Dynamic short crack growth in cortical bone.

It is well known for almost half a century that bones contain microcracks. Very little is known about the crack growth behaviour of very small cracks, e.g. the stage before they become macroscopically long. The aim of this work was to investigate the dynamic crack growth behaviour of sub-millimetre microcracks in cortical bone. It was found that slow stable crack growth occurs in specimens subjected to static loading conditions. Crack growth direction was dominated by the local fibre orientation of the bones. Crack angles varied between 10 and 36 degrees of the long axis of the bone. Short cracks were found to show periods of rapid growth followed by intervals of temporary crack arrest. Histological analysis showed that crack arrest occurred due to vascular canals in the bone. During these periods of crack arrest, crack opening displacements increased until the local strain was sufficient to overcome these features. These observations indicate a mechanism for growth of small cracks in bone at constant stress, involving microstructural barriers, time-dependent deformation of material near the crack tip and strain-controlled propagation.     

Slow crack growth in acrylic bone cement.

Slow crack growth in Perspex acrylic sheet (PMMA) and Simplex acrylic bone cement in air and water has been studied from a fracture mechanics viewpoint. It has been found that the crack velocity, V, for each material depends upon the intensification of stress at the tip of the crack. Experimental measurements have been made of V as a function of the stress intensity factor, K, at the crack tip, and a derived V, K relationship has been used to predict the times-to-failure of components made from PMMA and Simplex cement. Direct measurements of time-to-failure for PMMA have shown that the predicted values give a conservative estimate of the structural lifetime of the material.     

Investigation of fatigue crack growth in acrylic bone cement using the acoustic emission technique

Failure of the bone cement mantle has been implicated in the loosening process of cemented hip stems. Current methods of investigating degradation of the cement mantle in vitro often require sectioning of the sample to confirm failure paths. The present research investigates acoustic emission as a passive experimental method for the assessment of bone cement failure. Damage in bone cement was monitored during four point bending fatigue tests through an analysis of the peak amplitude, duration, rise time (RT) and energy of the events emitted from the damage sections. A difference in AE trends was observed during failure for specimens aged and tested in (i) air and (ii) Ringer's solution at 37 degrees C. It was noted that the acoustic behaviour varied according to applied load level; events of higher duration and RT were emitted during fatigue at lower stresses. A good correlation was observed between crack location and source of acoustic emission, and the nature of the acoustic parameters that were most suited to bone cement failure characterisation was identified. The methodology employed in this study could potentially be used as a pre-clinical assessment tool for the integrity of cemented load bearing implants.     

骨组织工程中的应力与生长

力学环境是骨组织所处的重要微环境之一 ,应力 (应变 )可促进细胞增殖 ,引起细胞骨架重排及细胞形态的变化 ,加速细胞基质矿化 ,刺激细胞因子及骨代谢激素的分泌 ,从而调节骨代谢、促进骨组织的生长与重建。但体外培养时 ,应力(应变 )水平和细胞反应程度间的关系及细胞间信号转导等的机制还不是十分清楚 ,而这些都是体外培养组织所必须解决的问题 ,因此 ,本文综述了应力对骨组织及成骨细胞、软骨细胞的影响与机理 ,对今后研究应力对骨组织工程化培养的影响具有十分重的意义     

Aspects of in vitro fatigue in human cortical bone: time and cycle dependent crack growth

Although fatigue damage in bone induced by cyclic loading has been recognized as a problem of clinical significance, few fracture mechanics based studies have investigated how incipient cracks grow by fatigue in this material. In the present study, in vitro cyclic fatigue experiments were performed in order to quantify fatigue-crack growth behavior in human cortical bone. Crack-growth rates spanning five orders of magnitude were obtained for the extension of macroscopic cracks in the proximal-distal direction; growth-rate data could be well characterized by the linear-elastic stress-intensity range, using a simple (Paris) power law with exponents ranging from 4.4 to 9.5. Mechanistically, to discern whether such behavior results from "true" cyclic fatigue damage or is simply associated with a succession of quasi-static fracture events, cyclic crack-growth rates were compared to those measured under sustained (non-cyclic) loading. Measured fatigue-crack growth rates were found to exceed those "predicted" from the sustained load data at low growth rates ( approximately 3 x 10(-10) to 5 x 10(-7) m/cycle), suggesting that a "true" cyclic fatigue mechanism, such as alternating blunting and re-sharpening of the crack tip, is active in bone. Conversely, at higher growth rates ( approximately 5 x 10(-7) to 3 x 10(-5) m/cycle), the crack-growth data under sustained loads integrated over the loading cycle reasonably predicts the cyclic fatigue data, indicating that quasi-static fracture mechanisms predominate. The results are discussed in light of the occurrence of fatigue-related stress fractures in cortical bone.     

Osteonal crack barriers in ovine compact bone

Bone is an anisotropic structure which can be compared to a composite material. Discontinuities within its microstructure may provide stress concentration sites for crack initiation, but act as a barrier to its propagation. This study looks specifically at the relationship between crack length and propagation in compact bone. Beam-shaped bone samples from sheep radii were prepared and stained with fluorochrome dyes and tested in cyclic fatigue under four-point bending in an INSTRON 1341 servo-hydraulic fatigue-testing machine. Samples were tested at a frequency of 30 Hz and stress range of 100 MPa under load control. Specimens were sectioned transversely using a diamond saw, slides prepared and examined using epifluorescence microscopy. Cracks in transverse sections were classified in terms of their location relative to cement lines surrounding secondary osteons. Mean crack length, crack numerical density and crack surface density were examined. Short microcracks (100 microm or less) were stopped at the cement lines surrounding osteons, microcracks of intermediate length (100-300 microm) were deflected as they hit the cement line, and microcracks that were able to penetrate through cement lines were longer (> 400 microm). These data show that bone microstructure allows the initiation of microcracks but acts as a barrier to crack propagation.     

Mechanisms of cell death in oxidative stress

Reactive oxygen or nitrogen species (ROS/RNS) generated endogenously or in response to environmental stress have long been implicated in tissue injury in the context of a variety of disease states. ROS/RNS can cause cell death by nonphysiological (necrotic) or regulated pathways (apoptotic). The mechanisms by which ROS/RNS cause or regulate apoptosis typically include receptor activation, caspase activation, Bcl-2 family proteins, and mitochondrial dysfunction. Various protein kinase activities, including mitogen-activated protein kinases, protein kinases-B/C, inhibitor-of-I-kappaB kinases, and their corresponding phosphatases modulate the apoptotic program depending on cellular context. Recently, lipid-derived mediators have emerged as potential intermediates in the apoptosis pathway triggered by oxidants. Cell death mechanisms have been studied across a broad spectrum of models of oxidative stress, including H2O2, nitric oxide and derivatives, endotoxin-induced inflammation, photodynamic therapy, ultraviolet-A and ionizing radiations, and cigarette smoke. Additionally ROS generated in the lung and other organs as the result of high oxygen therapy or ischemia/reperfusion can stimulate cell death pathways associated with tissue damage. Cells have evolved numerous survival pathways to counter proapoptotic stimuli, which include activation of stress-related protein responses. Among these, the heme oxygenase-1/carbon monoxide system has emerged as a major intracellular antiapoptotic mechanism.     

Mechanisms of tumor metastasis to bone

Bone metastases occur in approximately 80% of patients with advanced cancer. They are characterized by cancer cell growth and bone destruction that cause pain, fractures, anemia, and hypercalcemia. At diagnosis, bone metastases are usually incurable owing to their advanced development. However, the early stages in their formation are asymptomatic and begin as single micrometastatic cells from the blood stream. These cells can be detected by molecular analysis of bone marrow in approximately 30% of patients at the time of cancer diagnosis, but not all single micrometastatic cells develop into clinically significant bone metastases. A synergistic relationship exists between the micometastasis and the bone environment creating favorable conditions for the development and growth of disseminated tumor cells. Such bone metastases induce osteolysis or new bone formation, releasing growth factors and cytokines, which in turn amplify this pathological mechanism. The underling hypothesis, first proposed by Paget in 1889, is that the growth of disseminated tumor cells in bone is dependent on the fertility of the soil or bone itself. This article explores the most current opinions in this area of study and presents a comprehensive summary of the major factors involved.     

Mechanisms underlying hemodynamic and neuroendocrine stress reactivity at different phases of the menstrual cycle

This study examined the association of menstrual cycle phase with stress reactivity as well as the hormonal and neuroendocrine mechanisms contributing to cycle effects. Fifty‐seven women underwent a modified Trier Social Stress Test during the early follicular, late follicular, and luteal phases of the menstrual cycle. Greater increases in cardiac index (CI) and greater decreases in vascular resistance index (VRI) during speech were observed in the luteal phase relative to other phases, while greater increases in epinephrine (EPI) was observed during the late follicular and luteal phases compared to the early follicular phase. Luteal phase estradiol predicted luteal EPI reactivity but not CI or VRI reactivity, while luteal phase EPI reactivity predicted luteal phase CI and VRI reactivity. Thus, cycle‐related changes in EPI reactivity may be a stronger determinant of cycle effects on hemodynamic reactivity than sex hormones per se.     

Mechanisms of long and short term immunity to plague

Long and short term immunity to plague was produced in normal mice by using, respectively, an antibiotic resistant Yersinia pestis and Yersinia pseudotuberculosis. Both immunogens were used live. Passive serum transfer experiments, together with assays for the bactericidal activity of macrophages and delayed hypersensitivity tests, showed that the short term immunity was of a humoral nature and the long term immunity was cell mediated. The plague virulence markers of the two immunogens were: Y. pestis VW- F1+ P1+ P+; Y. pseudotuberculosis VW+ F1- P1- P-. The challenge organism was Y. pestis VW+ F1+ P1+ P+.     

Mechanisms of decrease in fatigue crack propagation resistance in irradiated and melted UHMWPE

Adhesive/abrasive wear in ultra-high molecular weight polyethylene (UHMWPE) has been minimized by radiation cross-linking. Irradiation is typically followed by melting to eliminate residual free radicals that cause oxidative embrittlement. Irradiation and subsequent melting reduce the strength and fatigue resistance of the polymer. We determined the radiation dose dependence and decoupled the effects of post-irradiation melting on the crystallinity, mechanical properties and fatigue crack propagation resistance of room temperature irradiated UHMWPE from those of irradiation alone. Stiffness and yield strength, were largely not affected by increasing radiation dose but were affected by changes in crystallinity, whereas plastic properties, ultimate tensile strength and elongation at break, were dominated at different radiation dose ranges by changes in radiation dose or crystallinity. Fatigue crack propagation resistance was shown to decrease with increase in radiation dose and with decrease in crystalline content. Morphology of fracture surfaces revealed loss of ductility with increase in radiation dose and more detrimental effects on ductility at lower radiation doses after post-irradiation melting.     

Calculation of porosity and osteonal cement line effects on the effective fracture toughness of cortical bone in longitudinal crack growth

Based on the microscopic analyses of cracks and correlational studies demonstrating evidence for a relationship between fracture toughness and microstructure of cortical bone, an equation was derived for bone fracture toughness in longitudinal crack growth, using debonding at osteonal cement lines and weakening effect of pores as main crack mechanisms. The correlation between the measured and predicted values of fracture toughness was highly significant but weak for a single optimal value of matrix to cement line fracture toughness ratio. Using fracture toughness values and histomorphometrical parameters from an available data set, matrix to cement line fracture toughness ratio was calculated for human femoral bone. Based on these calculations it is suggested that the effect of an osteon on fracture toughness will depend on the cement line's ability to compensate for the pore in an osteon. Matrix to cement line fracture toughness ratio significantly increased with increasing age, suggesting that the effectiveness of osteons in energy absorption may be reduced in the elderly due to a change in cement line properties.     

Slow crack growth behaviour of hydroxyapatite ceramics

Among materials for medical applications, hydroxyapatite is one of the best candidates in orthopedics, since it exhibits a composition similar to the mineral part of bone. Double torsion technique was here performed to investigate slow crack growth behaviour of dense hydroxyapatite materials. Crack rate, V, versus stress intensity factor, K(I), laws were obtained for different environments and processing conditions. Stress assisted corrosion by water molecules in oxide ceramics is generally responsible for slow crack growth. The different propagation stages obtained here could be analyzed in relation to this process. The presence of a threshold defining a safety range of use was also observed. Hydroxyapatite ceramics appear to be very sensitive to slow crack growth, crack propagation occurring even at very low K(I). This can be explained by the fact that they contain hydroxyl groups (HAP: Ca(10)(PO(4))(6)(OH)(2)), favouring water adsorption on the crack surface and thus a strong decrease of surface energy in the presence of water. This study demonstrates that processing conditions must be carefully controlled, specially sintering temperature, which plays a key role on V-K(I) laws. Sintering at 50 degrees C above or below the optimal temperature, for example, may shift the V-K(I) law towards very low stress intensity factors. The influence of ageing is finally discussed.     

Morphometrics of corneal growth in chicks raised in constant light

In this study we wish to augment our understanding of the effect of environment on corneal growth and morphology. To understand how corneal development of chicks raised in constant light differs from that of ‘normal’ eyes exposed to cyclic periods of light and dark, white Leghorn chicks were raised under either constant light (approximately 700 lux at cage top) or in 12 h light/12 h dark conditions for up to 12 weeks after hatching. To determine whether corneal expansion is uniform, some birds from each group received corneal tattoos for periodic photographic assessment. By 16 days of age, constant light corneas weighed less than light/dark regimen corneas [7.39 ± 0.35 mg (SE) vs. 8.47 mg ± 0.26 mg SE wet weight, P ≤ 0.05], and corresponding differences were seen in corneal dry weights. Spatial expansion of the corneal surface was uniform in both groups, but the rate of expansion was slower in constant light chicks [0.0327 ± 0.009 (SE) vs. 0.144 ± 0.018 (SE) mm² day⁻¹ for normal chicks, P ≤ 0.001]. At 1 day of age, there were 422 ± 12.5 (SE) stromal cells 0.01 mm⁻² in the central cornea and 393 ± 21.5 (SE) stromal cells 0.01 mm⁻²peripherally. Although this difference is not statistically significant, the cell densities in the central cornea were always larger than those of the peripheral cornea in all eight measurements over a 10.5-week period, and this difference is significant (P ≤ 0.008, binomial test). Light/dark regimen birds show no such consistent difference in cell densities between central and peripheral corneas. Thus, the density distribution of corneal stromal cells of chicks grown in constant light differs from that of normal chicks. Taken together, all these observations suggest that diurnal cycles of light and darkness are necessary for normal corneal growth.     

Mechanisms for oxidative stress in diabetic cardiovascular disease

Obesity, metabolic syndrome, and diabetes are increasingly prevalent in Western society, and they markedly increase the risk for atherosclerotic vascular disease, the major cause of death in diabetics. Although recent evidence suggests a causal role for oxidative stress in insulin resistance, diabetes, and atherosclerosis, there is considerable controversy regarding its nature, magnitude, and underlying mechanisms. Glucose promotes glycoxidation reactions in vitro, and products of glycoxidation and lipoxidation are elevated in plasma and tissue from humans suffering from diabetes, but the exact relationships between hyperglycemia and oxidative stress are poorly understood. This review focuses on molecular mechanisms of increased oxidative stress in diabetes, the relationship of oxidant production to hyperglycemia, and the potential interaction of reactive carbonyls and lipids in oxidant generation. Using highly sensitive and specific gas chromatography-mass spectrometry, molecular signatures of specific oxidation pathways were identified in tissues of diabetic humans and animals. These studies support the hypothesis that unique reactive intermediates generated in localized microenvironments of vulnerable tissues promote diabetic damage. Therapies interrupting these reactive pathways in vascular tissue might help prevent cardiovascular disease in this high-risk population.     

Mechanisms and models of endoplasmic reticulum stress in chondrodysplasia

Chondrodysplasias are a group of genetic disorders that affect the development and growth of cartilage. These disorders can result in extreme short stature, craniofacial defects, joint malformation, and early osteoarthritis; severely impacting quality of life for affected individuals. Many chondrodysplasias are caused by mutations in genes encoding cartilage extracellular matrix (ECM) proteins. These mutations typically result in synthesis of abnormal proteins that are improperly folded, and hence inappropriately retained within the endoplasmic reticulum (ER) of the cell, activating ER stress and the unfolded protein response (UPR), an adaptive cellular response to minimize production of the mutant protein and/or to enhance protein folding, degradation or export. If prolonged, activation of the UPR causes apoptotic cell death. Many human disorders have an underlying mechanism in UPR activation, and targeting ER stress pathways is showing promise for development of therapeutics for these conditions. Understanding and modeling the UPR in chondrodysplasia will be essential to advance such targeted approaches for the benefit of chondrodysplasia patients. The focus of this review is to compare the mechanistic sequelae of ECM protein mutations in chondrodysplasia that may cause chondrocyte ER stress and UPR activation, and to present current and future directions in chondrodysplasia disease modeling and therapeutic intervention. Developmental Dynamics 243:875–893, 2014. © 2014 Wiley Periodicals, Inc.