Browsing by Author "Spirakis, Thanos"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Open Access
    The application of the homogenisation method to the numerical modelling of cancellous bone
    (1995) Conway, Damian John; Mitchell, Greg; Spirakis, Thanos
    This thesis reports on an investigation into the viability of developing idealised numerical models of cancellous bone in order to make reasonable predictions about its macro- and microstructural mechanical behaviour using the homogenisation method. In total joint replacement, cancellous bone (the soft porous bone which lies below the articular surfaces at weight-bearing joints) provides the medium for the transfer of loads from the artificial component, or prosthesis to the hard, outer cortical bone. Although total joint replacement is usually a successful operation - providing relief from pain and often considerably improved joint function - loosening of the metal components remains a major obstacle to the long-term success of these operations. In the ongoing work to develop joints which are less susceptible to loosening, it has become necessary to study the micromechanics of cancellous bone in order to predict its response to the changing stress environments brought about by the inserted prostheses. Biomechanical engineers have used finite element analysis extensively in the analysis of reconstructed joints. However, owing to the prohibitively high computational costs associated with the microstructural modelling of cancellous bone, it is generally modelled simply as a homogeneous, isotropic material. It is not possible to accurately predict the mechanical response of cancellous bone to various implant conditions under this simplistic modelling assumption. Thus, alternative methods are being sought which will allow for more realistic modelling of cancellous bone. The homogenisation method is one such alternative. This method makes it possible to uncouple the analysis of some problem involving a composite material into an apparent global analysis and a local microstructural analysis. The apparent material properties of the periodically repeating composite microstructure are calculated, taking into account the structural heterogeneities of the composite. These properties are then used in the global analysis where the composite is treated as a continuum. The apparent global-level results are subsequently postprocessed to obtain the microstructural behaviour in any local regions of interest. The main aim of this research project was to investigate the applicability of the homogenisation method to the modelling of cancellous bone. The first part of this work involved an extensive literature study on the architecture and micromechanics of cancellous bone to investigate whether cancellous bone can indeed be modelled as an idealised composite material with a periodically repeating microstructure. The outcome of this investigation revealed that the structure and behaviour of cancellous bone is highly variable - depending on the patient, anatomic location and the level of density. However, certain regions of cancellous bone do have typical repeating architectures which have a major influence on the apparent mechanical behaviour of the bone. Thus it has been concluded that these specific regions of cancellous bone can be modelled by idealised structures, provided the observed microstructures and predominant deformations modes are well characterised in the models.
  • Thumbnail Image
    Item
    Open Access
    Numerical simulation of implant-bone interaction following cementless joint replacement
    (1996) Starke, Gregory Richard; Martin, John; Spirakis, Thanos
    The advent of cemented joint replacements has revolutionised the management of patients suffering from chronic arthritis. However, establishing a durable bond between the prosthesis and the surrounding bone remains a considerable problem. As a result, cementless implants have been developed. These components rely on the ingrowth of bone into a porous coating, which covers a portion of the component surface, to achieve the required mechanical interlock. Once mineralised bone tissue has formed within .the porous surface, a stable bond results which will be maintained by the normal bone turnover processes, thereby providing long term attachment. However, one of the problems associated with the use of cementless implants is the unpredictability of the extent of bone ingrowth. The process of osseo-integration is greatly influenced by the magnitude of the micro-motion between the implant and the surrounding bone. Large movements inhibit ingrowth, and may result in the formation of an interfacial fibrous tissue layer. In addition, interface strains will influence the early repair process and guide long term bone remodelling within this region. A numerical model for the prediction of bone formation within the porous surface has been developed. The evolution laws consider the early repair activity, possible fibrous tissue formation, and long term remodelling, as a function of the history of inelastic relative displacements and elastic interface strains. The model is based on the development of an isoparametric interface element, which is suitable for implementation into a non-linear finite element code. In the unbonded condition, the contact is governed by a Coulomb friction formulation. The position and shape of the Coulomb yield surface is altered according to the evolution equations, which govern the development of mineralised tissue within the surface porosity. The strain history and post-operative time are then used to develop a stimulus coefficient, which determines the course of the interface tissue development. If bone tissue is predicted, the subsequent interfacial material will be governed by a remodelling algorithm for the prediction of the long term response. If the bond strength is exceeded, fracture occurs and the joint may open or slide, thus returning to its original, unbonded, state. In the event of large micro-motions, the yield surface and material formulation are altered to include fibrous tissue. The model is used to predict the development of interfacial tissues at the porous surface of a tibial tray component, with a central peg and a PCA (Howmedica, Inc.) femoral stem. Although many factors influence interfacial tissue development, mechanical loads are assumed to be dominant. In the short term, the relationship between micro-motion and interface tissue response has been shown. However, long term remodelling of interfacial tissues has not been widely demonstrated and, therefore, additional experimental data is required to validate the current long term remodelling predictions.