Methods for medical applications

The outcome of a nonlinear finite element stress analysis of ceramic heads for artificial hip joints is presented. The analysis mainly covers the influence of taper friction, support conditions and trunnion modulus of elasticity on the hoop stress distribution at the surface of the head bore. The paper quantifies how much the maximum tensile stress decreases with increasing frictional coefficient, with stiffening of the support and with stiffening of the trunnion material. An appreciable rise of the maximum tensile upon unloading of the head is also shown for the case of cup support. The computational results are found in close correspondence with photoelastic measurements of taper pressures and encourage the use, for preliminary design purposes, of an approximate theoretical model retrieved from the literature. © 1995 by ASME.

The reduction of fractures by means of an Ilizarov's fixator is obtained by successively shortening or lengthening the rods. This entails that all reduction operations of the fracture stumps be performed with a series of empirical attempts, requiring great experience and manual dexterity in the surgeon. Moreover this process involves a long exposure of both physician and patient to potentially harmful radiation due to the continuous checking of the intermediate positions on the X-ray image intensifier. In order to overcome these limits a new device has been conceived, based on the application of three stepper-motors on three rods. Its basic principle is functionally very similar to Ilizarov's prototype. The relative motions between the two frames are carried out by controlling the three actuators with a computer, which processes the number of required steps on the basis of an algorithm, starting from a few inputs supplied by the surgeon. This article illustrates the functional kinematic study necessary for the complete automation of the reduction process. Also considered is the complex problem of the reduction trajectory definition, intended as a sequence of configurations of partial correction, obtained by formalizing in geometrical terms the empirical criteria followed by the orthopaedic surgeon in reducing fractures. Such a sequence is intended to be a suggestion for the surgeon who can visualize and possibly interact with the system to determine a trajectory harmless for the soft tissues surrounding the bone. © 1994.

Keywords: external fixator , fracture reduction , Kinematic analysis , robotics

The 'Bologna Boot' CFRP othesis was designed to help paraplegics to stand and to walk. The great advantage of the Bolonga Boot when confonted with previous otheses, is the absence of articulation blockage, since the leg is kept straight by moving the patient's center of gravity slightly forward. This displacement is achieved by a sloping sole. A strp transfers the unbalanced weight to a skeleton which includes a wedge of about 5 degrees; a leather sheath completes the boot. This work is focused on skeleton optimization. At the beginning a CFRP design was developed by using a high-modulus-carbon with a PMMA (Poly Methil MethAcrilate) thermoplastic matrix, to allow for minor modifications during test on patients. Then the skeleton was simplified by using a more complete set of measured loads. For this purpose, an high accuracy finite element and a computer program for sandwich optimization were developed. A high-modulus-carbon-epoxy sandwich rod was standardized for various groups of patient sizes; this structure is partially embedded in the polyurethane foam wede by using a low-modulus-carbon-epoxy interface. A PMMA-carbon collar completes the skeleton. This solution permits on-field modification of the skeleton to improve the patient' comfort with half the weight of the previous design.

An axisymmetric, mechanical analysis of conical press-fit ceramic heads is performed. The head strength and its fracture modes are assessed experimentally. The stress field is examined by finite element, strain gauge and photoelastic methods. An alternative head design, characterized by a cylindrical engagement with the stem, is analysed with the same techniques and its merits are explored. © 1990, Institution of Mechanical Engineers. All rights reserved.

A traditional knee-ankle-foot orthosis (KAFO) for myopathic patients has been studied for the assessment of loads and fatigue resistance. Starting from this basis a thermoplastic matrix carbon fibre reinforced plastic composite (CFRP) KAFO has been developed in order to reduce the weight. A finite-element simulation programme for deformation analysis was used to compare the behaviour of conventional and CFRP orthosis. There were no breakages either of the prototype or of its parts. The CFRP orthosis allows a weight reduction of more than 40 per cent. © 1990, Institution of Mechanical Engineers. All rights reserved.

Carbon fibre composites are being used as strong lightweight materials in knee supports.