Tribological Analysis of UHMWPE Tibial Implants in Unicompartmental Knee Replacements: From Retrieved to In Vitro Studies

Abstract

This study focuses on the identification of velocity accommodation mechanisms of UHMWPE tibial implants from unicompartmental knee replacements.A tribological analysis of two retrieved medial unicompartmental knee implants was performed. The first implant presented a high wear rate at 7years follow-up with the velocity accommodation mechanisms characterized by the formation of a plasticized thin and smooth 3rd body layer which was drawn until small UHMWPE particles were detached. The second implant presented good performances at 11years follow-up with a lower wear rate, with the velocity accommodation mechanisms characterized by the formation of a discontinuous 3rd body layer composed of small UHMWPE wear particles and biological molecules from synovial fluid.An important finding of the tribological analysis of the two retrieved UHMWPE tibial implants is that the 3rd body layer plays a key role in the wear rates, influencing the velocity accommodation mechanisms.An in vitro study of two identical UHMWPE tibial implants using two lubricating solutions, with different compositions and structures (a calf serum and a Gel-IN calf serum) was performed to explain the root causes of the major difference between the 3rd body layers formed in vivo.It was observed that the calf serum led to a high friction factor, as the molecules were unable to bind to the UHMWPE surface. However, the Gel-IN calf serum led to a lower friction factor, with the formation of a 3rd body layer on the UHMWPE surface due to protein and lipid adsorption.