Dentin shear strength: Effects of tubule orientation and intratooth location

Objectives. Dentin has a highly oriented tubule structure, and the tubule number density and area fraction of intertubular dentin vary with distance from the pulp. This investigation sought to determine the influence of tubule orientation on shear strength of dentin from samples derived at various intratooth locations. Methods. Third molars were sectioned and prepared to provide samples from two locations (center and cusp) and with one of three specific tubule orientations. In series 1, matched pairs of midcoronal samples were tested using two tubule orientations. A paired t-test was used for statistical analysis. In series 2, three samples from central and cuspal areas were tested using three different tubule orientations. A two-way ANOVA was used for statistical analysis. Each sample had dimensions of approximately 1×1×5 mm and was tested in a hydrated state by a single plane lap shear method. Results. The paired dentin samples of the midcoronal dentin in the two orientations had shear strengths of 72.4±15.6 MPa and 78.4±13.2 MPa, and were not significantly different (p>0.05; paired t-test). In the second series, samples from the center location with tubules parallel to the shear plane with applied force in directions rotated by 90° did not exhibit a significant difference (p>0.05), with an average value of 53.5±9.5 MPa. Samples oriented with tubules along the long axis of the specimen and tested with shear force applied perpendicular to the tubule direction had significantly higher (p0.05; two-way ANOVA) with respect to the three orientations (83.6±8.4; 85.2±13.8; 91.8±12.7 MPa). Cuspal areas were stronger than central areas in two of the three orientations tested. Significance. Results indicated that the shear strength differs in central and cusp areas and is dependent on dentin tubule orientation in the central area. Shear strengths were much larger than values reported in shear bond strength tests. This suggests that dentin shear strength is far in excess of dentin bond strengths using shear tests, and that fractures through dentin in such tests result from flaws or stress concentration in the dentin.