Stress-rate Modification of the Strength of a Resin-cemented Dental Ceramic

Objectives:  For many classes of dental ceramic material, adhesive cementation confers an increased resistance to fracture. The objective was to investigate the impact of resin-cement coating on the stress-rate dependency of the bi-axial flexure strength of a dental ceramic.

Methods: 300 nominally identical, 12.0 mm diameter and 1.0 mm thickness Vita VM7 disc-shaped ceramic specimens were silane coated and randomly allocated to ten groups. Five groups were coated with120±30 μm of Rely-X Veneer cement and five remained uncoated. Bi-axial flexure (BFS) strength was determined in a ball-on-ring configuration for a group of coated and uncoated specimens at each loading rate of 2.5, 10, 40, 160, 640 N.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. Flexural moduli for Rely-X Veneer cement at each stressing rate were determined from beams loaded in three-point-bending. Fractographic analyses of failed specimens were performed according to NIST protocols.

Results: The mean BFS of the uncoated specimens increased from 87.7±12.3 MPa loaded at 2.5 N.min^-1 to 97.7±9.9 MPa loaded at 640 N.min^-1 and was significantly influenced by stressing rate (P=0.06). Regression analysis demonstrated a loglinear relationship between stressing rate at BFS (R²=0.95). The resin coated specimens were significantly strengthened (P<0.001) but the magnitude of strengthening (58 to 73%) was sensitive to stressing rate (P<0.05). Fractographic analysis of failed specimens demonstrated competing failure mechanisms of Hertzian cone cracking and tensile crack extension for specimens loaded at 2.5 N.min^-1

Conclusion:  Clinically dental ceramic restorations may fracture under the application of low magnitude loads as a consequence of sub-critical (slow) crack growth. Our findings demonstrate significant reinforcement is conferred by resin-cement coating across a range of stress rates but the pattern of rate dependent fracture suggests the failure mechanisms differ.