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Query: UMLS:C0015672 (fatigue)
 51,768 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Glutaraldehyde-preserved porcine mitral leaflet tissue has been subjected to extended accelerated fatigue loading in Ringer's solution containing 0.15% glutaraldehyde. Five tissue test pieces were subjected to cyclic tensile stresses of 50 and 200 Gm. per square millimeter and to 300 million to 800 million accumulated fatigue cycles. Tissue disruption occurred in each of the fatigued test pieces. Tensile loading, apart from reducing the acuteness of the collagen waveform and thereby decreasing tissue compliance, does not contribute significantly to the disruption process nor its rate of occurrence. Compressive flexure occurring during the unloading half of the fatigue cycle, however, does induce damage in the tissue. Mechanisms involved in the disruptive processes have been identified by conducting simultaneous morphologic and stress/strain observations on both the fatigued and unfatigued tissues in their wet functional condition. This vulnerability of the preserved tissue to compressive flexure could well affect the long-term durability of the glutaraldehyde-preserved heterograft valve, and this possibility is discussed in relation to the clinical use of these valves.
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PMID:Fatigue-induced damage in glutaraldehyde-preserved heart valve tissue. 9 72

The influence of both imposed anodic potential in Ringer's solution and surface finish on the fatigue lives of annealed 316 type stainless steel and annealed pure titanium were measured and statistically compared to fatigue data run in air. The applied potentials in simulated extracellular fluid approximated conditions existing within the body while also producing the types of surface defects actually found on removed long time implants within the time interval of the accelerated R. R. Moor fatigue tests. Differentiating tests were run at single levels of applied cyclic stress well above the endurance limits. In Ringer's solution, the fatigue life of the 316 stainless steel decreased with increasing applied potential, and at +500 mV was significantly shorter than when run in air. At each condition, the 316 stainless steel was independent of initial surface finish. In contrast, the fatigue life of titanium improved rapidly with increasingly fine surface finishes. Furthermore, compared to air, the application of +500 mV in Ringer's solution improved the life of the rough surface finished material and markedly increased to number of cycles to failure for the electopolished specimens.
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PMID:The influences of electrical potential and surface finish on the fatigue life of surgical implant materials. 118 9

A scanning electron microscopy study is reported of the nature and morphology of fracture surfaces in pyrocarbons commonly used for the manufacture of mechanical heart-valve prostheses. Specifically, silicon-alloyed low-temperature-isotropic (LTI)-pyrolytic carbon is examined, both as a coating on graphite and as a monolithic material, following overload, stress corrosion (static fatigue), and cyclic fatigue failures in a simulated physiological environment of 37 degrees C Ringer's solution. It is found that, in contrast to most metallic materials yet in keeping with many ceramics, there are no distinct fracture morphologies in pyro-carbons which are characteristic of a specific mode of loading; fracture surfaces appear to be identical for both catastrophic and subcritical crack growth under either sustained or cyclic loading. We conclude that caution should be used in assigning the likely cause of failure of pyrolytic carbon heart-valve components using fractographic examination.
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PMID:On the fractography of overload, stress corrosion, and cyclic fatigue failures in pyrolytic-carbon materials used in prosthetic heart-valve devices. 157 36

1. The purpose of this investigation was to determine if alterations in extracellular calcium (Ca2+) influx by the dihydropyridine derivatives Bay K 8644 and nifedipine affected skeletal muscle fatigue. 2. Tetanic contractions (80 Hz, 100 msec) of frog sartorius muscles were evoked every sec for 3 min. Muscles were fatigued in normal Ringer's solution (NR), in NR containing 1 microM nifedipine of 10 microM Bay K 8644 or in low Ca2+ Ringer's. 3. In each case, the experimental conditions increased the rate and magnitude of fatigue. Rate constants of fatigue obtained during Bay K 8644, nifedipine and low Ca2+ conditions (-.0122 +/- .0016, -.0397 +/- 0022 and 0.0169 +/- .0064 sec-1, respectively) were significantly greater than NR (-.0104 +/- .0006 sec-1, p less than .05). In addition, tetanic forces developed at the end of the stimulation period under the experimental conditions (3.90 +/- 0.81, 1.21 +/- 1.40 and 2.04 +/- 1.10% of initial) were significantly less than NR (7.18 +/- 1.27%, p less than .05). 4. Caffeine contracture forces (10 mM) evoked immediately after stimulation were not significantly different between conditions. 5. These results suggest that alterations in sarcolemmal Ca2+ exchange has some influence on the fatigue process.
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PMID:Dihydropyridine effects on skeletal muscle fatigue. 172 73

The purpose of this investigation was to determine the effects of low extracellular calcium and calcium antagonists on skeletal muscle staircase and fatigue. Initial experiments revealed that, brief exposure (10 minutes) of single frog sartorius muscle to diltiazem, D-600 (5 and 30 microM) and low calcium Ringer's solution (LCR, calcium replaced by magnesium and EGTA) had little effect on isometric twitches evoked every 30 seconds. However, when stimulated at 1 per second for 15 minutes, the calcium antagonists significantly decreased the magnitude and time course of the staircase, whereas LCR decreased only the time course. Each experimental condition significantly increased the rate of fatigue while diltiazem and D-600 both increased the magnitude of fatigue. Following the stimulation period, caffeine (10 mM) elicited contractures from all muscles whereas high potassium (180 mM) elicited contractures from control muscles only. These results indicate that calcium channel antagonists depress the skeletal muscle staircase response. They also indicate that these compounds as well as LCR enhance the fatigue process. Extracellular calcium influx may therefore have some influence on skeletal muscle twitches during prolonged repetitive activity.
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PMID:Effects of low calcium and calcium antagonists on skeletal muscle staircase and fatigue. 217 74

isometric tension and maximum velocity of shortening of frog sartorious and biceps muscles were measured at varying pH and compared with the values obtained for muscles treated with DNFB. Both To and Vmax exhibited increase with increase in pH above neutral pH upto pH 9, and decreased as the pH was decreased up to 5. Muscle treated with DNFB at pH 7 showed about 30% decrement but these too improved at pH 9 to almost 105% and 130% respectively compared to untreated muscle at pH 7. Using the number of short duration tetanic contractions, which reduce To and Vmax by half, as an index for the onset of fatigue, high pH was found to have a positive effect in both normal and DNFB-treated muscle. Thus, the crucial factor for onset of fatigue is not a fall in ATP level but acidification and treatment with high pH Ringer's solution partially annuls the effect of acidosis, arising either naturally or from DNFB treatment. One additional role of creatine kinase activity to that of ATP regeneration is suggested to be the maintenance of neutral pH in the sarcoplasm.
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PMID:Effect of extracellular pH and DNFB treatment on the mechanical performance of frog skeletal muscle. 262 70

Two mixing methods, hand and vacuum mixing, were compared using high viscosity, high molecular, antibiotic containing polymethylmethacrylate kept at two different temperatures, 4 degrees C and 21 degrees C, prior to mixing. The mechanical properties, i.e., fracture strength, maximum deflection, modulus of elasticity and hardness, were improved by vacuum mixing when compared with hand mixing at both temperatures. The fatigue life was 10 times longer after vacuum mixing. Chilling prior to mixing made the mixing easier and improved the handling characteristics. Vacuum mixing delayed the setting time by 1 minute, but also decreased the peak temperature. Radiographic analysis showed that vacuum mixing mainly reduced the microporosity, but also the macroporosity. The mechanical properties deteriorated slightly after 2 months in Ringer's solution, but the differences between the mixing procedures remained unchanged.
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PMID:Bone cement improved by vacuum mixing and chilling. 357 37

A method of attaching ceramic femoral heads to metal stems has been developed and tested in the laboratory. Impact tests, fatigue tests in Ringer's solution at body temperature, and tests of the strength of fixation of head to stem, suggest that the method offers significant advantages over the taper-fit method of attachment.
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PMID:Laboratory development of a method of attaching ceramic femoral heads to metal stems. 370 68

Several reports have suggested that the nervous system can be affected by exposure to electric fields and that these effects may have detrimental health consequences for the exposed organism. The purpose of this study was to investigate the effects of chronic (30-day) exposure of rats to a 60Hz, 100-kV/m electric field on synaptic transmission and peripheral-nerve function. One hundred forty-four rats, housed in individual polycarbonate cages were exposed to uniform, vertical, 60-Hz electric fields in a system free of corona discharge and ozone formation and in which the animals did not receive spark discharges or other shocks during exposure. Following 30 days of exposure to the electric field, superior cervical sympathetic ganglia, vagus and sciatic nerves were removed from rats anesthetized with urethan, placed in a temperature-controlled chamber, and superfused with a modified mammalian Ringer's solution equilibrated with 95% O2 and 5% CO2. Several measures and tests were used to characterize synaptic transmission and peripheral-nerve function. These included amplitude, area, and configuration of the postsynaptic or whole-nerve compound-action potential; conduction velocity; accommodation; refractory period; strength-duration curves; conditioning-test (C-T) response, frequency response; post-tetanic response; and high-frequency-induced fatigue. The results of a series of neurophysiologic tests and measurements indicate that only synaptic transmission is significantly and consistently affected by chronic (30-day) exposure to a 60-Hz, 100-kV/m electric field. Specifically, and increase in synaptic excitability was detected in replicated measurements of the C-T response ratio. In addition, there are trends in other data that can be interpreted to suggest a generalized increase in neuronal excitability in exposed animals.
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PMID:Chronic exposure to a 60-Hz electric field: effects on synaptic transmission and peripheral nerve function in the rat. 626 52

Leaflet tissue specimens prepared from porcine aortic valves treated with glutaraldehyde at low and high pressures have been subjected to 0.45 x 10(9) accelerated fatigue cycles in Ringer's solution. The waveform or crimped property of the collagen fibres in the leaflet tissue is an essential requirement for its ability to resist localized deformation during repeated flexure. High pressure glutaraldehyde fixation of the whole valve eliminated the crimp structure and resulted in the formation of localized kink sites in the tissue specimen during repeated flexure. Eleven separate sites of serious tissue disruption were observed in the three fatigue specimens obtained from the high pressure-treated valve. In contrast to this only one site of serious disruption was observed in the three fatigue specimens obtained from the low pressure-treated valve. Here fixation preserved the fully crimped morphology of the collagen. It is expected that the long-term mechanical durability of glutaraldehyde treated aortic valves can be substantially increased if careful consideration is given to the pressure at which initial fixation is carried out.
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PMID:An 'in vitro' study of mechanical fatigue in glutaraldehyde-treated porcine aortic valve tissue. 678 50


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