Effect of occlusal contact size on interfacial stresses and failure of a bonded ceramic: FEA and monotonic loading analyses.

Objective: This study investigated whether contact area (i.e. facet size) would influence the loads necessary for subsurface radial crack formation in porcelain specimens bonded to a dentin analog material.
Methods: Dental porcelain discs (0.5 mm, 1 mm, and 1.5 mm thick) were bonded to compliant bases simulating dentin, and loaded with either a 1 mm, 2 mm, or 3 mm diameter aluminum piston until fracture. Pop-in of the subsurface radial crack from the bonded interface was detected using the acoustic emission (AE) method. Pre-test and post-test finite element analysis (FEA) was used to model the experiment and to calculate subsurface failure stresses.
Results: There were significant differences in loads sustained before fracture according to both the ceramic thickness and the piston diameter (p<0.05; ANOVA and post-hoc Scheffe). Failure loads were found to be proportional to the square of the porcelain thickness. For all thicknesses, significantly higher loads were sustained beneath the 3mm piston than beneath the 1mm piston. FEA calculated failure strengths for the 1 mm thick porcelain (calculated from experimental mean loads) differed significantly for loading with the 1mm piston (168 MPa) or 3 mm piston (60 MPa).
Conclusions: It appears that both ceramic thickness and contact facet size may be clinically controlled to increase load-bearing ability of all-ceramic crowns. Single value strengths may not accurately model bonded dental ceramics; adjustments such as with Weibull scaling may improve accuracy. These results further suggest that small spherical indenters do not create clinically analogous contact conditions.