## 4 Discussion
### Anterior-Posterior Shift and Medial-Lateral Tilt of the Patella  
For the anterior-posterior shift of the patella, relative to the femur, the results of [PERSON] et al. 's [31] in vitro experiments showed that with the flexion of the knee joint, the patella shifted backward; after 30 degree knee flexion, posterior movement rate of the patella increased, and the maximum posterior movement of the patella reached 32 mm. The results of [PERSON] et al. [29], based on the experimental dynamic finite element model, also show that the patella continues to shift backward with the knee flexion, and the maximum posterior movement of the patella is about 30 mm. The motion analysis results of patella anterior-posterior shift in this paper are consistent with those of the above studies; with knee flexion, patella continued to shift backward. Meanwhile, the analysis results of this paper gave the data of the posterior shift of the patella during high knee flexion. At 135 degree knee flexion, the posterior shift of the patella after the upper end, the lower end, and both ends of the patellar tendon were released to reach the maximum values of 61.82 mm, 62.57 mm, and 61.07 mm, respectively, which was 5%, 4%, and 6% lower than the no-release values of 64.97 mm. For the medial-lateral tilt of the patella relative to the femur with the flexion of the knee joint, the patella relative to the femur exhibits an inward motion pattern before and after release. At the same degree of flexion, the medial tilt angle of the patella relative to the femur was larger than that of the no-release model, the increase of the both ends release was the maximum (30%), and the increase of the lower end release was the minimum (10%). The relevant findings [27; 29; 31; 32] also showed that the patella, with the flexion of knee, tilted inward relative to the femur, which is consistent with the results of this study. At the same time, there are different research results for the medial-lateral tilt motion of the patella [33]. The results of [PERSON] et al. [33] showed that the patella continued to tilt outward with knee flexion, which may be related to the loading method that the quadriceps were divided into six bundles. There are differences in related studies, which may be related to the fact that the patella was less constrained by the femoral articular surface in the direction of patella medial-lateral tilt. Several limitations of the present study should be noted. In this study, only 30% of the patella was released, and others were not analyzed. The finite element model of the human knee joint after TKA was established in this paper, based on the consideration of all bone tissue and the main soft tissue of the knee; although the transverse patellar ligaments, which play an important role in the movement of the patella, are added, the effect of the joint capsule, hamstring, or gastrocnemius on knee joint activity was not considered. In the subsequent analysis of the knee joint, these tissues could be taken into account to make the model more in line with human physiology. The anterior cruciate ligament (ACL) has the functions of restricting the forward movement of the tibia and controlling the internal and external valgus of the knee [34; 35; 36]. The ACL is extremely vulnerable during exercise and it is difficult to repair itself. If not treated in time, the patient's daily activities will be seriously affected [37]. The posterior cruciate ligament (PCL), aside from providing restraint to posterior tibial translation, is reported to have a considerable role in providing rotational stability to the knee [38]. Injuries to the PCL are far less common than injuries to other knee structures, such as the ACL or the menisci [39]. In addition, it has been found that the clinical outcomes of PCL reconstruction are less satisfactory and less predictable relative to those of ACL reconstruction [39].