Assessing Periwound Moisture Using a Novel In Vitro Test Method

Introduction. While the determination of optimal moisture needed for the wound healing process is an enigma, the complications of excessive moisture are readily recognized by clinicians in the field of wound care.   Excess moisture can cause maceration of the periwound and degradation of the surrounding healthy skin due to the proteolytic enzymes found in chronic wound exudate. Maceration can impede healing and can potentially cause further deterioration of the wound bed. To date, a number of advanced wound care dressings have been designed to provide a moist wound environment, but little research has been done to understand the ability of the dressing to protect the periwound and surrounding healthy skin from excess exudate. Materials and Methods. An in vitro method was developed to examine the amount of exudate to which the periwound and healthy skin are exposed using a simulated wound model. After the 48-hour test, a noticeable difference was observed in the amount of fluid present on the simulated periwound between each of 4 dressings tested: an Active Fluid Management silver dressing (TRITEC Silver Milliken Healthcare Products, LLC, Spartanburg, SC), a hydroconductive dressing (Drawtex, Steadmed Medical, Fort Worth, TX), a nanocrystalline silver dressing (Acticoat Antimicrobial Silver dressing, Smith & Nephew, Memphis, TN), and a hydrofiber dressing (Aquacel, ConvaTec, Skillman, NJ). Gauze (Curity All Purpose Sponges, Covidien, Mansfield, MA) was also evaluated as a negative control. This fluid was quantified as a weight percent of exudate absorbed by the simulated periwound based on the initial weight of the simulated periwound. Results. After the 48-hour test, gauze exposed the periwound to the greatest amount of fluid (588% ± 41.89), followed by the hydrofiber silver dressing (205% ± 30.68), nanocrystalline silver dressing (179% ± 30.68), hydroconductive dressing (167% ± 13.51), and active fluid management dressing (31% ±4.09). Conclusion. The active fluid management dressings exposed the simulated periwound to the least amount of moisture, indicating that Active Fluid Management dressings provide greater protection of the periwound compared to the other dressings.

  In 1962, a study by George Winter¹ demonstrated that split-thickness wounds on pigs epithelialized faster when treated with an occlusive dressing compared to those left uncovered. Since then, there has been a dramatic shift toward moist wound care. Extensive research has demonstrated that acute wound exudate assists the wound healing process by promoting autolysis, supplying nutrients, enabling growth factor diffusion, and allowing cell migration.² However, chronic wound exudate contains higher levels of matrix metalloproteinases (MMPs) that can degrade the extracellular matrix, slow cell migration, and destroy tissue.³ Prolonged exposure of periwound skin and the surrounding healthy tissue to chronic wound exudate can lead to maceration which may delay wound healing or, in some cases, worsen the condition of the wound.^4,5 The wound care community agrees that too much or too little moisture can be detrimental to wound healing,⁶ but the optimal moisture level remains undefined. This leaves clinicians to rely on their best judgment to choose wound care dressings that maintain the proper moisture balance for a wide variety of wounds.

  Despite the need to understand optimal moisture, there has been little advancement in methods to measure the role of wound dressings in maintaining a moist wound environment while protecting the periwound from excess moisture. Previous studies have examined methods of monitoring the amount of moisture at the wound bed/dressing interface both in vitro and in vivo; however, none of these examined the moisture level specific to the periwound.^7-9 The test method described in this paper allows simple, quantifiable, and visual comparison of the moisture level within the simulated periwound when covered with different types of wound dressings. These findings could aid clinicians in selecting appropriate dressings for wounds with varying levels of exudate.

  Periwound protection wound model. This test method was designed to quantify the amount of moisture to which the periwound is exposed when different wound dressings are applied to a simulated wound model. The simulated wound model was constructed as follows: a hole 2.4 mm in diameter was created in the center of a polystyrene Petri dish (deep dish, 90 mm diameter) to simulate a wound. Silicone tubing (2.38 mm outer diameter x 0.8 mm inner diameter) was attached to the hole via the inside of the dish using silicone sealant. The simulated periwound was modeled using 2 layers of gauze cut into a 50 mm-diameter disk with a 10 mm hole in the center. The hole in the gauze was centered over the simulated wound on the Petri dish. The dressings were cut into 50 mm-diameter circles, placed on top of the wound and periwound, covered with 5 layers of 3-ply gauze as the secondary dressing, and secured with a 40 g weight (Figure 1). A syringe pump (New Era Pump Systems, Farmingdale, NY) delivered simulated wound fluid ([SWF]:142mM NaCl and 2.5mM CaCl₂ in DI H₂O)¹⁰ containing a blue dye through the hole in the dish via the silicone tubing. The fluid was pumped into the wound at a rate of 0.2 mL/hour to represent a highly exuding wound.¹¹

  The periwound gauze layer was weighed and photographed before and after the test and the percent weight gain due to fluid exposure was calculated using the following formula:
   % Periwound Weight Gain = Final Weight - Initial Weight    x 100
                                                         Initial Weight

  Each test was conducted over a 48-hour period with measurements taken at 4-hour intervals for the first 24 hours and the final measurement taken at 48 hours.

  Four different contact layer dressings were evaluated in this study: Active Fluid Management silver dressing (AFM) (TRITEC Silver Dressing, Milliken Healthcare Products, LLC, Spartanburg, SC), hydroconductive dressing (HD) (Drawtex, Steadmed Medical, Fort Worth, TX), nanocrystalline silver (NS) (Acticoat Antimicrobial silver dressing, Smith & Nephew, Memphis, TN), and hydrofiber dressing (HS) (Aquacel, ConvaTec, Skillman, NJ). Gauze (Curity All Purpose Sponges, Covidien, Mansfield, MA) was also evaluated as a negative control (Figure 2).

  Bulk Absorption. The bulk absorption was measured following the standard method.¹⁰ Dressings were cut into 50 mm x 50 mm squares, weighed, and placed in cups with a volume of simulated wound fluid equivalent to 40 times the initial sample weight. The dressings were incubated in the solution at 37°C for 30 minutes, removed from the solution, held vertically for 30 seconds to remove excess fluid, and reweighed to determine the final weight. Bulk absorption [g/m²] was calculated using the following formula:
  Bulk Absorption = Final Weight - Initial Weight
                                       Area of sample

  As shown in Table 1, the periwound percent weight gains for gauze, HS, NS, HD, and AFM dressings at the 48-hour time point were 587% (± 42), 205% (± 24), 179% (± 31), 167% (± 14), and 31% (± 4), respectively. Figure 3 shows the periwound layer at the 48-hour time point, where the higher amount of blue dye indicates a greater amount of moisture left on the periwound (bottom row).

  The bulk absorptions of the HS, NS, HD, and AFM dressings were 1,779 g/m² (± 60), 1,142 g/m² (± 67), 3,016 g/m² (± 43), and 727 g/m² (± 24), respectively (Table 2).

  It is expected that when the dressing reaches its absorptive capacity, the amount of fluid on the periwound will begin to increase. To examine the effect of saturation of the wound care dressing on periwound protection, the time point at which each dressing would have theoretically become saturated during the periwound protection test was calculated using the following equation, in which the density of the SWF is equal to approximately 1 g/m³:
  Time Until Absorptive Capacity Reached = Absorptive Capacity
                                                                      x Area of Dressing
                                                                         Fluid Flow Rate

  This calculated value is shown in the final column in Table 2. Figure 4 displays the results of the periwound protection test graphically with the theoretical saturation point noted on the x-axis. For the NS, HD, and HS dressings, the time of saturation appears to correspond with an increase in percent periwound weight gain. In contrast, the saturation of the AFM dressing appears to have had no effect on the percent periwound weight gain.

  Maintaining an optimal level of moisture at the wound/dressing interface is a challenge readily acknowledged in wound care literature and among wound care clinicians.¹² While the importance of maintaining moisture balance is understood, a method for assessing the impact different wound dressings have on the periwound moisture exposure is not currently available.

  The periwound protection model was developed to measure differences in the amount of moisture to which the periwound is exposed when using different wound dressings. Gauze was chosen as a negative control in this experiment because of its known ability to cause periwound maceration in clinical practice when used as a continuously moist dressing.¹³ As expected, the gauze exposed the simulated periwound to the greatest level of moisture within the initial 4-hour time period and continued to expose the periwound to increasing levels of moisture throughout the test. All other dressings tested showed measureable differences in the amount of moisture exposed to the periwound indicating the model is able to differentiate among the dressings.

  At each time point in the study, the AFM dressings exposed the periwound to the least amount of moisture compared to the other dressings. While the AFM dressings had the lowest bulk absorption compared to the other dressings, they were still able to minimize the amount of moisture left on the periwound. In contrast, when the HS, NS, and HD dressings reached their bulk absorption point, the periwound weight gain began to increase substantially. The difference observed between AFM and the other dressings may be attributed to the unique textile construction of the AFM dressing that moves moisture away from the wound and into the secondary dressing. Gauze does not have a mechanism for protecting the periwound skin. Due to its nature as an alginate, HS gels when wet, trapping the exudate next to the skin. The NS and HD dressings both have good wicking properties but this only provides for the moisture to move laterally, allowing moisture to spread to the periwound. They do not have an innate mechanism preventing moisture from transferring back to the wound. In this study, the AFM dressing was the only dressing that protected the periwound from excess moisture while transferring the moisture into the secondary dressings. These results support the practice of leaving AFM dressings in place while only changing the absorptive secondary dressings.

  Results of testing using this novel periwound protection model indicate that use of the advanced wound dressings results in less exposure of moisture to the simulated periwound than the gauze dressing. The AFM dressing exposed the simulated periwound to the least amount of moisture even though it had the lowest bulk absorption. As more advanced wound dressings are brought to market, it will be important to have tools to distinguish the moisture management capabilities among them. It is also important to translate these in vitro findings to in vivo results to determine if the differences shown in this benchtop experiment can contribute to differences in clinical outcomes.

The authors are from Milliken Healthcare Products, LLC, Spartanburg, SC.

Address correspondence to:
Kayla Perry
920 Milliken Rd M207
Spartanburg, SC 29303
Kayla.Perry@milliken.com

Disclosure: The authors disclose they are employees of Milliken Healthcare Products LLC.