TKA for Asian population
长岭隆二
日本九州大学医学部
In Asian population, anatomic characteristics of thelower extremities are lateral bowing of the femoralshaft and external rotation of the femoral condyle inthe coronal plane, and proximal tibia vara with medialshift of the tibial articular surface, and medialtorsion of the tibia. Suitable TKA system should bechosen, and operative technique should be modifiedtaking those anatomic characteristics intoaccount.
Gap control technique (ligament dependent cutprocedure) and proper medial soft tissue releases arethe essential for Asian people with varus deformity. Inthe gap control technique, distal femoral and proximaltibial cuts are made. Using a Tensor/Balance device,joint gap angle and distance are measured both inextension and in flexion. The joint gap is setrectangular in extension with the proper medial softtissue release. The order of releases was deep medialcollateral ligament (MCL), superficial MCL, pesanserinus, and semimembranosus. The superficial MCLwas released selectively. The joint gap angle betweenthe posterior condylar line and the tibial cut surfacein flexion is to be the external rotation angle of thefemoral component (anteroposterior cuts) relative tothe posterior condylar line. The size of the femoralcomponent is decided based on the joint gap both inextension and flexion, and the same rectangular gap ismade in flexion. It is easier to obtain deep flexionusing the posterior stabilized total knee system withround tibial post.
Three dimensional finite elementanalysis showed that round post can significantlyreduce the contact stress between the post and the camfrom engage angle to the maximum flexion angle. Thisresult has two meanings. First, the post-cam mechanismis safe, and the knee system can have longersurvival. Second, with the lower contact stress, theflexion-extension is smooth, and the knee can obtaindeeper flexion angle. The important techniques toobtain deep flexion are gap control technique, theproper posterior clearance, the larger posteriorcondylar offset, excessive external rotation of thefemoral component and the proper soft tissuereleases.
The posterior clearance consists of removalof osteophytes in the posterior cavity and free bodies,removal of the posterior femoral condyle that is notcovered with the posterior condyle (flange) of the femoral component and release of the posterior capsule. The bone that is not covered with the femoral component will impinge against the posterior lip of the tibial articular surface in deep knee flexion.
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