Summary

Our study demonstrates gender, pre high tibial osteotomy (HTO) age, pre HTO femoral varus deformity and post HTO joint line obliquity were not independent risk factors for early conversion to total knee arthroplasty in patients following medial open wedge HTO, however, under correction of a medial open wedge HTO was an independent risk factor.

Abstract

Introduction

Medial open wedge high tibial osteotomy (MOWHTO) in patients with medial compartment osteoarthritis aims to provide pain relief to avoid or delay the need for a primary total knee arthroplasty (TKA). A MOWHTO can produce a valgus deformity in the proximal tibia and potentially increase joint line obliquity (JLO). To restore a neutral mechanical alignment yet avoid abnormal JLO, some surgeons advocate double level osteotomy. However, there is controversy in the literature how JLO affects clinical outcomes and survivorship post MOWHTO. Multiple parameters have been studied to establish the risk factors for conversion to TKA in patients following MOWHTO. To our knowledge, there is no study investigating the impact of JLO and other risk factors on the timing of TKA after a MOWHTO. The aim of our study was to identify risk factors for early conversion to TKA following MOWHTO.

Methods

We retrospectively reviewed our institutions prospectively collected database identifying 159 patients who had conversion to TKA following a previous MOWHTO. Patients were divided into early (<5 years, 41 patients) and late conversion (>7 years, 85 patients) to TKA. Using mediCAD (Version 7.0) and PACS (AGFA Software 8.1.4.170) software we measured the hip-knee-ankle angle (HKA), mechanical medial proximal tibial angle (MPTA), mechanical lateral distal femoral angle (LDFA), weight bearing line percentage with medial side being 0 (WBL %), joint line congruency angle (JLCA) and posterior tibial slope (PTS). We calculated JLO using the Coronal Plane Alignment of the Knee (CPAK) classification, with JLO being the sum of MPTA and LDFA giving 3 groups: distal (<177), neutral (177-183) and proximal (>183). Risk factors for analysis were divided into pre and post MOWHTO. The inter-observer and intra-observer reliability were excellent. Univariate analysis followed by a logistic binary regression analysis was performed to identify independent risk factors. The p value for significance was set at ≤ 0.05.

Results

On univariate analysis there was no significant difference in pre HTO age (p=0.44), gender (p=0.68), KL grade (p=0.48), HKA >8º (p=0.45), MPTA (p=0.56), LDFA >90º (p=0.12), JLCA (p=0.21), and JLO (p=0.18) between the two groups. There was no difference in post HTO MPTA (p=0.18), JLO (p=0.24), and change in PTS (p=0.53) between groups. The incidence of WBL percentage <50% was higher (p=0.01) in the early group as compared to the late group. We included age at HTO, gender, pre HTO LDFA, post HTO JLO and post HTO WBL in our logistic binary regression analysis. Our overall model was statistically significant (p=0.04). We found that WBL percentage <50% was an independent risk factor (p=0.04) for early conversion to TKA with an OR of 2.67 (95% CI 1.04 to 6.82) as compared to those with WBL percentage 50-65%. Age, Gender, pre HTO LDFA, post HTO JLO were not independent risk factors for early conversion to TKA.

Conclusion

Under correction of a MOWHTO with WBL <50% was an independent risk factor for early conversion to TKR, while gender, pre HTO age, pre HTO femoral varus deformity and post HTO JLO were not independent risk factors.