Dental Function and the Chewing Cycle

Incisor teeth are tall and spatulate with narrow cutting edges and are used extensively by primates for peeling, paring and preparing food. Incisor crown height in Hominids reflects their relative importance for food preparation. Incisor tooth crowns wear away over a lifetime but tooth height above the alveolar bone is maintained for a long time by tooth eruption that compensates for wear. Human canine teeth are greatly reduced in size but are still the most prominent of the anterior teeth. Sexually dimorphic great ape canine teeth are fundamental to primate social life but also form a blade-like sectorial cutting complex with the adjacent lower premolar. Premolar and molar teeth have bicuspid or multicuspid surfaces that occlude precisely with teeth in the opposing jaw. Chewing is a rhythmic process that involves both opening and closing movements of the jaw in the sagittal plane as well as lateral movements. During the chewing cycle the lower jaw first falls away from centric occlusion towards the working side. From a position of maximum opening there is a fast closing stroke followed by a power stroke during which food is crushed between the teeth. An opening stroke completes the cycle. The entire chewing cycle is usually one continuous movement. The power stroke is the most variable part of the cycle and consists of two phases, During phase 1 (the buccal phase) the lower teeth move medially and upwards from their first contact with the upper teeth to the point of maximal intercuspation. In the shorter phase 2 (the lingual phase) the lower teeth continue to move medially but are displaced downwards as their buccal cusps slide against the palatal cusps of the upper teeth. Prominent canine teeth in both humans and great apes limit the lateral movements possible during the chewing cycle, so-called canine guidance.