Combining acetabular and femoral morphology improves our understanding of the down syndrome hip.

Abstract

Background: Hip instability is frequent in patients with Down syndrome. Recent studies have suggested that skeletal hip alterations are responsible for this instability; however, there are currently no studies simultaneously assessing femoral and acetabular anatomy in subjects with Down syndrome in the standing position. The aim was to analyze the three-dimensional anatomy of the Down syndrome hip in standing position. Methods: Down syndrome subjects were age and sex-matched to asymptomatic controls. All subjects underwent full body biplanar X-rays with three-dimensional reconstructions of their pelvises and lower limbs. Parameter means and distributions were compared between the two groups. Findings: Forty-one Down syndrome and 41 control subjects were recruited. Acetabular abduction (mean=52° [SD=9°] vs. mean=56° [SD=8°]) and anteversion (mean=14° [SD=8°] vs. mean=17.5° [SD=5°]) as well as posterior acetabular sector angle (mean=91° [SD=7°] vs. mean=94° [SD=7°]) were significantly lower in Down syndrome subjects compared to controls (P < 0.01). Anterior acetabular sector angle (mean=62° [SD=10°] vs. mean=59° [SD=7°]; P < 0.01) was significantly higher in Down syndrome compared to controls. The distributions of acetabular anteversion (P=0.002;V=0.325), femoral anteversion (P=0.004;V=0.309) and the instability index (P < 0.001;V=0.383) were significantly different between the two groups, with subjects with Down syndrome having both increased anteversion and retroversion for each of these parameters. Interpretation: Subjects with Down syndrome were found to have a significantly altered and more heterogeneous anatomy of their proximal hips compared to controls. This heterogeneity suggests that treatment strategies of hip instability in Down syndrome should be subject-specific and should rely on the understanding of the underlying three-dimensional anatomy of each patient.