Types of tooth movement, force magnitude and duration
Although it was previously thought that tipping of a single-rooted tooth (Fig. 1) occurred about a point almost midway along the root, rotation now appears to take place near the apical third within an elliptically shaped area. Half of the periodontal ligament is stressed, with maximum pressure created at the alveolar crest in the direction of movement and at the diagonally opposite apical area. For bodily movement and rotation,a force couple must be applied, loading uniformly the whole of the periodontal ligament in the direction of translation so both crown and root move in the same direction by equal amounts (Fig. 2). With extrusion, all of the periodontal ligament is tensed, but when a tooth is intruded the force is concentrated at the apex. An element of tipping is unavoidable with extrusion, intrusion and rotation.
For tooth movement to occur optimally, the force per unit area within the periodontal ligament should ideally not occlude the vascular supply yet be sufficient to induce a cellular response. A force should, therefore, be as light as possible for the movement intended, taking into account the root surface area over which it is spread. Optimal force ranges for various tooth movements are:
• tipping 30-60 g
• bodily movement 100-150 g
• rotation or extrusion 50-75 g
• intrusion 15-25 g.
Although tooth movement can occur in response to heavy forces, these should not be applied continuously; intermittent application may be clinically acceptable. Not only must a force be of sufficient magnitude to
effect the movement desired but it must also be sustained for sufficient time. For successful movement, a force must be applied for at least 6 out of 24 hours, and continuous application of light forces is optimal. This is favoured, because control of tooth movement and anchorage is facilitated while the risks of pulpal and radicular damage are minimised. Excessive mobility is avoided and movement is more efficient with less discomfort. Movement of the order of 1 mm in a 4-week period is regarded as optimal, with faster progress recorded in children than in adults. This is largely a con-sequence of the greater cellularity of the periodontal ligament, more cancellous alveolar bone and faster tissue turnover in a growing patient, which ensure a more rapid response to an applied force.