Relationship between displacement of the psoas major muscle and spinal alignment in patients with adult spinal deformity

Study Design: Cross sectional study. Purpose: To clarify the difference in position of the psoas muscle between adult spinal deformity (ASD) and lumbar spinal stenosis (LSS). Overview of Literature: Although it is known that the psoas major muscle deviates in ASD patients, no report is available regarding the difference in comparison with LSS patients. Methods: This study investigates 39 patients. For evaluating spinal alignment, pelvic tilt (PT), pelvic incidence (PI), sacral slope, lumbar lordosis (LL), PI-LL, Cobb angle, and the convex side, the lumbar curves were measured. For measuring the position of the psoas major at the L4/5 disk level, magnetic resonance imaging was used. The displacements of psoas major muscle were measured separately in the anterior-posterior and lateral directions. We examined the relationship between the radiographic parameters and anterior displacement (AD) and lateral displacement (LD) of the psoas major muscle. Results: AD was demonstrated in 15 cases with ASD and nine cases with LSS (p > 0.05). LD was observed in 13 cases with ASD and no cases with LSS (p < 0.01). The Cobb angle was significantly greater in cases with AD than in those without AD (p =0.04). PT, LL, PI- LL, and Cobb angle were significantly greater in cases with LD (p < 0.05). All cases with LD had AD, but no case without AD had LD (p < 0.001). The side of greater displacement at L4/5 and the convex side of the lumbar curve were consistent in all cases. Conclusions: Despite AD being observed in LSS as well, LD was observed only in the ASD group. Radiographic parameters were worse when LD was seen, rather than AD.