Introduction

The superficial musculoaponeurotic system, or SMAS, is often described as an organized fibrous network composed of the platysma muscle, parotid fascia, and fibromuscular layer covering the cheek. This system divides the deep and superficial adipose tissue of the face and has region-specific morphology. Anatomically, the SMAS lies inferior to the zygomatic arch and superior to the muscular belly of the platysma.

The fibromuscular layer of the SMAS integrates with the superficial temporal fascia and frontalis muscle superiorly and with the platysma muscle inferiorly. The SMAS is even often described as a fibrous degeneration of the platysma muscle itself. In reality, a precise anatomical definition of the SMAS is unclear and has been thoroughly debated since its first description by Mitz and Peyronie in 1976.[1]

Structure and Function

The SMAS connects the facial muscles to the dermis, and its purpose is to transmit, distribute, and amplify the activity of all facial muscles.[2] It has a close relationship with the most superficial fascial planes of the face and neck area. Macchi et al. describe the SMAS as a central tendon for a coordinated muscular contraction of the face and providing a functional role of movement for expression.[3][4] The SMAS can be considered as an aponeurotic 'mask' that overlies the key mimetic facial muscles as one of the five facial layers (skin, subcutaneous fat, musculoaponeurotic, loose areolar tissue, and periosteum). It is thus uniquely positioned to reflect and respond to global movements and thus expressions of the face.[5]

The key anatomical boundaries of the SMAS are the zygomatic arch superiorly and the platysma inferiorly [6]. Directly anterior to the SMAS is the subcutaneous superficial fatty tissue of the face, and directly deep to it is the parotidomasseteric fascia with an intervening potential space between the two connective tissue layers.

The SMAS is not uniform across its breadth; cadaveric dissection studies have led to the classification of two distinct types of SMAS occurring in distinct face areas.[7] Type 1 SMAS is the part of the SMAS in the region lateral from the nasolabial fold, whereas type 2 SMAS occupies the area medial to the nasolabial fold. The fundamental difference between these two areas of SMAS is the cellular architecture relevant to the underlying structures in these two areas. Tye 1 SMAS overlies the key facial expression muscles and is composed of large fat lobules, which are entwined in a network of fibrous septa. This area is relatively loose, enabling mobility of the underlying muscles. Type 2 SMAS, on the other hand, has only scarce fat cells and is thus a denser tissue with a more rigid connection to the tissue underlying it.[7]

Embryology

The superficial facial musculature originates from the second arch mesenchyme, which migrates during development and forms a pre-muscular lamina. This pre-muscular laminae ultimately gives rise to the mandibular, temporal, infraorbital, and cervical laminae during the eighth week of embryogenesis. The platysma muscle originates from a part of the cervical lamina, which encloses the inferior portions of the parotid gland and cheek, while the SMAS derives from the superior portion.[8]

Blood Supply and Lymphatics

The superficial musculoaponeurotic system receives its blood supply from the transverse facial artery, which also supplies blood to a broad region of the lateral malar area of the face. As this vessel courses directly through the SMAS, there is a risk of transection of this vessel during SMAS elevation. SMAS elevation is a step in facelift surgeries, technically known as rhytidectomy, and also for facial reconstruction following parotidectomy procedures.[9] Therefore great caution is vital to evade any imminent danger to the transverse facial artery and even the neurovascular structures that lie close to this area. 

The SMAS also partially receives arterial supply from the musculocutaneous perforators of the facial artery.[10]

The small lymphatic vessels lying deep to the SMAS mainly flow into the preauricular or submandibular lymph nodes, which then drain into the anterior cervical chain. These nodes are situated around the anterior jugular vein and then drain ultimately to the deep cervical chain, which is related to the internal jugular vein and descends to the mediastinum.

Nerves

The branches of the facial nerve (CN VII) are the most anatomically relevant nerves that lie in the vicinity of the SMAS and the facial muscles & associated fascial layers. The facial nerve exits the skull inferior to the tragus of the ear. The proximal branches of the facial nerve, primarily the temporal, zygomatic, and marginal mandibular nerves, course deep to the SMAS after exiting the parotid gland. Despite the anatomic disparity that exists, the superior masseteric retaining ligament and zygomatic ligament form a groove through which the upper zygomatic branch of the facial nerve traverses.[11]

Another nerve of relevance to the SMAS is the greater auricular nerve. This nerve originates from the cervical plexus, passes inferiorly to traverse the sternocleidomastoid muscle about 6 cm inferior to the auditory canal, and runs just deep to the SMAS along the course of the external jugular vein. 

The only nerves which traverse superficial to the SMAS are the sensory branches from the trigeminal nerve.

Muscles

The SMAS is considerably apparent in the buccal, temporal, zygomatic, and platysma regions. Thus, the corresponding muscles correlate or serve as an anatomical border to the SMAS. It lies directly anterior to the parotid gland and masseter muscle, which is a key muscle of mastication with a superficial and deep part.[12] The superficial part originates from the maxillary process of the zygomatic bone and the inferior border of the zygomatic arch. The deep part of the masseter originates from the inferior surface of the zygomatic arch. The two parts of the muscle unite into a common insertion at the angle and lateral surface adjacent to the angle of the mandible. The parotid gland itself lies in the region inferior to the zygomatic arch and extends posteriorly to the external auditory meatus and ramus of the mandible, and anteriorly is bounded by the masseter muscle.[13] The parotid gland is invested by a separate parotid fascia that is separated from the SMAS by a layer of intervening fat.[14]

The SMAS additionally invests smaller and more intricate muscles. This muscle group includes the orbicularis oculi, orbicularis oris, occipitofrontalis, and the levator labii superioris muscle. The orbicularis oculi are divided into orbital and palpebral sections, which are both attached in a complex manner to the surrounding structures of the eye. The orbital section is attached medially to the medial canthal tendon and laterally to the lateral palpebral raphe. Superiorly it interdigitates with the frontalis muscle.[15] The palpebral section is further subdivided into preseptal and pretarsal sections, which attach to the lacrimal sac and associated fascia, and the anterior and posterior lacrimal crests, respectively. The orbicularis oris muscle was previously considered a sphincter-like structure surrounding the mouth. Its deep fibers originate from the modiolus bilaterally, the fibrous confluence of the facial muscles.[16] 

The superficial fibers of the orbicularis oris originate directly from other muscles of facial expression - namely the depressor anguli oris, zygomaticus majos, zygomaticus major, levator labii superioris, levator labii superioris alaeque nasi and transversus nasi.[17] The interaction between the SAS and these underlying facial muscles is critical and central to its importance in facial expression and treatment of the signs of facial aging.[5]

The forehead, nasolabial folds, and nasal regions are usually not included under the SMAS, although literature reporting several anatomical variants involving these muscles are available.

Physiologic Variants

There are several physiologic variants of the SMAS, but the bulk of differences seem to exist due to the lack of uniform large-scale cadaveric studies employing histological and macroscopic dissection. The study by Khawaja et al. analyzed the SMAS during 800 facelift procedures. They used the term SMAFS which stands for the superficial musculoaponeurotic fatty system, instead of SMAS. From this research, they concluded that six definite SMAFS variants exist. These six variants are - membranous, fatty, mixed (membrane-fatty, fleshy-fatty, among others), island (broken), fleshy, and fibrous.

The variants described in this study are mainly due to the diversity in the deeper fatty layers of the SMAS. Some variants may be due to congenital anomalies or atrophy and breakage from repeated botox injections or even steroid use. They determined that the variants of SMAFS had an impact on the procedure and outcome of facelift surgery. The operative technique of plicating, lifting, debulking, and attaching the SMAFS to the bony periosteum must be according to the type of SMAFS present, which is necessary for suitable cosmetic and surgical success.[4]

Surgical Considerations

The SMAS plays a vital role in the rhytidectomy, frequently known as the facelift procedure. Surgical maneuvering and tightening of the SMAS allow for complete facial rejuvenation. The SMAS is more valuable to the enhancement of the lower third of the face than the midface. The clinical utility of the SMAS is primarily recognized in aesthetic surgery and employed in procedures to reverse age-related drooping of facial fat. These surgeries involve the superior elevation of the SMAS to elevate superficial dermal and muscular structures. Case reports suggest that 50% of all rhytidectomies involve at least some type of SMAS manipulation and dissection. The studies portray the clinical and surgical significance of the SMAS in cosmetic and dermatologic surgical procedures.[18][19][20] Furthermore, the infiltration of neuromodulation agents (e.g., botulinum toxin) to tighten the mimetic facial muscles can directly alter the conformational shape of the overlying SMAS, thus achieving the desired results in surgical intervention but in a less invasive manner.[5]

Clinical Significance

In addition to the ramifications in cosmetic surgery, the SMAS and other facial muscles can demonstrate involvement with several pathological processes. Denervation and atrophy can occur with pathologies that affect the facial nerve, including Bell palsy, myasthenia gravis, myotonic dystrophy, and some neoplasms. These neoplastic conditions include lymphoma, adenoid cystic carcinomas, and dermoid cysts. Facial muscles, including the SMAS, can demonstrate abnormalities following infection or trauma.[7]

Other Issues

As described previously, there have been numerous attempts to delineate and provide a clearer anatomical definition of the SMAS. However, inconsistencies exist regarding its anatomical location, general morphology, and basic terminology. There are even some researchers who doubt its existence. One of the main disagreements involves zygomaticus muscle investiture by the SMAS. Machi et al. reported through histological studies that the SMAS indeed invests in the zygomaticus muscle, while Gassner et al. argue that it does not. Another inconsistency often discussed is its continuity with the parotid fascia. Previous studies are somewhat inconclusive on this point.

In contrast, most recent studies concur that the SMAS is an entirely separate layer that lies superficial to the fascia of the parotid gland or parotideomasseteric fascia. The anterior relationship to the nasolabial fold is also controversial. Still, most agree there is continuity with the nasolabial fold and even extension into at least some part of the orbicularis oris. Despite these discrepancies, the SMAS certainly plays a vital role in providing facial elasticity and support while also facilitating the coordination of muscular activity of the cheek.[21]

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Disclosure: Zackary Whitney declares no relevant financial relationships with ineligible companies.

Disclosure: Megha Jain declares no relevant financial relationships with ineligible companies.

Disclosure: Felix Jozsa declares no relevant financial relationships with ineligible companies.

Disclosure: Patrick Zito declares no relevant financial relationships with ineligible companies.