## 3 Results
### Biomechanical Analysis of Tibia Under Flexion Movement  
The FEA was conducted to investigate the biomechanical characteristics of tibial flexion in normal and dynamic knee valgus poses (DKVPs). The Workbench was used to simulate the flexion movement under normal and dynamic genu valgus positions and analyze the stress distribution of the tibia. The finite element analysis method is as follows: Due to the volunteer's weight of 65 kg, vertical loads of 980 N, 1360 N, 1820 N, and 2360 N are applied to the meniscus area of the tibial plateau to simulate LLFM at 30\({}^{\circ}\), 60\({}^{\circ}\), 90\({}^{\circ}\), and 120\({}^{\circ}\). In normal posture, the inner and outer meniscus each bear 50% of the external force. For dynamic knee valgus posture, the lateral side of the tibial plateau bears 60% of the external load, while the medial side bears 40% of the external load. A rotational torque eM of 4 N \(\cdot\) m is applied to the proximal end of the tibia to simulate rotational motion during tibial flexion. A horizontal external load F of 50 N is applied on the left side of the proximal tibia to simulate the inward movement of the tibia under femoral traction during dynamic knee flexion. The 25 mm-long area of the distal tibia is set as a fixed constraint. The equivalent stress curves of the flexion motion in the different poses are shown in Figure 7.  
Figure 7(a) shows that under normal posture, as the tibia lengthened, the equivalent force first increased and then decreased, reaching a maximum at 250 mm. The curve before the peak was approximately a quadratic curve. In addition, the growth rate of the equivalent stress substantially accelerated as the bending angle increased. For the four different flexion angles, the stress concentration with the normal posture mainly occurred in the anterior part of the middle and lower tibia. The peak stress roughly occurred at the junction of the middle and lower third of the tibia. Figure 7(b) shows that under the DKVPs, the equivalent force also first increased and then decreased, reaching a maximum at 250 mm, as the length of the tibia increased. Unlike for normal posture, the curve before the peak is close to linear. The growth rate of the equivalent force also considerably accelerated as the bending angle increased. The stress concentration in the DKVP group mainly occurred on the inner side of the middle tibia, roughly distributed along the inner edge, gradually decreasing from the middle of the tibia to both ends. The reason for this is the imbalance in the torque between the internal and external extensor muscles of the hip,  
Figure 7: Equivalent stress curves for flexion from 30\({}^{\circ}\) to 120\({}^{\circ}\) in different postures. (**a**) Normal posture. (**b**) Dynamic knee valgus posture. which causes abnormal lateral force on the tibia. The maximum equivalent stress (MES) of the tibia and its plateau in the different poses is shown in Figure 8.  
As shown in Figure 8a, the MES in the DKVPs was larger than that in the normal pose under the same flexion angle. For 30\({}^{\circ}\) and 90\({}^{\circ}\) flexion, the MES in the normal posture and DKVPs were 19.1 MPa/35.0 MPa and 31.2 MPa/53.8 MPa, respectively. During bending from 30\({}^{\circ}\) and 90\({}^{\circ}\) to 120\({}^{\circ}\), the MES increased 29%, 22.69%, and 19.23%, respectively. Figure 8b shows that the equivalent stress of the medial tibial plateau (MTP) in the normal posture was higher than that on the lateral side, whereas the equivalent stress in the DKVPs was less than that on the lateral side. For 30\({}^{\circ}\) flexion, the equivalent stresses of the medial and lateral tibial plateaus in normal posture were 1.4 MPa and 1.8 MPa, respectively. Those of the DKVPs were 1.1 MPa and 1.2 MPa, respectively. The MES in the DKVPs was higher by 38.8%, 39.65%, 34.95%, and 27.52% compared with that in the normal posture group at the four different flexion angles.