Stress changes in the medial compartment
This study revealed that the MME is associated with increased stress loading on the medial compartment structures (femoral and tibial cartilage and meniscus), and this pressure was enhanced by the larger JLCA (VAEX compared with NTNE; medial compartment: femoral 3.96- fold, tibia 3.52-fold, meniscus 3.61-fold).
Interestingly, In terms of stresses on the medial compartment of the knee joint, there was no difference in the effect of alignment between the femoral and tibial sides and the meniscus (VANE compared with NTNE; medial compartment: femoral 2.62-fold, tibia 2.49-fold, meniscus 2.55-fold), while the effect of MME was greater on the femoral side (NTEX compared with NTNE; medial compartment: femoral 3.34-fold, tibia 1.60-fold, meniscus 1.93-fold). MME was found in 50% to 100% of patients with SONK, with the extent of medial meniscus extrusion correlating to the stage and volume of SONK lesions [3, 23]. Although SONK occurs in both compartments of the femur or tibia, most of it occurs in the medial femoral condyle . Based on the results of the present study, it is presumed that a large stress concentration may occur in the medial femoral condyle fastest when MME occurs.
Gokkus et al. analyzed the changes in stress distribution in the medial knee compartment depending on the different degrees of MME using the FE method . According to that report, the contact pressure on the medial compartment of the knee joint increased linearly as the degree of MME increased. Furumatsu et al. also reported that the amount of MME increases over time . The causality of lower leg alignment to MME remains controversial. Crema et al. reported that varus malalignment, meniscal tears, and cartilage damage were associated with MME  and Goto et al. reported a correlation between the degree of MME and knee varus alignment . By contrast, Erquicia et al. did not find any association between varus malalignment and meniscus extrusion . In actual clinical practice, there are some cases where MME is present but alignment is neutral, or cases where the JLCA is tilted to valgus direction in early knee OA patients.
As mentioned above, there are some cases in which the clinical symptoms are not severe and arthropathy changes do not progress much, even in the presence of MME . In the present study, the contact pressure on the medial compartment in the condition with valgus alignment of JLCA did not increase significantly compared to that in the control model, even if the medial meniscus was extruded (VGEX compared with NTNE; medial compartment: femoral 1.42-fold, tibia 1.17-fold, meniscus 1.65-fold). Considering from the results of the present study, patients who originally had valgus alignment of the JLCA, even if MME was occurred, may not have severe symptoms and may not progress to arthritic changes.
Based on the results of this study, the JLCA should be considered in the treatment of patients undergoing arthroscopic surgeries such as pull-out repair or centralization.
Stress changes in the lateral compartment
In examining the contact pressure on the lateral compartment, we found that MME increased the stress here as well as in the medial compartment (NTEX compared with NTNE; lateral compartment: femoral 1.80-fold, tibia 1.05-fold, meniscus 1.38-fold). Similar results have been reported in a biomechanical study of knee stress changes induced by MME that is caused by MMPRT in a cadaver knee . Specifically, in the condition of MME applied to valgus alignment, the contact pressure of lateral tibial cartilage reached 2.36-hold compared with that in the control (VGEX compared with NTNE; lateral compartment: femoral 1.11-fold, tibia 2.36-fold, meniscus 1.24-fold). This is considered a large increase compared to the increase of about 1.25-hold in the condition which the JLCA was tilted in the direction to valgus without MME (VGNE compared with NTNE; lateral compartment: femoral 1.00-fold, tibia 1.25-fold, meniscus 1.21-fold). Thus, MME in patients with the JLCA of valgus alignment may require attention to subsequent degeneration of the lateral tibial compartment.
This study had some limitations. First, the analysis was performed solely in the full-extension position. The change in the morphology of the femoral condyle during the knee flexion and extension movement and its translation movements during the movement are tolerated by the meniscus. In a case that causes MME, such as meniscus root tear, this tolerance disappears, eccentric movements occur in the medial compartment, and this may cause arthrosis as a cause independent of the increase in load distribution. Further, the contacts between the meniscus and the tibia were simulated as a complete fusion, but this does not reflect the variation in pressures investigated by this study. Knee joint degeneration could have been influenced by various factors, such as sex, body mass index, and coverage rate. Moreover, the knee joint has a broad variation in formation and degenerative change, but a single model was used to assess the impact of meniscus and alignment on the knee joint. Lastly, knee joint stress was assumed to be influenced by the hip joint, pelvis, and foot, but our model was constructed from the femur to the lower leg. This was because the model from the hip joint to the ankle joint was too large and could not be analyzed, but this model seemed to be acceptable, which was similar to that in a previous study [18, 21].