The Superficial Musculoaponeurotic System, often referred to as SMAS, is a continuous layer of tissue located within the face. This fibromuscular layer lies beneath the skin and the layer of subcutaneous fat. It plays a role in facial movement and maintaining contour.
Anatomy and Location
The SMAS is a unified fibromuscular layer positioned directly beneath the skin and subcutaneous fat, forming one of the five distinct layers of the face. It extends from the galea aponeurotica in the scalp, superiorly, to the platysma muscle in the neck, inferiorly. This continuous sheet also connects with the temporoparietal fascia at the zygomatic arch and becomes indistinct laterally below the zygomatic arch.
The composition of the SMAS varies across different facial regions, containing muscle fibers in some areas and fibrous or fibroaponeurotic tissue in others. It incorporates various facial muscles, including portions of the platysma, the orbicularis oculi, and the zygomaticus muscles. The SMAS also connects to the dermis through vertical septa, and it does not directly attach to bone.
The SMAS is particularly prominent in the buccal (cheek), temporal (temple), zygomatic (cheekbone), and platysma regions. Its key anatomical boundaries include the zygomatic arch superiorly and the platysma muscle inferiorly. Directly anterior to the SMAS is the superficial fatty tissue, while deeper to it lies the parotidomasseteric fascia, though a potential space exists between these two layers.
How the SMAS Functions
The SMAS serves as a unified system, connecting various facial muscles. This fibromuscular structure transmits the contractions of facial muscles to the overlying skin. It acts as an aponeurotic mask covering the mimetic facial muscles, which are responsible for expressing emotions.
The SMAS provides structural support and suspension for the overlying skin and subcutaneous fat. It helps to maintain the face’s contour and resists the downward pull of gravity. The ability of the SMAS to tension different parts of its connected layer and move them in a planar direction allows for the generation of diverse facial expressions.
SMAS and the Aging Face
Over time, the SMAS undergoes changes that contribute to facial aging. The elasticity and firmness of the SMAS gradually diminish due to factors like gravity, sun exposure, and the degradation of collagen and elastic fibers. This weakening of the SMAS’s supportive framework leads to a gradual descent of facial tissues.
The loss of elasticity and the downward movement of the SMAS contribute to the formation of common aging features. These include the development of jowls, which are sagging skin and fat below the chin or jawline. The deepening of nasolabial folds, the lines extending from the nose to the corners of the mouth, also occurs as the SMAS loses its ability to maintain tension and support. Additionally, laxity in the neck, often referred to as “turkey neck,” is a direct result of the SMAS and platysma muscle weakening and descending.
The SMAS in Cosmetic Surgery
The SMAS is a primary target in facial cosmetic procedures, especially facelifts, due to its role in facial structure and aging. Surgeons manipulate this layer to achieve a more youthful rejuvenation. The general principle involves lifting, tightening, and repositioning the SMAS layer along with the overlying skin to restore facial contours and address sagging.
Different surgical techniques exist to address the SMAS. SMAS plication involves folding and suturing the SMAS layer without making an incision. Another approach, SMAS imbrication, involves making an incision in the SMAS, with removal or transposition of a portion.
Deep plane facelifts involve lifting the skin and SMAS as a single unit for comprehensive rejuvenation of the midface, jawline, and neck by releasing retaining ligaments. The extended deep plane facelift further releases these ligaments for more extensive rejuvenation. These techniques aim to reposition the underlying structural changes rather than just tightening the skin, leading to longer-lasting and more natural results.