Abstract
Introduction
Post cam is useful to realize the intrinsic stability of a posterior-stabilized (PS) knee prosthesis replaced for a case with the severe degeneration. Some retrieval studies reveal the ultrahigh molecular weight polyethylene (UHMWPE) deformation or severe failure of the tibial post of PS knee. Strength of the tibial post of available design is obviously insufficient to prevent the severe deformation. The large size post might, however, shorten the range of knee motion. Therefore, minimally required size of the post should be clarified for polyethylene inserts. In the present study, we performed finite element (FE) 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.
Method
The shape of three commercially available knee prostheses, product A, B, and C was referred as PS knee prosthesis. The contour of the metallic femoral component and the UHMWPE insert were digitized by a computed tomography apparatus. Three dimensional finite elements were generated by modeling software (Simpleware, Ltd. UK) as four-node tetrahedral elements. In FE analysis, we used LS-DYNA ver.971 (Livemore Software Technology Corp. USA) as the software and Endeaver Pro-4500 (EPSON Corp. Japan) as the hardware. These bottoms of the tibial insert were fully constrained. The value of 30MPa was defined as yield stress of UHMWPE. 500N posterior load was applied to each femoral component at 10 degree hyperextension. Then, 1000N anterior load at 120 degree flexion, after tibial insert was located 10 degree internal rotation (Fig. 1). These loads were assumed to realize the two types of tibial post impingement under several kinds of knee motions. The distributed values of von Mises stress and plastic strain on the tibial post were shown as the results of the analysis.
Results
At the 10 degree hyperextension, these maximum values of von Mises stress were 24.5, 3.23, 27.09MPa on anterior aspect of tibial post of the product A, B, and C, respectively (Fig. 2). These plastic strains were 0.045, 0.001, 0.064. At the 120 degree flexion, these maximum values of von Mises stress were 33.67, 4.53, 27.03MPa on posterior aspect of the product A, B, and C, respectively (Fig. 3). These plastic strains were 0.28, 0.004, 0.061. The stress of product A was higher than yield stress of UHMWPE. The strain was obviously higher than that of product B and C.
Discussion
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. In the femoro-tibial articulation, the true-stress decreases with increase in load because the compressive deformation can widen the contact area on the UHMWPE. The true-stress in the tibial post, however, increases with increase in load because bending and tensile deformation reduces the section area. Therefore, 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.
