Association between body mass index-for-age and slipped capital femoral epiphysis: the long-term risk for subsequent slip in patients followed until physeal closure

BACKGROUND: Children who present with idiopathic slipped capital femoral epiphysis (SCFE) have an increased risk of developing bilateral disease. Predicting which patients will develop problems with bilateral hips is important for determining treatment algorithms. This is a retrospective observational study that evaluates the relationship and risk between body mass index (BMI)-for-age and unilateral and bilateral SCFE in patients followed until physeal closure. METHODS: This is a retrospective study of all patients with SCFE presenting to one institution from 1998–2005. Using the Center for Disease Control (CDC) references, BMI-for-age was calculated for each patient. The patients were followed up until complete closure of the bilateral proximal femoral physes, which was considered completion of the study. Statistical analysis for significant differences between groups was performed using the Kruskal–Wallis test for equality of populations. A logistic regression, controlling for age and gender, was used to identify BMI-for-age as a risk factor and to determine the significance of the odds ratios (ORs) for the relevant categorical variables—obese, overweight and healthy weight. RESULTS: Eighty patients (56 male, 24 female) presented to a single institution between 1998 and 2005 with a diagnosis of SCFE. The mean age of patients was 12.2 years at initial presentation (range 8.5–16). Forty-eight patients (32 male, 16 female) presented with unilateral SCFE, with 22 of the 48 patients having a BMI for-age percentile ≥95 %. Thirty-two patients (24 male, 8 female) presented with bilateral SCFE, with 29 of the 32 patients having a BMI-for-age percentile ≥95 %. Patients with a BMI-for-age ≥95 % had a significantly increased risk of presentation with bilateral slips (OR 4.83; relative risk [RR] 3.01; p < 0.05]. All but one patient in this study with bilateral SCFE or unilateral SCFE with subsequent contralateral involvement had a BMI-for-age ≥85 % (44 out of 45 patients). Additionally, the overall risk of developing bilateral SCFE until physeal closure with a BMI-for-age ≥95 % was significantly increased (OR 3.84; RR 2.02; p < 0.05; number needed to treat [NNT] 3.01). CONCLUSIONS: Previous work has established a relationship between BMI and SCFE. The CDC BMI-for-age growth charts more accurately measure obesity in the pediatric population compared to BMI and are therefore a more appropriate reference tool. This study demonstrates an association between obesity measured by BMI-for-age percentiles and SCFE. This study also demonstrates an association between BMI-for-age and risk for bilateral SCFE at presentation as well as overall incidence of developing bilateral SCFE in the obese pediatric population. By defining the at-risk population through BMI-for-age, physicians can screen the pediatric patient population and provide early strategies for therapeutic weight loss which may reduce the incidence of SCFE.