EXPERIMENTAL INVESTIGATION OF THE MECHANICAL

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  • THSE NO 2902 (2003)

    COLE POLYTECHNIQUE FDRALE DE LAUSANNE

    PRSENTE LA FACULT SCIENCES ET TECHNIQUES DE L'INGNIEUR

    Institut d'ingnierie des systmes

    SECTION DE GNIE MCANIQUE

    POUR L'OBTENTION DU GRADE DE DOCTEUR S SCIENCES

    PAR

    Bachelor of Engineering - Metallurgical, McGill University, Canadaet de nationalit canadienne

    accepte sur proposition du jury:

    Prof. I. Botsis, directeur de thseDr U. Belser, rapporteur

    Prof. M. Chiba, rapporteurProf. A. Curnier, rapporteur

    Dr D. Pioletti, rapporteur

    Lausanne, EPFL2003

    EXPERIMENTAL INVESTIGATION OF THE MECHANICALBEHAVIOUR AND STRUCTURE OF THE BOVINE

    PERIODONTAL LIGAMENT

    Colin SANCTUARY

  • I wish to express my gratitute to my thesis director, Prof. John Botsis, for his guidance,weekly meetings and fruitful discussions during the work of this thesis. A study of this naturewould not have been possible had it not been for his understanding, patience and kindness.

    I would also like to thank Prof. Alain Curnier for his comments and contribution throughoutthe progression of this work. His advice influenced the way in which I approached numerousproblems. Jrn Justiz also deserves particular thanks for his ongoing help and frequent advicethat contributed to the development of the project; remember myatt, Ticino and thenumerous nicknames we came up with to describe the PDL.

    The members of the PDL group deserve my thanks for their contribution on the medical andbiological aspects: Prof. Urs Belser, Dr. Anselm Wiskott, Dr. Susanne Scherrer, Dr. DieterBosshardt, and Dr. Tatsuya Shibata. I express my thanks also to the members of the jury fortheir comments. And to the Swiss National Science Foundation Grant no. 3152-055863.98,and REquip 2001 21-64562.01 for financing this study.

    The design of the machines and experiments would not have been possible without theexpertise of our workshop team, particularly to Gino Crivellari and Marc Jeanneret.

    To my LMAF colleagues with whom I have had many, many coffee breaks: Assa, Anna,Anne, Brian, Christian, Eric, Fabiano, Federico, Gabriel, Jarek, Joel, Larissa, Laurent,Matteo, Michel, Mostafa, Paola, Philippe B., Philippe Z., Stefan, Thomas G., Thomas R.

    To my friends who shared those many moments during my four years in Lausanne; momentsin the Alps, moments on/in the lake, countless moments. All these moments have made upfour very precious years of my life, and though they cannot be seen in this thesis, they verymuch fill the spaces between the lines of this work.

    A special thanks to my loving family. Look to the runes.

    A c k n o w l e d g e m e n t s

  • i

    One of the key problems in dental biomechanics is the prediction of tooth mobility underfunctional loads. Understanding tooth displacement due to load is becoming more importantas new solutions in dental restorations, prosthodontics and orthodontic treatments becomeincreasingly more advanced. The mechanical characterization of the alveolar bone, the toothand the periodontal ligament surrounding the root of the tooth, is necessary to predict toothmobility. The common assumption is that the periodontal ligament acts as the major elementin the stress distribution to the supporting tissues. Obtaining parameters that describeperiodontal ligament mechanical behaviour is a challenging problem. Isolating the tissue fortesting, the small size of the specimen, and the necessity to maintain, as much as possible,the ligament in vitro under normal physiological conditions, are all factors that contribute tothe complexity of the problem. The aim of this thesis is twofold and can be subdivided intotwo primary objectives. The first objective is to describe its morphology, anatomy, histologyand structure of the components in order to determine its geometric parameters at differentlength scales. The second objective is to determine its mechanical properties by identifyingkey parameters through shear and uniaxial tension-compression tests.Four studies are performed to describe the morphology, anatomy and histology of theperiodontal ligament. First, macroscopic and microscopic measurements of the tooth, bone,and the periodontal ligament are obtained. Second, a bovine first molar system isreconstructed in three dimensions from microcomputerized tomography scans. Third, themorphology of the ligament is observed during deformation using optical microscopy.Fourth, the histology of bovine periodontium tissue is investigated. In order to characterise the mechanical behaviour of the bovine periodontal ligament,custom- designed machines and gripping devices are constructed to subject specially-prepared tissue specimens to shear and uniaxial tension-compression experiments. Shearexperiments are performed on 2 millimetre thick transverse sections, and specimens of tooth-ligament-bone of approximately 8x5x2 millimetres for uniaxial experiments. All specimensare obtained from first molar sites of freshly slaughtered bovines. In both shear and uniaxialtesting, the specimens are subjected non-destructively to preconditioning, stress-relaxation,constant strain rate, and sinusoidal loading profiles before testing to rupture.The experiments reported in this thesis elucidate geometrical and mechanical characteristicsof the periodontal ligament. Concerning its geometry, a variation in collagen fibre orientationis observed in transverse sections, moreover the symmetry of the shear tests in the apico-coronal direction suggests the periodontal ligament is vertically isotropic. Uniaxialspecimens, however, may be considered to be transverse isotropic. Concerning itsmechanical behaviour, the periodontal ligament is nonlinear viscoelastic in that it exhibitsstiffening, nonlinear elasticity, and nonlinear pseudo-plastic viscosity. The interactionbetween various constituents of the periodontal ligament (collagen fibres, blood vessels,interstitial fluid etc...) during deformation contribute to the observed stress-strain responseof this tissue. A nonlinear viscoelastic model presented in the literature, the Power Law,adequately simulates the nonlinear behaviour of the periodontal ligament using finiteelement analysis.

    A b s t r a c t

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  • iii

    La prdiction de la mobilit de la dent est une des majeures difficults de la biomcaniquedentaire. Une meilleure comprhension des mcanismes de dplacement d'une dent souscharge est importante dans l'avancement des technologies dans les domaines des restorationsdentaires, des implants dentaires, et de l'orthodontologie. Afin de prdire le mouvementd'une dent, il est ncessaire de caractriser les proprits mcaniques de l'alveolus, de la dentet du ligament dentaire entourant la racine de la dent. Le ligament dentaire affecteconsidrablement la distribution de contraines dans les tissus entourant la dent. Lesparamtres qui dcrivent les proprits mcaniques du ligament dentaire sont ce jourinconnus. La reproduction in vitro des conditions physiologiques, la prparation del'echantillon, contribuent la difficult du problme. Le but de cette thse est, dune part decaractriser la morphologie, lanatomie, lhistologie et la structure du ligament dentaire afinde dterminer sa configuration gometrique, et dautre part, de dterminer ses propritsmcaniques en soumettant le ligament dentaire aux expriences de cisaillement et detraction-compression.

    Quatres tudes sont effectues pour dcrire la morphologie, l'anatomie et l'histologie duligament dentaire du bovin. Dans un premier temps, des mesures macroscopiques etmicroscope de la dent, de l'os et du ligament sont effectues. Dans un deuxime temps, unepremire molaire est reconstruite en trois dimensions partir de scans de lamicrotomographie computerise. Dans un troisime temps, la morphologie du ligamentdentaire est observe pendant sa dformation en utilisant la microscopie optique. Et dans unquatrime temps l'histologie du tissu du periodontium est examine.

    Afin de caractriser la mcanique du ligament dentaire bovin, un appareillage spcifique estconu et construit pour tester des chantillons de l'os-ligament-dent en cisaillement, et entension-compression. Les expriences de cisaillement sont effectues sur des coupestransversales de 2 millimtres dpaisseur, et de traction-compression sur chantillons ayantles dimensions d'approximativement 8x5x2 millimtres. Tous les chantillons sontsectionns d'une premire molaire obtenue d'un bovin immdiatement aprs abattage. Tantpour les tests de traction-compression que de cisaillement, les chantillons sont soumis unchemin de dformation non-destructif consistant en prconditionnement, relaxation, taux dedformation constant, et sinusoidal, avant d'tre dforms la rupture.

    Les rsultats prsents dans cette thse clarifient les caractristiques de la gometrie et lamcanique du ligament dentaire. Concernant sa gometrie, une variation de l'orientation desfibres de collagne est observe dans une coupe transverse de la dent. De plus, la symtriedes courbes de contrainte-dformation obtenues lors des expriences de cisaillement suggreune isotropie verticale. Cependant, le ligament dentaire serait transverse isotrope en ce quiconcerne les chantillons uniaxiaux. Concernant son comportement mcanique, le ligamentdentaire est viscolastique nonlinaire; c'est--dire nonlinaire lastique de durcissment, etune viscosit nonlinaire pseudo-plastique. Ce comportement est d l'intraction desconstituants du ligament dentaire (fibres de collagne, vaisseaux sanguins, fluide interstitieletc...) pendant sa dformation. En utilisant un modle nonlinaire viscolastique de lalittrature, la loi puissance, ce comportement peut tre simul numriquement en vuedanalyses par lments finis.

    V e r s i o n a b r g e

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  • v

    Acknowledgements

    Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .