Current concepts in wound dressings

Summary
Wound management has recently become more complex because of new insights into wound healing and the increasing need to manage complex wounds outside hospital. The wound is a synthetic environment in which numerous cellular processes are interlinked in the process of repair. Modern dressings are designed to facilitate the function of the wound rather than just to cover it. Principles of wound dressings are changing, especially in relation to debridement of wounds and control of the wound environment. Occlusive dressings which allow debridement in a fluid environment are equal to traditional wet or dry dressings. There are some advantages in allowing wounds to heal in a moist environment which facilitates cellular migration and epithelialisation. Straightforward surgical and traumatic wounds require low cost and low technology dressings. The place of more costly and complicated wound dressings has to be defined in terms of cost, labour saving and patient comfort, in addition to any putative advantages in the speed of wound healing.

Introduction
Wound management is a significant clinical and economic problem. Pressure sores alone account for a significant amount of health expenditure. A myriad of dressings have been applied to wounds since ancient times¹ and, today, many more new dressing materials are available.

Wound healing
Immediately after wounding, tissue repair begins with haemostasis. Healing then involves a complex chain of events coordinated by key cells, principally macrophages, which use an array of polypeptide growth factors. These newly identified factors act to induce migration and multiplication of cells, as well as the production of other growth factors. The interrelated processes of epithelialisation, angiogenesis, fibroplasia and collagen synthesis occur only when appropriate cells receive growth factor encoded signals.2 The modern view of the wound considers it to be a temporary 'organ' established to effect healing. Following this view, modern dressings are designed to nurture the cellular environment of the wound. Ultimately, dressing design may resemble the technology of tissue culture.

Function of a dressing
Traditionally, dressings fulfilled a number of functions: cosmesis, haemostasis, protection, support and absorption.

Previously, prolonged hospital stays for chronic wound treatment with multiple (and frequently painful) dressing changes were not unusual. Currently, there are insufficient resources to allow chronic inpatient treatment of all wounds and patients are less prepared to accept prolonged morbidity. New parameters for dressings have emerged. A dressing is no longer a passive adjunct to healing, but is an active element of wound management designed to debride the wound, control infection and promote healing. The ideal dressing must be easy to apply, painless to remove and require fewer changes (and less human resources). It must be robust enough to be used by outpatients.

Wound management
Comprehensive wound management includes prevention. Patients at risk of developing chronic wounds such as pressure sores or venous ulcers should be watched and receive appropriate preventive measures.³ Grading scales have been developed to estimate the risk, and surveillance of inpatients at risk has reduced the incidence of pressure sores.

The assessment of existing wounds includes consideration of factors which can affect healing, such as diabetes or ischaemia. Management consists of debridement, the control of infection and the promotion of natural healing.

Debridement
Not all wounds require surgical debridement, but necrotic tissue should be removed from the wound edges and cavity. Traditionally, wounds were debrided with 'wet to dry' dressings where layered gauze was allowed to dry and then removed with necrotic material attached. These dressings required frequent changes which were often painful. Recent developments in dressing technology have introduced hydrogels and other occlusive dressings. These allow in situ degradation of necrotic material which is then absorbed into the fluid phase of the dressing.

Control of infection
Great confusion arises in clinical practice between the significance of a positive wound swab and the risk of invasive infection. Wound healing is only significantly retarded when a sufficient bacterial load (>10⁵ bacteria/g of tissue) is present.⁴ In practical terms, an open draining wound without necrotic tissue will be colonised, but the bacteria present are usually insufficient in number to affect healing.⁵ In contrast to colonisation, invasive infection retards healing and leads to wound breakdown.

For most wounds, all that is required to reduce bacterial counts to tolerable levels is to clean away accumulated debris. Topical antiseptics can be used to irrigate, cleanse and debride wounds. In general, they do not promote the development of drug resistance, but they vary in antibacterial effect, and some species, especially pseudomonas, are resistant.

Cleansing non infected 'clean' wounds is best carried out with saline. For purulent wounds, a range of antiseptics is available⁶,⁷, but some have been suggested to delay healing. Chlorhexidine 0.05% is the most widely used antiseptic. It has a lower incidence of contact dermatitis and less tissue toxicity. Cetrimide 1% has a marked detergent action and is often used for soiled traumatic wounds. Its in vitro cytotoxicity makes it unpopular for routine use. Povidone-iodine is particularly useful against staphylococci, but is less effective against pseudomonas species. It is associated with contact hypersensitivity as well as toxicity from systemic absorption which limits its use as an irrigant in large, deep cavities. The use of hypochlorite solutions, such as Eusol, has declined due to concerns about tissue toxicity. The 'hypochlorite debate' is an example of how conclusions drawn from animal and in vitro data are occasionally mistakenly applied to human wound healing. Hypochlorite was found to be toxic to fibroblasts in vitro, but subsequent studies⁸ showed that the toxicity was concentration dependent and that lower concentrations were not toxic.

Systemic antibiotics do not penetrate necrotic tissue and have little to offer in the management of chronic wounds. Topical antibiotic applications should also be avoided since they are ineffective and foster multiple drug resistant strains of bacteria. Antibiotics are only appropriate when there is invasive infection, such as cellulitis.

Dressings (table 1)
An active dressing aims to establish an optimum microenvironment for healing the wound. It must maintain the wound temperature and moisture level, permit respiration and allow epithelial migration. Optimal wound temperature is required for the function of cells such as macrophages, neutrophils and fibroblasts. The respiration of these cells also requires adequate transfer of oxygen and carbon dioxide across the wound surface. The most favourable environment for the mobility and respiration of cells is a moist wound. Studies of the moist wound environment have shown enhanced epithelial migration, fibroblast function and collagen production. These findings supported the development of occlusive dressings.⁵

Semipermeable adhesive films
These dressings, including Opsite and Tegaderm, are permeable to gas and water vapour, but are a barrier to bacteria and water. They are generally reserved for the definitive closure of superficial, partial thickness wounds where comfort and ease of management are important. The dressings can be left in place for several days, but usually leak if exudate builds up. Removal is easier if the film is stretched before being pulled off.

Hydrocolloids
Dressings such as Comfeel and Duoderm are adhesive, water and gas impermeable membranes. When the inner layer comes into contact with exudate, it forms a gel. They provide an excellent seal around the edges of the wound and can protect pressure areas. The hydrocolloids absorb exudate and help to debride the wound. Patients should be warned that the wound may, at first, become smelly and appear to enlarge. The dressing needs to be changed when the gel leaks out, so to avoid frequent changes, the dressing should have a diameter at least 2 cm bigger than the wound.

The hydrocolloids can be used in the presence of necrotic material, but tend to have problems with overwhelming exudate buildup in large wounds or those where there is anaerobic colonisation.

Alginates
Kaltostat and other alginates are derivatives of seaweed. They are activated by wound exudate to produce a hydrophilic gel. Like hydrocolloids, alginates absorb the non cellular components of the exudate. Alginates provide a satisfactory dressing for lightly contaminated wounds and cavities. They are generally unsatisfactory in the presence of dry, necrotic tissue as there is no exudate to activate them. As alginates are not adhesive, they are easily removed by lavage, but must be held in place by another dressing. Depending on the amount of exudate, alginates can be changed twice a week.

Hydrogels
These gels include Intrasite and are based on starch polymers. They provide moisture to the wound and encourage debridement. Hydrogels are best suited to dry necrotic wounds, but they also absorb exudate while maintaining the products of tissue repair and degradation, including growth factors and lysosomes, in contact with the wound. If the wound is clean, the gel only needs to be replaced once or twice a week. Daily dressings may be needed if the wound is necrotic or infected.

Foams
Dressings such as Lyofoam and Alleryn are highly absorbent synthetic foams. Currently, their main applications are to absorb large volumes of exudate from discharging wounds, reducing the need for dressing changes. They can be used in combination with a hydrogel for necrotic wounds which require debriding.

Conclusion
Definitive protocols for these dressings have not yet developed. Currently, adhesive films are used for superficial wounds with hydrocolloids being used for shallow ulcers and alginates for the deep cavities. Foams are used as an adjunct for heavily exudative wounds. Hydrogels are used increasingly in the debridement of dry or necrotic ulcers, particularly when used with an adhesive film to retain the gel at the wound surface.

Straightforward surgical and traumatic wounds require simple, low cost dressings since healing is likely to be rapid and uncomplicated. Dressing choices for complicated wounds reflect a growing understanding of wound healing physiology. However, comparison of dressings continues to be largely anecdotal and management protocols are still evolving. Choices of dressings increasingly represent a cost/benefit analysis where the high cost of the latest dressing is balanced against savings in time and labour involved in dressing changes.