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DEVELOPMENT OF A VIRTUAL TESTING LABORATORY FOR LOWER LIMB PROSTHESIS
Get PDFThe introduction of computer-aided tools into the product development process allows improving the quality of the product, evaluating different variants of the same product in a faster way and reducing time and costs. They can play a meaningful role also in designing custom-fit products (especially, those characterized by a tight interaction with the human body), increasing the comfort and improving people’s quality of life.
This thesis concerns a specific custom-fit product, the lower limb prosthesis. It is part of a research project that aims at developing a new design platform centred on the digital model of the patient and his/her characteristics. The platform, named Prosthesis Virtual Laboratory (PVL), is being developed by the V&K Research Group (University of Bergamo) and integrates ICT tools and product-process knowledge. It provides two environments: one for prosthesis design (named Prosthesis Modelling Lab), both transfemoral and transtibial, and one for the prosthesis testing (named Virtual Testing Lab).
The main objective has been to embed within the Virtual Testing Environment numerical simulation tools to analyse the interaction between the socket and the residual limb under different loading conditions, allowing the prosthetist to automatically run the simulation and optimize socket shape. Simulation tools, such as Finite Element Analysis (FEA), permit to predict the pressures at the interface socket-residual limb, evaluate the comfort of socket and validate the socket design before manufacturing phase. However, the diffusion of simulation tools in orthopaedic laboratories is strongly limited by the high level of competence required to use them. Furthermore, the implementation of the simulation model is time consuming and requires expensive resources, both humans and technological, especially onerous for small orthopaedic labs. To effectively employ the numerical analysis in prosthesis design, the simulation process has been automated and embedded within the virtual design platform. Therefore, in such a context, the specific scientific objectives have been to:
• Critically analyse the state of the art with regard to methods and tools to evaluate socket-residual limb interaction.
• Identify the key issues to automate the simulation activities.
• Define a set of simulation rules and the Finite Element Analysis model.
• Implement and integrate within the new design platform the automatic simulation procedure.
• Test the integrated design platform with a case study.
• Identify future development trends.
Research activities have been organized into four main activities as follows.
The first activity consisted in an extensive analysis of the last two decades State of the Art on numerical models adopted to study residual lower-limb and prosthetic socket interaction. Starting from literature, the key issues of the simulation process (e.g., geometric models reconstruction, materials characterization, simulation steps, and boundary conditions), the methodologies and procedures have been identified. Particular attention has been also paid to the parameters commonly adopted to evaluate socket comfort. This phase played a fundamental role since it constituted the basis for the implementation of the embedded simulation procedure. It also permitted to highlight that current finite element models are stand-alone and not integrated with prosthetic CAD or Digital Human Modelling (DHM) systems.
In the second activity the tools and methods necessary to develop the embedded simulation module have been selected. By using these tools, it was possible to identify the simulation rules and the best practice procedures, which are fundamental to implement an automatic simulation module. Initially, the modelling tools have been considered since they provide the geometric models for the numerical analysis of the socket-residuum interaction and for the virtual gait analysis of the patient’s avatar. Then, particular attention has been paid on the choice of the FE solver, that has been made according to the results of preliminary FE models. They were implemented using two different solvers: Abaqus (commercial) and CalculiX (open-source). The latter has been experimented to verify the possibility to develop a design platform totally independent from commercial tools. However, according to the results, Abaqus has been chosen because it allows managing adequately simulation problems characterized by large deformations and difficult contact conditions, its results are comparable with those found in literature, and its scripting code does not require specific customization. The last considered tool was the Digital Human Modelling system (LifeMOD) since it permits to enhance the accuracy of the numerical analysis. By performing the gait simulation of the patient’s avatar, it provides the directions and the magnitude of forces and moments that act on the socket.
The third activity consisted in defining the architecture of the simulation module, implementing the module and the interfaces with the socket CAD tool (namely Socket Modelling Assistant-SMA) to get the geometric models of the involved parts (socket and residual limb) and with the DHM system to acquire forces acting on the socket during patient’s walking. The simulation module has been implemented using the Python language and the integrated environment works as follows. Once the prosthetist has created the 3D socket model, SMA acquires the input for the analysis (e.g., residual limb length, patient’s weight, friction coefficient, material properties), and produces the files required to generate the FE model. Abaqus automatically generates the FE model without any human intervention, solves the analysis and generates the output file containing the pressure values. Results are imported in SMA and visualized with a colour map. SMA evaluates pressure distribution and highlights the areas that should be modified. Geometry modifications are needed in the areas where pressure exceeds the maximum value and are carried out automatically by the system or by the prosthetist using the virtual tools available in SMA. Then, the system re-executes the simulation. Through this iterative process of adjustments, the socket shape is modified and optimized in order to eliminate undercuts, minimize weight and, especially, distribute loads in the appropriate way so that they can be tolerated for the longest period of time.
The fourth and last activity concerned the test and validation of the simulation module integrated within the new design platform, by considering a transfemoral patient. The new virtual process and the key issues of the simulation procedure have been tested starting from the patient’s data acquisition to the release of the socket using also data coming from the gait simulation with the DHM system. The geometric model of the residual limb has been reconstructed from MRI images and the socket has been modelled using SMA. Through an iterative process, the socket shape has been optimized until the pressure distribution on the residuum was consistent. Preliminary activity concerning the FE model validation has been performed comparing the pressure distribution experimentally acquired with pressure transducers over the residuum with the simulation results. To accomplish this task, the geometric model of the real socket has been acquired using reverse engineering techniques. Two numerical simulations have been implemented, they differ for the residuum geometric models adopted: from MRI and from 3D scanning. Preliminary results have been considered positive but improvements are necessary. As an example, some geometric inconsistencies, occurred during the acquisition of the geometric model of the residual limb, have reduced the accuracy of the final results. To complete the evaluation of the simulation model, a new residuum geometric model is needed and a refinement of the material model characterization is desirable.
To conclude, the simulation module embedded within Virtual Testing Laboratory has improved the prosthesis development process with the goal of assessing and validating the socket shape under different load conditions (static or dynamic) before the manufacturing phase. The testing phase of the new procedure has demonstrated the feasibility of the virtual approach for lower limb prosthesis design. The tests carried out permitted to highlight necessary improvements and future developments, such as the definition of a protocol to acquire the residual limb through MRI and 3D scan, refinement of the FE model (e.g., non-linear viscoelastic behaviour for soft tissues, friction coefficients), parallel computing to improve simulation performances, open-source solvers to implement a design platform totally independent from commercial systems, and a massive test campaign involving transtibial and transfemoral patients to fully validate the FE model and the design platform
FE Analysis of contact between residual limb and socket during simulation of amputee motion
No full textThe contact pressure at the socket–residual limb interface is the most important parameter to evaluate comfort of leg prosthesis. Experimental works analyzed this parameter for typical postures and during walking of an amputee; but experimental tests require a real prototype of the socket equipped with transducers. To optimize socket design, this work presents a virtual approach based on a digital avatar of the patient wearing lower limb prosthesis. Our approach considers the integration of two different types of simulation: the first one concerns the multi-body gait analysis, the second one the pressure evaluation at the socket–residual limb interface with FE analyses. The paper describes the model used, the simulation tools adopted and their integration. Finally the case study related to patient walking on flat floor is described
Physically based modelling and simulation to innovate socket design
No full textThis paper introduces a virtual laboratory to design prosthetic socket, which integrates a 3D CAD module, named Socket Modelling Assistant (SMA), specifically developed to create the socket digital model, and a CAE system to analyze the stump-socket interaction. Software tool, named Virtual Socket Lab (VSL), is part of a knowledge-based framework to design lower limb prosthesis centered on digital models of the patient or of his/her anatomical districts. The focus of this paper is on the definition of an automatic simulation procedure to study the stump-socket interaction and validate socket design. We first introduce the new design framework and main features of VSL. Then, we present a state of art on FE models adopted for residual lower-limb and prosthetic socket during last two decades highlighting key issues. Finally, the identified procedure and the integration strategy within SMA are described as well as preliminary results of the experimentation
An approach to integrate numerical simulation within KBE applications
No full textThis paper presents an approach to automate and integrate numerical simulations within knowledge-based engineering applications to improve the use of simulation tools, especially in small and medium enterprises. Main discussed issues concern: (a) the importance of the CAD model as an input of the process, (b) data migration from CAD to CAE tools, (c) model characterisation, (d) effects of parameters variations, (e) simulation steps, and (f) results evaluation. In addition, formalisation and representation of rules and procedures to automate the design process, in particular the simulation tasks, are presented. Two case studies are described to explain and verify the approach. The first case concerns the design process of centrifugal industrial fans, during which simulation tools are used to verify the correct sizing of impeller blades. The second one is related to a non-industrial process; actually, it regards a medical device, and more precisely lower limb prosthesis. Finally, main results are discussed
Digital Human Modelling to Analyse Virtual Amputee’s Interaction with the Prosthesis
No full textThis paper refers to the integration of simulations tools to
assess the design of prosthetic devices. We address issues
arising when the prosthesis needs to be virtually tested, i.e., the
gait of the virtual patient wearing the prosthesis. Therefore, we
integrate two different simulation tools: the first one to study
the interaction between socket and residual limb during the gait
and the second one to analyze the patient’s gait deviations.
Combining these numerical analyses, it is possible to
investigate the causes of gait deviations and suggest remedies,
both related to the prosthesis setup and the socket modeling. To
prove the validity of the approach, we implemented a Finite
Element Analysis model to analysis the stump-socket contact
and we assembled a low cost Motion Capture system to acquire
and elaborate patient gait. Preliminary results and remarks
conclude the paper
Numerical simulations and experimental data to evaluate residual limb-socket interaction
No full textFinite Element Simulations Integrated in Design Automated Procedures
No full textNumerical simulations can play a meaningful role in many industrial contexts, but a capillary diffusion of simulation tools is often limited by the high level of competence necessary for a profitable use, required resources and time expense to reach significant results. To improve dissemination a possible solution is to automate simulation activities and integrate them in an automated or semi-automated process. This paper discusses the issues related to this proposal by presenting two case studies and highlighting most important aspects. The first case is a typical engineering product application and concerns the integration of simulation in an automatic design process of industrial fans. The second one is related to a non-industrial process; in fact, it regards medical device and precisely lower limb prosthesis. Following issues will be discussed: importance of a CAD model as an input of the process, migration from CAD to CAE model; effects of parameters variations on the models, model characterization, simulation steps and results evaluation. It becomes clear the need to consider aspects that are typical of KBE (Knowledge Based Engineering) to formalize and represent rules and procedures to automate the design process and, in particular, simulation tasks
Inflatable tabular element for laying protective tarpaulins
No full textThe invention relates to an inflatable tubular element, which is inflated to a working shape from an initial rest shape in which the element is deflated or partially deflated. The tubular element is of modular type and has anchor points (801) for tubular elements of the same type, such that it may form a multiplicity of support structures for a protective tarpaulin, which may thus be laid in different manners, when such tubular element/s are coupled to said tarpaulin
An approach to integrate numerical simulation within KBE applications
No full textThis paper presents an approach to automate and integrate numerical simulations within knowledge-based engineering applications to improve the use of simulation tools, especially in small and medium enterprises. Main discussed issues concern: (a) the importance of the CAD model as an input of the process, (b) data migration from CAD to CAE tools, (c) model characterisation, (d) effects of parameters variations, (e) simulation steps, and (f) results evaluation. In addition, formalisation and representation of rules and procedures to automate the design process, in particular the simulation tasks, are presented. Two case studies are described to explain and verify the approach. The first case concerns the design process of centrifugal industrial fans, during which simulation tools are used to verify the correct sizing of impeller blades. The second one is related to a non-industrial process; actually, it regards a medical device, and more precisely lower limb prosthesis. Finally, main results are discussed
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