A Computer Modeling Study to Assess the Durability of Prophylactic Dressings Subjected to Moisture in Biomechanical Pressure Injury Prevention

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Ostomy Wound Management 2018;64(7):18–26 doi: 10.25270/owm.2018.7.1826
Dafna Schwartz, BSc; Ayelet Levy, MSc; and Amit Gefen, PhD

Abstract

The sacral area is the most common site for pressure injuries (PIs) associated with prolonged supine bedrest. In previous studies, an anisotropic multilayer prophylactic dressing was found to reduce the incidence of PIs and redistribute pressure. The purpose of the current study was to further investigate relationships between design features and biomechanical efficacy of sacral prophylactic dressings.

Using computer modeling, the anisotropic multilayer dressing and a hypothetical dressing with different mechanical properties were tested under dry and 3 levels of moist/wet conditions.  Sixteen (16) finite element model variants representing the buttocks were developed. The model variants utilized slices of the weight-bearing buttocks of a 28-year-old healthy woman for segmentation of the pelvic bones and soft tissues. Effective stresses and maximal shear stresses in a volume of interest of soft tissues surrounding the sacrum were calculated from the simulations, and a protective endurance (PE) index was further calculated. Resistance to deformations along the direction of the spine when wet was determined by rating simulation outcomes (volumetric exposures to effective stress) for the different dressing conditions. Based on this analysis, the anisotropic multilayer prophylactic dressing exhibited superior PE (80%), which was approximately 4 times that of the hypothetical dressing (22%). This study provides additional important insights regarding the optimal design of prophylactic dressings, especially when exposed to moisture. A next step in research would be to optimize the extent of the anisotropy, particularly the property ratio of stiffnesses (elastic moduli).

owm_0718_gefen_keypoints

A pressure ulcer (PU) or pressure injury (PI) as now termed in the United States and Australia is defined as a localized site of tissue damage that typically develops near a bony prominence as a result of sustained mechanical loads applied to soft tissues.1 The sacral area is the most common site for PIs associated with prolonged supine bedrest; the weight of the lower trunk and pelvis subject skin, fat, and muscle tissues enveloping the sacrum to sustained cell and tissue deformations.1-3 Clinical studies2,3 report that approximately 30% of these PUs are category 3 to category 4 or deep tissue injuries (DTIs), scaled according to international guidelines,1 that later evolve into open wounds that affect all tissue layers, down to the bone. In addition to the compressive and tensile loads that are present in weight-bearing sacral soft tissues, biomechanical analyses have identified shear loads as being dominant, given the high curvature of the (nearly rigid) sacral bone and stiffness gradients of the layered soft tissue structure.4 Computer modeling in particular shows these internal shear loads can be further intensified when patients slide downward in bed due to gravity (especially if the head of the bed is elevated) or during repositioning for PI prevention, comfort, or providing patient hygiene.

PI prevention is now an important focus of many health care organizations, government agencies, the medical industry, and academic research worldwide; efforts and resources are directed at finding preventive interventions that have consistently proven successful over time. One important successful preventive measure is the use of adequate prophylactic dressings. Santamaria et al5 conducted the first large-scale, randomized controlled trial (RCT), the “Border Trial,” in which Mepilex Border Sacrum (MBS) anisotropic multilayer dressings and heel dressings (Mölnlycke Health Care, Gothenburg, Sweden) were prescribed to trauma and critically ill patients (N = 440) in the hospital’s emergency department. The PU rates in the dressing group were compared to those of a control group receiving standard care including use of a low-air-loss bed, ongoing Braden risk assessments, and regular repositioning and skin inspections. Results of a similarly designed RCT with an equivalent sample size (N = 366) were reported by Kalowes et al.6 Both studies concluded that use of the aforementioned multilayer sacral and heel dressings, combined with preventive care, resulted in a statistically significant reduction in the incidence rate and severity of hospital-acquired PUs in intensive care patients. In another comparative cohort study,7 the Santamaria group found that the multilayer dressing variant having the same material composition as the MBS for protecting the heels was clinically effective in reducing intensive care unit-acquired heel PUs in a cohort of 150 patients. 

An adequate prophylactic dressing has a structure and composition that can absorb deformation energy (strain energy in engineering terms), especially shear loads, capturing this energy in the dressing structure and not in the tissues, as demonstrated in previously published biomechanical (computer modeling) studies by the current authors.8-11 The dressing is further designed to reduce the frictional forces between clothing and mattress, provide local cushioning, and redistribute bodyweight-associated pressures, as well as manage the microclimate and humidity of the skin.8-11 Likewise, the beneficial effect of the anisotropic feature of the multilayer dressing (termed deep defense by the manufacturer) has been described and characterized as stiffness with a directional preference (ie, the dressing is more flexible and stretchable in its lateral direction than in the longitudinal direction of the human body).10,11 Using the finite element (FE) method described in a previous study,9 the current authors simulated different clinically relevant scenarios and biomechanical characteristics of high-risk groups, such as frail elderly persons and persons with diabetes, and assessed the biomechanical performance and efficacy of the multilayer dressing in these scenarios.11 Their research consistently found a considerable reduction in exposure to skin and deep tissue deformations when the anisotropic multilayer dressings were applied, which may help explain the outcomes of the above RCTs from a scientific bioengineering perspective. 

Fluids tend to accumulate in a prophylactic dressing over time (even though the skin is intact) as a result of normal sweating, fever, or incontinence; these fluids affect the functioning of the dressing and the integrity and biomechanical properties of the skin.4 In particular, based on fundamental knowledge regarding how water absorption affects the strength and stiffness of porous materials (such as paper or cloth), the accumulation of fluids may affect different mechanical characteristics of a prophylactic dressing, including its stiffness properties and directional preference, the coefficient of friction and geometry. Each of these potential changes may impact the protective efficacy of the dressing over time. Accordingly, it is important to ensure that a prophylactic dressing continues to provide stable protection to tissues (including when wet) over the entire timespan of use, defining its durability. However, the effect of moisture on these dressings has not been tested. 

The purpose of the current study was to further investigate relationships between design features and biomechanical efficacy of sacral prophylactic dressings. Specifically, the authors aimed to simulate and study the performance of the MBS anisotropic multilayer dressing and a hypothetical dressing comprising the features of a similar dressing, but which become isotropic after absorbing fluids. The authors focused on the effect of wetness on the elastic properties of the dressings. The hypothesis was that the sustainability of the anisotropy characteristic in wet conditions is critical for the dressing to maintain its protective efficacy over time (ie, durability). 

Methods 

In order to investigate the biomechanical efficacy of sacral prophylactic dressings in protecting soft tissues after absorbing fluids, the authors developed 16 FE model variants representing the buttocks with either the anisotropic multilayer dressing or with a hypothetical dressing (see Table 1). The concept of the hypothetical dressing is based on commercially available dressings that were tested experimentally. However, to create comparable models of the MBS dressing and a hypothetical one, which needed to be identical in shape to the MBS but have different mechanical behavior (and hence, was not a “real” product), the hypothetical elastic moduli obtained in tests8,11 of some commercial products on the same geometric model that had been devised to represent the MBS were used. Specifically, the performance of the anisotropic multilayer dressing (which, according to manufacturer data, preserves its anisotropy feature when wet11) was compared to a hypothetical dressing that loses its anisotropy as fluid contents build up. The amounts of fluids are based on transepidermal water loss (TEWL) values taken from Kottner et al.12 These 3 fluid values include TEWL for lower back and buttocks of approximately 10 (g/[m2/h]), which corresponds to 0.025 (mL/[cm2/day]) in the dressing, and the higher levels (0.075, 0.15 [mL/(cm2/day)]) were taken in order to investigate a more substantial accumulation of fluids — for example, when excess perspiration or incontinence is present. Both types of dressings (anisotropic multilayer versus hypothetical) were tested for wetness levels when pure compressive bodyweight loading was applied and also for a combined compression and shear loading mode, consistent with the current authors’ previously published work.10,11 Effective and maximal shear stresses developed in the soft tissues at the sacral region in supine weight-bearing were systematically compared using a standard hospital mattress for each examined case (see Table 1).  

Geometry. A 3-dimensional (3D) anatomical model of the buttocks recently developed by the authors’ group for methodological, comparative sacral dressing studies10,11 was used in this work. Briefly, 76 T1-weighted axial magnetic resonance imaging (MRI) slices of the weight-bearing buttocks of a 28-year-old healthy woman were imported to the ScanIP module of the Simpleware software package (Synopsis Inc, Mountain View, CA) for segmentation of the pelvic bones and soft tissues.13 Details regarding the MRI machine, scan protocol, and medical ethical approval are available elsewhere.10 The authors focused on a volume of interest (VOI) of 27.8 cm x 17.4 cm x 5.6 cm, incorporating the sacral bone and surrounding soft tissues. This allowed researchers to optimize computer power and make the numerical calculations effective where the tissue distortion phenomena relevant to sacral pressure ulcers occurred (see Figure 1a). owm_0718_Gefen_figure1

As in the authors’ previously published work,8-11 the anisotropic multilayer dressing included 3 physical material layers in the modeling: polyurethane foam (PUR), a nonwoven (NW) layer, and the airlaid (AL) layer. Consistent with previous studies,8-11 the authors considered the innermost Safetac layer as a tied interface between the soft tissue component and the PUR foam layer, and the outermost “backing film” layer was represented as frictional sliding between the AL layer and the mattress (see Figure 1b).8-11 However, the shape of the anisotropic multilayer dressing was not adopted from the authors’ previous work; rather, it was recreated using the ScanIP module of Simpleware to comply with the newest anisotropic multilayer dressing design launched in 2017. To complete the generation of the model geometry, a flat foam mattress was added under the buttocks in the ScanIP module of Simpleware.

Numerical methods. Meshing of the tissues, multilayer dressings, and mattress model components was performed using the ScanIP module of Simpleware.13 Four (4)-node linear tetrahedral elements were used in all model components. In order to obtain optimal accuracy but minimize complexity of the numerical solution and the associated computational power, mesh refinements were applied locally at the skin-dressing and mattress-dressing interfaces. 

The FE simulations were set up using the PreView module of FEBio (Ver.1.19; University of Utah, Salt Lake City, UT), analyzed using the Pardiso linear solver of FEBio (http://mrl.sci.utah.edu/software/febio) (Ver. 2.5.0), and post-processed using PostView of FEBio (Ver. 1.10.2).14 Converging time steps were chosen for numerical data collection so that the resulting reaction force was within a 2% difference from the target reaction force (description to follow). The time for solving each simulation case, using a 64-bit Windows 8-based workstation with 2×Intel Xeon E5-2620 2.00 GHz CPU and 64 GB of RAM, ranged between 7 and 12 hours. 

Mechanical properties of the dressing and tissues. Constitutive laws and mechanical properties of the tissue components and the mattress were adopted from the literature. Specifically, the sacrum was assumed to be a linear-elastic isotropic material with elastic modulus of 7 GPa and a Poisson’s ratio of 0.3.15-17 The soft tissues were assumed to be nearly incompressible (Poisson’s ratio of 0.49) and nonlinear isotropic, with their large deformation behavior described by an uncoupled Neo-Hookean model with the following SED function W:

formulagefen

where Gins (the instantaneous shear modulus) is 2 kPa,17 λi (I = 1,2,3) are the principal stretch ratios, K (the bulk modulus) is 1 MPa , and J = det(F) where F is the deformation gradient tensor. Specifically, material constants reported by Oomens et al18 were used to calculate an effective soft tissue Gins comprised of 60% skin and 40% fat, as in the authors’ previous modeling work of the buttocks.10 

The anisotropic multilayer dressing has significantly different stiffness properties in the vertical versus the horizontal directions (anisotropy), while the hypothetical dressing has less distinct directional stiffness properties. The elastic moduli of the multilayer dressing at the X and Y directions (ie, the spinal and lateral directions, respectively) were measured in the authors’ laboratory and in the anisotropic dressing’s testing facilities with the authors maintaining oversight of the experimental protocol and data (see Table 2). The ratio between the elastic moduli at the Y direction over the X direction was found to be 6.6 for the anisotropic multilayer and approximately 1.8 for other commercially available dressings in the dry condition. The elastic moduli and the ratio of moduli at the Y direction over the X direction also were measured for the 3 levels of wetness in moist dressings: 0.025, 0.075, and 0.15 (mL/[cm2]) (see Table 2). For the hypothetical dressing in its wet conditions, a modulus ratio of unity was assigned based on measurements of commercial prophylactic dressings loaded with the above wetness levels and then mechanically tested in tensile loading at the X and Y directions. In other words, the hypothetical dressing was considered to become linear-elastic isotropic when wet (at any of the above 3 wetness levels), which is a potential material softening response known to exist in some wet porous materials (such as wet paper). A Poisson’s ratio of 0.258 was chosen for all dressings based on published experimental data.19 The mattress was considered isotropic linear-elastic, with an elastic modulus of 50 kPa and a Poisson’s ratio of 0.3, based on literature.8,9,20 

Body loads applied to the buttocks and boundary conditions. Downward displacements of 5.5 mm to 6.48 mm were applied on the top surface of the model in order to simulate the descent of the weight-bearing sacrum during supine bedrest or a 45˚ Fowler’s position, with the anisotropic multilayer or hypothetical dressings in the dry and 3 wet dressing conditions. A total reaction force of 40 Newtons was obtained in all simulations that represented approximately 7% of the total bodyweight of the subject; this was transmitted focally at the sacral region. Therefore, the comparison between all simulation cases was conducted under the same (7% bodyweight) conditions for consistency of outcome measures across the different model variants. In the combined compression and shear loading scenario (representing sliding in bed due to gravity), a horizontal displacement of the same magnitude was added in the Y direction. The bottom surface of the mattress was fixed for all motions, and tied interfaces were defined at the bone-soft tissue boundaries as well as between the soft tissues and the dressing. Frictional sliding was defined between the dressing and mattress, with the coefficient of friction set to 0.35.8,19 

Biomechanical outcome measures. Effective and maximal shear stresses within the VOI were compared across all simulation cases. Volumetric exposures of soft tissues below the sacrum (in the VOI) to elevated effective stresses also were compared and examined using stress exposure histogram (SEH) charts. As a final step after evaluating the volumetric exposure of soft tissues to stresses and plotting the SEHs, the protective endurance (PE) of the anisotropic multilayer dressing versus the hypothetical dressing (in percents) was calculated as follows: 1) the relative difference in the area under the SEH (A) for dry (d) and wet (w) cases of each dressing was calculated relative to the case in which no dressing was used: 

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2) the protective endurance (PE) of a given dressing was defined as: 

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Hence, the PE is an objective, standardized, and quantitative indicator of the preservation of biomechanical protection that a certain dressing provides to the soft tissues while being wet with respect to its (ideal) protective efficacy when it is dry. Because the present modeling is deterministic (and not probabilistic), each combination of dressing conditions (type of dressing, level of moisture, and loading mode as specified in Table 1) was simulated once. A detailed description of the chosen FE computer modeling and simulation approach is provided in the authors’ previous work.owm_0718_gefen_table1

Data management. The FE simulation data were directly imported to and post-processed using PostView (US National Institutes of Health, Washington, DC), a post-processor software designed to visualize and analyze results from a FEBio analysis.14 The displacement applied on the top surface of the model was increased incrementally for numerical convergence purposes, so the resulting reaction forces between the buttocks and the support surface were within <1.8% difference of the aforementioned target reaction force. The effective and maximal shear stresses data that developed in the soft tissues within a cubical VOI in a size of 9 cm x 9 cm x 2.5 cm were pooled for each dressing type in the dry and 3 wet conditions, under pure compression due to bodyweight and separately under combined compression and shear loading. Further, the volumetric exposure of soft tissues below the sacrum in the aforementioned VOI was compared to elevated effective stresses and a SEH was plotted per each dressing type and dryness/wetness condition. A PE index then was calculated for each dressing type according to the algorithm described. Because all simulation data are deterministic, no variability exists in the modeling outcomes per each specific case of input dataset; hence, statistical analyses were not applicable in this study (please see a detailed explanation in the authors’ previous work9). 

Results 

Although 16 FE models were created, results from only the 14 that provided sufficient convergence (see the “Data management” section for convergence criterion) were analyzed. Specifically, the simulation case for a 0.15 (mL/cm2) moisture level in the hypothetical dressing was excluded due to extremely low dressing stiffness properties in this scenario that caused numerical convergence problems and resulted in incomparable simulation data for this specific case. The prominent difference noted in the average effective stress and average maximal shear stress between the anisotropic multilayer dressing and the hypothetical dressing was for the transition from dry to a (mildly) wet condition. Specifically, effective tissue stresses at the sacral region were between 0.3 kPa and 0.75 kPa for both dressing types in their dry condition. Within that range, tissue stresses increased up to 18% for the wet hypothetical dressing, but only 9.9% for the wet anisotropic multilayer dressing. Interestingly, the simulated rise in fluid contents from the lowest level of 0.025 (mL/cm2) to the greater moisture loads (0.075 and 0.15 [mL/cm2]) had a minor influence on tissue stress levels near the sacrum (ie, an approximately 2% increase in tissue stresses for the 2 dressing types for the 0.3 kPa to 0.75 kPa and 0.2 kPa and 0.5 kPa effective and shear stress domains in tissues, respectively). 

Effective stress distribution in the soft tissues near the sacrum when the model was subjected to combined compression and shear loads in dry dressing conditions and in a wet condition of 0.075 (mL/cm2) moisture level is shown in Figure 2 for the 2 dressing types. The colored bar attached to the figures that show effective stress and shear stress distributions in soft tissues indicates high versus low stress exposure in tissues. The highest stress values (in kPa units) are depicted in warm colors (red/yellow) and the lowest values are shown in cool colors (blue/green). The colors between these 2 are equally divided according to maximum and minimum stress values. Peak effective stresses increased within the 0.3 kPa to 0.75 kPa domain by 11.6% and 3.4% in the wet hypothetical and anisotropic multilayer dressings, respectively (see Figure 2). Consistent with these data, the rise in maximal tissue shear stresses for the same moisture contents was within the same domain for both dressing types, 0.2 kPa to 0.5 kPa; however, the peak shear stress was shifted more for the wet hypothetical dressing condition than for the anisotropic multilayer dressing, a difference of 10.2% and 3.4%, respectively (see Figure 3). 

OWM_0718_Gefen_figure2OWM_0718_Gefen_figure3

Modeling calculations of cumulative volumetric exposures of the soft tissues under the sacrum to effective stress under combined compression and shear loading indicated the stress exposures within the entire 3D mass of soft tissues near the sacrum of the supine body were up to a peak of 1.2 kPa (see Figure 4a). However, the type of the dressing used affected the distribution of exposure to high versus low tissue stresses, predominantly for the wet conditions. Specifically, a comparison of the cumulative volumetric exposure of the soft tissues adjacent to the sacrum to effective stresses due to simultaneous compression and shear are shown in Figure 4a for both dressing types in dry and wet conditions (see Figure 4b). The wet hypothetical dressing caused a considerable rise in tissue stress levels, particularly within the 0.2 kPa to 1 kPa stress domain, whereas the anisotropic multilayer dressings did not (see Figure 4b). This is also evident and quantifiable by means of the PE values, following the algorithm of calculations specified in equations 2 through 4. These calculations show the PE of the anisotropic multilayer dressing is approximately 80%, compared to 22% for the hypothetical dressing. /OWM_0718_Gefen_figure4

Discussion 

A modeling framework was used to investigate the relationship between moisture in prophylactic dressings and the biomechanical protection provided by such dressings, and specifically, to compare the mechanical stresses developing in the soft tissues near the sacrum due to bodyweight loads in dry versus wet conditions for 2 specific types of dressings. The hypothesis was that the sustainability of the anisotropic characteristic of the dressing in wet conditions is critical for the dressing to maintain its protective efficacy. The present modeling work supported this hypothesis.

This study builds upon the authors’ previously published work with regard to the biomechanical modes of action and function of prophylactic dressings in PI and DTI prevention.10,11 The present study was conducted to further investigate biomechanical efficacy of dressings used prophylactically over the time of use after absorbing body fluids that accumulate in the dressing. 

Specifically, when comparing the tissue stress magnitudes developed at the different wetness levels, a difference in the transition from dry to a mildly wet dressing condition was observed, and stresses then approximately stabilized regardless of the additional increase in level of wetness. Importantly, this difference was considerably more pronounced for the hypothetical dressing, which underwent greater changes in mechanical properties with exposure to fluids. In contrast, the anisotropic multilayer dressing maintained its high modulus ratio, indicating strong anisotropy, which is a factor of its engineering design (see Table 2). owm_0718_gefen_table2

The simulation data indicate that any absorption of fluids in the dressing, even to a small extent, could potentially influence the mechanical properties and behavior of the dressing and subsequently its biomechanical efficacy in prophylaxis. Accordingly, the capacity of the anisotropic multilayer dressing to preserve approximately 80% of its ideal function (determined in dry conditions) even when wet (see Figures 2 through 4 and the related PE data) is highly important in real-world clinical scenarios, where patients are sweating and/or may have incontinence issues.

An explanation for the above differences between the 2 types of dressings is that while the anisotropic multilayer dressing continues to function as an anisotropic dressing with a directional stiffness preference when wet, the hypothetical dressing functions as a nearly isotropic dressing; hence, it has limited ability to protect the soft tissues, as reported in prior research.11 Specifically, the high modulus ratio (strong anisotropy) of the anisotropic multilayer dressing makes it much stiffer in the longitudinal direction of the dressing (the direction of the spine), which is also the direction of potential downward sliding of the body due to gravity (especially if the head of the bed is elevated). Because the anisotropic multilayer dressing is considerably stiffer along the line of the spine and given that it can maintain this property in wet conditions, the anisotropic multilayer dressing will act to preserve tissue shape and minimize tissue distortions in that longitudinal direction. By minimizing tissue distortions when wet, the anisotropic multilayer dressing effectively protects tissues of all patients, considering that normal perspiration always accumulates in the dressing, even if no other factors such as acute fever or incontinence caused (more substantial) fluid accumulation. The latter point is important, because a slightly wet dressing and a heavily wet (same) dressing behave similarly from a biomechanical perspective; thus, the dressing is able to provide a consistent, stable protective efficacy across the possible range of fluid accumulation, which makes a dressing reliably appropriate for PI prevention.  

This study introduced use of the PE index — an objective, quantitative, and standardized measure of the efficacy of prophylactic dressings subjected to altered conditions such as wetness accumulation. The PE is a straightforward method used for calculation once an adequate modeling framework is in place. Here, the PE was found to be distinctly greater for the anisotropic multilayer dressing when compared to the hypothetical dressing. This result remains consistent with the authors’ previous studies regarding the contribution of the anisotropy in the dressing structure to its biomechanical efficacy and highlights the importance of stable measures of efficacy in changing conditions such as exposure to perspiration or urine over the course of use.

In previously published work, the current authors compared the biomechanical PI prevention capacity of a hypothetical dry prophylactic dressing lacking directional stiffness preferences with that of the dry MBS anisotropic dressing.11 By means of the FE method, it was demonstrated that the anisotropic dressing design reduced tissue loads, particularly when shear forces were present at the dressing-support surface interface. In such conditions, the hypothetical isotropic multilayer dressing yielded 52% and 39% reductions in exposure to the low and high SED domains in tissues, respectively; whereas, the anisotropic MBS dressing reduced tissue exposures by as much as 60% in both domains. These results revealed the benefits of the anisotropy design feature for prophylactic dressings; however, the durability of the biomechanical protective performances in wet dressing states has not been investigated before the present work.  

Limitations

Assumptions and limitations are inevitable in any computational modeling work. First, the MRI anatomy used is that of a healthy adult. Hence, the selected anatomy does not necessarily represent pediatric patients and all the possible anatomical variations and risk factors in adults, particularly with respect to bony, malnourished, or obese patients who are at a risk for developing PIs.9,11 Likewise, pathophysiological changes in the mechanical stiffness of the tissues (eg, due to potentially existing scars or an evolving edema) were not considered. Moreover, the Poisson’s ratio of the dressing materials is an effective value for all layers (taken together), because it is technically problematic to measure Poisson’s ratios of very thin structures such as the individual layers of the dressing. Nevertheless, the authors believe these assumptions are reasonable, and they facilitated the systematic, standard and objective quantitative comparisons reported here. These data should be fundamental for computer-aided design of improved or new dressings and would help inform future methodological studies of efficacy of prophylactic dressings over time and use. 

Conclusion 

The purpose of this modeling study was to further investigate relationships between design features and biomechanical efficacy of sacral prophylactic dressings, particularly when exposed to moisture. The multilayer anisotropic prophylactic MBS dressing was found to exhibit superior PE, which was approximately 4 times that of the hypothetical dressing.  The present results are especially relevant to the common scenario of external shear loads applied to the sacral area that are caused by gravity pulling the body downward in bed, as well as the presence of moisture. A next step in research would be to optimize the extent of the anisotropy, particularly the property ratio of stiffnesses (elastic moduli). Although low anisotropy (toward an isotropic dressing) was shown to be less efficient in alleviating tissue loads in the model, an upper limit to the property ratio above which biomechanical effectiveness is not increased or even decreases should be considered. A second feature that interacts with the aforementioned stiffness property ratio is the ability of the dressing to evaporate or clear moisture, and hence maintain its anisotropy feature as well as its PE stability for hours and days, which requires laboratory studies of evaporation capacity of different dressing designs. 

References 

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5. Santamaria N, Gerdtz M, Sage S, et al. A randomized controlled trial of the effectiveness of soft silicone foam multi-layer dressings in the prevention of sacral and heel pressure ulcers in trauma and critically ill patients: the border trial. Int Wound J. 2015;12(3):302–308.

6. Kalowes P, Messina V, Li M. Five-layered soft silicone foam dressing to prevent pressure ulcers in the intensive care unit. Am J Crit Care. 2016;25(6):e108–e119.

7. Santamaria N, Gerdtz M, Liu W, et al. Clinical effectiveness of a silicone foam dressing for the prevention of heel pressure ulcers in critically ill patients: Border II Trial. J Wound Care. 2015;24(8):340–345.

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9. Levy A, Gefen A. Computer modeling studies to assess whether a prophylactic dressing reduces the risk for deep tissue injury in the heels of supine patients with diabetes. Ostomy Wound Manage. 2016;62(4):42–52.

10. Levy A, Gefen A. Assessment of the biomechanical effects of prophylactic sacral dressings on tissue loads: a computational modeling analysis. Ostomy Wound Manage. 2017;63(10):48–55.

11. Levy A, Schwartz D, Gefen A. The contribution of a directional preference of stiffness to the efficacy of prophylactic sacral dressings in protecting healthy and diabetic tissues from pressure injury: computational modelling studies. Int Wound J. 2017;14(6):1370–1377.

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17. Gefen A, Haberman E. Viscoelastic properties of ovine adipose tissue covering the gluteus muscles. J Biomech Eng. 2007;129(6):924–930.

18. Oomens CW, Zenhorst W, Broek M, et al. A numerical study to analyze the risk for pressure ulcer development on a spine board. Clin Biomech. 2013;28(7):736–742.

19. Call E, Pedersen J, Bill B, et al. Enhancing pressure ulcer prevention using wound dressings: what are the modes of action? Int Wound J. 2015;12(4):408–413.

20. Sopher R, Nixon J, McGinnis E, Gefen A. The influence of foot posture, support stiffness, heel pad loading and tissue mechanical properties on biomechanical factors associated with a risk of heel ulceration. J Mech Behav Biomed Mater. 2011;4(4):572–582.

Potential Conflicts of Interest: The study was supported by an unrestricted educational grant from Mölnlycke Health Care (Gothenburg, Sweden), from which Prof. Gefen received speaker honoraria. 

Ms. Schwartz is a master student in biomedical engineering; Ms. Levy is a doctoral student in biomedical engineering; and Prof. Gefen is a Professor of Biomedical Engineering, Department of Biomedical Engineering, Faculty of Engineering, Tel Aviv University, Israel. Please address correspondence to: Prof. Amit Gefen, Department of Biomedical Engineering, Tel Aviv University, Ramat Aviv 6997801, Tel Aviv, Israel; email: gefen@eng.tau.ac.il.

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