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BIOMECHANICS OF SKIN FLAPS
INTRODUCTION
Scientific and clinical research in the areas of wound healing and the biomechanics of skin flaps has changed skin flap design from erratic, unsure, and artistic to consistent, scientific, and artistic. Today, skin flap design emphasizes vascular patterns, skin physiology, and the biomechanical characteristics of cutaneous tissue. The unique mechanical properties of skin influence blood flow and flap survival and thus are integral in the design of local skin flaps. The study of soft tissue biomechanics has increased the precision and reliability of local cutaneous flaps, supporting the use of local flaps as the procedure of choice for the reconstruction of small and medium-sized facial defects that cannot be closed primarily.
BIOMECHANICAL PROPERTIES OF CUTANEOUS TISSUE
The mechanical properties of a material are described by the relationship that exists between a force applied to a specimen and the resultant deformation of the specimen as a function of time. In engineering, these mechanical properties are defined by the quantities stress (force per unit of original cross section) and strain (change in length divided by the original length). The stress/strain ratio thus measures the relationship between force and elongation by said force for a given cross-sectional area. The stress-strain relationship is independent of the dimensions of the specimen and is a property of the material itself. Uniform materials, such as titanium, have a linear stress-strain relationship. Their mechanical properties can be described by the proportionality constant (C = stress/strain). If the stress-strain relationship does not vary as a function of time, the material is said to be elastic.
Unlike many engineering materials, skin is a heterogeneous substance composed of a network of dissimilar materials. Skin is a living tissue, capable of proliferation, change, and response to physical stimulus. It is not surprising that the mechanical properties of skin are unique compared with other materials. For simplicity, skin can be said to have three basic mechanical properties: nonlinearity, anisotropy, and viscoelasticity.
Nonlinearity
This mechanical behavior of skin is attributable to its heterogeneous nature. Skin is composed of a series of interrelated networks that are intimately entwined. Structurally important components in the dermis include collagen fibers, elastic fibers, nerve fibers, capillaries, lymphatics, and ground substance. Skin without tension has collagen fibers distributed throughout the dermis in a haphazard, diffuse manner. Collagen is woven in a multidirectional array without preferential orientation of thick and thin bundles. Numerous connections between collagen bundles form a continuous network without visible free ends. There are no restrictive attachments among adjacent collagen bundles that seem free to glide, relative to each other. Elastic fibers loop spirally around collagen and attach at multiple points along each bundle. Elastic fibers function as a type of energy storage device, bringing stretched collagen back to a relaxed position. Structural proteins are found in the interstitial fluid that acts as a lubricant enabling movement on the one hand and acting as a buffer to resist rapid change on the other.
A typical stress-strain curve for isolated skin is shown in Fig. 3.1 . The shape of the curve is similar to stress-strain and force-advancement curves obtained from tests on skin flaps and other soft tissues. It is apparent from the graph that skin and skin flaps have nonlinear stress-strain relationships. The mechanical behavior can be divided into three separate regions: (1) an initial flat section in which considerable extension occurs with little force, (2) an intermediate section of rapid transition, and (3) a terminal section where little extension is possible despite great increases in applied force. Histologic examinations of the sequential stages of skin extension provide an explanation for the nonlinear nature of this graph. During initial deformation, randomly oriented collagen and elastic fibers are stretched in the direction of the applied force. Collagen fibers do not bear a significant burden until the bundle is completely straight in the direction of the applied force. As a result, there is little resistance to initial deformation, and the stress-strain relationship is nearly linear and elastic (region 1). As deformation progresses, additional collagen fibers are recruited into the load-carrying role and resistance rises. The low stress required for the high strain inherent in region 1 allows the small movements resulting from flexion and extension at joints, enabling freedom of movement and natural facial expression. Region 2 is the strain at which many collagen fibers transition from non–load carrying to a load-carrying role. At high-stress loads (region 3), virtually all the dermal collagen fibers are aligned in the direction of the applied force. At this point, no further deformation is possible because of the inextensible nature of fully oriented collagen. Wholly oriented collagen (region 3) preserves the structural integrity of skin by limiting deformation during accidental stresses. Raposio and Nordstrom studied the stress-strain curve of human scalp tissue during serial excision of the scalp and found scalp to be initially linear from 0 to 500 g, gradually reducing in compliance from 500 to 1500 g, and rapidly increasing in stiffness from 1500 to 5000 g.
Anisotropy
There are enormous individual variations in the extensibility of human skin—differences between the slim and obese, young and old, male and female. The shape of the force-advancement curve is further influenced by edema, inflammation, hormonal conditions, and body weight. It is the variation in skin tension within the same individual, however, that is the most interesting to the reconstructive surgeon. On the face, the skin is lax around the eyes and cheek, whereas it is taut on the nose, chin, and forehead. In each of these locations, there are directional (anisotropic) considerations for skin movement. In most regions of the body, there is skin tension in every direction, but the degree of tension is greatest parallel to the relaxed skin tension lines (RSTLs). Pierard and Lapiere showed that the presence of skin tension lines depends on the interaction between elastic fibers and collagen fibers and the anchorage of collagen fibers to each other. An incision made at a right angle to the RSTL will gape widely and is more likely to produce a widened or hypertrophic scar. The lines of maximal extensibility (LME) run perpendicular to the RSTLs and represent the direction in which closure can be performed with the least tension. , Therefore elliptical excisions should be performed parallel to the RSTLs to place the maximum closure tension parallel to the LME and local skin flaps should be designed so that the donor site closure is parallel to the LME.
RSTLs are not to be confused with Langer’s lines. Langer’s lines of cutaneous tension correspond to the orientation of forces that cause a circular puncture mark in the skin of a cadaver to distort into a fusiform shape. The orientation of Langer’s lines is often distinct from the creases formed in living subjects when the joints are placed in a relaxed position. RSTLs follow the longest and straightest furrows formed when the skin is relaxed and are not visible features of the skin. Rather, RSTLs derive from the act of pinching the skin and observing the furrows and ridges that form. When skin tension is preserved at the time of biopsy, a small number of extracellular collagen and elastin bundles have been demonstrated to run parallel to the RSTLs.
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