Purpose <p>To determine whether patient-specific instrumentation (PSI) reduces the angular deviation between planned and executed osteotomy lines, compared with the conventional freehand technique, using direct three-dimensional analysis of pre- and postoperative CT models.</p> Methods <p>A randomised study was conducted on 18 fresh-frozen human tibiae. Specimens were assigned to either PSI-assisted or conventional osteotomy groups. Osteotomies were planned on 3D CT models and executed by experienced surgeons. Pre- and postoperative CT scans were used to assess osteotomy angles and the distance from the cut to the joint line. The primary outcome was the difference between planned and executed osteotomy angles in the anteroposterior (AP) and lateral planes. Secondary outcomes included deviation in cut location and variability in results.</p> Results <p>Baseline characteristics were comparable between groups. PSI significantly reduced angular deviation in both the AP (2.8° ± 2.0 vs. 6.1° ± 5.6; <i>p</i> = 0.003) and lateral planes (2.1° ± 1.5 vs. 5.8° ± 6.0; <i>p</i> = 0.001). No significant difference was observed in the distance to the joint line (<i>p</i> = 0.136). The PSI group showed greater consistency and fewer outliers compared to the conventional group.</p> Conclusion <p>PSI significantly enhances the accuracy and reproducibility of osteotomy execution in medial opening wedge HTO compared with the freehand technique. These findings are limited to a cadaveric model and reflect technical accuracy at the cut level; any clinical impact requires confirmation in prospective in vivo studies. Clinical relevance: In this experimental cadaveric model, PSI demonstrated improved cut-level accuracy; potential clinical benefits remain to be established in patient cohorts. Level of Evidence: Level II – Randomised cadaveric study.</p>