2000 年 13 巻 1 号 p. 15-28
Analytical effects of an extended bridge length in Branemark implant system and biting force applied to the bridge length on a magnitude of vertical stress and the stress distibution in the cross sections of an abutment tooth and a gold screw placed at three different positions I-1 (near front site in the mandible), Ｉ-2 (middle site), I-3 (back site) were investigated in statics by the three-dimensional elastic finite element method (FEM) on a clinical size model. The average equivalent stress, vertical stress distributiton, and the stress gradient of the abutment tooth and screw increase as it chfanges from I-1 to I-2 and to I-3 positions, and the average vertical stress on the abutment tooth gives a small tensile, a large tensile and a large compressive stresses in that order.These stress values increase with increment in both the applied biting force and the extended bridge length. FEM solutions and their variations were found to be approximately true by static experiments performed on a 1.4-fold long stainless steel model. About one year after oral surgery, a clinical example showed that gold screws at I-1 and I-2 positions failed earlier than the one at I-3 position, which were contrary to FEM solutions in statics. A new dynamic biting machine was designed and manufactured to clarify the discrepancy between the solutions in statics and the failures in the clinical example. Dynamic biting tests repeated to about 2×10 cycles at max. were performed both on an abutment replica model at 20℃±1℃ in air. It was found that mechanical effects not only in statics but also in dynamics affect fracture behavior and loosening of gold screws at different positions, which are mainly controlled by the average vertical stress, the magnitude of vertical stress and its stress gradient in the cross section of a gold screw in statics and also by the number of repeated cycles in biting, the biting velocity, the stress amplide, the average stress in repeated biting in dynamics. Finally, the SEM fractography of failed gold screws in the clinical example and in the abutment tooth replica model tested by the biting machine were examined and differences in failure behavior in gold screws at I-1, I-2, I-3 positions were mechanically analyzed and discussed in both statics and dynamics.
