Experimental investigation of the mechanical behaviour and structure of the bovine periodontal ligament

One of the key problems in dental biomechanics is the prediction of tooth mobility under functional loads. Understanding tooth displacement due to load is becoming more important as new solutions in dental restorations, prosthodontics and orthodontic treatments become increasingly more advanced. The mechanical characterization of the alveolar bone, the tooth and the periodontal ligament surrounding the root of the tooth, is necessary to predict tooth mobility. The common assumption is that the periodontal ligament acts as the major element in the stress distribution to the supporting tissues. Obtaining parameters that describe periodontal ligament mechanical behaviour is a challenging problem. Isolating the tissue for testing, 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 to the complexity of the problem. The aim of this thesis is twofold and can be subdivided into two primary objectives. The first objective is to describe its morphology, anatomy, histology and structure of the components in order to determine its geometric parameters at different length scales. The second objective is to determine its mechanical properties by identifying key parameters through shear and uniaxial tension-compression tests. Four studies are performed to describe the morphology, anatomy and histology of the periodontal ligament. First, macroscopic and microscopic measurements of the tooth, bone, and the periodontal ligament are obtained. Second, a bovine first molar system is reconstructed in three dimensions from microcomputerized tomography scans. Third, the morphology 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 speciallyprepared tissue specimens to shear and uniaxial tension-compression experiments. Shear experiments are performed on 2 millimetre thick transverse sections, and specimens of toothligament- bone of approximately 8x5x2 millimetres for uniaxial experiments. All specimens are obtained from first molar sites of freshly slaughtered bovines. In both shear and uniaxial testing, 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 characteristics of the periodontal ligament. Concerning its geometry, a variation in collagen fibre orientation is observed in transverse sections, moreover the symmetry of the shear tests in the apicocoronal direction suggests the periodontal ligament is vertically isotropic. Uniaxial specimens, however, may be considered to be transverse isotropic. Concerning its mechanical behaviour, the periodontal ligament is nonlinear viscoelastic in that it exhibits stiffening, nonlinear elasticity, and nonlinear pseudo-plastic viscosity. The interaction between various constituents of the periodontal ligament (collagen fibres, blood vessels, interstitial fluid etc...) during deformation contribute to the observed stress-strain response of this tissue. A nonlinear viscoelastic model presented in the literature, the Power Law, adequately simulates the nonlinear behaviour of the periodontal ligament using finite element analysis.