Are There Differences Between the Upper and Lower Parts of the Superficial Musculoaponeurotic System? A Preliminary Biomechanical Study.

Abstract

The superficial musculoaponeurotic system (SMAS) becomes thinner and gradually disappears from the midface. In rhytidectomy, manipulation of the SMAS occurs in the lateral area, and previous research has focused primarily on the SMAS region as a whole. In this preliminary study, the authors compared the viscoelasticity of the upper and lower regions of the SMAS using biomechanical techniques. Two adjacent projection regions of the SMAS were designated: region 1 and region 2, representing the upper and lower parts, respectively. The SMAS tissues from 8 fresh-frozen cadaver heads were cut into 64 samples before biomechanical testing, and the following variables were recorded for subsequent analysis: stress-strain curve, elastic modulus, ultimate strength, and elongation at break. The stiffness of region 1 was markedly greater than that of region 2. Energy dissipation was greater in region 2. Elastic modulus and ultimate strength were significantly higher for region 1, and elongation at break was longer in region 2. The fit curve of the 2 regions deviated markedly. The biomechanical properties of the upper and lower regions of the lateral SMAS are functionally different. Such knowledge will help refine the planning and design of facial surgery and improve outcomes for patients who undergo rhytidectomy.