Objective To investigate causes and effects of medial meniscus extrusion in patients with knee osteoarthritis.Methods A total of 120 patients diagnosed as knee osteoarthritis between January 2011 and March 2012 were enrolled in this study,including 60 patients with medial meniscal extrusion confirmed by MRI (extrusion group) and other 60 patients without medial meniscal extrusion (control group).The extrusion distance of medial meniscus and tibiofemoral angle were measured on MRI.The correlation between tibiofemoral angle and extrusion distance were analyzed.The incidences of genu varum,medial meniscus injury and cartilage lesion of medial tibiofemoral joint were compared between two groups.The effect of medial meniscal extrusion on meniscus injury and effect of genu varum on meniscal extrusion were analyzed.Results In extrusion group,the extrusion distances of medial menisci ranged from 3.76 to 11.6 mm (average,8.3±1.79 mm); all patients had genu varum,and the tibiofemoral angle ranged from 174°to 181°(average,179.0°±2.2°); the incidence of medial meniscus injury was 50.0％ (30/60) in the anterior horn,93.3％ (56/60) in the body,and 93.3％ (56/60) in the posterior horn; the incidence of medial meniscus tear in the root of the posterior horn was 23.3％ (14/60); the incidences of cartilage degeneration in medial tibial plateau and medial femoral condyle both were 100％ (60/60); a significant negative correlation was observed between dimension of tibiofemoral angle and extrusion distance of medial meniscus.In control group,the extrusion distances of medial menisci ranged from 0 to 2.61 mm (average,0.57±0.80 mm); four patients had genu varum,and the tibiofemoral angle in all patients was 180°; the incidence of medial meniscus injury was 0 in the anterior horn,16.7％ (10/60) in the body,and 70.0％ (42/60) in the posterior horn; no medial meniscus tear was found in the root of the posterior horn; the incidence of cartilage degeneration was 26.7％(16/60) in medial tibial plateau and 30.0％ (18/60) in medial femoral condyle.The odd ratio of meniscus injury and the number of genu varum (extrusion group/control group) was 6.0 and 15.0,respectively.Compared with the control group,the incidences and severities of medial meniscus injury and cartilage lesion of medial tibiofemoral joint were higher in extrusion group.Conclusion Genu varum may be one cause of medial meniscal extrusion.Medial meniscal extrusion increases incidence of medial meniscus injury and has a significant influence on generation and development of osteoarthritis in medial tibiofemoral joint.