## 4 Discussion  
In this study, the most significant finding was that other compartments' contact stress and tibial bone stress, which are the most serious postoperative problems of UKA, are improved when using CFR-PEEK instead of UHMWPE. Recently, there have been many developments and improvements in orthopedic implants, which require significant attention and are surgically used to treat joint diseases and traumatic injuries, along with their associated complications [48]. For example, knee joint OA is very common in people over 60 years of age. For severe cases of knee OA, TKA is the standard type of care, relieving the patient's pain and allowing them to return to their ordinary daily activities [49; 50]. In spite of such improvement, the lifetime of the prosthesis is fundamentally limited, with infection, instability, aseptic loosening, periprosthetic fractures, and mechanical wear being the reasons for TKA revision [51; 52]. Consequently, many patients need to undergo TKA revision within 10 years after their primary surgery. Consequently, novel implants with advanced materials are being presented and evaluated to improve implant survival rates and reduce the burden of associated TKA revision [53]. PEEK and CFR-PEEK are popular as implantable orthopedic biomaterials used in spinal surgery, usually in the form of intervertebral cages [48; 54]. There has recently been growing interest in improvements in the affinity of PEEK and CFR-PEEK for bone ongrowth and ingrowth applications. CFR-PEEK is a biocompatible material and is sufficiently durable for long-term implantation. Its elastic modulus is very similar to that of bone, and it can endure prolonged fatigue strain [48]. In a previous study, PEEK and CFR-PEEK were investigated as substitute bearing materials for metallic and ceramic interfaces [54]. Research on simple pin-on-plate configurations, with PEEK or CFR-PEEK pins articulating against cobalt-chromium, zirconia-toughened alumina ceramic, and polymeric plates, has generally proved that CFR-PEEK has equivalent or better wear performance when compared to conventional polyethylene materials [55; 56]. Although PEEK does not appear to provide equivalent wear performance against hard bearings, pin-on-plate studies have shown that it may be a suitable material for articulation with another polymer [57]. As mentioned above, the currently used UHMWPE may cause worse problems for UKA than for TKA. Because UKA is also used as a surgical treatment for younger patients, the functionality of the tibial bearing is more important. In addition, as far as we know, there has been no research regarding the biomechanical effects in other compartments or with the tibial bone when using CFR-PEEK as a tibial bearing. We found that the lowest contact stress occurred on the lateral meniscus and tibial cartilage when using CFR-PEEK. In addition, the greatest contact stress was found during the stance phase, during which an axial force occurred. This trend was equivalent to those found in previous research [26; 58]. Because the contact stress is mainly affected by compressive loading, during the swing phase, where there is no compressive load, no difference was observed in the contact stress with the change in  
Figure 6: Comparison of von Mises stress in ROIs 1–5 with respect to different tibial bearing materials during the gait cycle: (**a**) ROI 1; (**b**) ROI 2; (**c**) ROI 3; (**d**) ROI 4; (**e**) ROI 5.  
material. In addition, the difference in contact stress for the different materials was greater in the lateral meniscus than in the articular cartilage, which is due to the anatomical difference between the lateral meniscus and tibial cartilage. A meniscus is a bumper for an articular surface, equivalent to a bearing; therefore, there is a greater difference in the lateral meniscus with respect to the change in material. Aseptic loosening of the prosthetic parts, particularly in the tibia, is one of the major failure modes of UKA [4].