The torsion test offers a new approach for evaluating CAD/CAM material bonding

This study assessed the validity of torsion test, by comparing it with shear test, in evaluating the bonding of resin composite core after different surface treatments to both zirconia and resin composite CAD/CAM blocks. In total, 108-resin composite core cylinders (3 mm × 3 mm) were prepared and stored in distilled water at 37 °C/48 h. The cylinders were subjected to no-treatment, sandblasting, or surface grinding. Cylinders from each treatment group were then bonded with either zirconia or resin composite CAD/CAM blocks using dual-cure resin cement and stored for 24 h. They were subjected to torsion (rotational speed, 0.25 rpm until failure) or shear (crosshead speed, 1.0 mm/min) tests (n = 9 for each core/block combination in each test). All fractured surfaces were observed by scanning electron microscopy, and the maximum principal stress (MPS) was assessed using finite element analysis (FEA). In the torsion test, the sandblasted composite core and resin CAD/CAM block showed significantly lower bond strength than the other groups (p < 0.05), but the shear test results showed no intergroup differences. While 39.89% of the specimens showed cohesion failures in the torsion test, 55.56% showed adhesion failures in the shear test. FEA demonstrated an even distribution of MPS from the outer edge to the center of the cement layer during the torsion test; however, MPS was unevenly concentrated at the bottom edge of the cement layer during the shear test. The torsion test was better than the shear test in bond testing because of the uniform tensile stress distribution and unidirectional displacement at the bond interface.