Ultrastructure and Nanomechanical Properties of CDJ

Study of natural interphases, transition zones or junctions may provide information necessary to engineer functionally graded materials, which can be used for restorative dentistry and other bioengineering applications. Objectives: This study was undertaken to demonstrate that the cementum-dentin junction (CDJ) is of a finite thickness with mechanical properties significantly different from dentin and cementum. Methods: Three - 2 mm thick transverse sections were obtained from the apical two-thirds of the roots of human molars (n=3). The samples were ultrasectioned at room temperature using a diamond knife and an ultramicrotome. The micro and ultrastructures of sectioned surfaces were evaluated under dry and hydrated conditions using AFM and mechanical properties determined by AFM-based nanoindentation techniques. Results: The CDJ predominantly consists of large collagen fiber bundles of 3–5 µm thick, perpendicular to the root surface, with CDJ thickness of 10-25 µm. Dry scanning revealed a valley between cementum and dentin with a depth of 250–500 nm. Scanning in nanopure water transformed the valley to a peak, with the height approximately the same as the depth of the valley. When the dehydrated sample surface was hydrated, absorption of water resulted in swelling of the collagen fibrils within the CDJ. Moreover, the relative hydration of the CDJ was higher than the adjacent cementum or dentin, suggesting that most of the collagen within the zone is not as highly mineralized as that commonly observed in dentin and cementum. AFM based nanoindentation (t-test, P<0.05) showed that the elastic modulus and hardness of the CDJ (14.2±1.0 GPa, 0.46±0.05GPa) under dry conditions were significantly lower than those of dentin (27.9±2.3GPa, P<0.001; 0.96±0.09GPa, P<0.001) and cementum (19.0±1.7GPa, P<0.001; 0.57±0.07GPa P<0.001). Conclusions: CDJ is an interphase of finite thickness with significantly lower mechanical properties than the adjacent hard tissues, dentin and cementum. Supported by NIH/NIDCR Grants T32DE07306 and P01DE09859.