Evaluation of biomechanical properties of human skin

We have investigated in vivo how various viscoelastic parameters that describe the mechanical properties of the human skin may vary with age. Accordingly, we have used a mechanical device that records the torsional extensibility of the skin. When submitted to a low torque, the time-response curve of the skin affords the determination of the immediate extensibility (UE), the immediate recovery (UR), the viscoelastic part of the deformation (UV), the elastic recovery (UR/UE), and the creep relaxation time (tau). Because the skin thickness varies with age and primarily governs the mechanical properties, it was measured through an ultrasound technique at the same sites (forearm) where the torque was applied. The results show that the skin maintains its thickness and extensibility up to the seventh decade as opposed to its elasticity or recovery capacities, which decrease from an early age. The viscous part of the deformation is constant through life, whereas the creep relaxation time decreases linearily with age. Except for skin thickness, no differences in these parameters between men and women were detected. The significance of these results are discussed in terms of structure alterations. The determination of the elastic recovery (UR/UE) appears to be a parameter of choice for illustrating skin aging.

Skin induration is difficult to quantify. In patients with systemic sclerosis (scleroderma), a clinical skin severity score (0 = normal skin; 3 = maximal induration) has been in use for several years for assessment of skin involvement and therapeutic outcome. The purpose of this study was to find a more objective method to assess skin hardness in patients with scleroderma. Finger pads, forearms, and thighs of 12 sequential nonselected patients with scleroderma and eight normal control subjects were scored by an observer blinded as to group, and quadruplicate determinations of the degree of skin induration at these anatomic sites were made with a hand-held type O durometer. Consistently, these anatomic sites except for the forehead showed a direct relation between the skin severity score and the hardness measured with the durometer. The best results were obtained from the index finger pad where, compared with uninvolved skin, an increased skin severity score was associated with higher durometer readings (p = 0.0032, p = 0.0002, and p = 0.0005 for skin scores of 1, 2, and 3, respectively). The durometer is an effective and reliable method to assess skin hardness.