Convened under the auspicious of esteemed endorsers - ISTA

Unicompartmental knee replacement components have gained favor because they replace onlythe most damaged areas of articular cartilage and the less invasive operation results in a fasterpatient recovery than traditional TKR. Additionally, they can provide a solution when a fullTKR is not yet needed. However, the wear magnitude of such implants is not well understood,primarily due the variation in design and the difficulty of testing them in knee simulatorsdesigned to test full TKRs. Modern innovative partial cartilage replacement knee componentswhich are typically even smaller and more bone conservative than unicompartmental implants,are even less common in testing with added challenges. This study investigates the fatiguecharacteristics of partial cartilage replacement knee components, and the wear of theUHMWPE bearing of a new, truly less invasive unicompartmental design by ArthrexInc./Florida.Fatigue testing was performed on MTS 858 MiniBionix machines. Two 12mm diameterUHMWPE tibial components were cemented into jigs at 0° posterior slope and were axiallyloaded at 2Hz for 10 million cycles (Mc) with a sinusoidal profile peaking at 60% of 8 averagehuman bodyweights (3800N) and a load ratio R of 0.1. Two femoral components were testedwith the same load profile at 10Hz for 10 million loading cycles (Mc). The femoral componentswere mounted at 15° flexion and only the anterior half of the implant was supported, replicatinga worst-case scenario where fixation had failed on the posterior half of the implant. Thisresulted in a large bending moment when force was applied that would fatigue the femoralimplant. Following the fatigue test, two full wear simulation tests were conducted on four12mm and four 20mm unicompartmental components on a four-station Instron-Stanmore forcecontrolknee simulator. The spring-based system to simulate soft-tissue restraining forces andtorques was adapted to operate the machine in a displacement control mode to achieve themotions of the medial compartment based on ISO 14243-3. The specimens were lubricatedwith bovine serum (20g/L protein, 37°C) and the simulator was operated at 1Hz. Liquidabsorption was corrected through passive-soak-control bearing inserts. The tibial specimenswere cleaned and weighed at standard intervals with the usual ISO test protocols.After 10Mc of fatigue testing, both tibial components had deformed by some flattening outbut were able to sustain the full load without failure and displayed average stiffness (over thewhole 10Mc) of 27,600±1,180 N/mm. Neither partially supported femoral component failed,and the femorals displayed average stiffness (over 10Mc) of 37,500 ±3,280N/mm. After 5Mcof wear testing, the 12mm tibial components displayed a wear rate of 4.56±1.45mg/Mc whilethe larger 20mm size wore at a lower 2.80±0.39mg/Mc.The results from the fatigue test suggest that this unicompartmental cartilage replacementdesign will not fail under simple axial loading, even under the extreme case where the tibialimplant is receiving the entire share of the load, and the femoral component is only partiallysupported. In the clinical application, of course some load-sharing with the native unworncartilage would occur, reducing the stresses on the implant. The results from the wear testshowed very low wear for tibial components of this design, lower than many successful TKRs.The larger size tibial components wore less likely due to reduced contact stress. Based on theresults of this test, an implant of this type could be a viable option prior to TKR.Thursday, October 7, 2010, 9:00-9:50Session A2: Alternate Bearings in THA 2file:///E|/ISTA2010-Abstracts.htm[12/7/2011 3:15:47 PM]