Objectives: Resin-cementation of all-ceramic restorations confers an increased resistance to fracture in service however the strengthening mechanisms are not fully understood. The strain-rate dependence of the strength of resin-cements and dental ceramic materials is acknowledged to differ significantly. Therefore the aim was to gain an insight into the resin strengthening mechanisms by observing the rate dependent fracture of a resin-ceramic bilayer.
Methods: 210 nominally identical Vita VM7 disc-shaped ceramic specimens (12.0 mm diameter and 0.9 mm thickness) were silane coated and randomly allocated to six groups (A-F) (n=35).Groups D-F were resin coated with a controlled thickness of Rely-X Veneer cement (150±30 μm). Bi-axial flexure (BFS) testing was performed in a ball-on-ring configuration at loading rates of 0.1 (Groups A and D), 1.0 (Groups B and E) and 10.0 (Groups C and F) mm min-1. Multilayered analytical solutions were used to calculate the BFS and the resultant data was analysed using factorial analyses of variance and Weibull statistics.
Results: The mean BFS of the uncoated specimens loaded at 0.1, 1.0 and 10.0 mm min-1 (Groups A, B and C) were 73.4±9.6, 85.5±18.4 and 95.1±25.5 MPa, respectively and differed significantly (P<0.001). Following resin-coating the mean BFS were increased by 82, 83 and 66% for Groups D, E and F respectively. A factorial analysis of variance demonstrated that resin-coating conferred significant reinforcement of the ceramic (P<0.01) but the magnitude of strengthening was sensitive to loading rate (P<0.01). Significant changes in the Weibull moduli were observed following resin-coating for all three loading rates investigated.
Conclusion: Modification of the pattern of rate dependent fracture of a dental ceramic following resin coating indicates a direct effect of resin infiltration on crack extension. The reinforcement of a dental ceramic by the resin cement is a function of loading rate.