Alveolar bone resorption and the center of resistance modification (3-D analysis by means of the finite element method).

The main goal of this research was to study the behavior of initial tooth displacements associated with alveolar bone loss situations when loaded by a force of 1 N. The analysis of displacements was carried out by the finite element method. Six 3-dimensional models of an upper central incisor (designated Geramy 391 to 396) with 1 to 8 mm of alveolar bone loss were formulated and used by the author. Center of rotation and center of resistance were located for the various stages of alveolar bone loss. The results revealed that the moment/force ratio (at the bracket level) required to produce bodily movement increases in association with alveolar bone loss. Bone loss causes center of resistance movement toward the apex, but its relative distance to the alveolar crest decreases at the same time. Greater amounts of displacements of incisal edge and apex were observed with increased alveolar bone loss for a constant applied force. Center of rotation of the tipping movement also shifted toward the cervical line. Among the many differences between orthodontic treatment of an adolescent and an adult patient is the presence of alveolar bone loss in the adult cases. Alveolar bone loss causes center of resistance changes as a result of the alterations in bone support. This necessitates modifications in the applied force system to produce the same movement as in a tooth with a healthy supporting structure.