Docking of the Femoral Head Following Closed Reduction for DDH: Does it Really Occur?

Background: It has been suggested that the femoral head can "dock" deeper into the acetabulum after initial closed reduction (CR) for developmental dysplasia of the hip (DDH). The purpose of this study was to quantify the interval change in femoral head position between immediate postoperative magnetic resonance imaging (MRI) and follow-up imaging at 3 weeks. Methods: A retrospective review of 29 patients (30 hips) who underwent CR and spica casting for DDH was conducted. Immediate postoperative and average 3-week follow-up MRI scans in spica were performed on all patients. On both scans, 2 blinded reviewers measured the following indices: the distance between the femoral head and the acetabulum on midcoronal and midaxial images, the displacement of the center of femoral head from Hilgenreiner's line in the coronal and axial plane, and the left-right displacement of the center of femoral head from Perkins line. Measurements were averaged between the 2 reviewers and the interval change in femoral head position between the immediate postoperative and follow-up scans were compared. Results: There were 26 female individuals and 3 male individuals in our series with a mean age of 7.6 months (range, 4 to 13 mo). Follow-up MRI scans were performed at an average of 23.8 days (range, 13 to 46 d). On the basis of the averaged measurements from both readers, the distance between the femoral head and the acetabulum decreased significantly on coronal measurement and on all 3 axial measurements between initial and follow-up MRI. In addition, the position of the femoral head became significantly more medial, more anterior, and more cranial relative to the acetabulum. The interrater correlation coefficient between both readers across all measurements was 0.731. Conclusions: These findings provide evidence that femoral head position within the acetabulum improves even over a short time period following initial CR for DDH, suggesting that the "docking" phenomenon may in fact occur. Level of Evidence: Level IV - therapeutic study.