Quantifying Muscle Size Asymmetry in Adolescent Idiopathic Scoliosis Using Three-dimensional Magnetic Resonance Imaging

Study Design: This is a case-control study of prospectively collected data. Objective: To quantify paraspinal muscle size asymmetry in adolescent idiopathic scoliosis (AIS) and determine if this asymmetry is (i) greater than observed in adolescent controls with symmetrical spines; and (ii) positively associated with skeletal maturity using Risser grade, scoliosis severity using the Cobb angle, and chronological age in years. Summary of Background Data: AIS is a three-dimensional deformity of the spinewhich occurs in 2.5% to 3.7% of the Australian population. There is some evidence of asymmetry in paraspinal muscle activation and morphology in AIS. Asymmetric paraspinal muscle forces may facilitate asymmetric vertebral growth during adolescence. Methods: An asymmetryindex [Ln(concave/convex volume)] of deep and superficial paraspinal muscle volumes, at the level of the major curve apex (Thoracic 8-9(th) vertebral level) and lower-end vertebrae (LEV, Thoracic 10-12(th) vertebral level), was determined from threedimensional Magnetic Resonance Imaging of 25 adolescents with AIS (all right thoracic curves), and 22 healthy controls (convex= left); all female, 10 to 16 years. Results: Asymmetryindex of deep paraspinal muscle volumes was greater in AIS (0.16 +/- 0.20) than healthy spine controls (-0.06 +/- 0.13) at the level of the apex (P< 0.01, linear mixed-effects analysis) but not LEV (P> 0.05). Asymmetryindex was positively correlated with Risser grade (r= 0.50, P< 0.05) and scoliosis Cobb angle (r= 0.45, P< 0.05), but not age (r= 0.34, P> 0.05). There was no difference in the asymmetryindex of superficial paraspinal muscle volumes between AIS and controls ( P> 0.05). Conclusions: The asymmetry of deep apical paraspinal muscle volume in AIS at the scoliosis apex is greater than that observed at equivalent vertebral levels in controls and may play a role in the pathogenesis of AIS.