Mechanical properties and microstructure of the superficial musculoaponeurotic system

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.