Analysis of stress-strain curves and histological observations on the periodontal ligament of impeded and unimpeded rat incisors at low velocities of loading

Abstract

Stress-strain curves of the periodontal ligament obtained by pulling a tooth part out of its alveolar bone in transverse sections of impeded and unimpeded rat mandibular incisors at low velocities (0.5mm/24h or 1.0mm/24 h) were analysed. The maximum shear stresses were always greater in the impeded than in the unimpeded incisors at the same loading velocity and the rate of increase in the maximum shear stress in association with increases in loading velocity were also greater in the impeded than in the unimpeded incisors. The tangent moduli and failure strain energy densities also showed similar tende ncies. Light and scanning electron microscopic observations showed that the periodontal ligament was sheared in its middle layer when loaded at either 0.5 or 1.0mm/24 h, and that the free surfaces of the sheared ligament were rather smooth with the fibres running mostly parallel to the long axis of the tooth. These results suggest that the periodontal ligament of the rat incisor could generate a variable resisting force according to the velocity of tooth movement, and thus automatically regulate the rate of tooth eruption. Sliding of the periodontal fibre bundles occurs in the middle layer of the ligament when loaded at approximate rates of eruption.