Visco-elasticity and anisotropy are important properties of the skin that relieve external forces. The anisotropy of the skin is important especially in the field of plastic surgery, because the direction of the incision influences scar formation. A practical method for understanding the skin’s anisotropy has, therefore, been sought. Several types of skin lines including the Langer’s line and relaxed skin tension lines have been proposed in the surgical field [1
]. Langer created a map of the directions in which wounds created on the skin of human cadavers using a round tipped awl would be stretched, and these are called Langer’s lines [1
]. Langer’s lines, however, are influenced by postmortem rigidity and not accurately applicable to living skin. Kraissl proposed lines called wrinkle lines, which are defined by tracing patients’ wrinkles based on photographs [4
]. He recognized that these lines were perpendicular to muscle action and scars in the wrinkles were least inconspicuous. However, wrinkle lines are age dependent. Borges proposed the relaxed skin tension lines (RSTLs) [2
]. These lines represent the direction of skin tension in a relaxed state, and are defined as follows. If the skin is pinched and then relaxed, a few deep linear furrows (RSTLs) perpendicular to the pinching direction are formed, while an S-shaped pattern is formed when the skin is not completely relaxed. Although wrinkle lines and the relaxed skin tension lines are considered to be the best guides for surgical incision [3
], these lines do not perfectly agreed with each other. So far, we do not have a complete understanding of the anisotropic properties of in vivo
It is commonly believed that skin anisotropy is due to the contraction of muscles [4
] and the dermal structure, the directions of which determine anisotropic extensibility and elasticity of the skin. There are many reports regarding the relationship between dermal structure and skin lines. In the case of non-extension examination, Cox demonstrated that collagen fibers, a major dermal structural protein, run along Langer’s lines [5
]. Recently, however, some papers demonstrated that the relationship between the direction of collagen fibers and that of Langer’s lines varies depending on the body site and motion [6
]. Pierad et al.
found thin elastic-like fibers, along the relaxed skin tension lines between collagen fibers [8
]. Alternatively, in the case of extension examination, stretching the skin promotes the running of collagen fibers [9
]. Redge demonstrated that stretching along Langer’s lines promotes the running of collagen fibers more than stretching across them [10
]. However, there are few reports regarding the anisotropic change of dermal structure under stretching along other directions than the Langer’s line. So far, we do not completely understand the structural mechanism of skin anisotropy. The main difficulty lies in the histological approaches. In the reports described above, excision of the skin results in a disappearance of the skin’s internal force. It is, therefore, important to make a noninvasive in situ
analysis of the change of dermal structure.
Polarization-sensitive optical coherence tomography (PS-OCT) and second-harmonic-generation (SHG) microscopy are promising solutions for this problem. Their methodologies are not only in vivo
and in situ
, but also capable of visualizing depth-resolved collagen property. SHG-microscopy enables in vivo
observation of dermal collagen fibers and analysis of its direction using the polarizing property of a SHG signal [11
]. Yasui et al.
described the evaluation of collagen fiber orientation using excised animal and human skin [12
]. However, the dimension of the evaluated area was of the order of submillimeters, which might be too small to facilitate a discussion of the more macroscopic properties of the skin, such as Langer’s lines and RSTLs. A few studies of in vivo
observation of human skin using SHG microscopy have been reported [14
]. However, there are no reports regarding in vivo
investigation of collagen orientation. Alternatively, PS-OCT enables the three dimensional analysis of birefringence due to collagen fibers and has a wide transversal measurement area. PS-OCT has been widely applied to in vivo
imaging in the ophthalmologic [17
] and dermatological fields [20
]. We have also evaluated the skin damage associated with photo-aging by measuring human dermal birefringence in vivo
] and have reported the relationship between the progress of wrinkle and depth-dependent dermal birefringence in the crow’s feet area in vivo
]. In both studies, depth-resolved en-face birefringence maps showed the heterogeneous distribution of birefringence in the skin in vivo
and it was indicated that this map would be useful for understanding skin anisotropy [22
In this study, we further examine the birefringence property of in vivo human skin by PS-OCT, especially in order to understand skin anisotropy. The birefringence of human skin is investigated under several conditions of mechanical stress, i.e. stretching and compressing in several directions. This study can provide additional knowledge about the relationship between dermal properties and the above-mentioned skin lines including Langer’s lines and RSTLs.