Bone bending, a feature of orthodontic tooth movement

A lthough the art of repositioning teeth has been practiced for centuries, the actual biologic mechanisms of tooth movement are not yet thoroughly understood.l OppenheimW 1911 interpretation of histologic sections is probably the most commonly used to explain the phenomenon. It describes bone resorption ahead of the tooth and apposition on the following surface without mentioning bone bending. A century ago, however, deformation or bending of the intraseptal alveolar walls was noted and had even been advocated as a theory of tooth movement.3 Recently, investigators have not only quantitated bone deflection at the alveolar margins, but Picton demonstrated that it could constitute as much as 25 per cent of initial tooth movement.4-6 Now bone bending is being suggested as an important mechanism linking the cellular and metabolic activity associated with tooth movement to the orthodontic force. Bassett postulates that electric potentials generated by the stress of bending may control cellular differentiation of bone.‘? * Justus and Luft? suggest that the linkage is mechanochemical, with a strain-induced change in solubility of the hydroxyapatite crystals acting as a chemical mediator. Epker and Frostlo, I1 have demonstrated that the alveolar wall we assume to be under pressure may actually be under tension from the strain resulting from bone deflection (as described later in Fig. 6)) thus questioning the concept of a simple tension and pressure relationship of tooth to alveolus. In addition, isotope studies of bone changes around moving teeth indicate areas of cellular activity well outside the periodontal ligament where response to tooth movement has been thought to be primarily a surface effect.12 The object of the present study was to determine whether intraseptal alveolar deformation is detectable and of a magnitude sufficient to contribute to the rapid initial tooth movement observed clinically in orthodontics.