2008 International ANSYS Conference

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1 2008 International ANSYS Conference Patient-Specific Orthopedics Simulation Using ANSYS Technologies N. Hraiech, E. Malvesin and M. Rochette ANSYS France M. Viceconti and F. Taddei Istituti Ortopedici Rizzoli, Bologna, Italy 2008 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary

2 Outline Introduction Clinical Applications of FE Problem and Contribution FE Generation using Planar Parameterization Morphing: Planar Parameterization Mesh Morphing using Radial Basis Functions (RBFs) for skin mesh RBF for volume mesh Numerical Results Conclusion and Future work 2008 ANSYS, Inc. All rights reserved. 2 ANSYS, Inc. Proprietary

FE for Clinical Diagnosis Osteoporosis Prediction of the risk of failure for osteoporotic

more accurate than current method based on DA (2D densitometry) AAA (Aortic Abdominal

failure From left to right: segmentation of the femur from CT dataset, finite element model

Courtesy of Laboratorio di Tecnologia Medica, Istituti Ortopedici Rizzoli, Bologna, Italy.

3 FE for Clinical Diagnosis Osteoporosis Prediction of the risk of failure for osteoporotic bones 3D geometry and bone mineral density from scan data Patient specific FE simulation much more accurate than current method based on DA (2D densitometry) AAA (Aortic Abdominal Aneurysms) Fluid Structure Interaction simulation from scan data Prediction of the risk of failure From left to right: segmentation of the femur from CT dataset, finite element model of the femur and non-homogenous material properties mapped on the mesh from CT data. Courtesy of Laboratorio di Tecnologia Medica, Istituti Ortopedici Rizzoli, Bologna, Italy. Geometry: Courtesy of LTSI INSERM, University Rennes I 2008 ANSYS, Inc. All rights reserved. 3 ANSYS, Inc. Proprietary

FE for Orthopedic Surgery Planning Hip Resurfacing Prosthesis Geometry and Material properties given by scan data Stresses and strains function of the prosthesis orientation angles Optimal

simulation More accurate, less invasive Minimization of the surgical risk Subject-specific muscle-skeletal model for the prediction of muscleforces during gait.

4 FE for Orthopedic Surgery Planning Hip Resurfacing Prosthesis Geometry and Material properties given by scan data Stresses and strains function of the prosthesis orientation angles Optimal orientation angles computed and delivered to the surgeon Orthopedics (Knee joint, Spine ) Simulation features integrated in a navigation system (patient specific geometry) Surgeon gesture simulation More accurate, less invasive Minimization of the surgical risk Subject-specific muscle-skeletal model for the prediction of muscleforces during gait. (Provided by Laboratorio di Tecnologia Medica, Istituti Ortopedici Rizzoli, Bologna, Italy). Montanari L,et al., J Biomech. 2006;39(S1):S46 Hip resurfacing prosthesis 2008 ANSYS, Inc. All rights reserved. 4 ANSYS, Inc. Proprietary

5 Problem and Contribution Problem: Patient-specific finite element (FE) models of long bones are useful in many clinical applications. Generation of FE models from patient s CT data in clinical practice needs to be automated, accurate and fast. Generation of FE from scratch is slow. Contribution: Fast and automatic approach for FE model generation based on morphing of existing FE model ANSYS, Inc. All rights reserved. 5 ANSYS, Inc. Proprietary

6 Method Overview Input: 3D FE generic mesh Patient s geometry User-defined anatomical landmark points Generic Mesh Method Patient s Geometry (STL) Output: Patient FE 3D mesh obtained by morphing Patient s Mesh 2008 ANSYS, Inc. All rights reserved. 6 ANSYS, Inc. Proprietary

Method: Femur Morphing Via Planar Parameterization

representation of an open 3D shape Simplification of a

morphing between 2 disks Morphed mesh is perfectly

Patient s geometry Surface to disk mapping Surface to

mesh 2D domain for patient geometry Resulting patient

7 Method: Femur Morphing Via Planar Parameterization Planar parameterization technique is an exact 2D representation of an open 3D shape Simplification of a 3D morphing problem on complex surfaces to a 2D morphing between 2 disks Morphed mesh is perfectly projected onto the 3D patient geometry Generic mesh Patient s geometry Surface to disk mapping Surface to disk mapping Mesh generation 2D domain for generic mesh 2D domain for patient geometry Resulting patient s mesh 2D morphing 2008 ANSYS, Inc. All rights reserved. 7 ANSYS, Inc. Proprietary

Planar Parameterization A parameterization of a surface is a one-to-one mapping

the circle We minimize a conformal energy for internal vertices: Example of a

[3] E T Mesh cot 2 2 1 f ( v2 ) f ( v3) cot 2 f ( v1) f ( v3) cot 3 f ( v1 ) f ( v2

8 Planar Parameterization A parameterization of a surface is a one-to-one mapping from a random surface to a fixed domain In our work : We constrain the boundary on the circle We minimize a conformal energy for internal vertices: Example of a planar parameterization; (left) generic mesh; (right) 2D parameter domain.[3] E T Mesh cot f ( v2 ) f ( v3) cot 2 f ( v1) f ( v3) cot 3 f ( v1 ) f ( v2 ) 2 [3]: ANSYS Mesh Morpher, release ANSYS, Inc. All rights reserved. 8 ANSYS, Inc. Proprietary

2D Morphing 2D morphing Based on RBFs 2D parameter domain of the generic mesh 2D parameter

2D Morphing 2D morphing 2D parameter domain of the generic mesh 2D parameter domain

11 Morphing via Radial Basis Functions (RBF) x old p 2 p 2 p 1 p 1 new? Linear regression with radial basis function x new f ( x old ) x old i k( x old,p i )w í ; k x old, p i exp x old 2 2 p i 2 n constraints: f(p i ) = p i, i = 1,, n w i : coefficient that represent the motion w as solution to p = Kw, with K ij = k(p i, p j ) ANSYS, Inc. All rights reserved. 11 ANSYS, Inc. Proprietary

2D Morphing using RBF Original 2D Mesh 2008 ANSYS,

2D Morphing using RBF New Position 2008 ANSYS,

2D Morphing using RBF Final Result 2008 ANSYS,

3D Morphing using RBF: Volumetric Representation Each element is defined by 10 nodes that

16 3D Morphing using RBF: Volumetric representation, Details & Result Generic mesh Result from morphing 1. Morph outer surface using 2D method. 2. Morph inner points using 1. and RBF interpolation ANSYS, Inc. All rights reserved. 16 ANSYS, Inc. Proprietary

Results (1) Generic mesh Surface to disk

using constraint points 2008 ANSYS, Inc.

Experimental Results (3) Generic Mesh Patient s geometry Resulting

Volumetric mesh 1.09 < Aspect Ratio <3.87 Result: 3D Patient s mesh 1.

20 Tetrahedral Mesh Evaluation The aspect ratio of a triangle is the length of the longest edge divided by the length of the shortest altitude Input: Volumetric mesh 1.09 < Aspect Ratio <3.87 Result: 3D Patient s mesh 1.11 < Aspect Ratio < ANSYS, Inc. All rights reserved. 20 ANSYS, Inc. Proprietary

21 Conclusion Fast and automatic approach for FE model generation based on morphing via planar parameterization. Preserves geometrical accuracy between the morphed surface mesh and the target geometry. Minimizes the distortion of the morphed volume mesh (high accuracy on FE results). Requires minimal user inputs. Validated on more than 20 tetrahedral meshes. Work in progress: o Tests on larger data base. o Apply the methodology for cerebral & abdominal aortic aneurysms. o Use spherical parameterization instead of planar parameterization ANSYS, Inc. All rights reserved. 21 ANSYS, Inc. Proprietary

David Wagner, Kaan Divringi, Can Ozcan Ozen Engineering

Internal Forces of the Femur: An Automated Procedure for Applying Boundary Conditions Obtained From Inverse Dynamic Analysis to Finite Element Simulations David Wagner, Kaan Divringi, Can Ozcan Ozen Engineering