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Biomedical subjects

A L Yettram

Publications and source records attributed to A L Yettram.

At least 19 recordsLinked to original sources
PubMed2004

Intramedullary nails with two lag screws.

OBJECTIVE: To investigate the structural integrity of intramedullary nails with two lag screws, and to give guidance to orthopaedic surgeons in the choice of appropriate devices. DESIGN: Alternative designs of the construct are considered, and the use of a slotted upper lag screw insertion hole is analysed. BACKGROUND: Intramedullary fixation devices with a single lag screw have been known to fail at the lag screw insertion hole. Using two lag screws is considered. It has also been proposed to use a slot in the nail for the upper lag screw to prevent the upper lag screw from sticking. METHODS: Bending and torsion load cases are analysed using finite element method. Consideration of both load conditions is essential. RESULTS: The results present the overall stiffness of the assembly, the load sharing between lag screws, and the possibility for cut-out to occur. CONCLUSIONS: While the slot for the upper lag screw might be advantageous with regard to the stresses in the lag screws, it could be detrimental for cut-out occurring adjacent to the lag screws. RELEVANCE: Comparative analyses demonstrate that two lag screws may be advantageous in patients whose cancellous bone quality is good and who impose large loads on the lag screw/nail interface. However, the use of two screws might pre-dispose to failure by cut-out of the lag screws. The addition of a slotted hole for the upper lag screw appears to do nothing significant to reduce the risk of such a failure.

Bone Nails↗
PubMed2003

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.

Bone Nails↗
PubMed2000

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.

Bone Screws↗
PubMed1998

Some factors that influence mechanical behavior of the left ventricle of the human heart in late systole: a feasibility study using finite element analysis.

In systole the left ventricle of the heart behaves mechanically in two modes simultaneously, passive and active. When in the former mode, the ventricle has to carry and react to the pressure increase within the cavity, while in the latter, force is generated within the myocardium itself through the contraction, i.e., active self-shortening, of the muscle fibers. When the deformations of these two opposing modes balance, isovolumic contraction occurs. After this phase, when the aortic valve has opened, the active mode dominates. Many models of the left ventricle under passive internal pressure have been reported, usually for analyzing the situation in diastole. Only a few attempts have been made to incorporate the self-activation effect present in systole. In this paper, a model for systole is described in which the active component has been treated by analogy to thermal stress analysis methods common in dealing with conventional engineering structures. The model was applied to the pressure and volume data for the ventricles of four patients with cardiac disease. A parametric study was then undertaken to investigate the influence of some of the mechanical factors on ventricular behavior. It was found that fiber angle has a very significant effect on the deformation of the ventricle and also that for ventricles with stiff myocardia, the passive component to the reaction of the ventricle was negligible.

Biomechanical Phenomena↗
PubMed1998

An analytical method for the determination of along-fibre to cross-fibre elastic modulus ratio in ventricular myocardium--a feasibility study.

The myocardium of the left ventricle of the heart is a fibrous structure, the fibres being wound helically anticlockwise on the epicardium and changing progressively through the wall thickness to being wound helically clockwise on the endocardium. At any point, therefore, the material can be considered as transversely isotropic. For mechanically modelling the ventricle, two independent material properties, the along-fibre and across-fibre moduli of elasticity are required. This paper describes a computer method for determining these by using a finite element model and matching cavity volume and ventricle length against values derived from cineangiographic and pressure data. The method is applied to a group of patients with various cardiac diagnoses and the modular ratios for their myocardia are derived.

Computer Simulation↗
PubMed1996

Stress and strain distribution within the intact femur: compression or bending?

The aim of this research was to test the hypothesis that the intact femur is loaded predominately in compression. The study was composed of two parts: a finite element analysis of the intact femur to assess if a compressive stress distribution could be achieved in the diaphyseal region of the femur using physiological muscle and joint contact forces; a simple radiological study to assess the in vivo deflections of the femur during one legged stance. The results of this investigation strongly support the hypothesis that the femur is loaded primarily in compression, and not bending as previously thought. The finite element analysis demonstrated that a compressive stress distribution in the diaphyseal femur can be achieved, producing a stress distribution which appears to be consistent with the femoral cross-sectional geometry. The finite element analysis also predicted that for a compressive load case there would be negligible deflections of the femoral head. The radiological study confirmed this, with no measurable in vivo deflection of the femur occurring during one legged stance.

Adult↗
PubMed1995

Loading of cells and a possible upper limit of load response with respect to strain energy density.

A model has been established to examine the effects of mechanically loading bone on the ionic distribution in osteocytes. The ionic concentrations are determined from the fluorescence of specific ionic indicators SPFI, PBFI, and SPQ. A deformation was induced in the calvariae of neonatal rats which were used as the experimental subjects. The ionic distribution was not uniform across the calvarial specimen, and with the aid of finite element stress analysis we examined the distribution of strain energy density within the specimen. A similarity between the pattern of distribution of strain energy density and the potassium and sodium ions was observed, whereas this was not the case with chloride ions.

Animals↗
PubMed1995

Cancellous bone stresses surrounding the femoral component of a hip prosthesis: an elastic-plastic finite element analysis.

The cancellous bone stresses surrounding the femoral component of total hip replacement were investigated using the finite element method. Four versions of a certain femoral hip prosthesis (the Freeman, Corin Medical), cemented, HA coated, press-fit, and press-fit with ridges were analysed. Each model was subjected to two separate load cases with the cancellous bone modelled as an elastic perfectly plastic material. The effect of bone quality was investigated by varying the cancellous bone stiffness. The resulting cancellous bone stress distributions were compared to that of the intact femur. The results were also compared to clinical subsidence data (published elsewhere) for the Freeman femoral prosthesis to determine if the initial cancellous bone stress distribution could be used to predict the migration of the various versions of this prosthesis. The results showed that the press-fit designs of prosthesis generated substantially higher cancellous bone stresses than the cemented and HA coated designs, and that these stresses were up to 6.5 times higher than found in the intact femur. For all forms of fixation the cancellous bone stress distribution was found to be insensitive to changes in the trabecular bone stiffness: thus poor quality cancellous bone is more likely to promote 'plastic' deformation, and therefore subsidence of the prosthesis. Comparison with the clinical migration data showed a good correlation and revealed that it may be possible to use the calculated initial cancellous bone stresses to predict the migration of the implant, and hence the probability of early and mid-term aseptic loosening.(ABSTRACT TRUNCATED AT 250 WORDS)

Biomechanical Phenomena↗
PubMed1995

Is stem length important in uncemented endoprostheses?

In an uncemented total hip replacement where the femoral stem is thinner than the medullary canal it may be hypothesized that the distal portion of the stem has no mechanical function. In this study an uncemented total hip replacement has been modelled mathematically using finite element analysis and mechanical tests of a similar system have been carried out loading implants in cadaveric proximal femora. Two implants have been tested mechanically; one with a full length stem and the second with the stem shortened, but three implants have been modelled with an additional intermediate length stem analysed. Additionally the finite element analysis has been done with a high neck resection and a standard level resection. The finite element model showed that the full length stem produced lower interface contact stress levels under the proximal neck, particularly when the nec was resected, and this was borne out by the mechanical testing.

Biomechanical Phenomena↗
PubMed1994

The response of bone to external loading regimens.

The results of Hert et al. have been re-examined using an external pin fixation technique involving New Zealand White rabbits. New bone formation was assessed with tetracycline epifluorescence in 49 animals. The animals were operated upon using inhalational anaesthesia, and pins were placed at opposite ends of the tibia of both hind limbs. Following recovery, oxytetracycline was given at a dosage of 25 mg kg-1 body weight intraperitoneally on the day of loading and on the fourteenth day of the loading regimen. The bones were loaded for 45 min per 24 h. Undemineralized specimens were sectioned and standardized photographs produced. The area of new bone was determined by a planimeter and by digitization. In dynamic loading the shape of the force/time curve is of little significance; the response is frequency dependent and for higher frequencies the response is greater than that from continuous loading; there was no statistical significance between the rapid relief loading regimens. For continuous loading the response is a function of the load value. In all cases more bone is laid down in immature subjects than in adults. With regard to osteoclastic activity, all loading increased the areas of resorption irrespective of character, in all adult animals intermittent loads produced a greater area of ostoeclastic resorption than static loads and produced a greater change as the number of load cycles per minute increased. The adult control animals had more resorptive activity than the immature animals although there was no statistical significance. Loading, irrespective of its character, led to a net increase of bone, the osteoblastic effects thus outweighing the osteoclastic effects.

Aging↗
PubMed1993

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.

Biomechanical Phenomena↗
PubMed1992

Factors influencing left-ventricular stiffness.

The aim of the study was to investigate the relative contributions of geometrical and material factors to overall left-ventricular cavity stiffness. Left-ventricular cavity shapes were reconstructed using a computer and the variation of myocardial elastic modulus was calculated, by the finite element method, through the passive phase of diastole when rising volume coincided with rising pressure. Geometric data were obtained from biplane cineangiography, with micromanometer pressure measurements, for ten patients with left ventricular disease. Dimensional analysis was applied to the initial and derived data from which the influences of myocardial compliance, wall thickness-to-long dimension ratio, and aspect ratio (long-to-short axes) were determined. The ratio between the volume elasticity and the myocardial modulus of elasticity, the normalized stiffness ratio (NSR), is proposed as a useful index of left ventricular mechanical behaviour in diastole. The volume elasticity of the chamber is dependent not only upon the myocardium elastic modulus and the wall thickness ratio, but also on the shape of the chamber. Changes in the thickness/radius ratio of the ventricle have less effect upon its distention than those in the long dimension/radius ratio. The left ventricle becomes more spherical in shape through diastole and hence becomes stiffer by this geometric mechanism.

Cineangiography↗
PubMed1990

Relation between intraventricular pressure and volume in diastole.

The pressure-volume curves for 10 patients with various types of heart disease were studied throughout mid to late diastole when both pressure and volume were increasing. The results were used to test a currently held theory that the form of this relation is exponential. It was found that for the patients examined this hypothesis was not valid.

Adult↗
PubMed1989

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.

Biomechanical Phenomena↗
PubMed1983

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.

Biomechanical Phenomena↗
PubMed1982

Structural analysis for the forces in the human spinal column and its musculature.

A method of structural analysis is proposed for a model of the thoracic and lumbar spine which involves body weight, external load, muscle forces, intervertebral reactions and disc and ligament elasticity. The procedure consists of three phases: (i) for an initial position the equations of equilibrium are solved using a linear programming technique which minimizes the total force in the system and produces intervertebral reactions and muscle forces, (ii) for an incremental change in configuration a structural analysis is performed to obtain a further set of intervertebral reactions, (iii) with the net values of intervertebral reactions, from combining the results of the previous two phases, a further application of the linear programming technique is made to obtain the muscle forces in the deformed positions. The results are presented for a subject, in a single step of forward flexion.

Biomechanical Phenomena↗
PubMed1980

Dependence of stem stress in total hip replacement on prosthesis and cement stiffness.

Two dimensional finite element analyses were used to determine the direct tensile stress distributions along the lateral edge of the stem of the femoral component of a total hip replacement. The efficacy of using two-dimensional analysis for three-dimensional composite structures was assessed, and the two-dimensional model was found to be suitable for parametic studies. Various combinations of stem and cement stiffnesses were investigated. In particular the influence of stem taper, cement stiffness, prosthesis stiffness and the effect of a plateau on the prosthesis stresses, were examined and compared. The most significant factor was found to be the modulus of elasticity of the stem material. The tensile stresses decrease with decreasing modulus. Cement stiffness and stem shape appeared to have far less effect on stem stress.

Bone Cements↗