Abstract: BACKGROUND: The medial meniscus extrusion beyond the edge of the tibial plateau can be observed on MR images of the medial meniscus of the knee joint during injury. It is worth exploring whether the external protuberance is related to the mode of medial meniscus injury and the degree of medial interventricular cartilage injury of the knee joint.
OBJECTIVE: To quantitatively measure the extrusion of medial meniscus under magnetic resonance imaging, and to study the factors affecting the value of medial meniscus extrusion, and whether it is related to the injury mode of medial meniscus and the degree of cartilage injury in medial compartment. 
METHODS: In this study, 117 patients diagnosed as knee meniscus posterior horn injury by sports injury in First Affiliated Hospital of Anhui Medical University were selected. They underwent knee joint MRI scan and knee arthroscopic meniscus surgery. The meniscus extrusion value (medial meniscus extrusion) was measured under MRI in each patient. The body mass index of each patient, whether due to acute injury, the mode of meniscus injury confirmed by arthroscopy and the grade of cartilage injury in the medial compartment were recorded from the medical record, and the correlation between the meniscus extrusion value and the above-mentioned factors was analyzed.
RESULTS AND CONCLUSION: (1) There were 31 (26.5%) acute injury patients with definite recall, 86 (73.5%) acute injury patients with no clear recall; there was a significant difference in the meniscus extrusion value between the two groups (P=0.001). (2) There were 56 (47.9%) patients with low body mass index (body mass index ≤ 25 kg/m2) and 61 patients (52.1%) with high body mass index (body mass index > 25 kg/m2); there was a significant difference in the meniscus extrusion value between the two groups (P=0.008). (3) According to the site of meniscus injury, there were meniscus posterior body injury group (60 cases, 51.2%), posterior horn injury group (37 cases, 31.6%), and posterior root fracture group (20 cases, 17.1%); there was a significant difference in the meniscus extrusion value between the two groups (P=0.000). (4) According to the injury confirmed by arthroscopy, patients with non-medial meniscus posterior root rupture were divided into compound meniscus fracture group (n=49) and non-compound meniscus fracture group (n=48); there was a significant difference in the meniscus extrusion value between the two groups (P=0.000). (5) The areas under receiver operating characteristic curve of moderate and severe cartilage injury in medial compartment and medial meniscus posterior root tearing predicted by medial meniscus extrusion were 0.794 and 0.942, respectively. (6) It is indicated that the size of medial meniscus extrusion reflects the presence or absence of acute trauma in the patient's disease course, their body mass index, and pattern of medial meniscus injury. Medial meniscus extrusion has a good predictive value for medial meniscus posterior root rupture. 

Key words: meniscus extrusion, meniscus tear, cartilage injury, arthroscopy, magnetic resonance imaging