Southwick angle measurements and SCFE slip severity classifications are affected by frog-lateral positioning
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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 (>1° difference between LHNA and SW). Of those, 70% had <5°, 24% had 5° to 10°, and 6% had >10° of error from the actual SW angle. Three per cent of LHNAs that had >10° error resulted from <10° 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 > 10° LHNA error; additional three-dimensional imaging should be considered.
KeywordsSCFE Slipped capital femoral epiphysis Southwick Computer modelling Hip Paediatric Juvenile Rare disease Radiographic measures
This study was supported by a Microgrant from the Rare Disease Foundation and an Undergraduate Student Research Award from Engineers in Scrubs.
Compliance with ethical standards
Ethics board approval was obtained from our institution for this study. All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.
Conflict of interest
The authors declare that they have no conflict of interest.
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