Cornelia Kober

Micromechanics-Based Conversion of CT Data into Anisotropic Elasticity Tensors, Applied to FE Simulations of a Mandible

Computer Tomographic (CT) image data have become a standard basis for structural analyses of bony organs. In this context, regression functions between stiffness components and Hounsfields units (HU) from CT, related to X-ray attenuation coefficients, are widely used for the definition of the (actually inhomogeneous and anisotropic) material behavior inside the organ. Herein, we suggest to derive the functional dependence of the fully orthotropic stiffness tensors on the Hounsfield units from the physical information contained in the X-ray attenuation coefficients: (i) Based on voxel average rules for the X-ray attenuation coefficients, we assign to each voxel the volume fraction occupied by water (marrow) and that occupied by solid bone matrix. (ii) By means of a continuum micromechanics representation for bone, which is based on voxel-invariant (species and whole bone-specific) stiffness properties of solid bone matrix and of water, we convert the aforementioned volume fractions into voxel-specific orthotropic stiffness tensor components. The micromechanics model, in combination with the average rule for X-ray attenuation coefficients, predicts a quasi-linear relationship between axial Young’s modulus and HU, and highly nonlinear relationships for both circumferential and radial Young’s moduli as well as for the shear moduli in all principal material directions. Corresponding whole-organ Finite Element (FE) analyses of a partially edentulous human mandible characterized by atrophy of the alveolar ridge show that volumetric strain concentrations/peaks within the organ are decreased when considering material anisotropy, and increased when considering material inhomogeneity.

Topography and morphology of the mandibular condyle during fixed functional orthopedic treatment --a magnetic resonance imaging study.

Background and Aim:Fixed functional orthopedic appliances used in 6 to 9 month-long treatments to correct distoclusion keep the mandible permanently in the therapeutically-desired protruded position. The principal aim of this approach is to achieve an increase in length of the lower jaw by stimulating mandibular growth. Ideally, adaptive mechanisms in adolescents and young adults result in condylar remodeling. Alternatively, however, therapeutically-undesirable alterations in the condyle position within the articular fossa may also occur. Thus the aim of this magnetic resonance imaging (MRI) study was twofold: 1) to verify the effects that the treatment with a fixed functional orthopedic appliance used to correct distoclusion has on the topographic relationship of mandibular condyle and glenoid fossa, and 2) to analyze morphologic changes in the condyle.Patients and Methods:Treatment progress in 20 patients was monitored by MRI at four defined points in time. Visual inspection and metric analysis were performed in three planes (axial, frontal, parasagittal) shown on the MRIs. 3D-reconstruction of the condyle surfaces based on the MRI data sets at hand was done in selected cases.Results:Upon assuming the therapeutically-desired position, the condyles were caudally and ventrally displaced from their centric position within the fossa. At the end of treatment, they had returned to their original position. When assessed laterally, statistical analysis revealed no significant differences between the joints on the right and left sides. Neither the anterior nor posterior joint space among all 40 joints exhibited significant changes in width compared to the baseline findings. Visual inspection in the axial, frontal, and sagittal planes revealed changes in the exterior form of 31 of the 40 condyles analyzed. On the whole, while morphologic changes were observed in all three planes, they were most marked in the axial plane. Metric analysis of the 2D-MRIs, on the other hand, revealed no significant changes in width, depth, or height in the plane in question. This is why we reconstructed the condyle structure three-dimensionally on a trial basis. For purposes of analysis, we super imposed the reconstructions of the condyle surfaces at the various control points on each other. By processing the data in this manner, an alternative approach for evaluating morphologic changes was created.Conclusions and Prospects:In patients treated with a rigid, fixed functional orthopedic appliance (FMA) for skeletal Class II malocclusion, both joints returned to a physiologic condyle-fossa relationship post-treatment. The improved occlusion was not achieved at the price of unphysiologic repositioning in the temporomandibular joint. Visual inspection suggested that morphologic changes in the condyle may have occurred as treatment progressed, but this was not confirmed by 2D metric analysis. However, by means of 3D-reconstruction of the condylar surfaces and their superposition, detailed visualization of adaptive mechanisms and their non-invasive evaluation in 3D may become feasible in clinical routine.ZusammenfassungHintergrund und Ziel:Mit festsitzenden funktionskieferorthopädischen Apparaturen wird der Unterkiefer während einer ca. 6 bis 9 Monate dauernden Behandlung zur Korrektur von Distalbisslagen permanent in einer therapeutischen Vorschublage gehalten mit dem Hauptziel, eine Vergrößerung der Unterkieferlänge über eine Stimulation des Unterkieferwachstums zu erreichen. Idealerweise erfolgen bei Jugendlichen und jungen Erwachsenen im Bereich der Kondylen Remodellationen als adaptive Mechanismen. Alternativ sind aber auch therapeutisch unerwünschte Änderungen der Kondylenpositionen innerhalb der Fossae articulares möglich. Ziel der vorliegenden magnetresonanztomographischen (MRT) Studie war es deshalb zu überprüfen, welche Auswirkungen einerseits eine Behandlung mit festsitzenden funktionskieferorthopädischen Apparaturen bei der Korrektur von Distalbisslagen auf die Topographie des Condylus mandibulae zur Fossa mandibularis hat; andererseits sollten die morphologischen Veränderungen der Kondylen analysiert werden.Patienten und Methoden:Die Behandlungsverläufe von 20 Patienten wurden zu vier definierten Zeitpunkten magnetresonanztomographisch kontrolliert. Eine visuelle Befundung und eine metrische Auswertung der MRTs erfolgten in drei verschiedenen Ebenen (axial, frontal, parasagittal). Die vorliegenden MRT-Daten dienten bei ausgewählten Beispielen als Grundlage zur dreidimensionalen Darstellung der Kondylusoberflächen.Ergebnisse:Durch das Einnehmen der therapeutischen Position wurden die Kondylen aus den Zentren der Fossae nach kaudal und ventral ausgelenkt. Bei Behandlungsabschluss waren sie wieder in die Ausgangspositionen zurückgekehrt. Die statistische Auswertung ergab auch im Seitenvergleich zwischen rechten und linken Gelenken keinen signifikanten Unterschied. Hinsichtlich der anterioren und posterioren Gelenkspaltbreiten ergaben sich bei allen 40 Gelenken bei Behandlungsabschluss keine signifikanten Veränderungen zum Ausgangsbefund. Die visuelle Befundung in der Axial-, Frontal- und Sagittalebene zeigte Variationen der äußeren Form bei 31 von 40 möglichen Kondylen. Grundsätzlich waren morphologische Veränderungen in allen drei Ebenen erkennbar, wobei sich diese insbesondere in der Axialebene darstellten. Die metrische Analyse der zweidimensionalen MRT-Darstellungen ergab in den befundeten Ebenen hingegen keine signifikanten Veränderungen von Breite, Tiefe und Höhe. Deswegen erfolgten exemplarische dreidimensionale Rekonstruktionen der Kondylenstrukturen. Die Oberflächenrekonstruktionen der Kondylen zu den einzelnen Kontrollzeitpunkten wurden zur Untersuchung der Unterschiede miteinander überlagert. Durch diese Aufbereitung des Datenmaterials wurde ein alternativer Ansatz zur Bewertung von morphologischen Veränderungen geschaffen.Schlussfolgerungen und Ausblick:Bei Patienten mit einer skelettalen Distalbisslage kommt es nach Abschluss einer funktionskieferorthopädischen Behandlung mit einer starren, festsitzenden Apparatur (FMA) zur Restitution einer beidseitig physiologischen Kondylus-Fossa-Relation. Die verbesserte Okklusion wird nicht auf Kosten von unphysiologischen Lageveränderungen in den Temporomandibulargelenken erzielt. Die visuelle Befundung der Magnetresonanztomogramme gibt Hinweise auf morphologische Veränderungen der Kondylen im Verlauf der Behandlung, die jedoch durch eine zweidimensionale metrische Analyse nicht bestätigt werden können. Mit Hilfe dreidimensionaler Oberflächenrekonstruktionen der Kondylen und deren Überlagerung könnte es jedoch routinemäßig möglich werden, adaptive Mechanismen detailliert zu visualisieren und noninvasiv am 3-D-Objekt zu bewerten.

Influence of different modeling strategies for the periodontal ligament on finite element simulation results

INTRODUCTION The finite element method is a promising tool to investigate the material properties and the structural response of the periodontal ligament (PDL). To obtain realistic and reproducible results during finite element simulations of the PDL, suitable bio-fidelic finite element meshes of the geometry are essential. METHODS In this study, 4 independent coworkers generated altogether 17 volume meshes (3-dimensional) based on the same high-resolution computed-tomography image data set of a tooth obtained in vivo to compare the influence of the different model generation techniques on the predicted response to loading for low orthodontic forces. RESULTS It was shown that the thickness of the PDL has a significant effect on initial tooth mobility but only a remarkably moderate effect on the observed stress distribution in the PDL. Both the tooth and the bone can be considered effectively rigid when exploring the response of the PDL under low loads. The effect of geometric nonlinearities could be neglected for the applied force system. CONCLUSIONS Most importantly, this study highlights the sensitivity of the finite element simulation results for accurate geometric reconstruction of the PDL.

Correspondences of hydrostatic pressure in periodontal ligament with regions of root resorption: A clinical and a finite element study of the same human teeth

INTRODUCTION The main objectives of this study were to generate individual finite element models of extracted human upper first premolars, and to simulate the distribution of the hydrostatic pressure in the periodontal ligament (PDL) of these models for evaluation of the risk of root resorption. METHODS The individual extracted teeth were from a previous in vivo study that investigated root resorption after application of continuous intrusive forces. The results of experimental examination and simulations were compared on these identical tooth roots. The applied force system was 0.5N and 1.0N of intrusive force. RESULTS The simulated results during intrusion of 0.5N showed regions near the apical thirds of the roots with hydrostatic pressure over the human capillary blood pressure. These regions correlated with the electron microscopies of previous studies performed in Brazil with the identical teeth. An increased force of 1.0N resulted in increased areas and magnitudes of the hydrostatic pressure. CONCLUSIONS The key parameter indicating beginning root resorption used in this study was an increased value for hydrostatic pressure in the PDL.

Metal-induced streak artifact reduction using iterative reconstruction algorithms in x-ray computed tomography image of the dentoalveolar region.

OBJECTIVE The objective of this study was to reduce metal-induced streak artifact on oral and maxillofacial x-ray computed tomography (CT) images by developing the fast statistical image reconstruction system using iterative reconstruction algorithms. STUDY DESIGN Adjacent CT images often depict similar anatomical structures in thin slices. So, first, images were reconstructed using the same projection data of an artifact-free image. Second, images were processed by the successive iterative restoration method where projection data were generated from reconstructed image in sequence. Besides the maximum likelihood-expectation maximization algorithm, the ordered subset-expectation maximization algorithm (OS-EM) was examined. Also, small region of interest (ROI) setting and reverse processing were applied for improving performance. RESULTS Both algorithms reduced artifacts instead of slightly decreasing gray levels. The OS-EM and small ROI reduced the processing duration without apparent detriments. Sequential and reverse processing did not show apparent effects. CONCLUSIONS Two alternatives in iterative reconstruction methods were effective for artifact reduction. The OS-EM algorithm and small ROI setting improved the performance.

Modelling and simulation of acrylic bone cement injection and curing within the framework of vertebroplasty

The minimal invasive procedure of vertebroplasty is a surgical technique to treat compression fractures of vertebral bodies. During the treatment, liquid bone cement gets injected into the affected vertebral body and therein cures to a solid. In order to investigate the treatment and the impact of injected bone cement, an integrated modelling and simulation framework has been developed. The framework includes (i) the generation of microstructural computer models based on microCT images of human cancellous bone, (ii) computational fluid dynamics (CFD) simulations of bone cement injection into the trabecular structure and (iii) non-linear finite element (FE) simulations of the subsequent bone cement curing. A detailed description of the material behaviour of acrylic bone cements is provided for both simulation stages. A non-linear process-dependent viscosity function is chosen to represent the bone cement behaviour during injection. The bone cements phase change from a highly viscous fluid to a solid is described by a non-linear viscoelastic material model with curing dependent properties. To take into account the distinctive temperature dependence of acrylic bone cements, both material models are formulated in a thermo-mechanically coupled manner. Moreover, the corresponding microstructural CFD- and FE-simulations are performed using thermo-mechanically coupled solvers. An application of the presented modelling and simulation framework to a sample of human cancellous bone demonstrates the capabilities of the presented approach.

Near-real time oculodynamic MRI: a feasibility study for evaluation of diplopia in comparison with clinical testing

AbstractObjectiveTo demonstrate feasibility of near-real-time oculodynamic magnetic resonance imaging (od-MRI) in depicting extraocular muscles and correlate quantitatively the motion degree in comparison with clinical testing in patients with diplopia.MethodsIn 30 od-MRIs eye movements were tracked in the horizontal and sagittal plane using a a TrueFISP sequence with high temporal resolution. Three physicians graded the visibility of extraocular muscles by a qualitative scale. In 12 cases, the maximal monocular excursions in the horizontal and vertical direction of both eyes were measured in od-MRIs and a clinical test and correlated by the Pearson test.ResultsThe medial and lateral rectus muscles were visible in the axial plane in 93% of the cases. The oblique, superior and inferior rectus muscles were overall only in 14% visible. Horizontal (p = 0,015) and vertical (p = 0,029) movements of the right eye and vertical movement of the left eye (p = 0,026) measured by od-MRI correlated positively to the clinical measurements.ConclusionsOd-MRI is a feasible technique. Visualization of the horizontal/vertical rectus muscles is better than for the superior/inferior oblique muscle. Od-MRI correlates well with clinical testing and may reproduce the extent of eye bulb motility and extraocular muscle structural or functional deteriorations. Key Points• Oculodynamic MRI technique helps clinicians to assess eye bulb motility disorders• MRI evaluation of eye movement provides functional information in cases of diplopia• Oculodynamic MRI reproduces excursion of extraocular muscles with good correlation with clinical testing• Dynamic MRI sequence supplements static orbital protocol for evaluation of motility disorders

3D-visualization of the temporomandibular joint with focus on the articular disc based on clinical T1-, T2-, and proton density weighted MR images

ObjectiveAn approach of 3D-visualization of the temporomandibular joint (TMJ) with special focus on the articular disc based on magnetic resonance imaging (MRI) was developed for the purpose of diagnosis support.Materials and methodsMandibular condyle and fossa were reconstructed as 3D-surfaces. Articular disc, retrocondylar tissue, and the lateral pterygoid muscle were visualized by means of direct volume rendering. By simultaneous visualization of both, the bony surfaces and the soft tissue, anterior disc displacement could be recognized in 3D-context. Additional superposition of the 3D-visualization with the original 2D-MRI slices allowed for a combination with conventional diagnostics. The method was tested for clinical T1-, T2-, and proton density weighted MRI data from four independent medical institutions.ResultsFor all cases, the skeletal anatomy could be reproduced. Applied validation approaches showed good results. Anterior disc displacement could be clearly depicted as well as the incidence of reduction of the disc. By several experienced observers, the approach was rated as significant.ConclusionAlthough partially non-standard in the clinical routine the new method provided promising results for efficient diagnosis support. Its validity in the medical practice, namely, its impact for dislocation/deformity of the mandibular disc will be further analyzed.

Globe restriction in a severely myopic patient visualized through oculodynamic magnetic resonance imaging (od-MRI)

Different mechanisms have been hypothesized as contributing to abduction deficit in high myopia: the size of the eye within the orbit, tightness of the medial rectus muscles, decompensation of longstanding esotropia, and inferior displacement of the lateral rectus muscle. Using oculodynamic magnetic resonance imaging, enhanced by computer-aided visualization, we demonstrate globe restriction by the medial orbital wall on abduction in a patient with high myopia.

Segmentation and visualization of the inner structure of craniofacial hard tissue

Abstract In the biomedical context, image segmentation is the process of dividing an image into different subregions, usually different organs or tissue types. But mostly, the gray values of the inner voxels and, by this, the major part of the information remain unused. The purpose of this work is a new way of segmentation and visualization of these voxels. Concerning treatment planning for clinical cases, we deliver a macroscopic view of bone respective tissue quality. For the simulation branch, we contribute to an improved individual inhomogeneous and anisotropic description of the material law.