The sternocleidomastoid (SCM) is the large, oblique strap muscle that forms the prominent cord visible on the lateral neck when the head is turned, running from the sternum and clavicle up to the mastoid process behind the ear. It is the primary muscle that rotates and laterally flexes the head, and when both SCMs contract together, they flex the cervical spine while simultaneously extending the upper cervical joints, producing the forward-head nodding motion. The SCM divides the neck into anterior and posterior triangles, making it a critical anatomical landmark.
| Origin | Anterior surface of the manubrium of the sternum; Medial third of the clavicle |
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| Insertion | Mastoid process of the temporal bone; Lateral half of the superior nuchal line of the occipital bone |
| Nerve Supply | Accessory nerve (CN XI), motor; Ventral rami of C2 and C3, proprioception |
| Blood Supply | Occipital artery; Superior thyroid artery |
| Actions | Unilateral: ipsilateral lateral flexion and contralateral rotation of the head; Bilateral: flexion of the cervical spine, protrusion of the head, and extension of the atlantooccipital joint (nodding 'yes'); Assists forced inspiration by elevating the sternum when the head is fixed |
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During whiplash injury and most neck trauma, the SCM is one of the first muscles to experience protective spasm, and myofascial trigger points here characteristically refer pain to the forehead, eye, and inner ear, mimicking tension headache, migraine, and even tinnitus.
Torticollis, a sustained involuntary contraction of the SCM causing the head to tilt toward and rotate away from the affected side, can be congenital (from perinatal SCM injury), acquired (from dystonia), or reactive (from muscle spasm). The accessory nerve is superficial in the posterior triangle of the neck just behind the SCM, making it vulnerable to damage during lymph node biopsy or neck dissection. SCM trigger points referring to the face and head are among the most overlooked causes of chronic headache in office workers.
The SCM is palpable as the diagonal cord on the lateral neck from the mastoid to the sternoclavicular junction, it stands out clearly when the subject rotates the head to the contralateral side against resistance. The sternal and clavicular heads can be felt separately at their origins.
Muscle fibre tears and protective spasm in the SCM following rapid acceleration-deceleration injury, producing neck pain, restricted rotation, and headache that can persist for weeks to months.
Fibrotic replacement of SCM muscle fibres from perinatal injury, possibly from forceps delivery or intrauterine positioning, presenting in infancy as a firm, non-tender mass in the SCM and a persistent head tilt that responds well to early physiotherapy stretching.
