The influence of the dentition on the shape of the mandible.

In the course of life the mandible undergoes substantial morphological and dimensional changes. The changes concerning mandibular angle, relation between the length of the mandibular corpus and ramus as well as roughness in the place of insertion of masseteric and pterygoid muscles do not depend upon age or sex but (apodictical) exclusively upon the dentition. In the complete dentition all masticatory muscles take part in chewing and grinding. The function of the temporal and masseter muscle is (reduced) weakened in partial or complete loss of teeth. The most evident morphological change of the lower jaw consists of an apparent elongation of its corpus resulting in a characteristic senile appearance. Measurements have been performed on 114 macerated mandibles of individuals ranging from 4 months to 68 years of age. The following measures have been taken: number of teeth in the lower jaw, angle between the corpus and ramus of mandible, length of the mandibular corpus, height of the mandibular ramus and angle of the mandibular incisure. The angle between the mandibular corpus and ramus is significantly lower in mandibles with 11 and more teeth (p < 0.001). The length of the mandible is significantly larger (p < 0.02) in mandibles with less than 11 teeth. The mandibular ramus is significantly higher in mandibles with more teeth (p < 0.01). There were no significant differences in the angle of the mandibular incisure within the groups (p > 0.05). The discriminant analysis reveals that 62.3% of the variability of mandibles is dependent upon the number of teeth. The rest of proportional variability of 37.7% is probably due to new circumstances and adjustment of the mandible in the period from tooth extraction.