Tubule orientation and the fatigue strength of human dentin

In this study the influence of tubule orientation on the strength of human dentin under static and cyclic loads was examined. Rectangular beams were sectioned from the coronal dentin of virgin extracted molars (N=83) and then loaded in quasi-static 4-point flexure or 4-point flexural fatigue to failure. The flexure strength, energy to fracture and fatigue strength were evaluated for specimens with the dentin tubules aligned parallel (θ=0°) and perpendicular (θ=90°) to the plane of maximum normal stress. Results from monotonic loading showed that both the flexural strength and energy to fracture of dentin specimens with θ=0° were significantly greater than those with θ=90°. Furthermore, the apparent endurance strength of dentin with θ=0° (44 MPa) was significantly greater than that of the dentin with θ=90° (24 MPa). The ratio of apparent endurance strength (for fully reversed loading) to the flexure strength for θ=0° and θ=90° was 0.41 and 0.28, respectively. Although the influence of tubule orientation was most important to mechanical behavior, the flexure strength and energy to fracture also decreased with an increase in tubule density. According to differences in the fatigue strength with tubule orientation, restorative practices promoting large cyclic normal stresses perpendicular to the tubules would be more likely to facilitate fatigue failure in dentin with cyclic loading.