Purpose In case of residual hip dysplasia (RHD) in children, pelvic radiographs are sometimes insufficient to precisely evaluate the entire coverage of the femoral head, when trying to decide on the need for further reconstructive procedures. Methods This study retrospectively compares the bony and the cartilaginous acetabular angle of Hilgenreiner (HTE) of 60 paediatric hips on pelvic MRI separated in two groups. Group 1 included 31 hips with RHD defined by a bony HTE > 20 degrees. Group 2 included 27 hips with a HTE < 20 degrees. They were compared by introducing a new ratio calculated from the square of cartilaginous HTE above the bony HTE on frontal MRI. The normal upper limit for this acetabular angle ratio was extrapolated from the published normal values of cartilaginous HTE and bony HTE in children. Results The acetabular angle ratio was statistically significantly increased in the hips with RHD with a mean value of 7.1 +/- 4.7 compared to the hips in the control group presenting a mean value of 2.1 +/- 1.9 (p < 0.00001). Conclusions This newly introduced ratio seems to be a helpful tool to orientate the further treatment in children presenting borderline RHD.