Abstract: BACKGROUND: Anterior cruciate ligament injury tends to lead to secondary meniscus injury and osteoarthritis. At present, there are few studies on the mechanics of meniscus and articular cartilage injury caused by anterior cruciate ligament injury. 
OBJECTIVE: To study the effect of partial rupture of the anterior cruciate ligament on the stress of medial and lateral meniscus and articular cartilage of knee joint by finite element analysis.
METHODS: The CT and MRI images of the knee joint of a healthy volunteer were selected, and the scan data were imported into Mimics, Geomagic and SolidWorks software. After registration and fusion, four kinds of three-dimensional knee joint models were established: models of intact anterior cruciate ligament, rupture of the posterior external tract of anterior cruciate ligament, rupture of the anterior internal tract of anterior cruciate ligament, and absence of anterior cruciate ligament. Finally, data were imported into Ansys software to apply four different modes of loads to the knee joint: Longitudinal loads of 750 N were applied to the top of the femur; longitudinal load of 750 N to the top of the femur and forward thrust of 134 N behind tibia; a longitudinal load of 750 N and a varus moment of 10 Nm were applied to the top of the femur to simulate genu varus; 750 N longitudinal load and 4 Nm internal rotation moment were applied to the proximal end of the femur to simulate knee internal rotation. The finite element analysis of biomechanical stress changes of the meniscus and articular cartilage of the knee joint was carried out.  
RESULTS AND CONCLUSION: (1) In the straight position of the knee joint, when the anterior medial tract of the anterior cruciate ligament was broken and the anterior cruciate ligament was missing under longitudinal loads of 750 N at the top of the femur, the total stress and peak value of meniscus increased significantly, but the stress distribution of the meniscus and the stress of articular cartilage did not change significantly. In longitudinal load of 750 N to the top of the femur and forward thrust of 134 N behind tibia, the fracture of the anterior internal tract of the anterior cruciate ligament increased the tibia forward, the compressive stress of posterior angle of the meniscus increased, and the stress of the articular cartilage did not change significantly. During simulating genu varus, the posterior angular stress of the lateral meniscus decreased, the stress of the medial meniscus increased, and the stress of articular cartilage slightly decreased when anterior cruciate ligament injuries were complete. When the anterior internal tract of the anterior cruciate ligament was broken or absent under knee internal rotation, the equivalent stress peak value of femoral cartilage and tibia cartilage shifted from medial cartilage to lateral cartilage, and the stress peak value of meniscus increased significantly. At this time, the anterior internal tract of the anterior cruciate ligament played a leading role in the rotational stability of the knee joint. (2) These results indicate that the risk of secondary meniscus injury in patients with anterior and medial anterior cruciate ligament band rupture was much higher than that in patients with posterior and external anterior cruciate ligament band rupture when the knee was in the upright standing position, varus and pronation, and there was no significant difference in the impact on articular cartilage.

Key words: anterior cruciate ligament injury, finite element analysis, biomechanics, meniscus, articular cartilage