Failures of restorations resulting from tooth fracture are one of the primary problems to lasting oral health. Fatigue cracks are often present in the dentin of restored teeth and their incidence increases with age of the patient. The primary objective of this investigation was to characterize the influence of microstructure, age and ethnicity on the crack growth resistance of human coronal dentin. Compact tension (CT) specimens were prepared from three regions (i.e. inner, middle and outer dentin) of the crown of 3rd molars obtained from young and old patients of the US and Colombia. Stable crack extension was achieved under Mode I quasi�static and cyclic loading. To understand the fundamental mechanisms of crack growth extension, the fracture surfaces were evaluated using scanning electron microscopy (SEM) and image processing techniques. Metrics related to the microstructure were correlated with the fatigue and fracture responses. Lastly, a hybrid approach was also used to quantify the contribution of toughening mechanisms to the overall toughness. Results from the fatigue crack growth experiments showed that deep dentin exhibited the lowest resistance to the initiation of cyclic extension, as indicated by the stress intensity threshold (DKth=0.8 MPa*m0.5), and the highest fatigue crack growth rate. Cracks in the inner dentin underwent incremental extension under cyclic stresses 40% lower than that required in peripheral dentin and grew 1,000 times faster than in peripheral dentin. In addition, the average fatigue crack growth rates increased significantly with tubule density, and with increasing patient age. There was no significant difference between the fatigue crack growth resistance of middle dentin obtained from patients of the US and Colombia. Results from the monotonic crack growth experiments showed that coronal dentin undergoes an increase in the growth resistance with crack extension. Stable crack growth initiated at a stress intensity of 1.35 MPa*m0.5 or greater, and the growth toughness ranged from 0.5 to 2.2 MPa*m0.5. The initiation toughness for outer dentin was approximately 60% higher than that for inner dentin. Furthermore, the fracture toughness of inner dentin (2.2�0.5 MPa*m0.5) was significantly lower than that of middle (2.7�0.2 MPa*m0.5) and outer dentin (3.4�0.3 MPa*m0.5). Overall, cracks oriented perpendicular to the dentin tubules exhibited the lowest crack growth resistance for both quasi-static and cyclic loading. Extrinsic toughening, composed mostly of crack bridging, was estimated to cause an average increase in the fracture energy of 26% in all three regions. The microstructural analysis revealed that a combination of toughening mechanisms were present during crack extension including, micro-cracking of the peritubular cuffs, crack bridging, branching and curving. The potency of these mechanisms of toughening varies with lumen occlusion, density and orientation. Based on these findings, dental restorations extended into deep dentin are much more likely to cause tooth fracture due to the decrease in the crack growth resistance with depth and greater potential for introduction of flaws. That knowledge will help to recognize the critical aspects of the present restorative treatments on the incidence of tooth fracture, and the possible need for new approaches in treatment of senior patients
