## 1 Introduction  
Uni-compartmental knee arthroplasty (UKA) has recently become a popular procedure owing to its excellent treatment of anteromedial osteoarthritis (OA), particularly for focal defect treatment in younger patients and in those with early localized OA of the knee [1; 2; 3]. Although several clinical studies have reported that UKA can mimic the motion of an intact knee [4; 5], and some authors have shown outstanding results over 10 years of follow-up with up-to-date designs [1; 6], failures with this procedure have been reported [7; 8]. The clinical outcomes of UKA have shown four main postoperative problems: malpositioning of the prosthetic parts, tibial bearing wear, OA progression in the remaining compartments, and localized tibia bone stress [7; 8; 9; 10]. Because of the recently strict selection of patients and advanced surgical techniques, the long-term survival rate post-UKA has been significantly enhanced [3]. In both types, fixed and mobile bearings, the wear of tibial bearings made from ultra-high-molecular-weight polyethylene (UHMWPE) is an important issue in the long-termperformance of UKA [8; 11]. The perceived requirement to lessen contact stress in a UKA design has led to the introduction of highly conforming and low-contact-stress designs. Although these implant geometries have indeed lessened contact stress on the articulating surface, the increase of conformity has tended to lead to an over-constraint in the joint, with a risk of mechanical failure due to secondary stresses occurring within the bearings [12]. Developments during the last decades have presented significant changes in manufacturing and sterilization methods, as well as material technologies, enhancing the mechanical properties, oxidative stability, and wear of UHMWPE [13; 14]. However, such an apparent trade-off between each of these factors with increasing cross-linkage is associated with a decrease in fatigue resistance [13; 14]. There have been several investigations of substitute materials for UHMWPE, including poly-ether-ether-ketone (PEEK) and carbon-fiber-reinforced PEEK (CFR-PEEK). [PERSON] et al. reported that highly conforming designs, such as total hip replacement and conforming UKA using CFR-PEEK, have shown low wear in in vitro knee wear simulations [15]. However, they recently stated that the wear rates of both PEEK and CFR-PEEK tibial bearings were nearly twice that of UHMWPE tibial bearings under comparable conditions for total knee arthroplasty (TKA) [16]. [PERSON] and [PERSON] reported that UKA with a CFR-PEEK tibial bearing provided lower volumetric wear rates than conventional metal-on-UHMWPE prostheses when tested under equivalent conditions [17]. Further research is required to substantiate whether CFR-PEEK is an appropriate substitute material for UHMWPE in terms of wear performance. However, as far as we know, there has been an information deficiency on tibia bone stress, which is an important criterion for aseptic loosening and anteromedial pain, when using PEEK and CFR-PEEK tibial bearing materials. Besides this, there has been no study that has evaluated contact stress on the other compartments with regard to the different materials. As previously mentioned, part malpositioning and wear problems can be overcome through proper surgical technique and a change in material, respectively. However, it is impractical to experimentally evaluate progressive OA on the other compartments and tibia bone stress problems based on a change in material. In addition, tibia bone stress is an important criterion for aseptic loosening and anteromedial pain [18; 19; 20]. The biomechanical effects of a change in tibial bearing material on the contact stress in the lateral meniscus and articular cartilage, and on stress on the tibia bone, can be evaluated in a practical manner using finite element (FE) analysis. Our prior study showed the biomechanical effect on varus/valgus conditions relating to UHMWPE and CFR-PEEK tibial bearing material in UKA.