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 51,768 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Recent reports have advocated the use of polytetrafluoroethylene (PTFE) suture for replacement or reinforcement of ruptured or elongated mitral valve chordae tendineae. The mechanical properties of PTFE (Gore-Tex) and other sutures were determined and compared to those of porcine mitral valve chordae. The results were analyzed to assess how closely chordal mechanical function may be simulated by synthetic suture materials. Chordae tendineae and suture samples were tested in uniaxial tension using an INSTRON Model 1000 at strain rates of 5 and 10 mm/min. The stress (g/mm2) was plotted versus strain, and the elastic modulus determined as the slope of the curve. Chordae tendineae exhibited a nonlinear viscoelastic stress/strain behavior. The elastic modulus of both suture types tested was significantly higher than that of the chordae. However, the PTFE suture did exhibit some viscoelastic characteristics (hysteresis and creep) that begin to approach the chordal behavior. Chordal viscoelastic behavior results from the inherent composite structure (collagen, elastin, endothelium, water, and ground substance). As yet, no synthetic materials are able to imitate this behavior with the appropriate tensile strength and fatigue resistant characteristics. At present, PTFE appears to be the best synthetic alternative for chordal replacement, due to its limited viscoelastic capabilities. Nevertheless, the need to more nearly approximate the mechanical behavior of mitral valve chordae tendineae with synthetic material warrants further investigation.
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PMID:Comparison of viscoelastic properties of suture versus porcine mitral valve chordae tendineae. 181 76

The mechanical properties of bones are governed by the same principles as those of man-made load-bearing structures, but the organism is able to adapt its bone structure to changes in skeletal loading. In this overview of the determinants of the strength and stiffness of bone, a continuum approach has been taken, in which the behavior of a macroscopic structure depends on its shape and size, and on the mechanical properties of the material within. The latter are assumed to depend on the composition (porosity and mineralization) and organization (trabecular or cortical bone architecture, collagen fiber orientation, fatigue damage) of the bone. The effects of each of these factors are reviewed. Also, the possible means of non-invasively estimating the strength or other mechanical properties of a bone are reviewed, including quantitative computed tomography, photon absorptiometry, and ultrasonic measurements. The best estimates of strength have been obtained with photon absorptiometry and computed tomography, which at best are capable of accounting for 90% of the strength variability in a simple in vitro test, but results from different laboratories have been highly variable.
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PMID:Determinants of the mechanical properties of bones. 184 37

Specimens of articular cartilage from the superficial and mid-depth zones of the human femoral head and the talus of the ankle joint were tested in tension in planes parallel to the articular surface and parallel to the predominant orientation of the superficial collagen fibrils. The tensile fracture stress of cartilage from both the superficial and mid-depth zones of the femoral head decreased considerably with age. The superficial zone decreased from 33 MPa at 7 years to 10 MPa by the age of 90 years, while the mid-depth zone decreased from 32 MPa at 7 years to 2 MPa by the age of 85 years. In contrast the fracture stress of both levels of cartilage from the talus of the ankle did not decrease significantly with increasing age. The tensile stiffness at 10 MPa of both the superficial and mid-depth zones of the femoral head decreased with age. That of the superficial zone decreased from 150 MPa at 7 years to 80 MPa at 90 years, while the mid-depth zone decreased from 60 MPa at 7 years to 10 MPa at 60 years. The stiffness of talar cartilage from the superficial zone decreased by 20%, while that of the mid-depth zone showed a slight increase in stiffness at 10 MPa with increasing age. There was no significant decrease in the tensile stiffness at 1 MPa with age for either the femoral head or talar cartilage. Based on the results of previous studies it is possible to conclude that the decrease in tensile properties seen in the femoral head results from a deterioration in the tensile properties of the network of collagen fibrils. It is suggested that progressive fatigue failure, perhaps with associated changes in the structure of cartilage due to altered chondrocyte metabolism, causes the reduction in tensile properties with age. The results offer a potential explanation for the observation that osteoarthritis commonly occurs in the hip and knee joints at an increasing incidence as age increases, while the condition only rarely occurs in the ankle joint except as a secondary event to trauma.
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PMID:Age-related changes in the tensile properties of human articular cartilage: a comparative study between the femoral head of the hip joint and the talus of the ankle joint. 195 24

Cross-linked collagen bioprostheses usually are designed to be inert and nonresorbable, resulting in fatigue and wear failure in high-stress environments. Eventual replacement of the implant, although minimizing strength loss during resorption, would result in a graft with reparative ability. Kangaroo tail tendon (KTT) partially cross-linked with glutaraldehyde (GA) was evaluated in vitro for resistance to bacterial collagenase digestion and in vivo for biocompatibility and resorbability in an intramuscular implant assay. Cross-linking was quantified by thermal denaturation studies. Incomplete cross-linking was achieved with concentrations of GA less than 0.1% (w/v). KTT cross-linked in greater than or equal to 0.05% GA were collagenase resistant being incompletely digested after 240 h. Cross-linking of KTT with low concentrations of GA resulted in partial collagenase resistance and slowed resorption.
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PMID:Collagen cross-linking and resorption: effect of glutaraldehyde concentration. 212 27

Pericarbon, a new generation pericardial valve, is characterized by a single, three cuspal shaped pericardium sheet, which is sutured to a second sheet lining the inner surface of the plastic, low profile stent. A coating of hemocompatible carbon film covers all the exposed, nonbiological surfaces. Optimal preservation of collagen and graft cells is achieved by fresh tissue glutaraldehyde fixation and cusp shaping without mold. Accelerated fatigue testing showed a duration of over 150 million cycles, a figure much higher than that observed when current pericardial and porcine valves were tested with the same apparatus. Results of long-term (greater than 7 months, average 10.5) implantation in 20 sheep (13 mitral, 7 tricuspid) disclosed no case of mechanical failure, mild to moderate primary calcification in older explants, and significant fibrous tissue overgrowth only in the tricuspid position. Transmission electron microscopy studies revealed collagen and elastic fiber integrity, no significant plasma protein insudation, and well-preserved graft cells. Re-endothelialization by host cells was a regular finding on scanning electronic microscopy. Early ultrastructural nuclei of calcification were seen mostly on collagen fibers. Pericarbon presents basic changes in pericardial valve design, and optimal morphological preservation is obtained after industrial processing. Accelerated fatigue tests in vitro show long duration. At medium long-term animal experimental follow-up, mechanical failure was not observed; significant host tissue reaction occurred in the tricuspid but not in the mitral position; primary calcification increased progressively with time and involved mainly collagen fibers.
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PMID:Evaluation of Pericarbon valve prosthesis: in vitro, ultrastructural, and animal studies. 251 86

The hypotheses that defective platelet structure and function is the basis for migraines is presented, with evidence explaining the biochemical, clinical, pathological, and pharmacological aspects of migraine. Platelets undergo 2 types of reaction, a shape change and a granule release reaction, releasing adenosine diphosphate (ADP) serotonin 5-hydroxy-tryptamine (5-HT), and thromboglobulin in response to collagen and thrombin. Platelets from migraine suffers contain more ADP, have more dense granules, and show some qualitative differences in their release reaction. Their platelets aggregate more readily when exposed to 5-HT, their platelet fibrinogen receptors have greater affinity, and their platelet membranes show altered viscosity. Some drugs that inhibit platelet aggregation, such as methysergide, aspirin, and amitryptylline, are beneficial in cases of migraine. Some migraine triggers, such as tyramine and catecholamines, are known to be vasoactive. The release by platelets of 5-HT may account for the visual aura or prodrome that migraine patients experience. Some migraine precipitating factors, such as stress, fatigue, hunger, certain foods, and hormones, may stimulate 5-HT release by platelets. Alterations in hormones, notably puberty, menstruation, oral contraceptive use, and menopause, are characterized by altered platelet aggregation and by onset of migraine in previously healthy people. Other arguments in favor of the platelet hypothesis involve prostacyclin deficit during menstruation and migraine associated with sudden decline in platelet numbers in cases of thrombocytopenic purpura and essential thrombocythemia.
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PMID:Migraine: the platelet hypothesis after 10 years. 270 Dec 86

The force required to pull sutures out of glutaraldehyde fixed bovine pericardium, for four different suture bites: 0.5 mm, 1 mm, 1.5 mm and 2.0 mm, was compared with the tissue strength. The mean suture pull-out force was significantly lower than the tissue strength for all bites, with a minimum value of 2.86 +/- 1.02 N for the 0.5 mm bite and a maximum value of 6.32 +/- 0.77 N for the 2.0 mm bite. The mean force which produced failure of the chemically modified pericardium was 15.49 +/- 8.48 N. The mean force at pull-out of the sutures lay on a regression line: force at failure = 1.68 + 2.25 x Bite. A video film of the experiments showed that the suture does not cut through the pericardium. It pulls a V-shaped band of collagen fibre bundles through the stationary pericardium. Eventually this band breaks away from the free edge of the tissue specimen. The specimens under uniaxial load failed by laminate debonding of two layers of tissue, rupture of the serosal surface layer followed by shear and fibre slippage. These results indicate that any suture which bears load, during the normal functioning of a heart valve substitute, will be a source of weakness, compared to the overall tissue strength. As a consequence the alignment/holding suture of the Standard Ionescu-Shiley valve and the modified stitch of the low profile valve are likely to be potential sites of fatigue failure.
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PMID:Pericardial heterografts: a comparative study of suture pull-out and tissue strength. 275 11

The purpose of the present investigation was to study the effects of both advancing age and life-long endurance training on the connective tissue and fibre composition of two types of rat skeletal muscle. In particular, additional evidence was sought on age- and training-induced transformations of muscle fibres, and on the significance of intramuscular collagen in muscle functioning. For this purpose a combined study of exercise and ageing throughout the life-span of the experimental animals was constructed. To gain a broad view of muscle responsiveness the muscle fibre types, the estimates of different biochemical and histological properties of collagen as well as of the passive mechanical properties of both a slow (m. soleus = MS) and a fast (m. rectus femoris = MRF) skeletal muscle were determined. The results can be summarized as follows: 1) The percentage of type I fibres in MS increased from about 60% in one-month-old rats up to about 90% in the untrained and almost 100% in the trained rats at ages between 4 and 10 months. In adult animals the proportion of type I fibres was significantly higher in the MS of the trained than untrained animals. In the slow area of MRF, the percentage of type IIA fibres increased from a mean value of below 40% in one-month-old rats to above 50% in the untrained and near to 70% in the trained rats at the age of 24 months. Correspondingly, the proportion of type IIB fibres decreased with both age and training. In both types of muscles, there was a tendency towards smaller cross-sectional areas for the predominant fibre type in the trained rats when compared to the untrained rats. Consequently, the long-term endurance training used did not reverse the age-related shift in muscle fibre composition but clearly accelerated the fibre transformation towards more fatigue-resistant muscle fibres with slower contractile speeds. Nevertheless, the slowing due to endurance training is not necessarily deleterious, as different mechanisms may be involved in these age- and training-related alterations. The former appears to involve degenerative changes in the neuromuscular system whereas endurance-type activity could assist in the maintenance of the low-threshold neural activity important for the expression of slow contractile characteristics. 2) The slow postural soleus contained more collagen compared with the fast locomotor rectus femoris muscle. The concentration of total collagen as well as the area-fractions of both endomysium and perimysium were larger in MS than in MRF.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:Effects of ageing and physical training on rat skeletal muscle. An experimental study on the properties of collagen, laminin, and fibre types in muscles serving different functions. 292 97

There is increasing evidence that platelets play an important role in the pathogenesis of acute ischemic heart disease. Therefore an understanding of factors that influence platelet performance is important. This study was undertaken (1) to characterize during evolving myocardial infarction platelet activity in the peripheral circulation and across the ischemic/infarcting myocardial compartment, the locus of presumed platelet hyperactivity, and (2) to evaluate the effects of prostacyclin (PGI2), a most potent antiplatelet agent and vasodilator. A total of 59 patients with evolving myocardial infarction were studied. Twenty-two patients were instrumented with arterial and coronary sinus catheters and received intravenous infusion of PGI2, 13 +/- 4.5 ng/kg/min (mean +/- SD), for 90 min. In 15 patients with anterior myocardial infarction, transcardiac platelet function and response to PGI2 were studied. Plasma levels of beta-thromboglobulin (beta-TG) and of thromboxane B2 (TxB2), in vivo measures of platelet activity, were elevated three- and 10-fold. 6-Keto-prostaglandin F 1 alpha, the stable end product of PGI2, was less than 10 pg/ml, reflecting a leftward shift of the TxB2/PGI2 ratio. Platelets circulating during evolving myocardial infarction ("ischemic platelets") were hyperaggregable in response to ADP and relatively resistant to PGI2, both in vivo and in vitro. Concentrations of platelet cyclic AMP and the cyclic AMP response to PGI2 were diminished. The platelet hyperreactivity, expressed by plasma beta-TG, platelet aggregation, and PGI2-induced inhibition of aggregation, was most intense early during infarct evolution and decreased with time. The increased platelet performance resulted in "platelet fatigue," indicated by decreased contents of beta-TG of the ischemic platelet and decreased TxA2 production in response to collagen. However, the ischemic platelet produced twice normal TxA2 in response to arachidonic acid (stimulus and substrate), demonstrating a heightened metabolic capacity. TxA2 was produced across the ischemic/infarcting compartment in 10 of 15 patients with anterior myocardial infarction. The antiplatelet effect of PGI2 was greatly diminished. In summary, the data define an abnormal pattern of platelet behavior during evolving myocardial infarction, characterized by a proaggregatory environment, heightened platelet reactivity in both the peripheral and coronary circulation, and relative resistance to PGI2. The clinical consequences of the data are that the patient in the acute phase of myocardial infarction may benefit from suppression of platelet function and requires significantly greater doses of PGI2 than normal subjects.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:Systemic and transcardiac platelet activity in acute myocardial infarction in man: resistance to prostacyclin. 293 81

Gamma-interferon appears to be a pivotal molecule in the immune system. Recombinant DNA technology has permitted the cloning and expression of the gene for gamma-interferon (gamma-IFN), leading to an exponential increase in the level of knowledge both in vitro and in vivo. Laboratory evidence suggests a clinical role for gamma-IFN in collagen vascular disease, chronic granulomatous infectious diseases, hematology, and medical oncology. Phase I trials have identified the toxicities; these toxicities are primarily clinical and include fever, fatigue, flu-like symptoms, and hypotension. Antitumor activity has been noted in the early development of the drug.
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PMID:Gamma-interferon: physiology and speculation on its role in medicine. 311 Mar 78


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