In clinical dentistry, since fracture is a major cause of tooth loss, better understanding of mechanical properties of teeth structures is important. Dentin, the major hard tissue of teeth, has similar composition to bone. In this study, we investigated the mechanical properties of human dentin not only in terms of mineral density but also using structural and quality parameters as recently accepted in evaluating bone strength. Aged crown and root dentin (age ≥ 40) exhibited significantly lower flexural strength and toughness than young dentin (age < 40). Aged dentin, in which the dentinal tubules were occluded with calcified material, recorded the highest mineral density; but showed significantly lower flexural strength than young dentin. Dentin with strong alignment of the c-axis in hydroxyapatite exhibited high fracture strength, possibly because the aligned apatite along the collagen fibrils may reinforce the intertubular dentin. Aged dentin, showing a high advanced glycation end-products (AGEs) level in its collagen, recorded low flexural strength. We first comprehensively identified significant factors, which affected the inferior mechanical properties of aged dentin. The low mechanical strength of aged dentin is caused by the high mineral density resulting from occlusion of dentinal tubules and accumulation of AGEs in dentin collagen.

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Dual-cured resin-based composites are gaining popularity as core build-up materials. Physical and elastoplastic characteristics of new experimental core build-up materials with variable filler contents (+2.5 and +5 wt%) were investigated using classic macroexperiments and nanoindentation. Flexural fracture strength increased from 91.7±11.8 to 114.1±9.1 and 116.6±14.7 MPa in the +2.5 and +5 wt% groups, respectively; similarly, elastic moduli increased from 12.6±0.9 to 15.4±1.2 and 15.3±2.0 MPa, respectively. Fracture toughness increased from 1.82±0.60 to 2.28±0.53 and 2.67±0.88 MPa•m 1/2 for the +2.5 and +5 wt% groups, respectively; indentation hardness increased significantly from 0.45±0.03 to 0.55±0.04 and 0.60±0.10 GPa, respectively. The addition of +5 wt% filler content led to a mechanically superior material; the addition of +2.5 wt% filler led to better reliability.