EP9.20 Increasing slip severity in slipped capital femoral epiphysis is linearly associated with decreasing femoral version in a 3:1 ratio

Abstract Introduction Unilateral slipped capital femoral epiphysis (SCFE) has previously been associated with asymmetric reduced femoral anteversion and increased rates of femoral retroversion in the affected hip compared to the unaffected side. However, the relationship between the severity of the SCFE deformity, measured by the Southwick slip angle, and femoral version has not been previously reported. Methods A retrospective study was conducted to evaluate preoperative pelvis radiographs and magnetic resonance imaging (MRI) of patients with unilateral SCFE treated at our institution from 2017 to 2025. Patients with incomplete imaging were excluded. A custom-developed and validated program (VirtualHip, Boston Children’s Hospital) was used to automatically measure femoral version in 3D following the Murphy method. Southwick angle was measured by a fellowship-trained orthopedic surgeon from X-ray frog views. Both hips of each patient were analyzed. Linear regression was used to assess associations between femoral version and Southwick angle, and paired Student’s t-tests were used to compare femoral version and slip angles between SCFE and contralateral control hips. Results Eighteen patients were included in the final study (18 SCFE, 18 controls). The average age was 11.5 years (range 8-14), 50% of patients were female, and the mean BMI was 27.1 (range 18.5-39.4). The mean femoral neck version was lower in hips with SCFE compared to the contralateral side (5.8° +/- 9.1° versus (17.5° +/- 10.5° °; p < 0.001), with a mean side-to-side difference of 11.8° +/- 10.1°. There was a higher prevalence of femoral retroversion (version <0°) in hips with SCFE (33.3%; p =0.019) versus the contralateral side (0%). The mean Southwick angle was higher in hips with SCFE compared to the contralateral side (26.6° +/- 15.9° versus 4.3° +/- 3.0° °; p < 0.001), with a mean side-to-side difference of 22.2° +/- 16.5°. Linear regression modeling found that each degree increase in Southwick slip angle was associated with a change in femoral version by -0.309° (R2 = 0.19, p < 0.01). Conclusion In patients with unilateral SCFE, the affected side is associated with an asymmetric reduction of femoral neck version and higher rates of retroversion compared to the contralateral normal side. For every 3° of slip, as measured by Southwick angle, there is an 1° decrease in femoral version. Significance This is the first report quantifying the relationship between SCFE severity and femoral version, which may aid in anticipating changes in rotational profile and identifying patients who may benefit from additional procedures.