Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Compressive Strength”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,765 records · Page 98Linked to original sources
PubMed2000

Displacement response of juvenile arthritic wrists during grasp.

OBJECTIVE: To analyze the displacement response of juvenile arthritic wrists during grasp in order to diagnose early ligamental laxity and facilitate early splinting. METHODS: X-rays of the wrists, made under standardized conditions, of 30 children with juvenile chronic arthritis (mean age 10.4 years, range 4.5-16.9) were analyzed after being digitalized. Osseous landmarks were identified, and coordinates were calculated from measured angles and lengths with an accuracy of 0.01'. Lunate and carpal-ulnar distance were obtained according to Youm, and ulnar variance according to Häfner. RESULTS: Overall, an increase in ulnar-lunate displacement and carpal narrowing and a decrease in ulnar variance were found. However, not all wrists responded to the same extent. Radial displacement of the lunate, though slight, was found in 2 wrists and the amount of ulnar displacement varied substantially (3.1% to 22.5%). The variance in amount of displacement could suggest that juvenile wrists do not respond to increased compressive forces to the same extent. CONCLUSION: The changes found are similar to those found in the healthy wrist. Furthermore, our findings suggest that the juvenile wrist acts in accordance with the generally accepted explanation for the development of malalignment in adult wrists. It seems that laxity of ligaments can be diagnosed early by the force grip maneuver during x-ray. It would have a significant impact on the moment of orthotic intervention as well as the design of the orthotic device. Further study along this line seems justified.

Adolescent↗
PubMed2002

Tensile destruction test as an estimation of partial proteolysis in fibrin clots.

We report a technique devised to evaluate the effects of partial proteolysis on the mechanical characteristics of acellular non-cross-linked fibrin clots. The destruction technique applies coaxial tension on mechanically preconditioned cylindrical molded clots and measures the number of mechanical failures vs the total number of samples at a given load (2, 3, and 4 grams force). We used different plasmin concentrations (0, 0.01, 0.02, 0.04, and 0.08 U/mL) in the bathing medium to cause partial proteolysis. We monitored the fibrinolysis process by measuring the amount of protein released in the bathing medium. Our results showed no difference in the creep function in all the groups studied. We compare our technique with compaction, a commonly used mechanical technique that compresses the sample by centrifugation, and found that our technique is capable of detecting minor changes of fibrinolysis (the results of the least square fit for the destruction test at 2 grams force, as a function of plasmin concentration, has a coefficient of determination of R(2) = 0.55), while compaction did not show a statistically significant difference in the same conditions, suggesting that each individual fibrin fiber bears load only under tension. Our findings suggest that when the fibers are cleaved their capacity to withstand stress is seriously challenged; thus, in principle, tensile destruction test can detect a minimal degree of proteolysis.

Blood Coagulation↗
PubMed1987

Nut-like oil seeds: food for monkeys, chimpanzees, humans, and probably ape-men.

The hypothetical hyperrobust australopithecine gnathic nutcracker adaptation is reexamined in light of ecobotanical information on edible wild nuts provided by the flora of tropical and subtropical Africa. The nut producing species are tree-forms. Those of the forest region do not as a rule produce fruits with edible mesocarps. In contrast, the woodland savanna species (particularly in the Zambezian region) characteristically provide an important whole fruit, i.e., a nutritious mesocarp in addition to edible oil-rich nut seeds. These fruits drop from the tree before they are fully mature and go through the final ripening phase on the ground. They are important seasonal foods for a variety of vertebrates, including primates, elephants, and antelope. Altogether the nuts exhibit a broad range of toughness values, measured here as strength under compression. The woodland nuts are not as tough (177-934 kg force, breaking load) as those of the tropical forest (192-1,673 kg force). The seed-predators of the woodland species include squirrels, baboons, warthogs, and parrots. Paleoecological analyses indicate that it was the woodland nuts that were probably available to Australopithecus boisei and A. robustus. Preliminary estimates of adult male gnathic nut-cracking capabilities suggest that A. boisei could have orally cracked a significant portion of the woodland nuts. In spite of this, ecobotanical data indicate that we can probably reject the hypothesis that these hominids were year-round gnathic nut-cracking specialists. Both the indirect and direct evidence support this conclusion.

Africa↗
PubMed2001

Modeling and remodeling in a developing artiodactyl calcaneus: a model for evaluating Frost's Mechanostat hypothesis and its corollaries.

The artiodactyl (mule deer) calcaneus was examined for structural and material features that represent regional differences in cortical bone modeling and remodeling activities. Cortical thickness, resorption and formation surfaces, mineral content (percent ash), and microstructure were quantified between and within skeletally immature and mature bones. These features were examined to see if they are consistent with predictions of Frost's Mechanostat paradigm of mechanically induced bone adaptation in a maturing "tension/compression" bone (Frost, 1990a,b, Anat Rec 226:403-413, 414-422). Consistent with Frost's hypothesis that surface modeling activities differ between the "compression" (cranial) and "tension" (caudal) cortices, the elliptical cross-section of the calcaneal diaphysis becomes more elongated in the direction of bending as a result of preferential (> 95%) increase in thickness of the compression cortex. Regional differences in mineral content and population densities of new remodeling events (NREs = resorption spaces plus newly forming secondary osteons) support Frost's hypothesis that intracortical remodeling activities differ between the opposing cortices: 1.) in immature and mature bones, the compression cortex had attained a level of mineralization averaging 8.9 and 6.8% greater (P < 0.001), respectively, than that of the tension cortex, and 2.) there are on average 350 to 400% greater population densities of NREs in the tension cortices of both age groups (P < 0.0003). No significant differences in cortical thickness, mineral content, porosity, or NREs were found between medial and lateral cortices of the skeletally mature bones, suggesting that no modeling or remodeling differences exist along a theoretical neutral axis. However, in mature bones these cortices differed considerably in secondary osteon cross-sectional area and population density. Consistent with Frost's hypothesis, remodeling in the compression cortex produced bone with microstructural organization that differs from the tension cortex. However, the increased remodeling activity of the tension cortex does not appear to be related to a postulated low-strain environment. Although most findings are consistent with predictions of Frost's Mechanostat paradigm, there are several notable inconsistencies. Additional studies are needed to elucidate the nature of the mechanisms that govern the modeling and remodeling activities that produce and maintain normal bone. It is proposed that the artiodactyl calcaneus will provide a useful experimental model for these studies.

Animals↗
PubMed2005

Structure and biomechanical properties of the trachea of the striped dolphin Stenella coeruleoalba: evidence for evolutionary adaptations to diving.

This study analyzes the structure and mechanical properties of the trachea of the striped dolphin Stenella coeruleoalba, one of the most common cetacean species. The cetacean trachea is made up of closed or semiclosed cartilaginous rings without a paries membranaceus. Our results indicate that the inner lining of the trachea contains erectile tissue in which several venous lacunae permeate the mucosa. We also observed and described the presence of peripheral neurons containing nitric oxide along the rim of the venous lacunae. Data obtained from compression and tensile tests and comparison with the pig and goat tracheas indicate a higher stiffness and a different, higher breaking point for the dolphin trachea. On the whole, our data suggest that the trachea of the striped dolphin possesses structural properties that allow rapid filling with blood, possibly in relation to dive activities, and also allow modifications due to increased pressure and immediate return to the original shape without risks of permanent bending or rupture, as would happen in a terrestrial mammal. As the organ undergoes intense pressure difference during descent to optimal foraging depth and subsequent rapid ascent to surface, especially in deep dives of hundreds of meters, the specific structural and biomechanical peculiarities of the trachea of the striped dolphin may represent an evolutionary adaptation to life in the water and to diving.

Animals↗
PubMed2003

Microenvironment regulation of extracellular signal-regulated kinase activity in chondrocytes: effects of culture configuration, interleukin-1, and compressive stress.

OBJECTIVE: To compare extracellular signal-regulated kinase (ERK) activity in response to interleukin-1 (IL-1) in chondrocytes under various culture configurations designed for the study of cartilage biology and repair, and also in response to dynamic load for chondrocytes in cartilage. METHODS: Isolated bovine articular chondrocytes were maintained in serum-supplemented medium under 4 culture configurations: high-density monolayer, attached to a cut surface of cartilage, within tissue-engineered constructs, or within intact cartilage explants. Samples were subjected to a change of medium with or without IL-1. Cartilage explants were also subjected to dynamic compression. RESULTS: In chondrocyte monolayers, both basal and IL-1-stimulated ERK activities were similarly elevated at 0.5 hours after medium change, diminishing by 74% after 16 hours. In contrast, chondrocytes in other culture configurations exhibited lower basal levels of ERK activity and a moderate activation of ERK in response to IL-1 that was sustained over the 16-hour treatment time. The dynamic component of loading of cartilage explants led to a 5-fold activation of ERK, compared with free-swelling controls, that was indistinguishable from the effects of IL-1. CONCLUSION: ERK signaling in response to IL-1 in chondrocyte monolayers exhibited a pattern that was distinct from that in other culture systems, suggesting that the extracellular matrix plays an important regulatory role in modulating the response to extracellular stimuli. Since IL-1 and dynamic loading have distinct effects on chondrocyte biosynthesis, signaling pathways other than ERK participate in the chondrocyte responses to these stimuli.

Animals↗
PubMed2005

Compressibility changes accompanying conformational transitions of apomyoglobin.

We used high-precision density and ultrasonic velocity measurements to characterize the native (N), molten globule (MG), and unfolded (U) conformations of apomyoglobin. The molten globule states that were studied in this work include the MG(pH4)(NaCl) state observed at pH 4 and 20 mM NaCl, the MG(pH4)(NaTCA) state observed at pH 4 and 20 mM sodium trichloracetate (NaTCA), the MG(pH2)(NaCl) state observed at pH 2 and 200 mM NaCl, and the MG(pH2)(NaTCA) state observed at pH 2 and 20 mM NaTCA. We used our densimetric and acoustic data to evaluate changes in adiabatic compressibility associated with the acid- or salt-induced N-to-MG, MG-to-U, MG-to-MG, and U-to-MG transitions of the protein. The N-to-MG(pH4)(NaCl) and N-to-MG(pH4)(NaTCA) transitions are accompanied by decreases in compressibility of -(3.0 +/- 0.6) x 10(-6) and -(2.0 +/- 0.6) x 10(-6) cm3 g(-1)bar(-1), respectively. The N-to-MG(pH2)(NaCl) and N-to-MG(pH2)(NaTCA) transitions are associated with compressibility changes of -(4.9 +/- 1.1) x 10(-6) and (0.7 +/- 0.9) x 10(-6) cm3 g(-1) bar(-1), respectively. We interpret these data in terms of the degree of unfolding of the various molten globule forms of apomyoglobin. In general, our compressibility data reveal significant disparities between the various equilibrium molten globule states of apomyoglobin while also quantitatively characterizing each of these states. Volumetric insights provided by our data facilitate gaining a better understanding of the folding pathways, intermediates, and kinetics of apomyoglobin folding.

Animals↗
PubMed2003

Compression of biocompatible liquid-filled HSA-alginate capsules: determination of the membrane mechanical properties.

Compression experiments between two parallel plates are performed on a series of biocompatible HSA-alginate capsules with two different membrane thicknesses. The capsule geometry and size as well as the average membrane thickness are first measured. The compression set-up is fitted with a sensitive force transducer that allows measurement of the compression force as a function of plate separation. The response of the capsule is analyzed by assuming different constitutive models for the membrane, where the shear and surface dilatation effects are accounted. An apparent area dilatation modulus is then computed for different values of the plate separation and required to remain constant as the capsule deformation increases. This allows identification of plausible constitutive laws for the membrane material.

Alginates↗
PubMed2004

Mechanical properties of desiccated ragweed pollen grains determined by micromanipulation and theoretical modelling.

The mechanical properties of desiccated ragweed pollen grains were determined using a micromanipulation technique and a theoretical model. Single pollen grains with a diameter of approximately 20 microm were compressed and held, compressed and released, and compressed to rupture at different speeds between two parallel surfaces. Simultaneously, the force being imposed on the pollen grains was measured. It has been found that the rupture force of pollen grains increased linearly with their displacement at rupture on average, but was independent of their diameter. The mean rupture force was 1.20 +/- 0.03 mN, and mean deformation (the ratio between the displacement and diameter) at rupture was 22 +/- 0.6%. Single pollen grains were modeled as a capsule with a core full of air and a non permeable wall. A constitutive equation based on Hookean law was used to determine the mechanical property parameters Eh (product of the Young's modulus and wall thickness), and the mean value of Eh of desiccated pollen gains was estimated to be 1653 +/- 36 N/m.

Ambrosia↗
PubMed2004

Gelation conditions and transport properties of hollow calcium alginate capsules.

The diameter, membrane thickness, and compression intensity of hollow Ca-alginate capsules were measured at different gelation conditions, such as the reactant concentration, dropping velocity, and gelation time. The optimum operation conditions for preparing capsules were determined at 100 g/L CaCl(2), 10 g/L sodium alginate (Na-alginate), a dropping velocity of 150 droplets/min, and a gelation time of 10 min. Diffusion of some saccharide and amino acid from bulk solution into capsules was investigated, and the diffusion coefficients were calculated by the developed mathematical model. All the tested substances can diffuse easily into the capsules. The combined diffusion coefficients of the capsule D(m) are 92-99% as large as their diffusion coefficients in pure water, while the diffusion coefficients in the capsule membrane D(1) are 60-95% as large as those. By employing polyethylene glycol (PEG) and bovine serum albumin (fraction V) (BSA(V)), the molecular weight cut-off of the capsule was determined. For linear macromolecules, hollow Ca-alginate capsules have a molecular weight cut-off of 4000. No diffusion of BSA(V) into the capsules was observed.

Alginates↗
PubMed2006

Immobilized fibrinogen in PEG hydrogels does not improve chondrocyte-mediated matrix deposition in response to mechanical stimulation.

The present investigation aims to explore the role of cell-scaffold interactions and whole cell compression in chondrocyte mechanotransduction using encapsulating poly(ethylene glycol) (PEG) hydrogel scaffolds and primary bovine chondrocytes. Scaffolds made from PEG hydrogels with immobilized fibrinogen molecules were seeded with chondrocytes and subjected to 15% dynamic compressive strain at 1-Hz frequency. Dynamic strain stimulation resulted in a 37% increase in the levels of sulfated glycosaminoglycan (sGAG) after 2 weeks of stimulation, when compared to static controls. Comparing results of the PEG-fibrinogen scaffolds with their respective PEG control group did not show significant differences between the two, even following 2 weeks of dynamic mechanical stimulation. Accordingly, these findings indicate that while cell deformations cause metabolic changes in chondrocytes seeded in PEG hydrogels, it is difficult to ascertain the role of matrix bioactivity in enhancing chondrocyte mechanotransduction in encapsulating scaffolds subjected to physical deformations. This study shows how interactions between mechanical stimulation and scaffold composition are evaluated using an experimental approach and customized biomaterial scaffolds.

Animals↗
PubMed2002

Triangular structure of the proximal femur.

The normal human skeleton is remarkably regular in form. Its constancy is demonstrated by the projection of well-recognized lines onto plain radiographs to define many of its relationships. The finding of the geometric symmetry of the normal proximal femur, demonstrated by the projection of an isosceles triangle onto its anteroposterior radiographic view, is presented. The base of this triangle extends between the trochanters and its apex lies at the center of the femoral head. This triangle may reflect the balance of bone modeling that takes place in the proximal femur as a result of the forces acting in the coronal plane. If so, it may then allow for the development of a mathematical expression for bone modeling in this plane.

Adolescent↗
PubMed2006

Mechanical contribution of the fibula to torsion stiffness in the lower extremity.

The role of the fibula in rotational stability of the lower extremity, taking into account the intact knee joint and tibia, to the best of our knowledge, has not been investigated. A cadaver study was designed to determine the torque transfer down the lower extremity, with and without the fibula. Six fresh, frozen human cadaver legs were used. The knee joint was left intact and the foot disarticulated. An external rotation up to 5 degrees , coupled with axial compression, was applied to the femur passing through the knee joint in extension and down the lower extremity to the fixed distal end, where torque (N m) was measured via a load cell. Each specimen was further tested with the fibula, cut 4 cm from the distal end, and finally with the entire fibula disarticulated at the proximal end and removed. To achieve 5 degrees rotation, torques reached 1.82 N m (SD, 0.66 N m). When the fibula was cut, the torque reduced a small yet significant amount of 5%. With the entire fibula removed, the torque reduction was significant and more pronounced at 11%. The fibula provided a small yet significant role in torsion stability. From this study, where the biomechanical characteristics of the fibula is explicated, it is hoped that this information will have use in further understanding the biomechanical role of the fibula, especially, in relation to the altered mechanics associated with lower limb pathology involving a deficient fibula.

Aged↗
PubMed2002

Effect of ball milling on the processing of bone substitutes with calcium phosphate powders.

Decreasing the microscale morphology of synthetic bone substitutes is of primary importance in order to enhance the morphology of the surface of the material, which is directly in contact with osteoconductive cells when it is implanted in bone. The aim of this study was to investigate the influence of ball milling of slurries on the microscale morphology of hydroxyapatite and tricalcium phosphate bone substitutes and the influence on their processing. Ball milling appeared to be a successful method in order to raise the sintering reactivity of the powders, that is, to decrease the sintering temperature and microstructural morphology of the material. However, it was demonstrated that ball milling had a great influence on dispersion, which became very difficult under long milling times because of dissolution of the calcium phosphate powders. Due to dissolution, ionic species were generated in the slurry and interfered with the dispersing agent. Moreover a reprecipitation process occurred simultaneously, and large particles of the most stable phase (HAP) formed. The presence of such large particles generated stress gradients and cracks in the material during the sintering stage.

Biocompatible Materials↗
PubMed2002

Effect of antibiotics on the properties of poly(methylmethacrylate)-based bone cement.

The goal of this study was to examine the relationship between gentamicin concentration in Palacos R bone cement (in the mass fraction range of 0-9.4%) and various properties of the cement. The results from the thermal, density, and static compression tests show that gentamicin favors the cement polymerization in its final steps, and forms a diphasic structure with the cement [poly(methylmethacrylate)] matrix. The static compression properties in the dry state are only slightly modified by the presence of the antibiotic. Concerning aging in water at 37 degrees C, two types of behavior can be distinguished: below a critical concentration, approximately 3-4%, the extraction of gentamicin by water is slow and there is only a slight change of static compression properties. In contrast, above this critical concentration, the gentamicin extraction is fast and almost complete after 48 weeks, and there is a considerable loss of static compression properties.

Anti-Bacterial Agents↗
PubMed2002

Predictive modeling of the mechanical properties of particulate hydroxyapatite reinforced polymer composites.

The influence of particle geometry and volume fraction on the mechanical properties of a hydroxyapatite (HAp) particulate reinforced polymer (poly-L-lactide and collagen) matrix composite was investigated through finite-element (FE) analysis. For cube-shaped (near sharp and curved corners and edges) embedded particles, it was found that the maximum stress concentration factor (SCF) in the matrix decreases with an increase of the HAp particle volume fraction (PVF). The maximum stress concentration in the matrix material containing a spherical inclusion has low sensitivity to the PVF. The compressive Young's modulus was found to be slightly dependent on particle shape, but very sensitive to PVF. Calculated stiffness values from FE analysis were compared to the experimental results available in the literature and with predictions from the Halpin-Tsai model.

Biomechanical Phenomena↗
PubMed2001

Time-dependent mechanical properties of HA/TCP particles in relation to morsellized bone grafts for use in impaction grafting.

In reconstructive surgery human bone defects are sometimes filled with the use of the impaction bone grafting technique. Currently different types of biomaterial particles are being developed as bone-substitute materials. Before these biomaterials can be applied their mechanical and biological behavior should be characterized. In this study the time-dependent mechanical behavior of biomaterial particles with different tri-calcium-phosphate/hydroxy-apatite (TCP:HA) ratios, particle sizes, and porosities is determined and compared to the behavior of human bone grafts, the latter being the standard material currently used to augment bone defects. The mechanical properties were assessed with the use of dynamic confined compression creep tests with a loading and unloading phase. Different graft material groups were tested, consisting of 100% human bone grafts, 100% biomaterial particles, and 50:50 weight mixtures of human grafts and biomaterial particles. No damage to the particles was observed by the impaction in the test chamber or by the dynamic load. Relative to the human graft material, the biomaterial particles hardly deformed under loading, were much stiffer, and showed almost no viscoelastic behavior. The mixtures showed intermediate results. Particle size and porosity influenced the behavior of the biomaterial particles. TCP:HA ratio did not have a great effect. The conclusion is that the application of these particles should be done with great care, as their mechanical behavior is drastically different than that of the human graft material. Mixing it with human bone grafts gave the material some biphasic, viscoelastic behavior that may be important for its biological response.

Biocompatible Materials↗
PubMed1981

The effect of carbon fiber reinforcement on contact area, contact pressure, and time-dependent deformation in polyethylene tibial components.

To study the mechanical behavior of a commercially available UHMWPE reinforced with carbon fibers, measurements of contact area, contact pressure and time-dependent deformation were made on tibial components from a contemporary total knee replacement loaded to physiological levels. For comparison, similar measurements were performed on identical components manufactured from plain UHMWPE. Contact area and pressure results reflected the increased stiffness of the carbon-UHMWPE material, with smaller contact areas and generally higher contact pressures versus the plain UHMWPE at the same load. Stresses in both materials under the contact area approached or exceeded the yield stresses for the materials when the physiological load was high or the radius of the femoral component indenter decreased (similar to flexion of the knee). Time-dependent deformation over a 24 h period was reduced significantly in the carbon-UHMWPE components for a high physiological load.

Arthroplasty, Replacement, Knee↗