Composite resin hygroscopic expansion cracking of glass-composite bilayers

Objective: This study examined in detail cracking of glass plates bonded to composite resin blocks when aged in water - simulating an all ceramic crown on a composite build-up. Investigated were crack development and crack dimensions with aging time and composite layer thickness. Methods: Glass slides (representing dental porcelain) were sectioned into 10 mm squares and polished to 0.9 mm thickness. The inner surface was particle abraded (50 micron alumina) at 0.5 MPa for 3 sec, cleaned and silanated (3M Ceramic Primer). Composite resin blocks (Z-100) of 10 mm sq were prepared then polished to 2.0 or 4.0 mm thickness. Glass plates were bonded to the composite blocks (Single Bond and Rely X Resin Cement) at a cement thickness of 100 microns. Samples (n=8) were aged in 100% humidity at 37 deg C and observed for spontaneous crack development. Controls remained at room temperature. Results: In aged samples cracking initiated after 7 days at the glass inner surface and propagated with time to the outer surface. Cracking occurred more rapidly in 2 mm vs 4 mm samples, developing for both into an extensive rectilinear pattern over 30 days with median dimensions a=b (p<0.01) of 0.96 (sd=0.17) mm. Calculated as a uniformly loaded plate in bending (r=2a, and assuming fracture at 0.10% strain), the theoretical non-uniform composite expansion creates glass stresses of > 140 MPa. Actual glass fracture stress may be lowered by the particle abrasion. Conclusions: All-ceramic crown performance may be compromised if luted to composite resin cores that have undergone hygroscopic expansion for less than 30 days. (Partially supported by NIDCR P01 DE10976)