ROUND TIBIAL POST CAN REDUCE CONTACT STRESS IN THE POST-CAM STRUCTURE IN THE POSTERIOR STABILIZED TOTAL KNEE ARTHROPLASTY

Abstract

Posterior stabilized (PS) type knee prosthesis characterized by Post-Cam structure as stabilizer has successfully been used in TKA worldwide, while failure and fracture problems of tibial insert made from polymeric material (UHMNWPE) are still important issues from clinical and mechanical points of view. It is therefore needed to understand the mechanical conditions of the tibial insert under different kinds of TKA motions. The aim of this study is to characterize the mechanical condition of tibial insert under contact between femoral component and tibia insert during flexional motion using dynamic 3-D finite element (FE) method. 3-D FE models of two different kinds of PS type prostheses clinically used were developed and stress analyses were performed from full extension to 135 degree knee flexion. Effects of the different Post-Cam structures on the stress states were investigated, and a guideline towards risk assessment of PS type prosthesis was discussed.

Three-D FE models of Stryker’s PS type knee prostheses, Scorpio Superflex and NRG, were developed base on their CAD data. The tibial post of Scorpio Superflex type knee prosthesis shapes angular, while NRG shapes round. Four nodes tetrahedral elements were used to construct the FE models. Nonlinear spring models were attached to the front and back of the tibial component to express the effect of soft tissues on the movement of real TKA knees. Vertical load and horizontal load were applied to the femoral and tibial components, respectively, to express a deep knee bending (squatting) motion. Flexion motion was introduced by rotation the femoral component from full extension to 135 degree. Internal rotation of 5, 10, 15 degrees were also introduced by rotating the tibial component simultaneously with the flexional motion.

Maximum Mises equivalent stress during knee flexion with 5, 10 and 15 degrees internal rotation of the tibial component of Superflex were much higher than that of NRG, especially at the flexion angle of 120 degree. NRG exhibited stress concentrations on both the Post and condylar surfaces and stress levels were much lower that that of Superflex. The maximum stress in NRG was found to be reduced to about half of Superflex. Mises equivalent stress distribution also showed that flexion with internal rotation generated higher stress concentrations on the condylar surfaces of both prostheses.

The analytical results well demonstrated that the design modification of the tibial insert of NRG effectively reduced the stress concentration with rotated tibial component. The lower stress level in NRG corresponds to the lower reaction force and hence lower resistance to flexional motion than Superflex. This implies that the round post is more suitable for deep flexion than the angular post.