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:
Large diameter femoral heads offer increased range of motion and reduced risk of dislocation. However, their use in total hip arthroplasty has historically been limited by their correlation with increased polyethylene wear. The improved wear resistance of highly crosslinked UHWMPE has led a number of clinicians to transition from implanting traditionally popular sizes (28mm and 32 mm) to implanting 36 mm heads. Desire to further increase stability and range of motion has spurred interest in even larger sizes (> 36 mm). While the long-term clinical ramifications are unknown, in-vivo measurements of highly crosslinked UHMWPE liners indicate increases in head diameter are associated with increased volumetric wear [1]. The goal of this study was to determine if this increase in wear could be negated by using femoral heads with a ceramic surface, such as oxidized Zr-2.5Nb (OxZr), rather than CoCrMo (CoCr). Specifically, wear of 10 Mrad crosslinked UHMWPE (XLPE) against 36 mm CoCr and 44 mm OxZr heads was compared. Ram-extruded GUR 1050 UHMWPE was crosslinked by gamma irradiation to 10 Mrad, remelted, and machined into acetabular liners. Liners were sterilized using vaporized hydrogen peroxide and tested against either 36 mm CoCr or 44 mm OxZr (OXINIUM(tm)) heads (n=3). All implants were manufactured by Smith & Nephew (Memphis, TN). Testing was conducted on a hip simulator (AMTI, Watertown, MA) as previously described [2]. The 4000N peak load (4 time body weight for a 102 kg/225 lb patient) and 1.15 Hz frequency used are based upon data obtained from an instrumented implant during fast walking/jogging and have previously been shown to generate measurable XLPE wear [2,3]. Lubricant was a serum (Alpha Calf Fraction, HyClone Laboratories, Logan, UT) solution that was replaced once per week [2]. Liners were weighed at least once every million cycles (Mcycle) over the duration of testing (∼ 5 Mcycle). Loaded soak controls were used to correct for fluid absorption. Single factor ANOVA was used to compare groups (a = 0.05).Introduction
Materials and Methods
