Silver Lining

10 April 2008 (Last Updated April 10th, 2008 18:30)

The search for better treatments for chronic wounds is ongoing. However, it has recently led to the rediscovery of a substance that has been recognised as a potent antiseptic since ancient times. Cathy Thomas Hess and Robert S Kirsner examine the clinical allure of silver.

During the past several decades, major advances have been made in the practice of wound care. Research has led to new insights and an understanding that a chronic wound has molecular, microbiological and clinical characteristics that distinguish it from an acute, healing wound. On a cellular and molecular level, these wounds possess features that oppose healing.

Infection, prolonged hypoxia, biofilms, trapping of growth factors, excess fibrin and senescent fibroblasts can all lead to wound chronicity. Bacteria play a particularly complex role in the microenvironment of a chronic wound.

Medical practitioners have actively looked at the coordinated cellular and biochemical events that take place in wound healing, and manufacturers of wound care products have partnered with practitioners to identify materials that assist in the management of both simple and complex wounds. At the same time, standards for describing and documenting skin and wounds are being disseminated in many forms to assist practitioners in documenting their assessments.

Looking back at the development of wound care over the decades, much progress has been made in accelerating wound healing:

  • Dressings have become available that manage wounds with various exudate levels, support autolytic debridement, promote granulation tissue and, ultimately, achieve wound closure.
  • Clinicians have realised that certain dressing types, such as occlusive dressings, assist in pain management.
  • Bioengineered skin products have become available to jump-start and stimulate the wound healing process in hard-to-heal or stalled wounds. Some of these constructs are available as dermal replacements as well as epidermal/dermal replacements.
  • Devices to manage the wound healing through the removal of wound fluid have become available.
  • Compression therapy has advanced. While still of value, the Unna Boot has now been joined by multi-layered, sustained, graduated compression wraps and devices to better assist edema control and wound healing.
  • Support surface technology has continued to promote optimal environments for pressure relief using pressure-reducing and pressure-relieving products.
  • Documentation systems for wound care have continued to be refined and made available to practitioners in paper form and as computerised record systems.
  • Novel biochemically active dressings have been developed, including dressings that release iodine and silver to decrease the bioburden in wounds, while minimising the cytotoxicity associated with commonly used antiseptics.

This article focuses on the use of silver dressings in wound care and their utility in managing a wound's bioburden.

ANCIENT REMEDY

Historically, silver has been used to treat infection going right back to ancient Greece, while in more recent times, the effectiveness of silver as an antiseptic has been well recognised and documented:

  • The ancient Greeks lined their eating and drinking vessels with silver to prevent infection.
  • The use of silverware became popular in the first century AD.
  • During bubonic plague outbreaks in Europe, parents were told to give their children silver spoons to suck on to prevent infection.
  • American settlers often put silver dollars in milk containers to delay its spoilage.
  • In 1884, a German obstetrician, Credé, discovered that a mild silver solution put into babies eyes at birth helped prevent eye infections.
  • Soldiers in the American Civil War swallowed whole silver dollars to ward off disease.
  • In the late 1930s, silver-containing medical products were used to treat numerous diseases until antibiotics came into general use.
  • In 1965, the antibacterial activity of compresses soaked with 0.5% silver nitrate applied to extensive burns was reported.
  • The value of silver nitrate against Pseudomonas aeruginosa was documented. Pseudomonas aeruginosa was thought to be a primary cause of death in patients with extensive burn wounds.
  • In 1968, documented complications associated with silver nitrate compresses led to the use of silver sulfadiazine (silver formulated with a sulfonamide).
  • In 1968, Fox reported silver sulfadiazine to be more effective than silver nitrate, particularly against Gram-negative bacilli.
  • In 1988, Larry C Ford, of the UCLA School of Medicine, and other researchers showed that destructive bacteria, viruses and fungus organisms are killed in minutes of contact with metallic silver.

The use of silver declined in the middle part of the 20th century, but its use increased again in the 1990s, when it was used in novel wound dressings.

SILVER REASSESSED

The appearance of multi-drug-resistant organisms has led researchers to reassess older antiseptics, including silver. While the use of antiseptics in wound management was once abandoned, because of the cytotoxicity observed, these observations were primarily made in cell cultures and acute wounds in animals, not in chronic non-healing wounds, where their benefits might outweigh their detrimental effect on the wound healing process.

In an effort to reduce the potential for cytotoxicity, products containing iodine and silver have been developed to decrease the bioburden in the wound, while minimising the cytotoxicity seen in commonly used antiseptics.

"Silver dressings provide a broad range of antimicrobial activity"

Silver compounds have been widely used as wound antiseptics, mainly in burns. Silver sulfadiazine (SSD) and silver nitrate have been among the most commonly used compounds. SSD is the most widely used treatment for the prevention of infection in patients with burn wounds. Combinations of SSD with cerium nitrate and nanocrystalline silver releasing systems (Acticoat®) have been developed to increase SSD's efficacy and reduce its toxicity. Newer silver formulations appear to increase the rate and degree of microbial killing and decrease exudate formation, while remaining active for days.

Animal studies examining the effects of SSD and silver nitrate on wounds have shown no significant effect on epithelisation rates. But SSD has been found to increase the rate of neovascularisation. In rats, silver compounds promote wound healing, reduce the inflammatory and granulation phases of healing, and influence metal ion binding. Geronemus et al found an increased reepithelisation rate in domestic pigs with the use of SSD. However, Leitch et al found that SSD inhibited wound contraction in an acute wound rat model. Likewise, Niedner et al found a slight and insignificant reduction in granulation tissue formation with silver nitrate.

While the use of silver compounds on burn patients is widely accepted, its use for treating other chronic wounds has been somewhat controversial.

Kucan et al examined the effects of SSD on bacterial counts in patients with infected chronic pressure ulcers. They found SSD to be effective in decreasing the bacteria in all the ulcers treated. In a randomised trial with venous ulcers, 1% SSD cream reduced the size of ulcers compared with placebo, while in another study it was found to be well tolerated and effective in wound cleaning and promoting granulation tissue formation.

Livingstone et al studied the effect of silver nitrate and an antibiotic solution (neomycin plus bacitracin) in reducing autogenous skin graft loss resulting from infection in patients with thermal injuries. They found both medications to be effective compared with the control group (Ringer's lactate solution), but the antibiotic solution was associated with the rapid emergence of drug-resistant organisms, while silver nitrate was not.

Nanocrystalline silver compounds have been found to increase the reepithelisation rate of meshed autografts and show promise as a treatment for other chronic wounds. The anti-inflammatory effects of silver could be associated with the vehicle, which reduces wound drying and inflammation – moist wounds have been found to be significantly less likely to be infected.

SILVER DRESSINGS

Silver dressings are topical wound care products derived from ionic silver. The dressings come in various delivery systems as well as different shapes and sizes. They release a steady amount of silver into the wound, which has an antimicrobial or antibacterial action. The silver is released from the dressing onto a wound's surface in proportion to the amount of exudate and bacteria in the wound.

Silver dressings are available in foams, hydrocolloids, barriers layers and charcoal cloth dressings. They may be used with topical and adjunctive therapies to decrease the bacterial load, contain exudate and optimise the appearance of the wound's granulation tissue. Silver dressings may also be used as primary or secondary dressings to manage minimal, moderate or heavy exudates for acute and chronic wounds, including burns, surgical wounds, diabetic foot ulcers, pressure ulcers and leg ulcers. They may be used under compression.

Silver dressings provide a broad range of antimicrobial activity: they reduce or prevent infection and alter metalloproteinases within wounds. However, clinicians who use the product should be mindful that silver may cause staining on wounds and intact skin as well as stinging or sensitisation.

PRACTICAL HEALING

When skin integrity is altered and a wound results, the healing process begins. This process is generally well orchestrated, leading to the repair of the injury. However, chronic wounds do not follow this complex healing model. Because of an impediment to the healing process, the wound becomes 'stuck' in the inflammatory or proliferative phase. Over time, key cells become senescent. Understanding and correcting the barriers to healing will spark the formation of granulation tissue, leading to the next stage of healing.

In recent the recent past, a multistep process has been needed to reverse the chronicity of wounds and move them toward healing. This process – called wound bed preparation – can be thought of as the global management of the wound to accelerate endogenous healing or to facilitate the effectiveness of other therapeutic measures. Wound bed preparation can accelerate the healing process and optimise the topical management and ancillary services for wound caring.

PREPARING THE WOUND BED

After assessing a patient and their wound, a clinician can develop a care plan that will prepare the wound bed for healing. The goals of wound bed preparation include removing necrotic or fibrinous tissue, reducing the total number of senescent or abnormal cells, decreasing exudate, decreasing the bacterial load and increasing granulation tissue. These goals are achieved through a multistep process employing strategies that are designed to improve the wound status.

BACTERIAL BALANCE

Most wounds contain a variety of organisms. The strategy of bacterial balance stresses the need for the clinician to recognise when the bacterial load has increased through a change in granulation tissue appearance and exudate amount.

In a chronic wound that is healing, the level of bacteria present is referred to as contamination/colonisation. This is a steady state of replicating organisms that maintain a presence in the wound but do not cause injury or delay the wound healing process.

For a patient with contamination/colonisation, the clinician should select topical therapies that can create and maintain a moist wound environment. Adjunctive therapies, in combination with absorbent topical management products, may assist the patient in achieving wound healing. For example, compression therapy may be combined with a moist wound dressing in a patient with a venous ulcer. Patient education for the care of the chronic wound is paramount in achieving and maintaining a healed wound.

The next level of bacterial load is critical colonisation. This can be characterised as replicating (infectious) organisms present in the wound that begin to cause a change in the wound's status. The clinician may see understated clinical features in the wound's appearance, including:

  • Foul or excessive odour
  • Absent or abnormal granulation tissue
  • A change in the colour of the wound bed from previous evaluations
  • Delayed healing
  • Friable granulation tissue
  • Severe or increased pain at the wound site
  • Excessive or increased serous exudate
  • Serous exudate with concurrent redness of surrounding periwound wound edges
  • Tunnelling or pocketing of the wound

The clinician should select topical therapies that will reduce the bacterial load (such as silver dressings), contain exudate and improve the qualities of the wound's granulation tissue. As with contamination/colonisation, the patient may benefit from adjunctive therapies, such as compression therapy, in combination with absorbent topical management products. The patient should be educated to help achieve and maintain a healed wound.

A wound infection can be characterised as organisms present in the wound and surrounding soft tissue that result in a host response and lead to non-healing or wound decline (an increase in wound size or pain, for example). Classic clinical signs and symptoms include:

  • Periwound and soft tissue edema
  • Periwound and soft tissue erythema
  • Fever
  • Foul odour
  • Severe or increasing pain at the wound site
  • Tenderness at the wound and periwound site and in surrounding soft tissue
  • Excessive and/or purulent drainage
  • Warmth in the surrounding soft tissue and periwound skin
  • Evaluated white blood cell count with an increase in newly developed cells (bands)

Appropriate systemic (oral or intravenous) antibiotics used with select topical therapies, such as silver dressings, can be employed to treat the bacterial infection, contain exudate and improve the qualities of the wound's granulation tissue. Consideration should be given to using adjunctive therapies, such as compression therapy, in combination with absorbent topical management products. Once again, the patient should be educated about achieving and maintaining a healed wound.

"The appearance of multi-drug-resistant organisms has led researchers to reassess older antiseptics, including silver."

DEBRIDEMENT

Through the process of removing devitalised tissue and foreign material from a wound, the clinician may also remove senescent and nonmigratory cells in and around the wound. Removing these may contribute to the release of available growth factors in the wound.

Debridement techniques include autolytic, biosurgical, enzymatic, mechanical and surgical proceedures. The type and frequency of the debridement technique chosen depends on the overall patient condition and treatment plan.

EXUDATE MANAGEMENT

Chronic wound fluid inhibits cell growth in culture and is associated with barriers to healing, including necrotic tissue and bacterial imbalance. Controlling the amount of exudate may improve healing by improving the migration of key cells, such as keratinocytes, fibroblasts and endothelial cells, as well as MMPs and other proteases. The type of exudate management device used will depend on the aetiology of the wound and may include compression therapy, mechanical or absorptive dressing management.

EASING THE BURDEN

The burden of chronic wounds is high. Associated morbidity, decreased quality of life and, in some cases, mortality are among the many reasons significant interest has been focused on the prevention and treatment of the most common types of these wounds.

Choosing silver to manage the wound's bioburden, based on the wound's appearance, may be appropriate. Topical silver has a broad-spectrum action that suggests it is efficient in treating chronic, refractory wounds. Different delivery systems may have advantages for individual patients, bacterial killing and exudate management.