フィクスチャーの長さが顎骨のひずみ分布に及ぼす力学的影響

―ヒト乾燥骨による実験的研究―

抄録

Osseointegrated implant are popularly accepted by today's general dental professionals as a deficiency prosthetic treatment with high predictability. Osseointegration is defined as a condition in which the bone structure comes in direct contact with the fixture. However, since osseointegration has no mechanism to buffer impact, as in the case of a natural tooth, and the force applied is transmitted directly to the bone, it provides dynamic characteristics different from those of a natural tooth. Most clinicians have empirically taken precautions to guard against failure of osseointegration caused by external force by increasing the number of fixtures to be inserted or inserting as long a fixture as possible.However, few experimental studies have investigated and elucidated the differences in stability of the fixture under various stresses generated by differences in fixture length. Therefore, the dynamic effects of differences in fixture length on the mandible was investigated using a method for measuring the degree of strain on the bone surface.
In this case, a static load was applied to the implant superstructure of an experimental model fabricated by inserting the fixture in a dried human mandible, and the findings obtained are reported here.
Four Brånemark system implants^® (available from Nobel Biocare Japan) were inserted between the right and left mental foramens of the mandibles in conformity with the generally practiced technique.Four fixtures were called R 1, R2, L1, and L2 from the right side.
The torque required for fixture insertion was 10.1±0.84 Ncm for the 7-mm fixture and 20.8±4.80 Ncm for the 15-mm fixture, and it was assumed that the latter could achieve strong fixation.
Validation was done for bone strain caused by the difference in the fixture major axis, and as a result, significant differences were observed in the alveolar parts and diaphysis corresponding to R 2 and L 1 when a load was applied to the median portion. When a load was applied to the left-side distal portion, significant differences were observed in the alveolar parts and diaphysis corresponding to L 1, L2, and R1. When a load was applied to the right-side distal portion, significant differences were observed in the alveolar parts and diaphysis corresponding to R 1, R 2, and L 2. Furthermore, in strains around the fixture head portion, significant difference was recognized.
Consequently, the results suggested it is desirable to use as long a fixture as possible whose top end reaches the compact bone to achieve strong fixation and to disperse stresses when fixtures are inserted.