Functional imaging and CFD analysis to perform maxillo-mandibular osteotomy

Thread 1. Computational Methods in Biomechanics and M e c h a n o b i o l o g y Results: The FE calculations resulted in a ratio of d(cylinder)/d(real) = 1.6 and 1.4. The results indicate that the deflection of a trabecula is overestimated by a factor of -1.5 by using a cylindrical model. Outlook: Bending tests of single trabeculas will be performed using a micro stepper, a force sensor and a LSM. The obtained displacements will then serve as crucial input parameters for the determination of the Young's modulus of single trabeculas using real volume FE models. References [1] Ulrich et al. Bone 1999, 25(1): 55-60. [2] Borah et al. The Anatomical Record 2001; 265(2): 101-110. [3] Lucchinetti et al. Journal of Materials Science 2000; 35(24): 6057-6065. 7072 We, 12:15-12:30 (P32) A simple logic based method for segregation of grey matter and white matter in 3D finite element human head model from CT scan data S. Sarkar 1, U.B. Ghosh 2, A. Roychowdhury 1. 1Department of Applied

Mechanics, Bengal Engineering and Science University, Shibpur, India, 2School of BioScience and Engineering, Jadavpur University, Kolkata, India Grey Matter and White Matter of human head exhibit different material behaviour, which necessitates separation of them in finite element model of head aimed at accidental brain injury study. Volumetric segregation of White Matter (WM) and Grey Matter (GM) from CT or MRI scan images of human head and subsequent 3D FE modeling of it is very difficult owing to the formation of innumerable small volumes [1]. A different approach, based on the principle of 'element-to-element material property definition from medical image data' has been utilized here to do that. A set of human head CT images in DICOM format, with resolution 0.391 0.391 5 has been used to produce a pixel co-ordinate vs. intensity (in Hounsfield Unit or HU) data set. Cerebrum and cerebellum have been modeled through standard image processing and subsequent FE model generation techniques, utilizing same CT images. HU ranges are specified for WM and GM in consultation with medical persons and literature. The data set is substructured into WM and GM and every WM and GM pixel is tagged with an element that it belongs to. Every element of cerebrum and cerebellum is then assigned as GM or WM element depending on the majority of corresponding pixels within it. A moderately fine meshing of human head (182,633 elements altogether) and a 0.391 0.391 1 gray value map (linearly interpolated from 5mm CT slice thickness) produce good agreement (around 90% 2D regional overlap) between a 2D slice and the corresponding FE section. References [1] Ruan J.S., Zhou C., Khalil T.B., King A.I. Techniques and applications of finite element analysis of the biomechanical response of the human head to impact. In: C.T. Leondes (Ed.), Computer Techniques and Computational Methods in Biomechanics, Biomechanic Systems -Techniques and Application. 2001 ; CRC Press LLC, Vol. 1, pp. 1-11. 7677 We, 14:00-14:15 (P35) Image-based biomechanical modelling in oral and maxillofacial practice D. Trikeriotis 1, P. Diamantopoulos 2. 1Oral and Maxillefacial Unit, IASO

General Clinic, Athens, Greece, 2Medical School, University of Athens, Greece The clinical importance of biomechanical computational methods has been established during the last decade, providing practical answers in a number of medical issues. Even though, the application of engineering computational techniques in clinical practice is still regarded as a research tool rather than a functional clinical facility. The purpose of this paper is therefore to present the experience obtained using image-based modelling techniques for the need of specific cases in oral and maxillofacial surgery. The process of producing such models in the treatment planning and their utility to the diagnostic and surgical procedures are presented. Medical imaging was utilised as an input source to software systems that provide virtual simulation and prototyping facilities, as well as a link to computer aided design and rapid manufacturing environments (MIMICS-SIMPLANT, Materialise N .V.). Particular attention was given to the application of those systems for pathologies requiring accurate 3D anatomical visualisation, pre-operative planning of surgical interventions, or design and manufacturing of custommade implants. Oral and maxillofacial representative cases were selected and treated: post-traumatic deformity, congetinal deformity, bone carcinoma, bone cyst, alveolar distraction osteogenesis. By applying image-based modelling techniques, a number of benefits were demonstrated within the clinical environment. In all cases, these methods led to a more accurate diagnosis of the pathology and the prediction of subsequent surgical manipulations. The time of the actual surgery was reduced and the treatment success rate was invariably maximised. In general, the experience obtained following treatment of the presented cases indicates that computa-

T1.14 Image-based Anatomical Modelling for CAD/FEA Applications

$427

tional techniques lead to a less invasive, more controlled, safe and effective surgery. This is possible by utilising relatively simple but reliable image-based biomechanical methods and without employing computer-assisted navigation or robotic surgery systems. 6798 We, 14:15-14:30 (P35) A route for digital design and manufacturing of customised maxillofacial implants S. Lohfeld 1, P. McHugh 1, D. Serban 2, D. Boyle 2, G. O'Donnell 2, N. Peckitt 3.

1National Centre for Biomedical Engineering Science and Department of Mechanical and Biomedical Engineering, National University of Ireland, Galway, Ireland, 2Department of Mechanical~Industrial Engineering, Galway-Mayo Institute of Technology, Galway, Ireland, 3 ComputerGen Implants Ltd., St Chad's House, Heeton Pagnell, Dencaster, UK Rapid Prototyping (RP) techniques as a part of Engineering Assisted Surgery TM enables manufacturing of customised implants and prostheses prior to surgical procedures. Using CT or MRI scans, a biomodel of the bone structure of an individual patient can be generated quickly on which a prosthesis can be designed. Though the prosthesis' shape will be based on the patient's individual bone structure, surgical procedures are not necessary until the prosthesis is implanted, which significantly reduces surgery time. Peckitt reported on such a prosthesis in [1]. However, significant improvements in accuracy and design were achieved by digitising the design route of an existing maxillofacial implant developed by Peckitt, and in working with a virtual biomodel the fabrication of a physical biomodel was avoided. As a result, several interchanges between physical and virtual biomodels, which are prone to introduce geometrical errors, were also avoided. The prosthesis was designed using CAE software, based on the virtual biomodel generated from the CT scans, which allows one to preserve the facial aesthetics very well even when large resections are necessary. Having digital models, a finite element analysis of the prosthesis was easily introduced into the design route to optimise the shape for mechanical requirements and find potential possibilities for weight reductions by a leaner profile. The analysis gave an estimation for the minimum wall thickness of the prosthesis, depending on the material used. Eventually, the prosthesis was rapid manufactured from titanium and a quality control system was defined to check the implant's dimensional accuracy. What resulted was a streamlined and completely computer based process for design and manufacture, allowing high accuracy and flexibility in the geometrical design of the implant. References [1] N.S. Peckitt. Stereoscopic lithography: customized titanium implants in orofacial reconstruction. A new surgical technique without flap cover. British Journal of Oral & Maxillofacial Surgery 37(5): 353-369. 5440 We, 14:30-14:45 (P35) Functional imaging and CFD analysis to perform maxillo-mandibular osteotomy W. Vos 1,3, J. De Backer 1,3, A. Devolder3, W. Okkerse 2, W. De Backer 3.

1University of Antwerp, Department of Physics, Antwerp, Belgium, 2 University Hospital of Antwerp, Department of Dentistry, Antwerp, Belgium, 3University Hospital of Antwerp, Department of Pulmonology, Antwerp, Belgium A maxillo-mandibular osteotomy is a method to restore the proper anatomic and functional relationship in patients with dentofacial skeletal anomalies [1]. Unlike many surgical procedures, the outcome of a maxillo-mandibular osteotomy depends not only on the surgical procedure but also on a multitude of factors that begin long before the actual surgery as well as on control of the variables long after surgery. A careful preparation (analysis of the soft tissue with clinical examination and supporting photographs, skeletal evaluation with standardized radiographs, and dental evaluation with study dental casts) is needed and a pre-operative simulation facilitates and increases the success rate of the surgery. However, a pre-operative simulation, performed in an articulator, is a time consuming process. Anatomical modeling based on computer tomography (CT) images coupled with finite element analysis (FEA) can assist in improving this procedure. High resolution multi slice CT scans of both the patient's maxillo-mandibular region of the skull and the patient-specific dental cast, are merged into one high definition 3D model with the Materialise MIMICS software. Within MIMICS both the maxillo- and mandibular dentoalveolar segments are virtually separated from the jaw, as required for the simulation of the osteotomy, and the complete model is exported. After the generation of a volumetric grid the model is imported in the ABAQUS FEA package where the repositioning of the dentoalveolar segments is simulated. This repositioning induces, due to the contact between the dentoalveolar segments and the remainder of the jaw, a movement of the mandibular condyles. With the ABAQUS FEA package the dentoalveolar segments can be put in a desired position while the movement of the mandibular condyles stays within accepted limits. The new position of

$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.

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

References [1] Patel P.K., Han H. and Kang N-H. Craniofacial Orthognathic Surgery. http://www.emedicine.com/plastic/topic177.htm, 2004.

7668 We, 15:15-15:30 (P35) An automatic method to generate patient specific finite element head model J. Ho, S. Kleiven. CTV - Centre for Technology in Health, Royal Institute of Technology, Huddinge, Sweden

7438 We, 14:45-15:00 (P35) Validation o f image-enhanced in vivo 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 periimplant 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 study o f maxillary expansion treatment using bone-anchors - Three dimensional finite element analyses Y. Fang 1, M.O. Lagrav~re 2, J. Carey 1, P.W. Major2, 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 expansion 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 ConeBeam 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 Hounsefield 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

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 expectationmaximization algorithm. The algorithm can compensate for intensity inhomogeneities 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 vivo reconstruction o f human coronary arteries '~S. Chatzizisis 1, P. Diamantopoulos 2, A. Matakos 3, G.D. Giannoglou 1. 1Cardiovascular 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 technique 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 SVHS 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 customdeveloped 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 technique 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.