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Purpose To clarify the effect of the radial tear of the lateral meniscus on the in situ meniscus force and the tibiofemoral relationship under axial loads and valgus torques. Methods Ten intact porcine knees were settled to a 6-degree of freedom robotic system, while the force and 3-dimensional path of the knees were recorded via Universal Force Sensor (UFS) during 3 cycles of 250-N axial load and 5-Nm valgus torque at 15°, 30°, 45°, and 60° of knee flexion. The same examination was performed on the following 3 meniscal states sequentially; 33, 66, and 100% width of radial tears at the middle segment of the lateral meniscus, while recording the force and path of the knees via UFS. Finally, all paths were reproduced after total lateral meniscectomy and the in situ force of the lateral meniscus were calculated with the principle of superposition. Results The radial tear of 100% width significantly decreased the in situ force of the lateral meniscus and caused tibial medial shift and valgus rotation at 30°–60° of knee flexion in both testing protocols. Under a 250-N axial load at 60° of knee flexion, the in situ force decreased to 36 ± 29 N with 100% width of radial tear, which was 122 ± 38 N in the intact state. Additionally, the tibia shifted medially by 2.1 ± 0.9 mm and valgusrotated by 2.5 ± 1.9° with the complete radial tear. However, the radial tear of 33 or 66% width had little effect on either the in situ force or the tibial position. Conclusion A radial tear of 100% width involving the rim significantly decreased the in situ force of the lateral meniscus and caused medial shift and valgus rotation of the tibia, whereas a radial tear of up to 66% width produced only little change. The clinical relevance is that loss of meniscal functions due to complete radial tear can lead to abnormal stress concentration in a focal area of cartilage and can increase the risk of osteoarthritis in the future.

Purpose Partial meniscectomy can cause osteoarthritic changes in knees, as inner portion as well as peripheral portion of meniscus is important. The hypothesis of this study was that the amount of the inner resection of medial meniscus affected the in situ forces through the meniscus and the tibial varus and external rotation under axial load. Methods Fourteen intact porcine knees were investigated with a six-degree of freedom robotic system and force/moment, and the three-dimensional path of intact knees were recorded by universal force sensor when an axial load of 300-N was applied at four different flexion angles (30°, 60°, 90°, and 120°). The same examination was performed on three phased inner resections (30%, 60%, and 90% width) of the medial meniscus. Finally, all paths were reproduced after total medial meniscectomy, and in situ forces of the medial meniscus were calculated based on the superposition principle. Changes in tibiofemoral varus/valgus and internal/external rotation alignment during an axial load were also calculated. Results In situ forces of the medial meniscus decreased according to the amount of meniscal resection at all flexion angles. The reduction was significant in knees with inner resections of > 60% width at all flexion angles and even of 30% width at a flexion angle of 120° ( p  < .05). Incremental changes in the tibiofemoral varus alignment increased depending on the inner resection width at all flexion angles ( p  < .05). Conclusion The amount of inner resection of the medial meniscus was related to reduction of its in situ forces and increment of the tibial varus rotation under axial load.