Strap muscles
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strap muscles of the neck infrahyoid muscles anatomy diagram
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Strap Muscles (Infrahyoid Muscles)
The strap muscles are the common clinical/anatomical name for the infrahyoid muscles - a group of four thin, flat, ribbon-like ("strap-shaped") paired muscles lying inferior to the hyoid bone, anterior to the thyroid gland, trachea, and larynx in the anterior neck (Scott-Brown's Otorhinolaryngology Head & Neck Surgery).
The four muscles
| Muscle | Attachments | Action | Nerve supply |
|---|---|---|---|
| Sternohyoid | Manubrium of sternum -> hyoid bone | Depresses hyoid | Ansa cervicalis (C1-C3) |
| Sternothyroid | Manubrium -> thyroid cartilage | Depresses larynx/thyroid cartilage | Ansa cervicalis (C1-C3) |
| Omohyoid | Scapula (via intermediate tendon) -> hyoid bone | Depresses hyoid | Ansa cervicalis (C1-C3) |
| Thyrohyoid | Thyroid cartilage -> hyoid bone | Elevates larynx / depresses hyoid depending on fixed point | Branch of C1 (travels with hypoglossal nerve, not true ansa cervicalis) |
Their common action is to depress the hyoid bone and larynx, which helps modify vocal resonance and assists swallowing/speech mechanics - Scott-Brown's Otorhinolaryngology Head & Neck Surgery.
Almost all of them receive motor supply from the ansa cervicalis (C1-C3), which enters the muscles at their inferior aspect - an important surgical landmark - Fischer's Mastery of Surgery, 8th ed.
Clinical relevance
- Thyroid/parathyroid surgery: The strap muscles (sternohyoid and sternothyroid) are retracted or, in large goiters, divided in their upper third to avoid injuring the ansa cervicalis, which enters lower down. They are reapproximated with sutures at closure - Scott-Brown's; Fischer's Mastery of Surgery.
- Thyroid cancer staging: Invasion of the strap muscles by thyroid carcinoma defines T3b extrathyroidal extension (AJCC), and gross invasion may mandate en bloc resection - Schwartz's Principles of Surgery, 11th ed.
- Fascial relations: They are separated from the true thyroid capsule by the perithyroidal (pretracheal) fascia - K.J. Lee's Essential Otolaryngology.
- Neck dissection landmarks: The medial border of Level IV (lower jugular) lymph nodes is defined relative to the strap muscles - Schwartz's Principles of Surgery.
- Tracheostomy: The strap muscles are divided or retracted at the linea alba (midline raphe between sternohyoid muscles) to access the trachea - Pfenninger and Fowler's Procedures for Primary Care.
- Muscle physiology: Strap muscles have a high proportion (~66%) of type I (slow-twitch) fibers, suited to sustained postural tone - Scott-Brown's.
If you want, I can go deeper into surgical technique for dividing the strap muscles during thyroidectomy, or into the ansa cervicalis anatomy/nerve supply in more detail.
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