1 Soft Tissue Biomechanics and Physiology
Summary
This chapter focuses on the biomechanical properties of the skin with respect to wound healing, importance of the aesthetic units and subunits of the face with respect to reconstruction, as well as the importance of relaxed skin tension lines with respect to design or your repair.
Keywords: biomechanics, subunits, relaxed skin tension lines, creep, wound healing, wound breaking strength
An understanding of the biomechanics of soft tissue, the vascular supply to skin flaps, the aesthetic units and subunits of the face, and the relaxed skin tension lines of the face is important in the design of an effective repair for a specific facial defect.
Skin is anisotropic and nonlinear and has time-dependent properties. Anisotropic means that the skin’s mechanical properties vary with direction. Relaxed skin tension lines (RSTLs) are the lines of minimal tension of the skin; incisions parallel to them are under the least possible tension while healing (Fig. 1.1). Perpendicular to the RSTLs are the lines of maximal extensibility (LMEs). A fusiform excision made parallel to the RTSLs and closed in the direction of the LMEs will result in the least closing tension and the best scar.
Skin is nonlinear; as it is stretched, progressively more force is required to deform it. These changes are typically described as a stress–strain curve, where stress represents force per unit area, and strain represents the change in length divided by the original length (Fig. 1.2). In section I of the curve, a relatively small stress produces a large strain. This section of the curve corresponds primarily to the deformation of the delicate elastic fiber network; the loss of these fibers with age or sun exposure results in a shift of the curve to the right. In section II of the curve, a progressively larger amount of force is required to stretch the skin, which correlates with a progressive change in orientation of the collagen fibers, from relatively random orientation to one parallel to the direction of the force. In section III of the curve, a large amount of force is required to obtain any increased length. The tension required to recruit more tissue at this point in the stress–strain curve is detrimental to wound healing and should be avoided by turning to other solutions, such as skin flaps or grafts, rather than primary closure.
Skin has time-dependent properties and is not totally elastic. Repeated stretching of a section of skin results in a response change termed “hysteresis,” in which the stress–strain curves are shifted to the right. Stress relaxation describes the decrease in skin tension seen over time if a segment of skin is stretched to a given length and maintained at that length. Creep describes the increase in length of skin seen over time when a given tension is applied to that segment of skin. The histologic and physiologic changes associated with creep are realignment of collagen fibers to a parallel orientation, fragmentation of elastic fibers, tissue dehydration by the displacement of fluid, and a migration of tissue in the direction of the vector of applied force. These properties are used in immediate tissue expansion. New skin is not formed in this procedure; rather, existing skin is recruited to allow moderate-sized defects to be closed with less tension.
