Unicompartmental knee replacement components have gained favor because they replace only the most damaged areas of articular cartilage and the less invasive operation results in a faster patient recovery than traditional TKR. Additionally, they can provide a solution when a full TKR 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 simulators designed to test full TKRs. Modern innovative partial cartilage replacement knee components which are typically even smaller and more bone conservative than unicompartmental implants, are even less common in testing with added challenges. This study investigates the fatigue characteristics of partial cartilage replacement knee components, and the wear of the UHMWPE bearing of a new, truly less invasive unicompartmental design by Arthrex Inc./Florida.

Fatigue testing was performed on MTS 858 MiniBionix machines. Two 12mm diameter UHMWPE tibial components were cemented into jigs at 0° posterior slope and were axially loaded at 2Hz for 10 million cycles (Mc) with a sinusoidal profile peaking at 60% of 8 average human bodyweights (3800N) and a load ratio R of 0.1. Two femoral components were tested with the same load profile at 10Hz for 10 million loading cycles (Mc). The femoral components were mounted at 15° flexion and only the anterior half of the implant was supported, replicating a worst-case scenario where fixation had failed on the posterior half of the implant. This resulted in a large bending moment when force was applied that would fatigue the femoral implant. Following the fatigue test, two full wear simulation tests were conducted on four 12mm and four 20mm unicompartmental components on a four-station Instron-Stanmore force-control knee simulator. The spring-based system to simulate soft-tissue restraining forces and torques was adapted to operate the machine in a displacement control mode to achieve the motions of the medial compartment based on ISO 14243-3. The specimens were lubricated with bovine serum (20g/L protein, 37°C) and the simulator was operated at 1Hz. Liquid absorption was corrected through passive-soak-control bearing inserts. The tibial specimens were 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 out but were able to sustain the full load without failure and displayed average stiffness (over the whole 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 5Mc of wear testing, the 12mm tibial components displayed a wear rate of 4.56±1.45mg/Mc while the larger 20mm size wore at a lower 2.80±0.39mg/Mc.

The results from the fatigue test suggest that this unicompartmental cartilage replacement design will not fail under simple axial loading, even under the extreme case where the tibial implant is receiving the entire share of the load, and the femoral component is only partially supported. In the clinical application, of course some load-sharing with the native unworn cartilage would occur, reducing the stresses on the implant. The results from the wear test showed 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 the results of this test, an implant of this type could be a viable option prior to TKR.