In vivo analysis of human skin anisotropy by polarization-sensitive optical coherence tomography

Skin anisotropy is an important issue for plastic surgeons and cosmetics science. Cleavage lines, such as Langer's lines and relaxed skin tension lines (RSTLs), have been proposed as keys to understanding skin anisotropy. Collagen, a dominant dermal structural protein, forms a fibrous structure believed to play an important role in skin anisotropy. There have been few reports, however, on the relationship between the orientation of collagen fiber and the direction of the cleavage line. Collagen fiber has birefringence, a property analyzable in skin in three dimensions by high-speed polarization-sensitive optical coherence tomography (PS-OCT). Here we used PS-OCT for an in vivo analysis of anisotropic changes in the dermal birefringence of mechanically deformed human skin. The dermal birefringence of the forehead increased significantly when the skin was shrunk perpendicular to the RSTL and increased significantly when the skin was shrunk parallel to the RSTL. En-face images of dermal birefringence revealed that both shrinking perpendicular to and stretching in parallel to the RSTL promoted the formation of a macro rope-like collagen structure. Moreover, the birefringent change under shrinking conditions perpendicular to the RSTL showed negative correlation to Ra, a skin roughness parameter. These results suggest that PS-OCT enables the in vivo evaluation of skin anisotropy.