Dynamic response of the monkey skull with loss of occlusal force

Abstract

We investigated the dynamic changes that occur in the cranium when occlusal and masticatory forces are not imparted on the jaw bones because of loss of teeth, even though there is opening and closing of the mandible. Electrical stimulation (30 or 40V) was applied to the central region of the masseter muscles on both sides of anesthetized adult Japanese monkeys fixed in a standing position, and the strains in the temporal bone, sphenoid bone, zygomatic bone, zygomatic arch, maxilla and mandible were measured when the animals were made to occlude. The experiment was carried out on three groups: 1) when they occluded on crowns that had not been reduced (controls), 2) when the crowns of all the maxillary and mandibular molars were reduced and they were made to open and close without any contact on the molars, although the anterior teeth were in contact (Group A), and 3) when the maxillary and mandibular anterior teeth were also reduced and they were made to open and close without contact of any of the teeth (Group B). Strain in the bones at this time resulted only from the forces of muscle contraction.
The amount of strain decreased in every bone when all the maxillary and mandibular crowns of both the anterior teeth and molars were reduced. The principal strain decreased in the temporal bone, sphenoid bone, zygomatic arch, maxilla and mandible to 52, 32, 40, 47 and 39%, respectively, compared with the controls. In contrast, strain in the zygomatic bone was little changed compared with the controls (94%).
Compared with the controls, the direction of the tensile component of the principal strain when teeth are lost is in the anteroposterior direction in the mandible, maxilla and zygomatic bone, while it is in the vertical (body) axis direction in the temporal bone and zygomatic arch. It is along the vertical axis in the sphenoid. That is to say, it rotates in the anteroposterior direction for the facial bones, and along the vertical (body) axis for the cranial bones. At that time, the center of rotation is in the sphenoid, because the direction of principal strain of this bone varies little regardless of the conditions under which opening and closing motions are carried out. Therefore the sphenoid bone creates a dynamic balance left, right and in the anteroposterior directions, while at the same time, playing an important role in transmitting the direction of displacement of the cranium to the cervical vertebrae. Shika Igaku (J Osaka Odontol Soc) 1997 Mar； 60(1)：25-34．