Although any open wound can be grafted, the most common indication for skin grafting a horse is to promote healing of a wound so large that it cannot heal by second intention. Grafting a wound that would heal by second intention may be more economical than a long period of bandaging. The two basic types of skin grafts are the pedicle graft and the free graft. A pedicle graft remains joined to the donor site by a vascular pedicle ensuring its viability and imparts a good cosmetic appearance because the graft is composed of all layers of skin. For horses, pedicle grafts are rarely used to cover wounds because mobilizing skin to advance a pedicle graft is difficult.
A free skin graft is a segment of skin that has no vascular supply because it has been detached completely from its donor site and relocated to a wound at another site. An autograft (or isograft; a graft transferred from one area to another on the same individual) is the most practical type of graft applied to wounds of horses. An allograft (or homograft; a graft transferred between two members of the same species) or xenograft (heterograft; a graft transferred from a member of one species to a member of another) is sometimes applied to a wound as a biological dressing. Free skin grafts can be classified as full-thickness or splitthickness.
Full- or split-thickness skin grafts can be applied to the surface of wounds (i.e., sheet grafts) or embedded within the wound (i.e., island grafts).
The graft rapidly attaches to the wound by fibrin and is nourished initially by imbibing plasma by capillary action into the lumen of the graft’s vessels. Within 48 to 72 hours, vessels and fibroblasts from the wound invade the fibrin. The graft is re-vascularized by day 5, and by day 10, the graft is attached firmly to wound. Skin grafts fail to be accepted because of infection, inflammation, fluid accumulation beneath the graft, or motion. Infection is, by far, the most common cause of failure. Inflammation is inherently present during second intention healing of wounds on the distal portion of limbs of horses, and therefore, grafts applied to wounds of horses may be more prone to failure than grafts applied to wounds of other species. Blood, serum or exudate accumulated beneath a graft prevents fibrin from attaching the graft to the wound and obstructs growth of capillaries from the wound into the graft. Shearing forces between the graft and the wound, caused by movement of the bandage, disrupt the fibrin seal and impair vascularization.
A fresh wound accepts a graft more readily than does a granulating wound, and immature granulation tissue accepts a graft more readily than mature granulation tissue. The bacterial status of a wound is assessed qualitatively. A wound should be assumed to be infected if it shows signs of inflammation, but a wound that shows no signs of inflammation may still be infected. Wound infection is most often caused by Streptococcal or Pseudomonas spp.
Streptococcae are nearly always susceptible to penicillin or other ß-lactam antibiotics, and pseudomonads are usually sensitive to aminoglycoside antibiotics. Topical administration of an antimicrobial drug is more successful in resolving infection than is systemic administration of the same drug, especially if the wound contains granulation tissue. Excising exuberant granulation tissue to the level of or slightly below the margin of the surrounding skin removes most bacteria and inflammatory cells.
The simplest method of grafting is island grafting, which is a technique in which a small disk or strip of skin is implanted into a granulating wound for the purpose of increasing the area of epidermis from which epithelialization can proceed. Types of island grafts applied to wounds of horses are punch grafts, pinch grafts, and tunnel grafts.
Pinch grafts are small disks of skin, harvested by tenting skin to create a cone of skin and then excising this cone to create a small disc, which is implanted in granulation tissue. The skin beneath the mane and the perineum are common donor sites for harvesting pinch grafts because scars at these locations are inconspicuous. Hair is clipped from the donor site, the site is cleaned, and desensitized by injecting local anesthetic solution subcutaneously. Skin is most easily tented by using a hypodermic needle with a bent point. The cone of skin is excised with a #11 scalpel blade to create a 3-mm diameter disk. The graft is placed on the wound,
with its epidermal side up, and a pocket into which the disk is implanted is created in the granulation tissue immediately distal to the graft by using a #15 scalpel blade inserted into the granulation tissue at an acute angle. The disk, which adheres to the wound by hydroscopic pressure, is pushed into the pocket using the scalpel blade used to make the pocket. By using this technique, the surgeon can create and implant the pockets without looking away from the wound. Grafting should begin distally and proceed proximally.
Punch grafts are full-thickness plugs of skin harvested and implanted into granulation tissue using skin biopsy punches. Punch grafts can be harvested with a punch directly from the horse, usually from the perineum, neck, or pectoral area. After preparing the donor site, grafts are harvested about 1 cm apart using a 5- to 8-mm diameter skin biopsy punch. The small wounds created by the punch can be closed with a suture or staple but are usually left unsutured to heal by second intention. Subcutaneous fascia is excised from each graft to expose dermal vasculature. One technique used to remove subcutaneous fascia from the graft is to rotate the skin biopsy punch until it penetrates the skin, and then, using thumb forceps, one edge of the graft is lifted to expose subcutaneous tissue, which is excised from the dermis.
Punch grafts can also be harvested from a section of full-thickness skin removed from the cranial pectoral region. The donor site is closed in one or two layers. Subcutaneous fascia is excised from the section of skin to expose the dermal vasculature, and plugs are harvested from the skin by using the biopsy punch.
The recipient holes in the wound are created about 5 or 6 mm apart. Creation of the holes should begin distally and proceed proximally. The recipient holes are created with a slightly smaller punch than that used to harvest the grafts to allow for contraction of the graft. The grafts are inserted into recipient holes with the graft’s epidermis oriented toward the surface of
The superficial, pigmented portion of pinch and punch grafts often sloughs during first week, exposing pale dermis. By three weeks, a red ring of advancing epithelium surrounds each graft and these rings increasingly enlarge until they converge to provide epithelial coverage for the entire wound. At least half of pinch and punch grafts can be expected to survive. The time required for the wound to become completely epithelialized is inversely proportional to the area covered with grafts.
Tunnel grafts are thin strips of full-thickness or split-thickness skin implanted into tunnels created in granulation tissue. Tunnel rafts can be harvested, using a variety techniques, from many areas and can be implanted with the horse sedated or anesthetized.
They can be harvested from the neck or cranial pectoral region with the horse sedated or from the ventral aspect of the flank with the horse anesthetized. After preparing the donor site, linear wheals, 2- to 3-cm wide and slightly longer than the width of the wound to be grafted are created by subcutaneously injecting isotonic saline solution, if the grafts are harvested with the horse
anesthetized, or local anesthetic solution, if the grafts are harvested with the horse sedated.
A straight intestinal forceps is applied to the base of the wheal so that skin protrudes above the jaws of the forceps, and the protruding skin is excised with a scalpel blade. The amount of skin that protrudes above the jaws determines the width and thickness of the graft. If the graft is full-thickness, the donor site should be closed with sutures or staples. If the graft is splitthickness, the partial-thickness wound created is left unsutured. Subcutaneous fascia is excised from full-thickness tunnel grafts to expose the dermis. Another method of harvesting full-thickness strips of skin is to remove a 2- to 3-cm wide, full-thickness section of skin from the cranial pectoral region, and after sharply excising subcutaneous tissue from the graft to
expose the dermal vasculature, the skin is divided into 2- to 3-mm wide strips. The wound on the chest is closed in one or two layers.
To implant a tunnel graft, the shaft of a thin alligator forceps is inserted through the granulation tissue, perpendicular to the long axis of the limb, at a depth of 2 to 3 mm. One end of the graft is grasped with the forceps, and the forceps and graft are pulled back through the wound.
Grafts are implanted about 2 cm apart. If the tunnel graft was implanted shallowly (i.e., at about 2 mm) the granulation tissue overlying the graft is likely to slough within a week exposing the re-vascularized graft. If the graft was implanted deeply (i.e., at 4 to 5 mm), granulation tissue overlying the grafts must be excised six to 10 days later, with the horse sedated or anesthetized.
Island grafting can usually be performed with the horse sedated and requires no expensive equipment and little expertise. Island grafts are often accepted by wounds that have little chance of accepting a sheet graft, such as inflamed wounds or wounds in areas of high motion. Rejection of one graft has no effect on acceptance of other grafts. Island grafting is usually reserved for small wounds and for circumstances where the epithelial scarring is not important to the owner. Harvesting island grafts is tedious, and the grafts can be applied only to granulating wounds.
Full-Thickness Sheet Grafting
A full-thickness graft is usually harvested from the cranial pectoral region, one of the few places on a horse where the skin is relatively mobile. The graft is usually harvested with the horse sedated, after desensitizing the donor site with local anesthetic solution. The donor site is sutured in one or two layers. After sharply excising subcutaneous tissue to expose the dermal vasculature, the graft is attached to the recipient site with staples, sutures, or cyanoacrylate glue. When attaching the graft with staples or sutures, the recipient site must be desensitized with local or regional anesthesia.
Full-thickness grafting is usually reserved for fresh, uncontaminated wounds because fullthickness grafts are not as readily accepted as split-thickness grafts because of their greater requirement for nourishment and because they have fewer exposed blood vessels. Fullthickness sheet grafting can be performed with the horse sedated without using expensive instruments, and wounds healed with a full-thickness skin graft are more cosmetic than are wounds healed with a split-thickness sheet graft or with island grafts, and they resist trauma better. Full-thickness grafts can be used to cover only small wounds because the horse’s lack
of redundant skin limits the availability of donor skin. The largest graft that can be harvested from the chest, while allowing the donor site to be easily closed with sutures, is an ellipse usually not wider than 8 cm. The owner should be cautioned that the donor site might dehisce but reassured that an open wound on the chest heals rapidly and cosmetically by contraction.
Split-Thickness Sheet Grafting
A split-thickness sheet graft is composed of epidermis and a portion of dermis. To harvest a split-thickness graft, the dermis is split by using a power-driven dermatome, a drum dermatome, or a free-hand knife. Although power-driven dermatomes and drum dermatomes allow grafts of precise width and thickness to be harvested, they are expensive, require skilled maintenance, and may fail to operate properly when proper function is critical.
Large grafts to cover large wounds of horses can be harvested with any one of a variety of free-hand dermatomes designed specifically for harvesting skin of human beings. The most easy to use hand-held knives have an adjustable roller in front of a disposable blade. An example is the Watson skin grafting knife (Watson Skin Grafting Knife; Padgett Instruments, Inc., 1520 Grand, Kansas City, MO). Because the adjustable roller controls the depth of cut, only a moderate amount of practice is required to become proficient in in the use of this knife.
A free-hand knife is far less expensive than a power-driven or drum dermatome, requires less maintenance, does not malfunction, and can be transported easily.
A split-thickness sheet graft can be harvested safely only with the horse anesthetized. The graft can be applied to the wound with sutures or staples while the horse is anesthetized, but if the horse is recovered from anesthesia before the graft is applied, the margin of the wound must be desensitized by using local or regional anesthesia or the margin of the graft can be glued to the wound’s margin with cyanoacrylate glue.
Split-thickness sheet grafts can be harvested in sheets large enough to cover wounds too large to be covered by a full-thickness graft, and they are more readily accepted than fullthickness grafts because they requires less nourishment and have more exposed vessels in the dermis. The appearance and durability of a wound healed with a split-thickness skin graft are poorer than that of a wound healed with a full-thickness skin graft. Harvesting a splitthickness graft is far less convenient and far more expensive than harvesting a full-thickness graft or island grafts because the horse must be anesthetized.
Meshing Sheet Grafts
A split- or full-thickness sheet graft can be applied to a wound as a solid or fenestrated sheet.
The primary reason to fenestrate, or to mesh a sheet graft is to enable the graft to cover a wound larger than the graft itself. Meshing a sheet graft also prevents fluid from collecting between the graft and the wound and allows antimicrobial drugs to contact at least a portion of the wound. Because it can expand, a meshed graft is better able to tolerate motion. A graft can be meshed with a scalpel blade or with a meshgraft dermatome, such as the relatively inexpensive, Padgett mechanical skin mesher (Mesh Skin Graft Expander, No. Z-PD-170; Padgett Instruments, Kansas City, MO).
A sterile, non-adherent dressing is applied to the grafted wound, secured to the wound with conforming rolled gauze, and a bandage is applied over the dressing. If the grafted wound is located in a region that is hard to immobilize, shearing forces can be decreased by securing the dressing to the wound with elastic, adhesive tape instead of rolled gauze, or a cast can be applied to completely immobilize the grafted portion of the limb. If casting the limb is impractical, a splint applied to the outside of a bandage may suffice. The bandage is usually not changed for 4 to 5 days after grafting, but if infection with virulent bacteria, such as Streptococcus and Pseudomonas spp., is a problem in the hospital, the bandage should be changed daily. A broad-spectrum antimicrobial agent effective against both ß-hemolytic Streptococcus and Pseudomonas spp. should be applied topically to the wound.
Storing Split-thickness Sheet Grafts
Skin grafts submerged in isotonic saline solution or lactated Ringer’s solution can be stored in a refrigerator for at least week. Using a culture medium, such as McCoy’s 5A medium, to which a small volume of serum has been added, extends the life of the stored graft for 3 weeks or longer. The horse’s own serum or a commercially available, antibody-free, equine serum should be used. Stored skin can be used to cover defects created by partial or complete loss of the primary graft. A stored graft is more readily accepted than a fresh graft because grafts stored for 24 hours or more undergo anaerobic metabolism, which causes release of metabolites that encourage rapid vascularization of the graft.
Jim Schumacher, DVM, MS, Dipl. ACVS, MRCVS
University of Tennessee
Knoxville, TN, USA
Western Veterinary Conference 2013
Dr Allison Maldonado é formado em Medicina Veterinária pela Universidade Federal do Paraná em 2002, atua desde então como Clínico e Cirurgião de Equinos, com enfase em Cirurgias Ortopédicas e do Aparelho Respiratório.