Effects of dressing type on 3D tissue microdeformations during negative pressure wound therapy: a computational study

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 epsilon(vol) at -50 mm Hg and -100 mm Hg 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., epsilon(vol), epsilon(dist), 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., et al., 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.