Summary

Myriads of wound‐care products are on the market, and making the right choice for a specific wound can be complex, especially because the scientific literature on wound‐care products in horses is sparse. To select the best possible dressing, the veterinarian must decide which material is needed to enhance healing. The clinician must then determine if antimicrobial therapy is required. Several antibacterial compounds are available for topical use and are often more effective than systemically administered antibiotics in the management of severe contamination, critical colonization, or infection. To serve the requirements of specific wounds, several combinations of antimicrobial drugs and dressings are available.

The wound milieu changes as healing progresses and so do the requirements for dressings. Continued assessment, based on a thorough clinical examination and sound knowledge of the healing process, is indicated, therefore, to maintain an optimal environment for healing.

Introduction

For many years, dressings were considered useful only for protecting the wound and removing exudate. The prevailing theory was that the wound, if kept dry, would heal uneventfully. This belief was overturned completely by the landmark study performed by George Winter and published in Nature in 1962.¹ This study showed that the moist environment under a polyethylene film increased the rate of epithelialization of experimental wounds in pigs compared to wounds that had been left exposed to air.¹ Although progress has been slow, the concept of moist wound healing is now widely accepted in wound care in humans.

Another critical turning point has been the gradual realization that major pathophysiologic changes take place in the wound bed as wounds heal (the reader is referred to Chapter 1 for more information on the physiology of wound healing). Consequently, no single dressing may support the processes of wound healing in all phases. Dressing should possess more than “plug‐and‐seal” capabilities – they should support healing by enhancing physiologic processes within the wound. The practitioner must, therefore, recognize the different phases of healing and have good knowledge of the properties of dressing materials to be able to select the best dressing for a specific wound at a specific stage in the healing process, from amongst the enormous assortment of available dressings.

The evidence‐based approach to choosing wound dressings in horses is hampered by the paucity of scientific evidence of how different dressings affect wound healing in this species. Today, thousands of wound‐care products are marketed, but few have been investigated in the horse. In most equine studies, the dressing under investigation has been used throughout all phases of wound healing, and most studies have been performed using experimental, excisional wounds. Accepting the premise that no single material can produce the optimum microenvironment for all wounds or for all the phases of healing, it becomes evident that such study designs are suboptimal for assessing how a wound‐care product would affect healing were it applied to an accidental wound in the phase of healing for which it was designed. Veterinarians are often forced to select a dressing based on studies performed in humans or laboratory rodents, or based on common sense and a pathophysiologic rationale. Extrapolating from studies performed using other species may not always be useful, because the pathophysiology of wound healing varies widely among different species. Horses are tight‐skinned and thick‐skinned animals,^2,3 and limb wounds in equids heal mainly by epithelialization, with contraction accounting for 15–20% of the wound closure.^4,5 In contrast, wounds of small mammals heal mainly by contraction, because small mammals are loose‐skinned and have the panniculus carnosus muscle layer that enhances contraction.⁶

Desirable dressing characteristics

Important properties of dressings are listed in Table 6.1.

Minimizing heat and fluid loss is important for maintaining a warm, moist wound environment, which maximizes the rate at which healing takes place (see the following section on Moist wound healing). It is generally desirable for the dressing to be non‐adherent so that pain is minimized and the healing tissues are not disrupted during dressing changes. Moreover, the dressing should be easy to use and cost effective. Although many modern dressings are more expensive than traditional materials, their use may result in more rapid response to treatment and reduced frequency of dressing change, which in turn, may considerably reduce the total cost of treatment.

Other characteristics important for dressings used in humans, such as the dressing’s attractiveness and ability to reduce odor, are of lesser importance in veterinary medicine.

Moist wound healing

Moist wound healing has repeatedly been shown to promote epithelialization and reduce scarring when compared to the healing of wounds in a dry environment.¹⁴ Use of dressings that provide moist healing not only promotes faster healing but also leads to the formation of a more cosmetic scar, which may be of great importance to some horse owners.

Providing moisture to the wound bed has the following beneficial effects on healing:

• Reduced dehydration and cellular death: wound repair requires the activity of a range of cells, such as neutrophils and macrophages, fibroblasts, endothelial cell, and myofibroblasts. The function of these cells in a dry wound is much reduced.
  
• Increased angiogenesis: cells required for angiogenesis thrive in a moist environment.¹⁵
  
• Enhanced autolytic debridement: moisture enhances survival of neutrophils and ensures that proteolytic enzymes responsible for autolytic debridement are carried to the wound.¹⁶
  
• Increased epithelialization: blood supply to dry, crusted wounds is reduced, and the scab produces a barrier to cellular migration, thus slowing rates of epithelialization.¹⁷
  
• Enhancing epithelialization is tremendously important in horses, where epithelialization plays a proportionally greater role in limb wound closure than does contraction.
  
• Decreased rates of infection: bacteria have been shown to pass through up to 64 layers of moist gauze. Conversely, occlusive dressings are designed to form a barrier to bacterial invasion.¹⁸ The warm, moist environment provided by occlusive dressings seems to favor the growth of pathogenic microorganisms leading to infection, but, although occlusive dressings have been shown to encourage microbial proliferation in wounds,¹⁹ the infection rate is actually lower under occlusive dressings than under conventional dry dressings.^20–22
  
• Decreased pain and iatrogenic trauma: the moist wound bed insulates and protects nerve endings, thereby reducing pain.²³ Furthermore, when using occlusive dressings, often fewer dressing changes are required, and therefore, discomfort to the patient and damage to the advancing epithelium are reduced.

Occlusive wound healing

Moist wound healing should be distinguished from occlusive wound healing. An occlusive dressing is impermeable to water vapor, whereas semi‐occlusive/semi‐permeable dressings have varying degrees of permeability to water vapor and allow gaseous exchange. Both occlusive and semi‐occlusive dressings cause retention of moisture in the wound bed. Semi‐occlusive dressings are constructed to allow some moisture to escape from the dressing while still maintaining a moist wound‐dressing interphase. They thereby provide the benefit of moist wound healing while minimizing the risk of maceration. Non‐occlusive dressings are freely permeable to water vapor and gases. As a dressing absorbs exudate, the combination (dressing plus exudate) may create a partially or fully occlusive shield over the wound (Figure 6.1). For instance, a gauze or cotton wool saturated with exudate may become fully occlusive and thereby exert different effects on the healing tissues than intended. Another example is a foam dressing, which is semi‐occlusive but not designed to handle the often thick exudate produced in the wounds of horses. Consequently, as the exudate accumulates in the foam, the dressing becomes occlusive and impermeable. Therefore, the frequency of dressing changes must be adapted to the clinical situation at hand rather than the theoretical fluid‐handling properties of a dressing.

Occlusion results in a mildly acidic pH and a relatively low oxygen tension in the wound bed. Angiogenesis is directed towards regions of low oxygen tension,²⁴ and because low oxygen tension also provides optimal conditions for proliferation of fibroblasts,²⁵ occlusive dressings enhance cellular activities involved in the formation of granulation tissue.

While generally beneficial in other species, the use of occlusive dressings may require a more circumspect approach in horses. A study investigating the effect of applying a fully occlusive dressing to experimental wounds on the limbs of horses showed that this type of dressing caused wounds to develop more exudate, take longer to heal and require more frequent trimming of exuberant granulation tissue (EGT) (11.5 times on average until the wounds were healed at 113 days) compared to control wounds dressed with a dry, non‐adherent gauze‐like pad (in which EGT was trimmed on average 3.5 times until wounds were healed at 53 days).²⁶

The adverse effects of occlusive dressings on the wounds of horses are, however, contested, since the authors of two case series involving six²⁷ and 28 horses²⁸ found that the use of a fully occlusive hydrocolloid dressing (Duoderm, Convatec) resulted in rapid formation of granulation tissue, which was suggested to be an advantage for horses with a severe laceration containing exposed bone or frayed tendons. Clearly, comparisons between small, superficial, experimental wounds and large, accidental wounds involving structures underlying the skin, must be made with caution.

Silicone gel sheet dressings, another form of fully occlusive dressing, have been shown to exert no negative effects on the healing of small, full‐thickness skin wounds created experimentally on the limbs of horses. In fact, while the silicone gel sheet did not accelerate the overall rate of healing, treated wounds showed less propensity to form EGT and a better quality of repair than did wounds dressed with a non‐adherent gauze pad.²⁹ Considering the divergent findings in studies using occlusive dressings for the management of wounds in horses, the effect of occluding equine wounds during the inflammatory and proliferative phases of healing appears to be poorly understood. Wounds should, therefore, be monitored closely if occlusion is anticipated, either as a result of the characteristics of the chosen dressing or the combined effect of the dressing and accumulating exudate.

To avoid the development of EGT, some authors have suggested that occlusive dressings should be used only during the inflammatory phase of healing and that their use should be discontinued at the earliest sign of formation of granulation tissue³⁰ or when the wound bed is covered with granulation tissue.²⁸ EGT developing beneath the occlusive dressing may need to be controlled by excision²⁶ or by other means (for more information on the management of EGT, the reader is referred to Chapter 15). Although it is occlusive, the silicone gel sheet should be considered as a unique treatment entity that differs from other types of occlusive dressings and can be used successfully throughout the proliferative phase of healing to control the development of EGT in the wounds of horses. Its mode of action and indications for use are described in more detail later in the chapter.

To bandage or not to bandage?

Bandages generally consist of three layers. The primary layer is the contact layer that lies directly against the wound, i.e. the dressings described in the following section. The secondary layer is padding that provides protection, immobilization and absorption of exudate. The tertiary layer is made of elastic and adhesive materials that serve to further immobilize, provide compression and fix the entire bandage to the limb. Bandaging techniques are described in more detail in Chapter 7.

The alternative to bandaging is to leave the wound uncovered. This causes a scab to form over the wound (Figure 6.2). Scabs are crusts of dried serum/blood containing erythrocytes, leukocytes, and platelets. They have beneficial effects in the sense that they provide a barrier against foreign material, minimize loss of fluids and proteins from the wound’s surface, span the surface of the wound thereby preventing it from expanding and may facilitate wound contraction. Scabs are, however, not the ideal “dressing” because they slow epithelialization.¹⁷ Moreover, unsightly scars may develop in wounds that are left unbandaged (Figure 6.3).³¹ A study, in which experimental limb wounds in horses were either “neglected” or treated with gauzed soaked in saline, showed that moisture accelerated healing.³² The limbs with the neglected wounds moreover became firmly and permanently swollen, and the author concluded that the lack of a support bandage had resulted in the initial cellulitis, which became organized over the course of the study.³² Bandages and casts applied to full‐thickness wounds at the distal extremity of the limb of horses have been shown to promote the formation of EGT,^31,33–35 possibly as a consequence of the effects mentioned in the section on moist wound healing. Consequently, leaving the wound uncovered may reduce the likelihood of having to trim EGT.^13,31 Moreover, it has been shown that leaving wounds exposed to air greatly reduces numbers of clostridia, Bacteroides, and Pseudomonas in the wound.³⁶

In horses, where financial constraints or practicalities (e.g., temperamental horses that resent application of bandages) have dictated discontinuation of bandaging, this author has successfully used treatment strategies where bandaging is discontinued altogether, or “alternating bandaging” is used (7 days with and 7 days without a bandage). The latter strategy serves to combine the benefits of leaving wounds exposed to air and those of moist healing. Both of these strategies should only be used in wounds that are covered with granulation tissue, and the owner should be instructed to leave the scab in situ. If using the approach of alternating bandaging, the scab should be gently removed by irrigating the wound before applying the bandage. If bandaging is discontinued completely, this author advises horse owners to remove the scab every 7–10 days to inspect the wound that is otherwise completely covered by the scab, to ensure that granulation tissue is healthy with no crevices or necrotic areas and that epithelial ingrowth at the wound’s margin is progressing as expected. Owners should be advised about the cosmetic consequences of not bandaging, and they should be warned that wounded limbs tend to swell when bandaging is discontinued, which may cause discomfort to the horse; swelling and discomfort can often be controlled by light exercise.

Selected products

Dressings are commonly classified according to the following: (1) their function in the wound (e.g., debriding, antibacterial, occlusive, absorbent, adherent); (2) the type of material used to produce the dressing (e.g., hydrocolloid, alginate, collagen, etc.); or (3) the physical form of the dressing (e.g., pad, film, foam, gel). In this chapter, dressings are classified according to their materials of composition into the following: gauze or gauze‐like (i.e., “dry dressings”), alginates, hydrofibers, hydrogels, hydrocolloids, polymeric foams, polymeric films, silicone gel sheets, and biologic dressings. The indications, contraindications, and instructions for use of the different types of dressings are shown in Table 6.2, while Table 6.3 highlights the selection of dressings for specific types of wounds.

For most types of dressings, one or more antiseptic/antibacterial subtypes are available. These dressings are described at the end of the chapter.

Gauze and gauze‐like dressings

Gauze has been used extensively to treat the wounds of horses, especially as wet‐to‐dry dressings for debridement. It is inexpensive, which is one of its main attributes. Gauze sponges are either woven or non‐woven. Woven sponges are traditionally made of 100% cotton fibers (threads) that are woven perpendicularly through each other, whereas non‐woven sponges are a synthetic blend of melded polyester and rayon. Non‐woven sponges are generally up to twice as absorbent as woven gauze and have a softer feel. Both woven and non‐woven sponges are prone to linting, but non‐woven sponges produce less lint. Both types of gauze are highly permeable, relatively non‐occlusive, at least when dry, and have a high absorptive capacity. A wound bed dressed with gauze is thus likely to desiccate.

Wet‐to‐dry dressings are still widely used in equine practice to debride wounds. It is important to understand that the debridement achieved with this type of dressing is non‐selective and removes healthy tissue and cells, such as epidermal cells and fibroblasts, as well as material requiring removal, such as fibrin and necrotic debris. Cross‐contamination may also be an issue with this type of dressing because the saturated gauze in a wet‐to‐dry dressing does not provide a barrier to the environment, thus allowing microorganisms to move into and out of the wound. To create a wet‐to‐dry dressing, gauze soaked in isotonic saline solution is placed within a wound with exudate. As the dressing dries, exudate sticks to it. The gauze is peeled off the wound after approximately 24 hours, and adhered debris and tissue are removed along with the gauze pad. Removal can be painful.³⁷ One to three applications of a wet‐to‐dry dressing have been recommended in exudative wounds;³⁰ continued use is contraindicated because the non‐selective nature of debridement strips away newly formed, fragile epidermis.

Using occlusive dressings (hydrogels or hydrocolloids) to assist autolytic debridement is a gentler, yet slower, approach to achieving a clean wound bed. An in vitro study showed that gauze, hydrofiber, hydrocolloid, and alginate dressings were the most efficient methods of debridement (measured as breakdown of protein in clotted equine blood). Hydrating the dressings in isotonic saline solution, rather than distilled water enhanced efficacy of debridement, and the authors suggested that isotonic saline solution was more efficient in donating water to the eschar.³⁸

The efficient debridement achieved with a traditional gauze dressing may be attributable to the fact that the gauze’s cotton fibers absorb fibrin;³⁹ fibrin absorbed into the gauze imbibes the saline solution, thereby increasing the surface area across which water can be donated to the eschar.

Dressing limb wounds with non‐hydrated gauze swabs was shown to induce the formation of EGT.¹³ Experimental, full‐thickness, 2.5‐cm × 2.5‐cm skin wounds, on the distal extremity of limbs of horses, dressed with gauze, showed greater and more prolonged retraction than unbandaged wounds during the first weeks after wounding. In bandaged wounds, EGT required regular trimming, whereas trimming was not necessary in unbandaged wounds. There was no significant difference in the total number of days to healing or the overall rate of healing in bandaged and unbandaged wounds (75 and 66 days to healing, respectively), provided that EGT was excised regularly from the wounds dressed with gauze.¹³

Roll gauze can be used to pack cavities, such as abscesses after lancing. The packing is extracted gradually over 3–4 days. Debris, necrotic material, and bacteria are evacuated from the cavity as the gauze is extracted. Several types of medicated roll gauze are available.

When adherence of the gauze to the wound is undesirable, for example to avoid disruption of healing tissues, a tulle gras dressing (gauze impregnated with paraffin, lanolin or similar emollient e.g., Jelonet from Smith & Nephew) (Figure 6.4) may be applied. Tulle gras dressings are non‐absorbent, and a secondary layer of absorbent material must be added over the dressing. Other forms of non‐adherent gauze‐like pads are the so‐called dry dressings (e.g., Melolin from Smith & Nephew and Telfa from Kendall) (Figure 6.5). These dry dressings are typically composed of some form of gauze pad or compressed cotton covered by a non‐adherent, thin, perforated polyester film (therefore sometimes also referred to as plastic film‐faced dressings). These dressings allow free exchange of fluid and gas and thus do little to support moist healing. They have limited absorptive capacity and are mainly indicated for sterile coverage of sutured wounds with minimal exudation. Sterile dressings help decrease the opportunity for bacteria to enter or exit wounds, thus reducing the risks for infection and cross‐contamination.

While acceptable for covering sutured wounds, dry dressings may be less ideal for wounds healing by second intention in horses. Several studies suggest that gauze‐like non‐adherent dressings, similar to traditional gauze,^13,26 stimulate the formation of EGT and thus may delay healing. In fact, in experimental studies on wound healing and wound management, where researchers intend to stimulate the formation of EGT, it has been demonstrated that 7–14 days of bandaging with a dry dressing is a consistent means of inducing the formation of EGT (Figure 6.6).^35,46,47

Taken together, the available evidence thus suggests that wounds of horses healing by second intention, especially wounds at the distal extremity of the limb, benefit from dressings that promote healing better than do traditional gauze and gauze‐like dressings.

Chitin and chitosan

Chitin, and its derivative chitosan, is a polysaccharide. Chitin is found in many naturally occurring organisms (e.g. fungi and yeast), and is the principal component in the exoskeleton of sea crustaceans. Chitosan, in contrast, is not found in large amounts in natural sources; therefore, the chitosan used for commercial products is derived from chitin through chemical or enzymatic treatment of shells of shrimp or crab. Chitin and chitosan are biocompatible and biodegradable agents, and they have been shown to exert positive effects on hemostasis and wound healing.

Chitosan is a hemostat; its fibers are positively charged, which enhances blood clotting by binding of the negatively charged red blood cells to the fiber. Chitosan fibers also polymerize with blood into a net‐like structure, which captures red blood cells and further enhances clotting. The hemostatic properties were evaluated in combat wounds, where the main indication for use was gauze failing to achieve hemostasis. In 97% of wounds, application of the chitosan dressing (HemCon Medical Technologies Inc.) caused cessation of bleeding or greatly improved hemostasis.⁴⁸

In addition to its hemostatic properties, chitosan exerts positive effects on several physiologic processes involved in the early phases of wound healing. It stimulates the initial inflammation by attracting leukocytes, and it enhances functions of polymorphonuclear leukocytes and macrophages, such as phagocytosis and synthesis of cytokines.⁴⁹ Chitosan gradually depolymerizes to release N‐acetyl‐β‐D‐glucosamine, which initiates fibroblast proliferation, helps in ordered collagen deposition and stimulates hyaluronic acid (HA) synthesis at the wound site, and it thereby may stimulate formation of granulation tissue. A chitin film was thus shown to lead to faster healing and a stronger wound than did gauze and a commercially available film (Opsite, Smith & Nephew).⁵⁰ In full‐thickness cutaneous wounds in a diabetic mouse model, treatment with chitin‐containing membranes resulted in an accelerated wound closure rate, probably based on an increase in angiogenesis.⁵¹ There are no studies on effects of chitin or chitosan on healing of wounds in horses. From a pathophysiologic rationale, chitin and chitosan dressings seem to be suited for dressing wounds with hemorrhage and wounds in the early phases of healing. Chitosan dressings are available in several formats (e.g. hydrogel, powder, rope/ribbon, film and pad/sheet), from several companies (e.g. HemCom Medical Technologies Inc., Trusetal Verbandstoffwek GMBH, and Aspen Medical).

Chitin and chitosan have some intrinsic antibacterial activity, but the material is also well suited for delivering antimicrobial compounds to burns and infected wounds.⁵² A proprietary dressing containing chitosan and silver nanoparticles exhibited significantly greater bactericidal activity in vitro than chitosan alone, and it reduced mortality from 90% to 14.3% in a Pseudomonas aeruginosa wound infection model in mice compared to gauze dressing.⁵³

Alginates

Alginates (Figure 6.7) are made from acids obtained from seaweed. The calcium salts of alginic, mannuronic, and guluronic acids are processed into non‐woven, biodegradable fibers. Alginates can absorb up to 20, or more, times their weight of a wound’s fluid. When an alginate dressing comes into contact with blood or exudate, ion exchange takes place between the calcium ions in the alginate and the sodium ions in blood or exudate. When sufficient calcium ions are replaced by sodium ions, the alginate fibers swell, partially dissolve, and form a gel. The gel provides a moist environment in the wound and, by doing so, promotes healing. Moreover, alginates have been shown to stimulate both inflammation and fibroplasia,⁵⁴ and from a pathophysiologic rationale they thus seem most suited to the inflammatory phase and the early proliferative phase of wound healing. A systematic review of dressings used to manage acute and chronic wounds in humans concluded that alginates were better than other modern dressing types for debriding necrotic wounds.⁵⁵ This author uses alginates until the wound bed is filled in with granulation tissue (Figure 6.8).

Due to their high absorptive capacity, alginates are useful in moderately to heavily exudative wounds. If used in wounds with little exudation, alginate dressings must be moistened with isotonic saline solution before being applied.

This author uses alginate dressing also over exposed bone, although the dressing is not marketed for that specific purpose. Formation of granulation tissue over bone denuded of periosteum is often a lengthy process because angiogenesis and fibroplasia must occur mainly from the soft tissues at the wound’s margin. Applying an alginate to exposed bone, however, seems to enhance capillary ingrowth from the bone marrow (Figure 6.9). In a study involving 25 human patients with a soft‐tissue defect exposing bone on the head and face, dressing the wound with an alginate compress, which was covered with a hydrocolloid dressing, provided significant advantages and resulted in good coverage of bone with high‐quality granulation tissue.⁵⁶

Alginates can also be used to achieve hemostasis. The high calcium content activates platelets and stimulates coagulation. Some alginates contain zinc, which enhances these prothrombogenic effects of the alginate.⁵⁷ Alginates have been used for hemostatic purposes after hemorrhagic surgery in humans, such as for packing the nasal cavity after trimming the nasal turbinates,⁵⁸ and to temporarily pack the alveolar socket after dental extraction.

Alginates moistened with saline solution containing 0.05% bupivacaine with adrenaline 1:200 000 have been applied to small abrasions and “road rash” in humans to temporarily relieve pain and reduce hemorrhage.⁵⁹ Although this use has not been reported in the veterinary literature, an alginate dressing used in this manner might be appropriate for treating horses with a similar injury.

Alginates are available as flat sheets suitable for covering superficial wounds and as ribbons or ropes that can be used to pack cavities (Figure 6.7). Alginate dressings are very conformable, especially when wet; and the sheet dressings can be folded and used to pack deeper wounds. The gelling action of the alginate results in varying degrees of loss of structural integrity of the alginate. The gelled dressing may resemble purulent material and should not be mistaken for that at dressing change. Because the gel does not adhere to the wound, dressing changes are pain‐free, and newly formed granulation tissue is not disturbed. Moreover, the gel may trap bacteria and thus exert an antibacterial effect. A study in rats showed that in cavitated wounds experimentally infected with Escherichia coli and Staphylococcus epidermidis, packing the wound with an alginate reduced dissemination of bacteria.⁶⁰

Hydrofibers

Hydrofiber dressings (Figure 6.10) consist of sodium carboxymethylcellulose fibers, a substance that can draw exudate away from the wound surface. Hydrofibers are very hydrophilic and, upon contact with fluid in the wound, they form a gel. Although hydrofiber dressings are very similar to alginates, they have been developed to provide superior absorbency; they absorb up to 30 times their weight in wound fluid, and retain the fluid within the interior of the dressing to reduce or eliminate lateral wicking. The dressing provides appropriate moisture at the wound‐dressing interphase with low risk of maceration, and therefore, an oft‐cited advantage is that the dressing can be left on the wound for several days. Another important difference between alginates and hydrofibers is that the latter have no effect on hemostasis.

A study comparing alginate and hydrofiber dressings for the management of chronic leg ulcers in humans found that the dressings exerted similar effects on healing. Hydrofiber dressings were, however, more cost effective, and healthcare workers found that hydrofiber dressings had superior handling properties during application and bandage change.⁶¹ Conversely, in a randomized study using alginates or hydrofiber dressings to cover wounds of 200 human patients after arthroplasty, skin health was better (i.e., fewer skin blisters), and patient comfort superior (i.e., less pain during removal of the dressing) in the alginate group.⁶² There are no studies evaluating the use of hydrofiber dressings in horses. In humans, hydrofiber dressings are mainly indicated for use in moderately to heavily exudating wounds. A secondary layer is required to keep the hydrofiber in place.

Hydrogels

Hydrated polymers or hydrogels are dressings composed of 90–95% water and 5–10% of a polymer or co‐polymer of natural gel‐forming agents such as alginates, carboxymethylcellulose, polyvinylpyrrolidone and pectin. Hydrogels are able to donate moisture to dehydrated tissue. Even though hydrogels are considered to be occlusive, they can absorb some wound fluid into the polymer matrix. The high water content of hydrogel dressings cools the wound, providing pain relief.

These dressings are particularly useful for rehydrating necrotic tissue and enhancing autolytic debridement and thus are indicated for dry wounds containing necrotic tissue or an eschar (Figure 6.11). Burns, in humans, to which hydrogel dressings were applied had earlier onset of epidermal healing than did uncovered burns.⁶³ This earlier onset of epithelial regeneration may have been the result of a shorter debridement phase.

Hydrogels should be applied only to wounds that do not show signs of infection, because they may increase the risk of microorganisms spreading to surrounding tissues.⁶⁴ Hydrogels with antimicrobial properties, such as hydrogel with hypertonic saline (e.g., Hypergel from Mölnlycke), with polyhexamethyl biguanide (PHMB) (e.g., Prontosan Wound Gel from B. Braun), or hydrogel containing silver (e.g., Normlgel Ag from Mölnlycke), may be used in infected wounds. A study in horses showed that wounds dressed with a fully occlusive hydrogel sheet throughout the entire course of healing took significantly longer to heal (113 days) than did wounds dressed with a non‐adherent gauze pad (53 days). The hydrogel stimulated formation of EGT, and hydrogel‐dressed wounds required significantly more excisions of EGT than those dressed with the non‐adherent gauze pad.²⁶ Consequently, such fully occlusive hydrogel sheet dressings may not be suitable for dressing the wounds of horses.