Thread 1. Computational Methods in Biomechanics and Mechanobiology T1.14 Image-based Anatomical Modelling for CAD/FEA Applications $425

Combination of CAS and Finite Element (FE) calculations leads to a new level of control on the surgical procedure. The additional information provided by the numerical methods may be used to enhance the surgical procedure in two different ways. First, FE-enhanced planning software may be used for the pre-operative evaluation of implants stability. This approach has a definitive advantage over current evaluation tools which are all post-operative. Secondly, numerical methods may be used to consider non-rigid tools and soft tissue deformations during surgical procedure. The potential enhancements of calculating tools deformations will be illustrated with femoral nailing. This treatment has become the technique of choice for most femoral shaft fractures. The surgery consists to the insertion of a long metallic bar in the femoral canal. Bone fragments are then attached to this bar via intra-cortical screws. Navigation is used to guide the surgeon during nail insertion and proximal screw locking. However, since the nail is deformed during its insertion, problems remain for the distal locking. Currently, distal screw position is defined with C-arm shots. In this project, FE calculations will be used to predict nail bending and the position of the distal screws. The benefits of this FE-guided surgery, when successfully integrated, will be: less X-rays radiations and faster surgical procedures

6546 We, 08:45-09:00 (P29) Is high accuracy anatomical modelling necessary for computer simulation of implant loosening in total hip replacement? A.B. Lennon 1 , J.R. Britton 1 , R.E MacNiocaill 2, P.J. Kenny 2, P.J. Prendergast 1 . 1 Trinity Centre for Bioengineering, Parsons Building, Trinity College Dublin, Ireland, 2Department of Orthopaedic Surgery, Cappagh National Orthopaedic Hospital, Dublin, Ireland

Image-based anatomical modelling has advanced to the stage that accurate 3D reconstructions from 3D digital imaging sources (e.g. CT and MRI) are becoming routine. However, for many common procedures, e.g. primary total hip replacement, it is prohibitively expensive to use such imaging modalities and planar radiographs are the norm for pre-operative planning. This study investigated whether 3D finite element meshes developed using information from post-operative planar radiographs could be useful in simulating implant loosening in total hip replacement. Scaling of a generic femur was performed based on measurements of extra- cortical width at the isthmus and the distance of the lesser trochanter to the isthmus. The implant position was reproduced in a FE pre-processor and the structure was meshed. Integration points within a specified distance of the extra-cortical surface were assigned cortical bone material properties based on a thickness measurement at the distal prosthesis tip. Remaining integration points were assigned either cancellous bone or PMMA material properties based on the results of a ray tracing test with a surface mesh representing the cement layer. Simulations of cement creep and fatigue damage accumulation were then carried out for 10 years of activity for 16 different patients (6 early revisions, defined as <10 years, and 10 unrevised hip replacements), each with a mesh created using this approach. Using predicted resultant migration as a ranking criterion, 5 out of the 6 early revisions were correctly identified and the revised group was found to have a higher average migration compared with the unrevised group (p=0.004). Planar radiographs were found to be a good source of geometric data for the purposes of predicting implant loosening in total hip replacement and higher accuracy is only likely to be required in exceptional cases. In conclusion, pre- operative planning simulations can be performed with planar radiographic data without the need for expensive 3D imaging procedures.

7052 We, 09:00-09:15 (P29) Application of the finite element method to investigate the stability of acetabular press-fit cups during impingement C. Voigt 1 , M. EIIguth 2, E. Steinhauser 3, C. KI6hn 2, R. Bader 4, R. Scholz 1 . 1 Department of Orthopaedic Surgery, University of Leipzig, Leipzig, Germany, 2Department of Mechanical and Power Engineering, Leipzig University of Applied Sciences, Leipzig, Germany, 3Department of Orthopaedic Surgery, Technical University of Munich, Munich, Germany, 4Department of Orthopaedic Surgery, University of Rostock, Rostock, Germany

Introduction: After total hip replacement impingement may occur between the implant components causing damage at the rim of the acetabular liner and shear stresses in the implant-bone interface of the cup. In the current study the finite element method (FEM) was applied to analyse the distribution of shear stresses in the acetabular implant-bone interface. Material and Methods: A finite element (FE) model of the proximal part of a femoral component and a standard acetabular press-fit cup (ESKA Implants) previously used in an experimental dislocation simulation by Bader et al. (2004) and Scholz et al. (2003) was generated using the FEM software Ansys (Ansys Inc.). The model incorporates non-linear material and sliding contact. Starting out from a CT data set a FE model of a hemi-pelvis was generated. This

was realized in part by a software script running in Mathematica (Wolfram Research) transforming a STL file of the pelvic bone into an Ansys input file. Another script was written in APDL (Ansys parametric design language) to automatically assigning material parameters to the FE model of the hemi- pelvis by simulation of bone density remodelling. For further studies a Ioadcase including the significant muscle forces acting on the pelvis was developed. Results: The finite element model was verified against test data of tilting mo- ment (resisting moment) measured at the acetabular cup during impingement by Bader et al. (2004) and Scholz et al. (2003). The distribution of shear stresses was contour plotted across the implant-bone interface. Discussion: The here presented FE model can be used for analysing the strength of the fixation of press-fit acetabular components during impingement and dislocation.

6671 We, 09:15-09:30 (P29) A general geometrical modeling procedure for biomechanical simulations, supporting pre-operative planning and post-operative examination E. Karatsis, I. Chalkidis, T. Charamis, G. Athanasiadis. Laboratory of Machine Elements & Machine Design, Department of Mechanical Engineering, Aristotle University of Thessaloniki, Thessaloniki, Greece

To date the typical way to create information for the modeling of a body area is the usage of CT or MRI scanners, in connection with special software used for computer aided design applications (CAD Software). The aim of this presentation is the introduction of a new modeling approach based on general Finite Element Models, supported by morphing functionality tools. Thereby parts from a general Finite Element Skeleton model can be modified and adapted to the patient's geometrical data using feature parameters. In a second step the geometrical modeling of selected parts for Finite Element Analysis (FEA) can be calibrated according to the needs of the research, such as operative preparation or post-operative examination of the results. The modeling procedure will be applied to the areas of the hip-joint and the knee, to analyze: - Geometrical compatibility of implants; - To validate surgery methodologies; - To validate the behavior and strength of bone-implant systems in a post-

operative environment. Additionally to the doctor's experience this knowledge can offer important information for a better preparation of a successful operation, as well as for an early recognition of possible dysfunctions in the post operative environment, which can raise both the financial and psychological cost of such operations.

7475 We, 09:30-09:45 (P29) On image-based meshing: New tools for generating numerical models from scan data E Young 1 , T. Beresford-West 2. 1School of Engineering and Computer Science, University of Exeter, Exeter, 2 Simpleware Ltd., Exeter, UK

Although a wide range of mesh generation techniques are currently available these, on the whole, have not been developed with image-based meshing for anatomical modelling in mind. Meshing from segmented 3D imaging data presents a number of challenges but also unique opportunities so that a conceptually different approach provides, in many instances, better results than traditional approaches. A mesh generation technique based on a unique in-house developed multi- part marching cubes algorithm with specifically designed multi-part smoothing algorithms will be presented. Uniquely this technique allows the user to: straightforwardly generate meshes of excellent quality (low element distortions) regardless of complexity; will mesh any number of structures simultaneously (handles multi-part junctions); and allow the user to seamlessly apply signal strength to material property mapping functions (e.g. Young's Modulus to Hounsfield Number mapping functions). Important concepts in smoothing of meshes such as topological preservation of data (for example to ensure preservation of connectivity), volume neutral smoothing (to prevent shrinkage of convex hulls) and convergence to geometry will be discussed. The approach has been applied to a very wide range of problems for both finite element analysis, CFD analysis and FSI problems which will be illustrated and advantages and disadvantages of this type of approach with alternative meshing techniques will be discussed.

5173 We, 11:00-11:15 (P32) Forearm muscle volumes can be accurately obtained from high resolution MRI C.M. Eng, S.R. Ward, L.H. Smallwood, G.D. Abrams, R.L. Lieber. Department of Orthopaedics, University of California and VA San Diego Healthcare System, San Diego, CA, USA

Upper extremity musculoskeletal modeling is becoming increasingly sophis- ticated and the need for subject-specific muscle architectural parameters is

$426 Journal of Biomechanics 2006, Vol. 39 (Suppl 1) Oral Presentations

increasing. One method of determining subject-specific muscle volume is magnetic resonance imaging. Purpose: To determine the validity of MRI-derived muscle volume in the human forearm. Methods: Seven cadaveric forearms were scanned using a fast spoiled gradi- ent echo pulse sequence with high spatial resolution (1 mm 3 voxels) on a 3T MR system. Pronator teres (PT), extensor carpi radialis brevis (ECRB), and extensor pollicis Iongus (EPL) muscles were manually segmented allowing volumes to be calculated. Forearms were then dissected, muscles were isolated, and muscle masses were obtained which allowed muscle volume to be computed. Intraclass correlation coefficients (ICC2.1) and absolute volume differences were used to compare measurement methods. Results: There was excellent agreement between the anatomical and MRI- derived muscle volumes (ICC = 0.97, average absolute error= 1.50±1.95 cm 3) when all 21 muscles were considered together. When individual muscles were considered, there was excellent agreement between measurement methods for PT (ICC=0.98, average absolute error=0.17±2.36cm3), ECRB (ICC=0.92, average absolute error=2.13±1.82cm3), and moderate agreement for EPL (ICC=0.65, average absolute error =2.20±0.83cm3). Discussion: MRI based measurements of muscle volume produce relatively small absolute errors (1.50cm3). This value, however, in some smaller muscle like EPL represents a larger fraction of the total muscle volume. In these cases, MRI derived volumes should be interpreted with caution. However, in general, high resolution MRI can be used to accurately measure forearm muscle volumes.

6556 We, 11:15-11:30 (P32) Evaluation of hip strength from radiographs T. J~ms& E Pulkkinen. Department of Medical Technology, University of Oulu, Oulu, Finland

The relationship between bone geometry and strength is important when devel- oping models for evaluating the risk of fracture. We have studied different two- dimensional geometrical and structural parameters from clinical and cadaver radiographs. The best predictor of a clinical hip fracture appeared to be the combination of medial calcar femoral cortical thickness (CFC), trochanteric BMD, neck/shaft angle (NSA), and Ward's BMD (r=0.72, p<0.001). The predictors of cervical and trochanteric fracture differed significantly. In an experimental cadaver study, we found that CFC and BMD have a similar effect on the mechanical strength of cadaver femurs as in the clinical hip fracture risk. Femoral neck fractures appeared to predominate at the lowest failure load levels, while trochanteric fractures are more common at high failure loads. At the lowest load quartiles, 94.7% of fractures in female and 62.5% in male were femoral neck fractures. At the highest quartiles, in contrast, only 52.6% of fractures in female and 33.3% in male were cervical fractures. NSA was the best predictor of fracture type, with higher values in subjects with cervical fractures. This finding was made in females (p<0.001) and males (p=0.02) and was consistent across all failure load quartiles. We have also studied the evaluation of trabecular bone structure from radio- graphs. Using gradient-based image processing, we were able to estimate trabecular bone. The trabecular bone area per total area at trochanter cor- related significantly with DXA-based BMD (r= 0.82, p < 0.001). This suggests that the method might be applied for structural analysis of trabecular bone. The results confirmed that the radiological measures of upper femur geometry and structure can be defined reliably from 2D radiographs and they improve the assessment of the risk of hip fracture and fracture type compared to BMD.

4195 We, 11:30-11:45 (P32) A fast framework for modelling the effective orthotropic elastic properties of cancellous bone D.H. Pahr 1, M. Charlebois 1 , '~ Chevallier 1 , H. AIImer 1 , W. Krach 2, E K. Zysset 1 . 1 Institute of Lightweight Structures and Structural Biomechanics, TU-Vienna, Austria, 2 CAE Simulation & Solutions GesmbH, Vienna, Austria

Numerical investigations of bone remodelling, bone fragility, bone-implant systems, etc. require an accurate knowledge of the mechanical behaviour of both cortical and cancellous bone tissue. State of the art voxel approaches apply effective isotropic cortical and cancellous bone stiffnesses which are based on bone mineral density measured by quantitative computer tomography (QCT). Especially in the case of cancellous bone, such an approach is not able to account for bone architecture and, therefore, for the orthotropic (direction dependent) material behaviour unless micron resolution is achieved and extremely large computer models are acceptable. This work extends and improves the effective isotropic approach. It introduces a morphology-based effective orthotropic cancellous bone material property. In this approach, the in- puts are high resolution computer tomography (HRCT) scans of the considered bone structure. The fabric is computed from the mean intercept length [MIL, Odgaard, 1996] distribution and the orthotropic elastic properties are predicted from a Zysset/Cournier morphology-elasticity relationship [Zysset, 2003]. The

novelty of the presented work is that the fabric evaluation is done for thousands of cancellous bone regions such that, for example, the orthotropic material property map of a whole proximal femur is obtained. Highly efficient fabric computation routines are developed and a framework is introduced which generates a visual map of the orthotropic material property and/or produces the input for a finite element model.. In that way, more accurate and more efficient numerical investigations of models containing human trabecular bone are achievable.

References Odgaard A., et.al. 1996. Fabric and elastic principle directions of cancellous bone

are closely related. J. Biomech. 5: 487-495. Zysset EK., 2003. A review of morphology-elasticity relationship in human trabec-

ular bone: theories and experiments. J. Biomech 36: 1469-1485.

5972 We, 11:45-12:00 (P32) Mechanical properties of the patellar cartilage by an inverse FE approach from MR-monitored compression tests

S. Knecht 1 , R. Luechinger 2, E. Stuessi 1 . 1Laboratory for Biemechanics, ETH Zurich, Switzerland, 2Institute for Biomedical Engineering, University and ETH Zurich, Switzerland

Introduction: Up to now, mechanical properties of the patellofemoral articular cartilage (AC) have been determined using destructive, such as (un)confined compression, or invasive techniques. The aim of this study is to bypass this drawback and to develop and validate a novel approach to determine elastic mechanical properties of the patellar cartilage. The method is based on individual Finite Element (FE) models generated from Magnetic Resonance (MR)-monitored compression tests of the entire joint. Materials and Methods: Equine patellofemoral joints were loaded with 410 N for 2 h within a clinical 1.5T Magnetic Resonance machine. Accurate geomet- rical representations of the patellar and femoral cartilage, obtained from the MR-images using a semi-automatic segmentation software, were meshed and imported into a commercially available FE software. In this full joint model, AC was modeled as linear elastic and isotropic material. The material parameters were calculated by an inverse FE approach, fitting the experimental data with the results of the FE calculation. The cartilage was subsequently characterized using common confined compression and indentation tests and biochemical analysis. Results: The analysis of the inverse FE approach revealed that the Young's Modulus E is insensitive to experimental errors and to modeling errors. Ex- perimental results of the patellar cartilage revealed an aggregate modulus of HA=0.55±0.08MPa and a Young's modulus of E = 1.36±0.30MPa. The results of the ongoing MR-controlled patellofemoral compression tests will be presented at the meeting. Discussion: Our method offers a new possibility to assess articular carti- lage based on its mechanical properties noninvasively and in vive. Since mechanical equilibrium properties are very sensitive to degenerative or adap- tive processes, such a method might help to improve the understanding of cartilage plasticity and to prevent widespread joint cartilage diseases such as osteoarthritis. Acknowledgement: We would like to thank the ISB for financial support.

5607 We, 12:00-12:15 (P32) Comparison of performances in a bending test between a real volume and an ordinary cylindrical FE model of a single trabecula

S. Lorenzetti 1, K. Oberhofer 1 , C. Sprecher 2, E. StLissi 1 . 1Laboratory for Biomechanics ETH, Z~rich, Switzerland, 2AO Research Institute, Davos, Switzerland

The mechanical and structural properties of trabecular bone samples and the importance of the trabecular architecture is well recognized using ~tCT analysis (e.g. refs [1] and [2]). However, the material properties of single trabeculas are still unknown, although they essentially influence the mechanical behavior of the entire spongiosa [3]. The objective of the present study is to determine the Young's modulus of single trabeculas using FE calculations and experimental bending tests. As subordinate target, the performance of the real volume FE model was compared with a standard cylindrical model. Method: Vertebrae from in vivo labeled (calcium green, xylenolorange) sheep were cut into slices. A laser-scanning-microscope (LSM) equipped with a two photon laser, was used to generate image stacks of single trabecula. The images were segmented using software AMIRA, the CAD body was built using Raindrop Geomagic and the FE calculation were performed with ANSYS. As boundary condition for the bending tests no displacement for the side planes was assumed, the Poisson ratio was set to 0.3 and the Young's modulus to 18GPa. A force was applied centrally at the surface of the trabecula. The displacement d of the opposite point was compared between the real volume and the ordinary cylindrical FE model. Taking the asymmetric shape into account, two points in an angle of 90 degrees on the sample were selected.