The Deep Fascia of the Head and Neck Revisited: Relationship with the Facial Nerve and Implications for Rhytidectomy

Abstract

Background:

The deep fascia is important in face-lift surgery, as it is relied on for protection of the facial nerve during surgical dissection. Lack of consistency with the term may be attributable to the term deep fascia having two different meanings. It is a specific type of connective tissue, classically thin, fibrous, and flat, whereas in the description of the layers of the face and neck, the deep fascia layer includes all the connective tissue deep to the superficial fascia layer. This cadaver study was undertaken to clarify the layered anatomy of the face and neck and its relationship with the facial nerve branches.

Methods:

Preliminary dissections and macrosectioning, followed by a conclusive series of standardized layered dissections, histologic analysis, and sheet plastination, were performed on 50 cadaver heads.

Results:

The deep fascia is thin in convex areas of the face and neck, and thicker in concave areas; it is interspersed with deep fat. The facial nerve branches, after emerging from the parotid gland, are embedded within the deep fascia, not deep to it. They transition from deep within the deep fascia at specific locations to course in the most superficial part of the deep fascia where they underlie their target superficial fascia muscles and are at risk from deep plane face-lift dissection.

Conclusions:

The deep fascia layer is a multilamellar fibrofatty layer of variable thickness, which includes the deep fat in which the facial nerve branches are embedded. In deep plane face-lift surgery, dissection must be performed in the most superficial level of this deep fascia layer.

The deep subcutaneous layer is called simply the deep fascia of the region where situated, e.g., deep cervical fascia.

—B. B. Gallaudet (1931)

The fascial layers of the head and neck have been the subject of controversy since their first description by Burns in 1811.^1,2 From a functional and surgical perspective, the soft tissue of the head and neck is considered in five concentric layers (Fig. 1): layer 1, skin; layer 2, subcutaneous fat; layer 3, superficial musculoaponeurotic system (SMAS)–platysma; layer 4, the deep plane with spaces and ligaments; and layer 5, the deep fascia.³ As a convention, the superficial fascia is considered as layers 2 and 3 together. The deep fascia (layer 5) is situated deep to the mimetic muscles (SMAS-platysma, layer 3), separated only by a virtual plane of spaces and ligaments (layer 4), which is understood to provide both gliding movement and stability. There is some ambiguity of the nomenclature, with layer 4 recently described as the “loose areolar connective tissue layer” including the deep fat compartments.⁴

Fig. 1.

Traditional textbook description of the layered anatomy of the face (left) and our current understanding (right). Traditionally, the concentric layers were as follows: layer 1, skin; layer 2, subcutaneous fat; layer 3, SMAS-platysma; layer 4, deep plane with spaces and ligaments; and layer 5, deep fascia. It is currently understood that the SMAS is not a complete anatomical layer and is only present in the form of the mimetic muscles but not the aponeurosis between the muscles. The deep fascia is not only a thin fibrous fascia, but a three-dimensional layer with fibrofatty and areolar tissue areas (spaces). The deep plane dissection (layer 4) is not a separate anatomical layer, but a potential dissection in the most superficial part of the deep fascia. Illustrations published with permission from Levent Efe. Copyright © 2022 Levent Efe.

The term deep fascia has two different meanings. For surgeons, deep fascia entities refer to thin but robust sheet-like connective tissues (eg, deep temporal fascia, sternomastoid fascia), which are typically depicted in standard anatomical illustrations as thin sheets of uniform thickness, in contrast to the surrounding adipose tissue of varying thickness.⁵ It is not well appreciated in the surgical literature that this description of sheet-like fascias differs from how classic anatomists of the nineteenth century originally used the term deep fascia, which was in the context of layers.² Classically, the term superficial fascia was ascribed to the superficial subcutaneous fat layer, which also contains the mimetic muscle layer (eg, the platysma muscle in its deep portion in the cervical area).^6–11 The term deep fascia, in contrast, referred to the connective tissue layer deep to the superficial fascia, surrounding and supporting the viscera (eg, investing layer of the deep cervical fascia).^6,7

In face-lift surgery, the deep fascia of the face and neck is of fundamental importance, as surgeons rely on this fascia to protect the facial nerve branches during dissection. Traditional teaching describes the facial nerve branches as being located deep to the deep fascia,^12–18 although in certain locations, facial nerve branches have been described to be superficial to, or even within, the deep fascia.^19–24 The facial nerve branches are usually reported from a two-dimensional perspective (ie, their course and relationship with anatomical structures are reported), and the depth of the nerve relative to the fascial layers has remained less well described. From a surgical perspective, the depth of the branches throughout their trajectory from the parotid gland to the mimetic muscles is of major importance, as surgical dissection is “safe” up to the point where the dissection plane encounters the nerve.

To improve the safety of deep plane face-lift procedures, the course of the facial nerve was studied, specifically, its three-dimensional relationship with the layered fascias of the face. This study elucidates the structure of the deep fascia and explains the difference, if any, between the deep fascia and the deep fat layer (Fig. 1). The relationship of each facial nerve branch to the deep fascia is examined.

PATIENTS AND METHODS

Ethical approval for the project was granted by the Human Ethics Advisory Groups of The University of Melbourne for the feasibility study and the Queensland University of Technology for the conclusive study (project nos. 14243 and LR 2021-4306-4761). The initial feasibility study was performed on 21 cadavers, 15 embalmed and six fresh-frozen (10 male and 11 female cadavers; mean age, 76 years). This involved layered dissections, face-lift dissections, and band-saw macrosections of the head and neck in various planes.

Based on this feasibility study, a definitive study was conducted on 29 cadavers. A series of standardized dissections was performed on one embalmed and 15 fresh (nonfrozen) cadavers (nine male and seven female cadavers; mean age, 78 years; body mass index, 26 kg/m²). In each cadaver, the two sides of the face were dissected with a different technique. A surgical deep plane face-lift dissection was performed on the first side to establish the surgical presentation of the deep fascia. This was done using the previously described surgical technique of the senior author (B.C.M.).^25,26 On the contralateral side, the following methodologic sharp layered dissection technique was used to identify the layered anatomy and its relationship with the facial nerve. With the neck extended, a transverse skin incision was made over the clavicle from the midline to the midcoronal line and extended through the skin (layer 1) and subcutaneous adipose tissue (layer 2) down to, but not through, the mimetic muscles (layer 3). Under loupe magnification, the skin and subcutis were then dissected off the superficial surface of the mimetic muscles. Subsequently, the platysma was incised at the level of the original skin incision. Sharp dissection was used on the underside of the mimetic muscles to allow the musculoaponeurotic layer (SMAS, layer 3) to be lifted. Next, the deep fascia (layer 5) was incised at the original incision site to the first sheet-like fascial layer or the surface of the deep muscles. The incision surface was then checked for any fascial subdivisions, to be sure no fascial layer was missed. Dissection along the deep surface of the deep fascia finally isolated this layer and its contents. Objective technical investigations were used to determine the layered anatomy in more detail in 21 cadavers:

1. Histologic analysis of full-thickness macrosections of the face and neck was performed in 73 samples from 11 fresh cadavers (six male and five female cadavers; mean age, 81 years).²⁷

2. Sheet plastination of the head and neck of 10 fresh cadavers were processed by von Hagens plastination in the axial, sagittal, and coronal planes using their latest technique (four male and six female cadavers; mean age, 67 years).²⁸ Sheet plastination is a tissue preservation technique used to allow anatomical study. In this technique, all water and fat are removed from thin tissue slices, which are then forcibly impregnated with E12 epoxy resin using a vacuum.

RESULTS

Superficial Fascia

The mimetic muscles, the deepest layer of the superficial fascia, separate the subcutaneous fat from the deep fat. The aponeurotic layer over the parotid gland, which is commonly termed parotid fascia or platysma-auricular fascia (PAF), was demonstrated to be in continuity with the platysma in all cases, not with the masseter fascia. When this layer (platysma-PAF) is removed, only a thin layer of parotid capsule remains, which is continuous with the masseter fascia. The continuity of the platysma with the PAF was confirmed with histologic analysis and sheet plastination, and a variable number of muscle fibers were identified within this layer (Fig. 2).

Fig. 2.

Demonstration of the continuity of the platysma with the PAF. (Above) Right hemiface of a fresh cadaver. The strong PAF is lifted with the platysma from an incision in front of the ear and turned over 180 degrees to demonstrate its deep aspect. The PAF is the evolutionary aponeurotic remnant of the platysma, which overlies the parotid in humans. Lifting this SMAS-platysma (layer 3) exposes the parotid parenchyma, which is now covered only by its thin capsula propria (parotid capsule). (Below, left) Histologic image of the lower face and neck. Note the dispersed muscle fibers (P) inside the PAF demonstrating the evolutionary remnant of the platysma involuted to a fibromuscular structure. Sheet plastination sections of the parotid-platysma in a coronal plane (center, right), showing the continuation of platysma over the sternocleidomastoid muscle with the PAF over the parotid, and in an axial plane (below, right), showing the continuation of platysma over the mandible with the PAF over the parotid.

Deep Fascia

The deep fascia layer is immediately deep to the superficial fascia. The surface of the deep fascia has the aspect of a sheet-like layer, but subsequent incision and dissection demonstrated that the sheet-like fibrous layer is part of a thicker fibrofatty layer that passively fills the space between the superficial fascia and the deep structures (eg, bone, muscles of mastication, glands) (Fig. 3). [See Video 1 (online), which demonstrates the deep fascia on three fresh cadavers with various body mass indexes.] The thickness of the deep fascia therefore varies according to the amount of space between the superficial fascia and the underlying structures (Fig. 4, left). Accordingly, the deep fascia is thin in areas where the underlying structures are convex. Over the cranium, upper temple, lower masseter, and submandibular gland, the deep fascia is thin but mobile and areolar, whereas over the sternocleidomastoid muscle (SCM), it is thin but closely adherent to the platysma fascia (cervical retaining ligaments), and over the parotid gland, it is even thinner (parotid capsule) closely adherent to the platysma and PAF. In contrast, the deep fascia is thicker in areas where the underlying structures are concave (lower temple to the upper masseter, the anterior midcheek, the anterior cervical triangle). These are largely the areas where the deep fat compartments have previously been described.

Fig. 3.

Dissection images of the deep fascia right side of a fresh cadaver. (Left) The deep fascia in situ. The deep fascia is thin over the cranium, upper temple, lower masseter, sternocleidomastoid, parotid gland, and submandibular gland, and it is thicker in the lower temple to the upper masseter, the anterior midcheek, and the anterior cervical triangle. (Center) After removal of the deep fascia. The buccal fat pad (BFP) is a separate entity altogether, and its temporal extension is seen. (Above, right) Close-up image of the same area in another cadaver. The deep fascia is reflected to show the underlying buccal fat pad and parotid duct (PD). Whereas the deep fascia features abundant connective tissue, the buccal fat pad lacks this and is more amorphous and pliable in contrast to the deep medial cheek fat (DMCF). (Below, right) Midline sagittal view of the deep cervical fascia after removal of the covering platysma muscle and removal of the right side of the deep fascia so that a cross-sectional view at the cut-end of the deep fascia is provided. The deep fascia is thin in the convex area over the thyroid and cricoid cartilages, but thick in the concave area overlying the hyoid bone. No separate deep fat layer could be distinguished from the deep fascia; they are one and the same layer.

Fig. 4.

Demonstration of the thickness of the deep fascia (in blue) filling the space between the platysma and the deeper structures. (Left) Axial macrosection through the neck at the level of the hyoid bone (C3) of a fresh cadaver. The deep fascia is a thin aponeurosis over the SCM but a thick fibrofatty layer that includes the deep fat of the neck in the other areas. (Center) Axial sheet plastination section through the neck at the level of the thyroid cartilage (C4). The deep fascia has a dense matrix of connective tissue. Not one but multiple sheets of connective tissue are visualized in the deep fascia. Up to 18 fascial sheets can be counted in the pretracheal area in this specimen (inset). Note how the connective tissue sheets of the deep fascia run parallel to the skin and SMAS-platysma and the connective tissue septa of the superficial fascia run perpendicular to the skin. (Right) Sagittal sheet plastination section through the masseter, mandible, submandibular gland, hyoid, and strap muscles. The deep fascia fills the space between the platysma and the deep muscles and extends continuously from the neck into the face, not stopping at the mandible as is traditionally described.

Video 1. This video is an animation of the effect of a deep plane dissection on the facial nerve branches within the masseteric fascia.

Download video file ^{(10.7MB, mp4)}

In areas where mimetic muscles are not present, the superficial fascia and deep fascia are in direct contact and macroscopically indistinguishable from each other notwithstanding loupe magnification. Microscopically, however, an important distinction exists, with the connective tissue architecture of the deep fascia being oriented parallel to the skin and that of the superficial fascia being oriented more perpendicular to the skin. These observations were confirmed by sheet plastination (Fig. 4, center and right) and by histologic analysis (Fig. 5). Not one but many sheets of connective tissue septae interspersed with adipose tissue were visualized dividing the deep fascia into more superficial and deeper layers (up to 18 different sheets in the neck). These different sheets generally merge at thinner areas, forming recognizable deep fascia entities (eg, masseter fascia, sternocleidomastoid fascia), and diverge at thicker areas, forming deep fat entities (eg, subplatysmal and interdigastric fat, deep cheek fat). Histologic analysis confirmed the arrangement of the multiple fascial sheets in a mille-feuille pattern, a pattern seen in multiple areas of the face (Fig. 5).

Fig. 5.

Full-thickness histologic specimens demonstrating the deep fascia of the different regions in the face. The area between the dotted lines is the deep fascia according to the classic understanding of deep fascia. The cranium and temporalis muscle were removed and are therefore not shown. Below each histologic image, an illustration depicts the general anatomy shown on histologic analysis. The deep fascia is depicted in blue. (Left) In the forehead between the frontalis (F) and the cranium, the deep fascia is thin. It is a multilaminated structure with up to numerous layers of connective tissue sheets (up to 41 in this specimen). Anatomical dissection could separate this deep fascia into separate layers to subcategorize them as “posterior frontalis fascia” for the superficial layers, “innominate fascia” for the middle layers, and the “cranial periosteum” for the deepest layers. However, these three entities are clearly parts of the same connective tissue entity known as the deep fascia. (Second from left) In the upper temple between the auricularis anterior muscle (AAM) and the temporalis muscle (removed), the deep fascia is also thin and similarly subcategorized as in the forehead, but instead of periosteum, the deepest layer is the deep temporal fascia. (Third from left) In the lower temple between the superficial fascia and the temporalis muscle (removed), the deep fascia gradually becomes thicker, with some of the fascial sheets splitting apart to give rise to the structures commonly known as the deep and superficial layer of the deep temporal fascia. However, even over the superficial layer of deep temporal fascia there is still a layer of deep fascia present in which the temporoparietal branches run. (Center) At the zygomatic arch between the superficial fascia and the zygomatic periosteum, the deep fascia is thick and fibrofatty. There is not simply zygomatic periosteum and the SMAS in this region. (Third from right) At the upper masseter between the superficial fascia and the masseter, the deep fascia is thick but becomes thinner toward the lower masseter. (Second from right) At the lower masseter between the superficial fascia and the masseter, the deep fascia is thin and multilaminated like the innominate fascia in the temple, allowing for an effortless dissection of the area (lower premasseter space). Usually, there is platysma muscle separating the superficial fascia from the deep fascia, but in this cadaver, the platysma did not come this high up. (Right) In the upper neck between the platysma and the strap muscles, the deep fascia is thick and multilaminated. Illustrations published with permission from Levent Efe. Copyright © 2022 Levent Efe.

The multiple fascial sheets fuse at the external surface of a muscle, gland, or major vessel, creating a smooth lining surface, such as the periosteum, muscle fascia, salivary gland capsule, and carotid sheath (Fig. 6). Accordingly, these lining fascias are not independent structures but are part of the surrounding deep fascia that even includes extensions of the fascia into the muscles/glands, which divide the muscle into fascicles (“perimysium”) and salivary gland into lobules (“capsula propria”).

Fig. 6.

Histologic specimens of the continuation of fascia. (Above) Superficial retinacula cutis (superficial fascia) extending into the orbicularis oculi and zygomatic major muscles to subdivide it in fascicles and continuing into the deep fascia as deep retinacula cutis. (Below) The deep retinacula cutis (deep fascia) extending into the parotid gland to subdivide it in lobules and extending into the platysma to continue as superficial retinacula cutis.

The previously reported deep fat compartments are part of the deep fascia, not separate entities. The buccal fat pad, however, is a distinct entity, being more pliable, amorphous, and yellow, as it largely lacks fibrous connective tissue, providing a filling rather than supporting function (Fig. 3, right).

Facial Nerve Branches

All facial nerve branches were identified within the deep fascia. No facial nerve branches were situated deep to the deep fascia (eg, in between the masseter muscle and its epimysium, muscle fascia). After passing through the parotid gland, the nerve branches first travel deep within the deep fascia and then gradually transition along their trajectory in the lateral face, to finally lie in the most superficial aspect of the deep fascia in the anterior face, innervating their target muscles from their deep aspect (with the exception of the levator anguli oris, buccinator, and mentalis muscles, which are innervated from their superficial aspect). The point at which the facial nerve branches reach their most superficial position within the deep fascia is generally at the point where terminal branches to the target muscles first appear: at the auricularis anterior muscle for the frontotemporal branches (ie, inferior temporal septum), at the peripheral edge of the orbicularis oculi muscle for the zygomatic branches (ie, zygomatic ligaments), near the anterior border of the masseter muscle for the buccal branches (ie, masseteric ligaments), and at the posterior border of the mandibular ligament (mandibular attachment of platysma, depressor anguli oris, and depressor labii inferioris) for the marginal mandibular branches. This trajectory was confirmed by histologic analysis for all facial nerve branches (Fig. 7) and by dissection for the frontotemporal branches (Fig. 8), the zygomatic and buccal branches (Fig. 9), the marginal mandibular branches (Fig. 10), and the cervical branches (Fig. 11).

Fig. 7.

Histologic images demonstrating the relative depth and gradual transition of the frontotemporal branches (I), zygomatic branches (II), buccal branches (III), marginal mandibular branches (IV), and cervical branches (V) within the deep fascia. Yellow circles mark the facial nerve branches. Green circles mark the parotid duct. Frontotemporal branches gradually transition from deep within the deep fascia (I.A) to midway in the deep fascia over the zygomatic arch (I.B and I.C), to finally lie directly underneath the auricularis anterior muscle, orbicularis oculi muscle, and frontalis muscle (I.D and I.E). Zygomatic branches exit the parotid gland at its anterosuperior border deep within the deep fascia (II.A and II.B), run over the masseter while splitting into multiple smaller branches, some of which will go over (II.C) and some of which will go under (II.D) the zygomatic major muscle. In nine of 12 cadaver dissections and seven of eight histologic cadaver investigations, at least one small zygomatic branch ran over the zygomatic major muscle. Finally, they transition to the suborbicularis oculi fat along the zygomatic ligaments (II.E and II.F). closely passing the zygomatic minor muscle. Buccal branches exit the parotid gland deep within the deep fascia proximally (III.A), but progressively transition to a more superficial position along their trajectory over the masseter (III.B and III.C). The buccal branches that innervate the upper lip levator muscles (eg, zygomatic minor and major muscles, levator labii superioris, levator labii superioris alaeque nasi) from their deep aspect, run over the maxilla within the most superficial part of the deep fascia (III.D), which in this region is the deep medial cheek fat (DMCF). The buccal branches that innervate the buccinator and levator anguli oris from its superficial aspect run essentially in the epimysium of the buccinator, at the deepest aspect of the subcutaneous fat layer (III.E). Marginal mandibular branches exit the parotid gland at its anteroinferior border, with some leaving the gland over the angle of the mandible whereas some leave below it (IV.A). The branches that overlie the masseter run deep within the deep fascia (IV.B), starting to transition superficially only where the deep fascia becomes thicker at the anterior border of the masseter (IV.C). Whereas most of the branches cross the facial vessels superficially, in two of 12 dissections and one of eight histologic investigations, a small additional branch was seen deep to the facial vessels (IV.D). Anterior to the facial vessels, the main terminal branch becomes exposed in the most superficial aspect of the deep fascia where it curves over the mandible and around the mandibular ligament to continue forward deep to the depressor labii inferioris and depressor anguli oris toward the mentalis muscle (IV.E). The main cervical trunk exits the parotid gland deep within the deep fascia (V.A, V.B, and V.C), to run inferiorly deep within the deep fascia along the anterior border of the SCM. Smaller terminal branches coming off the trunk transition more superficially to run forward in the most superficial layer of the deep fascia at the deep surface of the platysma, sending terminal branches innervating the platysma muscle along its entire deep surface (V.D). No branches were visualized running within the platysma muscle itself. AAM, auricularis anterior muscle (temporoparietal muscle); BFP, buccal fat pad; BM, buccinator muscle; CBFN, cervical branch of facial nerve; DAO, depressor anguli oris; DLI, depressor labii inferioris; F, frontalis muscle; MM, masseter muscle; OOc, orbicularis oculi muscle; P, platysma; PDM, posterior digastric muscle; PG, parotid gland; SMG, submandibular gland; TCN, transverse cervical nerve; TM, temporalis muscle; Z, zygomatic arch; ZCL, zygomatic ligament; ZMa, zygomatic major muscle; ZMi, zygomatic minor muscle. Illustrations published with permission from Levent Efe. Copyright © 2022 Levent Efe.

Fig. 8.

Right temple of a fresh cadaver demonstrating the surgical deep temple dissection after removal of the dermis and subcutaneous fat, hereby isolating the superficial temporal fascia. (Above) The pathway of the frontotemporal branches is demonstrated. At the level of the inferior temporal septum, three related changes occur: (1) the frontotemporal branches now run within the most superficial part of the deep fascia (innominate fascia) at the underside of the temporoparietal muscle (at this location: auricularis anterior muscle); (2) the deep fascia becomes less fatty and more a collection of fascial connective tissue sheets (ie, mille-feuille pastry organization of the deep fascia known as the innominate fascia); (3) a deep temple dissection occurs within this innominate fascia and lifts the most superficial part of the innominate fascia together with the superficial temporal fascia. This means that the nerve branches, which lie within the most superficial part of the innominate fascia, will be seen in the roof of a temple space dissection. (Below) When the superficial temporal fascia is turned over, it becomes clear that the most superficial part of the deep fascia with the frontotemporal branches are present at the deep surface of the flap. A deep plane dissection of the temple occurs within the natural plane of the deep fascia’s areolar tissue, mille-feuille pastry like organization (ie, the innominate fascia). Thus, the superficial part of this innominate fascia together with the frontotemporal branches will be seen in the roof of the dissection as part of the “superficial temporal fascia,” whereas it in fact is still part of the “deep” innominate fascia. This explains how the surgical understanding of the fascial layers is slightly different from the true anatomical in vivo situation.

Fig. 9.

Right masseter fascia from an obese cadaver (body mass index, 27 kg/m²) lifted off the masseter muscle. Note the nerve branches (arrows) traveling deep within the masseter fascia but not deep to it.

Fig. 10.

Right side of the mandible with the platysma elevated to demonstrate the deep fascia beneath it. The main marginal mandibular nerve is seen emerging from within the deep fascia to run in the most superficial part of the deep fascia, separated from the platysma only by a thin membrane of deep fascia. It then crosses over the mandible posterior to the mandibular ligament (ie, the mandibular attachment of the platysma, depressor labii inferioris, and depressor anguli oris), passing around it to continue deep to the depressor labii inferioris toward the mentalis muscle.

Fig. 11.

Right side of the lower face and neck. (Above) Before removal of a layer of deep fascia. (Below) After removal of a layer of deep fascia. Note how most of the deep fascia still underlies the nerves, indicating that the nerves are within the deep fascia. TCN, transverse cervical nerve.

DISCUSSION

This cadaver study clearly demonstrated that the layer deep to the superficial fascia, called the deep fascia, is not a thin aponeurotic layer but a thick, multilamellar layer that includes the deep fat and embeds the facial nerve branches.

Subdivision of the deep fascia into a more superficial “areolar layer” (layer 4, spaces and deep fat layer) and a “deep fascia proper” (layer 5) (eg, masseter fascia) is not possible because they are inseparable as complete layers, even with microscopic magnification of the histologic specimen (Fig. 5 second from right, and Fig. 6). Thus, the conclusion that the layer deep to the superficial fascia and mimetic muscles (“SMAS”) is the deep fascia as the classic anatomical descriptions had previously described: simply, the deep subcutaneous fat layer.²⁹ In contrast to elsewhere in the body, where the deep fascia is relatively thin, the variation of space between the flat mimetic muscle and the more complex deeper structures in the face and neck might secondarily stipulate the deep fascia to be locally thicker.

Deep Cervical Fascia

The deep fascia of the neck has traditionally been subdivided into three layers, a superficial layer (investing layer of the deep cervical fascia), a middle layer (pretracheal fascia), and a deep layer (prevertebral fascia).¹⁰ The investing layer is generally described as a continuous fascial sheath that completely encircles the neck and splits to envelop the two muscles of the posterior triangle (the SCM and trapezius) and two glands (the submandibular and parotid).^30,31 Nevertheless, others have described it as areolar connective tissue, not dense connective tissue.^32,33 Recent study of the investing layer using a reproducible, methodologic approach revealed that there is not a direct fascial linkage between the SCM and trapezius muscles posteriorly,³⁴ nor is there a direct fascial linkage between both SCMs anteriorly.³⁵ It was concluded that the investing layer of the deep cervical fascia “does not exist as a single fibrous sheet as traditionally depicted.” Instead, the presence of nonspecific fibrofatty connective tissue was described.

The current study confirms (1) the absence of a continuous aponeurotic sheet-like deep fascia in the neck, which is often depicted as a thin line in textbooks; (2) the presence of a deep fascial layer, which is thin only over the SCM and the anterior strap muscles but thicker in the other cervical areas; and (3) that the deep fat of the neck is part of this deep fascia and not a separate entity. Moreover, the subdivision of this deep fascia into superficial, medial, and deep fascial layers is not supported by layered dissection or by microscopic investigations.

Parotidomasseteric Fascia

The term parotidomasseteric fascia was introduced into plastic surgery long after classic anatomists had described a continuity between the parotid fascia and the masseter fascia.^11,20,36 Nonetheless, the hypothesis that the masseter fascia splits to invest the parotid gland has been opposed by numerous anatomists who wrote that the parotid is covered superficially by the platysma.^37–45 Jost and Levet argued in 1984: “classic anatomy created a false continuity between the parotid fascia and masseteric fascia.”⁴⁰ Levet later described that the pars zygomatica of the platysma has developed into the parotid fascia (ie, the PAF) in humans, confirming our findings.⁴¹ The dense adherence of the parotid capsule to both the masseter fascia and to the overlying platysma is explained by the embryonic development of the parotid bud within the deep fascia, tightly sandwiched between the platysma and the deeper muscles.^41,46

The current study confirms that there is a difference between the parotid capsule and the aponeurosis covering the parotid capsule. There is not a continuity of the fascia between the aponeurosis over the parotid and the masseter fascia. Instead, the aponeurosis over the parotid is continuous with the platysma and commonly contains muscle fibers. The masseter fascia is continuous with the parotid capsule posteriorly, the temporal fascia and innominate fascia superiorly, the deep cheek fat compartments anteriorly, and the deep cervical fascia inferiorly (Fig. 12). The term PAF was originally proposed as an alternative to the parotid fascia. It better describes this anatomical entity, as it implies continuity with the platysma and does not imply a deep fascial layer.⁴⁷ The term parotidomasseteric fascia could still be used for the deep fascia layer deep to the aponeurotic PAF: the parotid capsule continuing anteriorly as the masseter fascia. However, this is not the aponeurotic layer over the parotid that is sutured to in a face-lift procedure.

Fig. 12.

Artist illustration of the general architecture, variable thickness, and connectedness of the deep fascia. Note the variation of thickness of the deep fascia from over the masseter (MM) and SCM regions to more anteriorly. In the area of the lower masseter, this creates the so-called lower premasseter space, an area that is easy to dissect. However, in the area over the SCM, this results in the so-called cervical retaining ligaments, an adhesion zone between the muscle fascia of the platysma and the SCM. Illustration published with permission from Levent Efe. Copyright © 2022 Levent Efe.

The understanding that the PAF is part of layer 3 (SMAS) explains why inadvertent dissection deep to the PAF leads into the subplatysmal plane more anteriorly and not in a plane deep to the masseter fascia, which would be the case were the PAF continuous with the masseteric fascia. The continuity between the platysma and the PAF gives the surgeon the option to include this part of the fascia in the composite flap, provided the parotid capsule and parenchyma are preserved.⁴⁸ [See Video 2 (online), which demonstrates how an incision and dissection of the PAF leads to a subplatysmal plane and not a plane deep to the masseter fascia. (Courtesy of Dr. Andres Freschi.)] Although the PAF provides a strong handle to suspend the face-lift flap, it implicates the need for precise dissection with the potential risk for the development of parotid fistula.⁴⁹ Moreover, the posterior part of the PAF is commonly used as the place to secure the mobilized composite face-lift flap and might therefore be more useful if left in situ.

Video 2. This video demonstrates the deep fascia on three fresh cadavers with various body mass indexes.

Download video file ^{(64.3MB, mp4)}

Deep Fat Compartments

Rohrich and Pessa introduced the concept of deep fat compartments, as a sequel to their previous body of work in defining and expanding on the series of superficial fat compartments based on dye injection studies.^50–54 The existence of a deep fat layer in the face was previously proposed by Macchi et al., who termed it the deep adipose tissue, in contrast to the subcutaneous superficial adipose tissue.⁵⁵ This was remarkably also mentioned by Mitz and Peyronie in their original article on the SMAS.⁵⁶ The existence of a deep fat layer is consistent with similar findings in other areas of the body.^57–60 In addition, it is consistent with other mammals, as the superficial fascia reportedly invests the panniculus carnosus (evolutionary ancestor of inter alia (latin for “among other things”) mimetic muscles), whereas the deep fascia invests the deeper muscles.⁶¹

The current study confirms the existence of a deep fat layer. However, whereas some previous studies suggested that the deep fat layer or compartment is situated superficial to the deep fascia, our study confirms that the deep fat layer is part of the deep fascia, similar to the original descriptions by classic anatomists (Fig. 12).⁴ The definitive implication of this study is the recognition that in the layered system of the face, there is not a separate layer 4 as described. What was considered as a surgical layer 4 is actually a potential dissection plane within the superficial part of the deep fascia, which when dissected, opens up as the surgical spaces, a finding that will be elaborated on further in a separate article.⁶²

Facial Nerve Branches

To date, reports on the exact location of the facial nerve in relation to the deep fascia have varied significantly. Clinical observations have generally described the buccal, marginal mandibular, and cervical branches deep to the deep fascia, in the face as in the neck.^12–18 The exact location where the different nerve branches perforate the deep fascia has never been described, which can be explained by our current understanding. In clear contrast, multiple anatomical dissection studies have described these branches being in or superficial to the deep fascia.^19–24 In addition, embryologic studies have demonstrated the buccal and marginal mandibular branches developing superficial to the masseter epimysium at 8 to 17 weeks of development.⁶³

Our study confirms that the facial nerve branches travel within the deep fascia from where they emerge from the parotid gland to where they innervate their target mimetic muscles. However, unlike previously thought and described, the nerve branches do not transition abruptly; they simply cross the deep-plane dissection level at predictable locations, having continuously transitioned more superficially from their emergence from the parotid gland. [See Video 3 (online), which shows the marginal mandibular nerve crossing the mandible within the deep fascia.] Only the zygomatic branches that transition with the zygomatic ligaments were observed to transition abruptly at the periphery of the orbicularis oculi muscle.

Video 3. This video demonstrates how an incision and dissection of the PAF leads to a subplatysmal plane and not a plane deep to the masseter fascia. (Courtesy of Dr. Andres Freschi.).

Download video file ^{(67.7MB, mp4)}

Implications for Rhytidectomy

The realization that (1) the deep fascia is a three-dimensional layer, not a thin sheet; (2) the facial nerve branches are embedded within the deep fascia’s loose areolar fibrofatty layer, at one point even lying in its most superficial aspect directly underneath the mimetic muscles; and (3) a deep plane dissection involves dissecting in the most superficial part of the deep fascia (deep to the superficial fascia) has important surgical implications.

Visualization of a fibrofatty layer in the floor of a deep plane face-lift dissection does not imply a safe layer of deep fascia covering the nerves. The dissection could have been performed deep within the deep fascia and still there would be a smooth fibrofatty layer on the floor of the dissection! This is particularly important at the borders of surgical spaces such as at the anterior extent of the lower premasseteric space: as the thin masseter fascia becomes thicker and fibrofatty anteriorly, there is an increasing risk of dissecting within this fibrofatty layer instead of over it, potentially crossing the marginal mandibular nerve branches as they travel over the facial vessels (Fig. 13). [See Video 4 (online), which is an animation of the effect of a deep plane dissection on the facial nerve branches within the masseteric fascia. (Illustration published with permission from Levent Efe. Copyright © 2022 Levent Efe.)]

Fig. 13.

Dissection in the deep fascia can be performed superficial on the underside of the platysma or slightly deeper, with implications for where the dissection plane would cross the facial nerve branches. The more superficial dissection crosses the facial nerve more distally and is therefore safer. It is therefore advised to maintain visualization of the platysma in the roof of the dissection to safely dissect up to the level of the facial vessels. Illustrations published with permission from Levent Efe. Copyright © 2022 Levent Efe.

Video 4. This video shows the marginal mandibular nerve crossing the mandible within the deep fascia.

Download video file ^{(24.3MB, mp4)}

Rather than relying on the visualization of the deep fascia, as is the traditional practice, current findings advocate maintaining the dissection at the underside of the platysma, by dissecting within the most superficial part of the deep fascia (in theory, the plane between the deep fascia and the superficial fascia). The easiest, safest, and most efficient way of doing this is to commence the deep plane dissection in the lower premasseteric space, visualize the platysma in the roof of the dissection, and determine this as the safe plane. This dissection plane can then be maintained over the angle of the mandible down into the neck while keeping the dissection directly at the undersurface of the platysma muscle. This effectively prevents a dissection too deep in the deep fascia and thus the possibility that the dissection plane crosses the transitioning nerves prematurely (Fig. 13). The intrinsically horizontally oriented connective tissue of the deep fascia facilitates continuing this chosen depth of the dissection superiorly. The midcheek can be safely accessed by opening the prezygomatic space deep to the orbicularis oculi muscle, which will help to identify the origin of the zygomaticus major muscle, signaling the surgeon to follow a more superficial plane of dissection over the upper lip levator muscles.

CONCLUSIONS

This study demonstrated that the deep fascia layer is a fibrofatty connective tissue layer (1) that fills the space between the superficial fascia and the deeper structures, being thin only in certain areas, and thicker in all other areas; (2) includes the deep fat layer (including the deep fat compartments); (3) encloses the deep structures, including muscles (eg, SCM fascia), glands (eg, SMG capsule), visceral and vascular structures (eg, pretracheal fascia, carotid sheath), and also extends into these structures providing subdivisions (eg, perimysium in muscles, lobular subdivisions in glands); and (4) invests the facial nerve branches and cervical nerve branches up to where they perforate the superficial fascia to innervate mimetic muscles.

As deep plane face-lift dissection involves dissection within the deep fascia layer, it is fundamental to ensure this dissection is in the most superficial part of this fascia and not deep within it. Early identification of the underside of the platysma as the reference for the correct depth of this plane is most straightforward using the lower premasseteric space.

DISCLOSURE

The authors have no financial interests to disclose. No funding was received for this article.

ACKNOWLEDGMENTS

The authors acknowledge the donors and families of the Body Donor Programs of The University of Melbourne and the Queensland University of Technology who have made this study possible. Specific thanks to Matt Wissemann and Ian Mellor of the Medical Engineering Research Facility for assistance in the laboratory, and Quentin Fogg, associate professor, from The University of Melbourne and Cameron Brown, professor, from the Queensland University of Technology for supervision. The authors thank Erica Mu and Dr. Darryl Whitehead from the School of Biomedical Sciences of the University of Queensland, Tania Henderson and Felicity Lawrence from the Central Analytical Research Facility Histology Laboratory at the Queensland University of Technology, and Rory Bown for providing the pristine histology outcomes. The authors are grateful for the help of Dr. Vladimir Chereminskiy and Daniela Albinus from von Hagens Plastination for providing high-quality sheet plastination slices. Special thanks to the international authorities Drs. T. Gerald O’Daniel, Mario Pelle-Ceravolo, and Giovanni Botti and our local Melbourne Advanced Facial Anatomy Course faculty members Drs. Darryl Hodgkinson, Peter Callan, Tim Papadopoulos, and Naveen Somia for thorough revision of the article.