DENTIN SHEAR-STRENGTH - EFFECTS OF TUBULE ORIENTATION AND INTRATOOTH LOCATION

Objectives. Dentin has a highly oriented tubule structure, and the tub ule number density and area fraction of intertublar dentin vary with d istance from the pulp. This investigation sought to determine the infl uence of tubule orientation on shear strength of dentin from samples d erived at various intratooth locations. Methods. Third molars were sec tioned 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 orie ntations. A paired t-test was used for statistical analysis. In series 2, three samples from central and cuspal areas were tested using thre e different tubule orientations. A two-way ANOVA was used for statisti cal analysis. Each sample had dimensions of approximately 1 x 1 x 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 t wo 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 b y 90 degrees did not exhibit a significant difference (p>0.05), with a n average value of 53.5 +/- 9.5 MPa. Samples oriented with tubules alo ng the long axis of the specimen and tested with shear force applied p erpendicular to the tubule direction had significantly higher (p<0.05; two-way ANOVA) shear strength (78.0 +/- 8.5 MPa). The specimens from the cusp area did not exhibit a statistically significant difference ( p>0.05; two-way ANOVA) with respect to the th ree 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. Significan ce. Results indicated that the shear strength differs in central and c usp 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 i n excess of dentin bond strengths using shear tests, and that fracture s through dentin in such tests result from flaws or stress concentrati on in the dentin.