In this study, we confirmed that higher BMI is an important risk factor for DMMLs. Advancing age and radiographic factors (varus, steep MPTS) also contributed as risk factors for DMMLs. DMMLs is the result of a combination of systemic and local biomechanical factors. Thus, the predominant risk factors for DMMLs (varus, older age, and higher BMI) are consistent with those for development of osteoarthritis. Most importantly, the identification of relevant controllable risk factors other than age can be used to modify risk factors through conservative or surgical treatment, thereby reducing the risk of DMMLs and delaying the development of bone and joint.
Degenerative meniscus lesions is a slow degeneration process of meniscus biomechanics disturbed by repeated wear and heavy load in the knee. Most of the lesions are asymptomatic horizontal tears. The most common lesions site is the posterior horn or body of the medial meniscus with the highest percentage of the overall load, especially when the knee is flexed and the backward shear force is applied[4, 5]. Biomechanical studies showed that the medial meniscus plays an important role in the transfer of shear force and longitudinal loads. The percentage of total load carried by the posterior horn of the medial meniscus is the highest, especially in knee flexion and backward shear force. In clinical practice, the posterior horn of medial meniscus is also the most frequently injured site, which is consistent with our clinical data. Among the biomechanical risk factors that we evaluated, lower limb malalignment and overweight may be important influencing factors, which were important risk factors for the progression of osteoarthritis, which also associated with degenerative meniscus lesions. The results of this study showed that the average HKA of the Experimental Group was significantly lower than that of the Control Group, and the average BMI was significantly higher than that of the Control Group. In the varus patients with lower limb malalignment, the pressure load of the medial compartment was increased, and the risk of DMMLs was increased about 1.3-fold with each degree increase of varus. This finding has been confirmed by arthroscopy that there is a higher incidence of medial meniscus lesions in varus patients.
This study further confirmed the development process of meniscus injury osteoarthritis through which lower limb malalignment and obesity can lead to chronic overload. This overload increased the medial compartment load and pressure, and the increase in the medial meniscus load can easily lead to the tear of the medial meniscus (especially the tear of the posterior horn). Once the meniscus lost its key function in the knee, the increased biomechanical load on the articular cartilage may lead to cartilage loss, reduced joint space, and increased joint-line convergence angle, which afterwards exacerbated the lower limb malalignment. This was a process of mutual causality and vicious circle, but it was difficult to make clear the causal relationship among the lower limb malalignment, meniscus injury, and cartilage wear. During this degenerative change process of the knee, abnormality of the lower limb alignment may be basic role, more likely to cause DMMLs and was an early sign of knee osteoarthritis. Understanding the influence of these modifiable risk factors on meniscus injury and osteoarthritis can produce better prevention and treatment.
It is well known that the geometry of the tibial plateau has a direct influence on the biomechanics of the tibiofemoral joint. In load-bearing buckling motion, the longitudinal load and horizontal shear force component of the tibial platform were related to and positively correlated with the posterior tibial slope[12, 17]. PTS was a geometric factor with influences on the biomechanics of the knee. Relevant studies have proved that the increase of PTS would lead to the relative physiological forward shift of the tibia and the corresponding increase in the contact stress between the posterior horn of the medial meniscus and the posterior medial condyle of the femur, especially the degeneration and damage of the knee during rotation and flexion. After the anterior cruciate ligament was deficient or ruptured, the wedge shape form of the posterior horn of the meniscus may limit the tibia displaced forward and increase the stress on the posterior horn of the medial meniscus[19, 20]. For patients with posterior root tear of the medial meniscus, the posterior edge of the medial meniscus moved 8.56 ± 2.00mm backward on average when the knee flexed from 10° to 90°, which was significantly greater than the distance of posterior movement of the medial meniscus during flexion of the normal knee. If there were no injury to the posterior root of the medial meniscus, the posterior edge of the medial meniscus, which does not excessively move backwards, would generate contact stress with the posterior condyle of the femur when the knee flexion angle increases. The overload stress may be the potential factor for the high incidence of injury to the posterior horn of the medial meniscus. In conclusion, the high incidence of degenerative medial meniscus injury (especially the posterior horn) may be associated with the highest percentage of overall load carried by this site, especially in patients with increased longitudinal medial stress caused by varus and increased stress at posterior Angle of medial meniscus caused by MPTS. This was basically consistent with the results of this study, namely, the increase in MPTS was closely related to the increase in the incidence of degenerative medial meniscus injury. If we take these findings into account, we would expect an increase in MPTS to affect the shear force exerted on the posterior horn of the medial meniscus. Therefore, it was reasonable to assume that if the MPTS was increased, the risk of tearing the posterior horn of the medial meniscus would also be increased due to the relatively high shear force exerted on the meniscus. MPTS can be used as a potential radiological parameter to evaluate the possibility of DMMLs (especially the posterior horn) in patients with knee pain. The most important finding of this study was that the Experimental Group had steeper MPTS, and greater angle of the varus compared with the Control Group.
Rytter et al., in a retrospective case control study, found that patients with occupational kneeling had a higher incidence of degenerative meniscus lesions, and degenerative lesions were most common in the middle and posterior 1/3 of the medial meniscus, and the tear layer mostly involved the tibial side. According to the previous theoretical study, patients who squat frequently bear steep longitudinal loads and shear forces in the middle and posterior parts of the medial meniscus, and may be more prone to degeneration and damage. However, the results of this study showed that there was no statistical difference in the occupational kneeling between the two groups, which may have selection bias or limited number of cases.
Degenerative meniscus lesions, which can be considered as a process of aging or degeneration, was the most common form of meniscus injury in the middle and the old, and its incidence increased with age: the morbidity of women aged 50–59 was 16%, and that of men aged 70–79 was 50%. MRI usually showed linear high signal shadow in the meniscus. This high signal was the result of persistent mucinous degeneration, usually affected the articular surface. Therefore, surgery should not be the first choice for the treatment of degenerative meniscus lesions. The injury should be regarded as the degeneration that occurs with the growth of age. More attention should be paid to modifiable risk factors to minimize degeneration of meniscus and osteoarthritis.
The study may have the following limitations. First, this was a retrospective case-control study, and there may be some selection bias. The Control Group was not entirely volunteers without knee pain and discomfort, and included patients with knee pain or mass (such as skin soft tissue trauma, popliteal cyst). Secondly, patients included in this study were basically from Central China, and patients from other regions may differ.