Estimating dynamic skin tension lines in vivo using 3D scans

Abstract

Skin tension lines, defined as the lines of maximal tension, often provide guidelines for surgical incisions and pattern design of tight-fit, functional clothing. In this work, we are interested in developing methods for finding personalized, dynamic skin tension lines (DSTL) in a non-invasive manner. We base our study on the kinematic analysis of point markers that are colored on the skin. By tracking the motion-induced displacement of point markers, we locally analyze the skin deformation and numerically compute the maximum tension directions. Then, finding DSTL is transformed to the problem of finding continuous, interpolating locally computed lines of the maximum tension directions. Compared to existing methods, our method involves less invasive measurement on the skin, and is equipped with computational methods for identifying dynamic skin tension automatically. Consequently, our method is convenient to carry out, less prone to erroneous measurement, and repeatable. Our experiments have been successfully carried out on, though not limited to, lower body skins of male subjects.

Introduction

Langer’s line is a term used to define the direction within the human skin along which the skin has maximum tension. It is named after Langer, who punctured numerous holes at short distances from each other into the skin of a cadaver with an awl that had a circular-shaped tip, and noticed that the resultant punctures in the skin had ellipsoidal shapes. From this testing he determined “line directions” by smoothly connecting the longer axes of the ellipsoidal holes [1]. These lines, which are also termed as “cleavage lines”, show the directions of greatest skin tension. The surgical importance of Langer’s lines has been first noted by Kocher [2]. He advised that surgical incisions are carried out in the direction of Langer’s lines, because the scars would be least conspicuous and heal better. Since then, many surgeons have searched for the ideal guide to use for elective incisions, and there exist more than thirty differently named guidelines today [3].

One problem with Langer’s line has been that they represent lines of cleavage in cadavers and not lines of relaxed skin tension. Indeed, the facial musculature of a cadaver is often not relaxed, as some display a wide-eyed stare or wide-open mouth. Accordingly, surgeons have investigated less invasive methods to determine relaxed skin tension lines in-vivo, among them are Kraissl’s lines [2] and relaxed skin tension lines [4]. Kraissl’s line coincides with wrinkle lines, although not always, and tend to be perpendicular to the muscle action. Thus, Kraissl’s lines are found by studying the direction of underlying muscle fiber. Relaxed skin tension lines [4], [5] are the tension lines which follow the furrows formed when the skin is relaxed. In practice, they are derived from the act of pinching the skin and observing the furrows and ridges that are formed. Pinching parallel to the resting skin tension lines results in fewer and higher furrows than if it were done in the perpendicular direction. Although this method can provide personalized guidelines for elective incisions relatively easily, it may be subjective and may not be convenient, as it requires pinching.

We note that most of these lines are static in nature. That is, forces causing the tension on the skin are inherent in the skin or the body contents itself and not related to muscular action. However, surgeons and scientists agree that skin tension is dependent not only on the enclosed body content but also on joint movements. Similarly to the static tension lines, dynamic skin tension lines (DSTL) are defined as the maximum tension lines formed when the muscles are in action. To the best of our knowledge, the first work on the dynamic tension lines are by Bush et al. [6], who have measured the orientation of maximal strains on different location of the face with five different facial expressions. However, their experiments have been based on the excision of naevi from the face and neck, which is invasive and requires multiple subjects in order to be able to cover the entire face area.

The primary objective of this research is estimating personalized dynamic skin tension lines in vivo, in a non-invasive manner. We base our study on the kinematic analysis of point markers that are colored on the skin. By tracking the motion-induced displacement of point markers, we locally analyze the skin deformation and numerically compute the maximum tension directions. Then, finding DSTL is transformed to the problem of finding continuous, interpolating lines of the maximum tension directions. Smooth, continuous dynamic skin tension lines have been drawn on the skin surface by adopting interpolation and path-line integral over the marker meshes. Based on computational methods, our method is convenient to carry out, less prone to erroneous measurement, and repeatable. Our experiments have been carried out on, though not limited to, the skin on the legs of male subjects.