Tailoring Mechanical Testing to Chairside CAD/CAM Blocks

Objectives: Dimensions of most chairside CAD/CAM blocks for single unit restorations (i.e. 12x14x18mm³) impose important limitations in their mechanical testing (e.g. flexural strength (σ), fracture toughness (K_Ic), fatigue parameters), as several percent error may arise when using miniaturized bending bar fixtures (<20 mm span length). Therefore, the aim of this study was to evaluate the Ball-on-Three-Balls (B3B) biaxial strength test and the Compact-Tension (CT) test as suitable alternatives for mechanical testing of small CAD/CAM blocks.
Methods: Sixty plates (12x12x1.25mm³) were cut from C14 IPSe.maxCAD and C15 IPSe.maxZirCAD (Ivoclar-Vivadent) CAD/CAM blocks. After respective crystallization and sintering procedures, specimens were sequentially polished with SiC paper (Grit 600 down to 4000) on one surface. Initial biaxial strength (σ₀) was determined using the B3B fixture (n=30), with the polished surface loaded in tension. Obtained data was analyzed with a two-parameter Weibull distribution function. Cyclic fatigue testing (n=30) was conducted using the B3B assembly at sinusoidal loading with R(σ_min/ σ_max)=0.3 and 10Hz cycling frequency. Specimens were cycled in deionized water until fracture and the characteristic number of cycles (N₀) was determined. Weibull moduli of strength (m) and fatigue (m*) data were used to calculate the fatigue parameter n. Twelve CT specimens (12.5x12x2.5mm³) of each material were cut and notched for K_Ic determination, following the ASTM E 1820 standard. Specimens were fractured in tension and the results used to calculate K_Ic.
Results: See Table 1.
Conclusions: In light of the results, both evaluated methods in this study offered efficient alternatives for standardized mechanical testing of small-sized CAD/CAM blocks, not only limiting size-dependent inaccuracies, but also reducing specimen’s preparation time and cost.