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Research Papers

# Effects of Dressing Type on 3D Tissue Microdeformations During Negative Pressure Wound Therapy: A Computational Study

[+] Author and Article Information
R. Wilkes1

Kinetic Concepts, Inc., San Antonio, TX 78249rob.wilkes@kci1.com

Y. Zhao, K. Kieswetter

Kinetic Concepts, Inc., San Antonio, TX 78249

B. Haridas

Department of Biomedical Engineering, University of Cincinnati, Cincinnati, OH 45221-0048; Device & Implant Innovations, LLC, Mason, OH 45040

1

Corresponding author.

J Biomech Eng 131(3), 031012 (Jan 14, 2009) (12 pages) doi:10.1115/1.2947358 History: Received June 25, 2007; Revised February 18, 2008; Published January 14, 2009

## Abstract

Vacuum-assisted closure® (VAC® ) therapy, also referred to as vacuum-assisted closure® negative pressure wound therapy (VAC® NPWT), delivered to various dermal wounds is believed to influence the formation of granulation tissue via the mechanism of microdeformational signals. In recent years, numerous experimental investigations have been initiated to study the cause-effect relationships between the mechanical signals and the transduction pathways that result in improved granulation response. To accurately quantify the tissue microdeformations during therapy, a new three-dimensional finite element model has been developed and is described in this paper. This model is used to study the effect of dressing type and subatmospheric pressure level on the variations in the microdeformational strain fields in a model dermal wound bed. Three-dimensional geometric models representing typical control volumes of NPWT dressings were generated using micro-CT scanning of VAC® GranuFoam® , a reticulated open-cell polyurethane foam (ROCF), and a gauze dressing (constructed from USP Class VII gauze). Using a nonlinear hyperfoam constitutive model for the wound bed, simulated tissue microdeformations were generated using the foam and gauze dressing models at equivalent negative pressures. The model results showed that foam produces significantly greater strain than gauze in the tissue model at all pressures and in all metrics ($p<0.0001$ for all but $εvol$ at $−50mmHg$ and $−100mmHg$ where $p<0.05$). Specifically, it was demonstrated in this current work that the ROCF dressing produces higher levels of tissue microdeformation than gauze at all levels of subatmospheric pressure. This observation is consistent across all of the strain invariants assessed, i.e., $εvol$, $εdist$, the minimum and maximum principal strains, and the maximum shear strain. The distribution of the microdeformations and strain appears as a repeating mosaic beneath the foam dressing, whereas the gauze dressings appear to produce an irregular distribution of strains in the wound surface. Strain predictions from the developed computational model results agree well with those predicted from prior two-dimensional experimental and computational studies of foam-based NPWT (Saxena, V., , 2004, “Vacuum-assisted closure: Microdeformations of Wounds and Cell Proliferation  ,” Plast. Reconstr. Surg., 114(5), pp. 1086–1096). In conjunction with experimental in vitro and in vivo studies, the developed model can now be extended into more detailed investigations into the mechanobiological underpinnings of VAC® NPWT and can help to further develop and optimize this treatment modality for the treatment of challenging patient wounds.

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Copyright © 2009 by American Society of Mechanical Engineers
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## Figures

Figure 1

Schematic diagram of VAC® therapy (VAC® NPWT, KCI, San Antonio, TX). The foam occupies the void space in the wound and manifolds negative pressure across the wound bed. Multilumen tubing provides independent pathways for negative pressure/exudate removal and pneumatic feedback for control of the negative pressure at the wound. Exudate is collected in a disposable reservoir attached to the therapy unit. Image published with the permission of KCI Licensing, Inc.

Figure 2

Schematic of microdeformations in wound bed resulting from the combined effect of subatmospheric pressure applied to the tissue surface and contact between dressing microstructure and tissue. Image published with the permission of KCI Licensing, Inc.

Figure 3

SEM images of VAC® GranuFoam® Dressing at 30× (upper left) and 200× (upper right). The polyurethane foam microstructure consists of a continuous network of connected struts, created as bubble remnants during the foaming-polymer process. The pores between the bubble remnants are continuously interconnected. SEM images of USP Class VII absorbent gauze at 10.2× (lower left) and 100× (lower right). Gauze is a rectangular weave of strands that have been spun from individual cotton fibers. Image published with the permission of KCI Licensing, Inc.

Figure 4

Wound model micro-CT test cell apparatus cross sections: diagram (left) micro-CT scan (right). The negative pressure port is not visible in the micro-CT image. The scale bar is 5mm. Image published with the permission of KCI Licensing, Inc.

Figure 5

Micro-CT scan slices parallel to the tissue phantom surface for (upper) ROCF and (lower) gauze dressings at 0mmHg, −50mmHg, −125mmHg, and −200mmHg. The white specks are microspheres embedded into the tissue phantom. In (upper) the large arrow identifies a foam lying in the image plane; smaller arrows identify obvious examples of tissue microdeformation as it is drawn into the pore structure. In (lower) the large arrow identifies a typical gauze strand; smaller arrows identify examples of gauze strands migrating to common valleys to form macrostrands.

Figure 6

Reconstructions from micro-CT data of uncompressed dressings: ROCF (upper) and gauze (lower). Each surface consists of approximately 25,000 triangles and serves as a seed and boundary for volumetric tetrahedral grid generation and subsequent meshing for FEM.

Figure 7

Solid model of single layer of USP Class VII gauze (upper) and stack of seven layers meshed for FEM (lower)

Figure 8

FEMs of dressing-tissue interaction for (a) ROCF and (b) gauze prior to the application of negative pressure. The drape has been modeled as a rigid plate.

Figure 9

Contours representing vertical displacement in the ROCF and gauze at negative pressures of 100mmHg and 200mmHg

Figure 10

Contours representing dilatational strain (EVOL), distortional strain (EDIST), maximum principal strain (maximum tensile strain), minimum principal strain (maximum compressive strain), and maximum shear strain in the tissue for ROCF (left) and gauze (right) at −125mmHg.

Figure 11

Node population averages (±s.d.) for dilatational strain (EVOL), distortional strain (EDIST), maximum principal strain (maximum tensile strain), minimum principal strain (maximum compressive strain), and maximum shear strain in the tissue FEMs for ROCF and gauze dressings. N=1140 nodes (maximum of 10% of 11 400 for the tissue model), *=p<0.0001, ‡=p<0.0161, and †=p<0.0244.

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