TRACTION FORCES IN DEEP KNEE BENDING OF ROTATING PLATFORM TOTAL KNEE IMPLANTS AND IMPLICATIONS FOR WEAR

Abstract

A multibody dynamics program (LifeMOD/KneeSIM, LifeModeler, Inc., San Clemente, CA) was used to simulate knee bending. A PFC Sigma® (DePuy, Warsaw, IN) rotating platform (RP) posterior cruciate retaining total knee was subjected to two cycles of knee bending up to 130 degrees of flexion. The RP model (Free RP) included experimentally determined torsional frictional behaviour for the insert-tray bearing as a function of axial load and rotational speed. The analysis was repeated with the exact same implant design, but with the insert locked (Fixed RP) to the tray to prevent internal-external (IE) rotation (a theoretical design). IE rotation and tangential traction (frictional) forces were calculated over the contact patches and averaged at the centres of pressure in the medial and lateral compartments.

Cross-wise tangential traction forces were greater for the Fixed RP than for the Free RP design in both medial and lateral compartments. The tangential traction forces arising from rolling and sliding may cause delamination of the polyethylene, especially if they act cross-wise to the main direction of motion of the contact patches, in accordance with the strain-softening effect proposed as a mechanism of wear for multi-directional motion. Even though the amount of cross-wise motion in existing total knee arthroplasty designs has been shown to be limited, the present study indicates that cross-wise traction forces are greater in a theoretical design which is restrained from rotation at the RP bearing. These theoretical results lend support to the notion that a rotating platform design may reduce wear by reducing cross-shear traction forces between the femoral component and the tibial insert.