The Superficial Musculo-Aponeurotic System (SMAS): Does it Really Exist as an Anatomic Entity?

Background: The exact anatomical entity behind the term superficial musculoaponeurotic system (SMAS) remains poorly understood. The different interpretations of the term SMAS by anatomists, surgeons, and histologists have caused confusion. This article aims to provide clarity regarding this term and the relevant anatomy. Methods: A literature review was conducted to uncover the variety of descriptions of the term SMAS. A feasibility study, followed by a conclusive series of standardized layered dissections complemented by histologic analysis and sheet plastination, was performed on 50 cadaver heads (16 embalmed and 34 fresh; mean age, 75 years). Results: Most literature considers the SMAS as layer 3, that is, a musculoaponeurotic layer that separates the subcutaneous fat of the superficial fascia from the deep fat of the deep fascia. The authors’ dissections, histologic analysis, and sheet plastination demonstrated that layer 3 is present only where there are flat mimetic muscles and platysma-auricular fascia over the posterior part of the parotid gland as the evolutionary remnant of the platysma, but not between the flat mimetic muscles. Here, the subcutaneous fat is in direct contact with the deep fat without the interposition of a musculoaponeurotic layer 3. Conclusions: Because of the absence of a distinct and complete layer 3 connecting the flat mimetic muscles, the authors conclude that the SMAS as originally described does not exist as a specific anatomical entity. In retrospect, the surgically created compound layered flap composed of a variable thickness of subcutaneous fat, mimetic muscles (eg, platysma, orbicularis oculi), and a thin layer of deep fascia is what is known as the “SMAS.”