The effect of thermocycling on the strength of porcelain laminate veneer (PLV) materials

Abstract

Objectives. Postoperative cracking and resultant failure of porcelain laminate veneer (PLV) restorations are considered a possible consequence of thermal variations that the restoration is likely to encounter ‘in service’. The purpose of the study was to examine the impact of thermocycling on the development of surface flaws on the ‘fit’ surface of PLV restorations. Methods. Sets of 30 Vitadur-Alpha dentin porcelain discs (15 mm diameter, 0.9 mm thickness) were thermocycled on the glazed and unglazed surfaces at three different temperature regimes to simulate the conditions encountered in service. Mean fracture strengths, standard deviations and associated Weibull Moduli (m) were determined using bi-axial fracture (ball-on-ring). Results. The student- t-test analysis revealed a significant difference ( P<0.05) between the porcelain control group mean strength values exposed on the glazed and unglazed surfaces. The one-way analysis of variance revealed no significant differences between the means of the porcelain specimens exposed to different thermocycling regimes. However, a discontinuity existed at the lower strength values in the survival probability plots for porcelain specimen groups that were thermocycled. Significance. Large flaws on the surface of the specimens may become extended due to the thermocycling regimes imposed. The greater the tensile stresses imposed on the disc specimen surface by the thermocycling regimes employed the more likely the flaws are to become extended resulting in premature fracture. Consequently, the discontinuity in the survival probability distributions may be attributed to a different defect mechanism (possibly an extension of large surface flaws) superimposed on the distributions at these low values of strength.