Attachments
The infrahyoid muscles lie in two layers over the thyroid gland, larynx and trachea. Sternohyoid and omohyoid form the superficial layer; sternothyroid and thyrohyoid form the deep layer, which is effectively one muscle interrupted by the oblique line of the thyroid cartilage.
| Muscle | Lower attachment | Upper attachment | Nerve supply |
|---|---|---|---|
| Sternohyoid | Back of the manubrium and medial end of the clavicle | Lower border of the body of the hyoid | Ansa cervicalis (C1–C3) |
| Sternothyroid | Back of the manubrium, below sternohyoid, and first costal cartilage | Oblique line of the thyroid lamina | Ansa cervicalis (C2–C3) |
| Thyrohyoid | Oblique line of the thyroid lamina | Lower border of the greater horn and body of the hyoid | C1 fibres via the hypoglossal nerve |
| Omohyoid | Upper border of the scapula near the suprascapular notch | Lower border of the body of the hyoid, lateral to sternohyoid | Ansa cervicalis: superior belly from the superior root, inferior belly from the loop |
Omohyoid
Omohyoid has two bellies joined by an intermediate tendon. The inferior belly crosses the posterior triangle above the clavicle, and the tendon is held down towards the clavicle and first rib by a fascial sling. The superior belly then climbs almost vertically to the hyoid, crossing the carotid sheath at about the level of the cricoid cartilage (C6).
Actions
The infrahyoid muscles pull the hyoid bone and larynx downwards, returning them to rest after swallowing. By fixing the hyoid, they give the suprahyoid muscles a stable base from which to open the mouth against resistance.
- Sternohyoid and omohyoid depress the hyoid. Omohyoid may also tense the lower cervical fascia and so help keep the internal jugular vein open during deep inspiration, a proposed but unproven role.
- Sternothyroid pulls the thyroid cartilage and larynx downwards, returning the larynx to rest after swallowing.
- Thyrohyoid depresses the hyoid, or elevates the larynx towards the hyoid when the hyoid is fixed by the suprahyoid muscles.
Relations
The strap muscles are enclosed by the muscular part of the pretracheal fascia and lie deep to the investing layer of deep cervical fascia and the sternocleidomastoid. In the midline the two sternohyoids are separated by a thin raphe, the linea alba of the neck, which is opened to reach the thyroid gland or trachea.
Sternothyroid lies directly on the thyroid lobe. Its attachment to the oblique line holds the upper pole down, so an enlarging goitre tends to extend downwards behind the manubrium rather than upwards. The superior belly of omohyoid divides the anterior triangle into the carotid triangle above and the muscular triangle below. In the posterior triangle, the inferior belly separates the occipital triangle from the supraclavicular (omoclavicular) triangle.
Thyrohyoid lies over the thyrohyoid membrane. The internal laryngeal nerve and superior laryngeal artery pierce the membrane deep to it.
Blood supply and innervation
The ansa cervicalis is a nerve loop from the cervical plexus that supplies all the strap muscles except thyrohyoid. Its superior root carries C1 fibres that have travelled with the hypoglossal nerve and leave it as it hooks round the occipital artery. Its inferior root is formed by fibres from C2 and C3. The two roots join in a loop that lies on, or is embedded in, the anterior wall of the carotid sheath, usually at the level of the cricoid cartilage, though the height varies.
Thyrohyoid and geniohyoid are supplied by separate C1 branches that continue with the hypoglossal nerve beyond the superior root. Arterial supply is mainly from the superior thyroid artery, with the lingual artery contributing near the hyoid.
Clinical relevance
The strap muscles are separated in the midline, or divided, to expose the thyroid gland. Branches of the ansa cervicalis enter the muscles in their lower parts, so when division is needed for a large goitre the cut is made in the upper half to preserve the nerve supply.
The intermediate tendon of omohyoid lies over the internal jugular vein. The jugulo-omohyoid lymph node sits at this crossing and is an important node for the tongue. In neck dissection, omohyoid is a landmark between level III and level IV nodes.
The ansa cervicalis is used as a donor nerve for reinnervation of a paralysed vocal fold after recurrent laryngeal nerve injury, anastomosed to the distal recurrent laryngeal nerve.