## 4 Discussion  
A 3D model of the human tibia was constructed, and FEA was conducted to investigate the biomechanical characteristics and injury tolerance of the human tibia under flexion movement. The MES of the tibia was located at 250 mm under the normal and DKVPs. In  
Figure 11: Contact force and CSM of the tibia for different impact directions. (**a**) Contact force. (**b**) Section moment.  
Figure 12: The influence of size and age on the ultimate moment of the tibia. (**a**) The influence of size on the tolerance of tibial injury. (**b**) The effect of age on tibial injury tolerance. Note: x/y denotes the scaling ratio of the dimensions, where x indicates the scaling ratio of the tibial length, and y represents the scaling ratio of the lower limb width.  
terms of positioning, under the same conditions, the MES of DKVP was greater than that of normal positioning. Taking a 30\({}^{\circ}\) flexion as an example, the MES for normal and DKVPs were 19.1 MPa and 31.2 MPa, respectively. DKVP was more likely to cause tibial injury than normal posture. The stress was mainly concentrated on the inner side of the muscles and bones for the DKVPs, which may be due to the imbalance in the torque between the hip abductor and abductor muscles [15; 16]. The equivalent stress of the MTP in the normal posture was higher than that on the lateral side but less than that on the lateral side under DKVPs. With 30\({}^{\circ}\) flexion, the equivalent stresses of the medial and lateral tibial plateaus in the normal posture were 1.4 MPa and 1.8 MPa, respectively, and 1.1 MPa and 1.2 MPa under the DKVPs, respectively. This indicated that this side was more prone to shear injury due to the higher load on the lateral tibial plateau under the DKVPs [17]. This is consistent with the research findings of [PERSON] et al., who found that 64.1% of tibial plateau medial fractures were caused byeversion. The Wahlquist type C fractures caused by outward rotation force were significantly more than those caused by inward rotation force (\(p<0.05\)) [18]. The study by [PERSON] et al. also demonstrated that tibial plateau fractures may be caused by outward or rotational forces, which is consistent with the results of this study [19].  
In terms of tibial injury tolerance, the contact force reached a peak between 15 and 20 ms when the tibia was subjected to frontal impact. The ultimate contact force was approximately 4000 N, at which point the impact point reached the failure strain and fractures occurred. After impact, the equivalent stress in A2 remained high because the distal trabecular bone was fixed by the ankle joint and could not release stress through deformation or displacement [20]. If the impact was large enough, both the impact point and ankle might fracture. In addition, the maximum contact forces were larger for frontal than external impacts, at 4109 N and 3927 N, respectively. The ultimate torque with frontal impact was slightly larger than that of lateral impact, at 298 N-m and 277 N-m, respectively. The reason for the above finding might be the irregular cross-sectional shape of the tibia. Comparing the experiment with the impact test results from the cadaveric tibia specimens, the difference in the ultimate bending moment of the midsection impact was 2.4% and the maximum contact force error was 4.0%. The constructed model may be used for studying the impact injury tolerance limit of the tibia. The effects of tibial size and age on injury tolerance were also investigated. It was found that as the size of the tibia increased, its ultimate torque increased, and the tibia had the highest tolerance to injury at the age of 25 years. As age increased, bone loss gradually occurred, bone density decreased, and osteoporosis might occur [21; 22].