Functional evolution of the face

The anatomy of the face is more readily understood when considered from the perspective of its evolution and the function of its components (Fig. 6.1). Located at the front of the head, the face provides the mouth and masticatory apparatus at the entrance to the embryonic foregut, as well as being the location for the receptor organs of the special senses: eyes, nose and ears. The skeleton of the face incorporates a bony cavity for each of these four structures. Those for the special senses have a well-defined bony rim, in contrast to the articulated broad opening of the jaws covered by the oral cavity. The soft tissues of the face, integral to facial beauty and attraction, are in reality, dedicated entirely to their functions.

Fig. 6.1 Functional evolution of the facial skeleton, from the primordial vertebrate, fish through to the primate chimpanzee (center) and to the human. The facial skeleton supports four bony cavities whose size and location relate to their specific function. The eyes move to the front for stereoscopic binocular vision, while the nasal aperture is reduced, due to the lesser importance of olfaction. The ear remains in its original location, at the back of the face. The location of the orbits alters subsequent to cranial growth, which creates a new upper third of the face.

The soft tissue overlying each cavity undergoes modifications to form the cheeks, including the lips, the eyelids, the nose, and the ears. For each there is a full thickness penetration through the soft tissue, around which superficial facial muscles are located for control of the aperture of the functioning shutter. This is most evident for the lids and lips in the human. While the primary function of the sphincteric shutters is to protect the contents of the cavities, they are further adapted to a higher level of functioning for the additional roles of expression and communication. The degree of precision required for this important secondary function requires the muscles to be more finely tuned and the soft tissue fixation modified, to allow mobility. The balance between these two opposing functions, movement and stability, is integral to the facial structure. Aging brings with it a change of the youthful balance, leading to an altered expression on activity and at rest. It is a major surgical challenge to restore the youthful balance following rejuvenation surgery and to have normal dynamic appearance.

Regions of the face

The traditional approach to the face in thirds (upper, middle and lower) while useful, limits conceptualization, as it is not based on the evolving structure. The significant muscles of facial expression are all located on the front of the face (anterior aspect) predominantly around the eyes and mouth, where their effect is seen in communication. For these functional reasons the anterior aspect of the face contains the more delicate expressive areas, which are prone to developing aging changes (Fig. 6.2).

Fig. 6.2 Regions of the face. The fixed lateral face (shaded) overlies the masticatory structures and is separated from the mobile anterior face by the vertical line of facial ligaments (red). These ligaments are, from above: temporal, lateral orbital, zygomatic, masseteric and mandibular. The muscles of facial expression are within the anterior face. The midcheek is split obliquely into two separate functional parts in relation to the two adjacent cavities. The periorbital part above, (blue) and the perioral part below (yellow), share the midcheek and meet at the midcheek groove (oblique dotted line).

In contrast, the lateral face is relatively immobile as it passively overlies the structures to do with mastication, which are all deep to the investing deep fascia. These are the temporalis and masseter on either side of the zygomatic arch, along with the parotid and its duct. The only superficial muscle in the lateral face is the platysma in the lower third, which reaches no higher than the oral commissure. Internally, a distinct boundary separates the mobile anterior face from the lateral face. The vertically oriented line of retaining ligaments attached to the facial skeleton forms this boundary (Fig. 6.2).

From the perspective of priorities in rejuvenation surgery, the midcheek is the most important area of the face, because of its prominent central location between the two facial expression centers, the eyes and the mouth. The periorbital and the perioral parts overlap in the midcheek (Fig. 6.2). The periorbital part overlies the body and orbital process of the zygoma, while the perioral part overlies the maxilla, a bone of dental origin. The functional parts are inherently mobile and meet at the relatively immobile boundary that extends in an oblique line across the midcheek. This is the midcheek groove formed by the dermal extensions of the zygomatic ligaments (Fig. 6.3).¹

Fig. 6.3 The internal structure of the midcheek is revealed by its surface anatomy when aging changes are present. The two functional parts of the midcheek relate to the underlying cavities and are separated by the oblique line of the midcheek groove (3) which overlies the skeleton. The midcheek has three segments. The lid–cheek segment (blue) and the malar segment (green) are within the periorbital part and are adjacent to the nasolabial segment (yellow) in the perioral part, which overlies the vestibule of the oral cavity. The three grooves defining the boundaries of the three segments interconnect like the italic letter Y. The palpebromalar groove (1) overlies the inferolateral orbital rim and the nasojugal groove (2) overlies the inferomedial orbital rim, then continues into the midcheek groove (3). Mendelson BC, Jacobson SR. Surgical anatomy of the midcheek: facial layers, spaces, and the midcheek segments Clin Plast Surg 2008;35:395–404.

The soft tissue of the anterior face is further subdivided according to: where it overlies the skeleton and: where it overlies a bony cavity. The soft tissue is modified where it forms the lid and the mobile cheek because there is no underlying deep fascia. The transitions that define the part of the cheek overlying bone (the malar segment), and the mobile extensions (lower lid and the mobile cheek, nasolabial segment) are not visible in youth due to the shape of the youthful midcheek, which has a compacted rounded fullness. Subsequently, these transitions do become visible due to aging laxity in the midcheek.

The facial nerve in relation to regions of the face

The level in which the facial nerve branches travel relates to the region of the face (Fig. 6.4). In the lateral face below the zygomatic arch the branches remain deep to the investing deep fascia. In the anterior face (and above the lower border of the zygoma) the branches are more superficial in relation to their muscles. The transition in levels occurs at the retaining ligament boundary, which is the last position of stability before the mobile anterior face. The nerves are protected here as they course outward to their final destination

Fig. 6.4 The layers of the face. The five layers of the scalp are a prototype of facial anatomy and the simpler basis for the more complex structure elsewhere on the face. Layer 4 is the most changed layer, consisting of alternating spaces and ligaments. The course of the facial nerve changes level at the ligamentous boundary transition from the lateral to the anterior face. Mendelson BC, Jacobson SR. Surgical anatomy of the midcheek: facial layers, spaces, and the midcheek segments; Clin in Plast Surg 2008;35:395–404.

Layers of the face

The principles of facial structure can be summarized quite simply:

Fig. 6.5 The ligaments of the multi-link fibrous support system of the face can be likened to a tree. This system attaches the soft tissues to the facial skeleton; it links all layers of the face. The retaining ligaments are attached to the periosteum and deep muscle fascia and fan out via a series of branches into and through the SMAS. In the outer part of the subcutaneous layer, the increased number of progressively finer retinacular cutis fibers securely grasp the dermis.

It should be remembered that the complexity of the facial structure is entirely due to the bony cavities and their functional requirements. Transitional anatomy occurs at the boundary of the cavities, as in the scalp where the complexity of the glabella occurs where the forehead adjoins the orbital and nasal cavities. Here, the deeper facial muscles and related retaining ligaments attach to the skeleton.

Details of the layers

Layer one – skin

The structural collagen of the dermis is the outermost part of the fibrous support system and is intrinsically linked, both embryologically and structurally, with the collagenous tissue of the deeper layers. The thickness of the dermal collagen relates to its function, and tends to be in inverse proportion to its mobility. The dermis is thinnest on the eyelids and thickest on the forehead and nasal tip. The thinner, more mobile dermis is susceptible to an increased tendency for aging changes.

Layer two – subcutaneous

The subcutaneous layer has two components: (i) the subcutaneous fat, which provides volume and mobility, is supported by (ii) the fibrous retinacular cutis that connects the dermis with the underlying SMAS. Both components vary in amount, proportion and arrangement according to the specific region of the face.

In the scalp, the subcutaneous layer has a uniform thickness and consistency of fixation to the overlying dermis, whereas, over the face proper, the subcutaneous layer has considerable variation in thickness and attachment. In the high function mobile areas bordering an aperture such as the pretarsal part of the eyelid and the lips, this layer is compacted and subcutaneous fat is not present, so that the layer appears to be non-existent.

Each of the three midcheek segments has a distinctly different thickness of subcutaneous fat. The subcutaneous layer is thinnest in the lid–cheek segment adjacent to the lid proper. In the malar segment the layer is moderately thick and uniform, whereas it is markedly thicker in the nasolabial segment, which has the thickest layer of subcutaneous fat of the face. Where the subcutaneous fat is thicker, the retinaculum fibers are lengthened and more prone to weakness and distension. The thick subcutaneous fat in the nasolabial segment is named the malar fat pad, which is confusing terminology given that its position is predominately medial to the prominence of the zygoma in the perioral part of the midcheek^3,4 (Fig. 6.2).

Within the subcutaneous layer, the attachment to the overlying dermis is stronger than on its deep surface, due to the tree-like arrangement of the retinacular cutis fibers (Fig. 6.5). In superficial, i.e. subdermal, dissection of the subcutaneous layer, many fine retinacula cutis fibers are encountered. At the interface with the underlying layer 3, there are fewer, though larger fibers and less subcutaneous fat, which appears not to descend fully to the interface where it overlies the superficial muscles, orbicularis oculi and platysma.

This explains why surgically the subcutaneous layer can be more easily dissected off the outer surface of the underlying muscle layer (orbicularis oculi and platysma) than over other parts of layer 3.

The retinacular fibers are not uniform across the face, but vary in their orientation and arrangement according to the region. This variation mirrors the anatomy of the underlying 4th layer. As will be more apparent when the 4th layer is discussed, the line of retaining ligaments continue vertically through the subcutaneous layer to form septae, that form boundaries which compartmentalize between more mobile areas.⁵ Accordingly, where the subcutaneous layer overlies spaces (in the 4th layer) there are no vertically oriented subcutaneous ligaments extending through. In contrast, the retinacular fibers overlying the spaces have a predominantly horizontal orientation, being in strata-like layers that are less restrictive to underlying movement.

Clinical correlation 

The variation in the arrangement of the retinacular cutis fibers accounts for the variability in ease of subcutaneous dissection between different parts of the face. Where the subcutaneous dissection overlies a space and the retinacular cutis fibers are more horizontal, the subcutaneous layer tends to separate relatively easily, often with simple blunt dissection. Where the subcutaneous dissection directly overlies a facial ligament, the vertical septae are responsible for a firmer adhesion between the SMAS and the dermis. Sharp dissection is usually required for release here.

Layer three – musculo-aponeurotic

To fulfill its functional role, the face contains skeletal muscle within its soft tissue structure. These ‘intrinsic’ muscles of facial expression are fundamentally different to skeletal muscles beneath the deep fascia, which move bones, because they move the soft tissues of which they are a part. All the muscles of the face are within this layer, enclosed to a varying degree within a fascial covering and lining. The muscles are all derived from the embryonic second branchial arch. The muscle precursors migrated into the facial soft tissues in a series of laminae, each lamina being innervated by its own branch of the facial nerve. While the definitive muscles have subsequently lost continuity with their origin, the facial nerve branches remain, like the vapor trail of an airplane, as an indicator of the migratory path.

Fig. 6.6 Evolution of the facial muscles. The migratory path of the evolving muscles, including their connections and the multiple levels of the muscles, explain the definitive location of the facial nerve branches. The mandibular lamina splits into two trunks around the oral cavity. The upper trunk, the infraorbital lamina separates early for the developing midcheek while the mandibular lamina continues into the lower third. The two laminae later reconnect at the modiolus, which explains the two buccal trunks of the facial nerve. The infraorbital lamina in turn splits around the orbital cavity as well as branching to different depth levels.

In the anterior face, the migrated muscle masses are mainly located over and around the orbital and oral cavities. The double innervation of corrugator supercilii demonstrates the dual origins of the muscle from the supraorbital as well as the infraorbital migrating muscle mass.

In the prototype scalp, the third layer demonstrates key principles about the facial muscles. The superficial muscle, occipito-frontalis, moves the overlying soft tissues including the skin of the scalp and forehead. While the muscles have a minimal area of bony origin, which is remote (on the superior nuchal line), they have an extensive area of insertion into the overlying soft tissues.

The fibrous sheath enclosing the frontalis and occipitalis is continuous across the entire scalp, whereas the enclosed muscles are discontinuous. Where the fascia is present without intervening muscle, the superficial and deep layers of the sheath are apposed and fused to form the galea aponeurotica. This is the basis for the aponeurotic part of the 3rd layer. The superficial fascial layer is thin where it overlies the muscle, and in areas such as over the forehead, muscle fibers extend into the subcutaneous layer. In contrast, the deep layer of the fascia is thicker, more supporting and provides a gliding surface at the interface with the underlying 4th layer. The original description of the SMAS (superficial muscloaponeurotic system) in 1976 was essentially a description of this 3rd layer, as is applies to the mid and lower thirds of the face.⁶ The flat superficial muscle component predominates in some areas of layer 3, while in areas without muscle the aponeurotic element predominates.

When a scalp flap is elevated, the flap naturally separates from the periosteum following release of minimal attachments in layer 4. A scalp flap, being a natural fusion of the outer three layers, is a composite unit, both anatomically and functionally. The fibrous component of the outer three layers is the superficial fascia of the face. The SMAS is the deepest of the three layers of the composite unit. In the mid and lower face the composite structure is also present, although less obviously apparent.

Principle

A composite flap is a naturally integrated anatomical structure.

Layer 3 is a continuous generic layer of the face, which for descriptive purposes has different names to locate the particular part of the superficial fascia. Galea is the name of the scalp part and temporoparietal fascia where this layer extends over the temple, whereas over the orbital rim and upper cheek it is the orbicularis muscle and its fascia.

The definitive muscles in level 3 have a layered arrangement. The broad flat muscles form the superficial layer that covers the anterior aspect of the face: frontalis overlies the upper third and orbicularis oculi the middle third. The platysma, over the lower third extends onto the lateral face, presumably related to jaw movement, which functionally dominates the lower third. The superficial muscles are more closely related to the overlying subcutaneous layer than they are to the deeper structures. The superficial flat muscles have a minimal direct attachment to the bone. They are indirectly stabilized to the skeleton by a ligament, located at the lateral border of the muscles. The frontalis is fixed by the superior temporal ligament along the superior temporal line, the orbicularis oculi is stabilized by the main zygomatic ligament at its inferolateral border and the platysma is stabilized at its upper border by the upper key masseteric ligament (Fig. 6.15).

The composite three-layer structure of the face suggests it would be no less logical to use the SMAS as a ‘surgical carrier’ for the overlying soft tissue layers, than it is with a scalp flap.

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