$428 Journal o f Biomechanics 2006, Vol. 39 (Suppl 1) Oral Presentations

the dentoalveolar segments and the mandibular condyles, together with the stresses and strains in the jaw provide the data for a complete pre-operative analysis of the maxillo-mandibular osteotomy.

References [1] Patel P.K., Han H. and Kang N-H. Craniofacial Orthognathic Surgery.

http://www.emedicine.com/plastic/topic177.htm, 2004.

7438 We, 14:45-15:00 (P35) Val idat ion o f image-enhanced in v ivo microCT based FE models by strain gauge measurements

L. Muraru 1 , S.V.N. Jaecques 1, J. Demol 1 , I. Naert 2, J. Vander Sloten 1 . 1K.U. Leuven, Division Biomechanics and Engineering Design, Leuven, Belgium, 2E.g. Leuven, Department of Prosthetic Dentistry, BIOMAT Research Group, Leuven, Belgium

The present study belongs to a project which aims to analyse bone adaptive response around early loaded oral implants. Usually, ex vivo 3D microfocus CT qtCT) images are used when bone architecture or bone adaptation of small specimens are analysed by finite element models (FEM). In this study, FEM based on in vivo ~tCT images of guinea pig tibiae with titanium percutaneous implants were built to investigate stress and strain distribution in the peri- implant bone. We report the validation of such FEM by ex vivo strain gauge measurements on four FE-modelled tibiae with implants. A strain gauge was glued on the bone surface very close to the implant, on the tensile side. The implant was loaded in bending and axial strain was measured under two conditions: the distal end of the bone was either free or supported while the proximal end was embedded in resin. The experimental set-up was reproduced in the FE simulations. FEM of the tibiae were built following a standard triangulated language (STL) approach. From the FEM, strain over a group of elements corresponding to the measuring grid of the strain gauge was calculated and then compared with the experimentally measured strain. The FEM predictions are consistent with the experiment: the lowest, resp. highest strains were calculated and correspondingly measured for the same bones. For all bones, higher strains were measured and correspondingly estimated by FEM when the distal end of the bone was free. Strains predicted by FEM were within 30% of the measured strains when the distal end of the bone was free and within a range of 50% when the bone was supported. In literature, accuracies of 15% are reported for high-resolution ex vivo ~tCT based FEM. Our FEM results can be considered as a reasonable estimation taking into account the limited image quality of the in vivo ~tCT scans. Acknowledgement: EU FP5 project QLK6-CT-2002~)2442 IMLOAD

4441 We, 15:00-15:15 (P35) Biomechanical s tudy o f maxi l lary expans ion treatment using bone-anchors - Three dimensional f inite element analyses

Y. Fang 1 , M.O. Lagrav~re 2, J. Carey 1 , P.W. Major 2, R.W. Toogood 1 . 1Mechanical Engineering, University of Alberta, Edmonton, Canada, 2 Orthodontic Graduate Program, Faculty of Medicine and Dentistry, University of Alberta, Edmonton, Canada

A new maxillary expansion (ME) treatment has been recently developed to correct maxillary transverse deficiencies. This treatment uses bone-anchors (BA) instead of teeth as abutments to apply expansion load directly on palatal bones, which eliminates the disadvantages of traditional ME treatments such as undesirable tooth movement, lack of firm anchorage to retain suture ex- pansion and limitation of the palatal bone movement. Using three dimensional finite element analyses (FEA), this study provided an effective tool to help dentists to understand and predict the biomechanical response of craniofacial bones, and to evaluate optimal BA inclination angles. The present study developed a three-dimensional FE model based on Cone- Beam Computerized Tomography (CBCT) scan image data of a dry skull. The developed FE model represents accurate geometric features and bone material property distribution of the skull. It consists of 297,800 nodes and 877,400 hexahedral and tetrahedral elements. A total of 156 bone materials (125 for cancellous bones and 31 for cortical bones) were defined based on the Houn- sefield Unit (HU) values of CBCT image. A cylinder type BA (3 mm diameter) was modeled and inserted in the palatal bone. Transverse displacement of 4 mm was applied on the BA to simulate BA-ME. The displacement, stress and strain distribution in 25 reference regions in craniofacial and maxillary bones was analyzed. The stress in 7 craniofacial sutures and strain in 3 craniofacial sutures obtained from the FEA displayed similar patterns as that from literature and our maxillary expansion experiment using fresh skull. Six BA inclination angles, three in Transverse-vertical (TV) plane and three in 15 degree-vertical (15V) plane (The axis of BA was perpendicular, upward rotating and downward rotating 10 degree to the surface of palatal bones respectively), were modeled and evaluated. It was shown that placing BA in TV plane and upward rotating BA in 15V plane are optimal to reduce stress induced in the

BA osseointergration interface, which will lead to a decrease in BA failure rates and eventually help dentists' success on treatments.

7668 We, 15:15-15:30 (P35) An automat ic method to generate patient speci f ic f inite e lement head model J. Ho, S. Kleiven. CTV - Centre for Technology in Health, Royal Institute of Technology, Huddinge, Sweden

An automatic method to generate a patient specific finite element head model is proposed in this paper. A 3D MR image of a healthy patient is used as input. Segmentation of different tissues is performed by an in-house expectation- maximization algorithm. The algorithm can compensate for intensity inhomo- geneities and is able to classify background, bone, cerebral spinal fluid (CSF), gray matter, white matter and adipose tissue. Meshing of the head utilizes a voxel based method resulting in an all hexahedral finite element mesh. The nodal points are classified according to their materials and hence, elements can be assigned different properties. The resulting head model comprises a detailed model of the skull, CSF, brain and the internal membranes. At this stage, smoothing was only applied to the outer-most boundary of the brain. The geometric accuracy of the resulting finite element head model was good based on visual comparison with the volume-rendered images of the original MR data, and was used in a computational analysis involving a dynamic deformation simulating a surgical procedure. We also compared the mesh generated using our method against a previously developed head model adapted for impact biomechanics applications.

References K. van Leemput, F. Maes, D. Vandermeulen, P. Suetens. Automated model-based

tissue classification of MR images of the brain. IEEE Transactions on Medical Imaging 1999; 18(10): 897-908.

R. Schneiders, R. B~inten. Automatic generation of hexahedral finite element meshes. Computer Aided Geometric Design 1995; 12: 693-707.

7676 Th, 08:15-08:30 (P39) Real in v ivo reconst ruct ion o f human coronary arteries '~S. Chatzizisis 1 , P. Diamantopoulos 2, A. Matakos 3, G.D. Giannoglou 1 . 1 Cardiovascular Engineering and Atherosclerosis Laboratory, I st Cardiology Department, AHEPA University Hospital, Medical School, Aristotle University of Thessaloniki, Thessaloniki, Greece, 2Biomedical Modelling Unit, Department of Engineering and Design, School of Science and Technology, University of Sussex, UK, 3School of Polytechnics, Aristotle University of Thessaloniki, Thessaloniki, Greece

Intravascular ultrasound (IVUS) has become increasingly important in the imaging of coronary atherosclerosis. IVUS is a catheter-based imaging tech- nique that enables three-dimensional (3D) reconstruction of coronary arteries, thus permitting the reliable calculation of plaque burden. However, IVUS ignores the vessel curvature and the axial movements of the catheter. To overcome these limitations, a new in-vivo imaging technique is proposed that has been developed combining the information about vessel cross-sections, obtained from IVUS, with the information about vessel three-dimensional geometry, derived from biplane coronary angiogram (BCA). The introduction of the IVUS catheter in the coronary artery and the acquisition of two perpendicular BCA projections is the initial step of the method. The IVUS catheter trajectory and the coronary lumen outline are then semi-automatically marked at each 2D projection. IVUS is also performed and the ultrasound images along with the ECG signal for synchronization are recorded on a S- VHS videotape. The S-VHS ultrasound data are digitized and the end-diastolic frames (R-wave) are selected. The lumen and external elastic membrane (EEM) contours are then detected in each filtered image using a custom- developed semi-automatic algorithm based on active contour models. Each pair of lumen and EEM contour is placed perpendicularly to the catheter's 3D trajectory and is orientated in space according to the Frenet-Serret rules. The absolute orientation of the contours is established with back-projection of the reconstructed lumen from different rotational angles onto the angiographic images. Finally, the real lumen and wall 3D volume generation is implemented with interpolation of lumen and EEM contours, respectively. The described method was validated in 17 large segments of human coronary arteries and showed high in vivo feasibility and accuracy. The 3D reconstruction of coronary arteries comprises a reliable imaging tech- nique that can be utilized for plaque planimetric and volumetric analyses, real 3D geometry estimation and intracoronary flow simulation, with the application of Computational Fluid Dynamics (CFD) rules.