The purpose of this study was to assess the differences in proximal tibial deformity between closing wedge (CW) and opening wedge (OW) high tibial osteotomy (HTO) and their effects on the difficulty of total knee arthroplasty (TKA) conversion. Surgical simulations of CW-HTO and OW-HTO were performed on the same 3D computer-aided design knee models reconstructed from computed tomographic datasets of patients (median age 77 years; range 55-87 years; 40 knees) with medial osteoarthritis or osteonecrosis, and proximal tibial deformities were analysed. Subsequent TKA conversion was simulated in both CW and OW models, and the interference between the tibial implant and endosteal cortex was assessed. The difference in the metaphyseal-diaphyseal angle in the CW-HTO and OW-HTO groups was 0.3A degrees A +/- 0.2A degrees (p < 0.001). The mechanical axis in the CW-HTO group was shifted laterally by 1.8 +/- 1.1 mm relative to that in the OW-HTO group (p < 0.001) on the resection surface in TKA. Finally, the TKA tibial implant was substantially closer to the endosteal cortex in the CW-HTO group (mean 5.6 +/- 1.6 mm) than to that in the OW-HTO group (mean 7.3 +/- 1.6 mm) (p < 0.001). The difference in the post-operative angular deformities of the proximal tibia between CW-HTO and OW-HTO was considered to be clinically irrelevant. The risk of interference between TKA tibial implant and endosteal bone was greater after CW-HTO than after OW-HTO. For clinical relevance, the difference between the two techniques has little influence on subsequent TKA difficulty regarding the proximal tibial deformity, although preoperative planning is obligatory, particularly after CW-HTO, to prevent interference with the tibial implant. II.