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Technical Reports |
B Klucinec, PT, MS, ATC, is Physical Therapist, Joyner Sportsmedicine Institute Inc, 2525 9th Ave, Suite IA, Altoona, PA 16602 (USA). Address all correspondence to Mr Klucinec
M Scheidler, PT, MS, is Physical Therapist, Hancock Memorial Hospital and Health Services, Greenfield, Ind
C Denegar, PT, PhD, ATC, is Associate Professor, Department of Orthopedics and Rehabilitation, and Associate Professor in Kinesiology, Pennsylvania State University, University Park, Pa
E Domholdt, PT, EdD, is Professor and Dean, Krannert School of Physical Therapy, University of Indianapolis, Indianapolis, Ind
S Burgess, PT, MS, Assistant Professor of Physical Education, Ball State University, Muncie, Ind
Submitted February 22, 1999;
Accepted January 13, 2000
| Abstract |
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Key Words: Film dressings Hydrogel Transmissivity Ultrasound Wound care
| Introduction |
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Although the effects of various wound care dressings on diagnostic ultrasound imaging have been examined,25,26 there is a paucity of evidence regarding the transmissivity of these materials for therapeutic ultrasound. Therefore, the purpose of this in vitro study was to examine the ultrasound transmissivity of 4 hydrogel sheets and 4 film dressings used for wound care.
| Method |
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±SD), was used to deliver acoustic energy. The receiving transducer* was the same model as the delivering transducer. The receiving transducer was modified
so a bus net connector could be attached. With the bus net connector, the receiving sound head could be connected to an oscilloscope. The amount of sound energy received was read as peak-to-peak (peak-peak) voltage from a Gould digital storage oscilloscope
(Fig. 1). The beam nonuniformity ratio of each ultrasound applicator was specified by the manufacturer as 6:1. The ultrasound equipment was calibrated
prior to the taking of any measurements. All experiments were conducted with continuous wave ultrasound at a frequency of 3.3 MHz, which has become popular for superficial wounds12,20 and deep wounds.12
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in) in diameter, were fixed in the plate 17.8 cm (7 in) apart. A Plexiglas
platform was fitted to slide up and down on the rods. The bottom platform had a hole drilled in the center with the exact dimensions of the receiving sound head. The sound head was placed underneath the platform flush with the Plexiglas. The transducer was secured in foam to protect the crystal, held in place with a vise, and made level with the bottom platform. The transmitting transducer was housed in another piece of Plexiglas with the same dimensions as the bottom platform. This top piece of Plexiglas was aligned directly above the bottom platform and receiving sound head. The transmitting sound head was secured to the top plate using a Plexiglas collar, which was allowed to slide horizontally, to tighten around the sound head. This apparatus allowed for one movable piece during data collection. The top piece of Plexiglas, with the transmitting sound head attached to it, was only permitted to move vertically along the 4 aluminum rods. Between setups, the top plate was raised to remove one wound care product interface and add another. A 3-dimensional level was used to keep the Plexiglas plates and transducers parallel with each other.
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The hydrogels examined were CarraDres,|| Aquasorb Border,
Nu-Gel,** and ClearSite.
We used pig tissue in this study due to its physiologic similarity to human tissue.27 The tissue was cut fresh, frozen, then thawed prior to use. It was shaved to decrease attenuation of acoustic/ultrasound energy and to avoid air bubbles becoming trapped in the hair. The tissue dimensions were 11.5 x 8.2 cm, with 0.5 cm of subcutaneous fat. The same piece of tissue was used throughout this experiment. The experiment was conducted in a ventilated room at a temperature of 22°C.
Pilot Studies
A pilot study was conducted to test the intertrial reliability of obtained measurements. Five setups with 3 trials per setup were performed. A 5-minute rest interval was allotted between trials, and at least 5 minutes elapsed between setups. Each setup included elevating the top plate with the secured transmitting sound head, replacing it on the interposed tissue, and leveling it out. Voltage output readings from the first setup were lower compared with voltage output readings taken from setups 2 through 5. The readings from the first apparatus setup were consequently discarded. The mean (±SD) of the 12 acceptable trials between the 4 setups was 1.70±0.04 V. This measurement yielded a coefficient of variation of 2.15%, which we considered sufficient to proceed with the experiment.
A second pilot study was conducted to determine whether the order of presentation of the sound wave intensities caused sequence effects that would affect subsequent sound wave transmission. Three trials were conducted, with a 5-minute rest interval between trials. The interposed materials consisted of pig tissue between 2 gel pads. Similar peak-peak voltage output was found for each trial at each intensity, regardless whether the order of intensities was conducted from lowest to highest intensity (0.22.0 W/cm2), from highest to lowest intensity, or randomized. We concluded that any short-term transmissivity changes in the tissue in response to sonation were dissipated during the 5-minute rest interval. This pilot work supported our decision to use a single piece of pig tissue as a constant throughout the experiment.
Procedure
Three trials were performed at each of 5 intensities for baseline data collection and for each wound care product interface. The intensities were 0.2, 0.5, 1.0, 1.5, and 2.0 W/cm2. The baseline setup included a piece of pig tissue placed between the sound heads. Ultrasound gel was placed at each interface. Any large air bubbles in the gel at the receiving sound head interface were removed by a syringe. Following the collection of the baseline data, each of the 4 hydrogel dressings and the 4 film dressings were tested in randomized order. A thin layer of ultrasound gel was placed on the pig tissue before the film dressing or hydrogel was applied in order to prevent reflection of the acoustic energy. Air that became entrapped as the dressing was applied was removed by applying light pressure across the dressing with a sterile tongue depressor. A thin layer of gel was placed on the outer surface of the dressing to decrease sound energy impedance as a result of air.1,8,12,18,19,23,28 The wound dressings were handled only by the edges to avoid the accumulation of any body oils that could have interfered with the testing. Before applying the hydrogel dressing, the bottom plastic film was first removed. In cases where the hydrogel dressing was protected with a removable plastic film on both sides, the top film was also removed. Once the bottom plate and receiving sound head were level and the tissue sample with the wound dressing was in place, the top plate was lowered until the transmitting transducer contacted the material. The top plate was then made parallel with the bottom plate using a 3-dimensional level.
Three trials were done at each intensity for each interposed material. The interface was not touched until 3 trials at each intensity were recorded. The ultrasound unit was turned on and ramped to the desired intensity. Once the sine wave became stable (approximately 2 seconds after reaching the desired intensity), the peak-peak voltage output was recorded from the oscilloscope. The ultrasound unit was then shut off and a 5-minute cooling-off period followed. After data collection, the top plate was raised, and the interposed material was removed. The process was then repeated. Between setups, the apparatus and interface were examined for any debris that might interfere with the trials. After new ultrasound gel was placed at each interface and another dressing was applied to the pig tissue, the process was repeated. The same tester performed each setup. Another tester was responsible for ramping the ultrasound to the desired intensity.
Data Reduction and Analysis
We normalized our voltage data with the different wound product interfaces by using the voltages obtained at baseline. To normalize the voltage data, we first calculated the average baseline voltage at each intensity. Then, we converted each raw voltage value into a percentage of voltage by dividing by the mean baseline voltage for the corresponding intensity. The converted percentage of voltage data were then used in our statistical analyses.
We used a one-way analysis of variance (ANOVA) to compare transmissivity (expressed as percentage of baseline voltage) of the 9 interfaces, including 8 experimental conditions and the baseline of only gel and pig tissue. Post hoc comparisons were done with the Scheffé test. The alpha level was set at .05 for all analyses.
| Results |
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| Discussion |
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Various wound characteristics, conditions, and wound care treatment goals may influence which dressing type would be most appropriate for a particular wound.8,24,2830 For instance, hydrogel dressings absorb fluids and are used to manage moderate amounts of drainage.8,24,2830 Film dressings, in contrast, do not have absorption capabilities and can handle only minimal amounts of drainage.8,24,2830 In addition, hydrogel dressings may offer cushioning and a cooling sensation to the wound.24,28,30 Generally speaking, minimal, if any, irritation to the intact periwound tissue is expected on removal of a hydrogel, whereas the removal of a film dressing is thought to require greater care in those instances where the surrounding skin is fragile.28,29
Film dressings do not require a secondary dressing.24,2830 However, because exudate may pass through hydrogel dressings, using a secondary dressing is advised.24,2830 In contrast to hydrogel dressings, which are thought to be "unable alone to keep out bacteria,"29(p166) film dressings are thought to protect a wound from microbes and bacteria.24,2830 Some authors contend that film dressings may remain in place for up to 1 week, whereas hydrogels generally need to be changed every 3 to 4 days.28 Clinicians, we believe, should make clinical decisions on the type of wound care dressing used for ultrasound treatment based on attributes of the dressing, including the transmissivity of the dressing.
Our results have varied levels of agreement with the few results presented in the literature. We recorded that 47% of ultrasound energy was transmitted through Tegaderm compared with the baseline recording. This percentage is similar to the finding of 40% of Byl and colleagues.23 In contrast, Nussbaum21 reported that only 10% of "ultrasound power" was transmitted through an Opsite Flexigrid film dressing; we found that 31% transmitted through Opsite Flexigrid.
The only study of hydrogel transmissivity we found was of Geliperm|||| by Brueton and Campbell17 in 1987. The Geliperm was kept under water during the experiment17 and was attached directly to the face of the sound head.21 This procedure, which did not mimic clinical conditions, resulted in 95% transmission of the sound energy through the Geliperm. We were unable to obtain Geliperm for our study as it is currently available in Europe but not in the United States.8
Although we attempted to carefully control the experimental setup, we did not measure and control for pressure variations of the sound head. Pressure variation has been found to influence acoustic energy transmission.31 This potential for variation may have affected our results.
Our in vitro study did not mimic the varying wound conditions seen clinically in terms of wound depth, drainage amount and composition, whether a full-thickness wound should be filled with sterile normal saline solution1,1921 or a sterile gel,12,20 and so on. We were aware of these limitations but chose to control the experimental variables as much as was possible in order to focus on the transmission capabilities of the wound care products. The percentage of ultrasound transmission through a film dressing or hydrogel is not the only factor relevant to wound healing. However, clinicians can use our findings as a part of the clinical reasoning process that they use to select an optimal wound dressing.
There are other factors to consider in deciding which product or product type to use in the treatment of wounds with ultrasound, including cost, whether a sterile or clean technique is warranted, and the potential for cross-contamination. As we noted, one disadvantage of a hydrogel is its inability to keep bacteria out of the wound without the aid of a secondary dressing.29
An additional consideration with some of the hydrogels is that the clinician needs to decide whether to remove the outer polyethylene film during sonation, as we did during our study. This has been suggested for those occasions when clinicians want to increase the vapor transmission rate of the wound24,29 or when electrical stimulation with a reusable electrode is going to be conducted.12 If the outer film is removed during sonation, replacing the film on the dressing after treatment or covering the wound with another appropriate film to prevent dehydration of the dressing can be done. If a sterile technique is desired or there is the concern with cross-contamination, it may be appropriate to replace the sonated hydrogel dressing with a new dressing following treatment.
We recommend that future research should address the optimum amount of acoustic energy that is needed to be received in order to best assist aid the wound healing process and outcome. In addition, we suggest that further research is needed on the acoustic energy transmission of hydrogels with and without the polyethylene film in place.
| Summary |
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| Footnotes |
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Funding for this project was provided by the University of Indianapolis.
* Chattanooga Group Inc, 4717 Adams Rd, Hixson, TN 37343. ![]()
Methodist Hospital of Indiana Inc, Biomedical Engineering Department, 1701 N Senate Blvd, Indianapolis, IN 46202. ![]()
Gould Instrument Systems, Roebuck Rd, Hainault, Ilford, Essex, IG6 3UE England. ![]()
Rohm & Haas Co, Independence Mall W, Philadelphia, PA 19105. ![]()
|| Carrington Laboratories Inc, 2001 Walnut Hill Ln, Irving, TX 75038. ![]()
# 3M, Medical-Surgical Division, Bldg 275-4E-01, St Paul, MN 55144-1000. ![]()
** Johnson & Johnson Medical Inc, 2500 Arbrook Blvd, PO Box 90130, Arlington, TX 76004. ![]()

Smith & Nephew, Wound Management Division, 11775 Starkey Rd, Largo, FL 33773. ![]()

DeRoyal Industries Inc, 200 DeBusk Ln, Powell, TN 37849. ![]()

CONMED Corp, 310 Broad St, Utica, NY 13501. ![]()
|||| Geistlich Sons Ltd, Long Lane, Chester, CH2 2PF England. ![]()
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