Restoration of mechanical strength and morphological features of the periodontal ligament following orthodontic retention in the rat mandibular first molar

Biomechanical properties and morphological features of the periodontal ligament (PDL) in the rat mandibular molars were examined during orthodontic retention. Seventy-three male rats of the Wistar strain, 8 weeks of age, were used for biomechanical analysis and six rats for morphological analysis. An elastic band was inserted between the mandibular first and second molars for 4 days; after removal of the elastic band the interdental space was filled with resin for 4 and 8 days. The maximum shear stress, tangent modulus, and failure strain energy density of the PDL of the first molar in the experimental animals decreased markedly following application of an orthodontic force. They increased rapidly and were restored completely to the control levels by the 8th day after retention. Light microscopy showed severe compression and extension of the PDL in the experimental animals on the 8th day after retention. Birefringent collagen fibre bundles running across the compressed and expanded PDL were observed, although they appeared to be thinner with less insertions into the alveolar bone or cementum in the experimental animals than in the controls. This suggests that the periodontal collagen fibres were partially reorganized and rearranged during retention. The reorganization and rearrangement of periodontal collagen fibres seemed to be partly related to the restoration of mechanical strength of the rat molar PDL during the 8 days of retention.