We sought to determine what dimensional changes occurred from wear testing of a total knee implant, as well as whether any changes developed within the polyethylene subsurface. Three fixed bearing implants underwent wear simulator testing to 6.1 million cycles. Gravimetric analysis and micro-CT scans were performed pre-test, mid-test, and post-test. Wear volume and surface deviations were greater during 0–3.2 million cycles (91±12 mm3) than from 3.2–6.1 million cycles (52±18 mm3). Deviations (wear and creep) occurred across all surfaces of the tibial inserts, including the articular surface, backside surface, sides, and locking mechanism. No subsurface changes were found. The micro-CT results were a useful adjunct to gravimetric analysis, better defining the dimensional changes that occurred with testing and ruling out subsurface fatigue.
Implant wear continues to be a limitation of total knee replacement (TKR). Wear simulator studies are a valuable screening tool in new implant development. The purpose of this study was to determine the ability of micro-CT to prospectively measure wear in TKR implants during a wear simulator trial. Three identical cruciate-retaining, fixed bearing cobalt-chromium-molybdenum (CoCrMo) on conventional EtO-sterilized polyethylene TKA implants underwent wear simulator testing up to 3.2 million cycles using gait inputs; loaded-soaks were used to correct for fluid absorption. The implants were weighed and scanned with micro-CT (at 50 micron resolution) before and after testing. The gravimetric mass was converted to volume based on the density of polyethylene. Volume change due to wear was calculated from both the gravimetric and micro-CT methods. The pre- and post-wear test micro-CT geometries were co-registered and the deviations between the two were measured.BACKGROUND:
METHODS:
The most common bearing couple used in total knee arthroplasty (TKA) is ultra-high molecular weight polyethylene (UHMWPE) articulating against a CoCrMo alloy femoral component. Although this couple has demonstrated good clinical results, UHMWPE wear has been identified as one of the principal causes for long-term failure of total knee joint replacements1 indicating a need for improvements in TKA bearings technology. The wear resistance of UHMWPE can be improved by radiation crosslinking; however, in order to get the full benefit of this improved wear resistance, an abrasion resistant ceramic counterface is necessary.2 Since the radiation crosslinking degrades mechanical properties, it is also important to have an optimized radiation dose and subsequent processing. The purpose of this study was to evaluate the long-term wear performance of VERILAST Technology comprising two advanced bearing technologies, abrasion resistant OXINIUM femoral components (OxZr)3-4 and wear/strength optimized 7.5 Mrad crosslinked polyethylene (7.5-XLPE).5 Three component assemblies of LEGION(tm) cruciate retaining (CR) OxZr femoral components, 7.5-XLPE tibial inserts were tested on an AMTI knee simulator under displacement control at 1 Hz frequency as described previously.2 The tibial inserts were manufactured from compression molded GUR 1020 UHMWPE, radiation crosslinked to 7.5 Mrad dose, remelted to extinguish free radicals, and sterilized by EtO. The wear test was conducted for 45 Mcycle, which was considered to be a conservative estimate for the amount of cycles that would occur during 30 years of typical in-vivo use based on the relationship between patient age and the number of loading cycles as reported in the literature.6-8Introduction
Materials and Methods
