Radiographic Evaluation of a Threshold-Based Latent Medial Laxity Adjustment for Coronal Alignment After Medial Open-Wedge High Tibial Osteotomy

Background and Objectives: Postoperative alignment after medial open-wedge high tibial osteotomy (MOWHTO) reflects both osseous correction and load-dependent joint response. We evaluated the internal radiographic performance of a prespecified threshold-based latent medial laxity (LML) adjustment and explored alternative deduction models. Materials and Methods: Seventy-two consecutive eligible knees were retrospectively analyzed. LML, defined operationally as the difference between joint-line convergence angles on supine valgus-stress and standing radiographs, was treated as a composite load-and-stress response. One-third of LML was deducted from the target correction angle when LML was ≤5°, and one-half when LML was >5°. The primary outcome was 1-year hip–knee–ankle (HKA) error within ±1.0°. Change in mechanical medial proximal tibial angle (ΔmMPTA) served as a radiographic proxy for executed osseous correction. Alternative deduction rules were compared by internal counterfactual analysis with leave-one-out cross-validation for fitted models. Results: Fifty-two knees (72.2%; 95% CI, 60.4–82.1%) were within ±1.0° of target. Mean signed error was −0.38 ± 0.95° (p = 0.001), with Bland–Altman limits of agreement from −2.25° to 1.49°. Mean ΔmMPTA was 6.03 ± 2.59°, versus a planned osseous correction of 6.81 ± 1.41°; the execution proxy averaged −0.78 ± 2.13° (p = 0.003). The residual planning component averaged +0.40 ± 2.04° (p = 0.098). In the internal comparison, the threshold rule yielded an MAE of 0.76° and 72.2% within ±1.0°, versus 1.79° and 41.7% with no adjustment. An exploratory continuous model (deduction = −1.77 + 0.736 × LML) yielded an MAE of 0.66° and 79.2% within ±1.0° (leave-one-out MAE, 0.68°). Conclusions: Threshold-based LML adjustment reproduced the individualized 1-year HKA target within ±1.0° in 72.2% of knees. This simple method appears feasible for preoperative planning; however, the small target-relative undercorrection bias warrants further refinement and prospective comparative validation.