Triangles of the Neck — Anterior and Posterior Triangles, Subdivisions, Boundaries and Contents

The sternocleidomastoid (SCM) runs obliquely from the manubrium and medial clavicle to the mastoid process and superior nuchal line. On each side it divides the neck into two large regions: the anterior triangle in front of it and the posterior triangle behind it. Smaller muscles — the digastric above the hyoid and the omohyoid below it — then cut these two into six subdivisions that anatomists, surgeons and examiners use as a map.

The map is clinical, not decorative. StatPearls notes that carotid endarterectomy, thyroidectomy and cervical lymph node dissection all 'rely on these anatomic divisions' to give safe exposure and avoid injury to critical nerves and vessels. A lump is described by its triangle, a node biopsy is planned around the nerves in its triangle, and a central line is placed knowing what lies next to the internal jugular vein.

Each triangle has a roof and a floor. The investing layer of the deep cervical fascia forms the roof of both main triangles (with platysma and superficial fascia over it). Deeply, the mylohyoid floors the two suprahyoid triangles, the pretracheal fascia floors the carotid and muscular triangles, and the prevertebral fascia floors the posterior triangle — which is why the posterior triangle's floor carries the scalene muscles and the brachial plexus trunks emerging between them.

Submental triangle. Apex at the chin, base at the body of the hyoid, sides formed by the anterior bellies of the digastric, and a floor of the two mylohyoids meeting at the midline raphe. Its main contents are the submental lymph nodes and small tributaries of the anterior jugular vein. StatPearls notes the triangle 'lacks arteries and only contains few structures', so an abscess here can be drained through percutaneous incisions with a low risk of damage.

Submandibular triangle. Bounded by the inferior border of the mandible and the two bellies of the digastric, with mylohyoid as its floor. The submandibular gland occupies most of it. The facial artery (fourth branch of the external carotid) passes deep to the posterior belly of digastric and behind the gland, then hooks over the lower border of the mandible; the facial vein runs superficial to the gland. The submandibular lymph nodes lie alongside the gland, and the hypoglossal and lingual nerves run deeper in the triangle.

Common exam pathology in the submandibular triangle: sialadenitis, typically from a stone blocking Wharton's duct, causing painful swelling after meals. The commonest benign tumour of the submandibular gland is pleomorphic adenoma; the commonest malignant one is adenoid cystic carcinoma, with mucoepidermoid carcinoma second (although mucoepidermoid is the commonest salivary malignancy overall).

The carotid triangle is the 'busy' triangle and the most-asked. Its contents run vertically inside and around the carotid sheath, which encloses the common carotid artery (and the proximal internal and external carotids), the internal jugular vein and the vagus nerve. Within the sheath the artery lies medial (anteromedial) to the vein and the vagus lies between them.

Clinically, carotid sinus massage (gentle compression at the bifurcation) can terminate supraventricular tachycardia through the baroreceptor reflex. During carotid endarterectomy, handling the carotid sinus or the carotid branch of the glossopharyngeal nerve can cause profound bradycardia and hypotension; injury to the vagus in the sheath causes dysphagia, hoarseness and aspiration. The hypoglossal nerve and ansa roots are also at risk because they lie on the sheath.

The muscular triangle lies below the hyoid between the midline, the superior belly of omohyoid and the lower anterior border of the SCM. Its contents are the infrahyoid (strap) muscles — sternohyoid, sternothyroid, thyrohyoid and omohyoid — and, deep to them inside the pretracheal fascia, the thyroid and parathyroid glands, larynx and trachea.

• Thyroidectomy works in this triangle: the straps are retracted or divided to reach the gland.
• External laryngeal nerve lies near the superior pole with the superior thyroid vessels — the superior thyroid artery is ligated distal to its superior laryngeal branch to protect the artery and nerve. Unilateral injury gives voice fatigue and poor projection; bilateral injury of the superior laryngeal nerves abolishes the laryngeal cough reflex.
• Recurrent laryngeal nerve ascends in the tracheo-oesophageal groove; the commonest injury site is near the ligament of Berry. Unilateral injury → hoarseness; bilateral → vocal cord paralysis and airway compromise.

For the biochemistry of the gland that sits here, see thyroid hormone synthesis.

The four cutaneous branches of the cervical plexus (from the anterior rami of C2–C4) emerge together near the midpoint of the posterior border of the SCM — the so-called nerve point of the neck. That is where a superficial cervical plexus block is given, for example for thyroidectomy or carotid endarterectomy.

After leaving the skull through the jugular foramen, CN XI descends beside the internal jugular vein (usually crossing lateral to it near C3 — though it passes medial to the vein in a minority), supplies the SCM from its deep surface, then emerges from the posterior border of the SCM and crosses the posterior triangle downwards and backwards to pass deep to the trapezius. In the triangle it lies superficially — just under the investing fascia — among the posterior cervical lymph nodes.

That makes it the classic iatrogenic nerve injury of the neck. StatPearls: the most common cause of accessory nerve injury is iatrogenic, and it is 'most commonly seen following lymph node biopsies for diagnostic purposes in the posterior cervical triangle', with reported injury rates of 3 to 8%. Neck dissections, carotid and IJV surgery, and face-lifts are other causes.

• Shoulder droop and asymmetry of the shoulder line — the commonest sign.
• Pain and weakness of the shoulder; reduced ability to hold the arm abducted or elevate it overhead.
• Ipsilateral scapular winging from trapezius weakness, with trapezius wasting over time.
• SCM is usually spared: the nerve has already supplied it before entering the triangle.
• Some trapezius function may persist because of additional supply from C3–C4.

For the complete course and lesions of CN XI and the other cranial nerves, see cranial nerves.

Internal jugular vein cannulation is approached in the anterior triangle; StatPearls notes that it carries fewer complications than subclavian access, but the adjacent carotid artery means accidental arterial puncture or haematoma is the key risk. The IJV also mirrors right-heart pressure — the jugular venous pressure rises in heart failure.

1. Digastric innervation: anterior belly by the nerve to mylohyoid (V3), posterior belly by the facial nerve — reflecting first and second pharyngeal arch origin.
2. Stylohyoid travels with the posterior belly of digastric and also separates the carotid from the submandibular triangle; it is supplied by CN VII.
3. The superior belly of omohyoid separates carotid from muscular; the inferior belly separates occipital from subclavian.
4. The posterior triangle floor is prevertebral fascia — not pretracheal.
5. Hypoglossal crosses superficial to both carotids in the carotid triangle; the vagus stays inside the sheath.
6. CN XI injury after a posterior triangle node biopsy spares the SCM — the nerve has already supplied it.
7. Supraclavicular nodes sit in the subclavian (supraclavicular) triangle.