Abstract
Some retrieval studies reveal the ultrahigh molecular weight polyethylene (UHMWPE) deformation or severe failure of the tibial post of a posterior-stabilized (PS) knee. Strength of the tibial post of available design is obviously insufficient to prevent the severe deformation. Minimally required size of the post should be clarified for polyethylene inserts. In the present study, we performed finite element analysis assumed the mechanical conditions of a tibial post in a PS knee and aimed to design criterion of a post of polyethylene insert of a knee prosthesis. The shape of three commercially available knee prostheses was referred as PS knee prosthesis. Three dimensional finite elements were generated by modeling software as four-node tetrahedral elements. A 500N posterior load was applied to each femoral component at 10 degree hyperextension, and a 1000N anterior load was applied to each femoral component at 120 degree flexion after the tibial insert had been internally rotated 10 degrees. The distributed values of von Mises stress and plastic strain on the tibial post were shown as the results of the analysis. The stress of a smaller post was higher than yield stress of UHMWPE and the strain was obviously higher than that of other posts. Our results showed that plastic deformation may occur in the posterior aspect of a tibial post by impingement during common exercises like climbing up, or squatting. The design criterion including the post size must be revised the safety coefficient which realizes that the generated stress in the tibial post is sufficiently lower than the yield stress of UHMWPE.
