Distribution of stresses in the human skull.

A three-dimensional photoelastic model was reproduced from a human skull. The insertions of the superficial and deep portions of the masseter muscle, the temporalis muscle, the medial pterygoid muscle and the temporalis fascia were simulated with leather bonded to the appropriate areas. The following loading conditions were employed: (i) all three muscles were bilaterally loaded without occlusal loads; (ii) all three muscles were bilaterally loaded while occlusal loads were simultaneously applied on the maxillary teeth. The stress freezing technique was used to lock the resulting stress patterns into the model skull. The stress trajectories were observed and photographed in a circular polariscope. The mandibular closing muscles and the occlusal loads produced stresses which progressed through the maxilla, following a nasal, a zygomatic and a pterygoid route, while stress concentrations were seen in the frontonasal, zygomaticomaxillary and the pterygopalatine sutures. In the malar bone area the stresses were seen to branch superiorly to the frontozygomatic suture and posteriorly along the zygomatic arch to the zygomaticotemporal suture and the temporal bone. Generally, the stresses were concentrated in those areas of the skull where architectural reinforcement had been demonstrated by other methods.