A.L. Yettram

Intramedullary femoral nails: one or two lag screws? A preliminary study

Failures of proximal femoral nails that treat unstable femoral fractures have been reported. In this communication, a finite element model to include a proximal femoral nail within a fractured femur was used to carry out preliminary investigations into configurations of single or double lag screws. The effects of the different types of fracture were investigated. The results show that in order to share the load evenly between two lag screws, a good configuration seems to be to have a slightly larger screw above the lower screw. This also ameliorates stresses in the nail at the lag screw insertion holes. However, using two screws in this way can lead to large stresses in the cancellous bone in the femoral head, and these stresses may be significant in the initiation of cut-out.

The elastic stability of square perforated plates under combinations of bending, shear and direct load

Abstract This paper presents solutions for the elastic stability of plates containing perforations using the conjugate load/displacement method. Interaction curves for a variety of load cases and support conditions are presented. The effect of the size of a central square hole in a square plate on the elastic buckling load is demonstrated and, where possible, results are compared with the work of other authors.

The elastic stability of square perforated plates

Abstract A direct matrix method of analysis for the buckling loads on structures, which had already been proven on various structural forms, is applied to the examination of the stability of flat square plates with central square perforations. The results for a variety of boundary conditions are presented for a range of hole to plate size from 0 to 0.8. The method generally shows trends which are different to those predicted by other numerical methods, particularly over the range of the larger hole sizes. Possible reasons for these differences are suggested, but further work to obtain definitive results is clearly needed by all investigators.

The elastic stability of square perforated plates under bi-axial loading

Abstract A direct matrix method for the buckling loads on structures is applied to the examination of the stability of flat square plates with central square perforations under various combinations of bi-axial loading. Results are given for a range of hole to plate size from 0 to 0.8. The method generally shows results comparable with previous work and highlights interesting effects when one component of the bi-axial load is a high value of tension.

The forces acting on the human calcaneus

The forces acting on the human calcaneus are analysed for a static standing posture. An optimization procedure is invoked to solve the statically indeterminate set of forces which could arise from muscles, ligaments and reactions between the calcaneus and other bones and also the ground. Two alternative objective functions are investigated; that which returns no active muscle force is considered the more realistic, since electromyographic studies show that in static standing only minor muscle activity occurs to counteract body sway. This set of forces is then applied to the bone for finite element stress analysis to produce stress trajectories, which are then validated against both the trabecular configuration of the cancellous bone of the calcaneus and also the distribution of the orientations of the hydroxyapatite crystal major and minor axes.

Effect of interface conditions on the behaviour of a Freeman hip endoprosthesis

The femoral element of a total hip replacement is a composite structure of two, or perhaps three, components--the endoprosthesis, the bone and, where present, the cement. The interfacial conditions are such that complete structural continuity does not necessarily obtain. That this is so has often been suspected due to the observed loosening which can occur in vivo. In modelling the system, typically for finite element analysis, it has usually been considered to be monolithic, such that tensile and shear stresses could be transmitted across the interfaces as well as the normal compressive stress. Here the femoral component of a Freeman hip replacement is considered, implanted without bone cement, and analyses are carried out under monolithic, i.e. fully bonded, and non-bonded assumptions. Simultaneously the effect of retaining the neck of the femur, one of the features of using this particular prosthesis, is also examined.

Effect of myocardial fibre architecture on the behaviour of the human left ventricle in diastole

Stress and deformation results from finite element analyses are presented for a series of models of the human left ventricle. The myocardium has a complex anisotropic fibre structure, are made on both fibre orientation and on the ratio are made on both fibre orientation and on the ratio of the elastic moduli along and across the fibres. The results show that, at least in diastole, when the left ventricle is considered as a passive structure under the action of the internal blood pressure, the effect of the real fibre arrangement is generally to reduce deformation and also the direct stresses. In spite of fibre angle changing across the wall, the analyses correctly predict the lack of rotation of the left ventricle about the long axis.

Intramedullary nails: some design features of the distal end

Intramedullary nails are used to stabilise fractures of the proximal femur. The nail acts by transferring loads from the proximal fraction to the rest of the femoral shaft. The way in which this occurs depends to a large extent on the design of the distal end of the nail. This is not dissimilar to the situation with regard to load shedding (or load transfer) from the femoral component of a total hip replacement. A finite element model of a fractured femur with either a neck or a subtrochanteric fracture is set up to investigate the effects of nail length, nail distal stiffness and material stiffness on the structural behaviour of the system. Specifically what is considered is the influence of these parameters on the stress across the fracture and the normal pressure that the nail exerts on the endosteum of the femoral diaphysis. It is found that a longer nail could produce higher contact stress between the tip of the nail and the endosteum. Also, this contact stress is reduced when the distal region of the nail is made more flexible either by incorporating longitudinal slots or by using a material with a lower modulus of elasticity.

The elastic stability of stiffened plates using the conjugate load/displacement method

Abstract The conjugate load/displacement method of analysis for the determination of the elastic buckling loads of plates under various loading and support conditions has been developed to deal with stiffened plates. The theoretical justification for the approach is presented, along with any simplifying assumptions made. Results from this method are compared to those published by other workers for three particular applications and are found to give good agreement. The significance of the torsional rigidity of the stiffeners on the overall behaviour of the complete structure is highlighted.

Finite element analysis of intramedullary devices: the effect of the gap between the implant and the bone.

This paper examines the interaction interface between the implant and the bone for an intramedullary femoral nailing system using a finite element (FE) model and specifically considers the hypothesis that the local geometry at the interface is significant to the resulting localized contact stress between the medial and lateral aspect of nail and endosteum. Contact mechanics algorithms are used in the FE modelling technique that can be developed to deal with any form of intramedullary device for which contact at the bone—implant interface is important. Global stiffness data from the FE model are compared with available data from an experiment carried out on a construct of the bone and the device that uses intramedullary femoral nails. Acceptable agreement is obtained. The results demonstrate that the mechanical interface between the implant and the bone is significantly affected by the gap geometry and magnitude. In particular, larger gaps lead to greater concentrations of stress on the medial side, while the distribution of stress is more uniform at the lateral contacts. Furthermore, the results show that the gap can have a marked effect on the stresses that occur on the fracture plane.