External root resorption caused by luxation of rat molars

Following elevator luxation of the mandibular first right molar in 36 10‐weekold male Wistar SPF rats, three different kinds of external root resorption were observed. Surface resorption, found mainly on the sides where pressure was exerted, was recorded histometrically from 3 to 21 d after the trauma. Surface resorption increased considerably from the third to the tenth day, and no definite repair occurred within three weeks. The extension of areas with resorption decreased markedly in the apical direction on the root surface due to a tipping movement of the first molar during luxation. Inflammatory resorption was seen in association with an inflammation located between the first and second molars, and caused by the elevator. Replacement resorption (ankylosis) occurred on the pressure sides within 14–21 d after the trauma. Hyalinization of the periodontal membrane was seen on the pressure side. Osteoclastic bone resorption occurred on the pressure side as well as on the tension side and in the marrow spaces. Bone resorption later changed to bone formation and periodontal conditions became normal within three weeks. On the external surfaces of the mandible, periosteal bone formation was seen. No degenerative changes occurred in the pulp.