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

Abstract

INTRODUCTION: The exact anatomic 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 study aims to provide clarity regarding this term and the relevant anatomy.MATERIALS 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 histology and sheet plastination was performed on fifty cadaver heads (16 embalmed, 34 fresh, mean age 75 years).RESULTS: Most literature considers the SMAS as layer 3, i.e., a musculoaponeurotic layer that separates the subcutaneous fat of the superficial fascia from the deep fat of the deep fascia. Our dissections, histology and sheet plastination demonstrated that a 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 musculo-aponeurotic layer 3.CONCLUSION: Due to the absence of a distinct and complete layer 3 connecting the flat mimetic muscles, we conclude that the SMAS as originally concepted 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 (platysma, orbicularis oculi, …), and a thin layer of deep fascia, is what is known as the "SMAS".LEVEL OF EVIDENCE: No level of evidence is needed for Cadaver Study Articles.</p