下顎頸部骨折の発生機序に関する構造力学的研究

抄録

Fractures of the mandibular condylar processes are classified into intracapsular and extracapsular groups according to the location of the fracture lines. However, the mechanism underlying the difference between them has not been studied in the laboratory or at least not described in any publications.
To clarify this mechanism, we measured the stress and strain distributions and the deformation patterns under a static load applied from several directions to human mandibles and their replica models to which strain gauges were adhered. Furthermore, the cross-sectional characteristics of the test specimen and the internal strain of the condylar neck region were measured.
Consequently, the following conclusions were made:
1) The directions of principal stresses and the orders of their magnitude were very similar between the mandible and replica model and proved to be correlated. So, by using replica models we could determine the positions where the stresses were apt to concentrate and their directions.
2) The stress and strain distributions of the mandibles could be classified according to the direction of loading into two types. One was the type with longitudinal force to the mandibular body, with the stress being concentrated in the subcondylar region primarily and in the anterior region secondarily. The other was the type with the force lateral to the mandible; here the stress was concentrated at the loading point and anterior region primarily and in the subcondylar region secondarily.
3) With the loading londitudinal to the mandibular body, the stress behavior and the deformation pattern of the condylar neck could be divided into two types:(1) the bending stress type, observed when the condylar heads were fixed and the necks were bent buccolingually, so the neck projected inwardly, and (2) the longitudinal type, observed when the condyles were supported freely and the neck transferred the force to the condylar head such that the head rotated inwardly.
4) The cross-sectional area was largest in the mandibular angle section and smallest in the condylar neck section. The moment of inertia in all sections was larger about the horizontal than about the vertical axis. Among the evaluated data about the horizontal axis, in the condylar neck region the moment of inertia was smallest and at the angle region it was largest. About the vertical axis, in the condylar neck section it was smallest. The radius of gyration about both the horizontal and vertical axes was smallest in the condylar neck section.
5) The above experimental results revealed that the conditions in which the condylar head was most prone to fracture was with free end fixation and a mento-condylar direction of loading. Under these conditions, fifteen mandibles were subjected to the fracture test, and all the specimens fractured sagittaly or obliquely and only along the internal side of the condylar head under a mean static load of 92kg.