Orthodontic tooth movement: Mechanical stimulus, cellular reactions and numerical bone remodeling simulation

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Journal o f Biomechanics 2006, Vol. 39 (Suppl 1)

design. The objective of this study is to use finite element modelling to investigate the influence of porosity on osteogenesis within a load-bearing scaffold. To simulate the bone regeneration process, a three dimensional stochastic model for cell proliferation/migration based on random walk theory [1], is combined with a mechano-regulation model for tissue-differentiation [2]. Every iteration consists of two distinct parts: (i) biphasic analysis which determines the biophysical stimuli for tissue differentiation in the FE model and (ii) the cell migration/proliferation process that is performed in a lattice within each granulation element. If a certain number of mesenchymal stem cells (MSC) have a cell age greater than the minimum time for differentiation then a percentage (based on the differentiation rate) will differentiate into fibroblasts, chondrocytes or osteoblasts depending on the mechanical stimulus. In turn cells can also proliferate and migrate. At the end of this process new material properties are calculated or updated, based on the number of cells and tissue phenotype. Another iteration is then submitted and the process is repeated. An optimal scaffold porosity can be determined for a given loading environment. It is believed that the technique used for modelling cell migration and proliferation improves on previous diffusion models and holds the potential for optimization of porous scaffold design for tissue engineering. References [1] Perez MA, et al. Journal of Biomechanics, in review. [2] Prendergast PJ, et al. Journal of Biomechanics 1997; 30: 539-548. 5690 Th, 14:45-15:00 (P45) 3D simulation o f damage repair along a single trabecular strut B.M. Mulvihill 1, L.M. McNamara 2, P.J. Prendergast 1. 1Centre for

Bioengineering, Trinity College Dublin, Ireland, 2Department of Mechanical Engineering, National University of Ireland, Galway, Ireland Experimental studies have shown that mechanical stimuli, such as damage and strain, can produce biomechanical remodelling in bone [1,2]. Various theories have been postulated regarding the mechanisms by which these stimuli regulate the process [3,4]. Generally, computer simulations implementing such theories have only considered a single control stimulus. However, bone cells may be responsive to both stimuli [5]. Indeed a previous computational algorithm, which hypothesised a combined strain and microdamage stimulus (where damage removal was prioritised), successfully predicted Bone Multicellular Unit (BMU) remodelling [6]. However, this hypothesis has only been tested in 2D and the complex 3D development of a BMU has not yet been simulated. In this study we test the validity of this hypothesis using a 3D finite element model to represent a trabecular strut. To examine repair of microdamage by a BMU, a region of bone was defined to have a pre-existing damage level. Physiological loads were applied and stimuli were calculated using two different mechano-sensors: osteocytes and bone-lining cells. Material properties were changed depending on the magnitude of stimuli, where remodelling was limited to surface elements. It was found that, due to the number of biological factors involved, it is necessary to implement a rule based algorithm [7] to successfully predict damage repair along a trabecular strut using the remodelling hypothesis. It is envisaged that this algorithm will be applied to voxel-based finite element models of trabecular bone to test whether in vivo remodelling can be simulated. References [1] Burr et al. J Biomech 1985; 18(3): 189. [2] Carter. Calcified Tissue Int 1984; 36: $19. [3] Cowin and Hegedus. J Elasticity 1976; 6(3): 313. [4] Huiskes et al. Nature 2000; 405: 704. [5] Prendergast and Huiskes. In: Bone Structure and Remodelling, Odgaard and Weinans (Eds.). 1995, p. 213. [6] McNamara and Prendergast. Eur J Morphol 2005; 42(1/2): 99. [7] Shefelbine et al. J Biomech 2005; 38(1): 2440. 5941 Th, 15:00-15:15 (P45) Simulation o f bone adaptation in rat tibiae based on a local criterion J.C. van der Linden, J.H. Waarsing, H. Weinans. Dept. ef orthopaedics,

Erasmus Medical Center, Rotterdam, The Netherlands Various hypotheses on how trabecular bone adapts its structure to the mechanical environment have been simulated. Although the results of many of these studies look 'real', validation with real in-vivo changes remains a challenge. We have compared changes resulting from bone remodeling in the proximal tibia of an aging rat with the result of simulated bone remodeling, based on a local criterion, in a model of this rats' tibia. The proximal tibia of a ten-month-old female Wistar rat was scanned in an invivo micro-CT scanner (Skyscan1076), at week 0, 4, 14, 34 and 54. The first scan was used to make the 3D input model for the simulation, of 20 20 20 ~tm cubic voxels of bone(1000 MPa), marrow(200 MPa) and cartilage(500 MPa).

Oral Presentations The materials were assumed to be isotropic and linearly elastic. Loading was applied to the tibial plateau through a model of the femoral condyles and articular cartilage. Bone adaptation was simulated in a stepwise process. At each step the stress and strain distribution over the elements was calculated. Next, the architecture was adapted, based on the local Maximum Principle Strain (MPS) distribution. All surface elements, trabecular and cortical, of which the MPS was more than 1, 0.5 or 1.5 standard deviation (SD) below the average MPS in the spherical neighborhood (radius 80 to 320~tm) around the element were removed. An extra neighboring element was added to each surface element of which the MPS was more than 1, 0.5 or 1.5SD above this average. Bone volume fraction, trabecular thickness and structure model index were determined for the in vivo and the simulated datasets. In vivo, bone was lost especially in the more distal regions, preserved trabeculae slowly increased in thickness and aligned across the growth-plate. The extreme bone loss in the more distal regions was not found in the simulation model. However, the simulations showed changes in the trabecular architecture similar to the changes in vivo, including removal of entire trabecular struts and alignment of trabeculae. 5798 Th, 15:15-15:30 (P45) Orthodontic tooth movement: Mechanical stimulus, cellular reactions and numerical bone remodeling simulation C. Bourauel, A. Kawarizadeh, W. G6tz, A. J~ger. Department ef Orthodontics,

University of Bonn, Germany Biomechanical research in the field of orthodontics started in the early sixties with work on the mechanical behaviour of orthodontic appliances and the numerical analysis of teeth under orthodontic loading. Since then the experimental and numerical resources have increased significantly. Nevertheless we are still far away from a full understanding of how the orthodontic load is transferred into a mechanical stimulus within the tooth's socket, initiating bone remodeling processes. In this study, combined experimental, numerical, and immunohistochemical investigations were performed to identify the initial biological reactions to the application of orthodontic force systems. In an animal-experimental part, the upper first molars of 25 anesthetized rats were loaded with forces of 0.1N. The forces were recorded with a highresolution force/torque transducer, and were kept constant for 15 min, lh, 2h, 4h or 8h. Finite element (FE) models of part of the specimens were generated based on ~tCT scans to analyse stress and strain distributions around the roots under the same force systems as in the experiment using the FE package MSC.Marc/Mentat. The other specimens served for the histologic and immunohistochemical studies. Besides others the following factors were examined to quantify the cell biological reactions in the periodontium: proliferation marker PCNA, osteoblastogenese marker core binding factor alpha-1 (Cbfal/Runx2) and extracellular regulated kinases 1/2 (pERK1/2). Correlation of mechanical and biological reactions in the periodontal ligament (PDL) and the bone was done with respect to stresses and strains and the above factors. In selected areas under tension, the proportions of Runx2-positve and pERK1/2-positive cells increased within 8 hrs of loading, whereas these proportions in selected areas under pressure were significantly lower than those in control teeth. Moreover there were no significant changes in the number of PCNA-positive cells. The results of this combined biomechanical and histological study indicate that there is a direct correlation of calculated strain values in the PDL with the distribution of osteoclasts in the alveolar bone and the PDL. This study was supported by Medical Faculty, University of Bonn, German Orthodontic Society and German Israel Foundation. 7344 Th, 16:00-16:15 (P46) The biomechanical environment o f a bone fracture during normal walking S. Mishra 1, T.N. Gardner 55, M. Schuetz 1, R. Steck 1. 1School efEngineering

Systems, Queensland University of Technology, Brisbane, Australia, 2School of Sports and Exercise Science, University of Birmingham, Birmingham, UK, 3Oxford Orthopaedic Engineering Centre, University of Oxford, Oxford, UK It has been shown that magnitude and distribution of stress and strain environment within the bridging tissue (callus) at bone fracture sites profoundly influences healing [1,2,3]. Previous Finite Element Models (FEM) have estimated the mechanical environment in callus at single point of loading assuming that the mechanical environment at peak loading or peak axial displacements will correspond to the stresses which in turn will guide tissue differentiation. Since lower limb experiences complex stresses during gait cycle therefore, in the present study stress environment after four weeks of fracture has been simulated by developing FEM's at 10 discrete points of time during stance phase of the gait cycle. The geometry, material properties, boundary condition have been adopted from the previous study of Gardner et al [3]. Ten sets of displacement data were