Dynamic effects of lower limb mechanical-axis shift on medial meniscus posterior root radial tear

Introduction Lower limb mechanical alignment and medial meniscus posterior root radial tears (MMPRT) are considered risk factors for increased knee joint compartment loading, which may promote the development of osteoarthritis. MMPRT accounts for 21.5% of medial meniscus tears, with the medial meniscus posterior root radial tear (MMPRRT) being the most common type. Its occurrence is closely associated with dynamic loading of the knee joint during movement. This study aimed to establish a novel dynamic model of mechanical-axis shift and to investigate the biomechanical effects of mechanical-axis shift and tear length on MMPRRT under dynamic loading conditions. Methods One healthy male volunteer was recruited. The volunteer had a neutral mechanical alignment confirmed by full-weight-bearing lower limb radiographs (R_HKA 0.44°, R_mMPTA 88.61°, R_mLDTA 87.07°, R_mLDFA 88.77°, R_JLCA 0.30°). CT and MRI data of the right knee were acquired. A finite element model of the knee joint at neutral alignment (0°) was established. A novel method for constructing mechanical-axis shift was proposed: only the load application point at the femoral end was translated in the medial–lateral direction to simulate mechanical-axis shifts, without directly modeling anatomical malalignment or altering the basic geometry and ligament tension of the knee joint. A total of six finite element models of MMPRRT under different mechanical-axis shift conditions were established, including medial mechanical-axis shifts equivalent to 3°, 6°, and 9°, and lateral mechanical-axis shifts equivalent to 3° and 6°. Dynamic analyses were performed using ISO-14243-3 gait loading. The finite element model at neutral alignment (0°) was validated using one cadaveric knee specimen under one static loading condition (1150 N), with the medial compartment contact area as the validation metric. Results Under static loading, the error in medial compartment contact area between the finite element model and the cadaveric experiment was 5.72% for the intact meniscus condition. The peak contact pressure of each knee structure exhibited two peaks at 14% and 44% of the gait cycle. At neutral alignment (0°) and 14% of the gait cycle, the tear gap of the unstable posterior root radial tear was 48.75% larger than that of the stable tear. The maximum meniscal extrusion displacement was 2.69 mm, occurring at 44% of the gait cycle. A medial mechanical-axis shift elevated the medial peak contact pressure and prolonged the duration of high overall meniscal displacement, whereas a lateral mechanical-axis shift reduced the medial peak contact pressure and displacement. Discussion This study established a dynamic finite element model of the knee joint with mechanical-axis shift and investigated the effects of mechanical-axis shift and tear length on MMPRRT biomechanics. The results showed that a medial mechanical-axis shift was associated with increased medial compartment loading, meniscal displacement, and tear gap, whereas a lateral mechanical-axis shift was associated with reduced medial loading. These biomechanical associations suggest that mechanical-axis shift may influence the local mechanical environment of MMPRRT. However, the clinical relevance of these findings requires further validation through prospective clinical studies incorporating multi-subject models and patient data.