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Biomedical subjects

A M Ahmed

Publications and source records attributed to A M Ahmed.

At least 19 recordsLinked to original sources
PubMed1992

Ligament tension pattern in the flexed knee in combined passive anterior translation and axial rotation.

Twenty-two fresh-frozen specimens were used to measure tensions generated in selected bands of the major ligaments of the flexed knee (40-90 degrees) when an axially prerotated tibia is subjected to passive anterior shear and when an anteriorly pretranslated tibia is subjected to passive axial torque. The tensions were measured using the buckle transducer attached to the anteromedial band of the anterior cruciate ligament [ACL (am)], the posterior fibers of the posterior cruciate ligament [PCL (pf)], the long fibers of the medial collateral ligament [MCL (lf)], and in the total lateral collateral ligament [LCL]. The knee specimens were subjected to the combined motions in a 6-df passive loading apparatus. The results indicated that the joint resistance to anterior translation increased markedly with internal prerotation and only marginally with external prerotation. This increase in joint resistance, however, was associated with a decrease in ACL function. It has been inferred that the posterior structures, capsular and meniscal, contribute significantly to joint resistance when the tibia is prerotated in either sense. For internal prerotation, the interference between the medial femoral condyle and the central tibial eminence was found to be an additional mechanism of resistance to anterior translation. Also, it has been found that although the ACL (am) tension increased with internal rotation in the normal case, it decreased with internal rotation in the presence of an anterior pretranslation. It is concluded that ACL response to combined joint motion cannot be ascertained by a simple summation of its responses to individual motions.

Adult↗
PubMed1992

Thermal modeling of polymerizing polymethylmethacrylate, considering temperature-dependent heat generation.

A methodology for the modeling of unsteady heat conduction in polymethylmethacrylate (PMMA) during its exothermic polymerization is presented. The emphasis is on the formulation of a model for the volumetric rate of heat generation, including its temperature-dependent characteristics. Three parameters appear in the proposed model. The empirical determination of these parameters using Differential Scanning Calorimetry is demonstrated. The incorporation of the proposed model into finite volume methods is also demonstrated, in the context of unsteady, one-dimensional, radial heat conduction in cylindrical coordinates. In addition, the application of the proposed model to two test problems is presented and discussed. The results are encouraging, and the proposed methodology appears to be applicable to the thermal modeling of exothermic polymerization processes in general.

Bone Cements↗
PubMed1991

The role of axial rotation in the etiology of unilateral disc prolapse. An experimental and finite-element analysis.

This study was done to establish the relevance of axial rotation as a cause of disc degeneration in the lumbar spine and the role of facet asymmetry in the injury mechanism. It previously was shown that facet asymmetry does not affect the axial torque-rotation response of lumbar motion segments. This study, in both an experimental and a finite-element analysis, examined three important points previously not considered for lumbar motion segments subjected to axial torque: 1) the effect of facet asymmetry on the coupled motions; 2) the effect of combined geometric parameters on the segment response; and 3) the effect of facet asymmetry on the annular strains. Three different lumbar-coupling patterns were observed; however, they did not appear to be influenced by facet joint asymmetry. An oblique and flat compression facet may allow an increased motion-segment response, but in general, combined geometric parameters were found to have no effect on segment response. It was concluded that, without facet damage, the right or left side of the disc is not biased by a particular facet geometry to experience unusual levels of stress and strain, either as a result of increased axial rotation or any of the associated coupled motions.

Cadaver↗
PubMed1991

Effect of pulsed electromagnetic field on healing of experimental nonunion in rat tibiae.

To see the effect of Pulsed Electromagnetic Field (PEMF) on nonunited fracture healing, nonunion was induced in rat tibiae and PEMF was applied on it. Out of five different techniques utilised for inducing nonunion soft tissue interposition was found to be the most suitable and effective method of experimental induction of nonunion. Twenty eight experimental and 15 control rats were finally evaluated for the effect of PEMF applied for up to 8 weeks. After sacrifice of 8 experimental and 4 controls, 6 experimental and 3 controls, again 6 experimental and 3 controls and finally 8 experimental and 5 controls at 2, 4, 6 and 8 weeks respectively of PEMF application no significant difference as to the quality of healing was observed between the experimental and control animals. It was thus concluded that PEMF appeared to have no beneficial effect on the healing of nonunited fractures in experimental set-up.

Animals↗
PubMed1990

Dynamic stress response of the implant/cement interface: an axisymmetric analysis of a knee tibial component.

To determine the adverse effects on the implant/cement interface stresses caused by a dynamic load on the implant, an axisymmetric dynamic finite element analysis was performed for an idealized knee tibial component assuming perfect bonding at the interface. The component, consisting of a metal plate with a central stem, was subjected to a compressive load that varied with time as a terminated ramp function. At first, the reliability of the interface stress predictions was assessed by computing the effects of a number of method-related parameters, viz., the finite element mesh density, the assumed bone properties. Analysis was then performed considering the stem length, cement mantle thickness, and the type of implant metal as design variables. The analysis predicts high-frequency (600 Hz) stress oscillations of significant amplitude at those locations of the interface that are also subjected to high static stresses: near the stem-plate junction and the stem tip for normal stress, and at the stem tip for shear stress. However, the predicted stress amplitude has been found to be particularly sensitive to the assumed rise time of the input load function. With a rise time of 2.0 ms, an input condition considered to be severe enough to represent the most vigorous dynamic activity, the maximum stress augmentation, because of stress oscillations, is predicted to be less than 25%. In general, the design variables have been found to affect the static stresses much more than the dynamic ones. It has been concluded that for the cases studied, dynamic effects are relatively small and a static analysis is sufficient to characterize the interface stress condition.

Aged↗
PubMed1990

Prediction of stature from hand measurements.

A sample of 166 normal adult males and females was taken from different colleges of Assiut representing those living in upper Egypt. Each subject has been studied for measurements of stature (S), hand length (HL) and hand breadth (HB). The data were statistically analysed in order to assess the relationship between stature and hand measurements. The correlation matrix of the study indicates close similarity of the relationship between stature and hand measurements in both sexes and in both sides. A generalized multiple regression equation has been designed to estimate stature from values of hand length and hand breadth regardless of sex or side in the form: S = 34.5 + 5.77 HL + 2.7 HB +/- 5.1. This equation may be helpful to obtain approximate stature when there is difficulty in obtaining a direct measurement or where there is a chance print of a criminal or an amputated hand or arm.

Adult↗
PubMed1990

The effect of facet geometry on the axial torque-rotation response of lumbar motion segments.

The objective of this study is to examine the effect of the facet geometry on the axial torque-rotation response of lumbar motion segments. The three-dimensional facet geometry of 35 L2-3 and 35 L4-5 specimens has been quantified in terms of seven parameters from computed tomographic transverse sections taken at 2-mm intervals. The motion segments have then been subjected to axial torque, for a variety of preload conditions, to establish the axial torque-rotation response. There exists significant variation of facet geometry between the segment levels; however, the degree of facet asymmetry is identical for each level. No significant correlation exists between the lumbar facet geometry and the axial torque-rotation response. The facet joints do, however, act as a "positive stop" to axial rotation, regardless of their orientation. Therefore, axial rotation sufficient to cause disc injury may only be possible after facet failure.

Cadaver↗
PubMed1990

Investigation of the laminate structure of lumbar disc anulus fibrosus.

The structure of the lumbar disc anulus fibrosus was investigated using a layer-by-layer peeling technique and microscopic examination of various cut surfaces. Anulus specimens from spines of two different age groups and from two levels, L2-3 and L4-5, were examined. The vertebra-disc-vertebra units were subjected to intentional controlled dehydration to enhance the visual contrast between the white opaque fiber bundles and the translucent ground substance. The variations of the anulus structure with circumferential and radial locations were studied. The following principal structural features were quantified: 1) the anulus, excluding the transition zone, consists of 15 to 25 distinct layers, depending on the circumferential location, the spine level, and the specimen age; 2) in any 20 degrees circumferential sector, nearly half of the layers terminate or originate, thereby causing local laminate irregularities; 3) there are two identifiable mechanisms of layer interruption at these irregularities; 4) the thickness of individual layers varies both circumferentially and radially and increases markedly with age; and 5) the number of fiber bundles over the total height of the disc varies from 20 to 62, with an average interbundle spacing of 0.22 mm.

Adult↗
PubMed1989

A parametric axisymmetric model study on the interface motions in porous-surfaced tibial implants.

The effects of a number of variables on the interface relative motions in porous-surfaced tibial implants are investigated using a simplified axisymmetric finite element model. The parameters considered are contact or link spacing, height of the central metal stem, presence of a circumferential metal flange, presence of a UHMWP articular plate resting freely on or fixed to the metal base, resting of the prosthesis edge on the cortical shell, and type of the metal alloy. In order to represent the immediate post-surgical situation with no bone ingrowth, the interface between the bone and porous-surfaced metal is modelled by frictionless rigid links oriented normal to the interface. Cases are also studied in which the horizontal interface is assumed to be fixed while the vertical interface remains frictionless. The magnitudes of the interface motion are negligibly affected by the variation in the link spacing from 0.3 mm to about 3.0 mm. The interface relative motion is predicted to decrease in cases with a shorter central metal stem, with the addition of a circumferential metal flange, with the use of more rigid prosthesis, and with the addition of a UHMWP articular plate.

Humans↗
PubMed1989

Khat consumption: a pharmacological review.

The present review deals with the considerable body of evidence gathered in the last ten years on the clinical and experimental pharmacology of Khat. Khat effects are generally agreed to be of amphetamine-like type. In particular, Khat ingestion, like amphetamine ingestion, produces sympathetic activation, anorexia, euphoria, increased intellectual efficiency and alertness. These effects are mainly mediated by phenylalkylamines, such as cathinone and cathine, because the pharmacological actions of these agents and those produced by amphetamine almost overlap. In infra-human species cathinone is an effective positive reinforcer (i.e., it maintains self-administration). However, it would be inappropriate to infer from cathinone and cathine effects assessed in animals a high potential of abuse for Khat in humans; apart from other reasons the bulk volume of Khat leaves, limits the ingestion of high quantities of the active principles. Accordingly, in habitual consumers Khat dependence is probably mild, because craving and tolerance to the sympathomimetic and neuroendocrine effects of Khat are present, but there is no definite abstinence syndrome. Therefore, in our opinion, policies restricting the use of Khat should be adopted with caution, lest they simply change the pattern of drug abuse and increase the spread of more dangerous drugs.

Animals↗
PubMed1988

Arachidonic acid metabolism and casein secretion in lactating rabbit mammary epithelial cells: effects of inhibitors of prostaglandins and leukotrienes synthesis.

The metabolism of arachidonic acid (AA) in fragments of lactating rabbit mammary glands in vitro was studied by considering the distribution of 13-[14C]AA in the cells, and the effects of inhibitors of cyclooxygenase and lipoxygenase pathway on the basal and prolactin (PRL)-stimulated casein secretion. 13-[14C]AA was incorporated in all classes of lipids and PRL increased transiently the percentage of free fatty acid after 1 and 5 min. Ten microM ETYA (5,8,11,14-Eicosatetraynoic acid), a tetrayne analogue of AA inhibited prostaglandins F2 alpha (PGF2 alpha) production but not leukotrienes B4 and C4 (LTB4 and LTC4) production and increased basal casein secretion. 10(-4) M DCHA (Docosahexaenoic acid) a competitive inhibitor of prostaglandin-synthetase inhibited PGF2 alpha production but did not affect basal nor PRL-stimulated casein secretion. Fourteen microM indomethacin inhibited PGF2 alpha and LTC4 production and PRL-stimulated casein secretion. Ten microM NdgA (nordihydroguaiaretic acid) an inhibitor of lipoxygenase pathway, inhibited LTB4 and LTC4 production, increased basal level of casein secretion and inhibited PRL-stimulated casein secretion. Hundred microM caffeic acid, an inhibitor of glutathione-S-transferase (GST), a class of enzymes implied in the transformation of LTA4 into LTC4, had the same effect that NDGA on basal and PRL-stimulated casein secretion. These findings show that inhibitors of AA metabolites alter casein secretion.

5,8,11,14-Eicosatetraynoic Acid↗
PubMed1988

A stress compatible finite element for implant/cement interface analyses.

A new finite element has been developed to enforce normal and shear stress continuity at bimaterial interface points in order to alleviate the problem of high stress discontinuity predictions by the conventional displacement finite element method. The proposed element is based on a five node isoparametric quadrilateral element where the fifth node is located at the interface boundary of the element. A series of validation tests have been carried out to assess the correctness of the stress distribution obtained by the new element at interfaces of highly dissimilar materials. The results of the tests are compared to analytical solutions and to results from convergence studies performed by the conventional finite element method (SAP-IV). Overall, the proposed element has been demonstrated to have a very satisfactory degree of reliability, especially in view of the observed inability of the conventional method to yield interpretable interface stress values for most cases analyzed. Finally, the new interface element has been applied to the analysis of an axisymmetric model of the knee tibial implant. The superiority of the proposed element over the conventional one has been demonstrated in this case by a convergence study.

Bone Cements↗
PubMed1988

Brief footshock analgesia: long-lasting enhancement induced by cathinone, an amphetamine-like agent.

In view of the analgesic effects produced by cathinone (CATH), an amphetamine-like agent, and of the interaction of amphetamines with stressful environmental stimuli, the present study evaluated in rats the influence of CATH on the nonopioid analgesia induced by a brief electric footshock (FSA; 3 min of continuous 2.5 mA current). The influence of this combination on body temperature was also evaluated. CATH (5 mg/kg, i.p.) alone induced a brief and slight increase in latency during the hot plate test (HPT), but enhanced and prolonged the analgesic effect induced by FS. In addition, the presentation of the environment (shock box with unelectrified grid) where other rats received FS, caused CATH to induce a slow-rising analgesic effect for 180 min. A hyperthermic response paralleling the analgesic effect was observed in shocked and nonshocked rats receiving CATH. After 24 h, rats that had received both CATH and FS on the previous day showed prolonged latencies on the HPT before and after a 1-min presentation of unelectrified grid. These animals also showed an increased analgesic response to the subsequent application of a 15-second FS. At the same time no differences in body temperature were observed between treatment groups. These results suggest that CATH can interact with environmental stimuli to induce an analgesic effect, the time-course of which depends upon the intensity of the stimulus applied.

Alkaloids↗
PubMed1987

Force analysis of the patellar mechanism.

The aim of this study is an experimental evaluation of a force analysis of the patellar mechanism based on the assumption that patellofemoral contact is frictionless. At first, the geometric characteristics of contact surfaces, a prior knowledge of which is necessary for quantitative analysis, were measured from radiographs of 42 fresh-frozen knee specimens in the flexion range 0-120 degrees. The results were then used in the analysis to predict the relations between the forces acting on the patella. For the evaluation of the analysis, the ratio of the tension in the ligamentum patellae and the rectus femoris was measured in ten specimens during simulation of two knee functions: (1) "leg raising" against a resistance; and (2) "static lifting". The effect of flexion angle on the ratio is found to be rather complex. With increasing flexion, the ratio increases initially up to 30 degrees, then decreases up to 90 degrees, and finally increases again beyond 90 degrees. The ratio is above unity up to around 45 degrees and below that in the remaining flexion range. The analysis has been found to predict not only the characteristic variation of the ratio but also its magnitude with reasonable accuracy. It has been concluded that for an accurate prediction of the patellofemoral joint reaction, the force analysis needs to be based on the geometry of the contact surfaces. This implies that the mechanical consequences of surgical procedures involving tibial tubercle relocation cannot be inferred simply on the basis of their effect on the patellar mechanism angle, but that they also require consideration of their effect on the contact geometry.

Biomechanical Phenomena↗
PubMed1987

In-vitro ligament tension pattern in the flexed knee in passive loading.

Tensions generated in selected bands of the four major ligaments of the flexed knee (40-90 degrees) have been measured in vitro when the tibia is subjected to passive anterior translation and axial rotation with and without a compressive preload. The measurements were made in 30 fresh-frozen specimens using the buckle transducer attached to the anteromedial band of the anterior cruciate ligament [ACL (am)], the posterior fibres of the posterior cruciate ligament [PCL (pf)], the superficial fibres of the medial collateral ligament [MCL (sf)], and in the total lateral collateral ligament (LCL). Particular attention was placed on the evaluation of the performance of the transducer specific to such measurements in order to minimize the errors associated with the use of this transducer. The results indicate that, among the measured ligaments, substantial tension (greater than 20 N) is generated only in the ACL (am) in tibial anterior translation up to 5 mm. The tension pattern generated in response to tibial axial rotation, however, is complex and exhibits considerable variation between specimens. In general, both the MCL (sf) and LCL are tensed at all tested flexion angles, with the tension in external rotation being significantly greater than in internal rotation. At 40 degrees of flexion, the ACL (am) bears tension mainly in internal rotation, while at 90 degrees of flexion the PCL (pf) is tensed in both senses of rotation. The response of the LCL shows marked variation among specimens; very small tension (less than 15 N) is generated in internal rotation in 48% of the specimens, and in either sense of rotation in 20% of the specimens. The tension in the ACL (am) in internal rotation is invariably greater in those specimens in which LCL tension is negligible. This correlation between increased ACL (am) function and inadequate LCL restraint appears significant in terms of ACL injury and repair.

Humans↗
PubMed1987

A simple unconstrained dynamic knee simulator.

The design of a simple dynamic knee simulator is described. In the simulator the joint dynamics are reproduced in-vitro in a knee specimen by controlling the time-histories of the tensions in two flexible cables acting as lumped muscle group equivalents, without constraining the natural conjunct and passive motions of the specimen. The two cable tensions acting individually are used to control the active flexion/extension motion, while their simultaneous action is used to control joint compressive force. The characteristics of the electrohydraulic servo system acting under real-time microprocessor control are described. The system performance during simulation of an idealized level-walking function is evaluated.

Biomechanical Phenomena↗
PubMed1986

A finite element study of a lumbar motion segment subjected to pure sagittal plane moments.

A nonlinear finite element program has been developed and applied to the analysis of a three-dimensional model of the lumbar L2-3 motion segment subjected to sagittal plane moments. The analysis accounts for both material and geometric nonlinearities and is based on the Updated Lagrangian approach. The disc nucleus has been considered as an incompressible inviscid fluid and the annulus as a composite of collagenous fibres embedded in a matrix of ground substance. Articulation at the facet joints has been treated as a general moving contact problem and the spinal ligaments have been modelled as a collection of nonlinear axial elements. Effects of the loss of intradiscal pressure in flexion and of facetectomy in extension have been analyzed. Comparison of the predicted gross response characteristics with available measurements indicates satisfactory agreement. In flexion relatively large intradiscal pressures are generated, while in extension negative pressures (i.e. suction) of low magnitude are predicted. The stress distribution results indicate that the load transfer path through the posterior elements of the joint in flexion is different from that in extension. In flexion the ligaments are the means of load transfer, while in extension the load is transmitted through the pedicles, laminae and articular processes. In flexion, the inner annulus fibres at the posterolateral location are subject to maximum tensile strain. It is suggested that large flexion moment in combination with other loads is a likely cause of disc prolapse commonly found at this location of the annulus.

Humans↗