Worldwide 600,000 total knee reconstructions are performed annually, which has further spurred the investigation of the prostheses designs and materials. Currently, the most favored materials used in knee prostheses are CoCr alloy for the femoral and tibial components and the ultra-high-molecular-weight polyethylene polymer (UHMWPE) as the load bearing material for the tibial insert. UHMWPE has been in use for over 30 years due to its low wear rate, bio-inertness, and strong material properties.
UHMWPE tibial inserts are biocompatible, however, particulate UHMWPE debris resulting from wear can cause biological responses. In vitro studies can accurately quantify polyethylene wear rates and help modeling the wear processes in an effort to reduce the amount of debris generated and minimize its impact in vivo. Of particular interest are the wear-in phase of the prosthesis, which has the largest wear rate, and the transport pathways of the debris to the interfaces between bone and CoCr, where the harmful biological responses occur. The wear-in phase is the period of a high wear rate during the initial conformation of the UHMWPE to the metal femoral condyle. The common method of measuring in vitro wear rates with the gravimetric method of weighing the prosthesis before and after actuation lacks accuracy.
As an alternative, radioisotope tracing has been trialed and utilized for over 30 years in measuring the steady-state wear rates of commercial prostheses in vitro. In such work, recoil implantation has been used to introduce the tracer ⁷Be. This technique, however, is not well suited to studies of the initial wear-in phase of a prosthesis. This is because the high-energy recoil implantation of ⁷Be cannot label the first 28 µm of material without stopping foils. Therefore, the potential of the low-energy, direct ion implantation of the radioisotope ¹¹¹In has been studied in comparison with the established approach of recoil-implanting ⁷Be.