain thickness (P=.004). However, all-ceramic groups did not show a significant difference between the 2 different thicknesses of veneering porcelain (P=.198).

Conclusions

The all-ceramic crowns showed significantly higher success and survival rates after cyclic loading, but lower failure loads than metal ceramic crowns. The thickness of the veneering porcelain affected the failure load of the metal ceramic crowns, but not that of the all-ceramic crowns.

Section snippets

Material and methods

Two different coping designs based on the thickness of the incisal veneering porcelain were used (Fig. 1, Fig. 2; Table I). Each design was used for the 2 different systems (metal ceramic and all-ceramic), resulting in 4 experimental groups (n=10). The sample size was determined from a pilot study based on 2-way ANOVA (metal versus ceramic framework; long framework versus short framework), and a sample size of 6 in each combination (24 total) was deemed sufficient to have 90% power to detect

Results

Success, survival, and failure of the specimens under cyclic loading are summarized in Table II. Five specimens from the CL and CS groups were considered success, whereas only 1 from ML and none from MS were considered a success. According to the Fisher's exact test, the all-ceramic group showed significantly higher success (P=.003) and survival rates (P=.001) than those of the metal ceramic group after cyclic loading. The CL and CS groups had 3 and 1 failures, respectively, due to the presence

Discussion

For the present study, 2 null hypotheses were addressed for the testing of the ultimate failure load: (1) there would be no significant differences in the failure resistance between 2 different thicknesses of veneering porcelain in the individual crown systems, and (2) there would be no significant difference in the failure resistance between 2 different crown systems. The first null hypothesis was rejected for the metal ceramic system, but it was accepted for the all-ceramic system, which

Conclusions

Within the limitations of this study, the following conclusions were drawn:

  • 1.

    All-ceramic crowns tested showed significantly higher success and survival rates after the cyclic loading test than did metal ceramic crowns.

  • 2.

    Metal ceramic crowns showed significantly greater failure loads than the all-ceramic crowns following cyclic loading.

  • 3.

    The thickness of the incisal veneering porcelain affected the failure load of the metal ceramic crowns, but not that of the all-ceramic crowns. The metal

References (29)

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    Thus, improvement of the ceramic fracture strength is an important issue, especially in posterior teeth, and the IDS technique proved to be effective in increasing the fracture strength of ultrathin occlusal veneers made of PICN. However, the extrapolation of the results must include the limitations of a laboratory study that does not reproduce the clinical scenario, such as the oral environment or masticatory biomechanics (Shirakura et al., 2009). The following limitations of the present study can be cited: a) a constant axial load, which does not occur clinically, was applied to the specimen until fracture; b) the cyclic mechanical loading corresponded to an axial load applied by a pneumatic piston in the occlusal central area of the restoration, not simulating the clinical reality in which forces are applied in different directions; c) the small sample size of the study; however, the statistical analysis could demonstrate significant differences in fracture strength for specimens with and without IDS.

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This study was partially supported by a Greater New York Academy of Prosthodontics Student Grant.

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