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Sternocleidomastoid muscle

The sternocleidomastoid (SCM) muscle is a prominent paired muscle located in the anterolateral aspect of the neck. It plays a crucial role in head movement and serves as an important anatomical landmark for various clinical and imaging assessments. Its well-defined structure and superficial location make it easily palpable and visible, especially when the head is turned to the opposite side. The SCM is also clinically relevant for its involvement in torticollis and as a site for central venous access.

Function

  • Responsible for rotation of the head to the opposite side and flexion of the neck.

  • When both SCM muscles contract together, they flex the neck and help elevate the sternum during forced inspiration.

  • Unilateral contraction results in ipsilateral flexion and contralateral rotation of the head.

Origin

  • Manubrium of the sternum (sternal head).

  • Medial third of the clavicle (clavicular head).

Insertion

  • Mastoid process of the temporal bone.

  • Lateral portion of the superior nuchal line of the occipital bone.

Nerve Supply

  • Motor: Spinal accessory nerve (cranial nerve XI).

  • Sensory/proprioceptive: Ventral rami of C2 and C3 spinal nerves.

Artery Supply

  • Branches of the occipital artery.

  • Superior thyroid artery.

  • Suprascapular artery.

Vein Supply

  • Accompanying veins drain primarily into the external jugular vein.

MRI Appearance

  • T1-weighted images: SCM appears as intermediate to low signal intensity compared to surrounding fat.

  • T2-weighted images: SCM shows intermediate signal intensity.

  • STIR (Short Tau Inversion Recovery): SCM muscle shows low signal in normal conditions; high signal may indicate edema or myositis.

CT Appearance

  • The SCM appears as a soft tissue structure with attenuation slightly higher than fat but lower than bone.

  • Well-delineated borders allow for differentiation from adjacent vessels and fascial planes.

MRI images

Sternocleidomastoid muscle mri axial image

MRI image

Sternocleidomastoid muscle anatomy MRI coronal image -img-00000-00000

CT image

Sternocleidomastoid muscle anatomy CT axial image -img-00000-00000

CT image

Sternocleidomastoid muscle CT axial image-img-00000-00000