CHAPTER 18 Jim Schumacher, DVM, MS, Diplomate ACVS, MRCVS and Jacintha M. Wilmink, DVM, PhD Skin grafting is indicated for full‐thickness skin defects that cannot be closed using conventional suturing techniques or reconstructive surgical techniques and that require a long time to heal by contraction and epithelialization. This applies, in particular, to wounds at or below the carpus and hock because these wounds are incapable of significant contraction and because epithelialization of these wounds is slow and results in a fragile, unsightly scar. In those cases, grafting is less expensive than a lengthy period of bandaging. The most common type of skin graft applied to wounds of horses is the free skin graft. Techniques of free skin grafting include island grafting, sheet grafting, and Meek micrografting, which is a combination of sheet grafting and island grafting. Types of island grafts include pinch, punch, and tunnel grafts. Each type of free skin grafting has advantages and disadvantages, and the type selected, therefore, depends on the circumstances, such as the necessity for the best possible cosmesis, the size or location of the wound, available instrumentation, experience of the surgeon, and the horse owner’s finances. Skin grafting should be considered for full‐thickness skin defects that cannot be closed using conventional suturing techniques or reconstructive surgical techniques and that would require a long time to heal by contraction and epithelialization. This applies, in particular, to wounds at or distal to the carpus and hock because these wounds are capable of only limited contraction and because epithelialization of these wounds is slow and results in a fragile, unsightly scar (see Chapter 1). The purpose of this chapter is to describe the principles and various techniques of free skin grafting, the advantages and disadvantages of each technique, and the effects of grafts on the wound. Proper care of the wound, before and after grafting, is described so that failure of the graft can be avoided. The two basic types of skin grafts are the pedicle skin graft and the free skin graft. A pedicle skin graft, or skin flap, is a full‐thickness graft that remains joined by its vasculature to the donor site so that the graft does not depend on the vascularity from the recipient site to survive. Wounds healed with a pedicle graft have a good cosmetic appearance because the graft is composed of all layers of skin. Pedicle grafts are not as commonly applied to wounds of horses as they are to wounds of cats and dogs because mobilizing an adequate amount of skin on a horse to advance a pedicle graft is difficult.1,2 An axial‐pattern flap (i.e., a skin flap incorporating a direct cutaneous artery and vein into its base) can be transferred successfully to a wound at adjacent sites in humans and dogs,3,4 or to a remote wound by using microsurgical techniques to anastomose the flap’s vessels to local recipient vessels. Similar transfers in horses, using the circumflex iliac artery, have been unsuccessful, apparently because of reperfusion injury to the graft.5,6 A free skin graft is a section of skin that has been detached from its vascular supply and relocated to a wound at another site where it must create new vascular connections to the wound bed to survive. A free graft can be classified according to its source. The most common type is the autograft, or isograft, which is a graft relocated from one area to another on the same individual. An allograft, or homograft, is a graft transferred between two members of the same species, whereas a xenograft, or heterograft, is a graft transferred from a member of one species to a member of another. Although allografts and xenografts develop vascular connections to the recipient wound, the graft is eventually rejected because the host mounts an immune response against it.7,8 An autograft is the most common and practical type of graft applied to wounds of horses because the host mounts no immune response to the graft. An allograft or xenograft is sometimes applied to a wound on a horse as a biologic dressing.9–11 Free skin grafts can also be classified as full‐thickness or split‐thickness. A full‐thickness graft is composed of epidermis and the entire dermis, including adnexal structures, such as sweat glands and hair follicles, whereas a split‐thickness graft is composed of epidermis but only a portion of the dermis. The thickness of split‐thickness grafts can vary and is determined by the relative amount, rather than the absolute amount, of dermis included in the graft, because the thickness of the dermis varies between individuals and between regions on an individual.12–14 The thickness of equine skin on the trunk and limbs varies widely (between 1 and 5 mm).12 For example, the thickness of skin just proximal to the foot is about 5 mm, whereas the thickness of skin on the abdomen is between 1.5 and 2 mm (JW, personal observation). Full‐ or split‐thickness skin grafts are applied to the surface of wounds as sheet grafts or are embedded within the wound as island grafts. Each technique of grafting has its advantages and disadvantages, and the choice of technique depends on circumstances, such as the location and size of the wound, desired cosmetic appearance, available instrumentation, bias of the surgeon, and the financial resources of the horse’s owner. Nearly any type of tissue is capable of supporting a free graft, except bone devoid of periosteum, tendon devoid of paratenon, or cartilage devoid of perichondrium.15–18 Tissues less likely to accept a free graft are adipose tissue, joint capsules, and ligaments. The presence of small avascular areas of tissue within a wound incapable of accepting a graft, such as tendon, may still be covered with a graft owing to the phenomenon of vascular bridging, whereby a portion of a graft overlying a small, avascular area of the wound is revascularized by capillaries from the vascularized portion of the graft.17,19 Full‐thickness grafts are more effective than split‐thickness grafts at bridging avascular areas of the wound because they contain more dermis, which has superior collateral circulation.19 To accept a free graft, the wound must be vascularized and free of infection and necrotic tissue. Fresh wounds, created surgically or by accident, are the most capable of accepting a free graft because they are highly vascular and have not had time to become infected.14,20,21 Granulating wounds may also accept grafts, though not as readily as can fresh wounds. An advantage of postponing grafting of a large fresh wound until fibroplasia is well under way is that contraction rapidly decreases the size of the wound after granulation tissue forms, allowing it to be covered by a smaller graft. The graft is initially adhered to the wound by fibrin, produced by the wound, which binds to exposed collagen within the graft.15,22–24 Vessels and fibroblasts from the wound invade the fibrin clot within 48 to 72 hours. The graft is nourished initially by plasmatic imbibition, or plasmatic circulation, a process whereby transudate is imbibed passively, by capillary action, into the exposed lumen of the graft’s vessels, much like fluid is absorbed into a sponge.15,23,25 During the stage of plasmatic imbibition, which typically lasts about 48 hours, the graft has no blood supply of its own, and cells within the graft have depressed mitotic activity resulting from low oxygen tension. In this hypoxic state, metabolism becomes anaerobic, and metabolites accumulate. Thin grafts tolerate this ischemic phase better than do thick grafts. The graft becomes edematous and remains so until it revascularizes. After the first 2 days, new capillaries from the wound cross the fibrin matrix to anastomose with capillaries in the graft, a process called inosculation,15,22,25,26 which occurs mainly in the central region of the graft.27 Following inosculation, new capillaries from the wound invade other vessels pre‐existing within the graft and cut new vascular channels into its dermis, a process referred to as neovascularization. The graft revascularizes between the 4th and 5th day after being applied to the wound, and lymphatic circulation is re‐established by 7 days, which resolves edema.26 The graft can be considered accepted or to have “taken” when it has revascularized. The graft becomes firmly attached to the wound by the 10th day. Determining if a graft has been accepted can sometimes be difficult if the upper portion of the graft fails to vascularize and becomes desiccated. The desiccated tissue sloughs to reveal vascularized dermis (indicating that only the upper portion of the graft has failed to vascularize) or granulation tissue (indicating that the entire graft has failed to vascularize). Exposed dermis may closely resemble granulation tissue but can be differentiated from it by its paler color (Figure 18.1). Figure 18.1 The entire epidermis of this graft has sloughed, exposing pale dermis beneath it. This dermis may resemble granulation tissue. The epidermis of the graft becomes hyperplastic after grafting and may die and slough in some areas, exposing the pale dermis beneath it.16,21 Epithelial cells migrating from the hair follicles and the eccrine glands within the dermis soon cover the dermis. Split‐thickness skin grafts remain covered with flaky debris for several months, until the eccrine glands regenerate.21 They usually regain pigmentation beginning about 1 month after grafting (Figure 18.2). Hair begins to appear between 4 and 6 weeks,21 although it often grows abnormally long, perhaps because the origin of the graft is usually the trunk, where the local temperature is higher than that of the limbs and transfer of the graft to a cooler area (i.e., the distal portion of a limb) may stimulate hair growth.13,28,29 Figure 18.2 This split‐thickness skin graft is beginning to pigment. Split‐thickness grafts that have sloughed their epidermis are eventually epithelialized and usually begin to develop pigment by about 1 month after grafting. Nerves enter the base and margins of the graft and reconstruct the graft’s pattern of innervation by following vacated neurolemmal sheaths.30 Split‐thickness grafts become re‐innervated more rapidly but to a lesser extent than do full‐thickness grafts.15,30 Sensation to split‐thickness grafts of humans usually returns between 7 and 9 weeks but is incomplete and patchy.31 Humans sometimes develop hyperesthesia of the grafted wound,16,30 and judging from incidents where horses have mutilated a healed, grafted wound, this may also occur in some horses. Recoil of elastin fibers within the deep layers of the dermis causes a graft to contract immediately after harvest.32 This contraction is referred to as primary graft contraction. Full‐thickness grafts contract more than do split‐thickness grafts, and as the proportion of dermis within split‐thickness grafts increases, so does the amount of primary contraction. The significance of primary contraction is minor for sheet grafts because it is reversed when the graft is stretched and attached to the wound’s margin. Contraction of the wound after the graft has been accepted is referred to as secondary graft contraction, which in reality, represents contraction of the grafted wound rather than of the graft itself. The extent of wound contraction after grafting depends on the relative thickness of the graft’s dermis, the stage of healing of the recipient bed at the time of grafting, the percentage of area grafted, mechanical factors, and the species involved. Contraction of granulating or fresh wounds of humans and rats that receive a full‐thickness graft is less than that of those receiving a split‐thickness graft, even when the full‐thickness graft is thinner than the split‐thickness graft (a spit‐thickness graft removed from an area in which the skin is thick can be thicker than a full‐thickness graft removed from an area in which the skin is thin). This indicates that the deep portion of the dermis may suppress formation or contraction of myofibroblasts in the granulation tissue.33 In contrast, contraction of experimentally created limb wounds of horses did not differ between wounds grafted with full‐thickness skin and those grafted with split‐thickness skin.34 The stage of healing of these wounds, however, had an effect on contraction after grafting. Contraction was significantly less when fresh wounds were grafted than when granulating wounds were grafted, regardless of whether a split‐thickness or full‐thickness graft was applied. Skin grafts are sometimes applied to wounds of humans to prevent wound contraction because contraction can result in contracture, or distortion, of parts of the body.7,15 Conversely, skin grafts applied to granulating wounds of horses stimulate wound contraction rather than wound contracture,35 particularly when Meek micrografts (see Island grafting later in this chapter) are used.29 Contraction of wounds of horses should be considered beneficial because it accelerates wound closure and rarely results in contracture. The only contracture the authors have observed was around the proximal joints of a tail, from contraction of a wound near the tailhead. Stimulation of wound contraction in horses after grafting may result from the presence of certain cytokines in the dermis of the graft, although these have not yet been identified. Both authors have noted that skin grafts applied to chronic, slow‐healing wounds appear to speed contraction and epithelialization, even when the graft fails to be accepted. Healthy donor skin may stimulate healing of a chronic wound by supplying cytokines conducive to healing. Additionally and concomitantly, the reduction of chronic inflammation and, thus, the concentration of several contraction‐inhibiting mediators (e.g., prostaglandin [PG]E1, PGE2, interferon [IFN]γ, tumor necrosis factor [TNF]α, interleukin [IL]‐1, and IL‐6) while the wound is being prepared for grafting, could increase the rate of contraction.32 The superficial layer of exposed granulation tissue usually contains many leukocytes, which are not found beneath the leading edge of advancing epithelium. As the epithelium advances to cover the wound, the concentration of leukocytes in the granulation tissue decreases, and inflammation resolves. Reduction of the area of exposed granulation tissue, by grafts, is likely to have a similar effect on inflammation, which in turn accelerates contraction. The influence of skin grafts on contraction may also depend on mechanical factors. Fixing a sheet graft to a wound under slight tension may influence the granulation tissue by partially relieving the mechanical stress exerted on it by tissue surrounding the wound. This, in turn, favors apoptosis of myofibroblasts, as suggested by in vitro studies of fibroblasts in collagen lattices.36 The disappearance of myofibroblasts reduces contraction, however, and, although this is important in preventing contractures in humans, it is disadvantageous in horses, where wound contraction is beneficial. Whether fixation or certain characteristics of a sheet graft result in disappearance of myofibroblasts, and thus reduce contraction of equine wounds, is not known, but one author (JW) has the impression that wounds of horses contract less after receiving a split‐thickness sheet graft than after receiving split‐thickness island grafts (e.g., Meek micrografts). In general, the responsiveness of equine wounds to dermal factors supplied by grafts is mainly determined by the age of the granulation tissue and, therefore, by the concentration and state of differentiation of the myofibroblasts.34,37,38 Contraction is reduced when wounds are grafted soon after wounding when myofibroblasts are not yet present, and when grafting mature, fibrotic granulation tissue from which myofibroblasts have disappeared. Contraction is most obvious when wounds with healthy granulation tissue containing myofibroblasts are grafted. Failure of skin grafts to be accepted can be caused by infection, inflammation, fluid accumulation beneath the graft, and motion.13,16,24,25,29,32 Infection is reported to be the most common cause of failure in humans39,40 and horses.41 Although the surface of granulation tissue has a resident bacterial population, abundant blood vessels and phagocytic cells within the tissue act as a barrier against bacterial invasion.7,15,16,24,42,43 The wound becomes infected only when the tissue concentration of bacteria exceeds the ability of humoral and cellular defenses to destroy the microorganisms; this concentration is generally estimated to be 105 organisms per gram of tissue in a sutured wound or a wound covered with a graft. Skin graft survival is better correlated to the concentration of bacteria in the wound than to any other factor;39,40 therefore, a wound should not be grafted if it is found, by quantitative bacterial analysis, to contain more than 105 bacteria per gram of tissue. Even though performing quantitative bacterial analysis of a wound before grafting would be useful in helping to determine if the wound is capable of accepting a graft, this type of analysis is impractical for most clinicians. The type of bacteria found in the wound also is an important consideration because, for some bacteria, the concentration necessary to infect a wound can be much less than 105 per gram of tissue.24,42 β‐hemolytic streptococci, in particular, produce proteolytic enzymes that catalyze the conversion of plasminogen to plasmin, which digests fibrin and degrades the graft’s fibrinous attachment to the wound.24,44 Pseudomonads also weaken the graft’s fibrinous attachment by producing elastase, which specifically damages elastin in the graft’s dermis, to which fibrin attaches.24 Although infection is reported to be the most common cause of graft failure in horses,41 chronic inflammation, inherently present during second‐intention healing of wounds on the distal aspect of limbs of horses, may be at least as important because it deteriorates the quality of the granulation bed and results in the production of moderate amounts of purulent exudate containing several detrimental enzymes, which negatively influence graft acceptance.45,46 Grafts applied to wounds of horses may, therefore, be at greater risk of failure than grafts applied to wounds of other species. Wounds of ponies are less prone to chronic inflammation, and therefore may better accept a graft, though this has yet to be investigated. Accumulation of fluid, such as blood, serum, or exudate, beneath a graft prevents fibrin from attaching the graft to the wound bed and obstructs vascularization of the graft.17,25 Although a graft can survive for several days by imbibing nutrients from blood or serum, the graft perishes if capillaries from the wound are incapable of traversing the fluid to reach the graft within this time. To prevent the formation of a hematoma or seroma beneath it, the graft should be compressed firmly against the wound with a bandage, and meshing the graft before application allows escape of fluid that would otherwise accumulate (see Meshing sheet grafts later in this chapter). Hemorrhage from a wound grafted with pinch or punch grafts may dislodge the grafts. Moderate hemorrhage, however, is not considered by the authors to be a big detriment to grafting when applying a meshed sheet graft or Meek micrografts. Hemorrhage ceases quickly after these grafts are applied, possibly because more donor skin is applied to the wound using these techniques, and therefore, more dermal factors from the grafts that stimulate hemostasis are provided. For this reason, the surface of the granulation tissue can be excised at the time of grafting, and by doing so, slightly protruding or poor quality tissue, bacteria, and leukocytes are removed. A fresh, clean surface is created to which the graft is rapidly attached by fibrin. Grafting to a severely hemorrhaging wound should be avoided. If severe hemorrhage is expected after excising exuberant granulation tissue (EGT), excision should be done well in advance of grafting. If hemostasis cannot be achieved at the time of surgery, the graft can be stored for later application (see Storing split‐thickness sheet grafts later in this chapter). To establish vascular connections, the graft must remain immobile. Shearing forces between the graft and the wound, caused by movement of the bandage, disrupt the fibrin seal, impairing plasmatic imbibition and revascularization. Shearing forces occur when the wound is located in a highly mobile region, such as over a joint, but can usually be prevented by application of a splint or cast. Open grafting (i.e., grafting without bandaging) is a technique sometimes used to avoid shear forces and is especially useful when grafting a wound located in a region difficult to bandage, such as the trunk (see Aftercare of the recipient site later in this chapter). Shearing forces that occur during dressing changes can be prevented by avoiding changing the bandages during the first week after grafting, using non‐adherent dressings or foams, and by immobilizing the horse with sedation. Fibrin glue has been used to encourage graft acceptance in humans by increasing the strength of the adhesive bond between the graft and the wound bed.44 Fibrin glue enhances acceptance of skin grafts not only in this manner but also by providing hemostasis and enhancing phagocytosis. A trial using fibrin glue, made from equine fibrinogen, to attach autogenous, split‐thickness skin grafts to experimentally created wounds on the dorsal surface of the metacarpi and metatarsi of horses, however, did not support its use to decrease the incidence of graft failure.47 On the other hand, the authors’ clinical experience is that intentionally provoking very slight hemorrhage from a wound before grafting provides fibrin, which promotes attachment of the graft. The wound must be properly prepared to optimize a graft’s chances of survival. The most important factors to consider when evaluating a wound’s readiness for grafting are its vascularity and whether it appears free of infection.15 Grafting should be postponed if the vascularity of the wound seems inadequate to support a graft, if the wound appears to be infected or has an increased susceptibility to infection, or if the wound is chronically inflamed.46 Fresh tissue accepts a graft more readily than does granulation tissue, and new granulation tissue accepts a graft more readily than does mature granulation tissue, because vascularity diminishes as granulation tissue matures.42 Consequently, fibrous, poorly vascularized granulation tissue should be excised to below the margin of the skin edge so that new, vascular granulation tissue can form prior to grafting.13 Granulation tissue can usually be excised with the horse standing because granulation tissue has no innervation. When granulation tissue is allowed to proliferate for several days after wounding or after excision, the interval between application of the graft and the time at which it begins to vascularize (i.e., the phase of plasmatic imbibition) is reduced from about 48 to 24 hours, because sprouting capillaries capable of rapidly revascularizing the graft proliferate in the wound.48 A wound should be examined thoroughly for infection or susceptibility to infection prior to grafting. A wound, particularly a slow‐healing one or one with a draining tract, should be assessed for the presence of a foreign body and for evidence of damage to bone, ligaments, tendons, or synovial structures. Underlying bone should be examined radiographically for evidence of an osseous sequestrum or septic osteitis. Granulation tissue of slowly healing or non‐healing wounds, particularly those that appear pruritic, should be examined histologically for the presence of larvae of the equine stomach worm, Habronema, or for a cutaneous neoplasm, such as a sarcoid or carcinoma, because these may resemble granulation tissue (Figure 18.3). Figure 18.3 Some cutaneous neoplasms, such as this sarcoid on a pastern, resemble granulation tissue. The bacterial status of a wound to be grafted is usually assessed qualitatively, rather than quantitatively. Quantifying the concentration of bacteria in a wound is time consuming and often impractical, and, moreover, the type of bacteria is often more important than the concentration of bacteria.24 A wound is assumed to be infected if it shows signs of inflammation such as redness, swelling of the wound and surrounding area, and/or formation of exudate. Such wounds should not be grafted. Unfortunately, even wounds without signs of infection may contain a critical concentration of bacteria hindering acceptance of a skin graft.40 A wound that is suspected of being infected can be cultured for bacteria, and isolates tested for susceptibility to antimicrobial drugs. Infection is most often caused by Streptococcus or Pseudomonas spp. Resolving a streptococcal wound infection is usually relatively easy because this organism is nearly always susceptible to penicillin or other β‐lactam antibiotics.42,49,50 Occasionally, however, other bacteria in the wound secrete β‐lactamase, which inactivates most β‐lactam antibiotics. The efficacy of the β‐lactam antibiotic against streptococci is maintained, however, if potassium clavulanate, or clavulanic acid, an inhibitor of β‐lactamase, is administered concomitantly.49 Pseudomonas infection, which is also highly detrimental to grafts, can be recognized by the characteristic bluish‐green exudate and odor of grape juice produced by the bacteria. Pseudomonads are usually sensitive to silver sulfadiazine, aminoglycoside antibiotics, polymyxin‐B sulfate, fusidinic acid, or mafenide acetate.51 Using antimicrobial dressings or topical wound‐care products whose activity is not based on antibiotics helps prevent the development of resistant bacteria in the wound, such as methicillin‐resistant Staphylococcus aureus (MRSA) or Pseudomonas spp. (for more information on antimicrobial dressings, the reader is referred to Chapter 6; for more information on topical wound‐care products, the reader is referred to Chapter 5). Topical administration of an antimicrobial drug to an infected, granulating wound is usually more successful in resolving infection than is systemic administration of the same antimicrobial drug. Systemically administered antimicrobial drugs often fail to reach a therapeutic concentration within granulation tissue and at its surface, despite the good blood supply found in granulation tissue, because fibrin at the base of the granulation tissue prevents adequate penetration of the drug.15,52 Wounds infected with streptococci may be treated with topically applied fusidinic acid cream, and wounds infected with Pseudomonas can usually be treated successfully with topically applied silver sulfadiazine. Nevertheless, the most effective method of decreasing infection and/or chronic inflammation (i.e., the concentration of bacteria and leukocytes) is debridement. EGT can be excised to the level of or slightly below the margin of the surrounding skin, and for wounds containing granulation tissue that is not exuberant, the upper 2–3 mm of the surface should be excised.29 Most bacteria, inflammatory cells, and debris are simultaneously removed, improving local wound conditions. EGT should be excised several days before grafting, and a pressure bandage should be applied to control hemorrhage. When excision is performed at least 24 hours in advance of grafting, the time provided is sufficient for the development of budding capillaries that subsequently vascularize the graft.48 After excision, the bandage should be changed daily or every other day. The concentration of bacteria in the wound may be further reduced by topical application of a broad‐spectrum antimicrobial drug or dressing (as described earlier), which simultaneously decreases the inflammatory response. The chronic inflammation often present in wounds on limbs of horses can be further reduced with a single topical application of a corticosteroid, such as triamcinolone acetonide, 2 days before surgery.35 Reducing inflammation in this manner may improve graft acceptance. The potentially negative influence of the corticosteroid on angiogenesis is offset by reduction of the detrimental effect of inflammation on acceptance of the graft. A granulating wound capable of accepting a free graft is firm, flat, vascular, and red, and contains no more than a few clefts. It should not bleed easily, and discharge from the wound should be slight and serosanguinous or only slightly purulent (Figure 18.4).7 Adherence of a dressing to the wound bed by fibrin and the presence of advancing epithelium from the wound’s margin are useful indicators that a wound is free of infection and receptive to grafting. Figure 18.4 This granulating wound is firm, flat, vascular, and red, and is likely capable of accepting a free skin graft. At the time of grafting, the recipient bed should be prepared by clipping or shaving the hair surrounding the wound, taking care to first cover the wound with moistened gauze sponges. The skin surrounding the wound can be cleansed with surgical soap, but the wound itself should be cleansed only by rinsing it with isotonic saline solution or a balanced electrolyte solution, because detergent found in surgical soap and disinfectants is cytotoxic, increases the wound’s susceptibility to infection, and provokes more inflammation.53 The surface of the wound can then be rubbed with a gauze sponge to remove fibrinous debris, or its most superficial layer can be excised (i.e., 2–3 mm). This excision is best done before the graft is harvested to allow time for the wound to cease hemorrhaging. Although slight hemorrhage caused by superficial excision enhances formation of fibrin, which enables the graft to adhere to the wound surface, too much hemorrhage may hinder revascularization of the graft. The wound should not be scraped because this damages the surface of the wound and makes it irregular. The area selected for harvesting a graft depends on the intended method of grafting and whether the graft is to be harvested with the horse standing or anesthetized. Because harvesting a graft, especially one of split‐thickness, creates a blemish at the donor site, the site should be inconspicuous. The donor site should be prepared for aseptic harvest. Hair can be removed from the donor site by clipping, shaving, or chemical depilation. Shaving or depilating the hair improves the performance of a power dermatome. The direction of hair growth should be marked so that it can be matched to that of the skin surrounding the wound to which it is grafted. After the donor site is prepared for surgery, it should be rinsed with water or isotonic saline solution to remove residues of surgical soap and disinfectant, which are detrimental to the graft and the recipient site. Island grafts, sometimes referred to as implantation grafts or seed grafts, are small pieces of skin implanted either into or onto a wound.54–56 The different types of island grafts applied to wounds of horses are punch grafts, pinch grafts, tunnel grafts, and modified Meek micrografts. Except for the Meek micrografts, island grafts are implanted into granulation tissue of horses, to prevent shearing forces between the dressing and the graft. Island grafts are used to increase the area of epidermis from which epithelialization can proceed. Punch grafts are full‐thickness plugs of skin that are harvested and implanted into granulation tissue using skin biopsy punches. Punch grafts are harvested directly from the horse (Figure 18.5) or from a full‐thickness piece of skin (Figure 18.6), usually excised from the cranial pectoral region.57 Punch grafts harvested directly from the horse should be collected at a relatively inconspicuous site, such as the ventrolateral aspect of the abdomen, the perineum, or the portion of the neck that lies beneath the mane, because wounds created at the donor site heal with small scars. Figure 18.5 Punch grafts have been harvested directly from the neck of this horse. Figure 18.6 These punch grafts were harvested from a full‐thickness section of skin excised from the cranial pectoral region. Subcutaneous tissue was sharply excised from the section of skin to expose the dermis prior to harvesting the grafts with a biopsy punch. The donor site is prepared as described earlier (see Preparation of the donor site) and desensitized by subcutaneous administration of local anesthetic solution, or if the perineum is the donor site, by administering caudal epidural anesthesia. Grafts should be harvested about 1 cm apart and in a symmetrical pattern in an effort to improve the cosmetic appearance of the donor site (Figure 18.5).41 The small wounds created by the biopsy punch can be left unsutured to heal by second intention, but closing each wound with a staple or suture speeds healing and decreases scarring. Prior to implanting the grafts, subcutaneous tissue should be sharply excised from each graft to expose dermal vasculature and encourage plasmatic imbibition and inosculation.54,58 This can be done after the plug has been excised or while the plug is still attached to the horse.59 Because removing subcutaneous tissue from individual plugs of skin is tedious, some clinicians prefer to harvest punch grafts from a full‐thickness section of skin from which the subcutaneous tissue has been removed (Figure 18.6). This section of skin is usually harvested from the pectoral region, where it is relatively thin and mobile.57 A 10 × 4 cm section of skin provides a sufficient number of grafts to cover most wounds, and the resulting wound at the donor site can be sutured easily in one or two layers.60 A tie‐over bandage is sutured to the closed incision to decrease tension on the wound (Figure 18.7). The excised strip of skin is stretched and fixed, dermal side up, to a sterile piece of plastic, cardboard, or Styrofoam or to a polypropylene block or a sterile roll of elastic bandage, and subcutaneous tissue is sharply excised to expose the dermis (Figure 18.8). Plugs are cut from the skin with a 5–8‐mm diameter skin biopsy punch. The grafts are covered with a sterile gauze sponge moistened with isotonic saline solution until they are implanted into the wound (Figure 18.6). Figure 18.7 The wound created after excising a full‐thickness section of skin from the chest is sutured in one or two layers, and a tie‐over bandage is sutured to the closed incision to decrease tension on the wound. Figure 18.8 This section of skin has been stretched and fixed to a sterile piece of plastic. Subcutaneous tissue is being excised to expose the dermis. The recipient holes in the wound should be created before the grafts are harvested to ensure that hemorrhage has stopped before the grafts are implanted. The holes should be created in a symmetrical pattern, about 6 mm apart.57 The depth of the recipient holes should correspond to the thickness of the grafts.57 Creation of the holes should begin distally and proceed proximally so that the hemorrhage emanating from the holes does not obscure that portion of the wound that has not yet been perforated.58 Inserting a cotton‐tipped applicator into the holes prevents a blood clot from forming within and facilitates locating the holes for subsequent graft insertion.60 One can ensure a hemorrhage‐free wound for grafting by creating the recipient holes at least an hour in advance of implantation. In anticipation of primary graft contraction, the recipient holes should be created with a slightly smaller biopsy punch than that used to harvest the grafts. For example, if the grafts are harvested with a 6‐mm diameter biopsy punch, the holes should be created with a 4‐mm diameter biopsy punch. The holes should be irrigated to remove blood clots before inserting the grafts. Each graft is inserted into a recipient hole (Figure 18.9), using a hemostat or tissue forceps, with the graft oriented according to the direction of its hair growth. The grafted wound is covered with a non‐adherent dressing and then bandaged. Figure 18.9 The recipient holes in this granulating wound have been filled with punch grafts. About 60–75% of the grafts can be expected to survive.60 The superficial, pigmented portion of many of the grafts may slough between the 1st and 2nd week, exposing the pale, underlying dermis, which may be somewhat similar in appearance to surrounding granulation tissue. Difficulty in identifying many of the grafts and the presence of sloughed portions of graft on the primary dressing may cause the clinician to mistakenly assume that a large number of grafts have been lost. But by 3 weeks, the pale grafts can easily be recognized by the pink rim of migrating epithelium that surrounds them.57 The time required for epithelium to fully cover the remaining wound is inversely proportional to the area of the wound covered with viable plugs. Wounds to which punch grafts have been applied heal with an epithelial scar from which emerge sparse tufts of long hair, which often grow in various directions. The cosmetic outcome can be improved by orienting the grafts during implantation according to the direction of their hair growth. Punch grafting requires little expertise and neither expensive nor sophisticated equipment, it may be performed with the horse standing. The grafts are often accepted into recipient wounds that are not suitable for sheet grafting, such as wounds in regions of high motion.57 The grafts are independent of one another, and so, rejection of one graft has little or no effect on acceptance of other grafts. Punch grafting is usually reserved for small to moderate‐sized wounds and for circumstances in which the cosmetic appearance of the healed wound is not important. Pinch grafts, sometime referred to as Reverdin grafts, are small disks of skin harvested by excising an elevated cone of skin, which are then implanted into pockets created in granulation tissue.21,50,54–56,61 The perineum and the portion of the neck that lies beneath the mane are common donor sites for harvesting pinch grafts from horses because small scars created at these locations are relatively inconspicuous. The donor site is prepared and desensitized as described for punch grafting. A cone of skin is tented with a tissue forceps, a hypodermic needle with a bent point, or a suture needle, and excised with a scalpel blade.50,54–56,61 A disk of optimal size is about 3 mm in diameter.55 A disk of this diameter is thin toward its periphery but nearly full thickness toward its center. The grafts are covered with a sterile gauze sponge moistened with isotonic saline solution until they are implanted. As for punch grafting, implantation of pinch grafts should begin distally and proceed proximally so that the hemorrhage emanating from sites of implantation does not obscure the portion of the wound that has not yet been implanted. To implant a pinch graft, a #15 scalpel blade is stabbed into the granulation tissue at an acute angle to the wound to create a shallow pocket into which the pinch graft is inserted (Figure 18.10). The pockets are created about 3–5 mm apart.55 The graft is placed on the wound, proximal to the pocket, with its epidermal side up. It is pushed into the pocket using a straight suture needle, hypodermic needle, mosquito hemostat, or, most conveniently, with the scalpel blade used to make the pocket. Orientating the pinches according to the direction of hair growth has little effect on the cosmetic outcome, because hair growth from pinch grafts is usually sparse. To speed implantation, three or more grafts may be placed on the wound, each several centimeters proximal to the proposed site of implantation. While the grafts remain adhered to the wound by surface tension, the surgeon creates a pocket distal to a graft and pushes the graft into it using the same scalpel blade that was used to create the pockets. This technique enables the surgeon to create and implant the pockets in quick succession, without looking away from the wound. Alternatively, each graft can be inserted into the granulation tissue, without making a pocket, using a curved mosquito hemostat. The graft is grasped in the midepidermal region and inserted obliquely, from proximal to distal, into the granulation tissue, after which the hemostat is opened to create more space for the graft. This technique is much quicker than making separate pockets using a scalpel blade (T. Stashak, personal communication). Figure 18.10 A shallow pocket, into which a pinch graft can be inserted, is created by stabbing a #15 scalpel blade into the granulation tissue at an acute angle. The small, partial‐thickness wounds created at the donor site can be left unsutured to heal by second intention, but closing each wound with a staple or suture speeds healing and may decrease scarring. The grafted wound is covered with a non‐adherent dressing and then bandaged. The grafts initially appear as dark spots on the wound. The thin layer of granulation tissue overlying each graft sloughs, usually between the 1st and 2nd week,55 and, frequently, so does the superficial, pigmented portion of the graft. The exposed, pale dermis may be difficult to distinguish from surrounding granulation tissue, giving the impression that the entire graft has sloughed. By 3 weeks, however, grafts are surrounded by a pink rim of advancing epithelium, making them easily identified. Epithelium migrating from the wound’s margin and from the periphery of each graft rapidly coalesces to cover the entire wound. Even when conditions for grafting are unfavorable, at least 50–75% of the grafts are usually accepted. The healed wound is covered by epithelium containing scattered islands of hair. Pinch grafting, like punch grafting, requires little expertise and is economical because only basic instruments are required and the procedure may be performed with the horse standing. Pinch grafts, like punch grafts, can survive in a granulation bed that is not suitable for sheet grafting, such as a wound in an area of high motion.54,62 Like punch grafts, pinch grafts are independent of one another, and therefore, rejection of one graft has no effect on acceptance of other grafts. Pinch grafting is tedious, however, and therefore is usually reserved for small wounds. The cosmetic appearance of a wound healed with pinch grafts is poor (Figure 18.11).
Free Skin Grafting
Summary
Introduction
Classification of grafts
Requirements for graft acceptance
Physiologic events associated with graft acceptance
Revascularization
Graft regeneration
Graft contraction and wound contraction
Causes of graft failure
Infection
Inflammation
Accumulation of fluid beneath the graft
Motion
Preparation of the wound
Preparation of the donor site
Grafting techniques
Island grafting
Punch grafting
Technique
Acceptance and appearance
Advantages and disadvantages
Pinch grafting
Technique
Acceptance and appearance
Advantages and disadvantages
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