Residual hip dysplasia: A comparison of osseous and cartilaginous acetabular angles on MRI in children to guide further treatment

Introduction Despite improvements in early recognition and treatment of developmental dysplasia of the hip (DDH), residual hip dysplasia (RHD) continues to be a challenging clinical entity. Pelvic radiographs remain the standard in the follow up and evaluation of acetabular development, but can be insufficient to precisely evaluate the entire coverage of the femoral head, when trying to decide on the need for further reconstructive procedures in borderline RHD. We therefore introduced a new ratio comprised of the bony and cartilaginous acetabular angles measured on MRI in order to augment standard radiographs to aid clinicians in deciding if acetabuloplasty is needed. Materiel et methodes We retrospectively compared the bony and the cartilaginous acetabular angles of Hilgenreiner (HTE) in 30 children on pelvic MRI. Twenty children were followed up for DDH. The other 10 children had undergone a pelvic MRI for reasons other than DDH. RHD was defined by a bony HTE superior to 20°. Using standard radiographs, the 60 hips were separated into two groups: group 1 included the hips presenting RHD and group 2 the hips with a HTE below 20°. The hips in the two groups were then 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 (AAR) was set at five and was extrapolated from the published normal values of C-HTE and O-HTE in children. Resultats The AAR was statistically significantly increased in the hips with RHD with a mean value of 7.1 (min.: 1.6, max.: 22.5) compared to the hips in the control group presenting a mean value of 2.1 (min.: 0.1, max.: 8.4) ( P Discussion The cartilaginous part of the acetabulum seems to be an early and reliable predictor of acetabular development, as it's fully formed at birth and it is supposed to represent the bony margins of the acetabulum at adulthood after full ossification. The average C-HTE and O-HTE angles from the 27 healthy hips in group 2 corroborated with previous researches. An AAR above five means that not only the bony acetabular coverage is insufficient but also the cartilaginous part and therefore the hips have lower chance to normalize with growth. Moreover, an AAR from below five, even if RHD is seen on plain radiographs, indicates a sufficient cartilaginous coverage with an O-HTE that will correct with growth and we therefore renounce to perform an acetabuloplasty. Conclusion We recommend a pelvic MRI in children presenting borderline residual hip dysplasia on a plain radiography at the age of four and to perform acetabuloplasty if the AAR is superior to five.