A Review of Moisture-Control Dressings in Wound Care

Abstract

Wound healing progresses most rapidly in an environment that is clean, moist (but not wet), insulated, and protected from trauma and bacterial invasion. Selection of a dressing is based on specific wound characteristics: (1) the presence or absence of necrotic tissue, (2) bacterial load, (3) exudate volume, and (4) need for a bacterial barrier. Therefore, the optimal dressing should be able to control exudate without desiccating the wound surface; act as a bacterial barrier; allow atraumatic removal with no dressing left in the wound; and provide moisture vapor permeability sufficient to prevent overhydration of the wound and surrounding skin. There are a multitude of dressings on the market, and clinicians are frequently confused as to which type of dressing is the best for a particular wound. Effective dressing selection requires both accurate wound assessment and current knowledge of available dressings. The articles in the current supplement to the Journal of Wound, Ostomy and Continence Nursing review research about wound exudate management and the use of foam dressings.

Historically, it was believed that a dry wound environment, such as one achieved with exposure to air, promoted healing, lowered the risk for infection with gramnegative bacteria such as Pseudomonas species, and reduced costs. In fact, some burn units continue to expose burn wounds to warm dry air.¹ This practice pattern began to change around 1962, when Winter² demonstrated that epidermal healing was enhanced in a moist wound environment, as compared with wounds exposed to air. It has also been shown that a moist environment reduces eschar formation and shortens the time required for wound healing.³ The impact of moisture on wound healing was further investigated by a team of researchers in 1995.¹ They used a porcine model to evaluate the effects of various levels of hydration on healing of partial-thickness burn wounds.¹ In the study, wounds were subjected to dry, moist, and wet conditions. Wet conditions were achieved by immersing the wound in a saline-filled chamber, moist conditions were achieved by placing a hydrocolloid dressing over the wound, and dry conditions were achieved by placing dry

sterile gauze over the wound. All groups showed signs of re-epithelialization at day 3, but the rate of complete reepithelialization differed. Epithelialization in wet, moist, and dry wounds was completed by days 6, 7, and 8, respectively. Linear regression analysis of epidermal healing (ie, complete resurfacing of epidermis) showed that healing in both wet and moist wounds was significantly faster than that in dry wounds (P < 0.0001 and 0.005, respectively). In contrast, the speed of healing in moist and wet wounds was not significantly different (P > 0.075).⁴ This difference in healing rates is also reflected in the difference in total epidermal thickness on day 7; wounds managed in a wet environment, achieved an average epidermal thickness of 204 mcg, compared with 141 mcg for wounds managed in a moist environment and 129 mcg for wounds managed in a dry environment. Results of the study suggest that a wet environment may produce greater epidermal thickness than does a moist environment. However, the number of epidermal cell layers did not differ between the wounds managed in the wet or moist environments.

Multiple studies demonstrate that occlusive dressings have the potential to reduce pain, prevent wound deterioration (eschar development and/or extension of tissue damage), and promote rapid wound healing.^2,5–13 Data also suggest that a “liquid” wound environment can promote wound healing without causing maceration of the surrounding tissue, and wet or moist conditions are associated with less inflammation and scarring than are dry conditions (air exposure).³ Specifically, results of a study using an animal (porcine) model suggest that the depth of destruction in partial-thickness burns in pigs can be increased by a dry wound surface or minimized by a moist wound surface.³ Specifically, the final depth of injury was greatest in wounds exposed to air, but reduced in wounds managed in a chamber with 100% humidity, and minimized in wounds managed in a saline-filled chamber. Results also demonstrated the absence of dermal necrosis in wounds managed in a moist or wet environment. In contrast, dermal necrosis was the most common finding in wounds managed with a dry environment.

Another study evaluated the relationship between wound healing and dressings with variable water vapor transmission rates (WVTR).⁴ The researchers state that a low WVTR is a reliable indicator of a dressing's capacity to retain moisture and thus to provide an environment that supports healing. The authors also suggest that the WVTR of a dressing can serve as one predictor of healing. Although strong evidence correlating WVTR and wound healing rates is lacking, there are data indicating that healing rates for similar wounds follow patterns that can be predicted by the “moisture retention” characteristics of the dressing in use.⁴

It is now well accepted that a moist wound environment is more favorable to healing than is a dry environment. Nevertheless, data regarding the benefits of a “wet,” as opposed to “moist,” surface are contradictory. It might be assumed that, if a moist environment is better than a dry one, a wet environment would be better than a moist one. However, maceration of intact skin is sometimes seen with heavily exudative wounds and is frequently attributed to use of dressings with insufficient absorptive capacity. In evaluating the role of dressings in causing or exacerbating maceration, the clinician should remember that high volumes of exudate and maceration of intact skin are more commonly seen in wounds that are actively deteriorating or inappropriately managed. For example, appropriate management of a venous leg ulcer must involve sustained graduated compression, as well as appropriate wound dressings.⁴ Failure to control edema with compression therapy results in increased wound exudate and maceration of the surrounding skin. When the ulcer deteriorates because of the underlying issues with uncontrolled venous insufficiency and edema, the clinician may assume that the more visible maceration is the cause of the deterioration. Another point of potential confusion occurs when the advancing edge of new epithelium appears pale pink or almost white, sometimes leading to a mistaken assumption that this advancing epithelial border is a sign of maceration. If the clinician then switches to dry dressings to correct the problem, the advancing epithelial cells may become desiccated and the fragile new epithelium disrupted with dressing removal, again reinforcing the perception that maceration caused the deterioration.⁴ Some clinicians recommend that any change in topical therapy in response to apparent maceration should be delayed for 24 to 48 hours; during this time the new epidermis will become pink, whereas truly macerated tissue will remain white and wet to the touch.

Another adverse development commonly attributed to moisture retentive dressings is the development of hypergranulation tissue. Hypergranulation tissue occurs when granulation tissue extends above the surrounding skin surface during the proliferative phase of wound healing, as opposed to hypertrophic scarring, which involves continued deposition of collagen under intact skin during the maturation phase of healing. There is no evidence that either of these phenomena are linked to moisture retentive dressings.⁴ Zitelli¹⁴ demonstrated that hypertrophic tissue developing after Mohs micrographic surgery resolved naturally in the presence of a moist environment created by a hydrocolloid dressing. The only exception was hypergranulation tissue that was pedunculated and extended laterally over intact skin.¹⁴ Thus, the data currently available dispute the perception that hypergranulation tissue is a direct result of moist wound healing environments and further indicate that hypergranulation tissue can be effectively managed with moisture retentive dressings. In addition, a growing body of evidence indicates that moist wound environments reduce the elevation, area, and inflammation associated with the scarring normally seen in acute wounds.^15,16

Although the goals of topical therapy include effective control of wound exudate and prevention of tissue overhydration, they must be balanced against the need to maintain a moist wound surface. Thus, the clinician should be cognizant of research indicating that dressings with higher moisture-retention capacity are associated with fewer clinical infections, greater patient comfort, and reduced scarring.^3,12,15,16 Wound care clinicians should avoid desiccating therapies, such as heat lamps and dry gauze, and pay close attention to the moisture-retention properties of the products they are using.¹⁷

The WVTR has been identified as an effective and objective indicator of the moisture-retention capacity of various dressings.^18,19 Dressing WVTR is emerging as an objective indicator of a dressing's moisture-retention capacity, which is associated with positive outcomes in wound healing.⁷ When selecting dressings for wound management, the clinician must balance contact dressings designed for exudate management, such as alginates, hydrofibers, and moistened gauze, against cover dressings designed to maintain a moist wound surface (eg, moisture-retentive cover dressings or dressings that combine absorptive components in the inner layer with a highly moisture-retentive outer layer).^20-22

As noted previously, a moist wound environment (quantified as the balance between the volume of wound exudate and the dressing's absorptive capacity and WVTR) has been reported to have predictive validity for healing when all other variables are held constant.^23,24 The effects of moisture on epidermal regeneration include enhanced keratinocyte migration, proliferation, and differentiation. In addition, moisture encourages fibroblast proliferation, collagen synthesis, endothelial cell proliferation, new vessel formation (angiogenesis), and wound contraction. The effects on scar maturation (remodeling) include scar elevation, color, and inflammation.²⁵ A moist wound healing environment is associated with each of the following: (1) shorter and less intense inflammatory phase²⁵; (2) earlier proliferation, migration, and differentiation of keratinocytes, resulting in earlier restoration of the epidermal barrier function^2,4,18,19,26; (3) enhanced fibroblast proliferation and collagen synthesis, coupled with earlier angiogenesis²⁷; and (4) earlier wound contraction.¹⁹ Wound contraction and reduction in wound surface area are strong predictors of wound healing.^28-30

Although moisture is clearly critical for wound healing, the optimal level of moisture has not been clearly defined. It has been suggested that a dressing's WVTR may predict the rate of healing, although of course the moisture content at the wound surface is also affected by the volume of exudate. Dressings with a WVTR of <35 g/m²/h can be defined as moisture retentive.²³ The relationship between moisture-retentive dressings (dressings with low WVTR) and partial-thickness wound healing was evaluated, using reepithelialization at 7 days as an indicator of re-establishment of the epidermal barrier.^18,19 One study using a porcine model involved a total of 46 dressing trials: 9 different hydrocolloid dressings (28 trials), 2 transparent adhesive films (2 trials), 6 foam dressings (7 trials), and 1 gel dressing (1 trial).⁴ During each experiment, fresh wounds were immediately dressed with the study dressing and temperature and relative humidity were standardized and controlled. The WVTR was measured when the dressing had been in place for 24 hours using a ServoMed EP-1 Evaporimeter, with the results reported as grams of water transmitted through the dressing per square meter of dressing per hour (g/m²/h.). The mean WVTR was calculated on day 1, and findings were correlated with the return of epithelial barrier function (re-epithelialization) on day 7 after wounding. The researchers found an inverse relationship between WVTR and re-epithelialization rates. Specifically, moistureretentive dressings were positively correlated with restoration of the epidermal barrier (Table 1).

TABLE 1: Partial-and Full-Thickness Wound Healing in Swine as a Function of Dressing Category Water Vapor Transmission Rates

The effect of moisture-retentive dressings on fullthickness wound healing has also been investigated. One group of studies used 8 full-thickness excisional wounds on each of 39 swine in 10 different studies involving a total of 22 dressings.⁴ The types of dressings that were evaluated included 3 different hydrocolloid dressings (11 trials), dry gauze (4 trials), 2 transparent adhesive dressings (5 trials), and 1 foam dressing (2 trials). The moisture-retentive capacity of the various dressings was determined via WVTR measurements conducted on day 1 after wounding, and correlated to progress in healing at day 7 after wounding. Progress toward wound healing was operationally defined as the percentage of wound contraction as determined by image analysis from wound tracings. Lower dressing WVTR (enhanced moisture retention) was a significant linear predictor of enhanced healing for both partial-thickness and full-thickness wounds.⁴ However, WVTR should not be considered the only indicator of efficacy for a given dressing.

Impact on Infection

Historically, many clinicians believed moist wound environments would lead to increased incidence of wound infections. However, a retrospective review of 30 years of evidence suggests that wound care clinicians should be more concerned about the infection rates associated with less moisture-retentive dressings.¹² Data summarized in this review indicate that hydrocolloid dressings are associated with the lowest incidence of clinical infection (1.3%), foams are associated with a 2.4% infection rate, transparent adhesive films are associated with a 4.5% incidence of infection, and gauze is associated with a 7.1% incidence. Researchers and clinicians hypothesize that these findings can be explained by moisture-retentive dressings keeping the wound bed moist and viable, thus minimizing the formation of necrotic tissue, which is a known medium for bacterial growth.³¹ This issue is addressed in greater depth in another article in this supplement.

Impact on Pain

Acute and chronic wounds are frequently associated with pain, and wound care itself frequently causes or exacerbates pain. One advantage of moisture-retentive dressings is their ability to reduce pain. A number of studies have identified that dressings with greater moisture retention are associated with lower levels of pain.^26,32–34 This issue is addressed in greater depth in another article in this supplement.

Studies Related to Wound Healing with Specific Dressings

A number of studies have been completed comparing healing rates among similar wounds managed with different dressings. Allevyn (Smith & Nephew Wound Management Division, Largo, FL) hydrocellular foam was compared to gauze impregnated with zinc oxide and gelatin (the contact layer for Unna's Boot) (Graham-Field, Inc., Atlanta, GA) in a nonrandomized study involving 46 patients with leg ulcers caused by chronic venous insufficiency.³⁵ Eighteen patients were treated with the impregnated gauze as the wound contact layer, and the remaining 28 patients were treated with the hydrocellular foam dressing as the wound contact layer. In all cases, the wound contact dressing was covered with a single layer of gauze and a 2-inch elastic bandage applied in a pressure-gradient fashion from the base of the toes to the upper calf. Analysis of variance revealed significant differences between the hydrocellular group and the impregnated gauze group at the beginning of the study; specifically, the hydrocellular group included ulcers of longer duration and greater depth. In the hydrocellular group, 63% of the ulcers had been present for more than 1 year, as compared with 71% of the ulcers in the impregnated gauze group being present for less than 6 months. In addition, the ulcers in the hydrocellular group were larger at baseline than were the ulcers in the Unna Boot group. Although the greater severity of the ulcers in the hydrocellular group would seem to favor the impregnated gauze group, no significant difference in healing rates for the 2 groups were found. In fact, once the scores were adjusted to account for the difference in ulcer severity, healing rates were significantly better for the hydrocellular group. An examination of mean ulcer reduction rates revealed a statistically significant (P < 0.001) reduction in ulcer size at 8.11 weeks for the hydrocellular group (compared with 15.72 weeks for the impregnated gauze group).³⁵

A hydrocellular foam dressing (Allevyn) was compared with paraffin gauze in a study of 50 patients with partial-thickness, skin-graft donor sites.³⁶ Subjects acted as their own control; half of the area of the donor site wound was treated with the hydrocellular foam dressing and half with the paraffin gauze. The average time to complete epithelialization was almost twice as fast for donor wound segments managed by foam (5.46 days) than for wound segments managed by paraffin gauze (mean 9.0 days).

Allevyn hydrocellular foam was compared with Kaltostat calcium alginate (E. R. Squibb, Princeton, NJ) in a prospective randomized study involving 20 patients with standard coccygeal pilonidal excision wounds.³⁷ Wound assessments were carried out weekly and involved wound measurements (length, width, and depth) and wound photographs. Weekly biopsies (6-mm punch biopsies 1 cm from the wound edge) and regular bacteriologic swabs were also taken. No significant differences were found in rates of healing; the mean time to healing for the hydrocellular group was 56.7 days and the mean time to healing for the alginate group was 65.5 days. However, histologic analysis of the wounds demonstrated prolonged edema in the alginate group, and early tissue sections were characterized by the incorporation of alginate fibers surrounded by a giant cell foreign body reaction. This finding suggests that the alginate material was incorporated into the granulation tissue during the initial phases of healing, but the significance of this is unknown because there were no real differences in healing rates.

A hydrocellular foam (Allevyn Adhesive) was compared with a hydrocolloid dressing (Granuflex, E. R. Squibb) in a study of 60 pressure ulcers.³⁸ In this study, “non-dressing-related” adverse effects were reported for 10% of the patients managed with hydrocellular foam, as compared with 6% of the patients managed with a hydrocolloid dressing. The dressing was judged unsuitable in 3% of the patients managed with hydrocellular, as compared with 10% of the patients managed with hydrocolloid. Unfortunately, the authors did not clearly define the term “unsuitable,” although they did mentioned factors such as nonconformability and leakage. No pressure ulcer managed by the hydrocellular foam dressing failed to progress, and only 3% of patients experienced wound deterioration. In contrast, failure to progress occurred in 3% of the patients managed with hydrocolloid, and 6% experienced wound deterioration. “Rolling” of the dressing was not reported in any subject managed with hydrocellular foam, as compared with 6% of those managed with hydrocolloid. One patient managed with the hydrocellular foam dressing developed a localized rash, which did not occur in the hydrocolloid group. Dressing removal was associated with trauma to the wound bed in 3% of the patients managed with hydrocolloid dressing. Trauma did not occur with the hydrocellular foam.

Hydrocellular foam dressings were compared with hydrocolloid dressings in a cost-effectiveness study (n = 100) with a number of outcomes measures.³⁹ Adverse events were described in 3 of the patients managed with hydrocellular foam, but none were related to the dressing itself. Eleven patients managed with hydrocolloid dressings experienced adverse events, and 7 of these were related to the dressing. Wound healing outcomes differed significantly (P = 0.045) for these 2 groups; 23 of the 50 patients managed with hydrocellular dressings were healed at 56 days, as compared with 15 of the 46 patients managed with hydro-colloid dressings. This difference was noted even though the wounds in the hydrocellular group were significantly larger at baseline (P = 0.013).

Wound healing rates for hydrocellular foam dressings and hydrocolloid dressings were compared in a prospective, randomized study with 100 community-based patients and a parallel group design.⁴⁰ Healing rates for patients with venous ulcers managed with hydrocellular foam were reported as 13%, as compared with 7% for patients managed with hydrocolloid dressings. For the pressure ulcer groups, healing rates were 59% for patients managed with hydro-cellular foam, and 27% for patients managed with the hydrocolloid dressings. For wounds described as “other,” healing rates were 65% for the hydrocellular foam group and 59% for the hydrocolloid group. When all types of wounds were considered, healing rates were 46% for patients managed with hydrocellular foam and 33% for those managed with hydrocolloid dressings.

Two different foam dressings, Allevyn and Lyofoam (E. R. Squibb), were evaluated in a prospective, stratified, randomized clinical trial involving 61 wounds (pressure ulcers and other wounds).⁴¹ Eleven patients in the Lyofoam group withdrew before the end of the study, 4 because of wound infections unrelated to the dressing and 1 because of skin maceration directly related to the dressing. In addition, 1 was withdrawn because of lack of efficacy of the treatment, 1 because of a chest infection, 1 because of logistical problems, 1 because of heart failure, 1 because of urinary tract infection, and 1 subject died during data collection. In the Allevyn group, there were 9 withdrawals. Seven withdrawals were because of infections unrelated to the dressing, 1 because of pain of the periwound skin, and 1 because of a cavity wound that required a filler dressing. A total of 4 wounds healed during the study; 3 were managed by Lyofoam group and 1 by the Allevyn foam. There were no significant differences between the 2 groups with regard to patient comfort, and the researchers reported that both dressings performed well with respect to absorbency, with no significant difference in wear time. The investigators did not report quantitative data concerning reduction in wound size, but they stated that “almost all of the cases showed signs of improvement.” The lack of quantitative data limits the ability to draw firm conclusions from this study.

Spyrosorb foam dressing (CV Laboratories, Surry, United Kingdom) was compared with Granuflex hydrocolloid (ER Squibb) in a randomized study of 40 patients with grade 2 and 3 pressure ulcers.⁴² Randomization ensured that characteristics of the 2 groups were similar in most regards, but the average duration of wounds in the foam group was significantly longer than that for subjects in the hydrocolloid dressing group (56 vs. 21 days). Two subjects in the foam group (n = 20) withdrew because of issues unrelated to the wound. Among the remaining 18 wounds, 12 healed and 6 improved, yielding an overall improvement rate of 100%. In the hydrocolloid group, 10 subjects withdrew, including 6 who withdrew because of issues directly related to wound status (wound deterioration, hypergranulation, and discomfort). Among the 10 subjects who remained in the study, all wounds healed completely. Although similar numbers of dressings were used in both groups, a more favorable trend in wound healing was observed in patients treated with Spyrosorb than those treated with Granuflex. For this reason, the researchers suggested that greater numbers of hydrocolloid dressings would be required to achieve a healing rate comparable with that observed in the foam dressing group.

Another study compared a foam dressing (Spyrosorb) to Granuflex in 29 subjects with pressure ulcers.⁴³ Ten of the 13 wounds managed with Spyrosorb foam healed, and 14 of the 16 wounds managed with Granuflex healed. There was no significant difference in time to healing.

Two foam dressings, Allevyn Adhesive and Mepilex Border Safetac (Mölnlycke, Göteborg, Sweden) were compared in a 5-year descriptive study involving 2400 wounds in 1891 residents of 30 nursing homes.⁴⁴ The percentage of wounds healed was the same for both groups (63%); however, wounds in the Allevyn group were larger and took longer to heal than the wounds in the Mepilex group (mean area, 7.53 cm² as opposed to 5.5 cm²; healing time, 70.1 days as opposed to 39.2 days). The results of this study showed minimal differences between the 2 dressings; the overall closure rate was 53% for wounds managed with Allevyn Adhesive and 50% for wounds managed with Mepilex Border Safetac.⁴⁴

A study of a hydrocellular foam dressing (Allevyn Adhesive) in 81 community-dwelling patients with wounds included exudate volume as one outcome indicator.⁴⁵ Although this is not a direct measure of wound healing, a reduction in volume of a heavy exudate volume is commonly associated with improved wound status and progress toward healing. In the study, 21 wounds were described as having “copious volumes of exudate” at the beginning of the study; at the end of the study, only 4 wounds were classified as having copious amounts of exudate. Eighteen of the 81 wounds healed during the study.

An in vivo study of partial-thickness burn wounds used a porcine model to evaluate the effects of varied levels of wound hydration on wound healing. The 3 levels of hydration evaluated in the study were dry (wounds covered with dry gauze), moist (wounds covered with a hydrocolloid dressing), and wet (wounds managed with saline in a vinyl chamber). In a previous study using a full-thickness porcine wound model, granulomas developed in wounds managed with hydrocolloid dressings.⁴ In the current study, a larger number of inflammatory cells were found in the neodermis (subepithelium) of the “moist” wound group than in that of the “wet” wound group. Findings from days 5 and 7 after wounding suggest an exaggerated early inflammatory response, specifically a marked increase in the numbers of lymphocytes and neutrophils in the differential cell count on day 5 and significantly higher fibroblast counts on day 7. Nevertheless, the investigators point out that this finding must be interpreted with caution. Specifically, they state that the increased number of inflammatory cells may be attributable to use of a hydrocolloid dressing rather than to a lower level of hydration. They also note that the early popularity of hydrocolloid dressings was based in part on their apparent ability to promote angiogenesis and collagen synthesis, probably through the stimulation of macrophage activity. This increased inflammatory response merits additional study because the intensity and duration of the inflammatory response correlate positively to the volume of scar tissue produced during the proliferative phase of repair. Thus, full-thickness wounds managed with protocols or products associated with increased inflammation are at risk for increased scar tissue formation and, ultimately, for wound recurrence.¹

Summary

This review of the literature summarizes selected clinical and preclinical studies of various absorptive dressings to control exudate and maintain a moist wound surface. Moisture control is an essential component of topical wound therapy, and dressing selection exerts a clinically relevant effect on wound healing. Nevertheless, dressing selection is only one aspect of topical therapy; other components of wound bed preparation must be addressed as outlined in the acronym TIME, as discussed by Ayello in the preface to this supplement.

KEY POINTS

• ✓Much research has demonstrated that moisture control is a critical aspect of wound care.
• ✓The appropriate dressing can have a significant effect on the rate and quality of healing.
• ✓The appropriate dressing will help to minimize bacterial contamination and pain associated with wound care.