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Spinalis thoracis muscle

The spinalis thoracis is the largest and most consistent component of the spinalis subgroup of the erector spinae muscles. It is a paired, narrow, and elongated muscle that runs vertically along the thoracic spine, adjacent to the spinous processes. It acts as a powerful extensor and stabilizer of the vertebral column. Although often blended with the semispinalis and longissimus, it can usually be distinguished in the mid-thoracic region.

It plays a vital role in maintaining posture and resisting forward flexion, especially in the thoracic spine, and is clinically relevant in back pain, degenerative spine disease, and paraspinal pathology.

Synonyms

  • Spinalis of thoracic region

  • Thoracic spinalis muscle

  • Part of erector spinae (spinalis group)

Origin, Course, and Insertion

  • Origin:

    • Arises from the spinous processes of T11–L2 vertebrae

  • Course:

    • Fibers run vertically upward, closely applied to the spinous processes of thoracic vertebrae

    • Lies medial to the longissimus thoracis, forming part of the erector spinae mass

  • Insertion:

    • Inserts into the spinous processes of T1–T8 vertebrae (variable attachment, often T1–T4 or T5)

Relations

  • Anteriorly: Thoracic vertebral laminae and interspinous ligaments

  • Posteriorly: Thoracolumbar fascia and overlying trapezius/latissimus dorsi (upper thoracic)

  • Laterally: Longissimus thoracis muscle

  • Medially: Spinous processes of thoracic vertebrae

Function

  • Extends the thoracic vertebral column bilaterally

  • Assists in lateral flexion of the spine when acting unilaterally

  • Provides postural support and spinal stability

  • Works with longissimus and iliocostalis as part of the erector spinae group

Clinical Significance

  • May hypertrophy or atrophy in cases of chronic back pain or scoliosis

  • Involved in paraspinal muscle strain and myofascial pain syndromes

  • MRI assessment important in degenerative spine disease, trauma, and infection

  • Serves as a surgical landmark in posterior spinal approaches

MRI Appearance

T1-weighted images:

  • Muscle shows low-to-intermediate signal intensity

  • Fatty infiltration or chronic degeneration may show bright signal

T2-weighted images:

  • Muscle shows low-to-intermediate signal intensity

  • Edema, strain, or inflammation shows bright signal

STIR (Short Tau Inversion Recovery):

  • Normal muscle remains dark to intermediate

  • Pathological processes (strain, myositis, tumor infiltration) appear bright

T1 Fat-Sat Post-Contrast:

  • Normal muscle enhances mildly and homogeneously

  • Pathological muscle shows focal, diffuse, or rim enhancement depending on the condition

3D T2 SPACE / CISS:

  • Muscle appears with intermediate signal intensity, surrounded by bright CSF in spinal canal

  • Useful to differentiate paraspinal muscles from dural or neural lesions

CT Appearance

Non-Contrast CT:

  • Muscle appears as homogeneous soft tissue density adjacent to thoracic spinous processes

  • Chronic fatty degeneration shows low attenuation

  • Acute hematoma or calcification may alter density locally

Post-Contrast CT:

  • Muscle enhances mildly and evenly in normal state

  • Inflammatory or neoplastic involvement shows heterogeneous or nodular enhancement

  • Abscess presents as rim-enhancing lesion with central low density

MRI image

Spinalis thoracis muscle mri axial anatomy image-img-00000-00000

CT image

Spinalis thoracis muscle ct axial anatomy image-img-00000-00000