«Purpose». High tibial osteotomy (HTO) is a useful treatment option for osteoarthritis of the knee. Closing-wedge HTO (CW-HTO) had been mostly performed previously, but the difficulties of surgical procedure when total knee arthroplasty (TKA) conversion is needed are sometimes pointed out because of the severe deformity in proximal tibia. Recently, opening-wedge HTO (OW-HTO) is becoming more popular, but the difference of the two surgical techniques about the influence on proximal tibia deformity and difficulties in TKA conversion are not fully understood. The purpose of this study was to compare the influence of two surgical techniques with CW-HTO and OW-HTO on the tibial bone deformity using computer simulation and to assess the difficulties when TKA conversion should be required in the future. «Methods». In forty knees with medial osteoarthritis, the 3D bone models were created from the series of 1 mm slices two-dimensional contours using the 3D reconstruction algorithm. The 3-D imaging software (Mimics, materialize NV, Leuven, Belgium) was applied and simulated surgical procedure of each CW-HTO and OW-HTO were performed on the same knee models. In CWHTO, insertion level was set 2cm below the medial joint line [Fig.1]. While in OW-HTO, that was set 3.5cm below the medial joint line and passed obliquely towards the tip of the fibular head [Fig.2]. The correction angle was determined so that the postoperative tibiofemoral angle would be 170 degrees. The distance between the center of resection surface and anatomical axis, and the angle of anatomical axis and mechanical axis were measured in each procedure. Secondly, a simulated TKA conversion was operated on the each tibial bone models after HTO [Fig.3]. The distance between the nearest points of tibial implant and lateral cortical bone was assessed as the index of the bone-implant interference. «Results». The distance between the center of resection surface and anatomical axis was significantly shifted to the lateral side in CW group (0.62 ±2.95 mm lateral shift) than in OW group (0.93 ± 3.68 mm medial shift) (P<0.01). The angle of anatomical axis and mechanical axis was significantly increased in the CW group (CW: 0.77 ± 0.79 degree, OW: 0.49 ± 0.83 degree, P<0.01). In the simulation of TKA conversion, if thickness of the lateral cortical bone was 3mm, it was showed that the tibial implant was more interfered with the lateral cortical bone in CW group (2.77 ± 1.38 mm) than in OW group (4.32 ± 1.61 mm) (P<0.01). «Conclusions». The results suggested that bone deformity in proximal tibia after HTO might affect the difficulty of TKA conversion, particularly in the case of CWHTO