Assessment of the Biomechanical Effects of Prophylactic Sacral Dressings on Tissue Loads: A Computational Modeling Analysis

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Ostomy Wound Management 2017;63(10):48–55 doi: 10.25270/owm.10.4855
Ayelet Levy, MSc; and Amit Gefen, PhD

Abstract

The sacrum is the most susceptible anatomical site for developing pressure injuries, including deep tissue injuries, during supine lying. Prophylactic dressings generally are designed to reduce friction, alleviate internal tissue shear, manage the microclimate, and overall cushion the soft tissues subjected to sustained deformations under the sacrum. Using computational modeling, the authors developed a set of 8 magnetic resonance imaging-based, 3-dimensional finite element models of the buttocks of a healthy 28-year-old woman for comparing the biomechanical effects of different prophylactic sacral dressing designs when used during supine lying on a standard hospital foam mattress.

Computer simulation data from model variants incorporating an isotropic (same stiffness in every direction) multilayer compliant dressing, an anisotropic (directionally dependent stiffness properties) multilayer compliant dressing, and a completely stiff dressing were compared to control (no dressing). Specific outcome measures that were compared across these simulation cases were strain energy density (SED) and maximal shear stresses in a volume of interest (VOI) of soft tissues surrounding the sacrum. The SED and shear stress measurements were obtained in pure compression loading of the buttocks (ie, simulating a horizontal supine bed rest) and in combined compression-and-shear loads applied to the buttocks (ie, 45˚ Fowler position causing frictional and shear forces) on a standard foam mattress. Compared to the isotropic dressing design, the anisotropic dressing facilitated more soft tissue protection through an additional 11% reduction in exposure to SED at the VOI. In this model, use of the anisotropic compliant dressing resulted in the lowest exposures to internal tissue SED and shear stresses. Research to examine the clinical inference of this modeling technique and studies to compare the effects of prophylactic dressings on healthy volunteers and patients in different positions are warranted. 

 

A pressure ulcer (PU), now termed a pressure injury (PI) in the United States, is defined in the international guidelines1 as a localized injury to the skin and/or underlying tissues, usually over a bony prominence, resulting from sustained pressure (including pressure associated with shear). The soft tissues around the sacrum are known to be the most common anatomical site for patients to develop a PU while in bed.2 During prolonged supine bed rest, the weight of the lower trunk and pelvis is transferred through the pelvic bones to the mattress, subjecting subcutaneous fat and skin tissues under the sacrum to sustained intensified deformations. In cases where impaired mobility, sensitivity, or both are present, these sustained tissue deformations may exceed tissue tolerance levels, increasing the risk for PUs and particularly for a deep tissue injury (DTI), which is especially dangerous due to the potential extent of tissue damage.3 Because the sacrum is a major body weight-bearing site for a supine patient, and the sacral bone is not only rigid but also highly curved (almost “sharp”), the soft tissues in the sacral area are extremely susceptible to compressive tensional and shear loads. As shown by magnetic resonance imaging (MRI) and computer modeling studies, shear, which is naturally present in deformed soft tissues due to the complex curved anatomy and stiffness gradients between hard/soft and layered tissue structures, is substantially magnified when a patient is positioned in a Fowler position and starts sliding down in bed, during repositioning, or when spontaneous movements occur.4 

Minimizing magnitudes and exposure durations of mechanical compression and shear loads in soft tissues during weight-bearing, as well as reducing friction, have long been the main goals in PU prevention and management.1,4,5 According to clinical guidelines,1 conventional sacral PU prevention strategies mainly include manual periodical repositioning and use of pressure-redistributing mattresses, either static or alternating air pressure-based, which are designed to cushion the body of the patient and thereby minimize localized increased contact pressures between the skin and the support surface. However, several critical concerns are relevant regarding this approach; for example, how can it be determined whether reduced focal contact pressures on the skin also improve the mechanical state of the deep soft tissues in the context of DTI prevention? In addition, the effects of shearing forces that act on the internal soft tissues under the sacrum have not been evaluated for different prophylactic dressing designs. This is specifically relevant during manual repositioning or when a patient slides down in bed when the head of the bed is elevated due to gravity.   

The use of dressings as preventive (prophylactic) measures is a relatively new concept that is attracting attention in academia, medical settings, and the industry worldwide.6 Sacral prophylactic dressings generally are designed to reduce friction and shear with the mattress, manage the microclimate of the skin by maintaining an appropriate humidity level, and redistribute pressure by adding a layer of soft, cushioning materials between the skin and the mattress.6 In the past few years, several  clinical studies7,8 have demonstrated the efficacy of prophylactic dressings. For example, Santamaria et al7 showed significantly fewer patients developed PUs in the intensive care unit when prophylactic heel and sacral dressings were applied in the emergency department, compared to when dressings were not used prophylactically (5 versus 20 patients, P = .001). Other evidence regarding the biomechanical effects of prophylactic dressings has been generated based on laboratory testing of shear and frictional properties.9-12 Ohura et al10 used an experimental model consisting of porcine skin embedded with a force/shear sensor that demonstrated shear forces in the subcutaneous layer were reduced to within 31% to 45% when dressings were used, compared to a no-dressing scenario. Previously published mechanical studies13,14 from the current authors’ group have shown biomechanical efficacy of compliant multilayer heel dressings (the Mepilex® Border Heel dressing [Mölnlycke Health Care, Gothenberg, Sweden] design) by means of computational finite element (FE) modeling. Briefly, the FE method is a computational technique used to evaluate the mechanical deformations, strains, and stresses deep within complex structures of multiple materials and amorphous shapes. The complex structure is divided into thousands of small elements, each having a simple geometry, and then the differential governing equations that describe the mechanical interactions of the problem are solved numerically per each element with respect to its surrounding elements to form the solution of the entire structure. Apart from its ability to evaluate the mechanical states deep within the soft tissues in clinically realistic scenarios as opposed to superficial pressure mapping which merely captures pressure (but not shear) on the skin, flexibility and robustness are what make the FE method such an important approach in preclinical PI prevention research. However, it is important to recognize that the credibility of any model relies on the quality and realism of the input data, such as geometric factors, material properties, and boundary conditions.15 Biomechanical modeling should generally be developed based on relevant clinical observations and data in order to avoid threats to valid clinical inference. Hence, modeling work is complementary to clinical research, and, in the case of clinical studies of prophylactic dressings,7,8 it is particularly useful for explaining and interpreting the published clinical evidence of efficacy. As shown in the present study, modeling is specifically able to identify preferred design features and mechanisms of action of a prophylactic dressing technology that facilitate the reported clinically evident efficacy.7,8 

In the authors’ published modeling studies, the aforementioned multilayer dressing was found to have a beneficial effect on the mechanical states of the soft tissues of the heel of a single healthy subject, which may reduce the corresponding risk for heel ulceration (heel DTI) during supine bedrest.13,14 Specifically, the aforementioned computer simulation studies employed an MRI-based FE model of the heel resting on a hospital foam mattress and with either healthy or diabetic tissue properties to evaluate internal tissue deformations and mechanical stresses in the soft tissues that are distorted by the weight of the foot.13,14 Tissue deformations and stresses were compared between cases of the heel protected by a dedicated prophylactic dressing versus no protection and for the protected heel between multilayered and single-layer dressing designs. A 25.5% reduction of peak effective stresses occurred in the soft tissues of the heel in the healthy subject with the use of the multilayered prophylactic dressing that was modeled.13 The protective performance of the tested dressing was consistent across different plantar flexion positions of the foot subjected to various shearing forces.14 

In the current study, previous modeling techniques13,14 were applied to the sacral model to determine the biomechanical effects of different (sacral) prophylactic dressing designs on the state of mechanical loads in the soft tissues surrounding the sacrum for supine/Fowler posture on a hospital foam mattress. Furthermore, the modeling was expanded to investigate a new research question — namely, are there benefits in anisotropy of the dressing with regard to alleviation of internal soft tissue loads? Anisotropy, the characteristic of having directionally dependent stiffness (ie, the ratio of the force deforming the dressing over the actual deformation of the dressing) properties, is a design feature of the modeled anisotropic-compliant dressing. This is in contrast to conventional isotropy (ie, same stiffness in every direction) of other currently available prophylactic dressings. 

The aim of this study was to compare mechanical states of simulated soft tissues surrounding the sacral bone across scenarios involving an anisotropic-compliant dressing, an isotropic-compliant dressing, and a completely stiff dressing using a set of computational FE model variants of the 3-dimensional (3D) buttocks and prophylactic dressing structures during supine weight-bearing or a 45˚ Fowler position. Outcome measures assessed were strain energy density (SED) — an indicator of mechanical stress — and maximal shear stresses in a volume of interest (VOI) of soft tissues surrounding the sacrum. 

Methods

Eight (8) FE model variants were developed to investigate the effects of the design (particularly, the stiffness anisotropy) of prophylactic dressings on the mechanical states of the soft tissues at the sacral region in a supine weight-bearing position (see Table 1). The following conditions were simulated: 1) without a dressing, 2) with an isotropic multilayer compliant dressing, 3) with an anisotropic multilayer compliant dressing, and 4) with a completely stiff isotropic dressing. These mechanical states in soft tissues were simulated in pure compression loading of the buttocks (ie, simulating a horizontal supine bed rest) and in combined compression and shear loads applied to the buttocks (ie, 45˚ Fowler position). owm_1017_gefen_table1

Geometry. In order to develop a 3D, anatomically realistic geometrical model of the buttocks of a supine subject, 76 T1-weighted axial MRI slices were used. A 28-year-old healthy woman was scanned in a supine position, fully weight-bearing, on a designated rigid platform. Imaging was performed in a 1.5 Tesla MR system (MAGNETOM Aera, SIEMENS AG, Munich, Germany) utilizing T1-weighted images (TR/TE=550/10, field of view 42 mm × 420 mm, slice thickness 3 mm), at the Assaf Harofeh Hospital (greater Tel Aviv area), Israel. The MRI study was approved by the Institutional Review Board (Helsinki Committee) of Assaf Harofeh Hospital (Approval no. 190/14). The above MRI scan captured the entire region of the pelvis from the iliac crest to the shaft of the femurs. The image set was imported from the MRI to the ScanIP 3D image module of Simpleware® (Exeter, UK), where semi-automatic segmentation was performed in order to distinguish between the pelvic bones and soft tissue regions16 (see Figure 1a). owm_1017_gefen_figure1

Next, 3 of the 5 layers of the isotropic and anisotropic multilayer dressings were applied in the modeling — namely, the polyurethane foam (PUR), the nonwoven (NW), and the airlaid (AL) layers, using the 3D image module. The innermost Safetac® layer (Mölnlycke Health Care) then was added as a tied interface between the skin and the polyurethane foam, preventing these layers from penetrating or sliding across each other in the modeling, and the backing film layer also was represented to address frictional sliding with the elastic (foam) support (as reported in previous papers from the authors13,14). 

The present modeling challenge of representing the modes of action of the sacral prophylactic dressing involved allocating greater computational power than demonstrated in the authors’ previous work13,14 involving heel dressings. This is primarily due to the complexity of this 3D FE problem that includes elements with dimensions that vary from fractions of mm for the dressing components and up to tens of cm for the bone and soft tissue structures of the 3D buttocks. Accordingly, several measures were taken to simplify this large deformation problem to the extent that adequate numerical solutions could be obtained, despite the considerable challenge regarding the multiscales as explained previously. First, for these modeling procedures, skin, muscle and fat components were considered together and grouped as “soft tissue” structures. Second, the model volume for the FE analyses was decreased to include the dressing, the sacrum, and the surrounding soft tissues contained in the 3D block shown in Figure 1b. Adequate margins of soft tissue structures were intentionally kept around the dressing to avoid any boundary or edge effects (see Figure 1b). 

Next, a flat standard foam mattress was added under the modeled buttocks (and under the dressing, in cases where a dressing was applied). Final FE meshing also was performed in the 3D imaging module using 139 964 to 212 585 linear tetrahedral elements describing the bones and soft tissues as well as 1 636 013 linear tetrahedral elements describing the 3 physical layers of the multilayer dressings. Hence, the FE analyses, which are described here, were conducted using meshes that contained nearly 2 million elements, which was essential given the multiscale challenge, and specifically, for adequate numerical transition between the microscale of the layered structure of the dressing and the macroscale of the buttocks tissues.

Mechanical properties of the tissues and dressing. The constitutive laws and mechanical properties of all tissues were adopted from the literature based on empirical data.13,14 Specifically, the pelvic bone and femurs were assumed to be linear-elastic isotropic materials with elastic moduli of 7 GPa and Poisson’s ratios of 0.3. All soft tissues were considered together as 1 effective material as previously noted17 and were assumed to be nearly incompressible nonlinear isotropic materials, with their large deformation behavior described by an uncoupled Neo-Hookean constitutive model. 

The material constants reported by Oomens et al17 were used to represent the effective soft tissue stiffness, assuming that skin contributes 60% to the effective stiffness and the other 40% are attributed to fat. The PUR, NW, and AL layers of the isotropic multilayer dressing were considered isotropic linear-elastic materials with elastic moduli of 24 kPa, 150 kPa, and 30.6 kPa, respectively, based on measurements previously performed in the authors’ laboratory and recently reported.13,14 The Poisson’s ratio assigned to these dressing layers was 0.258 based on published experiments.12 In cases where the completely stiff isotropic dressing was used (variants 4 and 8), the PUR, NW, and AL layers of the stiff dressing were considered isotropic linear-elastic materials with elastic modulus of 1 MPa and a Poisson’s ratio of 0.258. The mattress was considered isotropic linear-elastic as well, with an elastic modulus of 50 kPa and Poisson’s ratio of 0.3, again based on literature.13,14,18 

The anisotropic multilayer compliant dressing design comprises anisotropy — directional stiffness properties that constitute a stiffer longitudinal behavior in the direction of the spine versus more compliant “wings” that facilitate lateral stretching of the dressing. To capture this anisotropy feature, the stiffness properties of the PUR, NW, and AL layers of model variants 3 and 7 of the isotropic dressing were increased by 45% only in the axial (Z) direction to replicate the longitudinal stiffness characteristic of the anisotropic dressing based on measurements preformed in the authors’ laboratory to quantify this anisotropy (see Table 1). 

Body loads applied to the buttocks model, shear, and friction conditions. Boundary conditions were chosen to simulate the descent of the weight-bearing pelvic bones during supine lying or a 45˚ Fowler position. The response of soft tissues to this descent was tested without and with each of 3 test dressings of the same shape. In all simulation cases, dressings were attached to exactly the same sacral region, ideally aligned, and symmetrically placed according to manufacturer’s guidelines (as these dressings would have been in a real-world scenario), as detailed in Table 1. 

In terms of other relevant constraints, the bottom surface of the mattress was fixed for all translations and rotations. Tied interfaces were defined between the bones and soft tissues as well as between the soft tissues and the PUR layer of the dressing to account for the full adherence properties of the Safetac layer of the dressings. Frictional sliding was defined between the AL layer of the dressings and the mattress, with the coefficient of friction set as 0.35 to simulate the low-friction effect provided by the backing film layer of the dressings.12 In model variants 1 and 5 (ie, simulations of the weight-bearing buttocks without a dressing), the coefficient of friction between the soft tissues and the mattress was set to be higher (0.4) because of the absence of the backing film.13

To simulate loading conditions, downward displacements in the range of 5.3 mm to 6.45 mm in all model variants were applied on the top surface of the reduced model volume (marked in Figure 1b) until the total reaction force acting back from the mattress reached 40 Newtons (roughly 7% of the total bodyweight of the subject), which were assumed to be transferred through this reduced model volume for the purpose of comparison across model variants. In model variants 5, 6, 7, and 8, the same extent of displacement also was applied in the axial (Z) direction, accounting for the shearing forces that may act due to sliding down in the bed (eg, when seated in bed in a 45˚ Fowler position) or due to some spontaneous movements or repositioning of the patient in the bed. The FE simulations all were created using the PreView module of FEBio (version 1.18), analyzed using the Pardiso linear solver of FEBio (version 2.3.1), and post-processed using PostView of FEBio (version 1.919) (University of Utah, Salt Lake City, UT). 

Biomechanical outcome measures. Volumetric exposures of the soft tissues adjacent to the sacral bone to sustained deformations were examined and quantified in terms of the strain energy density (SED) in these soft tissues within the reduced model volume (see Figure 1b). Briefly, SED is a scalar measure in units of mechanical stress (eg, kPa) that describes the spatial dispersion of the elastic energy that is stored in an object that undergoes deformation. It is a factor of the stiffness of the material and of the mechanical strains and stresses that develop in every point within the deformed object.

Data analysis. The SED data were pooled from the soft tissues for all the elements in a 67 mm x 55 mm x 20 mm soft tissue cube located immediately under the sacrum, which had been defined as the volume of interest (VOI) for the purpose of SED data comparisons across the model variants (see Table 1), as depicted in Figure 1d. Converging time steps were chosen for data collection, so the resulting reaction forces between the buttocks and the support were within less than a 2.4% difference from the aforementioned target reaction force. The SED in the VOI were analyzed across the model variants to determine whether additional biomechanical efficacy is present in the anisotropic multilayer dressing design in terms of alleviation of tissue loads with respect to a no-dressing situation, to an isotropic multilayer dressing case, or to a completely stiff dressing. These simulations were repeated in either pure compression or compression combined with shear loads (see Figure 1c) and compared quantitatively by calculating the volumetric exposures to SED in the soft tissues in the VOI per each simulation case (see Table 1). The details of the method of FE analysis are explained in the authors’ previous publication14 that includes explanations with regard to calculation and data processing techniques.

Results

Contact pressure distributions on the skin surface with and without the isotropic multilayered dressing are shown in Figure 2a. Similar to previously published results13,14 regarding contact pressure distributions under the heel protected by a border dressing, the isotropic multilayer dressing was able to reduce peak (maximal) contact pressures under the weight-bearing buttocks from 6 kPa to 2.9 kPa  (52%) when loaded in pure compression, resulting in a more uniform distribution of contact pressures between the skin and the mattress at the sacral region (see Figure 2a). owm_1017_gefen_figure2

Furthermore, with respect to a bare skin condition, the isotropic multilayer dressing consistently reduced the volumetric exposures of the soft tissues under the sacrum to sustained deformations across the entire range of SEDs from 0.1 kPa to 1.9 kPa (see Figures 2b, 3b, 4) and when loaded in either pure compression or in combined compression and shear. However, the anisotropic design, which is stiffer in the axial (Z) direction of the dressing, further reduced the volumetric exposures of the soft tissues under the sacrum to sustained large deformations when external shear was introduced, by an additional 11% with respect to the isotropic case (model variant 7) (see Figures 3, 4, and 5). Specifically, while the isotropic multilayer dressing lowered the average volumetric exposure of the soft tissues under the sacrum by 54% and 50% in the low (<0.5 kPa) and high (>0.5 kPa) SED domains, respectively, (as defined by Sopher et al18), the corresponding value for the anisotropic dressing was 61% (for both domains) (see Figure 5). Hence, the anisotropy feature of the anisotropic dressing facilitated more soft tissue protection (additional 11% reduction) in exposure to large tissue deformations. 

owm_1017_gefen_figure3owm_1017_gefen_figure4owm_1017_gefen_figure5

While in theory applying a completely stiff dressing on the sacral region may shield the underlying soft tissues in the sacral bone region from sustained deformations, in the cases where completely stiff dressings were tested (model variants 4 and 8), tissue deformations were found to have shifted laterally, resulting in increased stress concentrations in the soft tissues near the perimeter of the dressing rather than under the sacrum (see Figure 3d). For example, in the cases where combined compression and shear were applied, the maximal shear stress above the midpoint of the dressing and just below the sacral bone decreased from 0.2 kPa to 0.16 kPa (21.5%) when a completely stiff dressing was used (compared to the corresponding no-dressing case) but increased from 0.15 kPa to 0.5 kPa  (as high as 70%) upon analysis of the soft tissue volume above the perimeter of the dressing (see Figure 3). 

Discussion

In the present study, a set of MRI-based 3D FE model variants of the buttocks in a supine position was used to evaluate the design features and biomechanical effects of sacral dressings designed to prevent PIs and DTIs. Focusing specifically on stiffness and anisotropy of stiffness properties of these sacral dressings, volumetric exposures of soft tissues under the sacrum to tissue deformations and loads (quantified as elevated SED values) during supine lying when using dressings were determined. The biomechanical effects of an isotropic multilayer dressing, an anisotropic dressing, and a completely stiff dressing were compared when loaded in either pure compression or in combined compression and shear. The primary objective of the present study was to determine whether anisotropy of the prophylactic sacral dressing, which allows it to be more stretchable in the lateral (buttock cheeks) direction than along the direction of the spine, is beneficial in protecting the soft tissues from deformation-inflicted tissue damage. 

Similar to the authors’ previous modeling work regarding the risk for heel ulcers and the potential mitigating role of prophylactic dressings,13,14 prophylactic dressings were found effective in lowering exposure to sustained tissue deformations under the sacrum as well. Peak contact pressures and SED values decreased 50% to 61% compared to the no-dressing equivalent cases (see Figures 2 and 4). Multilayered sacral dressings not only provided extra cushioning and pressure redistribution under the weight-bearing buttocks, but they also deformed themselves (particularly in shear), given their compliant-stiff-compliant layered structure, which then diverted deformation and load from the tissues to the structure of the dressing itself. An additional advantage specific to the multilayer dressings that were modeled may be offered by the smooth backing film layer of the dressings, which facilitates decreased friction and hence, reduced shear loads in the underlying tissues.12,13 In addition, completely stiff isotropic dressings, which in theory could be effective in minimizing tissue deformations at the center of the dressing, were found to show a tradeoff effect, inflicting increased deformations and loads (SED values), especially elevated shear stresses in the soft tissues along the perimeter of the dressing. This was thought to be due to the sharp gradients in stiffness between the dressing material and the soft tissues, which was promoting shear in tissues at the borders. Hence, the concept of completely stiff dressings is not recommended for tissue protection; while stiff dressings will maintain the shape of tissues at the center of the dressing (much like a plaster cast will do), they may cause tissues at the border of the dressing to stretch and shear (see Figure 3d). 

The most important finding from this modeling study concerns the benefit of using anisotropy as a design feature in prophylactic sacral dressings. The anisotropic structure of the modeled anisotropic dressing resulted in the lowest exposures to tissue SED values, particularly when the buttocks model was loaded in combined compression and shear, a common scenario in patients who require head-of-bed elevation and frequent repositioning.14 The stiffness anisotropy of the anisotropic dressing (ie, modeled here using greater stiffness in the direction of the spine) was shown in this model to provide extra protection to soft tissues around the sacrum when shear loads are present. Specifically, in this model, the compliant stretch range in the lateral direction of the dressing (pointing toward the buttock cheeks) facilitated an extra 11% reduction in soft tissue exposure to SED at the VOI. The anisotropic dressing mitigated tissue deformations and loads under the perimeter of the dressing as well as directly under the sacrum (see Figure 3c) as opposed to the isotropic dressing, which did not allow tissues to expand laterally, leading to stress concentrations under the lateral borders of the isotropic dressing (see Figure 3d). The authors concluded the stiffness anisotropy may be a critically important design feature in multilayer dressings for prophylactic use.

In order to quantify the potential for tissue damage, the authors opted to use the SED measure, believing it is optimal for quantifying the exposure to sustained tissue loads as related to PI and in particular to DTI risks. In the literature, SED distributions have been experimentally correlated with the severity and extent of tissue damage in rodent model experiments.20 These findings made this scalar measure the first choice for characterizing tissue loads in the context of PI risks.18 Moreover, SED data are a weighed measure of all tissue deformation modes (ie, a strain tensor that includes compression, shear, and tension distortion components) with added tissue stiffness properties (ie, the stiffness tensor). This helps resolve the debate regarding which engineering load measures — strain or stress measures — are preferable for evaluating the risk for PIs in tissues.21 

Limitations

The clinical inference of this study design is unknown and any modeling inevitably includes assumptions and limitations, which should be discussed for completeness and interpretation of the findings. First, the authors chose to use the deformed (weight-bearing) anatomy of the buttocks to develop the initial geometric model, with the aim to focus on tissue deformations at the nearly weight-bearing configuration of the buttocks as relevant to a resting supine patient. If they had used a completely underformed (nonweight-bearing) anatomy, slightly greater exposures to SED (manifested as higher curves in the graphs in Figure 4) would have been expected. However, this additional tissue deformation would have been mostly due to the lateral tensile component (tissue stretching) from the spreading of the cheeks of the buttocks during the weight-bearing process rather than from the compression and shear components resulting from interactions between hard and soft tissues under the sacrum. That being said, for the purpose of comparison of cases with prophylactic dressings versus without them or for comparing isotropic to anisotropic dressing designs, a deformed anatomical buttocks model was believed to be reasonable and practical to use. Furthermore, to reduce the required computer power and simplify this extremely complex 3D large deformation FE problem, soft tissues were considered as 1 entity and used an effective soft tissue material. Using a distinct geometrical and mechanical representation of each tissue could have resulted in a more accurate resolution of tissue loads, and with the development of computational modeling technologies in this field, it will probably be attempted in the future. For example, with the soft tissues considered together into 1 “soft tissue” material as was done in this study, lower stresses were more likely to be noted in the “skin” region than had the skin (which is stiffer than the underlying fat and muscles) been represented as a separate tissue layer. In other words, the observed stresses at the skin surface are likely slightly less than they would be if tissue layers had been assessed separately, but subdermal tissue stresses, which were the focus of this work, are adequately evaluated. Hence, given that this study focused on the prevention of DTIs and the potential role of prophylactic sacral dressings in doing so, the resolution of superficial tissue loads (on the skin) could be reasonably compromised for the benefit of achieving the 3D representation of the large deformation of the entire buttocks. 

Lastly, although the geometrical model is anatomically accurate, it is based on an MRI scan of a single (healthy) subject, which is not necessarily representative of a patient at a high risk for PIs and DTIs. Future work should include modeling of the variations and changes in tissue structures and mechanical properties as associated with known risk factors for PIs (eg, type 2 diabetes).14 Likewise, more research is needed with regard to the variations in the shear loading schemes associated with the patient’s position in bed and guidelines for repositioning and general care, as well as to the use of advanced, more sophisticated support surfaces, which are commonly provided for at-risk patients. Specifically, more information is needed with regard to the interactions of sophisticated support surfaces with prophylactic dressings.

Conclusion

The FE modeling results obtained in this study suggest prophylactic sacral dressings may minimize exposure to sustained tissue deformations and as such protect tissues from PIs and DTIs during supine lying or a 45˚ Fowler position on a standard hospital foam mattress. Further, the differences in mechanical behavior observed between the modeled anisotropic, isotropic, and stiff dressings suggests the former may provide enhanced protection against unavoidable shear loads under the sacrum as well as in adjacent soft tissues underlying the perimeter of the dressing. Additional clinical research should be conducted to examine the clinical inference of this modeling technique and investigate the effects of prophylactic dressings on healthy volunteers and patients in different positions in bed or on operation tables, perhaps using force/shear sensors over and under the dressings. 

References 

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7. Santamaria N, Gerdtz M, Sage S, et al. A randomized controlled trial of the effectiveness of soft silicone foam multi-layered 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.

8. 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.

9. Forni C, Loro L, Tremosini M, et al. Use of polyurethane foam inside plaster casts to prevent the onset of heel sores in the population at risk. A controlled clinical study. J Clin Nurs. 2011;20(5-6):675-680.

10. Ohura T, Takahashi M, Ohura N Jr. Influence of external forces (pressure and shear force) on superficial layer and subcutis of porcine skin and effects of dressing materials: are dressing materials beneficial for reducing pressure and shear force in tissues? Wound Repair Regen. 2008;16(1):102–107.

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12. 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.

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14. 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.

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16. Simpleware® Ltd. ScanIP, +FE, +NURBS and +CAD Reference Guide ver. 5.1, 2012. Available at:  www.simpleware.com/software. Accessed September 26, 2017.

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

18. 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.

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Potential Conflicts of Interest: Dr. Gefen is Chair of the Scientific Advisory Board, Mölnlycke Health Care, Gothenburg, Sweden; and is funded by Mölnlycke Health Care for investigating the effects of dressing materials and designs on soft tissues during weight-bearing. 

 

Ms. Levy is a doctoral student; and Dr. Gefen is a Professor of Biomedical Engineering, Department of Biomedical Engineering, Faculty of Engineering, Tel Aviv University, Tel Aviv, Israel. Please address correspondence to: Amit Gefen, PhD, Tel Aviv University, Tel Aviv 6997801 Israel; email: gefen@eng.tau.ac.il.

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