UCSF

The aim of this study was to investigate the effect of reduced functional loads on the spatio-temporal changes in physico-chemical characteristics: structure, mineral composition, and mechanical properties of the bone-periodontal ligament-cementum complex using a rat model. In most vertebrate systems, mechanical loading of the dento-alveolar joint results in load-dependent adaptation of the hard (bone, cementum) and soft (periodontal ligament [PDL]) tissues of the joint and their interfaces. Within this load-mediated adaptation are load-independent changes due to innate physiological tooth drift. In order to investigate load-mediated effects due to reduced functional loads, two groups of six-week-old male Sprague-Dawley rats were fed nutritionally identical food in hard pellet (stiffness range: 127-158N/mm) or soft powder (stiffness range: 0.32-0.47N/mm) forms. Temporal adaptation was mapped by identifying physico-chemical changes of the bone-PDL-cementum complex in rats of ages six, eight, twelve, and fifteen weeks. Spatial adaptation was mapped by identifying physico-chemical changes at the coronal, middle, and apical portions of the bone-PDL-cementum complex in rats. Adaptation of the bone-PDL-cementum complex due to reduced functional loads was identified by mapping changes in PDL-collagen orientation and birefringence using histochemistry and polarized light microscopy, bone and cementum morphology using micro X-ray computed tomography, mineralization of the PDL-cementum and PDL-bone interfaces by X-ray attenuation, and changes in microhardness of bone and cementum by microindentation.

Structurally altered PDL orientation, decreased PDL collagen birefringence, and decreased apical cementum resorption were observed with age. In addition, a gradual increase in X-ray attenuation, owing to mineral differences, was observed at the PDL-bone and PDL-cementum interfaces for both groups, but without significant differences in the gradients due to reduced functional loads. Reduced functional loads resulted in significantly (p<0.05) lower microhardness of alveolar bone (0.93±0.16 GPa) and secondary cementum (0.803±0.13 GPa) compared to higher loads (1.10±0.17 GPa and 0.940±0.15 GPa respectively) at fifteen weeks. Based on the results from this study, occlusal loads differentially affect structural, compositional, and mechanical properties of local load-bearing sites of the bone-PDL-cementum complex, which could alter the overall biomechanical function of the dento-alveolar joint.