Proposal for an Expanded Classification of the Superficial Musculoaponeurotic System (SMAS) in the Human Forehead,...

The superficial musculoaponeurotic system (SMAS) is fundamental for facial soft tissue support and surgical rejuvenation. Although its morphology in the midface and neck is well characterized, the structure of its cranial extension to the forehead remains a subject of terminological uncertainty. The aim of this study was to conduct a detailed histological and immunohistochemical examination of the forehead supporting structures to characterize their morphology and propose an expanded, region-specific classification of the SMAS.

Full-thickness tissue specimens (n = 30) were obtained from five standardized facial regions (parotid, buccal, temporal, frontal, and cervical) from 12 male and 18 female body donors (aged 25-70 years). Specimens were processed for histological analysis using hematoxylin and eosin, van Gieson staining, and Masson's trichrome. Immunohistochemical staining for S100 protein was used to identify neural structures. Morphometric analysis was performed on digitized sections to quantify interseptal distances and the depth of superficial nerve trunks.

The analysis confirmed the established SMAS types (I-V) in the cheek, parotid gland, and neck, confirming the validity of the method. Two distinct, sequentially arranged structures were identified on the forehead, proposed as new types. Type VI (neurovascular arborization) is a discrete fan-shaped structures with a central collagen core surrounding a neurovascular bundle, showing positive S100 staining. These structures, spaced approximately 2.2 mm apart, function as true retaining ligaments. Type VII (fibroseptal) SMAS patterns is vertically oriented, purely fibrous septa (retinacula cutis) connecting the aponeurosis to the dermis, devoid of neural elements, and spaced approximately 9.2 mm apart. Importantly, the superficial nerve trunks were located at an average depth of only 1.09 mm (range: 0.57-1.97 mm) from the skin surface.

This study identified two novel SMAS patterns in the forehead-neurovascular arborization (type VI) and fibroseptal (type VII)-supporting the expanded functional seven-type classification of the SMAS. The extremely superficial location of the forehead nerves (average 1.1 mm) defines a critical "danger zone" for aesthetic procedures. These findings provide a refined anatomical basis for improving the precision and safety of both surgical and minimally invasive facial procedures.