Objective To determine the midterm clinical outcome in knee osteoarthropathy patients undergoing minimally invasive unicompartmental knee replacement(UKR).Methods A total of 402 patients (440 knees) undergone Oxford Ⅲ UKR from January 2006 to June 2010 were included.There were 165 males (177 knees) and 237 females (263 knees),at age of (58.3 ±4.7)years.Body mass index (BMI) was (24.14 ± 3.75)kg/m2.There were 232 left knees and 208 right knees.A total of 427 knees were diagnosed with medial compartment osteoarthrosis and 13 knees with idiopathic necrosis in the medial femoral condyle.Thirty-eight patients accepted bilateral simultaneous minimally invasive UKR and 364 patients unilateral minimally invasive UKR.Results were assessed with X-rays,American Knee Society Score (KSS),Western Ontario and McMaster Universities (WOMAC) osteoarthritis index,and Visual Analogue Scale (VAS).The varus deformity of the knee prior to surgery and the valgus degree after surgery were measured and recorded.Results Period of follow-up was (73.0 ± 1.9) months.Length of the surgical incisions was (5.0 ± 0.2) cm.No patients had intraoperative fracture,and no tibial plateau collapse,infection,thrombosis and aseptic loosening were present after operation.Mobile bearing dislocation occurred in four patients after operation,among which three had the operation of mobile bearing change and one had revision total knee replacement.One patient sustained medial collateral ligament injury during operation,two underwent a second arthroscopic surgery to remove the residual cement in joint cavity,and two had moderate pain in the operated knee after operation.Femorotibial angle (FTA) decreased from (183.6 ± 5.1) ° preoperatively to (174.3 ± 4.2) ° postoperatively in the coronal plane,and rectified varus deformity angle was (9.3 ± 1.2)°.KSS clinical and function scores were improved from (42.4 ± 2.9)points and (53.5 ±3.8) points to (92.9 ±3.8) points and (93.5 ±4.0) points respectively (P＜0.01).WOMAC score decreased from (47.5 ± 3.1) points preoperatively to (12.3 ± 1.5) points postoperatively (P ＜ 0.01).VAS dropped from (7.78 ± 1.85) points preoperatively to (1.56 ± 0.23) points postoperatively (P ＜ 0.01).Conclusion Minimally invasive UKR has the advantages with respect to trauma,anatomical destruction,pain,complications and rehabilitation,and the mid-term clinical outcomes are satisfactory.