External fixation is widely used in the fixation of fractures and limb deformities. The mechanical characteristics of a specific external fixator are major factors in determining the biomechanical environment at a fracture/osteotomy site and, hence, affect the healing process. Although the optimal biomechanical environment for healing of a fracture or an osteotomy is unknown, a specific range of interfragmentary motion exists which promotes healing. It is therefore desirable that the mechanics of an external fixator can be manipulated to enable the surgeon to control the range of interfragmentary motion. The characteristics of an external fixator are defined by a large number of variables. Therefore, to gain control over the degree of interfragmentary motion, an understanding of the effect of each variable and how it interacts with the others to determine the overall characteristics of the device is required. For the past two decades, individual components and whole-frame configurations have been studied in depth. This article provides a summary of previous work concerning the mechanics of external ring fixators and how they affect the biomechanical environment at the fracture/osteotomy site.
This article reviews the problems which are encounteredDetermination of their mechanical properties is essential if in defining the mechanical properties of natural tissues, natural tissues are to be replaced with materials which will and in replacing them with synthetic materials in the perform their required function without failing. In addition, human body. It describes how death, ageing, degeneration, natural tissues may also be used, sometimes after chemical pathology and individual variability influence the propermodification, as replacement materials. ties of natural tissues. Experimental problems arise fromThe next section is concerned with synthetic materials: these degradation and testing conditions; these are illustrated may be composites which are intended to mimic natural tissues by the properties of the nucleus pulposus of the interveror conventional engineering materials. Biological tissues are tebral disc. Replacement of natural tissues by graft matenatural examples of fibre-reinforced composite materials. rials and the products of tissue engineering is then 'Biomimetics' exploits the principles exploited by natural biodescribed. Synthetic replacement materials should be biomaterials to fabricate new materials which may have natural compatible, i.e. they should not cause adverse reactions in or synthetic components.1 Both the biomimetic and the conthe human body. However, polymers which hydrolyse in ventional engineering materials approach to replacing tissues the body fluids may be useful for implants which are have their problems which are covered in this section. The intended to have a limited life or for controlled release of article concludes with a brief summary of the current state of drugs. Synthetic implant materials may attempt to mimic the subject. natural tissues but there may be a problem of attaching them to the surrounding tissue. Artificial ligaments provide an example of implants of this kind. Total hip replacement is used to illustrate the successful use of conventional
This study demonstrates that clamping a tensioned wire can cause a reduction in wire tension. Tension (about 1275 N) was applied to a wire that was subsequently clamped, using cannulated bolts, to the steel half-ring of an Ilizarov external fixator. The tension in the wire was monitored before, during and after clamping. The apparatus was disassembled and the deformations in the wire caused by the clamps were measured. This experiment was repeated 15 times. When the wire was clamped to the frame, the wire tension was reduced by 22 +/- 7 per cent (mean +/- standard deviation, SD). The drop in wire tension was linearly correlated (r = 0.96; p < 0.001) with the deformation caused by the bolts. A finite element (FE) model of the wire was also constructed. The model was pre-stressed (tensioned), and the clamping effect replicated. This analysis showed that clamping the wire could be considered to squeeze the wire outwards (like toothpaste from a tube) and so reduce its tension during fixator assembly. To assess the magnitude of this effect in the clinical situation, the FE model analysis was repeated to replicate clamping a 1.8-mm-diameter wire to a 180-mm-diameter steel Ilizarov ring component. The analysis showed that for these conditions the tension reduced by 8-29 per cent. The results of this study highlight a general engineering problem: how can a tensioned wire be secured to a structure without an appreciable loss of tension? If the performance of the structure depends on the wire tension, this performance will change when the wire is secured.
Equations have been derived for calculating the stress distributions in a tapered reinforcing fibre in a composite material, i.e. for a fibre which does not have a uniform radius. In general, deformation of the matrix in a composite induces a radial (compressive) stress at the fibre surface and an axial (tensile) stress. The equations were solved for a circular conical, paraboloidal and ellipsoidal fibre embedded in a plastic matrix. Results were compared with the familiar results for a uniform cylindrical fibre (i.e. with a constant radius) for which the radial stress at the surface is zero. For a uniform cylinder, the axial stress increases linearly, from zero at the ends, to a maximum value at the centre. At the other extreme, the axial stress in a conical fibre was shown to be constant. The intermediate cases of a paraboloidal and an ellipsoidal fibre showed axial stress distributions lying between these two extremes.
The finite-element method was used to calculate the axial stress in an elastic fibre embedded in an elastic matrix to model a fibre-composite material. Axisymmetric models were created for cylindrical, ellipsoidal, paraboloidal and conical fibres embedded in a matrix and characterized by a fibre axial ratio, q. The effects of varying q, from 200 to 1000, and the ratio of the Young moduli of the fibre and the matrix, E f /E m , from 50 to 10 4 , were investigated. For a cylindrical fibre, the axial stress distribution along the fibre axis was similar for all values of q and E f /E m ; it was greatest at the centre, decreased steadily over most of the fibre length and fell rapidly to zero near the fibre end. For fixed q, the magnitude of this stress increased with increasing E f /E m , whereas for fixed E f /E m the variation with q was small. There was good qualitative agreement between these data and previous analytical models. The axial stress in the conical fibre was a minimum at the fibre centre and rose gradually to a maximum close to the fibre end. This was most pronounced for small values of q and at large values of E f /E m . Stress distributions for the paraboloid and ellipsoid lay between those for the cylinder and the cone. For small values of E f /E m , both the magnitudes and the axial distributions of axial stress were almost indistinguishable for all shapes of fibre and all values of q studied.
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