Wound dressings

Low adherent dressings

Low adherent dressings are cheap and widely available. Their major function is to allow exudate to pass through into a secondary dressing while maintaining a moist wound bed.

Most are manufactured in the form of tulles, which are open weave cloth soaked in soft paraffin or chlorhexidine; textiles; or multilayered or perforated plastic films.

They are designed to reduce adherence at the wound bed and are particularly useful for patients with sensitive or fragile skin.

Semipermeable films

Semipermeable films were one of the first major advances in wound management and heralded a major change in the way wounds were managed. They consist of sterile plastic sheets of polyurethane coated with hypoallergenic acrylic adhesive and are used mainly as a transparent primary wound cover.

Although they are impermeable to fluids and bacteria, they are permeable to air and water vapour, the control of which is dependent on the moisture and vapour transmission rate, which varies depending on the brand. It is through this mechanism that this dressing creates a moist wound environment.

Films are very flexible and are good for wounds on “difficult” anatomical sites—for example, over joints. They are unable to cope with large amounts of exudate, however, and may cause maceration of the skin surrounding the wound bed if they are used injudiciously.

Hydrocolloids

Sodium carboxymethylcellulose, gelatin, pectin, elastomers, and adhesives are bonded to a carrier of semipermeable film or a foam sheet to produce a flat, occlusive, adhesive dressing that forms a gel on the wound surface, promoting moist wound healing. Cross linkage of the materials used influences the viscosity of the gel under the dressing. This gel, which may be yellow and malodorous, may be mistaken for infection by the unwary. Hydrocolloids are virtually impermeable to water vapour and air and can be used to rehydrate dry necrotic eschar and promote autolytic debridement. They are reported to reduce wound pain, and their barrier properties allow the patient to bathe or shower and continue with normal daily activities without disturbing or risking contamination of the wound. Caution should be exercised when using hydrocolloids for wounds that require frequent inspection—for example, for diabetic foot ulcers.

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Hydrocolloid fibres are now available in the form of a hydrophilic, non-woven flat sheet, referred to as hydrofibre dressings. On contact with exudate, fibres are converted from a dry dressing to a soft coherent gel sheet, making them suitable for wounds with a large amount of exudate.

Dry, sloughy leg wound suitable for dressing with hydrogel

Hydrogels

Hydrogels consist of a matrix of insoluble polymers with up to 96% water content enabling them to donate water molecules to the wound surface and to maintain a moist environment at the wound bed. As the polymers are only partially hydrated, hydrogels have the ability to absorb a degree of wound exudate, the amount varying between different brands. They transmit moisture vapour and oxygen, but their bacterial and fluid permeability is dependent on the type of secondary dressing used.

Diabetic foot ulcer with maceration to surrounding skin suitable for dressing with alginate

Hydrogels promote wound debridement by rehydration of non-viable tissue, thus facilitating the process of natural autolysis. Amorphous hydrogels are the most commonly used and are thick, viscous gels.

Hydrogels are considered to be a standard form of management for sloughy or necrotic wounds. They are not indicated for wounds producing high levels of exudate or where there is evidence of gangrenous tissue, which should be kept dry to reduce the risk of infection.

Alginates

Alginates are produced from the naturally occurring calcium and sodium salts of alginic acid found in a family of brown seaweed (Phaeophyceae). They generally fall into one of two kinds: those containing 100% calcium alginate or those that contain a combination of calcium with sodium alginate, usually in a ratio of 80:20.

Alginates are rich in either mannuronic acid or guluronic acid, the relative amount of each influencing the amount of exudate absorbed and the shape the dressing will retain. Alginates partly dissolve on contact with wound fluid to form a hydrophilic gel as a result of the exchange of sodium ions in wound fluid for calcium ions in the dressing. Those high in mannuronic acid (such as Kaltostat) can be washed off the wound easily with saline, but those high in guluronic acid (such as Sorbsan) tend to retain their basic structure and should be removed from the wound bed in one piece.

Alginates can absorb 15 to 20 times their weight of fluid, making them suitable for highly exuding wounds. They should not be used, however, on wounds with little or no exudate as they will adhere to the healing wound surface, causing pain and damaging healthy tissue on removal.

Venous leg ulceration in background of chronic oedema suitable for dressing with foam

Foam dressings

Foam dressings are manufactured as either a polyurethane or silicone foam. They transmit moisture vapour and oxygen and provide thermal insulation to the wound bed. Polyurethane foams consist of two or three layers, including a hydrophilic wound contact surface and a hydrophobic backing, making them highly absorbent. They facilitate uniform dispersion of exudate throughout the absorbent layer and prevent exterior leakage (strike-through) due to the presence of a semipermeable backing.

Top left: Sloughy, infected arterial ulcer suitable for dressing with compound antimicrobial dressing (silver or iodine based). Top right: Gangrenous foot suitable for dressing with antimicrobial iodine impregnated dressing. Left: Malodorous malignant melanoma ulcer suitable for treatment with topical metronidazole

Polyurethane foam dressings are also available as a cavity dressing—small chips of hydrophilic polyurethane foam enclosed in a membrane of perforated polymeric film, giving a loosely filled bag.

Silicone foams consist of a polymer of silicone elastomer derived from two liquids, which, when mixed together, form a foam while expanding to fit the wound shape forming a soft open-cell foam dressing. The major advantage of foam is the ability to contain exudate. In addition, silicone foam dressings protect the area around the wound from further damage.

Antimicrobial dressings

Silver, in ionic or nanocrystalline form, has for many years been used as an antimicrobial agent particularly in the treatment of burns (in the form of silver sulfadiazine cream). The recent development of dressings impregnated with silver has widened its use for many other wound types that are either colonised or infected.

Iodine also has the ability to lower the microbiological load in chronic wounds. Clinically it is mainly used in one of two formats: (a) as povidone-iodine (polyvinylpyrrolidone-iodine complex), an iodophor (a compound of iodine linked to a non-ionic surfactant), which is produced as an impregnated tulle; and (b) as cadexomer iodine (a three dimensional starch lattice containing 0.9% iodine). Cadexomer iodine has good absorptive properties: 1 g of cadexomer iodine can absorb up to 7 ml of fluid. As fluid is absorbed, iodine is slowly released, reducing the bacterial load and also debriding the wound of debris. This mode of action facilitates the delivery of iodine over a prolonged period of time—thus, in theory, maintaining a constant level of iodine in the wound bed.

Caution is required in patients with a thyroid disease owing to possible systemic uptake of iodine. For this reason, thyroid function should be monitored in patients who are treated with iodine dressings.

Metronidazole gel is often used for the control of odour caused by anaerobic bacteria. This is particularly useful in the management of fungating malignant wounds. It may be used alone or as an adjunct to other dressings.

Unwanted effects of dressings

Maceration of the skin surrounding a wound may occur if a dressing with a low absorptive capacity is used on a heavily exuding wound. If the dressing is highly absorptive then more frequent dressing changes may be needed, in addition to investigation and management of the cause of the exudate (such as infection).

The skin surrounding a highly exuding wound may be further protected through the use of emollients (such as 50:50 mix of white soft paraffin and liquid paraffin) or the application of barrier films (such as Cavilon). Conversely, use of a highly absorptive dressing on a dry wound may lead to disruption of healthy tissue on the wound surface and cause pain when removed.

Abdominoperineal resection wound treated with vacuum assisted closure. The skin edges are protected with a barrier cream to prevent maceration

Allergic reactions are not uncommon: the dressing should be avoided, and the allergy may need to be treated with potent topical steroids. Tapes used to keep dressings in place are common causes of allergy. Many dressings require secondary dressings—for example, padding on highly exuding wounds—which may make them bulky. Secondary dressings should not be too tight, especially on patients with peripheral vascular disease.

Left: Allergy to dressing used to treat arterial leg ulceration. Note erythematous skin with sharply demarcated edges corresponding to the shape of the offending dressing. Right: Ulceration over the anterior aspect of the ankle caused by inappropriately tight bandage