Introduction: Treatment of medial tibiofemoral osteoarthritis with a high-tibial osteotomy (HTO) is most effective when the optimal angular correction is achieved. However, conventional instrumentation is limited when multiplanar correction is needed. Hypothesis: Use of patient-specific cutting guides (PSCGs) for HTO provides an accurate correction (difference < 2) relative to the preoperative planning. Materials and methods: Between February 2014 and February 2015, 10 patients (mean age: 46 years [range: 31-59]; grade 1 or 2 osteoarthritis in Ahlback's classification) were included prospectively in this reliability and safety study. All patients were operated using the same medial opening-wedge osteotomy technique. Preoperative planning was based on long-leg radiographs and CT scans with 3D reconstruction. The PSGCs were used to align the osteotomy cut and position the screw holes for the plate. The desired correction was achieved in the three planes when the holes on the plate were aligned with the holes drilled based on the PSCG. Preoperatively, the mean HKA angle was 171.9 degrees (range: 166-179 degrees), the mean proximal tibial angle was 87 degrees (86-88 degrees) and the mean tibial slope was 7.8 degrees (1-22 degrees). The postoperative correction was compared to the planned correction using 3D CT scan transformations. Intraoperative and postoperative complications were assessed at a minimum follow-up of 1 year. Results: The procedure was successfully carried out in all patients with the PSCGs. On postoperative long leg radiographs, the mean HKA was 182.3 degrees ( 180-185 degrees); on the CT scan, the mean tibial mechanical angle was 94 degrees (90-98 degrees) and the mean tibial slope was 7.1 (4-11 degrees). In 19 out of 20 postoperative HKA and slope measurements, the difference between the planned and achieved correction was < 2 based on the 3D analysis of the three planes in space; in the other case, the slope was 13 instead of the planned 10. The intra-class correlation coefficients between the postoperative and planned parameters were 0.98 degrees [0.92-0.99 degrees] for the HKA and 0.96 degrees [0.79-0.99 degrees] for the tibial slope. There were no surgical site infections; one patient had a postoperative hematoma that resolved spontaneously. Discussion: The results of this study showed that use of PSCGs in HTO procedures helps to achieve optimal correction in a safe and reliable manner. (C) 2017 Elsevier Masson SAS. All rights reserved.