PLASTIC DEFORMATION ANALYSIS OF UHMWPE TIBIAL INSERT OF PS TYPE KNEE PROSTHESES UNDER REPEATED DEEP FLEXIONAL MOTION

Abstract

Knee prostheses have widely been used for severely damaged knee with osteoarthritis or articular rheumatism. PS type knee prosthesis is one of typical artificial knee joint systems and characterized by possessing the post-cam structure to stabilize the motion of the knee at large flexion angles. Post is a projection placed on the surface of UHMWPE tibial insert, and severe fracture and wear of the post are sometimes reported. It is therefore very important to understand the stress state of the post under real flexion motions in order to prevent such damages. It is also well known that the contact and bearing surfaces of a human knee is subjected to very high force especially during deep knee flexional motion such as squatting, and it is naturally expected that the tibial insert of a knee prosthesis deforms plastically under such high force condition.

In this study, three dimensional dynamic finite element analysis of two types of PS knee prosthesis clinically used worldwide, Stryker’s Scorpio Superflex and NRG, are performed to characterize the plastic deformation behavior due to stress concentration generated in their tibial inserts under deep knee flexion motions. The new system NRG is recognized as a modified version of Superflex. Especially, the shape of the post is tried to be improved in order to reduce stress concentration and mobility. Continuous repeated flexional motion such as flexion-extension-flexion motion is considered in the analysis. Internal rotation of the tibial component and insert with flexional motion is also considered. It is found that severe stress concentration is generated in the post for both models and also in the condylar surfaces, and the stress concentration in Superflex is much higher and wider in NRG. Plastic deformation is therefore observed at these stress concentration points. The relationship between residual stress and plastic deformation in the tibial inserts is then discussed based on the analytical results.