IntroductionThe late detection of developmental dysplasia of the hip (DDH) will remain a major concern in some parts of the world until effective screening programs become available. With late diagnosis comes the need for open surgical reduction. Surgery is invariably followed by a period of immobilisation in a spica cast to prevent postoperative displacement. The goal of this study is to evaluate the effect of double-leg spica as compared to single-leg spica, on the risk of displacement after unilateral open reduction of the hip.Materials and methodsThis was a retrospective review of DDH patients from 2012 to 2016 and younger than 4 years of age, who had unilateral anterior open reduction. Patients who had one of the following were excluded: neuromuscular diagnosis, the addition of K-wire, and simultaneous bilateral open reductions. Demographic data were collected along with related clinical and radiographic variables. A total of 128 patients (162 hips) met the inclusion criteria; 93 were in the double-leg spica group, and 69 were in the single-leg spica group.ResultsThe mean age was 25.4 ± 8.1 months and the mean follow-up was 18.6 ± 11.6 months. Baseline characteristics were balanced between the two groups. There were three events of redislocation in the double-leg spica group as compared to one redislocation in the single-leg spica group. The difference did not reach statistical significance (p = 0.637, risk ratio 1.317, CI 0.736–2.356). The difference in subsequent disruption of Shenton’s line and hip migration of more than 29% was (p = 0.395, risk ratio 1.411, CI 0.892–2.234) and (p = 0.087, risk ratio 0.67, CI 0.417–1.078), respectively. Three patients had a greenstick distal femur fracture after double-leg spica and one after single-leg spica.ConclusionThese data suggest that including the contralateral hip in the cast after open reduction is not essential as it does not seem to improve stability.