Personalized modeling for real-time pressure ulcer prevention in sitting posture

, Ischial pressure ulcer is an important risk for every paraplegic person and a major public health issue. Pressure ulcers appear following excessive compression of buttock's soft tissues by bony structures, and particularly in ischial and sacral bones. Current prevention techniques are mainly based on daily skin inspection to spot red patches or injuries. Nevertheless, most pressure ulcers occur internally and are difficult to detect early. Estimating internal strains within soft tissues could help to evaluate the risk of pressure ulcer. A subject-specific biomechanical model could be used to assess internal strains from measured skin surface pressures. However, a realistic 3D non-linear Finite Element buttock model, with different layers of tissue materials for skin, fat and muscles, requires somewhere between minutes and hours to compute, therefore forbidding its use in a real-time daily prevention context. In this article, we propose to optimize these computations by using a reduced order modeling technique (ROM) based on proper orthogonal decompositions of the pressure and strain fields coupled with a machine learning method. ROM allows strains to be evaluated inside the model interactively (i.e. in less than a second) for any pressure field measured below the buttocks. In our case, with only 19 modes of variation of pressure patterns, an error divergence of one percent is observed compared to the full scale simulation for evaluating the strain field. This reduced model could therefore be the first step towards interactive pressure ulcer prevention in a daily setup. Highlights-Buttocks biomechanical modelling,-Reduced order model,-Daily pressure ulcer prevention

Finite element models of the thigh-buttock complex for assessing static sitting discomfort and pressure sore risk

Being seated for long periods, while part of many leisure or occupational activities, can lead to discomfort, pain and sometimes health issues. The impact of prolonged sitting on the body has been widely studied in the literature, with a large number of human-body finite element models developed to simulate sitting and assess seat-induced discomfort or to investigate the biomechanical factors involved. Here, we review the finite element models developed to investigate sitting discomfort or risk of pressure sores. Our study examines finite element models from twenty-seven papers, seventeen dedicated to assessing seating discomfort and ten dedicated to investigating pressure ulcers caused by prolonged sitting. The models' mesh composition and material properties are found to differ widely. These models share a lack of validation and generally make little allowance for anthropometric diversity

Predictive Dynamic Simulation of Healthy Sit-to-Stand Movement

This thesis situates itself at the intersection of biomedical modelling and predictive simulation to synthesize healthy human sit-to-stand movement. While the importance of sit-to-stand to physical and social well-being is known, the reasons for why and how people come to perform sit-to-stand the way we do is largely unknown. This thesis establishes the determinants of sit-to-stand in healthy people so that future researchers may investigate the effects of compromised health on sit-to-stand and then explore means of intervening to preserve and restore this motion. Previous researchers have predicted how a person rises from seated. However aspects of their models, most commonly contact and muscle models, are biomechanically inconsistent and restrict their application. These researchers also have not validated their prediction results. To address these limitations and further the study of sit-to-stand prediction, the underlying themes of this thesis are in biomechanical modelling, predictive simulation, and validation. The goal of predicting sit-to-stand inspired the creation of three new models: a model of biomechanics, a model of motion, and performance criteria as a model of preference. First, the human is represented as three rigid links in the sagittal plane. As buttocks are kinetically important to sit-to-stand, a new constitutive model of buttocks is made from experimental force-deformation data. Ten muscles responsible for flexion and extension of the hips, knees, and ankles are defined in the model. Second, candidate sit-to-stand trajectories are described geometrically by a set of Bézier curves, for the first time. Third, with the assumption that healthy people naturally prioritize mechanical efficiency, disinclination to a motion is described as a cost function of joint torques, muscle stresses, and physical infeasibility including slipping and falling. This new dynamic optimization routine allows for motions of gradually increasing complexity, by adding control points to the Bézier curves, while the model's performance is improving. By comparing the predictive simulation results to normative sit-to-stand as described in the literature, for the first time, it is possible to say that the use of these models and optimal control strategy together has produced motions characteristic of healthy sit-to-stand. This work bridges the gap between predictive simulation results and experimental human results and in doing so establishes a benchmark in sit-to-stand prediction. In predicting healthy sit-to-stand, it makes a necessary step toward predicting pathological sit-to-stand, and then to predicting the results of intervention to inform medical design and planning

Personnalisation des propriétés mécaniques des tissus mous du fessier humain par méthodes d'éléments finis et expérimentations In Vivo.

RÉSUMÉ Une escarre de pression est une dégénérescence des tissus mous qui représente un problème courant chez les usagers de fauteuil roulant et les patients immobilisés au lit. Elle résulte d’une compression prolongée des tissus mous, ce qui entraine l’interruption du flux sanguin et ainsi, une lacune en oxygène. Le fessier est l’une des zones les plus affectées par les escarres de pression, plus précisément les régions sous les protubérances osseuses telles que les tubérosités ischiatiques, les grands trochanters, le sacrum et le coccyx. Une des méthodes les plus prometteuses pour l’étude de la formation des escarres et de l’évaluation des coussins d’appui repose sur l’observation des contraintes et des déformations internes du fessier à l’aide de modèles par éléments finis. Malheureusement, ces modèles utilisent des propriétés mécaniques provenant d’expérimentations in vitro sur des animaux, souvent éloignées des propriétés du sujet modélisé, et sont validés uniquement par mesures expérimentales des pressions à l’interface entre le fessier et son support. Ces lacunes engendrent des imprécisions au niveau des contraintes et déformations internes, et limitent l’exploitation des modèles comme outil de recherche et de prévention clinique des escarres. Le présent projet visait à personnaliser les propriétés mécaniques d’un modèle par éléments finis du fessier à partir de mesures expérimentales de l’écrasement interne des tissus mous, et à vérifier l’intérêt de cette démarche sur la distribution des contraintes et des déformations internes lorsqu’appuyé sur une surface rigide et sur un coussin. Le premier objectif de ce projet était de développer un modèle biomécanique par éléments finis du fessier incorporant une loi de comportement hyperélastique et des propriétés tirées de la littérature. Pour ce faire, des images par résonnance magnétique (IRM) du fessier non-déformé (IRM #1) et déformé sur une surface rigide (IRM #2) d’un sujet sain en décubitus dorsal ont été acquises expérimentalement. Ces données ont permis, d’une part, de reconstruire en 2D la géométrie du fessier avant l’application du poids du corps (sur une des images IRM #1) et d’autre part, de mesurer les écrasements des tissus musculaires et adipeux en comparant les images IRM #1 et #2 correspondantes. Suite à l’acquisition des images IRM, des valeurs de pressions surfaciques ont été acquises expérimentalement sur le fessier déformé et ont été associées aux écrasements tissulaires mesurés sur les images IRM. Ces mesures d’écrasement tissulaire et de----------ABSTRACT For wheelchair users and bedrest patient, mechanisms of soft tissues deterioration such as pressure sores represent a severe health problem. Pressure sores are primarily caused by a long-term loading that causes a blood circulation interruption, thus resulting in a lack of oxygen for the soft tissues. The buttock represents the most affected regions by pressure sores, more precisely beneath bony protuberances such as the ischial tuberosities, the great trochanters, the sacrum and the coccyx. One of the most promising techniques to study the aetiology of pressure sores and to prescribe adequate seat cushion (or bed mattress) relies on the analysis of internal stresses and strains using finite element (FE) modeling. Unfortunately, FE models currently available use mechanical properties measured from in vitro experiments on animal and consequently, do not accurately represent the mechanical behaviour of a human buttocks. Moreover, these models are only validated using interface pressures between the buttock and the support surface. These limitations result in inadequate stresses and strains distribution inside the soft tissues, thus limiting the capacity to exploit the FE models as research and prevention tools for pressure sores. The purpose of this master degree project was to personalize the mechanical properties of the diverse soft tissue layers and to evaluate the necessity of the approach when the buttock lies on a rigid support and a foam cushion. The first objective of the project was to develop a biomechanical FE model of the human buttock that integrates hyperelastic material formulations for the soft tissues with material properties taken from the literature. A magnetic resonance imaging protocol was realized to acquire geometric data of a healthy human male buttock in non-weight-bearing (MRI #1) and weight-bearing (MRI #2) conditions. An axial slice from MRI #1 was then used to reconstruct the 2D contours of the pelvis and buttock soft tissues while vertical sagging of the muscle and adipose tissues were measured by comparing correspondent slices from MRI #1 and #2. After the MRI acquisition, interfaces pressures were acquired experimentally in weight-bearing condition and were associated to the corresponding vertical sagging. These vertical sagging and interface pressure were necessary to realize the second objective, the mechanical properties personalization. After the experimental acquisition, a detailed FE model of the buttock was realized by creating a 5 mm extrusion of the 2D reconstructed profile of the buttock and b

Lateral pressure equalisation as a principle for designing support surfaces to prevent deep tissue pressure ulcers

When immobile or neuropathic patients are supported by beds or chairs, their soft tissues undergo deformations that can cause pressure ulcers. Current support surfaces that redistribute under-body pressures at vulnerable body sites have not succeeded in reducing pressure ulcer prevalence. Here we show that adding a supporting lateral pressure can counter-act the deformations induced by under-body pressure, and that this ‘pressure equalisation’ approach is a more effective way to reduce ulcer-inducing deformations than current approaches based on redistributing under-body pressure. A finite element model of the seated pelvis predicts that applying a lateral pressure to the soft tissue reduces peak von Mises stress in the deep tissue by a factor of 2.4 relative to a standard cushion (from 113 kPa to 47 kPA) — a greater effect than that achieved by using a more conformable cushion, which reduced von Mises stress to 75 kPa. Combining both a conformable cushion and lateral pressure reduced peak von Mises stresses to 25 kPa. The ratio of peak lateral pressure to peak under-body pressure was shown to regulate deep tissue stress better than under-body pressure alone. By optimising the magnitude and position of lateral pressure, tissue deformations can be reduced to that induced when suspended in a fluid. Our results explain the lack of efficacy in current support surfaces and suggest a new approach to designing and evaluating support surfaces: ensuring sufficient lateral pressure is applied to counter-act under-body pressure

High volume ergonomic simulation of chairs

To understand what makes a chair comfortable or practical we need to test a large number of chairs, both good and bad. Due to the numbers involved we cannot achieve this with physical testing. Instead we use simpli ed ergonomic simulations. The sim- ulations presented here produce pressure maps within the range given the literature, along with several other measures of comfort and practicality. This was done sub- stantially faster than examples in the literature, permitting collection of thousands of results

Application of the Computational Design Synthesis framework for individualized car seats

The manufacturing capabilities of additive manufacturing allow great design freedom for mass customization of different products. This new solution space needs to be explored and served by engineers when designing individual variants of a product. Therefore, the methods of model generation for the individual variant with individual customer specific requirements must be improved to take advantage of this design freedom. This paper discusses the specific challenges of designing a customized car seat by showing its general process chain and the challenges associated with the design of foam replacement structures that offer the possibility to customize the stiffness of the cushion. A possible framework for the underlying digital process chain is then discussed. This framework manages model synthesis according to anthropometric data and safety requirements as conflicting requirements within various complex engineering correlations. In a case study, the chosen Computational Design Synthesis (CDS) framework is applied to the problem of designing an individualized car seat. Detailed descriptions of the concept for each block in the process chain are presented within the case study. The paper and conclusion discuss whether the framework meets the challenges of the application example and further steps for the project

COMBINED MUSCULO-SKELETAL MULTI-BODY DYNAMICS/FINITE ELEMENT ANALYSIS OF SEVERAL ERGONOMICS AND BIO-MECHANICS PROBLEMS

Within this thesis, two ergonomics (i.e. seating comfort and long-distance driving fatigue problems) and two structural bio-mechanics (i.e. femur-fracture fixation and radius-fracture fixation) problems are investigated using musculo-skeletal multi-body dynamics and finite element computational analyses. Within the seating comfort problem analyzed, a complete-body finite element model is constructed and used to assess the effect of seat geometry and seating posture on the feel of comfort experienced by a seated human. Within the long-distance driving fatigue problem, musculo-skeletal analysis is employed to assess the extent of fatigue experienced by a driver through the evaluation of level of activity of his/her various muscles. Within the femur-fracture fixation problem, physiologically realistic loading conditions associated with active daily activities (i.e. cycling) are employed within a finite-element frame work to assess fracture fixation performance and durability of the implant. Within the radius-fracture fixation problem, the analysis developed within the femur-fracture fixation problem is further related to indicate the effects of other types of loadings (associated with additional daily activities) and improved biological and structural material model are employed. For all cases studied in the present work, relevant experimental data are used to validate the computational procedure employed