The spinalis is the most medially placed and smallest column of the erector spinae, running between spinous processes rather than between spinous processes and transverse processes (longissimus) or between transverse processes and ribs (iliocostalis). Its thoracic portion is the most consistent; the cervical portion is inconstant and may fuse with semispinalis cervicis; the capitis portion typically merges with semispinalis capitis. The spinalis provides fine sagittal control of spinal extension.
| Origin | Spinalis thoracis: spinous processes of T10-L2; Spinalis cervicis: spinous processes of C5-T2 (inconstant); Spinalis capitis: often fused with semispinalis capitis |
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| Insertion | Spinalis thoracis: spinous processes of T2-T8; Spinalis cervicis: spinous processes of C2-C4; Spinalis capitis: occipital bone (blending with semispinalis capitis) |
| Nerve Supply | Dorsal rami of thoracic and cervical spinal nerves (medial branches) |
| Blood Supply | Posterior intercostal arteries; deep cervical artery |
| Actions | Bilateral: extension of the thoracic and cervical spine; stabilisation of the sagittal spinal alignment; the most medially placed erector spinae column acting closest to the instantaneous axis of rotation to provide extension moment with the smallest moment arm |
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The spinalis column is the most medial component elevated in posterior spinal approaches and is the first muscle layer encountered medial to the spinous process. Its preservation or careful reattachment in midline-splitting posterior approaches is important for preventing post-laminectomy kyphosis. In scoliosis surgery, the spinalis is examined for rotation along the concave side of the curve where it becomes stretched and thin. Its fatty infiltration on MRI correlates with thoracolumbar disc degeneration and chronic spinal instability patterns.
Denervation and denervation atrophy of the spinalis and adjacent multifidus from aggressive posterior dissection and prolonged retraction during multilevel laminectomy reduces segmental spinal extension control, contributing to progressive kyphosis particularly in the cervical spine; muscle-preserving approaches limiting retractor pressure and time reduce this risk.
