Mesh Generation from Point Cloud Data for Transtibial Prosthesis Socket Using Altair HyperMesh Chitresh Nayak
Vijendra Jain
Research Scholarship - Mechanical Engineering Malaviya National Institute of Technology, Jaipur, India
B.Tech, Department of Mechanical Engineering, Malaviya National Institute of Technology, Jaipur, India
[email protected]
[email protected]
Amit Singh
Himanshu Chaudhary
Mechanical Engineering Malaviya National Institute of Technology, Jaipur, India
Mechanical Engineering Malaviya National Institute of Technology, Jaipur, India
[email protected]
[email protected] Abstract
This paper presents a method for mesh generation and analysis of transtibial prosthesis socket from 3D digital data. It covers 3D scanning, post processing of point cloud, part digitization, mesh generation, trial with multiple prosthesis material, fine tuning the design. Altair HyperMesh software was utilized for Mesh generation and analysis. Material selection and best geometric profile are critical in the case of FEA of limb prosthesis for desired accuracy. Due to irregular and complicated profile of the limb and higher range of accuracy required for analysis, HyperMesh proves to be an excellent tool for prosthesis design and fitment. Keyword: Altair HyperMesh, Finite Element Method, Prosthesis Socket
1. Introduction The prosthetic socket is the elementary part for prosthetic performance. It is the primary interface between amputee limb and the prosthesis and transfers the body loads generated during the gait. Therefore, it plays a significant role in determining the quality of the fit. Commonly, there are two distinguishing approaches to investigate the stresses, namely, experimental measurements and theoretical analyses that have been used to study the stump socket interface conditions. Finite element analysis (FEA) has been identified as a powerful tool in learning the interface stress distribution for theoretical investigation that determines the patient’s comfort. The finite element method was presented in the last 1980s to design prosthetic socket. Steege et al. [1-5] established the first FE model of the BK residual limb and the use of FEA to predict interfacial pressure. They used the internal and external geometry of the residual limb based on transverse computer tomography (CT) scans. The socket was assumed to be rigid and fully connected with the stump. The bone, soft tissue and linear were meshed into 3D solid elements with linear elastic properties. Quesada et al. [6] reported the finite element (FE) model of an exoskeletal patellar tendon bearing (PTB) below knee socket which was designed and constructed to calculate the stresses in relation to some parametric variations. FE investigations were performed on the adjusted models to focus on the relative overcomes for stump-socket interface stress of changing the FE model of the prosthesis socket. Results
of the analysis indicated that use of lower modulus prosthetic materials and thinner fabrication reduced normal stresses between socket and stump. In all the above studies, it was found that not much work has been reported on mesh generation and analysis of transtibial prosthesis socket model. Further, material and mechanical properties using laser scanning, weight of the socket, and integration of CAD to evaluate the performance of socket through prosthetic use have not been considered earlier. This study aims to create mesh generation with different element size and element type. This paper attempts to provide a clear overall picture of the finite element mesh generation in the field of prosthesis and orthotics. 2. Creating CAD Model from point cloud data Utilize the method of reverse engineering involves the use of non-intrusive 3D scanner to get the digitized data points of lower limb and a CAD design system which is used to construct the CAD model of that residual limb based on the scanned points. CAD is the final stage of reverse engineering to design the socket but the main challenge is to create the model itself. Before starting of scan, number of points marked on the surface of model give all the attributes like humps, depressions, rounding’s etc. Once the model is created, the further process is to analyze it. The process of creating a model is basically scanning the socket using blue light steinbichler scanner to create point cloud data. For the present study a total of 24 scans were taken from different angles and with different model positions to scan the whole socket. After each scan, the previous scan was meshed with the help of automatic meshing of point to point (Black Dot) on the COMETPLUS mesh software. The final model of prosthesis socket is created as a surface on COMET Plus Software. This is then followed by post processing which is done on the prosthetics socket model and involves processes such as removal of unassociated area/noise, filling holes, segmentation of cloud, surface fitting and analysis. 3.
Method
A male right side traumatic transtibial amputee, 48 years of age, 176 cm height and 65 kg in weight, participated in this study. He had been using exo-skeletal transtibial prosthesis from last 24 years with PTB socket in Jaipur foot. The FEM analysis and optimization was done using Altair HyperWorks, version 10.0. A 3 mm thick fabricate glass fiber reinforced composite thermosetting plastic based on principle of PTB socket was used for the study. The complete assembly for the same is shown in Fig1. The CAD model of residual limb surface was obtained by digitizing a loose plaster cast using non tactile blue light scanner. The STL format surface data received from COMET plus was converted to IGES format. The detailed and geometrically accurate three-dimensional finite element model of socket was developed using the surface data of original model. Several trials were prepared by increasing number of element during meshing to achieve convergence.
Fig. 1 (a) Original socket (b) Scan Data (c) CAD socket
3.1
Geometry and shape acquisition
The external geometry of the prosthesis socket of residual limb surface was obtained from Bhagwan Mahaveer Viklang Sahayata Samiti (BMVSS) foot centre in Jaipur, India. Socket model is totally based on patient’s actual geometry and dimension. The patient is given cotton liner socks to wear with a uniform 5 mm thickness fitted between stump and socket. By tradition, CAD socket system usually involves a shape digitizer or 3D laser scanner which can get the shape of the stump and socket, but only the surface information. Some other techniques used to measure the geometry of stump surface and internal bones information are CT, MRI, X-ray and Ultrasound. For reducing soft tissues at the posterior regions, the subject wore an unrectified socket based on a loose plastic cast on the limb surface. The unrectified cast representing the residual limb surface was digitized and exported to prosthetic CAD software. 4. Finite element model Finite element technique is frequently used for the numerical analysis in biomechanics. FEM analysis is a computational approach for calculating the state of stress and deformation in the specific field. It is a useful method to understand the load transfer mechanics between residual limb and its prosthetic socket. FEA is achieved by dividing a complex problem into a finite number of smaller elements, which is not solved by analytical method. The accuracy of solution is mostly depending on number of elements of the model. FEM technique is applied to analyze the state of stress and elastic strain of male prosthesis socket in the proposed method. The CAD model of the prosthesis socket imported in IGES (Initial Graphics Exchange Specification) format and modified in HyperMesh v12.0.
Import the CAD model
Geometry cleanup
Mesh generation Pre-Processor (HyperMesh) Mesh quality check
Assign material and property
Loads and boundary condition Solver (RADIOSS)
Solve the model
Post-processor (HyperView)
Results
Fig.2 Flow chart of steps for FEM analysis on Altair HyperWorks
4.1 Mid-Surfaces: There were no geometric irregularities particularly free edges when the model was imported in HyperMesh. The geometry is a thin walled 3D structure and it contains outer and inner surfaces, and its two of the dimensions are very large in comparison to third dimension, shell mesh is chosen. So after visualizing, midsurface was extracted as shown in figure 3 and thickness of the geometry is virtually assigned to the 2D elements. Mathematically, the element thickness (specified by the user) is assigned with half in the + Z direction (element top) and the other half in the – Z direction (element bottom).
Fig.3 Extracting the mid-surfaces from outer and inner surfaces in HyperMesh 4.2 Mesh Generation: 2D elements are used for prosthesis socket design for mesh generation and divided into three basic element shapes are tria, quad and mixed. 4.2.1 Tria: There are two types of tria elements: Equilateral (trias) and Right Angled tria(R-trias) elements. R-trias are used only for specific applications such as mold flow analysis. 4.2.2 Quad: Quadrilateral elements have been proved to be useful for finite element and finite volume methods, and for some applications they are preferred to triangles or tetrahedra. Therefore quadrilateral and hexahedral mesh generation has become a topic of intense research. By connecting polygon nodes to their respective nodes on the object boundary, one gets a quadrilateral element mesh in the boundary region.
4.2.3 Mixed: The mixed mode element type is the most common element type used due to the better mesh pattern that it produces. Meshed models with different mesh types are as shown in figure 4. Note the more homogeneous mesh pattern resulting from “mixed” meshing
(a) Tria
(b) Quad
(c) Mixed
Fig.4 Meshed models with different element types 4.3 Element Quality Check In FEA modeling, element quality greatly affects the accuracy of the analysis results. The FEA modeler must take into consideration element quality, and thereby judge whether the analysis results are meaningful. There are many important quality parameters that need to be checked that affects the accuracy of the analysis results. Some of them are as follows: Warpage is the amount by which an element deviates from being planar. Since three points define a plane, this check only applies to quads, while tria elements do not have such problems. In this socket model warpage of up to five degrees is acceptable. Aspect Ratio is the ratio of the longest edge of an element to either its shortest edge or the shortest distance from a corner node to the opposing edge. Aspect ratios should rarely exceed 5:1. Skew of triangular elements is calculated by finding the minimum angle between the vector from each node to the opposing mid-side, and the vector between the two adjacent mid-sides at each node of the element. The minimum angle found is subtracted from ninety degrees and reported as the element’s skew. Chordal Deviation Curved surfaces can be approximated by using many short lines instead of a true curve. Chordal deviation is the perpendicular distance between the actual curve and the approximating line segments.
Jacobian measures the deviation of an element from its ideal or "perfect" shape, such as a triangle’s deviation from equilateral. The Jacobian value ranges from 0.0 to 1.0, where 1.0 represents a perfectly shaped element. In this case of Jacobian evaluation at the Gauss points, values of 0.7 and above are generally acceptable. In all three types of meshed models were inspected and compared based on the various quality parameters using check elements. As in mixed type mesh, there were less no. of distorted elements, and mesh generation is smoother so it is chosen for further modeling. After a number of iterations and inspections an element size of 5mm is chosen. The distorted/failed elements are corrected using various tools available in HyperMesh. The final FE model is prepared with 4367 elements and 4368 nodes as shown below in figure 5. The details of the mesh generation are given in table 1.
Fig.5 Final FE Model with element type mixed.
Table 1: Details of the FE model.
Parameters
Knee Socket
Weight of the Amputee
62Kg
Mesh Type
2D Shell – mixed Elements
No. of mesh element
4367
Element Size
5mm
No. of Nodes
4368
Internal Area (mm2)
105509.6
Max. Diameter(mm)
129.04
Length(mm)
355.36
Conclusions This study establishes the development of a suitable framework for modeling and analysis of prosthetic sockets using Altair HyperWorks. We have described state of the art mesh generation procedure using finite element method. This paper explores the power of Altair HyperWorks in the application of socket analysis and design. The socket being a very sensitive artifact, it demands critical design. Hence it is proposed to fine-tune the design and production employing various meshing options available in HyperMesh. Such a study helps prosthetist to select the most suitable socket material and best DFM (design for manufacturing) option. Authors hope this effort would offer optimum comfort to the patient in a customised way.
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Altair HyperWorks user manual.
