Combined Partial Knee Arthroplasty (CPKA) is a promising alternative to Total Knee Arthroplasty (TKA) for the treatment of multi-compartment arthrosis. Through the simultaneous or staged implantation of multiple Partial Knee Arthroplasties (PKAs), CPKA aims to restore near-normal function of the knee, through retention of the anterior cruciate ligament and native disease-free compartment. Whilst PKA is well established, CPKA is comparatively novel and associated biomechanics are less well understood. Clinically, PKA and CPKA have been shown to better restore knee function compared to TKA, particularly during fast walking. The biomechanical explanation for this superiority remains unclear but may be due to better preservation of the extensor mechanism. This study sought to assess and compare extensor function after PKA, CPKA, and TKA. An instrumented knee extension rig facilitated the measurement extension moment of twenty-four cadaveric knees, which were measured in the native state and then following a sequence of arthroplasty procedures. Eight knees underwent medial Unicompartmental Knee Arthroplasty (UKA-M), followed by patellofemoral arthroplasty (PFA) thereby converting to medial Bicompartmental Knee Arthroplasty (BCA-M). In the final round of testing the PKA implants were removed a posterior-cruciate retaining TKA was implanted. The second eight received lateral equivalents (UKA-L then BCA-L) then TKA. The final eight underwent simultaneous Bi-Unicondylar Arthroplasty (Bi-UKA) before TKA. Extensor efficiencies over extension ranges typical of daily tasks were also calculated and differences between arthroplasties were assessed using repeated measures analysis of variance. For both the medial and lateral groups, UKA demonstrated the same extensor function as the native knee. BCA resulted in a small reduction in extensor moment between 70–90° flexion but, in the context of daily activity, extensor efficiency was largely unaffected and no significant reductions were found. TKA, however, resulted in significantly reduced extensor moments, leading to efficiency deficits ranging from 8% to 43% in flexion ranges associated with downhill walking and the stance phase of gait, respectively. Comparing the arthroplasties: TKA was significantly less efficient than both UKA-M and BCA-M over ranges representing stair ascent and gait; TKA showed a significant 23% reduction compared to BCA-L in the same range. There were no differences in efficiency between the UKAs and BCAs over any flexion range and TKA efficiency was consistently lower than all other arthroplasties. Bi-UKA generated the same extensor moment as native knee at flexion angles typical of fast gait (0–30°). Again, TKA displayed significantly reduced extensor moments towards full extension but returned to the normal range in deep flexion. Overall, TKA was significantly less efficient following TKA than Bi-UKA. Recipients of PKA and CPKA have superior functional outcomes compared to TKA, particularly in relation to fast walking. This in vitro study found that both UKA and CPKA better preserve extensor function compared to TKA, especially when evaluated in the context of daily functional tasks. TKA reduced knee extensor efficiency by over 40% at flexion angles associated with gait, arguably the most important activity to maintain patient satisfaction. These findings go some way to explaining functional deficiencies of TKA compared to CPKA observed clinically

INTRODUCTION. There is strong current interest to provide reliable treatments for one- and two-compartment arthritis in the cruciate-ligament intact knee. An alternative to total knee arthroplasty is to resurface only the diseased compartments with discrete compartmental components. Placing multiple small implants into the knee presents a greater surgical challenge than total knee arthroplasty, and it is not certain natural knee mechanics can be maintained. The goal of this study was to compare functional kinematics in cruciate-intact knees with either medial unicondylar (mUKA), mUKA plus patellofemoral (mUKA+PF), or bi-unicondylar (biUNI) arthroplasty using discrete compartmental implants with preparation and placement assisted by haptic robotic technology. METHODS. Nineteen patients with 21 knee arthroplasties consented to participate in an I.R.B. approved study of knee kinematics with a cruciate-retaining multicompartmental knee arthroplasty system. All subjects presented with knee OA, intact cruciate ligaments, and coronal deformity ranging from 7° varus to 4° valgus. All subjects received multicompartmental knee arthroplasty using haptic robotic-assisted bone preparation an average of 13 months (6–29 months) before the study. Eleven subjects received mUKA, five subjects received mUKA+PF, and five subjects received biUKA. Subjects averaged 62 years of age and had an average body mass index of 31. Combined Knee Society Pain/Function scores averaged 102 ± 28 preoperatively and 169 ± 26 at the time of study. Knee range of motion averaged −3° to 120° preoperatively and −1° to 129° at the time of the study. Knee motions were recorded using video-fluoroscopy while subjects performed step-up/down, kneeling and lunging activities. The three-dimensional position and orientation of the implant components were determined using model-image registration techniques (Fig. 1). The AP locations of the medial and lateral condyles were determined by computing a distance map between the femoral condyles and the tibial articular surfaces. RESULTS. Knee kinematics during maximum flexion kneeling and lunging showed tibial internal rotation, and posterior lateral condylar translation for all three treatments (Fig. 2). All knees showed femoral external rotation and posterior condylar translation with flexion during the step activity (Fig. 3). In all three activities, knees with mUKA and mUKA+PF arthroplasty showed the most femoral external rotation and posterior translation, and knees with biUKA showed the least. DISCUSSION. Knees with tricompartmental arthroplasty usually sacrifice one or both cruciate ligaments and also exhibit kinematics which differ from the normal knee. In particular, tibiofemoral rotations are almost always significantly less than the normal knee, and often the femur translates forward with flexion over some portion of the motion arc. In contrast, knees with accurately-placed uni- or bi-compartmental arthroplasty exhibited stable knee kinematics consistent with intact and functioning cruciate ligaments. The patterns and magnitudes of tibiofemoral motion were more similar to natural knees in the mUKA and mUKA+PF groups than commonly has been observed in knees with total knee arthroplasty. These results demonstrate the potential to restore or maintain closer-to-normal knee kinematics by retaining intact structures and compartments. Knees with an intact lateral compartment had kinematics closer to normal than those where both tibiofemoral compartments were diseased/replaced

Introduction. Traditional Total Knee Arthpolasty (TKA) replaces all 3 compartments of the knee for patients diagnosed with OA. There might be functional benefit to replacing only damaged compartments, and retaining the normal ligamentous structures. There is a long history of performing multi-compartment arthroplasty with discrete components. Laskin reported in 1976 that good pain relief and acceptable clinical results were achieved at two years in patients with bi-unicondylar knee replacement [Laskin 1976]. Other authors also have reported on bi-unicompartmental knee arthroplasty achieving successful clinical outcomes [Stockley 1990; Confalonieri 2005]. Banks et al. reported that kinematics of bi-unicompartmental arthroplasties during gait demonstrated some of the basic features of normal knee kinematics [Banks 2005]. These reports suggest that a modular approach to resurfacing the knee can be successful and achieve satisfactory clinical and functional results. Objective. The primary objective of this study is to compare the functional outcomes of three patient groups treated for osteoarthritis. Methods. Subjects received either a modular, multicompartment knee arthroplasty (MKA) implanted with robotic-arm assistance(MAKO Surgical Corp., Fort Lauderdale, FL), a computer assisted TKA (TKA CAS) or a TKA implanted using traditional manual instrumentation (TKA T). Patients that were eligible to receive a TKA were randomly selected to receive computer assisted or traditional surgical technique and blinded to the type of TKA surgical technique utilized. We report post-operative functional outcomes including Range of Motion (ROM), Timed-up and go(TUG), and Quad strength at time intervals of 2 weeks, 6 weeks, 3 months and 6 months. The TUG test is a validated measure of patient mobility where a patient is asked to stand up from a chair, walk three meters turn around and sit back down [Boonstra, 2008]. The Quad strength assessment is measured with a hand held dynamometer (Lafayette Instruments, Lafayette, IN) while patient was seated with leg at 90 degrees flexion. The patient is asked to extend their knee while a physical therapist provides resistive forces to maintain static knee flexion. All tests were administered by one physical therapist. Results. Patients that underwent MKA saw significant increase in ROM post-operatively when compared to TKA CAS patients (P<0.009) and TKA T patients (p<0.003), Figure 1. Patients that underwent MKA also saw an increase in Quad Strength, however this was only statistically significant between the MKA and TKA CAS groups, (P<0.04), Figure 2. Patients that underwent MKA saw a reduction in TUG which indicated an improved mobility post-operatively, Figure 3. The reduced TUG was only statistically significant for MKA patients compared to TKA T patients (P<0.005). There was no statistical significance seen between the two TKA groups for any functional measure. Discussion. Initial findings do indicate a short term improvement in functional outcomes for MKA patients when compared to TKA patients. Additional data clinical and functional data is being collected and enrollment is continuing for this study