Browsing by Author "Starke, Gregory Richard"

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    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.
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    Validation of numerical prediction of bone ingrowth into cementless implants
    (1998) Galgut, Warren; Vaughan, Christopher Leonard (Kit); Starke, Gregory Richard
    Total joint replacement was pioneered by John Charnley in the late 1950's, and has since revolutionised the management of arthritis sufferers. By 1991, an estimated 5 million people had undergone hip replacements. Although relatively successful, the cemented components had some problems, and this led to the development of cementless implants. These implants depend on the ingrowth of bone into a porous coating, to produce a durable method of implant fixation which the normal bone turnover process will maintain. One of the problems with cementless implants is that the type and extent of tissue ingrowth into the porous coating is unpredictable. Movement of the implant relative to the surrounding bone may result in the formation of an interfacial fibrous tissue layer. Hence, numerical modelling has been used to predict tissue ingrowth into such implants. Numerical simulation has the advantage that comprehensive data can be extracted relatively quickly. The finite element method is a powerful tool that has become the preferred method of analysis, and takes into account critical factors such as implant design, bone properties, and loading conditions. However, these models have not been tested extensively. Little attention has been given to comparing numerical models with the actual findings of retrieval studies or radiological imaging studies. This study thus evaluates the potential of one such numerical model. Most numerical models analyse the stress patterns of a particular state of bone ingrowth (i.e. a static case). This model considered the development of the ingrowing material - a dynamic analysis of tissue changes over a period of time. A 2-dimensional, plane stress finite element model was used to predict the ingrowth of bone into the porous coating of the femoral stem of a hip implant. A side plate was incorporated to mimic 3-dimensional characteristics. The evaluation was achieved by comparing the predictions of the numerical model with plane X-ray images of seven patients with Zimmer Anatomic cementless hip implants. The X-rays were scanned at a high resolution, so as to be able to "magnify" the regions to be examined. Several algorithms were developed to analyse the images, and provide a quantitative assessment of the X-ray images. The algorithms were designed to identify regions of bony and fibrous tissue. This involved the identification of the interface between the implant and the surrounding bone, and the extraction of the grayscale values of the X-rays at this interface. Thereafter, various radiographic signs that indicate the presence of fibrous tissue or bony tissue were identified, and these were used to enhance the original grayscale plot. The resulting graph was then modified slightly so as to make its presentation comparable with the numerical model. Plane X-rays proved to be suitable for the task of identifying tissue types. These data were then compared with the predictions of the numerical model. A qualitative correlation was used, as this was deemed to be most appropriate. Several authors in the literature also found a quantitative approach to have limitations. Some agreement between the experimental findings and the numerical simulation was found to exist, although this was limited. The agreement was judged to be less than the "reasonable agreement'' that several studies in the literature concluded. The correlation is better described by "some agreement". Nevertheless, the finite element method was assessed as being a tool with great potential, and modifications to the present model may provide more reliable results. A time study was also undertaken, whereby the tissue density was evaluated at various periods after the operation. The study provided insight into the evolution of the implant-bone interface after surgery, and correlated well with the literature. The phases of repair and remodelling were evident, and it was assessed as being a valuable contribution to this work. The time study may prove to be a more useful method than those used in assessing the "static" images, and could even provide a prognostic tool in assessing implant stability over time.