Shear bond strength vs interfacial fracture toughness - Adherence to CAD/CAM blocks.

Objective: To compare shear bond strength (SBS) and interfacial fracture toughness (IK) results when assessing the effect of surface roughness and thermocycling on the adherence of a resin composite luting agent (RCLA) to a CAD/CAM resin composite block (RCB).

Methods: Tetric CAD HT along with the recommended bonding system, Adhese Universal and Variolink Esthetic LC, were used. Surface roughness was achieved with 600/320/60 grit SiC papers. Samples were stored 24h in 37°C water or thermocycled 10000× (5°C-55°C) prior to testing. Results were analyzed by univariate ANOVA and Scheffé modified t-tests (α=0.05). Fractured specimens were viewed with a stereo microscope and selected specimens with a scanning electron microscope.

Results: SBS results showed a significant difference between the 60 grit group and the other groups, both after 24h and thermocycling. A large number of SBS samples showed cohesive fracture or subsurface damage in RCB. Thermocycling led to a significant decrease in SBS in all groups. IK results showed no significant differences due to surface preparation after 24h storage in 37°C. After thermocycling, there was a significant difference between the 60 and the 600 grit groups. All K samples fractured adhesively at the RCB surface. K of the RCLA was significantly higher than IK of all groups.

Significance: The results endorse the use of fracture mechanics methodology for the assessment and characterization of adherence, while identifying difficulties in its implementation. The results suggest also that adherence to CAD/CAM RCB may be limited by the strength of the resin composite block - adhesive interface.