## 4 Discussion
Therefore, aseptic loosening of the tibial part has been attributed to abnormally low or high stress/strain of trabecular bone, both of which induce substantiating bone absorption. In addition, aseptic loosening is also caused by UHMWPE wear-debris-induced osteolysis. In the present research, we mostly evaluated the trabecular bone stress at the implant-bone interface and observed certain findings. We revealed that stress on the tibial bone ROIs 1-4 with CFR-PEEK and PEEK was greater than that with UHMWPE. Although the tibial part is made of titanium alloy, the CFR-PEEK tibial bearing is equivalent to bone in terms of its mechanical properties and may lessen the aseptic loosening in the tibial part with less of a stress-shielding effect. Some patients have problems with anteromedial pain over the proximal tibia in daily life, early after implantation, which almost always decreases during the first year. However, in a small number of patients, it persists and may even worsen. It is therefore important to understand why such pain occurs, why it spontaneously decreases in most cases, and why it remains and worsens in other cases. The elastic modulus of cortical bone, being even larger than that of trabecular bone, always leads to most of the load being transferred by the tibial baseplate. The abnormally high stress/strain on the proximal medial area of cortical bone has been used to explain the persistent pain post-UKA. Therefore, the level of stress was analyzed in ROI 5 with respect to the tibial bearing material applied. Our results may be explained by the change in bearing material being a good remedy for anteromedial pain, that is, the mechanical properties of CFR-PEEK are more similar to those of bone in comparison to UHMWPE. The stress increases in the cortical bone and the stress decreases in the trabecular bone around the tibial baseplate may lead to pain, inducing loosening of the tibial baseplate [59; 60; 61; 62]. Therefore, CFR-PEEK showed the potential to decrease anteromedial pain and loosening of the tibial baseplate when compared to PEEK and UHMWPE by way of its decreased stress on cortical bone and increased stress on trabecular bone. In addition, unlike contact stress occurring in other compartments, stress on the bone is influenced during the gait cycle. Stress is an important factor not only in terms of axial force but also in translation and rotation, unlike contact stress. We recently showed that a CFR-PEEK pedicle screw rod is a good device with regard to the degeneration in the adjacent segment in the lumbar spine as compared to titanium [39]. In other words, it is very beneficial for the mechanical properties of the implant to be equivalent to those of bone. It is important to emphasize some of the strengths derived from this research. First, unlike prior research on UKA, the FE model applied in this research included the femur and related soft tissues, as well as the tibia [18; 19; 36]. Second, unlike the conventional biomechanical model of UKA, this research included the application of the gait cycle loading condition, as opposed to the application of a vertical static loading condition [18; 19; 20; 36; 40; 59]. Third, the current research validated the intact model, and the UKA model was also validated by anterior tibial translation (ATT) and posterior tibial translation (PTT) comparisons with experiments from a prior study. Nevertheless, some limitations should be noted. First, the lateral compartment was regarded as an elastic material. In addition, no effects of anisotropy and viscoelasticity were considered. Second, the UKA designs and anatomy were regarded and virtually created and implanted from just one subject and one prosthetic product. Third, the UKA had been fully bonded and there was no consideration given in this paper to micro-motion that may occur between the PE bearing and the tibial part. Lastly, we only considered change in the tibial bearing material. In future studies, the relationship between tibial baseplates will be studied.