Hip Instability in Children With Spinal Muscular Atrophy: A Retrospective Study.

Purpose: Hip subluxation and dislocation are known problems for children with spinal muscular atrophy (SMA). Medical complexity of these children typically results in absence of monitoring and intervention for pathologic hips. Patterns of hip migration and acetabular morphology in SMA have not been described. This study examines the natural progression of hip instability across all types of SMA in a pediatric population.

Methods: Following institutional review board approval, a retrospective chart review of all children with SMA at our institution was performed. All x-rays taken before the age of 18 years containing adequate projections of the pelvis were measured for Reimer migration index (MI), acetabular depth ratio (ADR), and acetabular index (AI). Demographics and clinical data were collected including use of nusinersen, ambulatory status, contractures, and hip pain. Linear mixed effects model was fit to serial MI measures of individual hips with fixed effects consisting of SMA type, age at x-ray, and their interaction. ADR and AI measures were similarly modeled following conversion of raw values to z-scores based on the model developed by Novais et al Slope indicated rate of measure change as a function of age.

Results: Forty-five children (22 males) with SMA types 1 to 3 were included in this retrospective study. Six children were classified as type 1, 25 were type 2, and 14 were type 3. The interaction of age by SMA type was statistically significant (P=0.01), indicating a difference in the rate of hip subluxation between the 3 SMA types as measured by MI. By age 4, MI values were different from one another across all 3 groups (P<0.01). ADR decreased with age across all SMA types. The slopes of ADR regression lines were negative and statistically significant between the 3 groups (P=0.002). AI values were higher for all types of SMA, which is the opposite of expected in normal hips.

Conclusions: Hip subluxation occurs across all SMA types, most rapidly in SMA type 1. Regression lines of ADR and AI compared with those seen in unaffected populations suggest hips in children with SMA demonstrate a difference in morphology of the acetabulum and do not follow normal adaptive remodeling. As treatments advance and the population of SMA patients becomes more mobile, there is an increased need to monitor hip instability in children with SMA, thus making orthopaedic management an important consideration.