Mechanical Properties and Microstructure of the Superficial Musculoaponeurotic System

Abstract

Because of the widespread reliance on SMAS tightening procedures in present-day face lift surgery, a study was undertaken to examine the physical properties and microscopic structure of both virginal (40 specimens) and reoperated (8 specimens) SMAS tissue. The findings could be of practical value to the surgeon and are reported herewith: First, the SMAS is a composite fibrofatty layer comprising collagen and elastic fibers interspersed with fat cells. Second, microscopic appearance shows a considerable amount of elastic fibers in close relationship to the collagen fibers. Third, on scanning electron microscopy, the collagen fibers in the virginal SMAS show a convoluted appearance similar to that found in the dermis. In the reexcised SMAS tissue, there is some evidence of parallelization of the collagen fibers as seen in the stretched dermis. Fourth, mechanical testing (Instron), i.e., a series of loading/unloading tests at various rates and amplitudes, and stress relaxation tests were performed on samples of preauricular skin and SMAS. These indicated definite viscoelastic properties for both sets of specimens, with the tendency of an increased stiffness and a reduction in viscoelastic effects on repeated working of the samples. Overall, the mechanical behavior of both tissues was somewhat similar, the viscoelastic effects in SMAS being less pronounced. A nonlinear viscoelastic model is under development to represent the behavior of both tissues. The implications of these results may help to explain the slackening effect observed in some postoperative patients.

Mechanical characterisation of in vivo human skin using a 3D force-sensitive micro-robot and finite element analysis

“…Human skin is a complex material, which exhibits nonlinear stress-strain, anisotropic, and viscoelastic characteristics (Daly 1982;Schneider 1982;Silver et al 2001;Har-Shai et al 1996;Kvistedal and Nielsen 2009). In addition, skin in vivo is in tension, which varies according to location, age, and person (Alexander and Cook 1977).…”

Nonlinear elastic-viscoplastic constitutive equations for aging facial tissues

“…Previous research (Har-Shai et al 1996 has considered mechanical experiments to study the viscoplastic properties of the skin and the underlying supportive tissue SMAS (superficial musculoaponeurotic system). Mechanical constitutive equations were developed in a simple one-dimensional form (Rubin et al 1998) and in a more general three-dimensional form (Rubin and Bodner 2002).…”

“…2 Constitutive equations Rubin and Bodner (2002) developed nonlinear threedimensional constitutive equations for facial tissues which are valid for arbitrarily large deformations and which produce reasonable agreement with the experimental data of Har-Shai et al (1996). In that work, the tissue was modeled as a composite material with a fully elastic component and a dissipative component which contains both elastic and viscous elements (similar to a Maxwell model in viscoelasticity theory).…”

“…It was shown by Rubin et al (1998) that the experimental data of Har-Shai et al (1996) for cyclic loading and relaxation tests on relatively fresh excised samples of SMAS and skin can be modeled using elastic-viscoplastic constitutive equations without a fully elastic component for distortional deformations. However, as indicated by Rubin and Bodner (2002), the presence of such a fully elastic component is required to cause these tissues to return to their unstressed shapes when unloaded for sufficient time.…”

“…which have been determined by the available experimental data (Har-Shai et al 1996). More specifically, it is assumed that under load the deep fat will continue to flow and so the effect of distortional deformation on the elastic component was eliminated by taking m 2 = 0 for this tissue.…”