Dynamic responses in adult and infant monkey craniums during occlusion and mastication

Using occlusion without food as a basis, this study elucidated the dynamic responses and associated buffer mechanisms in the monkey cranium and its component bones during mastication. In addition, investigations were carried out on the relationship of these factors to the growth and development of the cranium. Using strain gauges, the masticatory buffer capacity and dynamic responses in the bone were investigated from the standpoint of the magnitude and direction of strain in the individual bones of the cranium. Compared with the occlusion without food, there were greater stresses during mastication in the bones of both the masticating and non-masticating sides of the cranium. These stresses were greater when masticating hard than soft foods, and greater in the cranium of the adult than infant monkey. On the masticating side of the cranium, the buffer effect to masticatory forces in the adult cranium was carried out by the inherent form of each bone in response to the firmness of the food, while in the infant cranium it was carried out by the entire cranium independent of the firmness of the food. On the non-masticating side of the cranium in both the adult and infant craniums, the zygomatic arch, temporal bone, and the bones of the temporomandibular joint region balanced the masticatory forces of the masticating side of the cranium, and played an important role in buffering these forces. The direction of the principal strain arising in the bones of the infant cranium corresponded to the direction of growth and development of the respective bone. However, in spite of the fact that large strains were observed in each bone on the non-masticating side during mastication of hard foods, the strains were very small on this side during mastication of soft foods. Consequently, it is necessary to masticate hard foods in order to promote the growth and development of the cranial bones. Viscoelasticity in the cranial bones could be explained by a three-element model.