Analysis of stress-strain curves in the rat molar periodontal ligament after application of orthodontic force

Abstract

Previous studies have shown that the mechanical strength of the periodontal ligament decreased markedly after application of an orthodontic force to the rat molars.13 However, an analysis of stress-strain curves obtained from transverse sections of the rat molars has not been made. The present study analyzed the stress-strain curves obtained from the mesial root of the rat mandibular first molar to evaluate the changes of the mechanical properties of the ligament after application of an orthodontic force. An elastic band was inserted between the mandibular first and second molars for 7 days. The maximum shear stress, elastic stiffness, and failure strain energy density were significantly less in the experimental group than in the control group, but the maximum strain was significantly greater. Histologic examinations of the transverse section after mechanical testing showed that the area of the ligament adhering to the mesial surface (compression side) of the socket bone was significantly less in the experimental group. It is suggested that orthodontic forces may cause changes in the constitution of the periodontal collagen, in osteoclastic activity in alveolar bone, and in mineralization patterns of Sharpey's fibers followed by reductions of the mechanical strength of the ligament, particularly on the compression side of alveolar bone.