Summary

This study demonstrates that the preoperative mechanical proximal tibial angle (MPTA) is highly predictive of the required correction angle in medial open wedge high tibial osteotomy, enabling more accurate and individualized surgical planning, which may enhance postoperative alignment and clinical outcomes for patients with knee osteoarthritis.

Abstract

Introduction

Medial open wedge high tibial osteotomy (MOWHTO) is widely performed for the treatment of medial, unicompartmental knee osteoarthritis associated with a varus alignment, aiming to move the pressure on the degenerated medial compartment to the undamaged lateral compartment. Accurate determination of the correction degree is crucial, with mechanical proximal tibial angle (MPTA), weight-bearing line percentage (WBL pct.), and hip-knee-ankle (HKA) angle being key parameters. Although a previous study showed a correlation between correction degree and preoperative (pre-) HKA angle and WBL pct., it did not find a correlation between correction degree and pre-MPTA. The study aims to validate conversion formulas (CFs) that predict correction angle from pre-parameters, especially focusing on the pre-MPTA in MOWHTO with MPTA-based surgical planning for knee osteoarthritis.

Methods

This retrospective study included 91 patients (93 knees) who underwent MOWHTO from January 2017 to June 2024. The inclusion criteria included patients with isolated medial compartment osteoarthritis (Kellgren-Lawrence grade ≥ II) and varus deformity. The exclusion criteria were multi-compartmental arthritis. Surgical planning utilized computer simulation software that set the correction angle to align MPTA to 93 degrees, and PSI was designed based on 3D reconstructed CT images. Intraoperative execution involved the use of 3D-printed polyamide jigs for precise osteotomy. Correlation and linear regression analysis were employed to evaluate possible associations among the correction angle, pre-parameters, and changes between pre-parameters and surgical planning parameters (Δ). The bootstrap method was performed to compare the parameters calculated from the CFs and postoperative parameters to verify the usefulness of CFs.

Results

Spearman's correlation coefficient analysis results showed that the correction angle, which represents the angle required to align the MPTA to 93 degrees, exhibited a perfect linear correlation with the pre-MPTA, ΔMPTA, ΔHKA angle, and ΔWBL pct. (|r| = 0.91 ~ 0.99, P < 0.01).
The correction angle showed the perfect linear correlation with pre-MPTA (y = 93.9000 - 1.0100x (R= 0.97)), ΔHKA angle (y = - 0.0703 + 1.0300x (R= 1.00)), ΔWBL pct. ( y = 0.2160 + 0.2260x (R= 0.94)), and ΔMPTA (y = - 0.0660 + 1.0400x (R= 1.00)).
The mean error for all parameters (correction angle, MPTA, HKA angle, and WBL pct.) calculated using the conversion formulas and the actual measured postoperative parameters was approximately 0, with values ranging from negative to positive within the 95% confidence interval (p-value = 0.05).

Conclusions

The pre-MPTA, ΔHKA angle, ΔWBL pct., and ΔMPTA are perfectly correlated with the correction angle, making these correlations useful for practical surgical planning. This method enables accurate and individualized planning by allowing surgeons to calculate correction angles using simple mathematical formulas, even without digital tools. The study's findings confirm the practicality and accuracy of using CFs in clinical settings, demonstrating strong agreement between calculated and actual postoperative outcomes.
Keywords: Open Wedge High Tibial Osteotomy, Surgical Planning, Correlation, Conversion Formulas, Mechanical Proximal Tibial Angle, Correction Angle