Measuring osteotomy wedge angle is more important than measuring wedge height in open wedge osteotomies around the knee in preoperative planning

Purpose: Preoperative planning for medial open wedge high tibial osteotomies (HTOs) and lateral open wedge distal femur osteotomies (DFOs) commonly uses wedge height to guide accurate correction. However, it is unclear if this parameter is influenced by intraoperative variations in osteotomy entry point or length. This study hypothesized that wedge angle remains constant during planning, while wedge height varies depending on hinge or entry points. Methods: Whole leg radiographs of 40 patients who underwent HTO or DFO (2018–2024) were analysed using digital planning software. For each HTO and DFO case, 27 and 21 osteotomy variants were created, respectively, by altering entry and hinge points, resulting in 960 simulations. Wedge angle, wedge height and osteotomy depth were measured for each variant. Correlations and regression analyses assessed the relationships among these variables, and a mathematical formula was developed to predict wedge height from wedge angle and osteotomy depth. Results: Wedge angle remained consistent across variants (mean deviation: 0.1 ± 0.1°), while wedge height showed variability (mean deviation: 0.7 ± 0.5 mm) influenced by entry and hinge points. Significant correlations were found between wedge height and opening angle (R = 0.83, p < 0.001) and osteotomy depth (R = 0.60, p < 0.001). Predicted wedge height closely matched actual values (R = 0.998, p < 0.001), with minimal error (−0.01 ± 0.1°). Conclusion: This study highlights that relying solely on wedge height for osteotomy planning in HTO and DFO is insufficient due to variations in entry and hinge points. The hinge angle proved to be the most reliable parameter. Intraoperative osteotomy depth measurements can help adjust wedge height for accurate limb alignment when deviations occur. Level of Evidence: Level V, simulation study.