Microstructural heterogeneity of the collagenous network in the loaded and unloaded periodontal ligament and its biomechanical implications.

The periodontal ligament (PDL) is a highly heterogeneous fibrous connective tissue and plays a critical role in distributing occlusal forces and regulating tissue remodeling. Its mechanical properties are largely determined by the extracellular matrix, comprising a collagenous fiber network interacting with the capillary system as well as interstitial fluid containing proteoglycans. While the phase-contrast micro-CT technique has portrayed the 3D microscopic heterogeneity of PDL, the topological parameters of its network, which is crucial to understanding the multiscale constitutive behavior of this tissue, has not been characterized quantitatively. This study aimed to provide new understanding of such microscopic heterogeneity of the PDL with quantifications at both tissue and collagen network levels in a spatial manner, by combining phase-contrast micro-CT imaging and a purpose-built image processing algorithm for fiber analysis. Both variations within a PDL and among the PDL with different shapes, i.e. round-shaped and kidney-shaped PDLs, are described in terms of tissue thickness, fiber distribution, local fiber densities, and fiber orientation (namely azimuthal and elevation angles). Furthermore, the tissue and collagen fiber network responses to mechanical loading were evaluated in a similar manner. A 3D helical alignment pattern was observed in the fiber network, which appears to regulate and adapt a screw-like tooth motion under occlusion. The microstructural heterogeneity quantified here allows development of sample-specific constitutive models to characterize the PDL's functional and pathological loading responses, thereby providing a new multiscale framework for advancing our knowledge of this complex limited mobility soft-hard tissue interface.

Response of the tooth-periodontal ligament-bone complex to load: A microCT study of the minipig molar.

One strategy evolved by teeth to avoid irreversible damage is to move and deform under the loads incurred during mastication. A key component in this regard is the periodontal ligament (PDL). The role of the bone underlying the PDL is less well defined. We study the interplay between the PDL and the underlying alveolar bone when loaded in the minipig. Using an Instron loading device we confirmed that the force-displacement curves of the molars and premolars of relatively fresh minipig intact mandibles are similar to those obtained for humans and other animals. We then used this information to obtain 3D images of the teeth before and after loading the tooth in a microCT such that the load applied is in the third linear part of the force displacement curve. We observed that at many locations there is a complimentary topography of the cementum and alveolar bone surface, strongly suggesting an active interplay between the tooth and the bone during mastication. We also observed that the loaded tooth does not come into direct contact with the underlying bone surface. A highly compressed layer of PDL is present between the tooth and the bone. The structure of the bone in the upper furcation region has a unique appearance with little obvious microstructure, abundant pores that have a large size range and at many locations the bone at the PDL interface has a needle-like shape. We conclude that there is a close interaction between the tooth, the PDL and the underlying alveolar bone during mastication. The highly compressed PDL layer that separates the tooth from the bone may fulfill a key shock absorbing function.