Relationship between Tooth-Movement Rate and Estimated Pressure Applied

The object of the experiments was to investigate the hypothesis that the pressure exerted by roots on the surrounding bone-periodontal-membrane complex, and not the force applied to the crown, is the critical factor in determining rate of tooth movement. As pressure is defined in terms of force per unit area, it was necessary to measure two quantities before any calculation of pressure could be made. These quantities are force and the projected area of the tooth root, provided that tipping and rotation are prevented. The distal movement of maxillary canine teeth was investigated in a group of twelveto fourteenyear-old male patients. In each case one canine was moved bodily while the other canine was tipped simultaneously as a comparison. A force of 450 gm. f. was exerted on each canine tooth by activating a helical torsion spring. A fixed point on a mandibular archwire was used as a reference point. Measurements of tooth movement and deflection of springs used in these experiments were accurate to 0.1 mm. The orthodontic experimental apparatus was similar to that used by Storey in his investigations in 1952 (E. STOREY, Aust. dent. J., 56:11, 1952). The projected areas of the roots of incisor teeth were determined with a planimeter after photographing extracted teeth normal to the axis, magnifying the image ten times, and tracing the outline. A study of the projected area of the mesial and distal surfaces and the labiolingual widths at the cemento-enamel junction and root lengths of incisor teeth showed that there was a highly significant relationship between the projected area of a root and these above-mentioned linear dimensions. Curves showing the rate of tooth movement of each patient against average force applied for bodily movement and tipping of the canine teeth were plotted (see fig.). The pressure is indicated for bodily movement. With decreasing force, and consequently pressure, due to the movement of the tooth and attached