Skin anisotropy in vivo and initial natural stress effect: a quantitative study using high-frequency static elastography.

Background: Most studies on skin anisotropy are carried out in an "average" context, e.g. with an extension/compression test on the skin in vivo, the elastic modulus being estimated for the stretched zone overall. Furthermore, the natural tension of the skin is not taken into account either in the experimental protocols or in the models studied. In this study, a battery of elastographic tests was carried out to investigate forearm skin anisotropy quantitatively by measuring local through-thickness strain in the dermis by means of high frequency elastography. The biaxial tensile effect and influence of the natural forearm skin tension were also analyzed.

Methods: The elastographic test was carried out using a combination of an extensiometer device and a real time ultrasound scanner. The extensiometer was used to apply a stress cycle, i.e. stretching, holding and release, on the internal face of the forearm in vivo. Parallel to the mechanical test, 2D real time ultrasound acquisitions were performed to track local displacements and to estimate local through-thickness strain using an elastographic algorithm. Local through-thickness strain kinetics were then extracted and used as a quantified indicator. We studied anisotropy in two stretching situations: stretching parallel and stretching perpendicular to Langer's lines. Elastographic tests were performed for two upper arm/forearm angles, i.e. outstretched and bent forearm, in order to check the natural skin tension effect.

Results: The results showed the effectiveness of elastographic tests to describe and quantify the anisotropic behavior of the forearm skin in vivo. Elastographic results were distinctly different according to forearm positions: the anisotropic behavior was reversed from the bent forearm to the outstretched forearm.

Conclusions: The local anisotropic behavior of the skin in vivo could be easily studied using the elastographic test. Nevertheless, the initial skin tension is an important parameter which strongly affects the mechanical behavior of the skin in vivo, in particular its anisotropic properties.