Not your ordinary bandage: Equine wound care for the 21st century

Equine wound care has often been based upon the premise of keeping the wound clean and dry, requiring frequent bandage changes when dealing with wounds that have a lot of exudate. Sometimes bandages have been left in place for longer periods of time, often resulting in a wound with a large amount of wound exudate. This was seen as a "necessary evil" when it was too difficult to change the bandage as often as it should be changed. Surprisingly, the wounds would often heal very well using this method, and would provide a sort of positive feedback allowing the practitioner to talk themselves into leaving the bandages on for longer periods of time. We now know that a regulated moist environment provides for much better wound healing.

Photo 1: Close up photograph of woven and non-woven gauze.

Moist wound healing concepts

Moist wound healing occurs when the wound exudate is allowed to stay in contact with the wound bed. Studies on people have shown that full thickness skin wounds kept in a moist environment re-epithelialize in approximately 12 to 15 days where the same wound exposed to the air will take 25-30 days to heal. The wounds are less inflamed, cause less itching, have less eschar formation, and are more likely to heal without scaring. Wound exudate in the absence of infection provides a substrate rich in enzymes, growth factors, and chemotactic factors. The enzymes are a byproduct of the breakdown of polymorphonuclear cells and macrophages. The enzymes allow the debridement of the devitalized tissue, improving the "foundation" for wound healing to proceed. This has been termed "autolytic debridement". Autolytic debridement occurs between 72 and 96 hours depending on the thickness of eschar and the size/location of the wound and is achieved under occlusive dressings, provided the wound bed remains in contact with the wound fluid. Growth factors provide a stimulus for the fibroblasts and epithelial cells. The chemotactic factors stimulate the migration of more neutrophils and macro-phages to phagocytize bacteria and debris while releasing enzymes to further promote autolytic debridement. A moist environment allows better migration of neutrophils and macrophages than a dry wound environment. Occlusion provides a constant thermal regulated environment leading to healthier cells and if the appropriate dressing is chosen, bacterial penetration is reduced or prevented. Disadvantages of moist healing include bacterial colonization, folliculitis, the possibility of trauma to peri-ulcer borders, and at least in people, allergies to the dressing material. There can be a fine balance between drying out of the wound and maceration of peri-wound tissue. Fortunately there are new dressings available to help the practitioner in determining what to use in each case to provide an optimum wound healing environment.

Photo 2: An amorphous gel is applied to a dry wound to provide a moist wound-healing environment.

Dressing choices

Many dressings have been used in the treatment of lacerations and abrasions in horses. Non-adherent dressings and gauze dressings are probably the most common dressings used. Both of these dressings are porous, allowing fluid transfer from the wound to the overdressing and from the outside of the bandage to the wound surface. If woven gauze (Photo 1, p. 7) is used, the wound exudate is quickly absorbed through the gauze and into the over dressing. This is termed vertical wicking. If non-woven gauze (Photo 1) is used, the wound exudate tends to flow to the edge of the gauze quickly and then is absorbed into the over dressing. This is termed horizontal wicking. The amount of moisture retention is dependent upon many factors including the amount of exudate and the type of gauze, the secondary dressing, and the frequency of dressing change. It is difficult when using a gauze dressing to maintain a moist wound-healing environment even if a wet to dry bandage is used.

Photo 3: Moist wound before and after applying calcium alginate dressing.

Occlusive dressings isolate the wound from the external environment providing many benefits over a simple gauze dressing. The occlusiveness of a dressing is measured by the evaporation of fluids from the wound surface through the dressing and ranges from minimally occlusive to completely occlusive. The benefits of occlusion include: rapid autolytic debridement with less necrotic tissue, a bacterial barrier, a waterproof barrier, a decrease in pain associated with wound/dressing, ease of use, fewer dressing changes and decreased wound healing time. The fears about occlusion are mainly centered around infection. All wounds are colonized with bacteria while they may not be actually infected. Infection in general refers to invasion of bacteria to the 105 power in live healthy tissue. Signs of infection include edema, erythema, induration and fever. While there are concerns, studies have shown that occlusive dressings are not associated with increased rates of infection. The ideal dressing should keep the ulcer bed continually moist and the surrounding skin dry, or more simply stated would be a dressing that will manage the amount of exudate present. This determination will be dependent on clinical judgement. The occlusive dressings addressed in this article are amorphous gels, calcium alginate dressings, foam dressings and hypertonic saline dressings.

Photo 4: Wound before using a foam dressing application. After 26 days, with the foam dressing application shows decreased granulation tissue and improved epithelialization without topical steroids.

The amorphous gels like Curafil are moist, medical grade gels that are intended to provide a moist wound-healing environment when used on a dry wound.(Photo 2, p. 7.) They are composed of water glycerin and a polymer. They are conformable, non-drying, convenient, bacterial free, and promote moist wound healing. They are available in either gel form or in a mesh-reinforced pad. The gels are completely occlusive and will allow a dry wound to become moist improving autolytic debridement, white blood cell migration, thermal regulation and subsequent improved wound healing. Once the wound has become moist, the gels are discontinued in favor of one of the other occlusive dressings.

Calcium alginate dressings like Curasorb are soft, non-woven fabric pads composed of sodium and calcium alginate, a derivative of seaweed. The calcium in the dressing interacts with sodium in the wound providing a wound exudate that stimulates the myofibroblasts as well as the epithelial cells while speeding wound homeostasis. The alginate dressings are used in moderate to heavily draining wounds and can absorb up to 20 times its weight in exudate reducing the frequency of bandage changes.(Photo 3, p. 8.) The dressing conforms to wound contours, applies easily, removes virtually painlessly, and intact. The good vertical wicking properties allow a reduction in maceration of healthy peri-wound tissue. The alginate dressings come in pads or rope that can be used for loose packing to fill deep wounds. They can be trimmed to size and are very conformable. Newer alginate dressings (Curasorb Zn) are provided with zinc embedded in the dressing as well to provide necessary elements for epithelialization.

Photo 5: Necrotic wound at presentation and after 96 hours of hypertonic saline dressing application. Note the healthy granulation tissue. This wound is now ready for a calcium alginate dressing.

In general, the alginates are used after the wound has been debrided and cleaned, or moistened by a gel. Use of alginates will stimulate granulation tissue and prepare the wound bed for a foam dressing. In wounds that lack excess exudate, but still need granulation tissue stimulation, the alginates can be pre-moistened prior to application.

Foam dressings, such as Hydrasorb, are semi occlusive dressings for use on moderately exudative wounds. They are generally used after healthy granulation tissue has formed. The use of a foam dressing will provide a moist wound environment improving epithelialization and minimizing granulation tissue formation. (Photo 4, p. 8). If there is a lot of exuberant granulation tissue present, the excess tissue should be trimmed back prior to dressing application. The foam dressings can be changed every four to seven days.

Table 1

Curasalt is a pre-moistened 20 percent hypertonic saline dressing that is primarily used to clean and debride wet necrotic wounds. These dressings are designed for infected or heavily exuding wounds only. (See Photo 5, p. 9.) The hypertonic saline works by osmotic action to desiccate the necrotic tissue and bacteria in a wound. The debridement is non-selective and must be carefully monitored to make sure the surrounding tissue is not damaged. Dressings should be changed every 24 to 48 hours at the onset of treatment, but can be left longer once the infection is under control. They are used before alginate dressings.

A gauze dressing with an anti-microbial agent bound to it such as Kerlix A.M.D. will provide an effective barrier to bacterial colonization and may reduce the bacterial load at the laceration site. Kerlix A.M.D. contains polyhexamethylene biguanide bound to a Kerlix Super Sponge. Polyhexamethylene biguanide (PHMB) is part of a class of cationic surface-active agents that have been used as preservatives in aqueous solutions and as disinfectants and antiseptics. Chlorhexidine di gluconate is one of the most recognized of the biquanides. Current uses of PHMB include use in cosmetics, contact lens solutions, baby wipes and pool sanitizers. Increased concentrations when impregnated onto fabric have shown the ability to suppress microbial growth when microbial contamination is present. Microbial death occurs by destabilization and disruption of the cytoplasmic membrane resulting in leakage of macromolecular components. This response is irreversible and the microbe cannot adapt or become "resistant" to the PHMB. These dressings are particularly useful in preventing bacterial infection in surgical incisions or where wounds are close to synovial structures (Photo 6) and deep penetration of bacteria would be catastrophic.

Photo 6: Wound over fetlock at presentation and at 24 days post treatment with an anti-microbial dressing.

Summary

The advantages of moist healing include; prevention of wound desiccation, increased re-epithelialization rate, prevention of eschar formation, decreased inflammation, enhanced autolytic debridement and a subsequent decrease rate of infection with occlusive dressings and cost efficiency. There are many options available for treating wounds, each with benefits at certain stages of healing. (See Table 1, p. 9) Clinical judgement will help the practitioner determine which dressing to use and at each stage of the treatment process.