The present study demonstrated that the maximum stress of medial plateau cartilage was higher than that of the lateral, and it increased with the increased angle of knee flexion. However, this characteristic did not apply to the meniscus and femoral cartilage. These novel results revealed the effect of different joint mobility loads on tibiofemoral cartilage stress. Our results indicated that more stress was concentrated on the edge of the removed meniscus and the maximum stress of the medial tibial plateau increased, which could explain the mechanical mechanism of progressive KOA.
Previous studies have shown that the medial meniscus is more important than the lateral, for it restrains uniplanar anterior loads on the tibia. Furthermore, compared with the lateral meniscus resection, the biomechanical changes of the medial meniscus resection are more significant, making it more likely to choose medial meniscus resection in clinical practice.
In this study, a high-fidelity three-dimensional finite element model of the knee joint (including bone, articular cartilage, meniscus, and major ligaments) after PMM was developed. The main purpose was to compare the effects of the different flexion angle, as well as internal and external rotation on the contact stress of the tibiofemoral joint. The results of finite element simulation of meniscus stress in the intact knee were similar to those of the previous health models, indicating the reliability of the results obtained by using this model. Although the finite element simulation only showed the transient response of the knee joint under compressive load, the trends observed indicated the potential for biomechanical changes that could result in development of KOA.
The finite element simulation results of the model straightening at 0° were similar to the data on peak stress in previous studies. However, as the flexion angle increased, the maximum stress of articular cartilage and meniscus became greater than the healthy knee model10,15. Moreover, the maximum stress gradually moved backward with the increased angle of flexion.
In case of 1150N load combined with 4Nm external rotation, the maximum stress on the outside of the tibial plateau was greater than the inside when the knee was flexed at 30° and 90°. The maximum stress on the tibial plateau cartilage at 0° and 60° was greater on the inside than the outside, and at 0, 30, 60 and 90°, the maximum stress on femoral condyle cartilage and the meniscus was greater on the inside than the outside.
The maximum stress value of the medial tibial plateau cartilage was 4.3-4.8 times that of the lateral when the knee was flexed at 0°. At the same time, the maximum stress value of the medial femoral condyle cartilage at 0°flexion and 0°flexion and external rotation was more than 8 times that of the outer side, except for 0°flexion and internal rotation, in which the medial side was only 0.65 times that of the outer side.
When combining internal and external rotation under different joint flexion degrees, in most cases, the maximum stress on the medial cartilage of the knee joint was greater than that on the lateral side. Our results showed that the significant increased stress on medial components (including cartilage and meniscus) was caused by the teared medial meniscus, which was consistent with the previous studies [17-18]. The stress concentration directly indicated that the abnormal overload might damage the risky area.
When vertical and forward loads are applied to the knee, the intact meniscus exhibits compression and displacement to provide adequate contact area between the cartilage of the femoral condyle and the tibial plateau. The meniscus bears stress, absorbs shock, and disperses stress through deformation. Previous studies have shown that the contact pressure value of the normal knee joint medial compartment in a normal person is greater than that of the lateral compartment, and the medial meniscus bears more mechanical effects. [19-20]
Similar to the results in previous health models, this study showed that the maximum stress value of the medial tibial plateau, including cartilage and meniscus, was greater than that of the lateral. Generally, the current study demonstrated that the maximum stress on the lateral meniscus was greater than that on the medial side, except in a knee flexion of 60°. However, the opposite occurred in tibial plateau cartilage, the maximum stress was larger on the medial side. The reason could be that the circumferential bearing capacity of the medial meniscus was weakened after partial resection of the medial meniscus, and thus the effect of shock absorption and pressure was attenuated, showing that the maximum medial contact stress of tibiofemoral articular cartilage was greater than the outer side.
When the knee is flexed and rotated internally and externally, the maximum displacement of the lateral meniscus is greater than that of the medial side. The maximum displacement of the meniscus increases with the rise the flexion angle. It is suggested that when the knee joint is flexed and rotated, the healthy side of the meniscus bears a larger load, which can reduce the stress load of tibiofemoral articular cartilage. Under different degrees of knee flexion and rotation, the removed medial meniscus only bears part of the load on the free edge. Therefore, the medial tibiofemoral articular cartilage carries most of the stress, and the maximum pressure on the medial femoral condyle and tibial plateau cartilage is greater than that on the lateral side.
Previous studies have shown that without the shock absorption by the meniscus, increasing the direct contact area between the femoral condyle cartilage and the tibial plateau leads to elevated stress, which can result in early cartilage degradation and early-onset osteoarthritis6. Although the characteristics of cartilage stress and meniscus displacement in the present model only reflect the transient response of the knee joint under compression load induction, previous studies have suggested that higher shear stress may cause early proteolytic degradation of the meniscus matrix and the tension of the articular cartilage may reduce the strength21. The peak shear stresses on the meniscus showed an obvious increase, and that on the cartilage was slightly increased.
Our results illustrated the compression of cartilage and meniscus after partial meniscus surgery in details. It was shown that with different degrees of flexion and rotation, more stress was concentrated on the medial tibial plateau and the edge of the meniscus. This increase in stress could lead to early proteolytic degradation of the meniscus matrix and articular cartilage, thereby reducing the tensile strength.
Our study showed the trend of biomechanical changes, which could potentially reveal the occurrence and evolution of KOA. Therefore, we infer that the medial tibiofemoral joint degeneration of KOA after PMM may be related to different joint motion angles. Hence, the biomechanical characteristics of the knee joint under different flexion and extension angles and different loads need further exploration to provide a biomechanical basis for sports rehabilitation after knee joint injury.
The limitations of this study should be addressed: this model is a single case study, and the finite element model of the same individual before and after surgery has not been constructed; the knee joint stresses of different injury types have not been compared and analyzed, and the stresses in complex sports have not been investigated.