Risk factors for early symptomatic femoroacetabular impingement following in situ fixation of slipped capital femoral epiphysis

In situ fixation of slipped capital femoral epiphysis (SCFE) results in residual deformity that can cause femoroacetabular impingement (FAI). It is unknown what factors could help differentiate patients who are more likely to become symptomatic. We performed a retrospective review of 55 hips treated with in situ pinning for SCFE and subsequent secondary deformity correction procedure for symptomatic FAI and compared them to 39 asymptomatic hips with SCFE deformity using multivariable analysis. Case patients were slightly older than controls (12.6 vs 11.3 years, p = 0.0002) but had similar BMI. The mean epiphyseal-diaphyseal angle was 56° in cases versus 44° in controls (p = 0.0019). Cases were significantly more likely to have obligate external rotation with hip flexion, external foot progression, flexion <90°, antalgic limp, and Trendelenburg lurch. On radiographs, most cases had a head-neck offset ≤0 mm, a distinct metaphyseal corner prominence, acetabular retroversion, and an alpha angle ≥60°. Most controls also had head-neck offset ≤0 mm. Pre-pinning, older age (OR = 1.98 per year, p = 0.0016) and initial epiphyseal-diaphyseal angle (OR = 1.04 per degree, p = 0.018) significantly increased the odds of having symptomatic FAI. Post-pinning, external foot progression increased the odds of symptomatic FAI by 10.48 (p = 0.017), and an alpha angle ≥60° resulted in 11.4 times higher odds of symptomatic FAI (p = 0.011). The linear correlation between epiphyseal-diaphyseal and alpha angle was poor (r = 0.28). Older age and initial epiphyseal-diaphyseal pre-pinning mildly increased the odds of eventual symptomatic FAI. This information can help the surgeon to predict which patients may develop symptomatic FAI.