Experimental investigation of the shear strengths of teeth in the region of the dentinoenamel junction.

The shear strengths of human incisors, canines, premolars, and molars and bovine incisors in the region of the dentinoenamel junction were tested. The median shear strength of all human teeth, 38.99 MPa, was not significantly different from that of bovine teeth, 37.40 MPa. Among the groups of human teeth, the highest median shear strength was obtained from mandibular premolars (46.15 MPa), and the lowest from mandibular canines (32.63 MPa). Physical treatments (cooling or drying) led to a significant reduction in shear strengths. A similar effect was found after exposure to amino butyrate (a caries-removing agent) but not after exposure to sodium hypochlorite (used in endodontic treatment). In all investigated groups of teeth, the fracture areas were mainly in dentin and never exactly at the dentinoenamel junction; indicating that the cohesion of dentin, not the adhesion between dentin and enamel, is the limiting factor in the shear strength.