Influence of a cylindrical crosshead on shear bond testing of composite-tooth interfaces.

The effect of a cylindrical crosshead on shear bond strength of composite to dentin was assessed by finite element analysis (FEA) after shear bond testing thirty mandibular molars restored with composite cylinders (Tetric EvoCeram-3 mm diameter) perpendicular to conditioned dentin. Cross-sectional geometric models were created with quadrilateral linear plane stress elements. Stress distributions for normal, shear, and major principal stresses were plotted. Mean shear bond strength for the experimental group was 23.9 MPa (+/-4.54), about 15-75% higher than values reported with other methods. FEA showed localized 'hot spots' (+/-16-20 MPa) at the corners of the base of the adhesive layer under a 20 N vertical load. Principal stresses across the composite-dentin interface were lower (+/-12-16 MPa), but significantly homogenous, approximating closely the nominal strength value. With uniform stress distributions across the adhesive layer, FEA confirmed that a cylindrical crosshead may be an optional tool to improve shear bond testing of dental materials.