Anatomy, Skin, Superficial Musculoaponeurotic System (SMAS) Fascia

Article Author:
Zackary Whitney
Article Editor:
Patrick Zito
Updated:
9/12/2018 8:16:48 PM
PubMed Link:
Anatomy, Skin, Superficial Musculoaponeurotic System (SMAS) Fascia

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 coordinated muscular contraction of the face and providing a functional role of movement for expression [3].

Embryology

The superficial facial musculature is derived from the second arch mesenchyme which migrates during development and forms a premuscular laminae. This premuscular laminae eventually gives rise to the mandibular, temporal, infraorbital, and cervical laminae during the eighth week of embryogenesis. The platysma muscle originates from part of the cervical lamina which encloses the inferior portions of the parotid gland and cheek, while the SMAS derives from the superior portion [4].

Blood Supply and Lymphatics

The transverse facial artery supplies blood to a large region of the lateral malar area of the face including the SMAS. Because this artery runs directly through the SMAS, there is a risk of transection of this vessel during elevation of the SMAS during certain facial procedures, and great care must be taken to avoid harm to not just the transverse facial artery, but to the other neurovascular structures that lie in close proximity to this area. The rich arterial supply of the SMAS also comes secondarily to the musculocutaneous perforators of the facial artery. Small lymphatic vessels located deep to the SMAS mainly drain to the preauricular or submandibular lymph nodes, then subsequently reach the anterior cervical chain.

Nerves

The branches of the facial nerve are by far the most anatomically relevant nerves that lie within proximity to the SMAS and the other facial musculature and fascial layers that associate with it. The facial nerve exits the skull inferior to the tragus of the ear, and its proximal branches, mainly the temporal, zygomatic, and marginal mandibular nerves course deep to the SMAS after exiting the parotid gland. Despite anatomic variation that exists, the superior masseteric retaining ligament and zygomatic ligament form a groove by which the upper zygomatic branch of the facial nerve passes [5]. Another nerve worth noting is the great auricular nerve, which originates from the cervical plexus, passes inferiorly to cross the sternocleidomastoid muscle about 6-cm inferior to the auditory canal, and courses just deep to the SMAS along the pathway of the external jugular vein. There are only sensory branches from the trigeminal nerve that course superficial the SMAS.

Muscles

The SMAS is evident in the buccal, temporal, zygomatic, and platysma regions. Thus, each of the corresponding mimetic muscles coordinates with, or at least serves as, an anatomical border to the SMAS. It also invests smaller and more intricate muscles, namely the orbicularis oculi, orbicularis oris, the occipitofrontalis, and the levator labii superioris muscle. The forehead, nasolabial folds, and nasal regions are not generally involved in the SMAS, although several anatomical variants involving these muscles have been described in the literature.

Physiologic Variants

Several physiologic variants of the SMAS have been described, but the majority of differences seem to exist due to lack of consistent large-scale cadaveric studies utilizing histological and macroscopic dissection. One study by Khawaja et al. analyzed the SMAS, or what they termed the superficial musculoaponeurotic fatty system (SMAFS), during 800 facelift surgeries. From this study they concluded that six distinct SMAFS variants exist, including 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 differences in the deeper fatty layers of the SMAS, and some variants may be due to congenital anomaly or atrophy and breakage from repeated botox injections or even steroid use. They determined that the variant of SMAFS has an impact on the procedure and outcome of facelift surgery and that the correct operative technique of plicating, lifting, debulking, and attaching the SMAFS to the bony periosteum according to the type of SMAFS present, is necessary for appropriate cosmetic and surgical success.

Surgical Considerations

The SMAS plays a key role in the rhytidectomy, commonly known as the facelift procedure. Surgical maneuvering and tightening of the SMAS allow for complete facial rejuvenation, although the SMAS is of more value to the improvement of the lower third of the face than the midface. The clinical value of the SMAS is broadly recognized in aesthetic surgery and is utilized in procedures to improve age-related drooping of facial fat, whereby the SMAS is pulled superiorly to elevate superficial dermal and muscular structures [6]. It has also been previously reported that over 50% of all rhytidectomies consists of at least some type of SMAS manipulation and dissection, which highlights the clinical and surgical significance of the SMAS in many cosmetic and even dermatologic surgical procedures [7].

Other Issues

As described previously, there have been numerous attempts to delineate and provide a more clear anatomical definition of the SMAS. However, inconsistencies exist not just regarding its anatomical location, but of its general morphology and basic terminology as well. There are even some who question its existence [8]. One of the main disagreements involves zygomaticus muscle investiture by the SMAS. Machi et al. reported through histological studies that the SMAS does indeed invest the zygomaticus muscle, while Gassner et al. argue that it does not [3][9]. Another inconsistency often discussed is its continuity with the parotid fascia. Earlier studies are rather inconclusive on this point, while the majority of recent studies agree that the SMAS is an entirely separate layer that lies superficial to the fascia of the parotid gland or parotideomasseteric fascia [8]. The anterior relationship to the nasolabial fold is also controversial, but most agree there is continuity with the nasolabial fold and even extension into at least some part of the orbicularis oris. Despite these inconsistencies, the SMAS certainly plays a significant role in providing facial elasticity and support, while also facilitating coordination of muscular activity of the cheek.