Multi-directional in vivo tensile skin stiffness measurement for the design of a reproducible tensile strain elastography protocol.

Background/aims: Elastography is a promising new medical imaging modality, displaying spatial distribution of biomechanical properties such as local tissue strain response to an applied stress. To develop a reproducible test protocol for skin elastography, the effect of various parameters on skin stiffness measurements was investigated.

Methods: The parameters investigated were: history of skin loading before test loading (preconditioning), direction of test loading (anisotropy) and posture (pre-stress). If a sample of skin is loaded, its stiffness will temporarily change. Finally, the reproducibility of skin stiffness and anisotropy measurements, using the developed techniques, was investigated.

Results: By measuring how the stiffness changed with different time delays between loading cycles, the time required for healthy skin to return to its original pre-loaded state was in the region of 125 s. A second finding, which supports and extends previous work, was that skin stiffness varied with direction, by an approximate factor of 2, and that anisotropy was less apparent with preconditioned skin than non-preconditioned skin. Study of the effect of posture showed that care needs to be taken over which stiffness measure is used. For example, measurement of the load at a given displacement was found to be highly dependent on posture, whereas measurement of the phase III stiffness was independent of posture.

Conclusion: It was shown that when the measurement variables and methods of analysis were standardised, skin stiffness could be measured reproducibly enough to distinguish between the stiffest and softest directions, and that these methods allowed formation of skin elastograms free from confounding influences.