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Biomedical subjects

G Jayaraman

Publications and source records attributed to G Jayaraman.

72 records · Page 4Linked to original sources
PubMed1991

The use of sublaminar cables to replace Luque wires.

Sublaminar wires have been used in conjunction with posterior instrumentation to stabilize the spine. Sublaminar wiring has fallen into disfavor because of an increase in neurologic complications with the Luque technique as well as wire breakage, dural tears, and difficulty of removal. A cable system consisting of two 49-stranded stainless steel cables connected to one malleable leader was designed to overcome these shortcomings. Biomechanical testing revealed that the maximum yield strength of a single stainless steel cable loop was 2.85-2.94 times greater than a double 0.05-in. stainless steel wire loop. The fatigue tests demonstrated that the stainless steel cables required 6-22 times more cycles to failure than the stainless steel wire. Many of the titanium cables failed immediately under higher loads (0-100 lb) because of slipping of the crimp. The preliminary clinical results after a mean of 19 months of follow-up of 245 cables are encouraging. There has been no breakage or loosening of the cables and no complications associated with the use of the cables. The stainless steel cables are very strong, but more important, the cable flexibility prevents repeated contusions to the spinal cord during insertion of the rods and tightening of wires. The cable conforms to the undersurface of the lamina. This may lead to a decrease in neurologic complications.

Biomechanical Phenomena↗
PubMed1989

Biomechanical analyses of surgical correction techniques in idiopathic scoliosis: significance of bi-planar characteristics of scoliotic spines.

Biomechanical analyses of Harrington distraction, Harrington distraction-compression, Cotrel and Luque correction techniques simulated mechanically on a three-dimensional mathematical model of scoliotic spines are developed and relationships between mechanical forces and achievable corrections are derived in terms of Cobb angle, vertebral inclination from the frontal plane, and bi-plane bending stiffness of motion segments. For all four systems, nomograms between Cobb angles and corrective forces with correction factors as parameters are prepared in terms of given bi-plane characteristics of scoliotic spines. Parametric studies to show the influence of the torsion plane bending stiffness of motion segments and vertebral inclinations from the frontal plane on the mechanical effectiveness of the surgical correction techniques are presented. The mechanical effectiveness of each of the four surgical correction techniques determined with the use of this model compares reasonably well with the clinical findings.

Biomechanical Phenomena↗
PubMed1988

Linear diffusion of lead in the intestinal wall: a theoretical study.

In this paper, the authors present a diffusion model of the intestinal wall to describe the transient overall uptake of lead ions across the epithelium of gastrointestinal mucosa and their subsequent diffusion in the underlying blood capillaries. The gastrointestinal mucosa is treated as a heterogeneous two-phase medium, consisting of a continuous extracellular phase and a dispersed cellular phase. The main mode of uptake is considered to be bulk diffusion, since transport of lead across the mucosal membrane occurs mainly by a passive diffusion process. The equations give the variation with time of concentrations of lead in both the phases across the intestinal wall. The mean concentration of lead available for absorption by the blood capillaries has been studied as a function of time. It is found that a steady state is reached in 19.45 min. It has been possible to estimate the intestinal membrane permeability for the lead molecule from this analysis: it is calculated to be 3.34 X 10(-3) s-1. Finally, the rate of lead uptake by the blood for different gut concentrations has been calculated.

Diffusion↗
PubMed1987

A theoretical model for water flux through the artery wall.

A two layer model for water flux through the artery is studied using a mathematical model based on the theory for the consolidation of water saturated soils. The matrix is considered to be constituted by two layers with different permeabilities and different elastic constants and the two systems of equations are coupled with the condition of continuity of pressure, total stress, solid displacement and fluid seepage velocity at the interface. The luminal pressure is considered to be harmonic in time. Exact solutions are obtained for displacements and pressures in both the layers. For large consolidation times, large pressure gradients are found to exist near the boundaries and at the interface. The heterogeneous model may not only be useful to understand the mechanics of transport in the physiological system but it will also help the bioengineers to choose proper implant materials to design artificial vascular organs for the purpose of prosthesis.

Arteries↗
PubMed1986

Mechanics of left ventricular aneurysm.

When a coronary artery is significantly occluded, the left ventricular myocardial segment, which is perfused by that coronary artery, will become ischaemic and even irreversibly infarcted. An acute infarct has very low stiffness and if it involves the entire wall there is a risk of rupture; however, in the absence of such a critical situation, fibrous tissue is laid into the infarcted myocardial segment. Such an infarcted fibrotic myocardial segment will not be able to contract, and so generate tensile stress. The surrounding intact myocardium will contract and generate wall stress, thereby developing a high intra-chamber systolic pressure; the chronically infarcted and fibrotic segment will have to sustain this high chamber pressure. Its loss of contractility and the resulting reduced systolic stiffness relative to the intact segment, will cause it to deform into a bulge; this is an aneurysm. When a left ventricular chamber with an aneurysm contracts during the systolic phase, some blood also goes into the aneurysm, and this decreases the stroke volume; since the aneurysm wall is passive, stagnant blood flow prevails in the aneurysm itself, which in turn can give rise to the formation of a mural thrombus. These serious consequences provide a justification for the analysis of an infarcted left ventricular chamber, in order to predict the size of the aneurysmic bulge. Such an analysis is presented in this paper. To determine the left ventricular wall deformation, and the stress arising from infarction of a wall segment (which leads to a ventricular aneurysm) the left ventricle is modelled here as a pressurized ellipsoidal shell. Deformations of infarcted wall segments are computed for several damaged wall-thicknesses in left ventricles of different shapes. The analysis involves a derivation of equations for wall-stress equilibrium with the chamber pressure, and myocardial incompressibility before and after infarct formation. The equations are solved by the Newton-Raphson method (using elliptical integrals of the first and second kind). Of significance are the prognostic implications of the results, presented in the form of graphs, showing the dependence of tensile stress and the bulge of infarcted wall-segments, on the extent of damaged wall-thickness and the angle of infarct. Scaled illustrations of the bulge shapes, for various degrees of infarcts, are provided. The results indicate that for rupture of the ventricle, the percentage of infarcted wall-thickness and the shape of the ellipsoidal left ventricular chamber play more dominant roles than the angle-of-damage, or the extent of the infarct.

Biomechanical Phenomena↗
PubMed1984

Reversing flow in the aorta: a theoretical model.

The viscous boundary layer in a reversing flow in a curved tube is analysed with a view to study the reversing flow in the aorta at the beginning of the diastole. The velocity in the core is taken to vary with time as in a dog's aorta. The flow is considered to be quasi-steady long before the time of reversal. Near the time of reversal, the flow is governed by the diffusion equation--a balance between the unsteady inertia terms and viscous terms. The solution which ensures the continuity of the displacement thickness at the time of take over from the quasi-steady solution to the diffusive solution, is obtained. The knowledge of the distribution of the shearing stress in the circulatory system is essential due to its relevance with regard to atheroma--a disease leading to the hardening of the arteries. With this in mind, the wall shear rate is obtained as a function of time at every cross-section of the tube. The shearing stresses at the inner bend and the outer bend are also plotted. The results are compared with those for the straight tube.

Animals↗
PubMed1984

A study of external skeletal fixation systems for unstable pelvic fractures.

This study was undertaken to determine the feasibility of constructing an anterior pelvic external fixator capable of resisting displacement of vertical shear fractures. Newly designed tapered thread pins for cancellous bone had better bone pin fixation and their greater 6-mm diameter provided more than twice the stiffness of Hoffmann 4-mm pins when tested in clusters. Using these pins and a more rigid anterior frame as one model, various pelvic fixators were tested to determine their resistance to vertical shear forces. These tests determined that this new fixator was 16 times stiffer than a Bonnel single anterior frame and five times stiffer than a double anterior frame (both constructed of Hoffmann components). Extrapolation from the data shows that 13 mm of posterior fracture displacement would occur with loads of one half body weight using the new fixator. Additionally it was noted that Hoffmann frames constructed with two 5-mm pins performed as well as those using three 4-mm pins.

Bone Nails↗
PubMed1983

Water transport in the arterial wall--a theoretical study.

Water transport in the arterial wall is studied using a mathematical model based on the theory for the consolidation of water saturated soils (Biot, 1941; Kenyon, 1976a). The intimal pressure is considered to be harmonic in time. Analytical results are obtained for both large and small consolidation times since both the situations are of physiological relevance. For large consolidation times, the filtration is confined to a thin boundary layer. Large pressure gradients exist within the boundary layer while the pressure gradient is negligible in the intermediate layer. Thus, the pulsatile flow is found to be confined to the boundary layer while a smaller mean flow exists throughout the wall.

Arteries↗
PubMedDate not supplied

The biomechanics of lateral knee bracing. Part I: Response of the valgus restraints to loading.

To better understand the role of preventive knee braces in injury prevention, a biomechanical study using fresh frozen cadaveric knees (N = 18) was conducted. Ligament tensions and joint displacements were measured at static, nondestructive valgus forces as well as low-rate destructive forces. After quantifying and establishing individual ligament contributions to valgus restraining function, knees were then braced with two different laterally applied preventive braces, the McDavid Knee Guard and the Omni Anderson Knee Stabler. The effects of lateral bracing were analyzed according to valgus force, joint line opening, and ligament tensions. Valgus applied forces, with or without braces, consistently produced medial collateral ligament (MCL) disruptions at ligament tensions surprisingly higher than the anterior cruciate ligament (ACL) and higher than or equal to the posterior cruciate ligament (PCL). Although large joint displacements were necessary for complete ligament failure, bundle disruption in the MCL, ACL, and PCL was noted at much smaller joint openings. In Part I of this study, no significant protection could be documented with the two preventive braces used. Also, four potentially adverse effects were noted: MCL preload, center axis shift, premature joint line contact, and brace slippage.

Adult↗
PubMedDate not supplied

The biomechanics of lateral knee bracing. Part II: Impact response of the braced knee.

In Part I of our study on lateral knee bracing, we evaluated brace effectiveness using cadaveric knees and two commercially available braces (McDavid and Omni). The results indicated that for low-rate dynamic valgus loading, neither brace provided significant protection against MCL injury. Also, four potentially adverse effects were noted. The goals in Part II were to: 1) determine the clinical significance of brace induced MCL preload; 2) define the functional character of an "ideal" brace; 3) design and validate a surrogate knee model for testing brace effectiveness; and 4) determine brace performance under impact loading using the surrogate knee and six commercially available brace types (manufactured by DonJoy, McDavid, Mueller, Omni Scientific, Stromgren-Scott, and Tru-Fit). Knee braces, modified to measure varus/valgus bending force, were used to determine MCL preload effects in 13 human volunteers. An anatomically correct surrogate knee model, instrumented to measure ligament/tendon tension and medial joint opening, was developed and validated using information from our previous cadaver studies and results of analyses on the effects of high strain rates (100% versus 1000% strain/sec) on MCL failure. Over 500 impact tests were performed on the surrogate knee in unbraced versus braced conditions. Tests were conducted for three impactor masses, two flexion angles, and free or constrained limb positions. Impact safety factors (ISF) were calculated for each test condition and brace type. An ISF of 1.50 (MCL load reduction of 30%) was considered significant.(ABSTRACT TRUNCATED AT 250 WORDS)

Athletic Injuries↗