A KNEE SIMULATION STUDY OF A DIRECTLY MOLDED TIBIAL INSERT WITH A CONTEMPORARY RESIN AND TIBIAL INSERT DESIGN

Abstract

Directly molding IB, MG and AGC UHMWPE tibial inserts has provided excellent clinical performance. This performance may be related to the oxidation resistance and higher fracture toughness provided by the direct molding process. Directly molded UHMWPE components have been reported not to oxidize after either nine years post irradiation aging on the shelf or after 11 years of implantation. Retrievals show that molded IB inserts to have lower oxidation, better polyethylene quality and less surface damage than machined IB II inserts. However, the IB, MG and AGC products were directly molded from 1900 UHMWPE resin which is no longer available. The question remains if directly molding resins other than 1900 in a contemporary modular design will provide the same benefits. We report here on the first knee simulation wear of a contemporary total knee system comprised of a directly molded 1020 esin tibial insert. This result will be compared to the knee simulation result of an IBII machined from 4150 extruded ro 4 Optetrak tibial inserts made by directly molding 1020 resin were tested on a 4 station Instron/Stanmore simulator at 1.4 Hz with a 2279 N maximum load and right knee kinematics. The lubricant was bovine calf serum with EDTA and sodium azide. Axial loads were applied from 0 to 40& #778; flexion and internal/external rotation was −3/+6 degrees. Location, type and area of surface damage, were evaluated every 1 million cycles (Mc).

The wear rate of the directly molded inserts was 6X less than reported for machined IB II inserts (2 vs 12 mg/million cycles respectively). There were no signs of delamination or pitting with either design. The more conforming Optetrak provided 52% reduction in wear area over the IB II (21 vs 32 % respectively). This demonstrates that resins other than 1900 may be directly molded in a contemporary and provide the same historical advantages.