Resin Cement Elasticity on the Strengthening Mechanism 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 hypotheses tested are 1) ceramic strength enhancement is dependent on the elastic modulus of the resin by improving stress transfer and 2) the degree of ceramic strength enhancement is dependent on the distribution of flaws resident on the ceramic surface. Methods: Three-point flexural moduli of a flowable, luting and hybrid composite resin were characterised. Dentine porcelain discs were air abraded, one group acted as a control and three additional groups were coated with 120±20μm of each resin prior to testing in bi-axial flexure. Further series of discs were air abraded, acid-etched, polished or polished and indented prior to resin coating with graded thicknesses of a resin luting cement. Testing was carried out under bi-axial flexure to enable examination of the strengthening on differing ceramic surfaces by a regression method. Results: All resins tested significantly increased the mean bi-axial strength and the associated strength increase was related to the resin elastic modulus (R²=0.9885) such that the longevity of all-ceramic restorations can be improved by using higher elastic modulus cements in clinical practice. The degree of strengthening demonstrated a reliance on the ceramic surface condition although discreet defect populations demonstrated a dependence on defect severity. Conclusions: The resin strengthening mechanism was a combination of the compensating Poisson contraction parallel to the crack surface and the cumulative interaction of stresses generated in adjacent flaws that may subsequently act to modify the stress intensity of the most critical defects. It is suggested the longevity of all-ceramic restorations can be improved by using higher elastic modulus cements in clinical practice. The previously proposed strengthening theories of crack bridging and polymerisation shrinkage have been shown to be incorrect.