Effect of Post-Processing Heat Treatment on the Crack Healing Behavior and Apparent Fracture Toughness of a Heat-Pressed Dental Ceramic

Esthetic ceramic restorations are in high demand but continue to be plagued by fracture due to their brittle nature. Flaws can be induced during the laboratory phase of fabrication and lead to premature failure of the restoration. Objectives: to investigate the effect of post-processing heat treatments on the crack healing behavior and strength of a heat-pressed dental ceramic (OPC^®3G^TM, Jeneric/Pentron). Methods: Bar-shaped wax patterns (24x4x2mm, n=30) were invested and heat-pressed according to the manufacturer’s recommendations in an automated pressing machine. After devesting, the specimens were polished to 1200 grit, annealed at 600°C for one hour, and furnace cooled. All bars were indented in oil with a Vickers indenter under a 9.81N load and randomly assigned to three groups. The strength of the control group was determined by three-point bend in a universal testing machine. The other two groups were subjected to a 30-minute heat treatment at either 800° or 825°C. The specimens were examined using 1000x optical microscopy and the indentation-induced cracks were no longer discernable on the surface of the specimen after the heat treatment. Results: ANOVA and Tukey’s test showed that the mean apparent fracture toughness of the group heat treated at 825°C for 30 minutes (3.247±0.480 MPa ^. m^0.5 ) was significantly higher (p<0.001) than that of the untreated control group (2.701±0.085 MPa ^. m^0.5 ) but not significantly different from that of the group heat treated at 800°C for 30 minutes (2.945±0.335 MPa ^. m^0.5 ). Conclusions: Heat treatment in air at 825ºC for 30 minutes eliminated visible cracks of a heat-pressable lithium disilicate glass-ceramic, which led to a significantly higher apparent fracture toughness compared to the untreated control group. Supported by NIH-NIDCR Grant No. DE 13570.