Methodology for long-term wear testing of total knee replacements

This article begins to address the validation requirements of wear testing on total knee replacements in a knee simulator. The knee simulator has four stations. The axial force is variable but reaches a maximum of 2.3 kN, Phy siologic anteroposterior shear force and rotational torques are supplied to the knee. The forces and displacements ars:limed to coincide with those of a typical gait cycle, Kinematics of the simulator are dependent on the typ e of knee being tested. Tests of designs with well known clinical histories were done to 10 million cycles. The relative amounts and types of Near sho wn by the designs were similar to that found in their clinical histories. W ear tracks on more conforming designs were larger and the penetration into the plastic appeared to be less. This did not necessarily mean that wear, a s measured by loss of material, was reduced an conforming designs. Delamina tion of the plastic was achieved only after aging the tibial components. We ar particles isolated from the lubricating fluid were similar in size and s hape to those isolated from in vivo specimens. However, the relative amount s of wear particle shapes were different depending on the design. At the st are of the tests, all, of the flexibly mounted tibial components showed Mor e motion than after 5 million cycles, indicating that the surface of the pl astic became more conforming. This study showed that knee wear similar to w ear observed in vivo can be reproduced in the laboratory, The parameters an d methods elucidated in this introductory study should form the basis for u se in preclinical wear tests of total knee replacements.