Tooth Mobility and Instantaneous Centers of Rotation

Abstract

Tooth mobility as it relates to orthodontics has not been thoroughly explored. The immediate aim of this project was to develop and test instrumentation to measure both tooth mobility and instantaneous centers of rotation under forces applied perpendicular to the tooth long axis. Two identical dial micrometers were selected and modified to allow variability in range, calibration, and contact pressure. All forces were delivered by mechanical force meters. A holding device was constructed to permit rigid positioning of a micrometer perpendicular to the long axis of any tooth in the arch and measure mobility at one point on the tooth. With an applied force of 500 gm., mobility measurements were made with the instrument on 44 teeth of six orthodontic patients. By repeating the analyses the instrument reproducibility was demonstrated. The pattern of tooth movement to applied force was explored and related to possible stress levels in tipping movements. A second holding device allowed both micrometers to contact a dental extension which was fixed to a maxillary central incisor. An incisor from each of four preorthodontic and two postorthodontic patients was analyzed. Over 200 clinical centers of rotation were calculated from measurements of tooth movements at two points on the tooth's long axis. Results from the first three patients were erratic. The rotation centers of the last three patients generally remained at midroot and also near the theoretical center of the tooth for all forces applied over 50 gm. More comprehensive studies on tooth mobility are recommended.