Improved Scar Outcomes with Increased Daily Duration of Pressure Garment Therapy

Abstract

Objective: Despite the development of a number of treatment modalities, scarring remains common postburn injury. To reduce burn scarring, pressure garment therapy has been widely utilized but is complicated by low patient adherence. To improve adherence, reduced hours of daily garment wear has been proposed.

Approach: To examine the efficacy of pressure garment therapy at reduced durations of daily wear, a porcine burn-excise-autograft model was utilized. Grafted burns were treated with pressure garments (20 mmHg) for 8, 16, or 24 h of daily wear with untreated burns serving as controls. Scar area, thickness, biomechanical properties, and tissue structure were assessed over time.

Results: All treatment groups reduced scar thickness and contraction versus controls and improved scar pliability and elasticity. Pressure garments worn 24 h per day significantly reduced contraction versus the 8- and 16-h groups and prevented alignment of collagen within the dermis.

Innovation: Though pressure garment therapy is prescribed for use 23 h per day, the need for almost continuous use has not been previously examined. Adjustable, low-fatigue pressure garments were developed for this porcine study to examine the role of daily duration of wear without confounding factors such as garment fatigue and patient adherence.

Conclusion: For maximum efficacy, pressure garments should be worn 23 to 24 h per day; however, garments worn as little as 8 h per day significantly improve scar outcomes versus no treatment.

Heather M. Powell, PhD

Introduction

Pressure garment therapy is commonly used to treat hypertrophic scarring. Pressure garments have been shown to reduce erythema, decrease scar height, and increase pliability of scars when applied following burn injuries. Patients are instructed to wear the garments for at least 23 h a day,^1–4 only removing them for hygienic measures, and to continue the therapy for 1–2 years.^2,3,5,6 However, the garments are reported to be uncomfortably hot and itchy, and have a high incidence of skin problems such as rash, tenderness, and blisters⁷ that often results in a lack of adherence to the therapy. One study reported that as few as 32% of the study population used the garments consistently.⁸ If the patients were able to remove the garments for longer periods of time during the day, it would likely decrease the prevalence of skin irritation, and improve patient adherence. Although 23 h a day is the standard requirement for the therapy,² there have not been any systematic scientific studies conducted to determine whether this wear time is necessary.

Additionally, the mechanism of action in pressure garment therapy is incompletely understood, though pressure is thought to work by decreasing collagen synthesis in hypertrophic scars.^9,10 In an in vivo study using Red Duroc pigs, applying pressure to hypertrophic scars resulted in a significant decrease in collagen I and III expression for 2 weeks following application.¹¹ Multiple in vitro studies have shown decreases in collagen expression when pressure is applied to fibroblasts in culture, and increases in matrix metalloproteinases (MMPs) 9 and 12.^12,13 Additional studies have been conducted to investigate how mechanical stimulation affects cell signaling in vitro. In one study, applying pressure to keratinocytes resulted in an increase in phosphorylated c-Jun N-terminal kinase (JNK 1) and mitogen-activated protein kinase kinases 3 and 6 within 5 min that decreased back to baseline within 20 min.¹⁴ The transient activation of these proteins suggests that further research is necessary to determine whether long periods of pressure are required to activate signaling pathways that can affect hypertrophic scarring. If shorter periods of pressure induce signaling cascades, then it is possible that 23 h a day of wear time is not necessary to decrease collagen expression, increase MMP expression, and achieve the observed benefits of pressure therapy.

The goal of this study was to investigate the effect of daily wear time of pressure garments on scar properties following burn injury. Full-thickness burn wounds were created on Red Duroc pigs, excised, and autografted with split thickness skin. One week after grafting, pressure was applied on the wounds either for 24, 16, or 8 h a day for 15 weeks. Untreated scars served as controls. Scar characteristics were monitored over time including scar depth, contraction, scar morphology, and biomechanics.

Clinical Problem Addressed

Pressure garments are the most common treatment for the reduction of scarring postburn injury; however, patient adherence to this therapy is low. One possible strategy to improve adherence is a reduced schedule of daily wear. Efficacy of pressure garment therapy at shorter daily durations must first be determined before modifications to the therapy protocol are implemented clinically.

Materials and Methods

Animal care and wound creation

All experiments were performed under a protocol approved by The Ohio State University Institutional Animal Care and Use Committee. Anesthesia of eight Red Duroc pigs was initiated with Telazol (Zoetis, Florham Park, NJ) and maintained with isoflurane. Before wounding, the dorsal trunk was shaved and cleaned with two alternating chlorohexidine/70% isopropanol scrubs (Butler Schein, Columbus, OH). Full-thickness burn wounds were then created on the dorsum by pressing a custom 1 × 1 inch metal stylus heated to 200°C against the skin for 40 s.¹⁵ Four wounds were created on each side for a total of eight wounds per pig. Split thickness skin was harvested from the dorsum using a Zimmer Air Dermatome (Zimmer, Warsaw, IN), meshed 1:1.5, and applied to the excised burn wounds. The wounds were covered with an antimicrobial dressing (Restore™; Hollister, Inc., Libertyville, IL) and then packed with sterile Hydrasorb^® surgical sponges (Carwild, Inc., New London, CT), which were held in place with sterile spandex and skin surgical staples (Henry Schein, Melville, NY). A fiberglass cast (3M Healthcare, St. Paul, MN) was formed over the dorsum and secured with Vetrap™ (3M Healthcare) and Elastikon^® tape (Johnson & Johnson, New Brunswick, NJ). The pigs were supplied with NOVAPLUS Fentanyl patches (Watson Pharmaceuticals, Inc., Parsippany, NJ) for 3 days following the surgery for pain management, and the bandages were removed 1 week after wounding.

Pressure garment therapy

Low fatigue pressure garments were fabricated from Powernet fabric with the finished edged of the fabric parallel to the circumference of the pig to reduce fabric fatigue.¹⁶ Custom-made garments were constructed using two layers of Powernet with ends secured with Velcro^® to allow for pressure adjustments during the study. Pressure was applied immediately after bandages were removed at 1 week postgrafting. The garments were applied circumferentially around the trunk of the pigs and maintained at 20 mmHg over the wounds. Pressure was measured daily using a Kikuhime pressure sensor (MediGROUP, Melbourne, Australia) and garments were removed and replaced with freshly laundered garments every 3 days. Site assignment was stratified to ensure each pig contained all treatment groups and a control and that each site (cranial to caudal) was equally assigned to a specific treatment group or control to remove anatomic location of the wound as a factor in observed outcomes. Treatment groups included either 24, 16, or 8 h of pressure application per day with wounds that did not receive any pressure serving as controls (n = 16 per group).

Scar contraction

Scar area was measured at 1, 8, and 15 weeks after grafting. The scar margins were traced onto a transparent film, which was then scanned with a ruler in the field of view and imported into ImageJ. The area of the scar at each time point was measured, then divided by the area of that scar 1 week after grafting to obtain a percent area. The average percent area is reported for each treatment group ± standard deviation (SD). Pig growth over time has historically resulted in a mean 215% increase in skin surface area 15 weeks postgrafting.¹⁷ Therefore, scars at 15 weeks that have less than a 215% increase in area have undergone contraction.

Surface roughness

The surface roughness of the scars was measured at week 15 and expressed as the mean roughness depth, or Rz. Aquasil Ultra XLV dental impression material (DENTSPLY Caulk, Milford, DE) was applied to each scar in vivo to obtain a permanent negative impression of the surface texture of the scars. Phase2Gel dental alginate (Accu-Cast Dental, Bend, OR) was then used to fill the dental mold, resulting in a positive impression of each scar. The alginate was sectioned into slices, and the cross sections were imaged with a ruler in the field of view and imported into ImageJ. The surface of the scar was then traced, and the five largest wrinkles were measured and averaged to obtain one Rz per cross section. The Rz from three cross sections was averaged to obtain one average Rz per scar. Average Rz was reported ± SD.

Scar morphology and total scar thickness

Six millimeter biopsies were taken from the scars at 1, 8, and 15 weeks postgrafting. Biopsies were marked with a line drawn across the scar in the cranial-caudal orientation before excision to ensure consistent orientation for sectioning. The biopsies were embedded in OCT™ (Fisher Healthcare, Houston, TX), frozen, and later cryosectioned. The sections (10 μm thick) were then stained with Masson's Trichrome (Sigma-Aldrich, St. Louis, MO) to visualize general scar morphology. The stained histological cross sections were also used to measure scar depth. Sections were scanned with a ruler in the field of view and the images were imported into ImageJ. Total scar thickness was measured from the top of the stratum corneum to the interface between the dermis and subcutaneous fat. Three measurements were averaged to obtain one scar depth per wound, reported as average scar depth ± SD. At the final time point, excised tissue from each scar was embedded in paraffin wax and sectioned en face to assess collagen fiber orientation. Blocks were sectioned 7 μm thick and stained with Picrosirius Red stain (Electron Microscopy Sciences, Hatfield, PA). Collagen fiber orientation at the 15-week time point was measured using ImageJ with an angle of 90 indicating the collagen was aligned in the dorsal-ventral direction. A minimum of 50 fibers were measured per sample with nine samples analyzed per group.

Scar biomechanics

In vivo biomechanics were measured on normal pig skin and scars using a Biomechanical Tissue Characterization device (BTC-2000™; SRLI Technologies, Franklin, TN). The BTC-2000 applies a vacuum to the skin and measures displacement of the skin in response to the negative pressure. The resulting displacement versus time and pressure versus displacement curves can be used to obtain several biomechanical properties of the scars, including viscoelastic deformation (displacement of the skin during constant vacuum, expressed in mm), elastic deformation (deformation of the skin during increasing vacuum, expressed in mm), ultimate deformation (total deformation during testing, expressed in mm), stiffness (the slope of the pressure versus deformation curve, expressed in mmHg/mm), energy absorption (the area under the pressure versus deformation curve, expressed in mmHg*mm), and elasticity (the recovery of displacement immediately following removal of the vacuum, expressed as % recovery).

Statistical analyses

Statistical analyses were performed using SigmaPlot version 14.0. Statistically significant differences were detected using either Student's t-test or a one-way analysis of variance with a posthoc test of Tukey. Statistical significance was considered at p < 0.05.

Results

Scar appearance

Overall, wounds treated with pressure tended to be smoother and less contracted than control grafts (Fig. 1). However, wounds that only received pressure for 8 or 16 h a day appeared more contracted and rougher than wounds that received continuous pressure for 24 h a day. Quantification of scar area revealed that all pressure treated groups had significantly reduced contraction (greater area) than control scars at 8 and 15 weeks after grafting (Fig. 2A). Additionally, wounds that received pressure 24 h a day had significantly reduced contraction than the wounds that received pressure for either 8 or 16 h a day (Fig. 2A). However, only scars that received pressure therapy for 16 or 24 h per day showed significantly reduced surface roughness compared to control scars (Fig. 2B).

Figure 1.

Improved clinical appearance of full-thickness burns treated with split-thickness autografts with and without pressure garment therapy. Shown are representative pictures of grafts treated with pressure 24 h a day, 16 h a day, 8 h a day, and controls that did not receive pressure. Applying pressure for 24 h a day resulted in visibly smoother and less contracted scars compared to those that received pressure for fewer hours a day. Scale bar in the bottom right is the same for all images (2 cm). Color images are available online.

Figure 2.

Reduced contraction and surface roughness with increased duration of pressure garment therapy. (A) Scar area as a function of time and treatment condition. Area is normalized to the area of each individual scar at 1 week. Control scars had significantly reduced area compared with all pressure treated scars; however, scars that received pressure for 24 h per day exhibited significantly greater area than reduced durations of daily wear. (B) Surface roughness at week 15 expressed as Rz. Only scars that received pressure for 24 or 16 h per day exhibited significant improvements in surface roughness compared with control scars.

Scar morphology

One week after grafting, trichrome stained histological sections showed large areas densely populated with cells with relatively smaller regions of collagen in all groups (Fig. 3). Collagen content increased over time, and by the final time point all treatment groups showed evidence of dense collagen in the superficial dermis with less dense collagen in the deeper dermis. Pressure treated scars at 15 weeks tended to have reduced density of collagen than control scars (Fig. 3). There was no correlation between scar thickness and duration of daily wear, although all pressure treated scars were significantly less thick than control scars (Fig. 4).

Figure 3.

Collagen deposition in scars over time. Shown are representative trichrome stained histological sections at 1, 8, and 15 weeks postgrafting. All treatment groups showed an increase in collagen deposition (blue) and a decrease in cell cytoplasm (red) over time. Control scars showed denser collagen than pressure treated scars at 15 weeks. Scale bar in the lower right is the same for all images (1 mm). Color images are available online.

Figure 4.

Pressure garment therapy reduces scar thickness. Depth of scars measured from histological sections at 15 weeks postgrafting. Pressure application significantly reduced the depth of the scars, regardless of daily pressure duration.

Picrosirius Red stained sections at week 15 showed that collagen fibers in control scars were highly aligned in the dorsal-ventral orientation, while scars in the 24-h pressure group had a semi-cross hatched pattern (Fig. 5A, D), similar to the pattern observed in uninjured dermis. Quantification of fiber alignment showed most fibers were ±10° from vertical (dorsal-ventral direction) in the control group with a decreasing fraction of highly aligned fibers associated with increased durations of garment wear. At 24 h/day, a bimodal distribution of orientation was observed with the two major fiber orientations approximately perpendicular to one another (Fig. 5E).

Figure 5.

Collagen fibril structure is altered by pressure garment therapy. Picrosirius stained, en face histological sections at 15 weeks postgrafting in the (A) 24 h, (B) 16 h, and (C) 8 h per day groups along with (D) controls. All images are oriented such that the top of the image is dorsal and the bottom ventral on the pig. Scars that received pressure 24 h a day tended to have collagen fibers oriented in a cross-hatched pattern. Scars treated for 8 h per day or with no treatment had significant dorsal-ventral orientation of the collagen fibers (Scale bar = 200 μm). (E) Representative histograms of fiber orientation showing greater fiber alignment in the dorsal-ventral direction in scars not treated with pressure garment therapy. As scars were treated with greater durations, fiber alignment in a single orientation was reduced until orientation was bimodal. Color images are available online.

Biomechanics

At 15 weeks postgrafting, scars in the 24-h pressure group showed improvements in all measured biomechanical properties compared to control scars (Fig. 6). Scars that received reduced daily wear (8 or 16 h a day) showed significant increases in elastic and ultimate deformation, and energy absorption, reaching levels that were equivalent to normal porcine skin (NPS) (Fig. 6C, D). Softer, more compliant tissues exhibit higher energy absorption values, whereas lower energy absorption indicates firmer tissue; thus, the significantly higher energy absorption of pressure treated scars compared with controls demonstrates a softening of tissue due to pressure treatment. Consistent with this, stiffness was decreased in all pressure treated scars compared with controls (Fig. 6D). However, applying pressure 8 and 16 h a day did not result in improvements in viscoelastic deformation or elasticity (Fig. 6C, D).

Figure 6.

Mechanical characterization of scars and normal pig skin (NPS) in vivo. Example graphs obtained from BTC-2000™ of deformation versus time (A) and pressure versus deformation (B). BTC-2000™ data collected 15 weeks postgrafting (C, D). While applying pressure 24 h a day improved all measured biomechanical properties, decreased duration of daily wear did not result in improvements in viscoelastic deformation or elasticity compared with controls. BTC, Biomechanical Tissue Characterization. Color images are available online.

Discussion

The mechanical environment surrounding a wound is a critical factor in predicting scar outcomes. Off-loading of tension within an incisional wound has previously been demonstrated to reduce vascular density, myofibroblast populations, and transforming growth factor-β1 expression leading to a significant reduction in scarring.^18,19 The mechanical environment within a burn wound is inherently complex with variations in strain as a function of wound shape, depth, and location. The application of pressure garments results in compressive forces both perpendicular to and tangential to the surface of the skin, effectively reducing the tension on the tissue independent of wound geometry. Pressure garment therapy has previously been shown to reduce scar contraction and total scar thickness, while improving pliability and elasticity in porcine burn scar models.^17,20,21 Pressure has also been shown to significantly decrease total collagen content within porcine scars.¹¹

The porcine burn-autograft model has been shown to result in reproducible scar formation, and is intended to model the current clinical standard of care for full-thickness burn wound treatment.²² The current study investigated application of pressure to developing burn scars beginning 1 week after grafting with split-thickness autograft, as our previous study demonstrated that early application of pressure garment therapy was safe and provided a significantly greater benefit than application at later time points.¹⁷ This is consistent with results of a meta-analysis of clinical studies, which reported better outcomes with earlier initiation of pressure garment therapy.^4,23 In clinical practice, however, initiation of pressure garment therapy generally occurs much later, with delays encountered due to time required for garment fabrication and clinicians' perceived fragility of the grafted burn wound.²⁴ While pressure garment therapy can have beneficial effects, even when it is not initiated early in the healing process, there is little high-quality evidence to support its efficacy in treatment of existing hypertrophic scars or keloids.^23,25

In the current study, scar contraction and collagen alignment within the dermis were significantly reduced when pressure garments were applied continuously versus treatment times of 8 or 16 h per day and untreated controls. The removal of the pressure garments and the associated elimination of external compressive forces likely resulted in a reestablishment of tension within the developing scar during those periods. Collagen alignment scaled with duration of daily wear, with highest alignment observed when no treatment was given, and lowest alignment when pressure garments were worn 24 h per day. This supports the hypothesis that removal of the garments allows for tensile forces to increase within the tissue. Scar contraction and alignment of collagen within the scar was previously observed following pressure garment cessation, with scars previously treated with pressure garment therapy contracting to approximately the same size as nontreated scars within 4 weeks of garment removal.²⁰

All groups receiving pressure garment therapy were thinner and more pliable than control scars. This observation is likely linked to both the abundance and alignment of collagen. The control group with no mechanical off-loading had the thickest scar tissue and highest alignment of collagen within the dermis. Prior studies showed that collagen gene expression was not significantly altered by pressure garment therapy¹⁷; however, compression was shown to reduce total collagen content.¹¹ The presence of tension within the developing scar may have altered the susceptibility of collagen to degradation. Uniaxial tension was shown to decrease the degradation of collagen by collagenase¹² and MMP²⁶ in vitro. Thus, the off-loading of tension resulting from the pressure garments may have allowed the collagen to be broken down by MMPs present in the dermis, resulting in thinner, more pliable scars.

While scar reduction was observed in the 8 and 16 h groups, it is important to note that the garments utilized were specially designed to exhibit very low fatigue during daily use and to be adjustable, such that the target pressure magnitude could be met throughout the course of the study.¹⁶ In a prior study, custom-fit garments lost as much as 20% of their applied pressure within 24 h of wear, especially in areas where pressure was greatest.¹⁶ A number of studies have also reported reductions in applied pressure of ∼50% within 1 month of wear and laundering.^2,10,27–29 While the low-fatigue study garments used in the current study remove fatigue as a confounding factor, the efficacy of reduced duration of wear using standard pressure garments may be different than the outcomes reported here with the low-fatigue garments. With the loss of pressure magnitude generally observed in standard garments, it is anticipated that at some point during daily wear, pressure will fall below therapeutic levels and may blunt potential differences in outcomes between fully compliant usage and reduced wear. Nevertheless, the current study reinforces the importance of maintaining pressure on the developing scar for maximum scar prevention and the need to monitor pressure beneath pressure garments to ensure therapeutic levels are being reached and maintained.

Innovation

Low-fatigue adjustable garments, which deliver consistent pressure over time, were utilized to examine the role of daily duration in efficacy of pressure garment therapy. For the first time, it was demonstrated that 24 h/day, which approximates the current clinical standard-of-care, is required for maximum benefit. Pressure garment therapy at 24 h/day significantly reduced scar thickness, contraction and collagen alignment, and improved scar pliability and elasticity. The data support the increased benefits of long durations of daily garment wear. The correlation between duration of daily therapy and improved scar outcomes may help motivate patients to increase adherence to the prescribed therapy.

Key Findings

• Low fatigue pressure garments worn for 8, 16, or 24 h per day significantly reduce scar contraction, thickness, and stiffness

• Compared to 8 and 16 h per day, continuous wear (24 h/day) significantly reduced contraction and enhanced tissue elasticity

• Continuous wear prevented collagen alignment within the dermis

Abbreviations and Acronyms

BTC

Biomechanical Tissue Characterization

JNK 1

c-Jun N-terminal kinase

MMP

matrix metalloproteinase

NPS

normal pig skin

Rz

mean roughness

SD

standard deviation

Acknowledgments and Funding Sources

This project was supported by the Shriners Hospitals for Children Medical Research Grants #85100 and #85400 (HMP). The authors would also like to thank the Special Shared Histology Facility at The Shriners Hospital for Children-Cincinnati, the Campus Microscopy and Imaging Facility at OSU, and The Ohio State University Laboratory Animal Resource staff.

Author Disclosure and Ghostwriting

No competing financial interests exist. The content of this article was expressly written by the authors listed. No ghostwriters were used to write this article.

About the Authors

Dr. DeBruler was a Graduate Research Fellow in the Department of Materials Science and Engineering at Ohio State University (OSU) working on scarring and skin biomechanics. She is currently a Research Scientist at MilliporeSigma developing antibodies for cancer detection.

Ms. Baumann is a PhD candidate and Presidential Fellow at Ohio State majoring in Biomedical Engineering.

Mr. Jacob C. Zbinden is a PhD candidate in the Department of Biomedical Engineering at Ohio State currently studying cardiovascular tissue regeneration.

Dr. Blackstone is a Research Scientist in the Department of Materials Science and Engineering at OSU working on biomaterials for tissue regeneration and wound healing.

Dr. John Kevin Bailey is a board certified general and hand surgeon and Professor of Surgery at Wake Forest University School of Medicine.

Dr. Dorothy M. Supp studies abnormal scarring and skin regeneration and is the Director of Research at the Shriners Hospitals for Children—Cincinnati and an Adjunct Research Professor in the Department of Surgery at University of Cincinnati.

Dr. Powell is an Associate Professor of Materials Science and Engineering and Biomedical Engineering investigating wound healing, tissue regeneration, and scarring.