Introduction

Cervical fascia plays a critical role in stabilizing, supporting and enclosing major structures in the neck and transmitting mechanical forces emitted by neighboring muscles. Fascia also acts as a barrier to disease spread. Understanding these fascial planes is crucial for generating an accurate differential diagnosis of pathology seen on cross sectional exams. As a landmark in surgery, fascial planes aid surgeons in the preservation of important anatomical structures. It is generally used to identify neighboring or enclosed structures and is considered safe for dissection due to its lack of major vessels. However, despite its anatomic importance, there is significant variability in the description of cervical fascia in current literature. Controversy exists on what constitutes layers of the cervical fascia, and the correct nomenclature.

Lack of coherency in the naming, descriptions, and classifications of cervical fascia can present a problem for learners of anatomy, radiology, and surgery. Furthermore, existing textbooks and literature typically convey cervical fascia in 2-dimensional (2D) images, further simplifying the 3-dimensional (3D) nature and behavior of fascial planes. Visualization and depth perception of fascial layers is key to preserving neurovascular and soft tissue structures of the head and neck and staying safe during surgical intervention. While live dissections can serve to supplement 2D atlases, cervical fascias can be difficult to view and isolate on cadaveric dissection due to their fibrous and often thin nature. Anatomically, accurate 3D models can therefore serve to model fascial planes commonly identified as diagnostic and surgical landmarks. In this manuscript, we utilize existing anatomical literature, surgical and neuroradiological expertise, and an experienced medical illustrator to construct an anatomically accurate model for the fascial planes of the head and neck.

Methods

A detailed review of the methodology used to create this model has been previously published. The CT of a healthy 29-year-old female was segmented, imaging artifacts were removed, the cervical structures enclosed in cervical fascia, including musculature, vasculature and relevant organs were segmented and overlayed with the appropriate fascia layers. The results were then refined using the 3D illustration program. Anatomical accuracy was further refined using descriptors found in literature. Primary cervical fascial layers, their subdivisions, and secondary fascia structures including the carotid sheath were added to the model. Divisions and subdivisions of the fascial layers were described in detail in the text below, with relevant references to surgical landmarks and terminology. The model was optimized for online viewing and manipulation by the medical illustrator. The model was then uploaded and made available on an online viewer (Medreality, Thyng LLC, IL).

Results

Superficial cervical fascia

The superficial cervical fascia is described as a thin layer of connective tissue between the dermis and the deep cervical fascia. In many texts, it is an extension of the platysma and envelops the cutaneous sensory nerves, blood vessels, and lymphatics. In the plastic surgery literature it is often referred to as the superficial musculoaponeurotic system (SMAS) when discussing face lift procedures. It takes origin at the mandible and inserts into the superficial fascia of the skin over the pectoral muscles. It is often considered to be a part of the platysmal layer in the neck. It extends to the perimeter of the neck housing the subcutaneous tissue and vasculature.

Deep cervical fascia

The deep cervical fascia is generally divided into three fascial layers: the superficial layer of the deep cervical fascia, commonly called the investing layer, the middle layer of the deep cervical fascia, commonly called the pretracheal layer, and the deep layer of the deep cervical fascia, commonly called the prevertebral layer. It acts to compartmentalize structures of the head and neck, encircling the entirety of the neck and extending between the face and pectoral regions. Subdivisions of the deep cervical fascia are depicted in Figures 1 and 2 . Anatomical spaces created by the various layers of cervical fascia are depicted in Figure 3 .

Anatomic illustration of the deep cervical fascial layers within the infrahyoid neck. (A) Illustration depicting structures enclosed within the prevertebral fascia (teal), (B) Illustration depicting structures enclosed within the carotid sheath (blue) and neighboring alar fascia (pink), (C) Illustration depicting structures enclosed within the muscular (green) and visceral (orange) components of the pretracheal fascia and (D) Illustration depicting structures enclosed within the investing fascia (purple).

Anatomic illustration of the deep cervical fascial layers in the suprahyoid region. (A) Illustration depicting the prevertebral fascia extending to the skull base (teal). The alar fascia (pink) is located anterior to the prevertebral fascia. The two layers demarcate the danger space. (B) Illustration depicting the alar fascia (pink) extending laterally to join the carotid sheath (blue). Within the carotid sheath, the internal carotid artery (ICA, red), internal jugular vein (IJV, light blue) and CN X (yellow) can be appreciated. (C) The buccopharyngeal fascia, a division of the pretracheal fascia overlaying the buccinator muscle (orange) and the tensor vascular styloid fascia (dark green) extending from the tensor veli palatini (TVP) to the styloid process. (D) Illustration depicting muscular structures enclosed within the investing fascia (purple).

Anatomical and potential spaces created by the cervical fascial layers.

Superficial layer of the deep cervical fascia (Investing Layer)

The superficial layer, or investing layer, of the deep cervical fascia is described as enclosing the entirety of the neck ^, It forms superior attachments at the skull base, mandible, and clavicle. It extends inferiorly to the pectoral and axillary region, joining the thoracic fascia covering the pectoralis major muscle. ^, Anteriorly, its boundaries include the mandible, hyoid bone and sternum. Laterally, it is bounded by the clavicle and scapula, and posteriorly by the occipital bone and trapezius. In the anterior cervical region, it is well-defined and easily dissected, growing increasingly thin as it approaches the lateral and posterior cervical regions. Several sensory nerves are described to pierce this fascial layer at various levels, with the supraclavicular nerves entering subcutaneous layer caudal to the nerve point, where the other three superficial branches of the cervical plexus emerge from the posterior border of the sternocleidomastoid. The layer is said to grow thinner and less distinct as it descends. ^, It ultimately inserts on the ventral plane of the manubrium of the sternum and membrana sterni anteriorly and the cervical spinal processes via the nuchal ligament posteriorly. ^{, ,} According to Levitt et al. and Grodinsky et al., it splits, crossing the anterior triangle of the neck and forming a sheath around the sternocleidomastoid and around the trapezius before attaching to the vertebral spines. ^, However, some authors disagree with this description, including the later published Gray’s textbook description. It has been stated by subsequent investigators that the superficial layer of the deep cervical fascia does not have a direct attachment to the cervical vertebrae. Gray’s describes the layer between the SCM and trapezius as areolar tissue rather than connective fascial tissue. Below level C2, the loose connective tissue of the investing layer becomes less defined, instead a nonfibrous membrane exists between the two muscles. Most studies agree that the fascia becomes less defined after C1. ^{, , ,}

Middle layer of the deep cervical fascia (pretracheal layer)

Also called the pretracheal layer, the middle layer of the deep cervical fascia encloses the infrahyoid muscles and visceral space. For this reason, many authors will separate the pretracheal layer into two compartments: the muscular compartment and the visceral compartment. Occasionally, texts will describe the entire fascia as the visceral fascia but most commonly, the visceral fascia is termed to describe the fascia around the visceral compartment whereas the infrahyoid fascia is termed to describe the muscular compartment.

Though variations exist in its definition, the pretracheal layer is generally described as the fascia layers around the trachea or in proximity to the thyroid gland. The fascia arises at the base of skull on the posterosuperior aspect, and the thyroid, hyoid bone, and thyroid cartilage anteriorly. It inserts at the dorsal aspect of either the manubrium of the sternum or the clavicle. ^, Inferiorly, it extends to fuse with the pericardium. When considered as two compartments, the visceral compartment encloses the thyroid gland, parathyroid glands, recurrent laryngeal nerve, and esophagus, extending superiorly past the hyoid and to the pharynx. The infrahyoid compartment surrounds the neighboring infrahyoid muscles: the sternohyoid, sternothyroid, and omohyoid. Below the level of the hyoid, the pretracheal fascia is limited to the lateral border of the omohyoid muscles, in which the omoclavicular triangle is formed. It then turns posteriorly and blends into the carotid sheath on its lateral limit. ^, On the posterior end it extends superiorly to blend with the buccopharyngeal fascia. Above the level of the hyoid, the visceral compartment is also known as the pharyngeal mucosal space.

Deep layer of the deep cervical fascia (prevertebral layer)

Referred to as the prevertebral layer, the deep layer of the deep cervical fascia encloses the prevertebral muscles, scalene muscles, and the levator scapulae muscles. It encloses the vertebral column, longus coli, longus capitis, scalenes, levator scapulae, paraspinal muscles, phrenic nerve, and cervical/brachial plexus. ^, According to Grodinsky et al. it spans to the tip of the transverse process of the vertebral bodies encasing the prevertebral space both anteriorly and posteriorly. On the other hand, Feigl describes the fascia as fusing with the superficial layer of the deep cervical fascia at the trapezius muscle. All agree that the prevertebral fascia wraps around the prevertebral muscles and defines the prevertebral space. It can be identified as the floor of the posterior triangle. Within it, the vertebral artery maintains direct communication with the intervertebral foramen, where the prevertebral space and epidural space connect. The prevertebral fascia lies superficial to the brachial plexus and subclavian artery, enclosing the sympathetic trunk in a thin double layer of fascia. This fascia continues laterally and forms a sleeve – the axillary sheath, around the brachial plexus, subclavian artery, and axillary artery, creating a closed compartment as the neurovascular bundle passes the distal clavicle and extends to the upper extremities. ^, When infections do spread into the parapharyngeal, retropharyngeal, and danger space through natural occurring defects, lymphatic, and small vascular channels in the middle deep fascia plane, this is the last deep fascial plane before reaching the spine and potentially the epidural space.

Alar fascia (intercarotid fascia)

Significant controversy exists on whether the alar fascia is a true fascia or an extension of the prevertebral fascia. ^{, , ,} It is described as a fascial layer between the visceral/ buccopharyngeal fascia and prevertebral fascia, serving as a connecting band between the two carotid sheaths, posterior to the esophagus. As an independent structure, the alar fascia is said to extend across the midline posterior to the pharynx, esophagus, and visceral fascia, fusing with the prevertebral fascia at the tips of the transverse processes, where both layers are attached. ^, This posterior fusion occurs at the level of C1 and the alar fascia is otherwise not well defined superiorly. ^{, ,} From the transverse process, a portion of the alar fascia continues anterolaterally, joining the mesh of fascia layers contributing to the carotid sheath, to form its medial wall. ^{, ,} This fascia follows the course of carotid arteries anteriorly. As a result, the midline viscera will pass through the alar fascia as they course posteroinferiorly.

Across the majority of studies, the alar fascia was determined to be a well-defined and distinct midline structure, down to level C6 inferiorly. ^{, ,} Grodinsky et al. found it extends to level of C7 vertebrae where it joins the visceral fascia. Some describe this fusion at the level of T2. ^, In addition to the inferior attachment, according to Feigl, in about half of the cases, the alar fascia and the buccopharyngeal fascia fuse posterior to the pharynx or at the lateral border in between the two carotid sheaths, and in the other half, there is no further connection between these two layers. In general, these fascia layers are documented to merge together and are largely indistinguishable from each other inferiorly, after fusion, beyond the level of T2. ^, The space created between the alar fascia and the buccopharyngeal/visceral fascia defines the retropharyngeal space. It remains separate from the prevertebral fascia inferiorly, blending into the endothoracic fascia within the superior mediastinum. ^, While these anatomic boundaries exist and understanding is crucial to delineate infectious spread through the danger space into the mediastinum, radiographic delineation is difficult to separate due to the thin nature of the fascial planes and displacement by the abscess.

Carotid sheath

The carotid sheath is a fibrous network of connective tissue enclosing the internal carotid artery, internal jugular vein, and the vagus nerve. Each structure is encased in its own layer of connective tissue, frequently described as a tunnel. ^, Cranial nerves IX, XI, and XII briefly travel within the carotid sheath above the hyoid bone before piercing the sheath anteriorly. Significant variation exists in the description of fascia layers which contribute to the carotid sheath. While there exists variability in which exact layers contribute, all publications agree it receives contributions from multiple fascial layers. ^{, , , ,} Grodinsky et al. describes the carotid sheath as primarily receiving fibers from the alar fascia and most agree the alar fascia serves to connect the carotid sheaths bilaterally. ^{, ,} Attachments of the carotid sheath are also heavily debated. While it is agreed that the carotid sheath inserts near the jugular foramen, there is some variation on its insertion site, described as either at the medial face of the vaginal process or anteriorly to the vaginal process and posterior to the jugular foramen and carotid canal. ^, Most authors will identify the carotid sheath origin at the posterior rim of the jugular foramen. ^{, ,} The vagus nerve exits the skull through jugular foramen, medially to the internal jugular vein. Carotid artery exits via the carotid canal which is anterior and slightly medial to the jugular foramen. The internal jugular vein is described to travel anterolateral to the common carotid artery bilaterally. The vagus nerve most commonly descends in between and posterior to the great vessels. This relationship can be useful in distinguishing schwannomas of vagus nerve and sympathetic chain using imaging. Although it is not common in our surgical practice to see the vagus nerve anterior to the great vessels in either side, in a study of 50 cadavers, Hojaij et al., found that caudal to the level of carotid bifurcation, the vagus nerve was anterior to the great vessels in 18 cadavers on the right and 34 cadavers on the left.

Masseteric fascia

Covering the masseter muscle, the masseteric fascia has been described as a superficial sheet extending to the lower border of the zygomatic arch superiorly and to the posterior edge of the ramus of the mandible inferiorly. The parotid gland, which covers most of the posterior aspect of the masseter muscle, contains a superficial layer of fascia which extends from the mastoid tip. These fascial layers together are often termed the parotidomasseteric fascia.

Parotid fascia

The parotid fascia forms a fibrous network around the parotid gland and is composed in part by the superficial layer of the deep cervical fascia. It envelopes the posterior side of the gland in an “open book” fashion. The upper two thirds of the fascia is described as thick, whereas the lower one third of the parotid fascia is described as thin, blending respectively with the superficial cervical fascia and platysma fibers. It attaches posteriorly to the anteromedial aspect of the mastoid tip and posterosuperiorly to the inferior aspect of the tragal pointer. Anteriorly, it fuses with the masseteric fascia, and together they are often called the parotidomasseteric fascia. ^, The branches of the facial nerve lie deep within the parotid fascia, with the facial nerve trunk located in the pretragal area.

The parotid fascia is composed of two distinct layers, a superficial and a deep layer. The superficial layer is a continuation of the superficial cervical layer, where the deep layer is a continuation of the deep cervical layer and is often referred to independently as the parotidomasseteric fascia. While the superficial layer provides a landmark for identifying the facial nerve trunk, the deep layer covers and protects the facial nerve during surgical dissection. The parotid fascia remains in close proximity to a network of structures including fascial layers within the cheek, termed the superficial muscular aponeurotic system (SMAS). ^{, ,} The SMAS separates subcutaneous fat from the parotidomasseteric fascia, serving as an extension of the superficial cervical fascia onto the face. The plane over the parotid fascia is commonly used as a plane of dissection when raising a sub-SMAS flap to minimize the risks of damage to the auriculotemporal nerve and limit the development of Frey’s syndrome. Moreover, entering the parotid fascia can aid in identifying the location of the facial nerve trunk medially.

Temporoparietal fascia

The temporoparietal fascia, also termed the superficial temporal fascia or temporal aponeurosis, is a continuation of the SMAS superiorly extending to the frontalis and orbicularis oculi. ^, It extends from the upper border of the zygomatic arch to the superior temporal line over the temporalis muscle. ^, This fascia has been recognized to be distinct and independent of the deep temporal fascia, and separated from it by the temporal fat pad as it descends to the zygomatic arch. ^, Branches of the facial nerve, including the temporal branch, are embedded within this fascia. ^,

Discussion

Fascial planes aid in identifying and defining boundaries within the supra and infra hyoid head and neck. While names and descriptions vary across literature, the clinical, radiographic, and surgical applications for understanding fascial layers remain constant. The real and potential spaces created serve not only as surgical landmarks during head and neck surgery but also offer avenues of pathological spread for infection and disease. Potential spaces are typically compressed or collapsed in normal anatomy. However, they can become filled or displaced by masses and infections. The location, extent, and direction of displacement can all be indicative of the origin of pathology. Identifying abnormal pathology in these spaces on imaging therefore becomes a valuable clinical tool in diagnosing and treating disease. Finally, a deep understanding of the fascial layers can provide critical landmarks for safe and effective surgeries in the head and neck.

Infections to certain anatomical spaces can be life-threatening. This includes the carotid sheath, PPS, submandibular, retropharyngeal and danger spaces. Infections originating in these spaces can spread locally and traverse spaces, facilitated by the interconnections created by fascial planes. The carotid sheath, which courses from the skull base to the mediastinum, can be involved by pathologies of the PPS. The retropharyngeal space and danger space communicate with the PPS and carotid sheath. In the suprahyoid region, the PPS is in direct communication with the visceral fascia, peritonsillar space, parotid, masticator, submandibular and mandibular spaces. ^, The interconnected nature of these spaces allow pathologies to spread beyond the site of origin, resulting in life threatening conditions such as mediastinitis.

Ludwig’s angina is a potentially fatal, rapidly progressive bacterial cellulitis involving the floor of mouth. Initially described by Wilhelm Fredrich von Ludwig in 1836, it often presents with neck swelling, tooth pain, bilateral submandibular edema and posterior displacement of the tongue. It primarily arises from polymicrobial odontogenic infections and initially impacts the submandibular space. ^, When left untreated, it can spread contiguously to the sublingual and retropharyngeal spaces where it can subsequently constrict the airway. Advancements in antibiotic therapy and drainage have largely replaced the need for invasive surgical intervention. In early stage localized disease, antibiotics are recommended. Late and advanced disease compromising the airway is commonly managed via tracheostomy under local anesthesia in addition to incising and draining the underlying infection.

Cervical fascias play an important role as surgical landmarks in neck dissections, where fascial planes can be utilized to compartmentalize the neck and guide lymph node excision. This is especially useful in clinically N0 disease, in which there is no obvious gross pathology within the lymphatic tissue. The largely avascular nature of fascial planes allows for easy and safe dissection. In general, subplatysmal skin flaps incorporating the superficial cervical fascia are elevated from the mandible to the level of the clavicle, preserving the superficial layer of the deep cervical fascia. In levels 1A and 1B, submental and submandibular lymph nodes can be identified for excision. Extra care should be taken to preserve the marginal mandibular nerve at approximately the level of the facial notch. The marginal mandibular nerve can be seen coursing over the superolateral surface of the gland under the superficial layer of the deep cervical fascia. Entering this fascia at the inferior border of the gland and developing a space superiorly between the fascia and nerve laterally and the gland medially, helps retract the nerve with the fascia above the mandible and keep it safe away of the dissection field. Division of this fascia also frees the submandibular gland from this investing fascia, allowing the surgeon to bring this and the perifacial nodes down from the inferior border of the mandible.

Continuing the dissection to the digastric muscles, the superficial layer of the deep cervical fascia can continue to be divided over the digastric muscle, all the way back to the sternocleidomastoid muscle. Similarly, the superficial layer of the deep cervical fascia can be divided at the anterior border of the sternocleidomastoid, allowing the muscle to be retracted posteriorly and the nodal packet enveloped by the cervical fascia to be retracted anteriorly. Along the upper half of the muscle, proximal to the atlas, precaution must be taken to avoid the spinal accessory nerve which pierces the muscle in this region. With the sternocleidomastoid muscle retracted, the internal jugular vein can be identified within the carotid sheath. This can be further dissected to free the upper and middle jugular lymph nodes, from the carotid sheath. A deep understanding of these different fascia compartments in the neck is crucial in performing a safe neck dissection surgery.

An understanding of fascial planes is also important in reconstruction of head and neck defects Resting superficial to the buccopharyngeal fascia lies the buccal fat pad. When the buccopharyngeal fascia is incised and the buccal space carefully dissected, this fat pad can be utilized as a vascularized fat flap in reconstruction of mucosal defects in the oral cavity. It has an average thickness of 6 mm and can provide 10 cm ² of coverage. It receives ample vascularization from the facial artery, maxillary artery, superficial and temporal artery, and rich capillary plexuses. The buccal fat flap serves as a reliable flap for reconstruction especially among those with compromised wound healing. Preserving its capsule is key in its utility in reconstruction. Its proximity to the recipient site and low morbidity levels makes it ideal in some oral cavity reconstructions.

The temporoparietal fascia, a superior continuation of the SMAS, can also be utilized in head and neck reconstruction. Thin and highly vascularized, it has minimal risk of necrosis. Complications of harvest include alopecia, injury to the frontal branch of facial nerve, hematoma, temporal hollowing, and scar formation in the temporal region. This flap can be harvested just above the parotid fascia and utilized to reconstruct superficial parotidectomy defects without facial nerve sacrifice to fill the parotid defect. Once the gland is excised, the harvested flap can be sutured to the zygomatic periosteum and the parotidomasseteric fascia. While it is a simple and reliable procedure, given its thin volume its utility is limited to small defects seen in superficial and partial partoidectomies. ^,

Since its initial report in 1977, a detailed understanding of the SMAS has improved the results of facial rejuvenation surgery. The SMAS-platysma face lift, otherwise known as a “deep plane” facelift, has improved the cosmetic outcomes after rhytidectomy by providing effective means to suspend soft tissue beyond tightening of the skin. Although several variations exist, most descriptions of this technique call for elevation of the subcutaneous fat and platysma muscle in addition to skin. During this procedure, the SMAS is dissected and lifted in an upward or lateral direction, lifting the submental region and reducing the need for submental fat removal. The flaps can then be transposed, plicated or imbricated to maximize patient satisfaction.

Conclusion

Understanding cervical fascial planes has significant implications in clinical and surgical outcomes. The spaces they create help compartmentalize structures and limit pathological spread. At the same time, interconnectivity between fascial layers can serve as routes of spread for infections and understanding these connections is important in treating deep neck space infections. A firm understanding of the fascial planes described above is crucial to maintaining surgical safety and following the appropriate dissection planes in several commonly performed head and neck surgeries, including neck dissection and rhytidectomy. While typically described as two primary fascial layers, the superficial fascia and the deep cervical fascia, variations exist in terminology used to describe each. This can create confusion among trainees, and knowledge of the appropriate planes and spaces is best obtained through 3D visualization such as the model created in this study.

Financial material and support

Internal departmental funding was used without commercial sponsorship or support.

Model QR code