Why a zero CTE mismatch may be better for veneered Y-TZP structures

Objective

Compare residual stress distribution of bilayered structures with a mismatch between the coefficient of thermal expansion (CTE) of framework and veneering ceramic. A positive mismatch, which is recommended for metal-ceramic dental crowns, was hypothesized to contribute to a greater chipping frequency in veneered Y-TZP structures. In addition, the multidirectional nature of residual stresses in bars and crowns is presented to explore some apparent contradictions among different studies.

Methods

Planar bar and crown-shaped bilayered specimens with 0.7 mm framework thickness and 1.5 mm porcelain veneer thickness were investigated using finite element elastic analysis. Eight CTE mismatch conditions were simulated, representing two framework materials (zirconia and metal) and six veneering porcelains (distinguished by CTE values). Besides metal-ceramic and zirconia-ceramic combinations indicated by the manufacturer, models presenting similar mismatch values (1 ppm/°C) with different framework materials (metal or zirconia) and zirconia-based models with metal-compatible porcelain veneers were also tested. A slow cooling protocol from 600 °C to room temperature was simulated. The distributions of residual maximum and minimum principal stresses, as well as stress components parallel to the long axis of the specimens, were analysed.

Results

Planar and crown specimens generated different residual stress distributions. When manufacturer recommended combinations were analysed, residual stresses obtained for zirconia models were significantly higher than those for metal-based models. When zirconia frameworks were combined with metal-compatible porcelains, the residual stress values were even higher. Residual stresses were not different between metal-based and zirconia-based models if the CTE mismatch was similar.

Significance

Some conclusions obtained with planar specimens cannot be extrapolated to clinical situations because specimen shape strongly influences residual stress patterns. Since positive mismatch generates compressive hoop stresses and tensile radial stresses and since zirconia-based crowns tend to be more vulnerable to chipping, a tensile stress-free state generated with a zero CTE mismatch could be advantageous.