Proposal and validation of an angular External Fixation Index (aEFI) for standardized reporting of gradual angular deformity correction: a time-normalized descriptive metric

BACKGROUND: ​ Gradual correction of genu varum using circular external fixators is well-established. Although the External Fixation Index (EFI) is widely used in linear bone lengthening, no standardized, time-normalized metric exists for angular deformity correction. This study introduces the angular External Fixation Index (aEFI) as a descriptive tool and evaluates internal consistency within a clinical cohort treated with oblique proximal tibial corticotomy (OPTC). METHODS: ​ A prospective cohort study included 22 patients (30 knees) who underwent gradual genu varum correction using OPTC and circular external fixation. The aEFI was calculated as total duration of external fixation (weeks) divided by achieved angular correction (degrees). Radiographic evaluation included medial proximal tibial angle (MPTA) and hip-knee-ankle angle measured preoperatively and at final follow-up. Functional outcomes were assessed using the Western Ontario and McMaster Universities Osteoarthritis Index, and the Stanmore Limb Reconstruction Score. RESULTS: At a mean follow-up of 16.5 ± 3.25 months, coronal alignment was restored in all knees. Mean fixation duration was 27.4 ± 6.2 weeks, with a mean aEFI of 1.89 ± 0.41 weeks/degree. An inverse association was observed between correction magnitude and aEFI (r = -0.88, p < 0.001), reflecting the reduced proportional effect of fixed treatment phases with larger corrections. Functional scores improved, and minor pin-tract infections occurred in 20% of knees and resolved conservatively. CONCLUSION: ​ The proposed aEFI serves as a standardized, descriptive, time-normalized metric for reporting treatment duration relative to angular correction. External validation across different constructs and deformity patterns is warranted.