Cracking the problem of premature failure of all-ceramic restorations
Objectives: Resin luting all-ceramic restorations results in increased performance, however, the strengthening mechanism and the role of the mechanical properties of the resin are not fully understood. The authors have previously proposed the existence of a resin-ceramic hybrid layer and the hypotheses tested are ceramic strength enhancement is dependent on the elastic modulus of the resin and conferred by the characteristics of the resin-ceramic hybrid layer. Methods: Three-point flexural moduli of a flowable, luting and hybrid composite resin were characterized. 240 porcelain discs were air abraded, one group acted as a control and three additional groups were coated with 120±20mm of each resin prior to bi-axial flexure testing. 300 dentine porcelain discs were polished to a P4000 grade abrasive paper and 150 were centrally indented at 9.8N. A further 150 discs were alumina air abraded. Groups of 30 specimens were coated with resin cement thicknesses varying from 0 to 250 ± 20mm before bi-axial flexure testing. Results: All resins significantly increased the mean strength and the associated strength increase was related to the elastic modulus of the resin (R2=0.9885) so the hypothesis was accepted. Regression analysis enabled calculation of the magnitude of 'actual' strengthening for a theoretical 'zero' thickness of resin cement on each surface texture. Conclusion: The combination of Poisson constraint and the creation of a resin inter-penetrated layer sensitive to the elastic modulus of the resin may provide an explanation of the strengthening mechanism. Accounting for resin bulk strengthening, resin cement coating significantly increased the mean strength which was attributed to a resin-ceramic hybrid layer that is sensitive to surface texture.