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) modelling. 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

Finite element analysis of the stump-ischial containment socket interaction: a technical note

The role of the above-knee socket is to ensure the load transfer via the coupling of residual limb-prosthesis with minimal discomfort and without damaging the soft tissues. Modelling is a potential tool to predict socket fit prior to manufacture. However, state-of-the-art models only include the femur in soft tissues submitted to static loads neglecting the contribution of the hip joint. The hip joint is particularly challenging to model because it requires to compute the forces of muscles inserting on the residual limb. This work proposes a modelling of the hip joint including the estimation of muscular forces using a combined MusculoSKeletal (MSK)/Finite Element (FE) framework. An experimental-numerical approach was conducted on one femoral amputee subject. This allowed to i) model the hip joint and personalize muscles forces, ii) study the impact of the ischial support, and iii) evaluate the interface pressure. A reduction of the gluteus medius force from the MSK modelling was noticed when considering the ischial support. Interface pressure, predicted between 63 to 71 kPa, agreed with experimental literature data. The contribution of the hip joint is a key element of the modelling approach for the prediction of the socket interface pressure with the residual limb soft tissues

Pressure Ulcers and Dressings: A Strain Sensitivity Analysis of the Boundary Conditions of a Finite Element Model

Recently, a new bi-layer dressing was proposed by Urgo RID to reduce the healing time of pressure ulcers (PU). This dressing was numerically evaluated in previously published work. In the current work, the influence on the maximal shear strains of modelling parameters such as the dressing local geometry, the pressure applied by the gauze inside the wound, the wound deepness, and the mattress stiffness, was assessed. A sensitivity analysis was performed on these four parameters. Among all experiments, the mean maximal Green--Lagrange shear strain was 0.29. The gauze pressure explained 60% of the model response in terms of the volume of tissues under strains of 0.3, while the wound deepness explained 28%. The mattress had a significant, but low impact, whereas the dressing local geometry had no significant impact. As expected, the wound deepness was one of the most influential parameters. The gauze turned out to be more significant than expected. This may be explained by the large range of values chosen for this study. The results should be extended to more subjects, but still suggest that the gauze is a parameter that might not be neglected. Care should also be taken in clinical practice when using gauze that could have either a positive or negative impact on the soft tissues' strains. This may also depend on the wound deepness

In vivo identification of the mechanical properties of thigh tissues from FreeHand Ultrasound for the numerical investigation of loads at the socket/residual limb interface of amputee people

Prosthetic sockets are custom-designed and are decisive for functionality and comfort of limb prosthesis. To ensure load transmission and stability, high interface stresses are applied. Several computer models of the socket/residual limb assembly have been designed to identify critical stress accumulations that may induce discomfort or trauma. However, clinically relevant personalized Finite Element (FE) models represent a bottleneck. While material definition of tissues is critical for the evaluation of socket/tissues interactions, personalization of deep Soft Tissue (ST) material properties remains challenging. For clinical purpose, it has to be simple, in vivo and thus non-invasive. This work explores the feasibility of freehand ultrasound with an inverse method for assessing mechanical properties of the ST of the thigh. To be relevant for socket design, the residual strain from donning socket was simulated by compressing tissues (5% of initial circumference)

Femoral residuum/socket kinematics using fusion between 3D motion capture and stereo radiography

Measurement of femur motion relative to the socket in gait in TF amputated patients can be a good indicator of prosthesis outcome. Our model, using low dose bi- planar radiography and motion capture, gives a prediction of femur motion (AA, EF, IER) in line with the sparse results found in the literature

Subject Specific Finite Element Mesh Generation of the Pelvis from Biplanar X-ray Images: Application to 120 clinical cases

Several Finite Element (FE) models of the pelvis have been developed to comprehensively assess the onset of pathologies and for clinical and industrial applications. However, because of the difficulties associated with the creation of subject-specific FE mesh from CT scan and MR images, most of the existing models rely on the data of one given individual. Moreover, although several fast and robust methods have been developed for automatically generating tetrahedral meshes of arbitrary geometries, hexahedral meshes are still preferred today because of their distinct advantages but their generation remains an open challenge. Recently, approaches have been proposed for fast 3D reconstruction of bones based on X-ray imaging. In this study, we adapted such an approach for the fast and automatic generation of all-hexahedral subject-specific FE models of the pelvis based on the elastic registration of a generic mesh to the subject-specific target in conjunction with element regularity and quality correction. The technique was successfully tested on a database of 120 3D reconstructions of pelvises from biplanar X-ray images. For each patient, a full hexahedral subject-specific FE mesh was generated with an accurate surface representation

Prediction of muscle forces in residual limb during walking: comparison of transfemoral and Gritti–Stokes amputations

Evaluation of muscle forces is relevant to understand walking strategies of amputated subjects. Such results could be implemented in finite element modelling to study the interaction between the residual limb and the socket by adding muscles to the model to improve pressure prediction at the interface. Further studies are needed to accurately estimate the percentage of vertical loads supported by the ischial support and to evaluate the results with, for example, EMG data

Combining Freehand Ultrasound-Based Indentation and Inverse Finite Element Modelling for the Identification of Hyperelastic Material Properties of Thigh Soft Tissues

Finite Element Analysis (FEA) is a numerical modelling tool vastly employed in research facilities to analyse and predict load transmission between the human body and a medical device, such as a prosthesis or an exoskeleton. Yet, the use of Finite Element Modelling (FEM) in a framework compatible with clinical constraints is hindered by, amongst others, heavy and time-consuming assessments of material properties. Ultrasound imaging opens new and unique opportunities for the assessment of in vivo material properties of soft tissues. Confident of these advances, a method combining a freehand ultrasound probe and a force sensor was developed in order to compute the hyperelastic constitutive parameters of the soft tissues of the thigh in both relaxed (R) and contracted (C) muscles configurations. Seven asymptomatic subjects were included for the experiment. Two operators in each configuration performed the acquisitions. Inverse FEM allowed for the optimisation of an Ogden’s hyperelastic constitutive model of soft tissues of the thigh in large displacement. The mean shear modulus identified for configurations R and C were respectively 3.2 ± 1.3 kPa and 13.7 ± 6.5 kPa. The mean alpha parameter identified for configurations R and C were respectively 10 ± 1 and 9 ± 4. An analysis of variance showed that the configuration had an effect on constitutive parameters but not the operator.Proteor ParisTech BiomecAM chair progra

Contribution à la modélisation du membre résiduel des personnes amputées de membre inférieur pour la conception personnalisée de l’emboiture fémorale

The socket connects the prosthesis to the residual limb of the amputee. For transfemoral amputations, it must allow the transfer of mechanical actions while being conformable. However, its design remains mainly qualitative. The pressures at the socket/residual limb interface can be evaluated using the Finite Element (FE) method. However, the design of such models is a challenge for both the reconstruction of geometries, the characterization of tissue behavior and the definition of loadings. This work presents a personalized FE model of the interaction of the residual limb and the socket. Measurements for the characterization of the tissue behavior but also for the definition of loadings, using a musculoskeletal model, supply the EF model. This model was compared with the literature and then used to estimate the influence of certain parameters on the pressures.L’emboîture fait le lien entre la prothèse et le membre résiduel des personnes amputées. Pour les amputations transfémorales, elle doit permettre le transfert des actions mécaniques tout en étant confortable. Cependant, sa conception reste principalement qualitative. Les pressions à l’interface emboîture/membre résiduel peuvent cependant permettre d’évaluer les emboîtures grâce à la méthode des éléments finis (EF) Toutefois la conception de tels modèles est un défi tant pour la reconstruction des géométries, la caractérisation du comportement des tissus ou la définition du chargement. Ce travail présente un modèle EF personnalisable de l’interaction du membre résiduel et de l’emboîture. Des mesures pour la caractérisation du comportement des tissus mais aussi pour la définition du chargement, grâce à un modèle musculosquelettique, alimentent le modèle EF. Celui-ci a été comparé à la littérature puis exploité pour l’estimation de l’influence de certains paramètres sur les pressions

Contribution to the modelling of the residual limb of people with lower limb amputation for personalized design of the socket

L’emboîture fait le lien entre la prothèse et le membre résiduel des personnes amputées. Pour les amputations transfémorales, elle doit permettre le transfert des actions mécaniques tout en étant confortable. Cependant, sa conception reste principalement qualitative. Les pressions à l’interface emboîture/membre résiduel peuvent cependant permettre d’évaluer les emboîtures grâce à la méthode des éléments finis (EF) Toutefois la conception de tels modèles est un défi tant pour la reconstruction des géométries, la caractérisation du comportement des tissus ou la définition du chargement. Ce travail présente un modèle EF personnalisable de l’interaction du membre résiduel et de l’emboîture. Des mesures pour la caractérisation du comportement des tissus mais aussi pour la définition du chargement, grâce à un modèle musculosquelettique, alimentent le modèle EF. Celui-ci a été comparé à la littérature puis exploité pour l’estimation de l’influence de certains paramètres sur les pressions.The socket connects the prosthesis to the residual limb of the amputee. For transfemoral amputations, it must allow the transfer of mechanical actions while being conformable. However, its design remains mainly qualitative. The pressures at the socket/residual limb interface can be evaluated using the Finite Element (FE) method. However, the design of such models is a challenge for both the reconstruction of geometries, the characterization of tissue behavior and the definition of loadings. This work presents a personalized FE model of the interaction of the residual limb and the socket. Measurements for the characterization of the tissue behavior but also for the definition of loadings, using a musculoskeletal model, supply the EF model. This model was compared with the literature and then used to estimate the influence of certain parameters on the pressures