Purpose:

The Superficial Musculoaponeurotic System (SMAS) is a heterogenous supportive lining for facial soft tissue. It has favorable inherent viscoelastic properties which resist deformational forces and provides a platform to surgically correct the effects of aging.   The face has a predictable pattern of aging related to regional areas of soft tissue descent tethered by points of fixation. The SMAS likely has similar regional differential properties which contribute to this soft tissue descent. The purpose of this study was to define the regional viscoelastic properties of the SMAS in relation to the aging face and its surgical correction in facelift surgery.

 Method:

Fresh cadaver heads were evaluated within 72 hours of death. Cutaneous landmarks were marked prior to dissection and translated onto the hemiface SMAS flap prior to elevation. Regional samples of SMAS were divided into 3 hemiface groupings: lateral, mid cheek and medial. All samples were evaluated ex vivo for bursting strength, stress relaxation and creep with the Instron System tensiometer (Model 5565, Canton, MA). Bursting strength was defined as the force (N) required for tissue rupture, with bursting pressure (psi) calculated using force per tissue unit area. Stress relaxation and creep were respectively defined as the tension (N) created with a constant stretch (mm) over time, and the distance of tissue stretch (mm) with a constant load over time.

 Results:

Twelve fresh cadaver hemi-facial specimens were included in the study (N=12). The average age of the cadavers was 55. The SMAS flap had a uniform thickness of 5.2 mm in each region of the face. There were consistent regional differences in the viscoelastic properties of the SMAS. A significant reduction in bursting strength and bursting pressure was seen in a lateral (38.73 N / SD 20.33) to medial (14.56 N / SD 6.56) direction (p<0.0001). Likewise, stress-relaxation and creep had significant regional differences. The lateral SMAS specimens were found to inherently relax to 45 % of their original tension and elongate 15% when subjected to predetermined facelift conditions. Mid cheek and medial SMAS had a significant increase in failure rate, stress relaxation and creep when subjected to similar forces as those placed on the lateral SMAS.

 Conclusion

The strength of the SMAS decreases from lateral to medial on the face, which correlates with the pattern of aging. The force required to deform the SMAS varies by region, but once deformed the SMAS responds uniformly. The lateral SMAS in facelift surgery can reliably withstand the load for facial soft tissue repositioning, though increased tension may contribute to early recurrence in facelift surgery.