膝内側側副靭帯損傷の生体力学的検討

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  • BIOMECHANICAL STUDY OF MCL INJURIES

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To gain insight into the mechanism of MCL injury, we performed both experimental measurements and theoretical model analyses. In the initial experiment, the strain distributions over the entire MCL in a rabbit's femur-MCL-tibia complex were measured using a photoelastic coating method. The results of tensile testing revealed that the isochromatic line was concentrically higher near the tibial insertion of the MCL than in other areas, and rupture began with the avulsion of the periosteum at this site in all cases. With application of a valgus moment, the isochromatic line concentrated on the medial femoral condyle, and changed over the medial epicondyle. Next, histological studies were performed on the rupture sites. Histological observation noted that, for the specimen after a tensile test, rupture was observed in the nonmineralized fibrocartilage zone mainly at the MCL tibial insertion site, while for the specimen after valgus bending, rupture was present over the medial epicondyle from the region of the medial femoral condyle. Thus histological findings demonstrated that rupture sites and increased strain concentration sites correlated closely. Histological inspection further revealed that collagen fibers were directly inserted and anchored into osseous tissue at the femoral insertion, and the development of mineralized fibrocartilage was greater at the femoral insertion than at the tibial insertion. Investigations of actual MCL injuries showed that the ruptures commonly occurred on the femoral side irrespective of its mechanical strength. This suggested that the mechanism of MCL injury could be induced by particularly large tensile force on the femoral insertion side when a valgus moment was applied to the knee joint. We therefore attempted to introduce stress distributions on the MCL with mathematical models when a tensile force and a valgus moment were applied. In the first simulation, a constitutive equation for the MCL composite was formulated on the assumption that the MCL can be idealized as being composed of a homogeneous matrix in which densely distributed extensible fibers are embedded. Using the finite element method, we performed a simulation whose results showed that stress concentration was located in the tibial or in the femoral insertion site respectively, when a simple tensile force or a valgus moment was applied. Next, using the free body force and mechanical funicular diagram, we calculated tension on the MCL during application of valgus moment. The results demonstrated that when a valgus moment was applied, an imbalance tensile force was generated at the femoral insertion. An impingement phenomenon on the medial femoral condyle was seen as well. This phenomenon explained the higher incidence of MCL injuries on the femoral side seen in the clinical setting.

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