Grafting the Burn Wound
If wounds are deep (full-thickness) or extensive, spontaneous re-epithelialization is not possible. Therefore, coverage of the burn wound is necessary until coverage with a graft of the patient’s own skin (autograft) is possible. The purposes of wound coverage are to decrease the risk for infection; prevent further loss of protein, fluid, and electrolytes through the wound; and minimize heat loss through evaporation. Several methods of wound coverage are available; some are temporary until grafting with permanent cov-erage is possible. Wound coverage may consist of biologic, bio-synthetic, synthetic, and autologous methods or a combination of these approaches.
The main areas for skin grafting include the face (for cosmetic and psychological reasons); functional areas, such as the hands and feet; and areas that involve joints. Grafting permits earlier functional ability and reduces contractures (shrinkage of burn scar through collagen maturation). When burns are very exten-sive, the chest and abdomen may be grafted first to reduce the burn surface.
Granulation tissue fills the space created by the wound, cre-ates a barrier to bacteria, and serves as a bed for epithelial cell growth. Richly vascular granulation tissue is pink, firm, shiny, and free of exudate and debris. It should have a bacterial count of less than 100,000 per gram of tissue to optimize graft take. If the wound is not ready for skin grafting, the burn wound is excised and allowed to granulate. Once the wound is excised, a wound covering is applied to keep the wound bed moist and promote the granulation process.
Biologic dressings have several uses. In extensive burns, they save lives by providing temporary wound closure and protecting the granulation tissue until autografting is possible. Biologic dress-ings are commonly used in patients with large areas of burn and little remaining normal skin donor sites. Biologic dressings mayalso be used to débride wounds after eschar separation. With each biologic dressing change, débridement occurs. Once the biologic dressing appears to be “taking,” or adhering to the granulating surface with minimal underlying exudation, the patient is ready for an autograft.
Biologic dressings also provide temporary immediate coverage for clean, superficial burns and decrease the wound’s evaporative water and protein loss. They decrease pain by protecting nerve endings and are an effective barrier against water loss and entry of bacteria. When applied to superficial partial-thickness wounds, they seem to speed healing. Biologic materials can be left open or covered. They stay in place for varying lengths of time but are re-moved in instances of infection or rejection.
Biologic dressings consist of homografts (or allografts) and heterografts (or xenografts). Homografts are skin obtained fromliving or recently deceased humans. The amniotic membrane (amnion) from the human placenta may also be used as a biologic dressing. Heterografts consist of skin taken from animals (usually pigs). Most biologic dressings are used as temporary coverings of burn wounds and are eventually rejected because of the body’s immune reaction to them as foreign.
Homografts tend to be the most expensive biologic dressings. They are available from skin banks in fresh and cryopreserved (frozen) forms. Homografts are thought to provide the best in-fection control of all the biologic or biosynthetic dressings avail-able. Revascularization occurs within 48 hours, and the graft may be left in place for several weeks. Cost, availability, and the pos-sibility of transmission of disease limit the use of homografts.
Amnion is less expensive and is available in hospitals with burn centers and specialized tissue banks, which obtain and process it in cooperation with obstetric services. However, amnion grafts do not become vascularized by the patient’s vessels and can be left in place only for short periods.
Pigskin is available from commercial suppliers. It is available fresh, frozen, or lyophilized (freeze-dried) for longer shelf life. Pigskin impregnated with a topical antibacterial agent such as sil-ver nitrate is also available. Pigskin is widely used for temporary covering of clean wounds such as superficial partial-thickness wounds and donor sites. Although pigskin does not vascularize, it will adhere to clean superficial wounds and provides excellent pain control while the underlying wound epithelializes.
Problems with availability, sterility, and cost have prompted the search for biosynthetic and synthetic skin substitutes, which may eventually replace biologic dressings as temporary wound coverings. Currently the most widely used synthetic dressing is Biobrane, which is composed of a nylon, Silastic membrane combined with a collagen derivative. The material is semitransparent and sterile. It has an indefinite shelf life and is less costly than homograft or pigskin. Like biologic dressings, Biobrane protects the wound from fluid loss and bacterial invasion.
Biobrane adheres to the wound fibrin, which binds to the nylon–collagen material. Within 5 days, cells migrate into the nylon mesh. Generally, adherence to the wound surface correlates directly with low bacterial counts. When the Biobrane dressing adheres to the wound, the wound remains stable and the Bio-brane can remain in place for 3 to 4 weeks. Biobrane dressings (Fig. 57-4) readily adhere to donor sites and meticulously clean débrided partial-thickness wounds; they will remain until spon-taneous epithelialization and wound healing occur. Biobrane can be laid on top of a wide-meshed autograft to protect the wound until the autograft epithelium grows out to close the interstices. As the Biobrane gradually separates, it is trimmed, leaving a healed wound.
Biobrane is also useful for intermediate or long-term closure of a surgically excised wound until an autograft becomes avail-able. Like biologic dressings, Biobrane should not be used over grossly contaminated or necrotic wounds. Removal of Biobrane after several weeks is similar to but easier than removal of a vas-cularized allograft and leaves a bleeding granulation bed that readily accepts an autograft.
Another fairly new temporary wound covering is BCG Matrix. This dressing combines beta-glucan, a complex carbohydrate, with collagen in a meshed reinforced wound dressing. Beta-glucan is known to stimulate macrophages, which are vital in the in-flammatory process of healing. BCG Matrix is a temporary wound covering intended for use with partial-thickness burns and donor sites. It is applied immediately after cleaning and débridement. If the burn wound surface remains free of infection, BCG Matrix can be left in place until healing is complete.
Several other synthetic dressings are available for burn wound care. Op-Site, a thin, transparent, polyurethane elastic film, can be used to cover clean partial-thickness wounds and donor sites. This dressing is occlusive and waterproof but permeable to water vapor and air; this permeability not only provides protection from microbial contamination but also allows for the exchange of gases, which occurs much more quickly in a moist environment. Other synthetic dressings used for burn wounds include Tegaderm, N-Terface, and DuoDerm. Burns that are between superficial and deep partial thickness in depth can be treated with a promising new temporary biologic covering, TransCyte, a material composed of human newborn fi-broblasts, which are cultured on the nylon mesh of Biobrane. The thin silicone membrane bonded to the mesh provides a moisture vapor barrier for the wound. TransCyte is used to treat burns in which the depth is indeterminate. TransCyte delivers a variety of biologically active proteins, which may benefit the wound healing process. Research has shown that wounds treated with TransCyte healed more quickly and with less hypertrophic scarring than burns treated with the traditional silver sulfadiazine protocols (Noordenbos, Dore & Hansbrough, 1999).
In an attempt to develop the ideal burn wound covering product, dermal substitutes have been created. Two such products are In-tegra Artificial Skin and Alloderm.
Artificial skin (Integra) is the newest type of dermal substitute. A dermal analog, Integra is composed of two main layers. The epi-dermal layer, consisting of Silastic, acts as a bacterial barrier and prevents water loss from the dermis. The dermal layer is composed of animal collagen. It interfaces with the open wound surface and allows migration of fibroblasts and capillaries into the material. This “neodermis” becomes a permanent structure. The artificial dermis is biodegraded and reabsorbed. The outer silicone mem-brane is removed 2 to 3 weeks after application and is replaced with the patient’s own skin in the form of a thin epidermal skin graft. Long-term effects of Integra include minimal contracture formation. The graft site is very pliable, almost eliminating the need for repeated cosmetic surgery. Most importantly, Integra has resulted in less hypertrophic scarring (Fig. 57-5), thus eliminating the need for compression devices once the burn wound has healed. The use of Integra is increasing the survivability of burns and im-proving the functional and cosmetic qualities of the healed burn (Winfrey, Cochran & Hegarty, 1999).
Another promising dermal substitute is Alloderm. It is processed dermis from human cadaver skin, which can be used as the der-mal layer for skin grafts. When a donor site (the area from which skin is taken to provide a skin graft for another part of the body) is harvested for an autologous skin graft, both the epidermal and dermal layers of skin are removed from the donor site. Alloderm provides a permanent dermal layer replacement. Its use allows the burn surgeon to harvest a thinner skin graft consisting of the epi-dermal layer only. The patient’s epidermal layer is placed directly over the dermal base (Alloderm). The new graft is then treated according to the burn unit’s protocol. Use of Alloderm has also resulted in less scarring and contractures with healed grafts; donor sites heal much more quickly than conventional donor sites be-cause only the epidermal layer has been harvested. This is impor-tant when donor sites are limited because of extensive burns (Luterman, 2000).
Autografts remain the preferred material for definitive burn wound closure following excision. Autografts are the ideal means of cover-ing burn wounds because the grafts are the patient’s own skin and thus are not rejected by the patient’s immune system. They can be split-thickness, full-thickness, pedicle flaps, or epithelial grafts. Full-thickness and pedicle flaps are commonly used for recon-structive surgery, months or years after the initial injury.
Split-thickness autografts can be applied in sheets or in postage stamp–like pieces, or they can be expanded by meshing so that they can cover 1.5 to 9 times more than a given donor site area. Skin meshers enable the surgeon to cut tiny slits into a sheet of donor skin, making it possible to cover large areas with smaller amounts of donor skin.
These expanded grafts adhere to the re-cipient site more easily than sheet grafts and prevent the accumu-lation of blood, serum, air, or purulent material under the graft. However, any kind of graft other than a sheet graft will contribute to scar formation as it heals. Using expanded grafts may be neces-sary in large wounds but should be viewed as a compromise in terms of cosmesis.
If blood, serum, air, fat, or necrotic tissue lies between the re-cipient site and the graft, there may be partial or total loss of the graft. Infection and mishandling of the graft, as well as trauma during dressing changes, account for most other instances of graft loss. Using split-thickness grafts allows the remaining donor site to retain sweat glands and hair follicles and minimizes donor site healing time.
Use of cultured epithelial autograft (CEA) is common at several burn centers. This involves a biopsy of the patient’s skin in an unburned area. Keratinocytes are then isolated and ep-ithelial cells are cultured in a laboratory. The original epithelial cell sample can multiply to 10,000 times its original size over 30 days. These cells are then attached to the burn wound. Vary-ing degrees of success have been reported, and results are en-couraging. However, the disadvantages of the CEA are that the grafts are thin and fragile and can shear easily. Research has shown that the outcomes of use of CEA are not as positive as once thought. The quality of burn scars is better, but patients have longer hospital stays and higher hospital costs and require more surgical procedures than those treated by traditional methods. In addition, patients require more reconstructive procedures in the first 1 to 2 years postinjury. Therefore, CEA use is very limited and reserved for burn patients whose donor sites are limited (Noordenbos et al., 1999).
Occlusive dressings are commonly used initially after grafting to immobilize the graft. Occupational therapists may be helpful in constructing splints to immobilize newly grafted areas to prevent dislodging the graft. Homografts, heterografts, or synthetic dressings may also be used to protect grafts. The graft may be left open with skin staples to immobilize it, which allows close observation of progress.
The first dressing change is usually performed 3 to 5 days after surgery, or earlier in the case of purulent drainage or a foul odor. If the graft is dislodged, sterile saline compresses will help prevent drying of the graft until the physician reapplies it.
The patient is positioned and turned carefully to avoid dis-turbing the graft or putting pressure on the graft site. If an ex-tremity has been grafted, it is elevated to minimize edema. The patient begins exercising the grafted area 5 to 7 days after grafting.
A moist gauze dressing is applied at the time of surgery to maintain pressure and to stop any oozing. A throm-bostatic agent such as thrombin or epinephrine may be applied directly to the site as well. The donor site may be treated in sev-eral ways, from single-layer gauze impregnated with petrolatum, scarlet red, or bismuth to new biosynthetic dressings such as Bio-brane or BCG Matrix. Some burn centers are using the Acticoat dressing on donor sites. Despite the type of donor site covering, donor sites must remain clean, dry, and free from pressure. Be-cause a donor site is usually a partial-thickness wound, it will heal spontaneously within 7 to 14 days with proper care. Donor sites are painful, and additional pain management must be a part of the patient’s care.