Nonuniformity in Periodontal Ligament: Mechanics and Matrix Composition.

The periodontal ligament (PDL) plays a critical role in providing immediate response to abrupt high loads during mastication while also facilitating slow remodeling of the alveolar bone. The PDL exceptional functionality is permitted by the unique nonuniform structure of the tissue. Two distinct areas that are critical to PDL function were previously identified: the furcation and the dense collar. Despite their hypothesized functions in tooth movement and maintenance, these 2 regions have not yet been compared within the context of their native environment. Therefore, the objective of this study is to elucidate the extracellular matrix (ECM) structure, composition, and biomechanical function of the furcation and the collar regions while maintaining the 3-dimensional (3D) structure in the murine PDL. We identify significant difference between the collar and furcation regions in both structure and mechanical properties. Specifically, we observed unique longitudinal structures in the dense collar that correlate with type VI collagen and LOX, both of which are associated with increased type I collagen density and tissue stiffness and are therefore proposed to function as scaffolds for tooth stabilization. We also found that the collar region is stiffer than the furcation region and therefore suggest that the dense collar acts as a suspense structure of the tooth within the bone during physiological loading. The furcation region of the PDL contained more proteins associated with reduced stiffness and higher tissue remodeling, as well as a dual mechanical behavior, suggesting a critical function in loads transfer and remodeling of the alveolar bone. In summary, this work unravels the nonuniform nature of the PDL within the 3D structural context and establishes understanding of regional PDL function, which opens new avenues for future studies of remodeling, regeneration, and disease.

Initial orthodontic tooth movement of a multirooted tooth: a 3D study of a rat molar.

OBJECTIVE: To elucidate the 3D interactions in the tooth-PDL-bone complex immediately after application of orthodontic forces and their implications on tooth movement and function. METHODS: A special visualization method using microCT allows us to directly image in 3D the movements of a multirooted molar tooth inside the alveolar bone as well as the collagenous network of the PDL. Using fresh, unstained rat mandibular 1st molar under mesial loads of 0.5-1 N, we address basic concepts in orthodontics during the initial stages of orthodontic movement. RESULTS: We show that immediately after the application of orthodontic load, direct distinct contacts between the tooth and the bone form in the furcation area. These contacts limit tooth movement and interfere with whole body translation. Only localized sites of highly compressed PDL between the root surfaces and the bone were observed. In general, the collagenous network of the PDL appeared loose and not densely packed in the compressed side. On the tension side, the fibers maintained their overall orientation without any significant extension of the fibers. CONCLUSIONS: Localized direct contact areas between the tooth roots and the bone at the furcation already form within a few minutes of orthodontic tooth movement. This direct and localized bone involvement guides the movement trajectory and provides a mechanism for the miscorrelation found between force levels and tooth movement during the initial stages of an orthodontic tooth movement.