Histological, SEM and three-dimensional analysis of the midfacial SMAS - New morphological insights

The superficial musculoaponeurotic system (SMAS) of the midface has a complex morphological architecture, and a multitude of controversial opinions exist regarding its in vitro appearance and clinical relevance. The aim of this study was to investigate the three-dimensional architecture of the midfacial SMAS.

Histological and SEM analyses were performed on tissue blocks of the skin, subcutaneous tissue and mimic musculature of the midfacial region between the anterior parotid gland pole and lateral to the nasolabial fold and tissue blocks of the skin, subcutaneous tissue and parotid fascia. Blocks were collected postmortem from six formalin-fixed donor bodies. Serial histological sections were made, stained with Azan and digitized. Three-dimensional reconstructions and visualization of the tissue blocks were performed using AutoCAD.

Two different SMAS architectures were found in the midfacial region: parotideal (type IV) and preparotideal (type I) SMAS. Type I SMAS showed three-dimensional interconnecting fibrous chambers embracing fat tissue lobules that cushioned the space between the skin and mimic musculature. Fibrous septa divided the mimic musculature surrounding the muscular bundles. Beneath the mimic muscular level, SMAS septa were oriented parallel to the muscular plane. Above the mimic muscular plane, SMAS septa were oriented perpendicularly, inserted into the skin. Type IV SMAS showed a parallel alignment of the fibrous septa to the skin level, anchoring the skin to the parotid fascia, presenting lymphatic nodes in the fat tissue compartments. The fat cells of the SMAS were enveloped in a fibrotic membrane at the border of the fibro-muscular septa. The SMAS blood supply comprised two subcutaneously epimuscularly spreading anastomosing vascular systems.

Midfacial SMAS represents a functional unit with physical and immunological tasks appearing in two different morphological architecture types. A well-defined nomenclature is needed to prevent controversy.