Abstract: BACKGROUND: The incidence of medial collateral ligament injuries in the knee joint is easy to lead to secondary meniscus and cartilage damage, and long-term chronic damage can lead to the occurrence of osteoarthritis. At present, there are few studies on the mechanics of meniscus and articular cartilage injury caused by medial collateral ligament rupture. 
OBJECTIVE: To investigate the effect of different degrees of medial collateral ligament injury on the biomechanics of meniscus and cartilage of knee joint.
METHODS: The CT and MRI examinations of the knee joint of a healthy volunteer were performed to obtain the image data. The scanning data were imported into Mimics, Geomagic, and Solidworks software in turn. After registration and fusion, a 3D model of normal knee joint was established. On this basis, models of medial collateral ligament injury in different degrees of knee joint were simulated, which were divided into four groups, including: (1) medial collateral ligament was intact; (2) deep medial collateral ligament fracture; (3) superficial medial collateral ligament fracture; (4) complete rupture of medial collateral ligament. Finally, Ansys software was introduced to apply three modes of loads to the knee joint: (1) 10 N·m valvaration torque was applied to the top of the femur. (2) A 4 N·m internal torque was applied to the top of the femur. (3) A 4 N·m external torque was applied to the top of the femur. The effects of four groups of models on knee biomechanics under different loads were analyzed.
RESULTS AND CONCLUSION: (1) In the extension position of the knee joint, when a 10 N·m valgus torque was applied to the knee joint, the overall stress of the posterolateral meniscus increased with different degrees of medial collateral ligament injuries, while the stress of the articular cartilage did not change significantly. The peak stress of the posterolateral meniscus increased significantly with superficial medial collateral ligament rupture. (2) In the knee extension position, when a 4 N·m internal rotation torque was applied to the knee joint, the overall stress of the medial and lateral meniscus increased after different degrees of medial collateral ligament injury. When superficial medial collateral ligament rupture occurred, the peak stress of the meniscus shifted from the anterior horn of the medial meniscus to the anterior horn of the lateral meniscus. (3) In the knee extension position, applying a 4 N·m external rotation torque to the knee joint, the peak stress of the posterolateral meniscus increased more significantly than that of the medial meniscus, and the stress of the articular cartilage changed less. (4) These results show that the risk of meniscus injury secondary to superficial medial collateral ligament rupture is much higher than that of deep medial collateral ligament rupture when the knee is in extension, and the lateral meniscus is more vulnerable to injury than the medial meniscus. Both superficial medial collateral ligament and deep medial collateral ligament play an important role in the rotational stability of the knee joint. 

Key words: knee joint, medial collateral ligament, meniscus, finite element analysis, biomechanics