The influence of foot posture, support stiffness, heel pad loading and tissue mechanical properties on biomechanical factors associated with a risk of heel ulceration

The frictional energy absorber effectiveness and its impact on the pressure ulcer prevention performance of multilayer dressings

Pressure ulcers including heel ulcers remain a global healthcare concern. This study comprehensively evaluates the biomechanical effectiveness of the market-popular ALLEVYN® LIFE multilayer dressing in preventing heel ulcers. It focuses on the contribution of the frictional sliding occurring between the non-bonded, fully independent layers of this dressing type when the dressing is protecting the body from friction and shear. The layer-on-layer sliding phenomenon, which this dressing design enables, named here the frictional energy absorber effectiveness (FEAE), absorbs approximately 30%-45% of the mechanical energy resulting from the foot weight, friction and shear acting to distort soft tissues in a supine position, thereby reducing the risk of heel ulcers. Introducing the novel theoretical FEAE formulation, new laboratory methods to quantify the FEAE and a review of relevant clinical studies, this research underlines the importance of the FEAE in protecting the heels of at-risk patients. The work builds on a decade of research published by our group in analysing and evaluating dressing designs for pressure ulcer prevention and will be useful for clinicians, manufacturers, regulators and reimbursing bodies in assessing the effectiveness of dressings indicated or considered for prophylactic use.

The Protocol of Ultrasonic Backscatter Measurements of Musculoskeletal Properties

This study aims to introduce the protocol for ultrasonic backscatter measurements of musculoskeletal properties based on a novel ultrasonic backscatter bone diagnostic (UBBD) instrument. Dual-energy X-ray absorptiometry (DXA) can be adopted to measure bone mineral density (BMD) in the hip, spine, legs and the whole body. The muscle and fat mass in the legs and the whole body can be also calculated by DXA body composition analysis. Based on the proposed protocol for backscatter measurements by UBBD, ultrasonic backscatter signals can be measured in vivo, deriving three backscatter parameters [apparent integral backscatter (AIB), backscatter signal peak amplitude (BSPA) and the corresponding arrival time (BSPT)]. AIB may provide important diagnostic information about bone properties. BSPA and BSPT may be important indicators of muscle and fat properties. The standardized backscatter measurement protocol of the UBBD instrument may have the potential to evaluate musculoskeletal characteristics, providing help for promoting the application of the backscatter technique in the clinical diagnosis of musculoskeletal disorders (MSDs), such as osteoporosis and muscular atrophy.

Impact of Stoma Baseplate Convexity on Tension and Compression Around the Stoma Site: A Finite Element Analysis

For patients living with intestinal or urinary stomas, skin barriers play an essential role in protecting the peristomal skin and preventing peristomal complications. Convex baseplates press into the peristomal skin and are suitable for retracted stomas that do not protrude, peristomal skin with creases, folds, or dips, and stomas where frequent leaking can occur with flat pouching systems. However, there is a lack of data on the magnitude and location of tension applied to the abdomen by convex baseplates. We evaluated the impact of a range of convex baseplates applied to a simulated stoma site. A comparative finite element analysis investigation was conducted to evaluate the impact of eight different convex stoma system baseplates applied to an idealised flat abdomen, representing skin, subcutaneous tissue, and musculature layers. The baseplates considered had varying convexity with depths of 3.5 mm and 7 mm and internal structural diameters between ~30 mm and ~60 mm. The convex product range provided tension in the skin (maximum principal strain) and compression through the fat layer (minimum principal strain). Large differences in the locations and magnitudes of skin tension and fat layer compression were seen between the baseplates under analysis, with both the depth and diameter of convexity influencing the strain experienced across the abdominal topography. The results generated highlight the importance of having an appropriate range of convexity products available and selecting an appropriate option for use based on the stoma type and condition of the peristomal skin.

Engineering the perfect mattress: The influence of substrate mechanics on deep tissue stresses in supine

BACKGROUND

With increasing global interest in sleep hygiene, sleep ergonomics is an area that has been largely understudied. During sleep individuals turn over during the night to restore blood flow in occluded blood vessels, indicating that control of local tissue pressure may play a role in improving sleep comfort. This study investigates the influence of mattress stiffness on tissue compressive stresses during supine lying.

METHODS

A subject-specific 3D finite element (FE) model of the pelvis area has been developed to simulate supine lying on substrates of varying firmness. Constitutive parameters for the adipose-skin tissue and muscle-organ tissue were calibrated using a novel application of the inverse finite element method.

FINDINGS

The compressive stress was consistently greatest in the muscle interfacing the sacrum at 18.5 kPa on the soft foam, and 30.9 kPa on the firm foam. From soft to firm, the compressive stress increased by 67% at the sacrum, 20% at the ischium, 42% at the lesser trochanter, and 50% at the skin.

INTERPRETATION

The non-linearity of the foam substrate had a pressure distributing effect, relieving the peak compressive stresses at the sacrum, indicating that it may be possible to design arrays of foam substrates that can provide most efficient pressure relief.

Finite Element Tissue Strains Computation to Evaluate the Mechanical Protection Provided by a New Bilayer Dressing for Heel Pressure Injuries

OBJECTIVE

Pressure injuries (PIs) result in an extended duration of care and increased risks of complications for patients. When treating a PI, the aim is to hinder further PI development and speed up the healing time. Urgo RID recently developed a new bilayer dressing to improve the healing of stages 2 and 3 heel PIs. This study aims to numerically investigate the efficiency of this new bilayer dressing to reduce strains around the PI site.

METHODS

The researchers designed three finite element models based on the same heel data set to compare the Green-Lagrange compressive and maximal shear strains in models without a PI, with a stage 2 PI, and with a stage 3 PI. Simulations with and without the dressing were computed. Analysis of the results was performed in terms of strain clusters, defined as volumes of tissues with high shear and compressive strains.

RESULTS

Decreases in the peak and mean values of strains were low in all three models, between 0% and 20%. However, reduction of the strain cluster volumes was high and ranged from 55% to 68%.

CONCLUSIONS

The cluster analysis enables the robust quantitative comparison of finite element analysis. Results suggest that use of the new bilayer dressing may reduce strain around the PI site and that this dressing could also be used in a prophylactic manner. Results should be extended to a larger cohort of participants.

Definition and evaluation of a finite element model of the human heel for diabetic foot ulcer prevention under shearing loads

Diabetic foot ulcers are triggered by mechanical loadings applied to the surface of the plantar skin. Strain is considered to play a crucial role in relation to ulcer etiology and can be assessed by Finite Element (FE) modeling. A difficulty in the generation of these models is the choice of the soft tissue material properties. In the literature, many studies attempt to model the behavior of the heel soft tissues by implementing constitutive laws that can differ significantly in terms of mechanical response. Moreover, current FE models lack of proper evaluation techniques that could estimate their ability to simulate realistic strains. In this article, we propose and evaluate a FE model of the human heel for diabetic foot ulcer prevention. Soft tissue constitutive laws are defined through the fitting of experimental stretch-stress curves published in the literature. The model is then evaluated through Digital Volume Correlation (DVC) based on non-rigid 3D Magnetic Resonance Image Registration. The results from FE analysis and DVC show similar strain locations in the fat pad and strain intensities according to the type of applied loads. For additional comparisons, different sets of constitutive models published in the literature are applied into the proposed FE mesh and simulated with the same boundary conditions. In this case, the results in terms of strains show great diversity in locations and intensities, suggesting that more research should be developed to gain insight into the mechanical properties of these tissues.

A review of foot finite element modelling for pressure ulcer prevention in bedrest: Current perspectives and future recommendations

Pressure ulcers (PUs) are a major public health challenge, having a significant impact on healthcare service and patient quality of life. Computational biomechanical modelling has enhanced PU research by facilitating the investigation of pressure responses in subcutaneous tissue and skeletal muscle. Extensive work has been undertaken on PUs on patients in the seated posture, but research into heel ulcers has been relatively neglected. The aim of this review was to address the key challenges that exist in developing an effective FE foot model for PU prevention and the confusion surrounding the wide range of outputs reported. Nine FE foot studies investigating heel ulcers in bedrest were identified and reviewed. Six studies modelled the posterior part of the heel, two included the calf and foot, and one modelled the whole body. Due to the complexity of the foot anatomy, all studies involved simplification or assumptions regarding parts of the foot structure, boundary conditions and material parameters. Simulations aimed to understand better the stresses and strains exhibited in the heel soft tissues of the healthy foot. The biomechanical properties of soft tissue derived from experimental measurements are critical for developing a realistic model and consequently guiding clinical decisions. Yet, little to no validation was reported in each of the studies. If FE models are to address future research questions and clinical applications, then sound verification and validation of these models is required to ensure accurate conclusions and prediction of patient outcomes. Recommendations and considerations for future FE studies are therefore proposed.

Acute Skin Failure in the Critical Care Patient

OBJECTIVE

The purpose of this research was to build on previous work regarding predictive factors of acute skin failure (ASF) in the critically ill population.

METHODS

Researchers conducted a retrospective case-control study with a main and validation analysis. Data were extracted from the New York Statewide Planning and Research Cooperative System. For the main analysis, there were 415 cases with a hospital-acquired pressure injury (HAPI) and 194,872 controls without. Researchers then randomly selected 100 cases with a HAPIs and 300 controls without for the validation analysis. A step-up logistic regression model was used. Researchers generated receiver operating characteristic curves for both the main and validation analyses, assessing the overall utility of the regression model.

RESULTS

Eleven variables were significantly and independently related to ASF: renal failure (odds ratio [OR], 1.4, P = .003), respiratory failure (OR, 2.2; P = < .001), arterial disease (OR, 2.4; P = .001), impaired nutrition (OR, 2.3; P = < .001), sepsis (OR, 2.2; P = < .001), septic shock (OR, 2.3; P = < .001), mechanical ventilation (OR, 2.5; P = < .001), vascular surgery (OR, 2.2; P = .02), orthopedic surgery (OR, 3.4; P = < .001), peripheral necrosis (OR, 2.5; P = .003), and general surgery (OR, 3.8; P = < .001). The areas under the curve for the main and validation analyses were 0.864 and 0.861, respectively.

CONCLUSIONS

The final model supports previous work and is consistent with the current definition of ASF in the setting of critical illness.

Prevention of pressure injury in the operating room: Heels operating room pressure injury trial

The objective was to evaluate the efficacy of multi‐layered silicone foam (intervention) compared with transparent polyurethane film (control) in preventing heel pressure injuries caused by surgical positioning of individuals undergoing elective surgery. It was designed an intra‐patient, open, parallel, randomised controlled trial was conducted in a university hospital in southern Brazil, from March 2019 to February 2020, with patients undergoing elective surgeries of cardiac and gastrointestinal specialties. The patients who met the selection criteria constituted, simultaneously, a single group receiving the intervention and active control, through paired analysis of the cutaneous sites (right heel and left heel). The outcome was the occurrence of PI, within the follow‐up period was 72 hours. Brazilian Registry of Clinical Trials: RBR‐5GKNG5. There was analysis of 135 patients/270 heels, with an overall incidence of 36.7%. The pressure injury incidence was significantly lower in the intervention group (26.7%), compared with the control group (P = .001); relative risk of 0.57. In the intervention group, the estimated pressure injury‐free time (survival) was 57.5 hours and in the control group, 43.9 hours. It was concluded that Multi‐layered silicone foam (intervention) is more efficacious than transparent polyurethane film (control) in the prevention of pressure injuries caused by surgical positioning of individuals undergoing elective surgery.

Heel Pressure Injuries in the Adult Critical Care Population

Patients in critical care units have a multitude of diseases and conditions that contribute to their illness and as such are susceptible to comorbid conditions such as heel pressure injuries. Prevention is a key strategy to avoid heel pressure injury occurrence. Risk factor identification can help a clinician identify those patients at risk for a heel pressure injury requiring timely prevention strategies. The purpose of this article is to raise awareness regarding the critical care patient's vulnerability to heel pressure injuries and strategies that can help avoid their occurrence or expedite their healing if occur.

Finite Element Analysis Modeling of a Novel Silicone Dressing

Introduction In the United States (US), pressure injuries are believed to affect over 2.5 million people. The prevalence of pressure ulcers in the European Union (EU) is believed to be 13.7%. Recent guidelines have recommended the consideration of polyurethane foam dressings as part of pressure injury prevention strategies. This study assesses the reduction in tissue strain and stresses associated with the use of a new silicone foam dressing.  Methods Finite element analysis (FEA) models were used to investigate the ability of silicone foam dressings to reduce tissue stress and strain energy density (SED) in the regions adjacent to the sacral bone. The loading modeled on the dressings was for combined compression and shear (modeling a patient lying in a 45° Fowler's position). Nine commercially available silicone foam dressings and a no-dressing control were modeled. Results FEA modeling showed that all silicone dressings tested, including Tegaderm™ Silicone Foam (TSF; 3M Health Care, St. Paul, MN) dressings, achieved reductions in tissue distortional stress and SED relative to no-dressing conditions. The use of silicone foam dressing results in a lower volume of tissue at higher stresses and deformation compared to no dressing. Conclusion The results presented indicate that TSF may provide an appropriate option for pressure ulcer prevention programs.

Risk factors for developing heel ulcers for bedridden patients: A finite element study

BACKGROUND

The heel is one of the most common sites of pressure ulcers and the anatomical location with the highest prevalence of deep tissue injury. Several finite element modeling studies investigate heel ulcers for bedridden patients. In the current study we have added the implementation of the calf structure to the current heel models. We tested the effect of foot posture, mattress stiffness, and a lateral calcaneus displacement to the contact pressure and internal maximum shear strain occurring at the heel.

METHODS

A new 3D finite element model is created which includes the heel and calf structure. Sensitivity analyses are performed for the foot orientation relative to the mattress, the Young's modulus of the mattress, and a lateral displacement of the calcaneus relative to the other soft tissues in the heel.

FINDINGS

The models predict that a stiffer mattress results in higher contact pressures and internal maximum shear strains at the heel as well as the calf. An abducted foot posture reduces the internal strains in the heel and a lateral calcaneus displacement increases the internal maximum shear strains. A parameter study with different mattress-skin friction coefficients showed that a coefficient below 0.4 decreases the maximum internal shear strains in all of the used loading conditions.

INTERPRETATION

In clinical practice, it is advised to avoid internal shearing of the calcaneus of patients, and it could be taken into consideration by medical experts and nurses that a more abducted foot position may reduce the strains in the heel.

Bone Position and Ligament Deformations of the Foot From CT Images to Quantify the Influence of Footwear in ex vivo Feet

The mechanical behavior of the foot is often studied through the movement of the segments composing it and not through the movement of each individual bone, preventing an accurate and unambiguous study of soft tissue strains and foot posture. In order to describe the internal behavior of the foot under static load, we present here an original methodology that automatically tracks bone positions and ligament deformations through a series of CT acquisitions for a foot under load. This methodology was evaluated in a limited clinical study based on three cadaveric feet in different static load cases, first performed with bare feet and then with a sports shoe to get first insights on how the shoe influences the foot's behavior in different configurations. A model-based tracking technique using hierarchical distance minimization was implemented to track the position of 28 foot bones for each subject, while a mesh-morphing technique mapped the ligaments from a generic model to the patient-specific model in order to obtain their deformations. Comparison of these measurements between the ex vivo loaded bare foot and the shod foot showed evidence that wearing a shoe affects the deformation of specific ligaments, has a significant impact on the relative movement of the bones and alters the posture of the foot skeleton (plantar-dorsal flexion, arch sagging, and forefoot abduction-adduction on the midfoot). The developed method may provide new clinical indicators to guide shoe design and valuable data for detailed foot model validation.

How patient migration in bed affects the sacral soft tissue loading and thereby the risk for a hospital-acquired pressure injury

Head‐of‐bed (HOB) elevation is a common clinical practice in hospitals causing the patient's body to slide down in bed because of gravity. This migration effect likely results in tissue shearing between the sacrum and the support surface, which increases the risk for pressure injuries. StayInPlace (HillRom Inc.) is a commercial migration‐reduction technology (MRT) incorporated in intensive care bedframes. Yet, the effects of migration‐reduction on tissue shear stresses during HOB elevation are unknown. We analysed relationships between migration and resulting sacral soft tissue stresses by combining motion analysis and three‐dimensional finite element modelling of the buttocks. Migration data were collected for 10 subjects, lying supine on two bedframe types with and without MRT, and at HOB elevations of 45°/65°. Migration data were used as displacement boundary conditions for the modelling to calculate tissue stress exposures. Migration values for the conventional bed were 1.75‐ and 1.6‐times greater than those for the migration‐reduction bed, for elevations of 45° and 65°, respectively (P < .001). The modelling showed that the farther the migration, the greater the tissue stress exposures. Internal stresses were 1.8‐fold greater than respective skin stresses. Our results, based on the novel integrated experimental‐computational method, point to clear biomechanical benefits in minimising migration using MRT.

Modifiable patient-related factors associated with pressure ulcers on the sacrum and heels: Secondary data analyses

AIM

To explore factors associated with the presence of category I-IV pressure ulcers on the sacrum and heels.

DESIGN

Cross-sectional, secondary data analysis using data collected from the Landelijke Prevalentiemeting Zorgproblemen (LPZ) project, a multicentre prevalence study including nursing home residents and community care clients (N = 4,842) in the Netherlands in 2017.

METHODS

A single binary logistic regression model was designed to identify factors associated with the presence of pressure ulcers. Additionally, a multiple binary logistic regression model including modifiable explanatory factors associated with the presence of pressure ulcers was designed.

RESULTS

Impaired mobility, friction and shear (evaluated using the Braden Scale) are significantly associated with the presence of both sacral and heel category I-IV pressure ulcers. Incontinence-associated dermatitis is significantly associated with category I-IV sacral pressure ulcers.

CONCLUSION

In pressure ulcer prevention, nursing interventions should focus on frequent repositioning and mobilization while avoiding exposure of the skin to friction and shear. The need to consider incontinence-associated dermatitis, incontinence and moisture as important factors in pressure ulcer risk assessment is confirmed.

IMPACT

Pressure ulcers occur when skin and tissues are deformed between bony prominences and the support surface in a sitting or lying position. They are the result of a complex interaction between direct causal factors and a wide range of indirect factors. Recognition of these factors influences risk assessment guidance and practice. Knowledge of skin-specific factors at the patient level, modifiable by nursing interventions, enables a better targeted and tailored preventive approach.

Why is the heel particularly vulnerable to pressure ulcers?

In this article, the vulnerability of the soft tissues of the heel to pressure ulcers (injuries) is explained from a biomechanical engineering perspective, and emerging technologies for protecting the heel, particularly low-friction garments, are reviewed. Sustained deformations in the soft tissue of the weight-bearing posterior heel cause progressive cell and tissue damage due to loss of homeostasis in the cells, as the cytoskeleton and plasma membranes of the affected cells lose integrity and functionality. This deformation damage onsets and evolves rapidly when there is no relief of the tissue distortion (e.g. in supine motionless lying). Hence, prevention should be timely and be applied across all patient populations that are at risk. In particular there is a need to protect tissues from the action of frictional forces that are shearing not only the skin but also the deep tissue structures of the heel. The internal anatomy and physiology of the posterior heel, the common hospital conditions (lying supine, head of the bed elevated) and medical conditions involving neuropathy and perfusion impairments may impose specific risk for heel (pressure) ulcers. There is growing evidence that low-friction-fabric garments may provide added benefits in preventing heel ulcers when used in addition to standard clinical and technology-supported pressure ulcer prevention strategies, as the low-friction fabric structures absorb frictional forces before these are able to considerably distort the susceptible heel tissues.

Investigation of the relationship between localized cumulative stress and plantar tissue stiffness in healthy individuals using the in-vivo indentation technique

This study was conducted to determine whether prolonged and repetitive exercise stiffens the plantar soft tissue. Healthy female subjects in their early 20s with a similar body mass index but different majors (13 engineers (controls) and 13 ballet dancers) were recruited. Tissue thickness was measured using ultrasound, while peak stress, stress distribution, and center of pressure were obtained Zebris^® pressure mat. Stiffness was evaluated using a custom-made tissue indentation system. F-test and independent sample T-test were used to determine significant differences between the two groups. No significance was found in the thickness of the second sub-metatarsal head (MTH) and heel between the two groups. In the second sub-MTH, the ballet group showed higher peak stress, loading rate, and stiffness than the control group. Conversely, in the heel region, all the results were higher for the control group. The results of this study quantify the impact of exercise on the stiffness of plantar soft tissue and confirm that even healthy individuals who do prolonged and repetitive exercise have stiffer plantar soft tissue.

Sonographic Assessment of Heel Pad Thickness in Patients With Poorly Controlled Diabetes

The aim was to compare the heel pad thickness (HPT) in diabetic patients with high biochemical parameters (fasting blood sugar [FBS], hemoglobin A1c [HbA1c], and lipid profile) with nondiabetic counterparts. A total of 438 subjects made up of 216 diabetics with high biochemical parameters (poorly controlled) and 222 apparently healthy subjects were recruited. The HPT, FBS level, HbA1c values and lipid profile, and duration of diabetes mellitus were assessed. Results showed that the mean HPT was 13.33 ± 1.29 mm in the control subjects and 16.79 ± 1.84 mm in diabetics. The HPT among diabetics differed significantly from the control group ( P < .05). The mean value of HbA1c in the control group was 5.4 ± 1.3 compared to diabetics with values of 8.53 ± 2.1. The values of HbA1c among diabetics were significantly higher than that of the control group ( P < .05). HPT had a significant linear relationship with HbA1c among the diabetic subjects ( r = 0.42, P < .05).

Phantom testing of the sensitivity and precision of a sub-epidermal moisture scanner

The majority of pressure ulcers (PUs) including deep tissue injuries (DTIs) are preventable, and even reversible if detected in their early phase. One of the greatest barriers in PU prevention is that clinicians traditionally depended on subjective and qualitative techniques, particularly routine visual skin assessments that would only document existing, macroscopic PUs/DTIs, rather than preventing them or detecting them at their microscopic phase. At the early phase of cell damage, when a forming PU is still microscopic, there is a local increase in extracellular fluid contents within affected tissues, which is called sub-epidermal moisture (SEM). This new understanding has led to an emerging technology, a SEM Scanner (BBI LLC, Bruin Biometrics) that has been designed to effectively examine the health status of tissues, by measuring local changes in the biophysical SEM marker. In the present work, the SEM Scanner was tested under controlled laboratory conditions to experimentally determine its sensitivity and precision in identifying small (1 mL) water content changes in phantoms of the human heel and skull/face, which simulated common PU development scenarios. In both phantom configurations, the locally increased water contents resulted in consistent, statistically significant elevated SEM readings, which confirms that the SEM Scanner is able to detect fluid content changes that are as small as 1 mL. In agreement with a simplified theoretical (mathematical) SEM model, which was also developed here, changes in water contents had a consistent trend of effect on SEM delta values, which increased with each 1 mL increment in intra-tissue-substitute water contents.

The biomechanical protective effects of a treatment dressing on the soft tissues surrounding a non-offloaded sacral pressure ulcer

Patients who are immobile endure prolonged bodyweight-related compressive, tensional and shear loads at their body-support contact areas that over time may lead to the onset of pressure ulcers (PUs). Approximately, one-third of the common sacral PUs are severe and classified as category 3 or 4. If a PU has occurred, off-loading is the basic, commonly accepted clinical intervention; however, in many situations, complete off-loading of sacral PUs is not possible. Minimising the exposure of wounds and their surroundings to elevated mechanical loads is crucial for healing. Accordingly, in the present study, we aimed to investigate the biomechanical effects of the structural and mechanical properties of different treatment dressings on stresses in soft tissues surrounding a non-offloaded sacral PU in a supine patient. Using a novel three-dimensional anatomically realistic finite element modelling framework, we have compared performances of three dressing designs: (a) The Mepilex Border Sacrum (MBS) multilayer anisotropic silicone foam dressing (Mölnlycke Health Care), (b) an isotropic stiff dressing, and (c) an isotropic flexible dressing. Using our newly developed protective efficacy index (PEI) and aggravation index (AI) for assessing prophylactic and treatment dressings, we identified the anisotropic stiffness feature of the MBS dressing as a key design element.

Effects of humidity on skin friction against medical textiles as related to prevention of pressure injuries

Sustained pressure, shear forces, and friction, as well as elevated humidity/moisture, are decisive physical factors in the development of pressure injuries (PIs). To date, further research is needed in order to understand the influence of humidity and moisture on the coefficient of friction (COF) of skin against different types of medical textiles. The aim of this work was to investigate the effects of moisture caused by sweat, urine, or saline on the resulting COF of skin against different textiles used in the medical setting in the context of PI prevention. For that purpose, we performed physical measurements of static COFs of porcine skin followed by finite element (FE) computational modelling in order to illustrate the effect of increased COF at the skin on the resulting strains and stresses deep within the soft tissues of the buttocks. The COF of dry skin obtained for the 3 textiles varied between 0.59 (adult diaper) and 0.91 (polyurethane dressing). In addition, the COF increased with the added moisture in all of the tested cases. The results of the FE simulations further showed that increased COF results in elevated strain energy density and shear strain values in the skin and deeper tissues and, hence, in an increased risk for PI development. We conclude that moisture may accelerate PI formation by increasing the COF between the skin and the medical textile, regardless of the type of the liquid that is present. Hence, reduction of the wetness/moisture between the skin and fabrics in patients at a high risk of developing PIs is a key measure in PI prevention.

Prevention of heel pressure ulcers among adult patients in orthopaedic wards: an evidence-based implementation project

BACKGROUND

Immobility and prolonged bed rest often lead to heel pressure ulcers in patients. A point prevalence audit undertaken in the orthopaedic wards of a Singapore tertiary hospital reported that 6 out of 30 patients who were audited had mild to blanching redness on their heels.

AIMS

The evidence-based project sought to achieve 80% compliance from nurses to perform heel off-loading practice and a 50% reduction in the occurrence of heel pressure ulcers.

METHODS

The project, lasting two years, was undertaken in two orthopaedic wards and utilized a pre- and post-implementation audit strategy using the Joanna Briggs Institute on-line 'Practical Application of Clinical Evidence System' and 'Getting Research into Practice' programs. Implementation occurred in four phases and involved a sample consisting of 30 adult patients.

RESULTS

Nurses' compliance with performing heel off-loading techniques increased. The post-implementation audit showed 93.3% compliance of nurses undertaking heel off-loading techniques in the subsequent four follow-up audits. Meanwhile, the compliance with documentation increased from 63.3% to 86.7%. The project resulted in more than 50% reduction in stage one heel pressure ulcers.

CONCLUSION

The implementation of heel off-loading techniques significantly reduced the incidences of heel pressure ulcers in orthopaedic wards.

Nonlinear Elasticity of the ECM Fibers Facilitates Efficient Intercellular Communication

Biological cells embedded in fibrous matrices have been observed to form intercellular bands of dense and aligned fibers through which they mechanically interact over long distances. Such matrix-mediated cellular interactions have been shown to regulate various biological processes. This study aimed to explore the effects of elastic nonlinearity of the fibers contained in the extracellular matrix (ECM) on the transmission of mechanical loads between contracting cells. Based on our biological experiments, we developed a finite-element model of two contracting cells embedded within a fibrous network. The individual fibers were modeled as showing linear elasticity, compression microbuckling, tension stiffening, or both of the latter two. Fiber compression buckling resulted in smaller loads in the ECM, which were primarily directed toward the neighboring cell. These loads decreased with increasing cell-to-cell distance; when cells were >9 cell diameters apart, no such intercellular interaction was observed. Tension stiffening further contributed to directing the loads toward the neighboring cell, though to a smaller extent. The contraction of two neighboring cells resulted in mutual attraction forces, which were considerably increased by tension stiffening and decayed with increasing cell-to-cell distances. Nonlinear elasticity contributed also to the onset of force polarity on the cell boundaries, manifested by larger contractile forces pointing toward the neighboring cell. The density and alignment of the fibers within the intercellular band were greater when fibers buckled under compression, with tension stiffening further contributing to this structural remodeling. Although previous studies have established the role of the ECM nonlinear mechanical behavior in increasing the range of force transmission, our model demonstrates the contribution of nonlinear elasticity of biological gels to directional and efficient mechanical signal transfer between distant cells, and rehighlights the importance of using fibrous gels in experimental settings for facilitating intercellular communication. VIDEO ABSTRACT.

Mitigating the damaging effects of tissue distortions by using a low-friction heel protector

This article reports the finding of a small non-controlled evaluation over a 2-week period in three different care settings: a residential care home, an acute stroke unit and a community intermediate care hospital. At initial recruitment 30 patients were identified by clinical assessment as being at high risk of developing a heel pressure ulcer. Further inclusion criteria were identifying heels that had signs of pressure damage occurring, blanching and non-blanching erythema, blistering and category 2 ulceration. In all, 15 patients fully completed the evaluation over a 14-day period. The mean age was 86 years. The low-friction bootee was worn constantly while in bed and seated in chairs, only being removed for heel checks and hygiene care. No patients were independently mobile during the evaluation; products were not worn to walk in due to a risk of falling-patients in the community hospital who had to mobilise for rehabilitation removed the bootees for this activity. All had pressure mapping and ultrasound of pedal pulses prior and after evaluation by the tissue viability specialist nurse. Results of pressure mapping showed a reduction of peak heel pressures on application of the bootees and a final review of reduction in visual signs of heel damage, reduced pain, increased comfort and ease of use. These results indicate that a standardised care pathway approach to heel protection using low-friction heel bootees is effective in all care settings for the reduction and prevention of heel pressure damage.

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

The sacral region is the most common site for pressure injuries (PIs) associated with lying in bed, and such sacral PIs often commence as deep tissue injuries (DTIs) that later present as open wounds. In complex patients, diabetes is common. Because, among other factors, diabetes affects connective tissue stiffness properties, making these tissues less able to dissipate mechanical loads through physiological deformations, diabetes is an additional biomechanical risk factor for PIs and DTIs. A preventive measure with established successful clinical outcomes is the use of sacral prophylactic dressings. The objective of this study has been to expand our previous work regarding the modes of action and biomechanical efficacy of prophylactic dressings in protecting the soft tissues adjacent to the sacrum by specifically examining the role of a directional stiffness preference (anisotropy) of the dressing while further accounting for diabetic tissue conditions. Multiple three-dimensional anatomically detailed finite element (FE) model variants representing diabetic tissue conditions were used, and tissue loading state data were compared with healthy tissue simulations. We specifically compared soft tissue exposures to elevated internal shear stresses and strain energy densities (SED) near the sacrum during supine weight bearing on a standard (foam) hospital mattress without a dressing, with a prophylactic dressing lacking directional stiffness preferences and with an anisotropic dressing. Our results have clearly shown that an anisotropic dressing design reduces the peak tissue stresses and exposure to sustained tissue deformations in both healthy and diabetic cases. The present study provides additional important insights regarding the optimal structural and material design of prophylactic dressings, which in turn, informs clinicians and decision makers regarding beneficial features.

NUMERICAL ANALYSIS OF THE FOOT IN HEALTHY AND DEGENERATIVE CONDITIONS

The aim of this work is the development of a 3D numerical model of the foot that allows evaluating the influence of degenerative phenomena on the foot mechanical functionality. Such degenerative phenomena induce histo-morphological alterations and significant modification of the plantar soft tissue mechanical properties, as stiffening and lower damping capabilities. The finite element model of the foot is developed starting from the analysis of biomedical images. Different constitutive models define the mechanical response of the biological tissues. Because of the major role of plantar soft tissue in the here proposed analysis, a specific visco-hyperelastic constitutive formulation is provided considering the typical features of the tissue mechanics, as geometric and material non linearity, almost incompressible behavior and time-dependent phenomena. Constitutive parameters are identified by the analysis of experimental data from in vitro and in vivo mechanical tests, leading to the identification of a range of constitutive parameters for healthy and degenerative conditions. Numerical analyses are developed to investigate the influence of the progression of the degeneration on the distribution of stress and of strain within foot tissues during static standing. Numerical results show the increase of stress values with the appearance of degenerative conditions, showing the typical stiffening phenomenon. The mechanical response of the plantar soft tissue during specific loading condition and the influence of degenerative phenomena on foot mechanics can be evaluated with numerical analysis.

Heel pressures with generic and focused rigid heel cast devices while in a static supine and seated position

Objective:

To identify if the choice of material used to make focused rigid cast (FRC) and generic heel cups has an effect on heel pressure in healthy individuals during a static supine and seated position.

Method:

A repeated measure design was used to compare the effect of two focused rigidity heel devices made from different materials (3M semi-rigid and Benecast FLEX) and a generic polymer gel heel cup device on barefoot heel pressures. Subjects had heel pressures taken while barefoot and with the three different heel devices while in a supine and seated position using the device.

Results:

We recruited 32 healthy participants (21 females, and 11 males). When comparing Benecast FLEX and 3M semi-rigid focused rigidity casts with barefoot and the generic heel cups significant reductions in pressures were seen in all areas while seated and in the distal area while supine. However, there was no statistical difference between the two FRC devices, or between barefoot and the generic heel cup in either position.

Conclusion:

This study demonstrates that FRC heel devices effectively reduced heel pressures in healthy individuals and therefore could be used in practice when a reduction in pressure is required for the management of heel pressure ulcers in bedbound or chair-bound patients.

Declaration of interest:

Materials were supplied from Benecare Medical (who agreed to supply 20 rolls of Benecast FLEX), and the podiatry department of Kent Community Health NHS Trust (who agreed to supply some of the materials).

Effects of different heel angles in sleep mode on heel interface pressure in the elderly

BACKGROUND

The heels are one of the most common sites of pressure ulcers, and the incidence rate in the elderly aged 70 years or older is high. Although there is literature on heel interface pressure, the heel interface pressure of the elderly in different postures has not yet been explored, which will be investigated in this study, as well as the effects of different foot positions. Their skin conditions will also be examined.

METHODS

Twenty-five females and twenty-six males, 70 years old or older, are evaluated while lying down, with only their naked foot in its natural position on a mattress, as well as placed on a standard or pressure-relieving mattress in different positions. The moisture, sebum content, and elasticity of the skin of the heel are tested.

FINDINGS

The heel of most of the participants is positioned at a 60°-69° or 90°-99° angle to the support surface. The heel interface pressure is the greatest when the foot is upright. The age, weight, and body mass index have no significant impacts. The moisture and sebum content are extremely low while elasticity is normal.

INTERPRETATION

The relaxed position of the foot is in neutral external rotation and upright positions. A greater amount of pressure is experienced when the foot is upright. The pressure-relieving mattress is more effective for reducing heel pressure but may not apply to all cases. Finally, the skin of the heel is dry and lacks sebum, which implies greater risk of developing heel sores.

Investigation of the mechanical behaviour of the plantar soft tissue during gait cycle: Experimental and numerical activities

The aim of this work is to investigate the mechanical response of the plantar soft tissue from the heel strike to the midstance, developing both experimental and numerical activities. Using force plates and motion tracking system, the dynamic and kinematic data of 10 subjects are evaluated. The average kinematics data obtained from the experimental tests are assumed as boundary and loading conditions for the computational analyses. A three-dimensional virtual solid model of the foot is developed from the analysis of Digital Imaging and Communications in Medicine images from computed tomography and magnetic resonance. Constitutive formulations that interpret the mechanical response of the biological tissues are defined. Because of the major role of plantar soft tissue in the proposed analysis, a specific visco-hyperelastic constitutive formulation is provided considering the typical features of the tissue mechanics. The three-dimensional numerical model permits to evaluate the capability of the plantar soft tissue to redistribute the deformations, especially during the midstance, and to define quantitative aspects related to the energy absorption. The numerical results highlight the stress distribution from the heel strike to the midstance. The values of stress and strain reached are more intensive during the midstance, when there is a single support of the foot.

The biomechanical efficacy of dressings in preventing heel ulcers

The heels are the most common site for facility-acquired pressure ulcers (PUs), and are also the most susceptible location for deep tissue injuries. The use of multilayer prophylactic dressings to prevent heel PUs is a relatively new prevention concept, generally aimed at minimizing the risk for heel ulcers (HUs) through mechanical cushioning and reduction of friction at the dressing-support interface. We used 9 finite element model variants of the posterior heel in order to evaluate the biomechanical performance of a multilayer dressing in prevention of HUs during supine lying. We compared volumetric exposures of the loaded soft tissues to effective and maximal shear strains, as well as peak stresses in the Achilles tendon, without any dressing and with a single-layer or a multilayer dressing (Mepilex(®) Border Heel-type), on supports with different stiffnesses. The use of the multilayer dressing consistently and considerably reduced soft tissue exposures to elevated strains at the posterior heel, on all of the tested support surfaces and when loaded with either pure compression or combined compression and shear. The aforementioned multilayer design showed (i) clear benefit over a single-layer dressing in terms of dissipating tissue strains, by promoting internal shear in the dressing which diverts loads from tissues; (ii) a protective effect that was consistent on supports with different stiffnesses. Recent randomized controlled trials confirmed the efficacy of the simulated multilayer dressing, and so, taken together with this modeling work, the use of a prophylactic multilayer dressing indicates a great promise in taking this route for prevention.

Using an alternating pressure mattress to offload heels in ICU

The heel continues to be one of the most common sites of pressure damage. This article reviews the anatomy and physiology of the heel and explores significant risk factors, including those found in the critically ill patient. Interventions to prevent heel pressure ulceration by offloading the heel are explored. An evaluation of the Nimbus 4 alternating pressure mattress was undertaken within an intensive care unit (ICU) to consider the efficacy of its unique Wound Valve Technology, which is designed to help prevent heel pressure ulceration. During the evaluation period none of the patients using the Nimbus 4 developed a pressure ulcer. Staff observed that the Wound Valves provided effective pressure redistribution and, although the cells frequently needed to be adjusted, patient safety was maintained throughout. The Wound Valves were most effective on patients who were less prone to voluntary movement.

Instrumento para avaliação da integridade tissular dos pés de portadores de diabetes melittus

OBJETIVO: Construir e validar uma escala de avaliação dos pés de portadores de diabetes mellitus, a partir dos indicadores da NOC para o resultado "Integridade Tissular: pele e mucosas". MÉTODOS: Utilizou-se da enfermagem baseada em evidências para realização deste estudo. A partir da questão: "Quais indicadores devem ser utilizados para a avaliação dos pés de pacientes com diabetes mellitus?" foi realizada uma busca em bases de dados e livros-textos e, posterioremente, a construção da escala e a avaliação por enfermeiros peritos. RESULTADOS: A versão final do instrumento consiste em 20 indicadores para avaliação das condições da pele e pêlos, circulação sanguínea, sensibilidade, temperatura e pressão plantar dos pés. CONCLUSÃO: O instrumento de avaliação proposto integridade tissular dos pés de portadores de diabetes melittus foi construído e validado por enfermeiros peritos com nível excelente de aceitação.

Investigations on the viscoelastic behaviour of a human healthy heel pad: In vivo compression tests and numerical analysis

The aim of this study was to investigate the viscoelastic behaviour of the human heel pad by comparing the stress–relaxation curves obtained from a compression device used on an in vivo heel pad with those obtained from a three-dimensional computer-based subject-specific heel pad model subjected to external compression. The three-dimensional model was based on the anatomy revealed by magnetic resonance imaging of a 31-year-old healthy female. The calcaneal fat pad tissue was described with a viscohyperelastic model, while a fibre-reinforced hyperelastic model was formulated for the skin. All numerical analyses were performed to interpret the mechanical response of heel tissues, with loading conditions and displacement rate in agreement with experimental tests. The heel tissues showed a non-linear, viscoelastic behaviour described by characteristic hysteretic curves, stress–relaxation and viscous recovery phenomena. The reliability of the investigations was validated by the interpretation of the mechanical response of heel tissues under the application of three pistons with diameter of 15, 20 and 40 mm, at the same displacement rate of about 1.7 mm/s. The maximum and minimum relative errors were found to be less than 0.95 and 0.064, respectively.

A three-dimensional inverse finite element analysis of the heel pad

Quantification of plantar tissue behavior of the heel pad is essential in developing computational models for predictive analysis of preventive treatment options such as footwear for patients with diabetes. Simulation based studies in the past have generally adopted heel pad properties from the literature, in return using heel-specific geometry with material properties of a different heel. In exceptional cases, patient-specific material characterization was performed with simplified two-dimensional models, without further evaluation of a heel-specific response under different loading conditions. The aim of this study was to conduct an inverse finite element analysis of the heel in order to calculate heel-specific material properties in situ. Multidimensional experimental data available from a previous cadaver study by Erdemir et al. ("An Elaborate Data Set Characterizing the Mechanical Response of the Foot," ASME J. Biomech. Eng., 131(9), pp. 094502) was used for model development, optimization, and evaluation of material properties. A specimen-specific three-dimensional finite element representation was developed. Heel pad material properties were determined using inverse finite element analysis by fitting the model behavior to the experimental data. Compression dominant loading, applied using a spherical indenter, was used for optimization of the material properties. The optimized material properties were evaluated through simulations representative of a combined loading scenario (compression and anterior-posterior shear) with a spherical indenter and also of a compression dominant loading applied using an elevated platform. Optimized heel pad material coefficients were 0.001084 MPa (μ), 9.780 (α) (with an effective Poisson's ratio (ν) of 0.475), for a first-order nearly incompressible Ogden material model. The model predicted structural response of the heel pad was in good agreement for both the optimization (<1.05% maximum tool force, 0.9% maximum tool displacement) and validation cases (6.5% maximum tool force, 15% maximum tool displacement). The inverse analysis successfully predicted the material properties for the given specimen-specific heel pad using the experimental data for the specimen. The modeling framework and results can be used for accurate predictions of the three-dimensional interaction of the heel pad with its surroundings.