Introduction. Quadriceps performance following total knee arthroplasty (TKA) is a critical factor in patient satisfaction that can be significantly affected by implant design (Greene, 2008). The objective of this study was to compare quadriceps efficiency (QE) following TKA with a medial-pivot system (EVOLUTION®, MicroPort Orthopedics Inc., Arlington, TN, USA) to non-implanted control measurements. Methods. Five cadaveric leg specimens with no prior surgeries, deformities, or disease were obtained. Each was placed in a custom closed chain device and loaded to simulate a heel-up squat from full-extension to deep flexion (approximately 115°) and back to full extension. Quadriceps force (FQ) and ground reaction force (FZ) were measured, and the ratio of the two was calculated as the quadriceps load factor (QLF). QFLs are inversely related to QE, with higher QFLs representing reduced efficiency. Each specimen was then implanted with a medial-pivot implant by a board certified orthopedic surgeon and force measurements were repeated. Mean pre- (represents control values) and post-implantation QFLs were compared to determine any differences in QE throughout the range of motion. Results. Mean QFLs were not statistically different for pre- and post-implantation measurements throughout loading (Figure 1). QE was increased in the post-implantation measurements compared to pre-implantation between approximately 80° and 115° flexion and reduced between 5° and 80°. The mean peak post-implantation QFL was 5% less than that measured pre-implantation. Discussion. Quadriceps muscles were least efficient during peak flexion (80°–115°) when FQ was highest during both pre- and post-implantation measurements. The similar QE seen between the pre- and post-implantation measurements for most of the range of motion could be a result of the system design, which seeks to mimic the kinematics of the normal knee (Schmidt, 2003). The observed nearly-linear change in the FQ through 75° is likely due to the combination of the medial spherical radius and the conformity of the medial tibial insert socket that provides a constant moment arm on which the extensor mechanism can act. The primary driver of decreasing efficiency of the extensor mechanism is the increasing moment arm of the load with increasing flexion. The second increase in FQ in deep flexion (>110°) for the implanted measurements is likely due to the smaller closing radius on the femoral component in this range. These preliminary data have the potential to be significant clinically in that decreased QE may result in increased quadriceps forces manifesting in anterior knee pain or patient fatigue. Additionally, increased QE may play a role in rehabilitation and return to activities of daily living. The current results show the medial-pivot system may increase QE during peak flexion and does not significantly reduce QE during midflexion when compared to control. In-vivo testing is needed to confirm if these results translate to clinical practice

Introduction. Total knee replacement (TKR) implant designs and materials have been shown to have a significant impact on tibial insert wear. A medial-pivot (MP) design theoretically should generate less wear due to a large contact area in the medial compartment and lower contact stresses. Synovial fluid aspiration studies have confirmed that a first generation MP TKR system (ADVANCE®, MicroPort Orthopedics Inc., Arlington, TN, USA) generates less wear debris than is seen with other implant designs articulating against conventional polyethylene (CP). Objectives. The objective of this study was to evaluate the in vitro wear rate of a second generation MP TKR system (EVOLUTION® Cruciate-Sacrificing, MicroPort Orthopedics Inc., Arlington, TN, USA) using CP tibial inserts and compare to previously published values for other TKR designs with CP and first or second generation crosslinked polyethylene (XLPE) tibial inserts. Methods. In vitro wear was assessed for five MP CP tibial inserts, each loaded for 5 megacycles (Mc) of simulated gait in accordance with ISO 14243–3. Insert cleaning and wear measurements were performed every 0.5 Mc in accordance with ISO 14243–2. Manufacturer websites and the MEDLINE database were searched for previously published in vitro wear rates for other TKR designs used in combination with CP and first or second generation XLPE inserts. Second generation XLPE inserts are those with additives or additional manufacturing, such as sequentially annealed and irradiated XLPE (X3®, Stryker, Mahwah, NJ, USA) and vitamin E infused polyethylene (E1®, Biomet, Warsaw, IN, USA). All TKR designs utilized cobalt-chrome (CoCr) femoral components, except Legion-Verilast that included Oxinium™ femoral components (Smith & Nephew, Memphis, TN, USA). Results. The mean wear rate for the MP system (2.0+0.2 mg/Mc) was less than half the wear rates reported for other TKR designs using CP inserts (Figure 1). The wear was also reduced or similar to those reported for all but three designs used in combination with XLPE inserts (Figure 2). Interestingly, wear rates for the MP system were approximately one-third of those reported for E1 and X3 used in combination with the Scorpio and Triathlon CR TKR systems (Stryker, Mahwah, NJ, USA). The main limitation to the current study is the use of literature comparators. While the comparison studies were all conducted using similar methods on knee wear simulator machines, there were some experimental differences that could potentially impact wear rates (e.g. diluted vs. non-diluted serum, gait patterns, types of testing machines). Conclusions. In vitro wear for a second generation MP TKR system was similar or lower than what has been previously reported for other TKR systems used with CP or XLPE tibial inserts. These results suggest that implant design may play a larger role in TKR wear debris generation than the material used for the tibial insert

Prior implant designs have relied on a four-bar link theory and featured J-curve femoral components intended to recreate femoral rollback of the native knee, but this design could lead to anterior femoral sliding or paradoxical motion. Recent kinematic analyses of the native human knee have shown the medial compartment to be more stable to anteroposterior translation than the lateral, resulting in a “medial pivot” motion as the knee flexes. “Medial pivot” designs in total knee arthroplasty were introduced in the 1990s to attempt to re-create this motion. They consist of an asymmetric tibial insert with a highly congruent medial compartment and less conforming lateral compartment. The femoral component has a single radius of curvature and a high degree of conformity. In vivo fluoroscopic studies have shown medial pivot designs to be successful in achieving its intended motion, while other cruciate-retaining designs had a higher incidence of paradoxical anterior translation and lateral condylar lift-off. Furthermore, numerous investigations have shown medial pivot designs to have excellent outcomes and survivorship at up to 10 years post-operatively. However, the contention in this debate that medial pivot designs avoid the need for ligament balancing is incorrect. Appropriate ligament balancing remains a crucial aspect of any successful total knee arthroplasty and is no less important based on the implant design utilised. In the Methods section of all prior reports using a medial pivot design, the authors have noted that appropriate ligament balancing was obtained both in flexion and extension consistent with the recommended technique with other primary TKA implant designs. From a kinematic standpoint, this makes absolute sense. If a patient has a valgus imbalance with loose medial structures, then as the knee is brought into flexion the femur will not maintain congruency and contact with the conforming tibial surface – thus the medial pivot motion will be lost. Thus, balancing remains critical. Lastly, although not the focal point of this debate, whether re-creation of a medial pivot motion in total knee arthroplasty actually improves patient outcomes remains an area of debate. A recent investigation by Warth and Meneghini, et al. demonstrated that re-creation of a medial-pivot pattern intra-operatively did not correlate with patient-reported outcomes at 1-year post-operatively. Thus, although the concept of a medial pivot design has merit, whether this will consistently improve outcomes and patient satisfaction remains to be seen