The Human Labial Superficial Musculoaponeurotic System (SMAS): Anatomy, Imaging, and Biomechanical and Sensory Implications-A Narrative Review.

Abstract

The superficial musculoaponeurotic system (SMAS) is a fundamental concept in facial anatomy and surgery, but its organization within the human lips remains incompletely defined. This narrative review critically reassesses the anatomical evidence for a labial SMAS and its regional configuration. Published anatomical, histological, and imaging evidence was critically reviewed. The literature review is complemented by original qualitative histological and imaging observations of the human lips, included exclusively for morphological correlation with the published anatomical evidence and not as an independent quantitative study. The available evidence does not demonstrate a classical, readily separable laminar SMAS within the lips. Instead, it supports a three-dimensional connective tissue network extending from the oral mucosa to the skin and intimately incorporating the orbicularis oris muscle. This organization is associated with neurovascular structures and may have biomechanical and sensory implications, although these functional interpretations are not directly demonstrated by the present material. The labial connective tissue architecture differs from the classical laminar SMAS described in other facial regions and may represent its regional equivalent. Further three-dimensional anatomical studies are required to establish this interpretation conclusively. By defining the normal fibromuscular architecture of the lips and the limits of its visualization, this review provides an anatomical framework for interpreting lip imaging while emphasizing that a labial SMAS is not currently a validated diagnostic or prognostic marker.