The effect of thermocycling on the fracture toughness and hardness of core buildup materials

Thermocycling has been shown to cause surface degradation of many dental materials, but its effect on the fracture toughness and hardness of direct core buildup materials is unknown.This study was designed to determine the effect of thermocycling on the fracture toughness and hardness of 5 core buildup materials.Fifteen specimens were prepared from each of the following materials: Fluorocore, VariGlass VLC, Valiant PhD, Vitremer, and Chelon-Silver. American Standard for Testing Materials guidelines for single-edge notch, bar-shaped specimens were used. Ten specimens of each material were thermocycled for 2000 cycles; the other 5 specimens were not thermocycled. All specimens were subjected to 3-point bending in a universal testing machine. The load at fracture was recorded, and the fracture toughness (K(IC)) was calculated. Barcol hardness values were also determined. Data were analyzed with 1-way analysis of variance and compared with the Tukey multiple range test (P.05). Pearson's correlation coefficient was also calculated to measure the association between fracture toughness and hardness.Fluorocore had the highest thermocycled mean K(IC) and Valiant PhD the highest non-thermocycled K(IC). Chelon-Silver demonstrated the lowest mean K(IC) both before and after thermocycling. One-way analysis of variance demonstrated significant differences between conditions, and the Tukey test showed significant differences (P.05) between materials for both conditions. Most specimens also showed significant hardness differences between conditions. Pearson's correlation coefficient indicated only a mild-to-moderate correlation between hardness and fracture toughness.Within the limitations of this study, the thermocycling process negatively affected the fracture toughness and hardness of the core buildup materials tested.