In our study, we found that the stress at the knee joint at the 7-degree and 10-degree valgus deformity of the lateral knee joint is significantly less than the stress at the corresponding deformity angle of the medial platform (both P < 0.05), but not significant when the deformity is 3 degrees. We also found that as the deformity angle increases, the increase in the medial platform is more prominent than that of the lateral platform. Our research also showed that the stress on the lateral plateau is significantly higher at valgus deformities than the stress on the lateral plateau measured at neutral position as well as the corresponding data measured on the medial plateau.
Femoral fractures are common injuries in traumatic orthopedics. Open or closed reduction and internal fixation is the standard treatment options for displaced femoral fractures. However, during the closed reduction and intramedullary nailing, the fractures can not be always reduced anatomically, and various residual fracture deformity and complications may occur. Winquist et al.[23] reported that 520 cases of femoral fractures were treated with intramedullary nail, and 8% of the patients had external rotation deformity of more than 10 degrees. Braten et al.[24] measured the anteversion angle of bilateral femur in 110 patients with interlocking intramedullary nails for unilateral femoral fractures, and 21 (19%) of them had rotation deformity. The change in the mechanical axis of the lower limbs after fracture malunion can lead to the changes in the tension of the ligaments and joint capsules, and patella malacia in a long time. If not corrected in time, it can affect the biomechanical characteristics of knee and ankle joints and ultimately result in TA, following impaired knee function. Maquet[25] also believed that the uneven distribution of stress on the joint surface and the excessive stress concentration is one of the important causes of traumatic arthritis of the knee. Therefore, malunion and malalignment of the lower limb is an important contributing factor to traumatic arthritis of knee joint.
When malunion and malalignment occurs after femoral fracture, the contact characteristics of knee joint will be altered. In patients with knee varus deformity, the mechanical axis of the lower limbs inclines from the center of the knee to the medial plateau, which causes the stress distribution within the joint to be redistributed, so that the load is mainly concentrated on the medial plateau of the knee. Our findings reveal that the stress on the medial plateau of tibia is significantly higher at varus deformities when compared with those measured at the neutral position, and significantly higher than the corresponding stress values on the lateral plateau. When the knee is at valgus position, the mechanical axis is offset laterally and the lateral compartment load increases. Studies have shown that the high stress load on joints can increase the damage pressure of articular cartilage, accelerate the wear of articular cartilage, and is an important factor leading to cartilage degradation[26]. Therefore, in order to reduce the risk of long-term arthritis in patients, orthopedic surgeons should reduce the fractures anatomically to avoid the malalignment and malunion of lower limbs fractures[27].
Varus or valgus deformity around the knee can cause knee progressive degeneration. Brouwer et al.[28] analyzed the data of 1501 patients (2664 lateral knees), and found that there was a significant correlation between malalignment of lower extremities and the occurrence and progression of arthritis, especially in overweight patients. However, Some studies have found that the deformity of varus or valgus within 5 degrees can be accepted after treatment. Brown and Sarmiento et al.[29] found 5 degrees of varus or valgus at any level to be cosmetically and functionally acceptable. When Bryant [30] studied the biomechanical effect of residual deformity after total knee arthroplasty, they found that the deformity angle of 5 degrees had no effect on the patellofemoral contact characteristics. Our research showed that the stress change of the knee joint was not remarkable within the deformity angle of 3 degrees, but highly remarkable at 7 or 10 degrees of valgus or varus deformity. Therefore, our data supported postoperative corrective deformity within 3 degrees to maintain necessary biomechanical balance of knee joint. It is a pity that we had no data on the 5 degrees of deformity. In the future, clinical cohort study with long-term follow-up data are needed to ascertain our finding, to continue investigation of the relationship between deformity extent and the responsive traumatic arthritis, and to define the more reasonable and clinically applicable cut-off value for deformity.
Alignment of the lower limb is a prerequisite for ensuring a reasonable distribution of joint stress. Under normal conditions, the lower limb mechanical axis is from the center of the hip joint to the center of the ankle joint, passing through the center of the knee joint, so that the load and stress on the knee joint can be reasonably dispersed [31]. However, in a human gait, the center of body gravity will move to the contralateral side during the stance phase, with resultant increase of compressive force in the medial compartment of knee. Therefore, it is likely that this imbalanced stress change between medial and lateral platforms cause the final skewed stress distribution and the secondary osteoarthritis. Moreover, during this process, a varus deformity plays a more critical role, compared to valgus deformity. Therefore, in orthopaedic practice, varus deformity should be corrected or avoided as a priority, with aims to prevent the occurrence of potential osteoarthritis.
In biomechanics research, the contact position, contact area and contact stress between joint surfaces are often measured. At present, there are a variety of experimental measurement methods for joints, including 3D photoelastic measurement, pressure sensing and finite element analysis[32–33], but these methods have a large error in the measurement of joint stress and the operation method is relatively complex. Therefore, we choose pressure-sensitive film technology, which can not only directly measure the pressure value, but also can measure the contact area, intuitively see the distribution of pressure, has obvious advantages.
This study has some limitations. Firstly, the sample size is small due to the limited source of specimens, which will reduce the credibility of experimental data (type II error). Secondly, this study is based on cadaver specimens and can not take the action of muscles into consideration. Thirdly, dynamic load on the knee joint was not taken and the current model can not simulate the stress distribution during knee motion. The last, We excluded specimens of osteoporosis, but in clinical practice, distal femoral fractures are very common in older adults with osteoporosis, which may affect the external validity of clinical work. However, the current findings guarantee further biomechanical studies on the stress distribution of knee joint under dynamic load, clinical studies on the quantitative relationship between residual malunion and traumatic arthritis, and the endeavor to reduce complex fractures anatomically in closed fashion or minimally invasive way.