Southwick angle measurements and SCFE slip severity classifications are affected by frog-lateral positioning

Slipped capital femoral epiphysis (SCFE) is a hip disorder where the femoral head slips relative to the neck at the physis. Appropriate treatment of SCFE depends on the severity of the slip, commonly categorised using the Southwick (SW) angle. The SW angle is measured in the frog-lateral leg position, which can be painful and potentially unattainable for patients. The purpose of this study is to determine how errors in frog-lateral radiograph positioning affect measured SW angles and slip classifications. Models of SCFE hips were produced from one CT scan of a normal hip; 360 deformities were created. SW angles were measured from a simulated frog-lateral position. Femoral lateral head-neck angles (LHNA; equivalent to SW in incorrect frog-lateral plane) were measured over a range of 837 incorrect frog-lateral leg positions with positioning errors in flexion and/or internal/external rotation. Seventy-six per cent of all imaging position-deformity combinations had error in the reported angle (> 1A degrees difference between LHNA and SW). Of those, 70% had < 5A degrees, 24% had 5A degrees to 10A degrees, and 6% had > 10A degrees of error from the actual SW angle. Three per cent of LHNAs that had > 10A degrees error resulted from < 10A degrees of positioning error. If the patient is limited in flexion or external rotation, more diagnostic testing should be considered if error in the reported slip measurement would affect treatment decisions or if accurate severity classification is needed for research. Small positioning errors in moderate and severe slips can cause a > 10A degrees LHNA error; additional three-dimensional imaging should be considered.