Physical Strength Properties of CAD/CAM Hybrid Composite Resin Blocks

Objectives: Hybrid composite resin blocks (HCR-Bs) for dental CAD/CAM systems have higher physical strengths than light-cured composite resins because HCR-Bs are polymerized at high pressure and temperature. While most studies investigated flexural strength of HCR-Bs, their tensile strength has been scarcely evaluated despite being a brittle material. This study aimed to investigate physical strength properties of commercial HCR-Bs by examining their flexural and tensile strengths.
Methods: Ten commercial HCR-Bs were tested as follows:
Flexural strength test: HCR-Bs were sectioned into slabs (14mm-length, 4mm-width, 1.2mm-thickness; n=10), stored in water (37°C, 24h), and subjected to 3-point flexural testing (Instron universal testing machine; support span: 12mm, crosshead speed: 1mm/min).
Indirect tensile strength test: HCR-Bs were milled into cylindrical specimens (4mm-diameter, 4mm-height; n=10) using a CAD/CAM milling machine (DWX-51D, DGSHAPE), stored in water (37°C, 24h), and subjected to indirect tensile testing (Instron universal testing machine; load applied diametrically to specimens, crosshead speed: 1mm/min).
Data obtained were used to calculate correlation coefficients between flexural and indirect tensile strengths.
Results: Indirect tensile strength values (74 to 100MPa) of HCR-Bs were less than half of their flexural strength values (170 to 271MPa).
A positive correlation was found between flexural strength and indirect tensile strength for all HCR-Bs (correlation coefficient=0.715).
Conclusions: Being a brittle material, HCR-Bs tended to show higher strength against compressive stress than their resistance to tensile stress. Accordingly, in the flexural test, fractures of HCR-B slabs occurred at a point that was subjected to tensile stress, which is on the opposite side from the surface with a loading point. As demonstrated by the test results, among the HCR-Bs, those with higher tensile strength have higher flexural strength.
This study suggests that high resistance to tensile stress is required for designing HCR-Bs with high physical strength properties.