The posterior cervical triangle houses an important nodal basin in the spread of several cancers in the head and neck, particularly cutaneous malignancies of the scalp. A safe and effective Level V neck dissection necessitates thorough understanding of the neurovascular structures housed within the region. Conventional 2D anatomical representations offer insights into the named structures but fall short in illustrating the spatial relationships crucial in surgery. Here, we aim to develop an anatomically precise 3D virtual model of the posterior cervical triangle and its constituent structures. Musculature and neurovasculature were segmented from the computerized tomography (CT) angiogram of a healthy 29-year-old female. Literature review of cadaveric studies was performed to identify the most common variants, relevant surgical relationships, and usual dimensions of structures contained in the model. Both radiographic and anatomic data were used to inform the creation of a 3D model. This accurate 3D anatomical model will serve to supplement the education of practicing and aspiring head and neck surgeons.
Introduction
The posterior neck contains muscles responsible for rotation and extension of the neck and spine, as well as movement and support of the shoulder girdle. The posterior cervical triangle is a division of the neck bounded by the sternocleidomastoid muscle (SCM), the trapezius muscle, and the middle one-third of the clavicle. This triangle houses lymphatic tissue that serves as an important nodal basin for potential lymphatic spread of cancers in the head and neck. It contains several important neurovascular structures, including the spinal accessory nerve (CNXI), the brachial plexus, the third segment of the vertebral artery, the phrenic nerve, and sensory nerves innervating the skin. ,
Cervical lymphadenectomy, or neck dissection, is the most common indication for surgery in the posterior cervical triangle and is used for oncologic control of cancers with metastatic disease in the cervical lymph nodes. The first systematic approach to neck dissection was described in 1906 by Dr. Crile and was later popularized and refined by Dr. Martin in the 1950s. This technique, which has become known as the radical neck dissection, entails surgical excision of all lymphatic tissue from the neck (Levels I-V), as well as sacrifice of CNXI, the internal jugular vein, and the sternocleidomastoid muscle. Patients undergoing radical neck dissection often experienced significant shoulder dysfunction and winging of the scapula due to loss of CNXI innervation and transection of the sternocleidomastoid muscle. In patients undergoing bilateral radical neck dissections, significant venous congestion and chronic lymphedema occurred with sacrifice of the bilateral internal jugular veins.
Lymphographic research conducted in the 1950s and 1960s revealed that cancers spread to predictable nodal basins depending on their head and neck site of origin. For example, a study conducted by Dr. Fisch in 1964 revealed that contrast injected into the retroauricular skin first spread to the lymphatics of Level Va and Level IIb. With improved understanding of nodal drainage patterns and literature to support more limited dissection, the radical neck dissection has now largely become a procedure of last resort. Sacrifice of CNXI, the internal jugular vein, and the sternocleidomastoid muscle is performed only in cases in which these structures are grossly involved by tumor and no nonoperative treatment options are available.
First introduced by Dr. Suarez in the 1960s, the selective neck dissection seeks to remove lymphatic tissue from the regions which are most likely to harbor metastatic disease from the primary tumor’s site of origin, while preserving lymphatic tissue unlikely to contain metastatic deposits. Cutaneous malignancies of the auricle, posterior scalp, and neck can metastasize to lymph nodes in the posterior triangle (Level V), and Level V neck dissection is most often performed for this indication. , Although less likely, cancers of the thyroid, nasopharynx, and oropharynx, can also metastasize to Level V. Excision of lymph nodes from Level V is most frequently part of posterolateral neck dissection, a selective neck dissection first described by Dr. Rochlin in 1962. This procedure entails removal of all lymph nodes in Level V, as well as those in Levels II, III, and IV.
Neck dissection is a requisite skill for the aspiring head and neck surgeon. Thus, high-quality educational materials describing this intricate anatomic region are essential. Traditional 2D anatomic representations offer learners general knowledge of structures housed within the region. However, they fall short in their ability to accurately represent the nuanced spatial relationships existing among the structures encountered in surgery. Throughout the past 2 decades, 3D models have been applied successfully in anatomy education. The paradigm shift from traditional 2D anatomical representations to 3D models has improved medical training. , An anatomically accurate representation of the posterior neck in 3D can therefore serve as a powerful educational tool. This study aims to develop a 3D virtual model of the anatomical structures within the posterior triangle of the neck based upon peer reviewed anatomic studies. Such a model will equip aspiring and practicing surgeons with a strong understanding of anatomical relationships in this region.
Materials and methods
The methodology of this manuscript is described in a previous publication. Briefly, muscles adjacent and deep to the posterior triangle were segmented from the CT scan of a healthy 29-year-old female. Contents of the posterior triangle, including nearby nerves and vasculature were also segmented for this model. Structures were uploaded to a 3D illustration program and artifacts were removed. Anatomical depictions were then refined through extensive literature review with special attention paid to reliable surgical landmarks used in structure identification. While noteworthy variations described in the literature are discussed in the Results section, the most common variant was utilized for construction of the model. Factors considered for selecting the most common variant included anatomical studies with large specimen count, high-quality anatomical descriptions, and detailed figures. The 3D model was further refined by an experienced medical illustrator and uploaded to an online viewer (Medreality, Thyng LLC, IL).
Results
The posterior triangle of the neck
The posterior cervical triangle is bounded by the posterior border of the SCM muscle anteriorly, the anterior border of the trapezius muscle posterolaterally, and the superior aspect of the middle one-third of the clavicle inferiorly ( Figure 1 ). The triangle’s apex is formed superiorly by the intersection of the SCM and trapezius muscle at the superior nuchal line of the occipital bone. The “floor,” or deep extent of the posterior triangle, is formed by the prevertebral fascia overlying the levator scapulae, splenius capitis, anterior scalene, and middle scalene muscles.
As it pertains to neck dissection, the posterior cervical triangle is known as Level V, which represents 1 of 6 total anatomic surgical levels described by head and neck oncologic surgeons. Note, different naming conventions are utilized by anatomists, radiologists, and radiation oncologists. Level V is further divided into 2 sub-levels by the horizontal plane of the inferior border of the cricoid cartilage. The region superior to the cricoid cartilage constitutes Level Va, or the occipital triangle. The region inferior to the cricoid cartilage defines Level Vb, or the subclavian triangle, Table 1 . The occipital triangle is larger in size and contains the spinal accessory and occipital lymph nodes. The subclavian triangle, also called the supraclavicular triangle, houses supraclavicular lymph nodes and a segment of the subclavian artery and vein.
| Boundary | Radiological criteria | Surgical criteria | Radiation oncology criteria |
|---|---|---|---|
| Level V | |||
| superior | Superior Nuchal Line | Superior Nuchal Line | Skull base |
| Inferior | Middle 1/3rd of clavicle | Middle 1/3rd of clavicle | Middle 1/3rd of clavicle |
| Lateral/posterior | Anterior border of the trapezius muscle | Anterior border of the trapezius muscle | Anterior border of the trapezius muscle |
| medial | Prevertebral Fascia | Prevertebral Fascia | Prevertebral Fascia |
| Anterior | Posterior border of the sternocleidomastoid muscle | Posterior border of the sternocleidomastoid muscle | Posterior border of the sternocleidomastoid muscle |
Muscles of the posterior triangle
Sternocleidomastoid muscle (SCM)
The SCM is a paired muscle that originates inferiorly as 2 distinct muscular heads at the manubrium and clavicle. The 2 muscle bellies join in their course to insert on the mastoid process and lateral aspect of the superior nuchal line. The muscle separates the anterior cervical triangle from the posterior cervical triangle. When contracted unilaterally, the SCM causes ipsilateral lateral neck flexion and contralateral neck rotation. Bilateral contraction results in cervical flexion, especially in the supine position, and elevates the thorax in deep inspiration. The SCM receives its vascular supply from the SCM branch of the occipital artery at its superior one-third, from the suprascapular artery at its inferior one-third, and the superior thyroid artery at its middle one-third. The muscle is innervated by CNXI. ,
Trapezius
The trapezius muscle is a paired muscle located superficially in the posterior neck and back. As suggested by the name, when viewed jointly, both trapezius muscles form a shape resembling a trapezoid. In the midline, the large muscle spans from the occiput to the lower thoracic spine. Laterally, it extends to the acromion process. The muscle is comprised of 3 divisions: the descending (upper), transverse (middle), and ascending (lower) parts. Each subunit of the muscle contains fibers oriented at unique angles and with different points of insertion. Each part, therefore, contributes a different biomechanical influence in the muscle’s composite action. Broadly speaking, the function of the muscle is to elevate, retract, rotate, and stabilize the scapula. , , The descending segment, which serves as the posterior border of the posterior cervical triangle, originates at the superior nuchal line and nuchal ligament. The muscle fibers exit their site of origin with a vertical orientation before assuming a more transverse orientation near their insertion on the lateral one-third of the clavicle.
The transverse segment of the muscle contributes muscle fibers with the greatest diameter and load-bearing capacity. The muscle fibers of this segment originate at the spinous processes of C7-T1 and insert onto the lateral scapular spine. , Lastly, the ascending part of the muscle originates at the T2-T5 spinous processes. The fibers course superolaterally and attach to the medial end of the scapular spine.
The muscle is innervated by CNXI and receives its blood supply from the transverse cervical, dorsal scapular, and posterior intercostal arteries, which run on the ventral surface of the trapezius. , The innervating CNXI traverses the posterior triangle of the neck and courses inferolaterally before innervating the trapezius. Some studies describe CNXI continuing anteriorly along the trapezius muscle before piercing it. Other studies have described variations of the CNXI that pass underneath the anterior border of the trapezius, close to the site of its clavicular attachment. , Variability exists in descriptions of the number of branches of the CNXI that go on to innervate the trapezius, ranging from only innervation through the main trunk of the CNXI to 5 branches.
Omohyoid muscle
One of the 4 paired strap muscles, the omohyoid is comprised of an inferior and superior belly adjoined by an intermediate tendon. The inferior belly originates at the superior border of the scapula, medial to the scapular notch. , , From its origin, the inferior belly of the omohyoid muscle travels in an anterosuperior trajectory. At approximately the level of the cricoid cartilage, the inferior belly passes deep to the sternocleidomastoid muscle and terminates at the intermediate tendon. The intermediate tendon is attached to the clavicle by the omoclavicular fascia. From the intermediate tendon, the superior belly emerges and runs anterosuperiorly to insert onto the inferior border of the hyoid bone, immediately lateral to the insertion of the sternohyoid muscle. , , The omohyoid muscle passes superficially to the anterior and middle scalene muscle. When contracted, the omohyoid muscle depresses the hyoid bone and larynx. Both bellies receive innervation from branches of the ansa cervicalis. , The principle vascular supply to the muscle is through branches of the superior thyroid artery ,
The aforementioned description of the omohyoid muscle is accurate in approximately 85% of cases. However, several variations in the origin, course, and insertion of the omohyoid muscle have been documented in the literature. While the inferior belly of the omohyoid muscle is most consistently noted to originate near the scapular notch, some studies report the clavicle to be another, less frequent, site of origin. , Several studies also report duplication of the omohyoid muscle unilaterally or bilaterally. , In some instances, the superior belly of the omohyoid muscle has also been reported to merge with the sternohyoid muscle before inserting on the hyoid. , , More uncommonly, the superior belly may receive accessory fibers from the sternum or have anomalous insertion sites. The superior belly can be either absent or multiheaded. ,
Scalene muscles
The 3 paired scalene muscles (anterior, middle, and posterior) each originate from the transverse processes of the cervical vertebrae and insert on the first 2 ribs. When contracted, they exert an upward pull on the ribs, serving as an accessory muscle in respiration. They also assist with anterior neck flexion when contracted bilaterally and lateral flexion when contracted unilaterally. These muscles are positioned deep to the SCM and are covered by prevertebral fascia.
Anterior scalene muscle
The anterior scalene muscle originates from the anterior tubercles of the transverse processes of the C3-C6 vertebrae. , , The muscle fibers travel anterolaterally to their insertion on the superior aspect of the first rib at the scalene tubercle. , The anterior scalene muscle inserts on the first rib between the groove of the subclavian vein anteriorly and the subclavian artery posteriorly. The anterior scalene muscle is innervated by the ventral rami of the cervical spinal nerves and receives its vascular supply primarily from the inferior thyroid artery.
The subclavian vein courses superficially to the anterior scalene muscle. The phrenic nerve also courses superficially within the fascia overlying the muscle, passing between the muscle posteriorly and the subclavian vein anteriorly. The roots of the brachial plexus emerge between the anterior and middle scalene muscles, deep to the subclavian artery. Absence of the anterior scalene muscle, as well as variable relationships to the brachial plexus and subclavian artery, have been described.
Middle scalene muscle
The middle scalene muscle originates from the transverse process of the C2-C7 vertebrae. Its muscle fibers insert on the superior aspect of the first rib posterior to the impression of the subclavian artery. Importantly, the brachial plexus and the subclavian artery emerge from between the anterior and middle scalene muscles. The principal blood supply to the middle scalene muscle is from the ascending cervical artery, which arises from the thyrocervical trunk.
The first 2 roots comprising the long thoracic nerve, which classically originates from the ventral roots of C5-C7, either emerge between the middle and posterior scalene (most commonly), pierce the middle scalene obliquely, or emerge from the anterior aspect of the muscle. In either variant, the nerve goes on to travel dorsal to the brachial plexus on its way to innervating the serratus anterior muscle.
The dorsal scapular nerve originates from the ventral root of C5, occasionally sharing a common trunk with the long thoracic nerve. It enters the middle scalene muscle before taking a more dorsal course deep to the posterior scalene to innervate the rhomboid major and minor muscles, as well as the levator scapulae muscle.
Posterior scalene muscle
The posterior scalene muscle originates from the posterior tubercles of the transverse processes of C4-C6 and inserts onto the second rib. , The principal blood supply to the posterior scalene muscle is the ascending cervical artery.
Splenius muscles
Splenius capitis
Coursing obliquely in the posterior neck, the splenius capitis runs from the mastoid process to the spinous processes of C7-T4. , , , Contraction of the muscle facilitates head extension and rotation. , , The vascular supply to this muscle appears to be variable. When present, the suboccipital artery of Salmon supplies the splenius capitis. The suboccipital artery of Salmon is a branch of the vertebral artery and is present in 48-67% of individuals. , Individuals without this artery receive supply from unnamed branches of the occipital artery. Posterior rami from C2 and C3 spinal nerves innervate the splenius capitis. The prevertebral fascia overlying the splenius capitis contributes to the floor of the posterior triangle.
Splenius cervicis
The splenius cervicis muscle exists between the deeper erector spinae and semispinalis muscles and the more superficial serratus posterior superior, rhomboid, and trapezius muscles. It sits in close relationship to splenius capitis. The splenius cervicis is present bilaterally, arising from the transverse processes of the atlas and axis and the posterior tubercle of the third cervical vertebra. It courses inferomedially and inserts into the spinous processes of T3- T6. It functions unilaterally to rotate the upper cervical vertebra and bilaterally to extend the corresponding vertebrae. , It receives primary innervation from the lateral branch of the dorsal rami.
Erector spinae muscles
Longissimus cervicis
The longissimus cervicis is a long, slender muscle that originates at the posterior tubercles of the C2-C6 transverse processes and courses inferiorly between the longissimus capitis and longissimus thoracis, inserting into the transverse process of T1-4/5 vertebrae. It functions as part of the deep muscles of the back to extend the spine. It is innervated by the lateral branches of the C2 cervical dorsal ramus. ,
Spinalis cervicis / colli
A definitive anatomical understanding of the spinalis cervicis (colli) is not well-documented. It is inconsistently represented as a composite paramedian muscle originating from C2-4 spinous processes and inserting into the lower part of the ligamentum nuchae and the spine of the C7 vertebra, and less frequently at T1-2. It functions to extend and laterally bend the cervical vertebrae. , It is innervated by the lateral branches of the cervical dorsal rami, , and receives vascular supply from the occipital, deep cervical, and vertebral arteries. ,
Levator scapulae
The levator scapulae is a muscle originating at the transverse process of C1-C4. , It inserts on the superior medial border of the scapula. It is typically split into 2 slips. The ventral slip inserts on the ventral surface of the subscapularis or the serratus posterior superior. The dorsal slip runs along the splenius capitis and inserts on the medial angle of the scapula. The muscle is variably innervated by either the dorsal scapular nerve or the anterior rami of spinal nerves C3, C4, and C5.
Transversospinalis muscles
Semispinalis capitis
The semispinalis capitis is a large extensor muscle of the head and neck. It is described as a broad, wide, and thin muscle. It extends from the articular processes of C4-C6 vertebrae to the transverse process of C7-T6. It inserts on the occipital bone between the nuchal lines. The muscle is innervated by the medial branch of C2 and receives additional innervation from branches of the occipital nerve.
Semispinalis cervicis
The semispinalis cervicis muscle is comprised of numerous muscle bundles that extend from the spinous process of 1 vertebra to then insert on the transverse processes of another vertebra. The slender muscle bundles originate from the posterior portion of the T1-6 transverse vertebrae and span approximately 5 or 6 transverse processes before inserting onto the spinous processes of the C2-5 vertebrae, bilaterally. Akin to all the transversospinal muscles, the semispinalis cervicis functions to extend the neck. , The muscle is innervated by the medial branches of the dorsal rami of their corresponding spinal nerves and supplied by the occipital, deep cervical, and vertebral arteries. ,
Neurovasculature of the posterior triangle
Spinal accessory nerve (CN XI)
CNXI provides motor innervation to the SCM and trapezius muscles. CNXI is classically described as composed of a spinal and cranial root. The spinal segment of CN XI originates from the rootlets exiting from the lateral gray matter at levels C1-C5. These rootlets combine and, together, course superiorly to enter the cranium via the foramen magnum. , The spinal segment courses along the floor of the posterior cranial fossa where it joins with the cranial segment, which is comprised of rootlets arising from the dorsolateral medulla. The segments travel together as CN XI and exit through the jugular foramen. , As the nerve exits the jugular foramen, it courses into the neck between the internal carotid artery and internal jugular vein, where it splits into an internal and external branch. The internal branch receives the cranial fibers and anastomoses with the vagus nerve at the level of the transverse process of the C1 vertebra, providing descending fibers that aid in motor function of the larynx and pharynx. , The external branch receives the spinal portion of CN XI and goes on to innervate the trapezius and SCM muscles. ,
The course and innervation patterns of the external branch of CNXI have been studied extensively and are modeled in Figure 2 . The external branch of CNXI courses through the lateral neck, passing predominantly superficial (67%-96%) to the internal jugular vein. Other variations include passing through the internal jugular vein, posterior to the internal jugular vein, and splitting into anterior/posterior limbs across internal jugular vein. , CNXI continues through the neck, passing the transverse process of C1 anteriorly (77.5%), with lateral (20.5%) and medial (2.5%) variations also described. Next, the nerve travels towards the superior portion of the SCM near the occipital artery after descending medial to the styloid process and coursing past the stylohyoid and digastric muscles. Once at the SCM, several variations of CNXI innervation and branching exist. Kierner et al. described that CN XI innervation of the SCM can occur through penetration (54%) or without penetration (46%) of the muscle. Similar findings were noted by Lee et al., where penetration of the SCM was observed in 54.1% of neck dissections. Shiozeki et al. suggest that when CNXI courses posterior to the IJV, these variants tend to penetrate the SCM muscle with its branches, while anterior coursing patterns tend not to give off penetrating branches.
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