Paraspinal muscle architecture in adolescent girls: A diffusion tensor imaging study

Forces applied to the skeleton during development are key moderators of growth. Paraspinal muscles apply forces directly on the spine; quantifying their architecture is crucial to determining the magnitude and direction of applied forces, which will ultimately inform the diagnosis, prevention, and treatment of developmental spinal disorders. Here we aim to develop a non-invasive method to quantify in-vivo, side-to-side and regional variation in paraspinal muscle architecture, i.e. fascicle orientation, length, and curvature. T1-weighted and diffusion-tensor MRI scans were conducted on 16 female adolescents (mean ± SD age: 13.3 ± 1.8 years). Using anatomically constrained fibre tractography, multifidus and longissimus muscle fascicles were reconstructed across 10 vertebral levels (T5-L2). Muscle-fascicle orientation angle, length, and curvature were determined. Linear mixed-effects models were used (α = 0.05) and bonferroni post-hoc tests were applied when appropriate. Multifidus' fascicle orientation angles ranged from 13.3 ± 5.9° to 20.7 ± 7.7°, with greatest angles in the lower-mid thoracic spine. Longissimus' fascicle angles ranged from 7.7 ± 4.3° to 14.8 ± 4.6°, with greatest angles in the upper mid-thoracic and lumbar spine. For both muscles, the shorter the fascicle, the greater the fascicle orientation angle, with small but significant left-right differences in fascicle orientation angle and lengths observed at most vertebral levels (p ≤ 0.05). Fascicle curvature was greater in multifidus than longissimus (p ≤ 0.05), with the highest curvatures found near the fascicle ends in both muscles, producing 'S'-shaped fascicles. We provide a novel in-vivo method to non-invasively quantify paraspinal muscle fascicle orientation, length, and curvature; and have shown significant side-to-side and regional variation in these outcomes in children with typically developing spines.