A biomechanical approach to craniofacial shape in primates, using FESA.

The head of a land-living vertebrate is exposed to the forces of acceleration, in particular the permanent earth acceleration (= gravity) and the muscle-generated bite and chewing forces. In mammals, at least, the latter seem to play the dominant role. Bite forces are applied to the teeth and close the circle of forces by passing through the facial skeleton to the insertions of the mandibular adductors. With the aid of three-dimensional Finite Element Systems Analysis (FESA), the stress flows in homogenous bodies are investigated, whereby the braincase, the orbits and the nasal channel are taken as preconditions. The muscle insertions are varied systematically. The resulting stress flows in all cases turn out to be very similar to the bony structures of a skull. Little or not stressed parts of the available homogenous body indicate the external surface or hollow spaces (= sinuses) inside the skull. The possible applications of forces (i.e., the forms and positions of the dental arcade in relation to the braincase) determine the pathways along which the forces are transmitted. It seems that the factors mentioned above as preconditions represent the selective pressures exerted by the lifestyle of the animal and its environment (= ecological conditions). The results that can be obtained by our deductive approach in comparison to inductive, experimental procedures are discussed briefly.