Failure of Dentin: Development of a Fracture-mechanics-based Framework

Methods: Measurements of the in vitro fracture toughness were performed on compact-tension specimens (N=16) in Hank's Balanced Salt Solution using dentin extracted from elephant tusk (to allow for larger specimen size); five different orientations (two perpendicular and three parallel) with respect to the dentinal tubules were examined. Unnotched cantilever beams (N=30) were used for stress-life fatigue testing at frequencies of 2-20 Hz.

Results: The fracture toughness was found to be ~55-65% higher for cracks propagating parallel, as opposed to perpendicular, to the dentinal tubules, i.e., K_c ~2.7 MPam^1/2 (s.d.=0.1) compared to 1.6 MPam^1/2 (s.d.=0.1). This anisotropy in toughness has its origins in several toughening mechanisms, most notably from crack bridging. Indeed, theoretical estimates suggest contributions of the order of 0.1 MPam^1/2 and 0.1-0.4 MPam^1/2, respectively, from collagen fibril and uncracked-ligament bridging (in "parallel" orientations), as compared to 0.3 MPam^1/2 from microcracking (in all orientations). Corresponding stress-life results indicated the presence of a frequency-dependent fatigue limit of ~25-45 MPa at 10⁶-10⁷ cycles, with lifetimes decreasing with increasing mean stress at a given stress amplitude.

Conclusions: The fracture toughness of dentin is clearly orientation-dependent, with statistically significant worst-case (lowest) values in orientations perpendicular to the tubules. The fatigue results provide a basis for defining safe-unsafe regimes for dentin fracture and can be used to develop a framework for predicting fatigue-related lifetimes for the failure of this material.

Supported by NIH/NIDCR Grant P01DE09859.