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

We have recently demonstrated that noncalcific tissue damage can lead to significant collagen degradation in clinically explanted bioprosthetic heart valves (BHVs). In the present study we quantified the early response of glutaraldehyde treated bovine pericardium (GLBP) to cyclic tensile loading to begin to elucidate the mechanisms of noncalcific tissue degeneration in BHV biomaterials. GLBP specimens were cycled at 30 Hz to a maximum uniaxial strain of 16% (corresponding to approximately 1-MPa peak stress), with the loading direction parallel to the preferred collagen fiber (PD) direction. After 30 x 10(6) cycles, specimens were subjected to biaxial mechanical testing, then cycled until 65 x 10(6) cycles. The results indicated a permanent change in the unloaded tissue dimensions of +7.1% strain in the PD direction and -7.7% strain in the cross fiber direction (XD) after 65 x 10(6) cycles and an increase of the collagen crimp period from 40.6 to 45.2 microm by 65 x 10(6) cycles (p = 0.05). Fourier transform IR spectroscopy analysis indicated that cyclic fatigue of GLBP leads to both collagen conformational changes and early denaturation. Furthermore, no significant changes in areal strain were found under 1-MPa equibiaxial stress, indicating that cyclic loading changed the collagen fiber orientation but not the overall tissue compliance. These observations suggest that while deterioration of collagen begins immediately, fiber straightening and reorientation dominates the changes in the mechanical behavior up to 65 x 10(6) cycles. The present study underscores the complexity of the response of biologically derived biomaterials to cyclic mechanical loading. Improved understanding of these phenomena can potentially guide the development of novel chemical treatment methods that seek to improve BHV durability by minimizing these degenerative processes.
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PMID:Response of heterograft heart valve biomaterials to moderate cyclic loading. 1516 8

Damage of the annulus fibrosus is implicated in common spinal pathologies. The objective of this study was to obtain a quantitative relationship between both the number of cycles and the magnitude of tensile strain resulting in damage to the annulus fibrosus. Four rectangular tensile specimens oriented in the circumferential direction were harvested from the outer annulus of 8 bovine caudal discs (n = 32) and subjected to one of four tensile testing protocols: (i) ultimate tensile strain (UTS) test; (ii) baseline cyclic test with 4 series of 400 cycles of baseline cyclic loading (peak strain = 20% UTS); (iii & iv) acute and fatigue damage cyclic tests consisting of 4 x 400 cycles of baseline cyclic loading with intermittent loading to 1 and 100 cycles, respectively, with peak tensile strain of 40%, 60%, and 80% UTS. Normalized peak stress for all mechanically loaded specimens was reduced from 0.89 to 0.11 of the baseline control levels, and depended on the magnitude of damaging strain and number of cycles at that damaging strain. Baseline, acute, and fatigue protocols resulted in permanent deformation of 3.5%, 6.7% and 9.6% elongation, respectively. Damage to the laminate structure of the annulus in the absence of biochemical activity in this study was assessed using histology, transmission electron microscopy, and biochemical measurements and was most likely a result of separation of annulus layers (i.e., delamination). Permanent elongation and stress reduction in the annulus may manifest in the motion segment as sub-catastrophic damage including increased neutral zone, disc bulging, and loss of nucleus pulposus pressure. The preparation of rectangular tensile strip specimens required cutting of collagen fibers and may influence absolute values of results, however, it is not expected to affect the comparisons between loading groups or dose-response reported.
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PMID:Mechanical damage to the intervertebral disc annulus fibrosus subjected to tensile loading. 1565 55

This study investigates the effect of structural changes of collagen fibrils on the bonding durability of a total etch luting resin (Super-Bond C&B) and a self-etching luting resin (Panavia F 2.0) to dentin. An atomic force microscope (AFM) was used to observe structural changes of intact dentin collagen fibrils after acidic conditionings of two bonding systems. After 90 d water storage and 15,000 thermal cycles (TC) as artificial aging, micro-tensile bond strength (microTBS) was utilized to evaluate the bonding durability of the two bonding systems to dentin. microTBS after 1 d or 90 d water storage without TC were separately measured in control groups. A cross-banding periodicity of about 67 nm along collagen fibrils was seen on demineralized intertubular dentin surfaces in AFM images. For both luting resins, thermal cycling decreased (p < 0.05) microTBS of 1 d and 90 d, compared to controls. Scanning electron microscope and transmission electron microscopic examinations revealed that the top and bottom of hybrid layer (HL) were weak links in the bonding interface over time. The results suggest that the top of HL contains disorganized collagen fibrils from the smear layer which degrade over time. AFM results indicate that the demineralized intact collagen fibrils beneath the smear layer were not denatured during acidic conditioning. However, these collagen fibrils may be structurally unstable due to poor infiltration by resin or loss of resin protection within the HL over time, reducing the long-term microTBS. This process was accelerated by thermal fatigue cycling.
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PMID:Effect of structural change of collagen fibrils on the durability of dentin bonding. 1576 38

Previous work by others have suggested the occurrence of one or more chemical or metabolic 'markers' for ME/CFS including specific amino acids and organic acids and a number of unidentified compounds (CFSUM1, CFSUM2). We have shown elsewhere that CFSUM1 is partially derivatised pyroglutamic acid and CFSUM2 partially derivatised serine and have suggested and demonstrated that the analytical methods used were unsuitable to identify or to accurately quantify urinary metabolites. We have now made a detailed analysis of plasma and urinary amino acids and of urinary organic acids from patients with ME/CFS and from three control groups. Fasting blood plasma and timed urine samples were obtained from 31 patients with CFS, 31 age and sex-matched healthy controls, 15 patients with depression and 22 patients with rheumatoid arthritis. Plasma and urinary amino acids and urinary organic acids were determined using established and validated methods and data compared by statistical analysis. None of the previously reported abnormalities in urinary amino acids or of organic acids could be confirmed. Results however provide some evidence in patients with ME/CFS for underlying inflammatory disease and for reduced intramuscular collagen with a lowered threshold for muscle micro-injury. These factors in combination may provide a basis for the fatigue and muscle pain that are the major symptoms in these patients.
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PMID:Urinary and plasma organic acids and amino acids in chronic fatigue syndrome. 1599 88

Terminal sterilization of bone allografts by gamma radiation is often essential prior to their clinical use to minimize the risk of infection and disease transmission. While gamma radiation has efficacy superior to other sterilization methods it also impairs the material properties of bone allografts, which may result in premature clinical failure of the allograft. The mechanisms by which gamma radiation sterilization damages bone tissue are not well known although there is evidence that the damage is induced via free radical attack on the collagen. In the light of the existing literature, it was hypothesized that gamma radiation induced biochemical damage to bone's collagen that can be reduced by scavenging for the free radicals generated during the ionizing radiation. It was also hypothesized that this lessening of the extent of biochemical degradation of collagen will be accompanied by alleviation in the extent of biomechanical impairment secondary to gamma radiation sterilization. Standardized tensile test specimens machined from human femoral cortical bone and specimens were assigned to four treatment groups: control, scavenger treated-control, irradiated and scavenger treated-irradiated. Thiourea was selected as the free radical scavenger and it was applied in aqueous form at the concentration of 1.5 M. Monotonic and cyclic mechanical tests were conducted to evaluate the mechanical performance of the treatment groups and the biochemical integrity of collagen molecules were assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The native mechanical properties of bone tissue did not change by thiourea treatment only. The effect of thiourea treatment on mechanical properties of irradiated specimens were such that the post-yield energy, the fracture energy and the fatigue life of thiourea treated-irradiated treatment group were 1.9-fold, 3.3-fold and 4.7-fold greater than those of the irradiated treatment group, respectively. However, the mechanical function of thiourea treated and irradiated specimens was not to the level of unirradiated controls. The damage occurred through the cleavage of the collagen backbone as revealed by SDS PAGE analysis. Irradiated specimens did not exhibit a noteworthy amount of intact alpha-chains whereas those irradiated in the presence of thiourea demonstrated intact alpha-chains. Results demonstrated that free radical damage is an important pathway of damage, caused by cleaving the collagen backbone. Blocking the activity of free radicals using the scavenger thiourea reduces the extent of damage to collagen, helping to maintain the mechanical strength of sterilized tissue. Therefore, free radical scavenger thiourea has the potential to improve the functional life-time of the allograft component following transplantation.
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PMID:Free radical scavenging alleviates the biomechanical impairment of gamma radiation sterilized bone tissue. 1602 98

In normal daily activities, ligaments are subjected to repeated loads, and respond to this environment with creep and fatigue. While progressive recruitment of the collagen fibers is responsible for the toe region of the ligament stress-strain curve, recruitment also represents an elegant feature to help ligaments resist creep. The use of artificial intelligence techniques in computational modeling allows a large number of parameters and their interactions to be incorporated beyond the capacity of classical mathematical models. The objective of the work described here is to demonstrate a tool for modeling creep of the rabbit medial collateral ligament that can incorporate the different parameters while quantifying the effect of collagen fiber recruitment during creep. An intelligent algorithm was developed to predict ligament creep. The modeling is performed in two steps: first, the ill-defined fiber recruitment is quantified using the fuzzy logic. Second, this fiber recruitment is incorporated along with creep stress and creep time to model creep using an adaptive neurofuzzy inference system. The model was trained and tested using an experimental database including creep tests and crimp image analysis. The model confirms that quantification of fiber recruitment is important for accurate prediction of ligament creep behavior at physiological loads.
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PMID:Biomechanical study using fuzzy systems to quantify collagen fiber recruitment and predict creep of the rabbit medial collateral ligament. 1606 Mar 55

Osteocytes, which are embedded in bone matrix, are the most abundant cells in bone. Despite the ideal location of osteocytes to sense the local environment and influence bone remodeling, their functions, and the relative importance of these functions, remain controversial. In this study, we tested several hypotheses that address the possibilities that population densities of osteocyte lacunae (Ot.Lc.N/B.Ar) correlate with strain-, remodeling- or metabolism-related aspects of the local biomechanical environments of mid-third diaphyseal equine radii and third metacarpals from skeletally mature animals. Ot.Lc.N/B.Ar data, quantified in multiple cortical locations, were analyzed for possible correlations with (1) structural and material characteristics (e.g., cortical thickness, percent ash, secondary osteon population density, mean osteon cross-sectional area, and predominant collagen fiber orientation), (2) strain characteristics, including prevalent/predominant strain magnitude and mode (tension, compression, shear), (3) hypothesized strain-mode-related microdamage characteristics, which might be perceived by osteocyte 'operational' networks, and (4) variations in remodeling dynamics and/or metabolism (i.e. presumably higher in endocortical regions than in other transcortical locations). Results showed relatively uniform Ot.Lc.N/B.Ar between regions with highly non-uniform strain and strain-related environments and markedly heterogeneous structural and material organization. These results suggest that population densities of these cells are poorly correlated with mechanobiological characteristics, including local variations in metabolic rate and strain magnitude/mode. Although osteocytes hypothetically evolved both as strain sensors and fatigue damage sensors able to direct the removal of damage as needed, the mechanisms that govern the distribution of these cells remain unclear. The results of this study provide little or no evidence that the number of osteocyte lacunae has a functional role in mechanotransduction pathways that are typically considered in bone adaptation.
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PMID:Spatial distribution of osteocyte lacunae in equine radii and third metacarpals: considerations for cellular communication, microdamage detection and metabolism. 1633 Aug 78

In limb bone diaphyses, it is hypothesized that collagen and extra-fibrillar mineral are aligned differently in relatively simple loading conditions (e.g., habitual longitudinal compression) when compared to complex or potentially deleterious strain environments (e.g., habitual shear or tension). These putative differences in collagen/mineral organization might be adaptations that enhance toughness and fatigue resistance by controlling the direction of microdamage propagation. This study examined relationships between the non-uniform strain distribution of wild deer calcanei and elastic anisotropy of cortical bone specimens in three preparations: (1) demineralized (collagen only), (2) deproteinized (mineral only), and (3) untreated. Using simulated functional loading, the following strain data were obtained from the dorsal "compression", plantar "tension", and medial and lateral ("neutral axis") cortices of one calcaneus of each of seven pairs: (1) peak strain magnitude, (2) prevalent/predominant strain mode (compression, tension, shear), and (3) principal strain orientation with respect to the bone's long axis. In the contralateral calcanei, elastic anisotropy ratios (ARs) were calculated using acoustic velocity (longitudinal and transverse) measurements from a pair of orthogonally sliced specimens (representing each of three preparation types) from each cortex. In a separate set of seven adult calcanei, predominant collagen fiber orientation (CFO) was measured using circularly polarized light (CPL) in the four cortical locations. Results showed that, in general, elastic anisotropy was significant in each region, with ARs being significantly different from isotropy (where AR=1.0). Compared to CFO, mineral orientation more strongly influenced this anisotropy, which was most notable in the plantar "tension" cortex. High correlations (r values from -0.675 to -0.734, P<0.05) were found between collagen anisotropy obtained from acoustic data when compared to the CPL data. Significant correlations of mineral and collagen anisotropy were also found between strain mode, magnitude, and orientation (all r values approximately -0.750). The habitual compression, tension, and shear (neutral axis) regions also had different collagen/mineral organizations, which may be important in accommodating the well-known disparity in the mechanical properties of bone in these loading modes.
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PMID:Dissociation of mineral and collagen orientations may differentially adapt compact bone for regional loading environments: results from acoustic velocity measurements in deer calcanei. 1645 55

Pulmonary arterial hypertension (PAH) is a pathological condition of the small pulmonary arteries. PAH is characterized histopathologically by vasoconstriction, vascular proliferation, in situ thrombosis, and remodeling of all 3 levels of the vascular walls. These pathologic changes result in progressive increases in the mean pulmonary-artery pressure and pulmonary vascular resistance, which, if untreated, leads to right-ventricular failure and death. PAH can be associated with multiple conditions or risk factors (eg, collagen vascular diseases, liver disease, human immunodeficiency virus, congenital heart disease, or ingestion of certain medications or toxins) or it can be idiopathic. Up to 10% of the idiopathic cases are familial. Regardless of the etiology, the clinical presentation, histopathologic lesions, and response to therapy are all similar. Early in the disease process, the signs and symptoms of PAH are often subtle and nonspecific, making diagnosis challenging. Patients most often present with progressively worsening dyspnea and fatigue. An extensive evaluation is indicated to diagnose PAH, decipher its etiology, and determine long-term treatment goals. Transthoracic echocardiogram is an excellent screening tool to evaluate PAH, but every patient requires a right-side heart catheterization to help stage the disease and guide therapy. Prior to a decade ago, clinicians were only able to offer symptomatic therapy to this challenging group of patients. Earlier diagnosis, rapidly advancing understanding of the pathogenesis, and an increasing number of treatment options have changed the course of PAH, which was once thought to be invariably fatal.
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PMID:Diagnosis and management of pulmonary arterial hypertension: Implications for respiratory care. 1656 91

The long-range reversible deformation of vertebrate arteries is primarily mediated by elastin networks that endure several million deformation cycles without appreciable fatigue. To determine how elastin contributes to the composite arterial properties, we studied the three-dimensional microstructure and biomechanics of isolated elastin. We initially estimated the sensitivity of these studies by comparing two elastin isolation protocols, autoclaving and alkali-extraction, and measured their effect on isolated elastin using uniaxial tests and histology. These studies show that autoclaved tissues have a trend for higher modulus (900.79+/-678.02 kPa) than alkali-extracted samples (417.74+/-162.23 kPa)albeit with higher collagen-proteoglycan impurities, and (2) greater optical density (78.6+/-9.1%) than alkali-extracted groups (46.2+/-5.9%), suggesting that autoclaving is superior to alkali-extraction for biomechanical tests on elastin. Using these data we show that an isotopic Mooney-Rivlin model cannot adequately represent arterial elastin. The neo-Hookean model, with coefficient 162.57 (+/-115.44) kPa for autoclaved and 76.94 (+/-27.76) kPa for alkali-extracted samples, fits the uniaxial data better. Autoclaved elastins also show linear stress-strain response and equal stiffness in circumferential and axial directions suggesting equal number of layers in these directions and that elastin may help distribute tensile stresses during vessel inflation. Histology of autoclaved and control porcine arteries reveals axial elastin fibers in intimal and adventitial layers but circumferential medial fibers. We propose an orthotropic material symmetry for arterial elastin with two orthogonally oriented and symmetrically placed mechanically equivalent fibers. An exact form of the constitutive equation will be obtained in a future study.
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PMID:Determination of strain energy function for arterial elastin: Experiments using histology and mechanical tests. 1664 25


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