Proclination of lower incisors: a design to maximize food penetration and minimize torque

Human upper and lower incisors are both tilted forward in the sagittal plane. Previous theoretical and in vitro studies have investigated how proclination may help the teeth either to penetrate or to fracture food more effectively or both. We study the proclination of lower incisors in relation to efficiency and to the protection it may offer from potentially damaging torque forces. Lateral cephalographs from 57 normal human subjects were traced. In one study, a line was drawn joining the centre of the condyle to the tip of the lower incisor. The results showed the lower incisor is oriented so that it is closely parallel to the arc of a circle centred at the condyle. In another study, lines were drawn joining the tips of upper and lower incisors at different openings. Each line showed the direction of the force that must be used to bite an object held between the tips of the incisor teeth. Its direction was compared with the direction of the long axis of the lower incisor when the mandible was graphically rotated open. The results showed the long axis of the lower incisor is closest to the direction of the bite force at 12 degrees and 15 degrees of jaw openings (roughly 20-25 mm incisal separation). This is the opening where the maximum incisal force is normally produced. The findings suggest that to reduce the torque, lower incisors implanted or relocated during orthodontic treatment should be oriented parallel to the closing arc.