What is the SMAS (Superficial Musculoaponeurotic System)?

The Superficial Musculoaponeurotic System (SMAS)

The Superficial Musculoaponeurotic System (SMAS) is a complex fibrous network that connects facial muscles to the dermis, divides facial fat into superficial and deep layers, and serves as a critical surgical landmark to avoid facial nerve injuries during facial procedures.

Anatomical Structure and Composition

The SMAS consists of three distinct sublayers:

1. Superficial layer: A thin membrane directly connecting to the septa in the subcutaneous fat 1
2. Intermediate layer: Contains connective tissue, adipose tissue, and facial muscles 1
3. Deep layer: Connective tissue in contact with the sub-SMAS structures 1

This fibromuscular layer effectively divides the subcutaneous fat into two distinct layers:

• Superficial fatty layer (above the SMAS)
• Deep fatty layer (below the SMAS) 2

Regional Variations

The SMAS demonstrates significant morphological heterogeneity across different facial regions, with five distinct types identified:

• Type I: Preparotideal region 3
• Type II: Chin and lip region 3
• Type III: Eyelid region 3
• Type IV: Temporal and parotideal region 3
• Type V: Cervical region 3

These segments have distinct surgical characteristics determined by:

• Presence of deep gliding planes
• Adhesions through fibrous insertions 4

Anatomical Connections

The SMAS forms a continuous network throughout the face and neck:

• Connects to the superficial temporal fascia cranially
• Connects to the platysma caudally
• Incorporates the frontal muscle and peripheral parts of the orbicularis oculi 5

Clinical Significance

The SMAS is particularly important in facial surgical procedures for several reasons:

1. Surgical landmark: Serves as a critical reference point to avoid facial nerve injuries during facial surgeries 5

2. Facelift procedures: Understanding the segmental nature of the SMAS is essential for successful deep plane face lift procedures 4

3. Imaging assessment: Ultra-high frequency ultrasound (U-SMAS) can visualize the different morphological characteristics of the SMAS across facial regions, aiding in pre-procedural planning 3

Controversies in SMAS Anatomy

Despite its clinical importance, several aspects of SMAS anatomy remain debated:

• The exact extent of the SMAS throughout the face
• Whether the SMAS extends into the upper lip
• The continuity between the SMAS and the superficial temporal fascia 5

Historical Context

The term "superficial musculoaponeurotic system" was first introduced by Mitz and Peyronie in 1976, based on surgical findings during facelift procedures. The "musculo+aponeurotic" terminology derives from the histological presence of muscle cells (including the risorius) and aponeurotic cells within the same surgical layer 2.

The SMAS concept evolved from earlier descriptions of the superficial fascia in the head and neck region, which was previously described as dividing the subcutaneous tissue and enclosing voluntary muscles in its deep portion 2.

Understanding the complex anatomy of the SMAS is essential for facial surgeons to achieve optimal outcomes in facial rejuvenation procedures while minimizing the risk of complications.