The Relationship Between Screw Placement and Southwick Angle Remodeling in Slipped Capital Femoral Epiphysis

BACKGROUND: In situ pinning (ISP) is the standard treatment for slipped capital femoral epiphysis (SCFE). While spontaneous remodeling of the femoral head following ISP is recognized, the factors influencing this remodeling, specifically the impact of sagittal screw placement, remain unclear. This study aimed to investigate whether screw placement is associated with changes in the Southwick angle 1 year postoperatively and to determine the optimal insertion position to facilitate remodeling. METHODS: We retrospectively reviewed 59 hips in 47 patients treated with ISP using a single screw between 1983 and 2024 who had a minimum follow-up of 1 year. A metric defined as screw position (%) and the Southwick angle were measured on Lauenstein lateral radiographs immediately postoperatively and at 1 year. The relationship between Southwick angle improvement and variables including screw position (%), age, BMI percentile, type, onset, fixation method, and Southwick angle just after the surgery was analyzed using the Spearman rank correlation and multivariable linear regression analysis. RESULTS: The mean Southwick angle improved significantly from 31.8 degrees immediately postoperatively to 25.1 degrees at 1 year (P<0.05). Spearman rank correlation showed a significant negative correlation (ρ=-0.43, P<0.01) between screw position (%) and angle improvement, indicating that anterior screw placement was associated with greater correction. Multivariable linear regression analysis identified the magnitude of the immediate postoperative Southwick angle and screw position (%) as significant independent predictors of remodeling (P<0.05). However, the effect of screw position on this improvement was also relatively small (regression coefficient=-0.31). CONCLUSIONS: An anterior screw placement was identified as a significant predictor of Southwick angle improvement. However, given the limited clinical significance of the angular improvement, we fundamentally recommend central screw placement to ensure safety while avoiding posterior screw placement. LEVELS OF EVIDENCE: Level III-retrospective case series.