[Bone statics of the face: the frontal-sphenoid-pterygoid bone pillars as biomechanic equivalents of the mandible].

In general biomechanics, trabecular osteo-architectonics is considered as a response to the mechanical strains present in the bone (compression or traction). The facial and cephalic skeleton is no exception to this law. The study of the bony architectionic provides the proof, since we have demonstrated in the fixed facial bone mass (at the limits of the facial skeleton and the skeleton of the base of the skull) the biomechanical equivalent of the mandible: the cranio-facial pterygo-spheno-frontal bony pillars. Paired, symmetrical and with a medio-frontal junction, they represent an embryological, anatomical and functional entity. Their ossification (mixed) may be compared to that of the mandible. They are made up of directional trabecular bone which may be analysed by stereology, electron microscope scanning and by strain gauges. Each pillar is made up of: - A part free of musculature corresponding to the supra-orbital frontal arch, which receives the four vertical pillars (naso-maxillary and maxillo-malo-frontal) of the superficial system of the face which transmit to the supra-orbital frontal arch the ascending mechanical strains of mastication registered in the spongy hard palate. The supra-orbital frontal arch is the biomechanical homologue in the fixed facial mass of the tooth bearing bony base of the mandible. - A spheno-pterygoid part, into which are inserted the alimentary muscles of the cephalic region (masticators, velar and superior pharyngeal). There is further functional homology between the ascending ramus of the mandible and the pterygoid greater wings and apophyses complex of the sphenoid. The 2 pterygo-spheno-frontal pillars also participate in the osseous statics of the orbit and the osseous statics of the temporo-mandibular joint. The basi-sphenoid plays a role of first importance in the general statics of the cephalic region. Thus the two pterygo-spheno-frontal pillars, biomechanical homologues of the mandible in the fixed facial mass, underline the functional interdependence of the osseous, dental and muscular systems of the cephalic region, a region which should be viewed as being in a stade of permanent dynamic equilibrium during growth and in the adult.