Ultrastructure and Nanomechanical Properties of Cementum

The mechanical response of a material is defined by its native ultrastructure, and understanding this relationship is essential for tissue engineering. Objective: To relate the mechanical properties of cementum to its ultrastructure. Methods: Three - 3mm thick transverse sections from apical two-thirds of the roots of human molars were ultrasectioned at room temperature using a diamond knife and an ultramicrotome. The sectioned samples were characterized using AFM and nanoindentation techniques. Confocal laser scanning microscopy (CLSM) was performed on 75-80µm thick ground sections (n=3) prepared from the same roots of the molars. Results: AFM images showed that the older growth bands or lamellae were thicker (3-6µm) than the newer lamellae (1-3µm). The lamellae consisted of highly organized densely packed mineralized collagen fibrils perpendicular to the root surface. In between any two lamellae loosely packed mineralized collagen fibrils were either oriented randomly or at an angle relative to the fibrils within the adjacent lamellae. Additionally, Sharpey’s fibers perpendicular and originating from the root surface 6-8µm in thickness were observed. Under wet conditions these fibers assumed a bundle-like shape as a result of hydration. The CLSM results demonstrated 5-10µm³ spatial distribution of lacunae-canaliculi system within the lamellar-like regions. Owing to the variation in ultrastructure, dynamic stiffness mapping showed a significant variation in the storage modulus (9-14GPa) of cementum. Under hydrated conditions the variation existed but the storage modulus was significantly lower (3-7GPa). Conclusions: Overall, cementum is an inhomogeneous composite material, with a varying modulus. The observed high and low moduli were caused predominantly by the orientation and packing density of the mineralized collagen fibrils within specific regions of the material. Supported by NIH/NIDCR Grants T32DE 07306 and P01DE09859.