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At least 865 records · Page 48Linked to original sources
PubMed2004

An alternate conformation of the hyperthermostable HU protein from Thermotoga maritima has unexpectedly high flexibility.

The homodimeric HU protein from the hyperthermophile Thermotoga maritima (HUTmar) is a model system which can yield insights into the molecular determinants of thermostability in proteins. Unusually for a thermostable protein, HUTmar exists in a structurally heterogeneous state as evidenced by the assignment of two distinct and approximately equally populated forms in solution. Relaxation measurements combined with chemical shift, hydrogen exchange, and nuclear Overhauser enhancement data confirm the main structural features of both forms. In addition, these data support a two-state model for HUTmar in which the major form closely resembles the X-ray structure while the very flexible minor form is less structured. HUTmar may therefore be a new example of the small class of hyperthermostable proteins with unexpected flexibility.

Amino Acid Sequence↗
PubMed1998

Random coil conformation of a Gly/Ala-rich insert in IkappaB alpha excludes structural stabilization as the mechanism for protection against proteasomal degradation.

Peptide segments of multiple glycine and alanine residues prevent the proteolytic degradation of ubiquitinated proteins by the proteasome. The structure of a Gly/Ala-rich insert in IkappaB alpha was probed by nuclear magnetic resonance (NMR) spectroscopy, comparing IkappaB alpha samples with and without Gly/Ala-rich insert. Narrow 1H-NMR resonances at chemical shifts indicative of random coil conformations were observed in the difference spectrum. circular dichroism (CD) measurements further confirm that the mechanism of protection against proteolytic degradation is not based on structural transition or stabilization caused by the Gly/Ala-rich segment. In addition, most of the N- and C-terminal residues outside the ankyrin repeats in wild-type IkappaB alpha were found to be flexibly disordered.

Alanine↗
PubMed1999

The structural flexibility of the preferredoxin transit peptide.

In order to obtain insight into the structural flexibility of chloroplast targeting sequences, the Silene pratensis preferredoxin transit peptide was studied by circular dichroism and nuclear magnetic resonance spectroscopy. In water, the peptide is unstructured, with a minor propensity towards helix formation from Val-9 to Ser-12 and from Gly-30 to Ser-40. In 50% (v/v) trifluoroethanol, structurally independent N- and C-terminal helices are stabilized. The N-terminal helix appears to be amphipathic, with hydrophobic and hydroxylated amino acids on opposite sides. The C-terminal helix comprises amino acids Met-29-Gly-50 and is destabilized at Gly-39. No ordered tertiary structure was observed. The results are discussed in terms of protein import into chloroplasts, in which the possible interactions between the transit peptide and lipids are emphasized.

Amino Acid Sequence↗
PubMed1998

The effect of knee flexion angle on knee stiffness values obtained using the Leicester arthrometer.

Knee stiffness was measured in 20 patients with a unilateral ACL-deficiency at 0 degree, 20 degrees and 65 degrees of knee flexion, respectively. The reduction in stiffness when the ACL deficient knee was compared with the normal contralateral knee was 46, 39 and 39%. The differences between these values were not statistically significant. The data raise the possibility that the angle at which instrumented measurement is made may not be critical provided the deficient knee is compared with the normal knee at the same angle.

Adult↗
PubMed1998

"Biological" internal fixation of long bone fractures: a biomechanical study of a "noncontact" plate system.

Based on existing knowledge of noncontact plates, an experimental prototype of a nonperiosteal contact internal fixation implant ("noncontact internal fixator") has been designed. The construct rigidity of osteotomised synthetic composite femora, fixed with the noncontact fixator and a reamed, statically-locked intramedullary nail were compared in axial compression, two plane bending and torsion in four types of diaphyseal fractures. With the exception of axial loading in the presence of extensive comminution, the fixation stability provided by the noncontact fixator is significantly higher than that of the tested intramedullary nail. Any degree of cortical contact between the two main fragments is important for the stability of this nonperiosteal contact fixation system under axial load. Appropriately-designed "internal fixators" could provide not only a number of biological and technical advantages, but also fixation stability comparable and in certain aspects superior to that of other fixation methods.

Biomechanical Phenomena↗
PubMed2000

A synthetic fiber-reinforced stentless heart valve.

There is strong evidence that failure of bioprosthetic and synthetic valves occurs as a consequence of high tensile and bending stresses, acting on the leaflets during opening and closing. In stented prostheses, whether synthetic or biological, the absence of contraction of the aortic base causes the leaflets to be subjected to an unphysiological degree of flexure, which is also related to calcification. However, a stentless synthetic valve, which has a flexible aorta base, can be a good alternative for stented synthetic valves. Moreover, fiber-reinforcement is assumed to lead to a decrease of tears and perforation as a result of reduced stresses in the weaker parts of the leaflets in their closed configuration. The manufacturing method for a stentless, fiber-reinforced, synthetic valve is presented. Prototypes are tested in a pulse duplicator system. The results show that the mean systolic pressure difference is very low, while the high regurgitation (up to 26%) is probably caused by a too small coaptation area of the leaflets.

Aortic Valve↗
PubMed2000

A continuous pure moment loading apparatus for biomechanical testing of multi-segment spine specimens.

An apparatus is described that enables the application of continuous pure moment loads to multi-segment spine specimens. This loading apparatus allows continuous cycling of the spine between specified flexion and extension (or right and left lateral bending) maximum load endpoints. Using a six-degree-of-freedom load cell and three-dimensional optoelectronic stereophotogrammetry, characteristic displacement versus load hysteresis curves can be generated and analyzed for different spinal constructs of interest. Unlike quasi-static loading, the use of continuous loading permits the analysis of the spine's behaviour within the neutral zone. This information is of particular clinical significance given that the instability of a spinal segment is related to its flexibility within the neutral zone. Representative curves for the porcine lumbar spine in flexion-extension and lateral bending are presented to illustrate the capabilities of this system.

Animals↗
PubMed2000

Patellar cartilage deformation in vivo after static versus dynamic loading.

The objective of this study was to test the hypothesis that static loading (squatting at a 90 degrees angle) and dynamic loading (30 deep knee bends) cause different extents and patterns of patellar cartilage deformation in vivo. The two activities were selected because they imply different types of joint loading and reflect a realistic and appropriate range of strenuous activity. Twelve healthy volunteers were examined and the volume and thickness of the patellar cartilage determined before and from 90 to 320s after loading, using a water excitation gradient echo MR sequence and a three-dimensional (3D) distance transformation algorithm. Following knee bends, we observed a residual reduction of the patellar cartilage volume (-5.9+/-2.1%; p<0.01) and of the maximal cartilage thickness (-2.8+/-2.6%), the maximal deformation occurring in the superior lateral and the medial patellar facet. Following squatting, the change of patellar cartilage volume was -4.7+/-1.6% (p<0.01) and that of the maximal cartilage thickness -4.9+/-1.4% (p<0.01), the maximal deformation being recorded in the central aspect of the lateral patellar facet. The volume changes were significantly lower after squatting than after knee bends (p<0.05), but the maximal thickness changes higher (p<0.05). The results obtained in this study can serve to validate computer models of joint load transfer, to guide experiments on the mechanical regulation of chondrocyte biosynthesis, and to estimate the magnitude of deformation to be encountered by tissue-engineered cartilage within its target environment.

Adult↗
PubMed2000

In vitro axial preload application during spine flexibility testing: towards reduced apparatus-related artefacts.

Presently, there is little consensus about how, or even if, axial preload should be incorporated in spine flexibility tests in order to simulate the compressive loads naturally present in vivo. Some preload application methods are suspected of producing unwanted "artefact" forces as the specimen rotates and, in doing so, influencing the resulting kinematics. The objective of this study was to quantitatively compare four distinct types of preload which have roots in contemporary experimental practice. The specific quantities compared were the reaction moments and forces resulting at the intervertebral disc and specimen kinematics. The preload types incorporated increasing amounts of caudal constraint on the preload application vector ranging from an unconstrained dead-load arrangement to an apparatus that allowed the vector to follow rotations of the specimen. Six human cadaveric spine segments were tested (1-L1/L2, 3-L2/L3, 1-L3/L4 and 1-L4/L5). Pure moments were applied to the specimens with each of the four different types of compressive preload. Kinematic response was measured using an opto-electronic motion analysis system. A six-axis load cell was used to measure reaction forces and moments. Artefact reaction moments and shear forces were significantly affected by preload application method and magnitude. Unconstrained preload methods produced high artefact moments and low artefact shear forces while more constrained methods did the opposite. A mechanical trade-off is suggested by our results, whereby unwanted moment can only be prevented at the cost of shear force production. When comparing spine flexibility studies, caution should be exercised to ensure preload was applied in a similar manner for all studies. Unwanted moments or forces induced as a result of preload application method may render the comparison of two seemingly similar studies inappropriate.

Artifacts↗
PubMed2001

2D calculation method based on composite beam theory for the determination of local homogenised stiffnesses of long bones.

A calculation method using the finite element technique is presented. Its main objective was to determine strains, stresses and more particularly stiffnesses in any cross section of a tibia, thus enabling the localisation of tibial torsion in vivo. Each tibial cross section was considered to be a non-uniform cross section of a composite beam with arbitrary orientation of fibres. The determination of stresses, strains and stiffnesses within a composite beam cross section has been defined by solving a variational problem. The validation of this method was performed on a tibial diaphysis of which each cross section was assumed to be the cross section of a composite beam made of orthotropic materials with orthotropic axes of any orientation with respect to the principal axis of the bone. The comparison of the results, from our model and that of a three-dimensional one, was performed on each nodal value (strains, stresses) of the meshed cross section as it was impossible to obtain local stiffnesses by experimentation. The good agreement between the results has validated our finite element program. Actually, this method has enabled to treat directly 2D geometric reconstructions from CT scan images with a good accuracy to determine locally the homogenised mechanical characteristics of human tibia in vivo, and particularly to quantify torsional tibial abnormalities of children without approximation of the shape of the cross section and by calculating the real moment of inertia J. The importance of the fibre orientation with regards to the stiffness values has been emphasised. This 2D method has also allowed to reduce CPU time of the 3D modelling and calculation.

Algorithms↗
PubMed2001

The effect of elastin damage on the mechanics of the aortic valve.

Porcine bioprosthetic heart valves degenerate and fail mechanically through a mechanism that is currently not well understood. It has been suggested that damage to the elastin component of prosthetic valve cusps could be responsible for changes in the mechanical function of the valve that would predispose it to increased damage and ultimate failure. To determine whether damage to elastin can produce the structural and mechanical changes that could initiate the process of bioprosthetic valve degeneration, we developed an elastase treatment protocol that fragments elastin and negates its mechanical contribution to the valve tissue. Valve cusps were mechanically tested before and after digestion to measure the mechanical changes resulting from elastin damage. Elastin damage produced a decrease in radial and circumferential extensibility (from 43 to 18% strain radially and 12 to 7% strain circumferentially), with a slight increase in stiffness (1.3-2.6kN/m for radial and 10.6-11.9kN/m for circumferential directions). Digestions with trypsin, which does not cleave elastin, confirmed that the changes in mechanics of the circumferential samples were likely due to the nonspecific removal of proteoglycans by elastase, while the changes in the radial samples were indeed due to elastin damage. Removing the mechanical contribution of elastin alters the mechanical behavior of the aortic valve cusp, primarily in the radial direction. This finding implies that damage to elastin will distend the cusps, reduce their extensibility, and increase their stiffness. Damage to elastin may therefore contribute to the degeneration and failure of prosthetic valves.

Animals↗
PubMed2001

Estimation of the Young's modulus of articular cartilage using an arthroscopic indentation instrument and ultrasonic measurement of tissue thickness.

We evaluated whether the use of cartilage thickness measurement would improve the ability of the arthroscopic indentation technique to estimate the intrinsic stiffness of articular cartilage. First, cartilage thickness and ultrasound reflection from the surface of bovine humeral head were registered in situ using a high-frequency ultrasound probe. Subsequently, cartilage was indented in situ at the sites of the ultrasound measurements using arthroscopic instruments with plane-ended and spherical-ended indenters. Finally, full-thickness cartilage disks (n=30) were extracted from the indented sites (thickness=799-1654microm) and the equilibrium Young's modulus was determined with a material testing device in unconfined compression geometry. We applied analytical and numerical indentation models for the theoretical correction of experimental indentation measurements. An aspect-ratio (the ratio of indenter radius to cartilage thickness) correction improved the correlation of the indenter force with the equilibrium Young's modulus from r(2)=0.488 to r(2)=0.642-0.648 (n=30) for the plane-ended indenter (diameter=1.000mm, height=0.300mm) and from r(2)=0.654 to r(2)=0.684-0.692 (n=30) for the spherical-ended indenter (diameter=0.500mm, height=0.100mm), depending on the indentation model used for the correction. The linear correlation between the ultrasound reflection and the Young's modulus was r(2)=0.400 (n=30). These results suggest that with large indenters, knowledge of the cartilage thickness improves the reliability of the indentation measurements, especially in pathological situations where cartilage thickness may be significantly lower than normal. Ultrasound measurements also provide diagnostically important information about cartilage thickness as well as knowledge of the integrity of the superficial zone of cartilage.

Animals↗
PubMed2001

A nonlinear anisotropic model for porcine aortic heart valves.

The anisotropic property of porcine aortic valve leaflet has potentially significant effects on its mechanical behaviour and the failure mechanisms. However, due to its complex nature, testing and modelling the anisotropic porcine aortic valves remains a continuing challenge to date. This study has developed a nonlinear anisotropic finite element model for porcine heart valves. The model is based on the uniaxial experimental data of porcine aortic heart valve leaflet and the properties of nonlinear composite material. A finite element code is developed to solve this problem using the 8-node super-parameter nonlinear shells and the update Lagrangian method. The stress distribution and deformation of the porcine aortic valves with either uniform and non-uniform thicknesses in closed phase and loaded condition are calculated. The results showed significant changes in the stress distributions due to the anisotropic property of the leaflets. Compared with the isotropic valve at the same loading condition, it is found that the site of the peak stress of the anisotropic leaflet is different; the maximum longitudinal normal stress is increased, but the maximum transversal normal stress and in-plane shear stress are reduced. We conclude that it is very important to consider the anisotropic property of the porcine heart valves in order to understand the failure mechanism of such valves in vivo.

Animals↗
PubMed2002

Design and validation of a surrogate humerus for biomechanical testing.

At present biomechanical testing of fracture plating strategies is conducted using animal or cadaveric whole bone models. This may introduce experimental error into these studies. This communication summarises the design and validation of a novel bone and fibre-reinforced plastic construct conceived to minimise intra-experimental error. A tubular surrogate humerus was produced with dimension and strength matched to that of the human humerus. Bone inserts placed into the wall of the tube allow for the fixation of the plates with bone screws. Three-point bending tests of the flexural rigidity of the surrogate humerus (EI=100.1 (SD 6.0)Nm(2)) showed it to be comparable to the human humerus. Further, pull-out tests of the screws showed that the bone slots adequately mimicked the whole bone scenario. This testing construct will be used for a comparative study of humeral plating techniques.

Animals↗
PubMed2003

A novel technique for four-point bending of small bone samples with semi-automatic analysis.

The design of this system takes into account the following parameters: the effects of excessive strain; specimen depth and width; span-depth ratio; overhang; rate of loading; measuring displacement from the crosshead position; radius of contact points; and the distance between the contacts. The contacts are radius edges rather than cylinders, although this is not to be recommended for multiple cycles because of wear problems, it does make the system as stiff as possible (allowance is made for this compliance). Equations are presented that allow the surface stress and strain to be calculated for each time point semi-automatically. The combination of stiff contacts and strain equations based on crosshead position not mid-span displacement is novel, this allows rapid analysis with minimum intervention from the user and gives results that are in agreement with those in the literature.

Automation↗
PubMed1997

An inverse dynamics modeling approach to determine the restraining function of human knee ligament bundles.

During knee motion, the fiber bundles of ligaments are nonuniformly loaded in a recruitment pattern which is different for successive knee-joint positions. As a result, the restraining functions of these ligaments are variable. To analyze the relative restraint contributions of the fiber bundles in different knee-joint positions, a new method was developed. Its application was illustrated for the cruciate ligaments of one knee-joint specimen. The methods developed to estimate bundle forces comprise five steps. First, the three-dimensional motions of a knee specimen are measured for anterior-posterior forces, using Röntgen Stereophotogrammetric Analysis. Second, bone-ligament-bone tensile tests are performed to evaluate the mechanical properties of these structures in several relative orientations of the bones. Third, multiple fiber bundles are identified in each ligament, based on the main fiber orientations. Fourth, the nonlinear force-length relationship of each functional bundle, as defined by a stiffness and a recruitment parameter, is determined by combining the multidirectional tensile tests with a multiline-element ligament model. Finally, the information obtained is combined in a whole-joint computer model of the knee, to determine the internal forces in the initial kinematic experiment, using an inverse dynamics approach. The technique appeared to be extremely time consuming and technologically involved. However, it was demonstrated to be useful and effective. The preliminary results reveal that the fiber bundle restraints are extremely sensitive to the knee flexion angle and the restraining forces are highly variable within the ligaments. For both cruciate ligaments, a gradual transition was demonstrated in load transfer from the posterior bundles to the more anteriorly positioned ones during knee flexion. Furthermore, it appeared that relatively high forces were carried by only a few fiber bundles at each flexion angle. Based on these preliminary results, it is concluded that the determination of forces in multiple ligament bundles is important for the understanding of failure mechanisms of ligaments. In particular, alternate loading of different fiber bundles suggests that successful operative reconstruction of the cruciate ligaments may not be achieved simply by a one-bundle preparation.

Algorithms↗
PubMed1997

Scanning electron microscopy of human cortical bone failure surfaces.

Undecalcified samples extracted from human femoral shafts are fractured by bending and the fracture surfaces are examined with a scanning electron microscope (SEM). The investigation is performed on both dry and wet (hydrated with a saline solution) specimens. SEM micrographs show patterns in many respects similar to those observed in fractography studies of laminated fiber-reinforced synthetic composites. In particular, dry and wet samples behave like brittle and ductile matrix laminates, respectively. An analysis carried out on the basis of the mechanisms that dominate the fracture process of laminates shows that a reasonable cortical bone model is that of a laminated composite material whose matrix is composed of extracellular noncollagenous calcified proteins, and the reinforcement is constituted by the calcified collagen fiber system.

Adult↗
PubMed1997

Trabecular bone adaptation to variations in porous-coated implant topology.

Trabecular bone adaptation adjacent to porous-coated platen implants embedded within canine distal femoral metaphyses was evaluated following 24 weeks of daily compressive loading. The in vivo experimental model delivered controlled loads to five different platen implant topologies with variations in platen shape and porous coating distribution. Adaptive changes were evaluated based on three-dimensional stereological analyses of trabecular bone architecture underneath each platen and non-destructive mechanical tests of platen construct stiffness. Fully coated cylindrical platen designs possessed the highest construct stiffness in both tension and compression. Changes in local trabecular bone morphology were also found to be significantly influenced by platen implant topology. Cylindrical platens with fully coated bottom surfaces were associated with greater decreases in trabecular bone volume and connectivity than cylindrical platens with smooth bottom surfaces or fully coated conical platens. Comparisons to site-matched contralateral control bone volumes across all platen designs indicated significant decreases in the average bone volume fraction, trabecular plate number, and connectivity within experimental samples, but no change in trabecular plate thickness. In addition, analyses of microstructural anisotropy revealed a 20 degrees or 20.2 degrees trabecular reorientation towards the axis of loading in experimental tissue. This study demonstrates that trabecular bone adaptation near porous-coated surfaces is influenced by variations in local implant topology and provides insight into specific mechanisms of implant-mediated microstructural adaptation.

Analysis of Variance↗