Adult spinal deformity (ASD) affects 60% of aging adults, with many researchers and clinicians acknowledging the importance of the lumbopelvic musculature. This work investigated morphometry of lumbopelvic muscle in ASD patients in upright functional postures in comparison to supine using upright magnetic resonance imaging (MRI), with foundational feasibility studies in asymptomatic individuals. A secondary aim explored markers of muscle activity from upright MRI.

Subjects (asymptomatic:​ 6 lumbar, 6 lumbopelvic, 7 upper arm;​ ASD:​ 8 lumbopelvic), were scanned in a 0.5T upright MRI (MROpen, Paramed) in various upright and supine postures. Measures included muscle morphometry (cross-sectional area (CSA), position) for the multifidus/erector spinae, psoas major, gluteus, and iliopsoas L3/L4-S4/S5, and bony geometry. Repeatability was assessed using intraclass correlation coefficient (ICC(3,1)), effects of posture and muscle activity were evaluated by ANOVA(p<0.05), and relationships between muscle and bony geometry were evaluated by correlations.

Promising repeatability (average ICC(3,1) 0.85) illustrated feasibility of upright lumbopelvic imaging. Standing to supine had select muscle and level dependent effects, with some asymptomatic to ASD variation. For example, gluteus CSA changed up to 17% (S4/S5) in both groups. Standing to flexion effects included up to 11% decrease in multifidus/erector spinae CSA in both groups. Psoas major level dependent effects on CSA (L3/L4 increase 30%, L5/S1 decrease 20%) were observed in asymptomatic individuals. This suggests supine MRI is generally representative of upright musculature, except for select instances and flexion postures which may need more careful consideration, such as for biomechanical modeling.

Standing to supine affected sacral slope, pelvic tilt (PT), and L3-S1 lumbar lordosis (LL). With flexion, ASD patients had 14° less L3-S1 LL change and 9° greater pelvic PT change which suggests preferential hip flexion due to lumbar disease/pain. Relationships between muscle and geometry were individual-specific and highlighted that overarching relationship may not be valid to assume across groups. Muscle activity detection using quantitative upright MRI in the upper arm provided confidence in studying muscle during upright/active postures.

Overall, this work highlights the importance of considering ASD lumbopelvic musculature in upright functional postures and helps inform our foundational knowledge for future mitigation and treatment of ASD.