The Utility of Posterior Sloping Angle in Predicting Contralateral Slipped Capital Femoral Epiphysis

Abstract

Prophylactic pinning of the uninvolved side after unilateral slipped capital femoral epiphysis (SCFE) is controversial. The posterior sloping angle (PSA) has previously been proposed to predict contralateral slip. The purpose of this study was to determine whether the PSA can predict subsequent slip after unilateral SCFE, and if so, whether a sex difference exists. A retrospective case-control study was performed comparing 51 patients who initially presented with unilateral SCFE and subsequently developed contralateral slip (Bilateral) with 51 patients who had unilateral SCFE only (Unilateral). Data collected included age, sex, ethnicity, and PSA. The patients in the Bilateral group had significantly higher PSA (14.5±6.1 vs. 10.6±5.3, P=0.001) and were younger (11.3±1.5 vs.12.3±1.2, P<0.001) than the patients in the Unilateral group. A receiver-operating characteristic curve demonstrated that the threshold for pinning a contralateral hip with PSA >12.66 yields an area under the curve of 67%. When the analysis was repeated with respect to sex, girls in the Bilateral group had significantly higher PSA (15.9±6.3 vs. 10.1±6.0, P=0.002) and were younger (10.7±1.1 vs.11.9±1.0, P<0.001) than the girls in the Unilateral group. Among boys, these associations were not significant. A receiver-operating characteristic curve demonstrated that the threshold for pinning a contralateral hip with PSA >13 in girls yields an area under the curve of 76%. PSA is predictive of contralateral slip in patients presenting with unilateral SCFE. However, it is more predictive in girls, and we recommend considering prophylactic pinning in girls with PSA >13. Level III.