Background:The potential benefit of arthroscopic surgery for osteoarthritic knee associated with medial meniscus tear is controversial.This study was conducted to determine the effect of pre-operative medial meniscus extrusion (MME) on arthroscopic surgery outcomes in the osteoarthritic knee associated with medial meniscus tear during a minimum 4-year follow-up.Methods:This was a retrospective review of a total of 131 patients diagnosed with osteoarthritic knee associated with medial symptomatic degenerative meniscus tear who underwent arthroscopic surgery from January 2012 to December 2014 and were observed for more than4 years.Patients were classified into two groups:MME ≥3 mm (major MME group,n =54) and MME ＜3 mm (non-major MME group,n=77).Clinical assessments,including the West Ontario and McMaster Universities Osteoarthritis Index (WOMAC) score,and radiographic assessments,including the Kellgren-Lawrence (K-L) grade and medial joint space width (JSW),were evaluated pre-operatively and at final follow-up.The longitudinal changes of clinical and radiographic parameters (WOMAC and the medial JSW change,K-L grade progression) were compared between groups unadjusted and adjusted for age,sex,and body mass index.Four-year survival rates (without progression to knee replacement [KR]) were also evaluated using a log-rank test and Cox proportional hazard regression modal.Results:Major MME was present in 41％ of patients.After a minimum 4-year follow-up,the mean WOMAC total and pain scores improved significantly in both groups.However,the medial JSW and K-L grade worsened significantly.Patients with pre-operative major MME worsened more in WOMAC total (adjusted mean difference [MD] 3.800,95％ confidence interval [CI]:0.900,11.400;P =0.037) and function (adjusted MD 3.100,95％ CI:0.700,6.300;P =0.038) scores than patients with pre-operative non-major MME,and no significant difference was observed in WOMAC pain and stiffness score between groups.The group with major MME had significantly higher joint space narrowing (adjusted MD-0.630,95％ CI:-1.250,-0.100;P=0.021) and K-L rate progression (adjusted mean relative risk [RR] 1.310,95％ CI:1.100,1.600;P =0.038) than the group with non-major MME.There was a significantly more KR progression in patents with major MME compared with those with non-major MME (adjusted RR 3.100,95％ CI:1.100,9.200;P =0.042 and adjusted hazard ratio 3.500,95％ CI 1.100,9.500;P =0.022).Conclusions:Osteoarthritic knee patients associated with medial meniscus tear with non-major MME are more responsive to arthroscopic surgery in terms of the clinical and radiologic outcomes and survival for at least 4-year follow-up;however,in terms of pain relief,arthroscopic surgery in patients with major MME is also beneficial as well as in patients with non-major MME.