Structural and Mechanical Properties of the Transparent Dentin Region in the Tooth Root

Dentin is altered in an age-related manner, such that transparent dentin in the tooth root forms during the aging process. Notably, fractures are more likely to occur in aged teeth. However, few studies have examined the mechanical properties of aged dentin. The purpose of this study was to investigate the structural and mechanical properties of the transparent dentin region, by analysis of the mineral density, using micro-computed tomography （micro-CT）, the tensile strength, and nanohardness to understand tooth fracture. As experimental samples, 10 caries-free, human molars were extracted from patients aged >50 years because of periodontal disease. As controls, bovine lower central incisors were extracted immediately after sacrifice from animals estimated at 2–2.5 years of age. Mineral density was quantified with X-ray micro-CT analysis of transparent human dentin and normal bovine dentin. Specimens were prepared from each tooth （human and bovine） and subjected to tensile strength testing; maximum load was used to calculate strength, and mean tensile strength of the specimens was calculated in each group. Finally, the nanohardness and Young’s modulus of intertubular dentin of the upper and apex root regions were investigated. All results were compared by paired Student’s t-test （α=0.05）. The mineral density of transparent dentin was significantly greater than that of normal dentin. Importantly, the tensile strength of the transparent regions was significantly lower than that of the normal regions, whereas the nanohardness and Young’s modulus of transparent intertubular dentin were significantly greater than those of normal intertubular dentin. Fractures of the teeth remain a major problem in clinical practice. Notably, transparent dentin is weaker than normal opaque dentin, which may explain the occurrence of apex root dentin fractures in aged individuals.