Finite element modeling for predicting the contact pressure between a foam mattress and the human body in a supine position

The biomechanical efficacy of a hybrid support surface in protecting supine patients from sacral pressure ulcers

Support surfaces are the most important pressure ulcer/injury prevention technology available to clinicians for protecting their at-risk patients. A hybrid support surface marries the benefits of reactive and active support surfaces, by using high-quality foam material inside inflatable air cells. When used in its "static mode", it is a constant low air pressure mattress which delivers pressure redistribution in response to patient bodyweight and movements, by maximising the immersion and envelopment performance of the support surface. When used in its powered "dynamic mode", this system further delivers alternating pressure care via the connected foam and air cells. Modes of action of hybrid support surfaces were never studied quantitatively before, excluding through the limited scope of interface pressure mapping. In this work, we developed a novel computational modelling framework and simulations to visualise and quantify the state of soft tissue loading at the buttocks of a supine patient positioned on a hybrid support surface, in both the static and dynamic modes. We found that the dynamic mode effectively shifts deep concentrated soft tissue loading from under the sacral bone (towards the sacral promontory) to the tip of the sacrum (coccyx) and vice versa, and thereby, generates a deep tissue offloading effect.

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.

The Influence of Mattress Stiffness on Spinal Curvature and Intervertebral Disc Stress-An Experimental and Computational Study

Sleeping support systems can influence spinal curvature, and the misalignment of the spinal curvature can lead to musculoskeletal problems. Previous sleep studies on craniocervical support focused on pillow variants, but the mattress supporting the pillow has rarely been considered. This study used a cervical pillow and three mattresses of different stiffnesses, namely soft, medium, and hard, with an indentation load deflection of 20, 42, and 120 lbs, respectively. A novel electronic curvature measurement device was adopted to measure the spinal curvature, whereby the intervertebral disc loading was computed using the finite element method. Compared with the medium mattress, the head distance increased by 30.5 ± 15.9 mm, the cervical lordosis distance increased by 26.7 ± 14.9 mm, and intervertebral disc peak loading increased by 49% in the soft mattress environment. Considering that the pillow support may increase when using a soft mattress, a softer or thinner pillow is recommended. The head distance and cervical lordosis distance in the hard mattress environment were close to the medium mattress, but the lumbar lordosis distance reduced by 10.6 ± 6.8 mm. However, no significant increase in intervertebral disc loading was observed, but contact pressure increased significantly, which could cause discomfort and health problems.

Recurrent neural network to predict hyperelastic constitutive behaviors of the skeletal muscle

Hyperelastic constitutive laws have been commonly used to model the passive behavior of the human skeletal muscle. Despite many efforts, the use of accurate finite element formulations of hyperelastic constitutive laws is still time-consuming for a real-time medical simulation system. The objective of the present study was to develop a deep learning model to predict the hyperelastic constitutive behaviors of the skeletal muscle toward a fast estimation of the muscle tissue stress.A finite element (FE) model of the right psoas muscle was developed. Neo-Hookean and Mooney-Rivlin laws were used. A tensile test was performed with an applied body force. A learning database was built from this model using an automatic probabilistic generation process. A long-short term memory (LSTM) neural network was implemented to predict the stress evolution of the skeletal muscle tissue. A hyperparameter tuning process was conducted. Root mean square error (RMSE) and associated relative error was quantified to evaluate the precision of the predictive capacity of the developed deep learning model. Pearson correlation coefficients (R) was also computed.The nodal displacements and the maximal stresses range from 70 to 227 mm and from 2.79 to 5.61 MPa for Neo-Hookean and Monney-Rivlin laws, respectively. Regarding the LSTM predictions, the RMSE ranges from 224.3 ± 3.9 Pa (8%) to 227.5 [Formula: see text] 5.7 Pa (4%) for Neo-Hookean and Monney-Rivlin laws, respectively. Pearson correlation coefficients (R) of 0.78 [Formula: see text] 0.02 and 0.77 [Formula: see text] 0.02 were obtained for Neo-Hookean and Monney-Rivlin laws, respectively.The present study showed that, for the first time, the use of a deep learning model can reproduce the time-series behaviors of the complex FE formulations for skeletal muscle modeling. In particular, the use of a LSTM neural network leads to a fast and accurate surrogate model for the in silico prediction of the hyperelastic constitutive behaviors of the skeletal muscle. As perspectives, the developed deep learning model will be integrated into a real-time medical simulation of the skeletal muscle for prosthetic socket design and childbirth simulator.

The biomechanical efficacy of a dressing with a soft cellulose fluff core in protecting prone surgical patients from chest injuries on the operating table

Pressure ulcers are soft‐tissue damage associated with tissue exposure to sustained deformations and stress concentrations. In patients who are proned for ventilation or surgery, such damage may occur in the superficial chest tissues that are compressed between the rib cage and the support surface. Prophylactic dressings have been previously proven as generally effective for pressure ulcer prevention. In this study, our goal was to develop a novel computational modelling framework to investigate the biomechanical efficacy of a dressing with a soft cellulose fluff core in protecting proned surgical patients from chest pressure ulcers occurring on the operating table, due to body fixation by the Relton‐Hall frame. We compared the levels of mechanical compressive stresses developing in the soft chest tissues, above the sternum and ribs, due to the trunk weight, whilst the body is supported by the Relton‐Hall frame pads, with versus without the prophylactically applied bilateral dressings. The protective efficacy index for the extremely high stresses, above the 95th‐percentile, were 40.5%, 25.6% and 24.2% for skin, adipose and muscle, respectively, indicating that the dressings dispersed elevated soft‐tissue stresses. The current results provide additional support for using soft cellulose fluff core dressings for pressure ulcer prophylaxis, including during surgery.

Société de Biomécanique Young Investigator Award 2021: Numerical investigation of the time-dependent stress-strain mechanical behaviour of skeletal muscle tissue in the context of pressure ulcer prevention

BACKGROUND

Pressure-induced tissue strain is one major pathway for Pressure Ulcer development and, especially, Deep Tissue Injury. Biomechanical investigation of the time-dependent stress-strain mechanical behaviour of skeletal muscle tissue is therefore essential. In the literature, a viscoelastic formulation is generally assumed for the experimental characterization of skeletal muscles, with the limitation that the underlying physical mechanisms that give rise to the time dependent stress-strain behaviour are not known. The objective of this study is to explore the capability of poroelasticity to reproduce the apparent viscoelastic behaviour of passive muscle tissue under confined compression.

METHODS

Experimental stress-relaxation response of 31 cylindrical porcine samples tested under fast and slow confined compression by Vaidya and collaborators were used. An axisymmetric Finite Element model was developed in ABAQUS and, for each sample a one-to-one inverse analysis was performed to calibrate the specimen-specific constitutive parameters, namely, the drained Young's modulus, the void ratio, hydraulic permeability, the Poisson's ratio, the solid grain's and fluid's bulk moduli.

FINDINGS

The peak stress and consolidation were recovered for most of the samples (N=25) by the poroelastic model (normalised root-mean-square error ≤0.03 for fast and slow confined compression conditions).

INTERPRETATION

The strength of the proposed model is its fewer number of variables (N=6 for the proposed poroelastic model versus N=18 for the viscohyperelastic model proposed by Vaidya and collaborators). The incorporation of poroelasticity to clinical models of Pessure Ulcer formation could lead to more precise and mechanistic explorations of soft tissue injury risk factors.

Computational studies of the biomechanical efficacy of a minimum tissue deformation mattress in protecting from sacral pressure ulcers in a supine position

Sustained soft tissue exposure to localised deformations is a trigger for the formation of pressure ulcers. Immersion and envelopment are critical benchmarks that determine comfort and the pressure ulcer risk mitigation, as they have considerable influence on tissue stress concentrations near bony prominences. In the present study, we developed a computer modelling framework for quantifying the extent by which optimal envelopment disperses tissue stress concentrations near the sacrum. To compare the risk of developing a sacral pressure ulcer while lying supine on a regular foam mattress with respect to lying on a specialised, minimum tissue deformation mattress (which closely conforms to the body contours), we used a three‐dimensional anatomically‐realistic model of the adult female buttocks. The strains and stresses in the subdermal soft tissues reached peak values of 65% and 2.4 kPa for the regular mattress, respectively, but always remained below 45% and 1.2 kPa for the minimum tissue deformation mattress, which indicates longer safe times for supine support on the latter mattress. Our work demonstrates that alleviation of localised, sustained stress concentrations through good immersion and envelopment of the support surface protects from pressure ulcers, and has the potential to relieve chronic pain which is associated with the pressure ulcer risk.

Proposal and validation of polyconvex strain-energy function for biological soft tissues

BACKGROUND

Mechanical simulations for biological tissues are effective technology for development of medical equipment, because it can be used to evaluate mechanical influences on the tissues. For such simulations, mechanical properties of biological tissues are required. For most biological soft tissues, stress tends to increase monotonically as strain increases.

OBJECTIVE

Proposal of a strain-energy function that can guarantee monotonically increasing trend of biological soft tissue stress-strain relationships and applicability confirmation of the proposed function for biological soft tissues.

METHOD

Based on convexity of invariants, a polyconvex strain-energy function that can reproduce monotonically increasing trend was derived. In addition, to confirm its applicability, curve-fitting of the function to stress-strain relationships of several biological soft tissues was performed.

RESULTS

A function depending on the first invariant alone was derived. The derived function does not provide such inappropriate negative stress in the tensile region provided by several conventional strain-energy functions.

CONCLUSIONS

The derived function can reproduce the monotonically increasing trend and is proposed as an appropriate function for biological soft tissues. In addition, as is well-known for functions depending the first invariant alone, uniaxial-compression and equibiaxial-tension of several biological soft tissues can be approximated by curve-fitting to uniaxial-tension alone using the proposed function.

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.

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.

A Systematic Review of Continuum Modeling of Skeletal Muscles: Current Trends, Limitations, and Recommendations

Finite elasticity theory has been commonly used to model skeletal muscle. A very large range of heterogeneous constitutive laws has been proposed. In this review, the most widely used continuum models of skeletal muscles were synthetized and discussed. Trends and limitations of these laws were highlighted to propose new recommendations for future researches. A systematic review process was performed using two reliable search engines as PubMed and ScienceDirect. 40 representative studies (13 passive muscle materials and 27 active muscle materials) were included into this review. Note that exclusion criteria include tendon models, analytical models, 1D geometrical models, supplement papers, and indexed conference papers. Trends of current skeletal muscle modeling relate to 3D accurate muscle representation, parameter identification in passive muscle modeling, and the integration of coupled biophysical phenomena. Parameter identification for active materials, assumed fiber distribution, data assumption, and model validation are current drawbacks. New recommendations deal with the incorporation of multimodal data derived from medical imaging, the integration of more biophysical phenomena, and model reproducibility. Accounting for data uncertainty in skeletal muscle modeling will be also a challenging issue. This review provides, for the first time, a holistic view of current continuum models of skeletal muscles to identify potential gaps of current models according to the physiology of skeletal muscle. This opens new avenues for improving skeletal muscle modeling in the framework of in silico medicine.

Numerical study to evaluate the effect of a surface-based sensor on arterial tonometry

Arterial tonometry is a widely used non-invasive blood pressure measurement method. In contrast to the cuff-based method, it is possible to obtain a continuous pressure profile with respect to systolic and diastolic pressures using this method. However, due to a requirement of arterial tonometry-that a sensor needs to be placed directly above a blood vessel-placement error is inevitable if the measurement device is only capable of measuring local regions. This study assumed that the plate sensor is flexible, thus reducing the placement error. We investigated the pressure distribution along the wrist surface rather than the local region through the contact simulation between the flexible plate sensor and the wrist. As a result, we concluded that there is a unique pressure distribution for any specific wrist, regardless of the length and position of the plate, and that it is possible to measure the blood pressure using the response at the wrist surface to the pressure inside the radial artery.

Technologies to monitor the health of loaded skin tissues

There are many situations where the skin and underlying soft tissues are compromised by mechanical loading in the form or pressure, or pressure in combination with shear. If sustained, this can lead to damage in the tissues particularly adjacent to bony prominences, resulting in chronic wounds. An array of bioengineering technologies have been adopted to assess the integrity of loaded soft tissues. This paper aims to review these approaches for the quantification, simulation and early detection of mechanically-induced skin damage. The review considers different measurements at the interface between the skin and support surface/medical device, involving pressure, shear, friction and the local microclimate. The potential of the techniques to monitor the physiological response of the skin to these external stimuli including biophysical measurement devices and sampling of biofluids are critically analysed. In addition, it includes an analysis of medical imaging technologies and computational modelling to provide a means by which tissue deformation can be quantified and thresholds for tissue damage defined. Bioengineering measurement and imaging technologies have provided an insight into the temporal status of loaded skin. Despite the advances in technology, to date, the translation to clinical tools which are robust and cost effective has been limited. There is a need to adapt existing technologies and simulation platforms to enable patients, carers and clinicians to employ appropriate intervention strategies to minimise soft tissue damage.