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

Objective 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. Methods 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. Results Seventy-six per cent of all imaging position-deformity combinations had error in the reported angle (>1° difference between LHNA and SW). Of those, 70% had 10° of error from the actual SW angle. Three per cent of LHNAs that had >10° error resulted from 10° LHNA error; additional three-dimensional imaging should be considered.