b) Posterior Triangle

The posterior triangle is bounded by the anterior border of trapezius posteriorly; the posterior border of sternocleidomastoid anteriorly, and inferiorly by the middle third of the clavicle (between the insertions of sternocleidomastoid and trapezius). It is subdivided into two sub-triangles (occipital and supraclavicular) by the omohyoid muscle which passes inferolaterally about 2.5 cm above the clavicle. The roof of the triangle is formed by the deep cervical fascia; the floor by the prevertebral fascia which overlies the splenius capitis, levator scapulae and scalene muscles.

Occipital Triangle

The occipital triangle is the superior triangle, lying above the omohyoid muscle. Its boundaries are therefore the trapezius posteriorly, the sternocleidomastoid anteriorly and the omohyoid muscle inferiorly. The roof is formed by the deep cervical fascia and overlying skin; the platysma crosses the inferior part of the triangle. The floor is formed by multiple prevertebral muscles; the splenius capitis, levator scapulae, middle and posterior scalene (from superior to inferior).

Structures within the occipital triangle

The occipital triangle is notable for containing the spinal accessory nerve, which emerges from within sternocleidomastoid about midway between its insertions and passes inferoposteriorly to reach the deep border of trapezius. The upper and middle trunk of the brachial plexus cross the triangle at its inferior extent above the middle scalene muscle. Level V lymph nodes also lie within the triangle, extending from the mastoid process above along the posterior border of sternocleidomastoid.

Supraclavicular Triangle (Supraclavicular Fossa)

The supraclavicular triangle is bounded above by the omohyoid, inferiorly by the clavicle and medially by the sternocleidomastoid. It contains the supraclavicular fossa, which is a hollow of various depth depending on the build of the person. The roof is similar to the occipital triangle, formed by the skin, superficicial and deep fascia, and platysma. The floor contains the first rib, which curves posteriorly, and the middle scalene muscle which passes from the rib superiorly.

Structures within the subclavian triangle

The subclavian artery crosses the inferior part of the triangle, posterior to the attachment of the anterior scalene and superior to the first rib, and enters the axilla. The trunks of the brachial plexus lie superior and posterior to the subclavian artery. The subclavian vein is anterior, divided from the subclavian artery by the attachment of the anterior scalene muscle. Smaller nerves (supraclavicular, transverse cervical and the nerve to subclavius) also lie within the triangle.
Supraclavicular lymph nodes are considered to be inferior deep cervical nodes. They lie between the subclavian vein and the internal jugular vein.

Radiotherapy Definition of the Supraclavicular Fossa

The supraclavicular fossa contains lymph nodes that drain the breast and the neck, and is important to cover in radiotherapy fields designed to treat nodal metastases. From this point of view:

  • The medial part of the fossa is formed by the trachea and the thyroid. It includes the carotid sheath.
  • The floor of the fossa is formed by the middle scalene muscle
  • The inferior extent of the fossa is to the subclavian vessels
  • The lateral extent of the fossa is the lateral part of the insertion of the middle scalene on to the first rib.
  • The superior extent of the fossa is the omohyoid muscle.