Rationale for preoperative planning in the supine position based on variability in the joint line convergence angle before and after open-wedge high tibial osteotomy.

Aims: In advanced medial knee osteoarthritis, lower limb alignment is assessed using weightbearing imaging, which reflects the joint line convergence angle (JLCA). However, successful lateral load transfer with open-wedge high tibial osteotomy (OWHTO) can reduce the JLCA by decreasing the lateral and increasing the medial joint space on weightbearing imaging, making the correction less predictable. This study aimed to demonstrate that OWHTO planning using preoperative supine imaging yields more reproducible correction than weightbearing imaging and to identify radiological predictors of excessive JLCA change (> 2°). Methods: We retrospectively analyzed 115 knees of 96 patients who underwent OWHTO planned using supine imaging. Radiological parameters, including the hip-knee-ankle angle (HKAA) and arithmetic HKAA, were measured using preoperative and postoperative long-leg weightbearing radiographs. Knees were categorized into three groups by ΔJLCA: ΔJLCA < -2° (decreased); -2° ≤ ΔJLCA ≤ 2° (unchanged), and ΔJLCA > 2° (increased). Logistic regression and receiver operating characteristic curve analysis were used to determine radiological factors associated with ΔJLCA. Results: The unchanged JLCA group comprised 84.3% of knees, with similar preoperative weightbearing and supine JLCAs (2.6° and 2.5°, respectively). The postoperative weightbearing JLCA was 2.5°. The decreased JLCA group included 11.3% of knees, and the postoperative weightbearing JLCA (mean 2.6°) was closer to the preoperative supine (mean 4.1°) than the weightbearing JLCA (mean 5.5°). Preoperative arithmetic HKAAs were identical in the unchanged and decreased JLCA groups (3.9° varus). The increased JLCA group comprised only 4.3% of knees. Preoperative weightbearing JLCA > 4.9° and varus HKAA > 8.8° were significantly associated with decreased JLCA, with areas under the curve of 0.86 and 0.72, respectively. Conclusion: Postoperative weightbearing alignment following OWHTO is better predicted by preoperative supine than weightbearing imaging. Furthermore, large preoperative weightbearing JLCA (> 4.9°) and severe varus HKAA (> 8.8°) are associated with decreased postoperative JLCA.