Tensile strength of human dentin as a function of tubule orientation and density

Purpose: This study investigated the ultimate tensile strength (UTS) of human dentin as a function of tubule orientation and density. Materials and Methods: Slabs of dentin (ca 0.7 mm thick) were obtained from human third molars by either transversally or longitudinally sectioning the crowns with an Isomet saw. The slabs were gently trimmed to reduce the central area of the coronal dentin to a cross-sectional area of approximately 0.5 mm². The longitudinally sectioned slabs were either trimmed from the mesial and distal sides or from the occlusal and pulpal aspects to permit the tensile load to be applied either parallel or perpendicular to the tubule orientation. The transversally sectioned specimens were obtained at several distances from the pulp and were used to evaluate the effects of tubule density. The trimmed specimens were tested in tension on a Kratos testing machine at 0.5 mm/min. After failure, the UTS of each was calculated and expressed in MPa. The fractured ends of the transversally sectioned specimens were viewed under SEM to calculate the number of tubules per mm² at the site of fracture, and its relation with the UTS was investigated by regression analysis. Results: The UTS of dentin is higher when the load is applied perpendicular to the tubule orientation (80 ± 13 MPa) than when applied parallel to tubule orientation (58 ± 11 MPa, p < 0.05). There was a tendency for dentin to be weaker as the number of tubules at the site of fracture increased, although this relationship was not statistically significant (R² = 0.051, p 0.05). Conclusion: The UTS of dentin is dependent on the tubule direction. Dentin tends to be weaker as the number of tubules per area increases.