Current Reconstructive Surgery (Lange Current)

Book Preface

Achievement of adequate wound repair, resolution of healing, and prevention of pathologic scarring is dependent not only on a complete understanding of this process but also on an understanding of wound biomechanics. This chapter will first focus on certain features of wound biology and biomechanics where early intervention may prevent healing complications and problem scarring. The fundamentals of wound repair, emphasizing choice of incision, suture, and technique will be explored to further enhance decision making at the time of tissue restoration. In some cases, failure of resolution of the normal proliferative response, which occurs at the resolution phase of normal wound healing, can lead to proliferative scarring; however, prevention steps are possible provided the surgeon is knowledgeable about the pathogenesis of hypertrophic scars. Due to the challenging treatment of hypertrophic scars and keloids, both surgical and nonsurgical treatments will be discussed in addition to prevention strategies.

PRINCIPLES OF WOUND HEALING

Wound Biomechanics

The plastic surgeon must plan each case relative to the biomechanical properties of skin. The principle constituent of skin is collagen, which is a structural protein organized in sheets with distinct spatial structural components. Elastin fibers are also present in smaller amounts, giving the skin both elasticity and viscoelastic properties. In other words, in response to constant force, the skin will stretch and not fully return to its unstretched state. Skin Tensions. Placement of skin incisions and resultant scar formation are dependent on both static and dynamic skin tensions. Static tension is the inherent force which stretches the skin over the underlying structures when the body is motionless. 1 The static tension varies enormously within individuals based on anatomic location—for instance, it is quite high over the sternum and minimal in the groin. As a person ages, not only does skin relax and lose elasticity, but the magnitude of tension also changes. As mentioned earlier, the collagen fibers have a spatial orientation, and this organization is refl ected in static lines of maximal skin tension called Langer lines. An incision along one of these lines will result in a tensionfree closure with minimal static forces pulling against the scar. The outcome is a narrow, fine, camoufl aged scar. In contrast, placing an incision against the directional orientation of static skin tension will result in a wider, more visible scar. This is the sequela of continuous high-magnitude forces pulling at the healing wound margins. 2

There has been an increased understanding at the cellular level of the impact of mechanical forces on signal transduction mediated through fi broblast attachments to the collagen matrix via integrin receptors. 3 , 4 Fibroblasts respond to tension with cellular proliferation, and collagen synthesis, while fi broblasts relieved of tension undergo programmed cell death (apoptosis). 5 If tension is sustained within a healing wound over several weeks, the excessive fi brosis may lead to pathologic scarring. Dynamic skin tensions are caused by a variety of changing forces such as voluntary muscle activity and movement across joints. These tensions may cause contractures in areas of high skin mobility if incisions are placed perpendicular to Langer lines. 2 The skin in these areas must offer a significant amount of extensibility and, since scars lack elasticity, mobility is restricted.

Tensile Strength. The breaking strength of a wound is a measure of the maximum amount of stress that a wound can withstand without dehiscing. Tensile strength is breaking strength per unit of cross-sectional area. The rate of which a wound gains tensile strength has served as a useful index of wound healing. 6 Tensile strength is dependent on collagen accumulation and its organization. Although collagen accumulation is maximal at 17 to 20 days postwounding, tensile strength is only a fraction of its original value. This can be attributed to lack of cross-linking and organization. Over 6 months, collagen synthesis and breakdown occurs, with a gradual improvement in collagen organization and increase in wound strength. At 2 weeks, wounds have gained less than 10% of their eventual strength, and at 6 weeks,
wound strength is still only 30% to 50% of normal. Wounds never completely gain the strength of unwounded skin. In fact, at the end of a year, a wound is only about 80% of its original strength. 7 Most absorbable sutures retain their tensile strength for only 3 weeks, and if the incisions are closed under tension, scar widening will not be prevented. An alternative, more effective strategy is to place permanent sutures which will stay in at least 6 months.