THE LEVELS OF THE NECK:
Netter Plate Number: 23

Level Ia: Submental triangle
Boundaries:
Anterior belly of the digastric muscle and the hyoid bone.

Level Ib: Submandibular triangle
Boundaries:
Body of the mandible and the anterior and posterior bellies of the digastric muscle.

Level II: Upper jugular nodes
Boundaries:
Anterior: lateral border of the sternohyoid muscle.
Posterior: posterior border of the sternocleidomastoid muscle
Superior: skull base
Inferior: Level of the hyoid bone (clinical), carotid bifurcation (surgical)
*Level II is divided by the accessory nerve into levels IIa (anteriorly) and IIB (posteriorly).

Level III: Middle jugular nodes
Boundaries:
Anterior: lateral border of the sternohyoid muscle.
Posterior: posterior border of the sternocleidomastoid muscle
Superior: hyoid bone
Inferior: cricoid notch (clinical), omohyoid muscle (surgical)

Level IV: Lower jugular nodes
Boundaries:
Anterior: lateral border of the sternohyoid muscle.
Posterior: posterior border of the sternocleidomastoid muscle
Superior: cricoid notch (clinical), omohyoid muscle (surgical)
Inferior: clavicle

Level V: Posterior triangle
Boundaries:
Anterior: posterior border of the sternocleidomastoid muscle
Posterior: anterior border of the trapezius muscle
Inferior: clavicle

Level VI: Nodes of the anterior compartment - pretracheal, paratracheal, precricoid lymph nodes
Boundaries:
Lateral: lateral border of the sternohyoid muscle
Superior: hyoid bone
Inferior: suprasternal notch

Modifications to the radical neck dissection include the following:
Type I: The spinal accessory nerve is preserved.
Type II: The spinal accessory nerve and the internal jugular vein are preserved.
Type III: The spinal accessory nerve, the internal jugular vein, and the sternocleidomastoid muscle are preserved.
Extended radical neck dissection: Resection of lymph node groups and/or additional structures not included in the classic neck dissection is performed.

**The above diagram is an approximation based on anatomy.