Pseudo-dynamic analysis of a cemented hip arthroplasty using a force method based on the Newmark algorithm

The main goal of this work was to develop an approached model to study dynamic behavior and prediction of the stress distribution in an in vitro Charnley cemented hip arthroplasty. An alternative version of the described pseudo-dynamic procedure is proposed by using the time integration Newmark algorithm. An internal restoring force vector is numerically calculated from the displacement, velocity, and acceleration vectors. A numerical model of hip replacement was developed to analyze the deformation of a dynamically stressed structure for all time steps. The experimental measurement of resulting internal forces generated in the structure (internal restoring force vector) is the second fundamental step of the pseudo-dynamic procedure. These data (as a feedback) are used by the time integration algorithm, which allows updating of the structure's shape for the next displacement, velocity, and acceleration vectors. In the field of Biomechanics, the potentialities of this method contribute to the determination of a dynamically equivalent in vitro stress field of a cemented hip prosthesis; implant fitted in patients with a normal mobility or practice sports. Consequences of the stress distribution in the implant zone that underwent cyclic fatigue loads were also discussed by using a finite element model. Application of this method in Biomechanics appears as a useful tool in the approximate stress field characterization of the peak stress state. Results show a peak value around two times the static situation, more for making possible the prediction of future damage and a programed clinical examination in patients using hip prosthesis.     

Tectorial structures on the inner ear sensory epithelia of Proteus anguinus (Amphibia,caudata)

The tectorial structures of the inner ear of the proteid salamander Proteus anguinus were studied with transmission and scanning electron microscopy in order to analyze the ultrastructure of the otoconial membranes and otoconial masses of the maculae and the tectorial membrane of the papilla amphibiorum. Both otoconial and tectorial membranes consist of two parts: (1) a compact part and (2) a fibrillar part that joins the membrane with the sensory epithelium. Masses of otoconia occupy the lumina above these membranes. There are two types of calcium carbonate crystals in the otoconial masses within the inner ear of Proteus anguinus. The relatively small otoconial mass of the utricular macula occupies an area no greater than the diameter of the sensory epithelium, and it is composed of calcite crystals. On the other hand, the enormous otoconial masses of the saccular macula and the lagenar macula are composed of aragonite crystals. In the sacculus and lagena, globular structures 2–9 m?m in diameter were discovered on the lower surfaces of the otoconial masses above the sensory epithelia. These globules show a progression from smooth-surfaced, small globules to large globules with spongelike, rough surfaces. It is hypothesized that these globules are precursors of the aragonite crystals and that calcite crystals develop similarly in the utriculus. The presence of globular precursors in adult animals suggests that the formation of new crystals in the otoconial membranes of the sacculus and lagena of Proteus is a continuous, ongoing process.     

Cytoplasmic streaming in internodal cells ofNitella under centrifugal acceleration: a study done with a newly constructed centrifuge microscope

Summary We constructed a new centrifuge microscope of the stroboscopic type, with which the cytoplasmic streaming inNitella internodal cells under centrifugal acceleration was studied. Under moderate centrifugal acceleration (ca. 50–100×g), the direction of cytoplasmic streaming in an internodal cell ofNitella is parallel to the direction of the subcortical fibrils. The speed of endoplasm flowing contiguous to the subcortical fibrils is neither accelerated nor retarded by moderate centrifugal acceleration. The endoplasmic flow, however, stops suddenly following an electrical stimulus. The endoplasm contiguous to the subcortical fibrils is immobilized transiently at the time of streaming cessation induced by an electrical stimulus under centrifugal acceleration at 50–100×g, even at 900×g. It is suggested that transitory cross bridges between the immobilized endoplasm and the subcortical fibrils are formed at the time of streaming cessation. The bulk endoplasm flows as a whole in the direction parallel to that of the subcortical fibrils and stops promptly upon electrical stimulation. Soon after the stoppage the bulk endoplasm starts to flow passively in the direction parallel to that of the centrifugal acceleration as a result of the centrifugal force.Abbreviations APW artificial pond water - CMS centrifuge microscope     

Deformation analysis of microcapsules compressed by two rigid parallel plates

Alginate-poly(l)lysine-alginate microcapsules with a diameter of 200–300 μm were produced and subjected to compression experiments using two rigid parallel plates. The deformed shape was observed, and the relationship between the applied force and the displacement of the plate was measured. A practical analysis method is proposed for the prediction of mechanical properties of microcapsules, namely, the changes in force, transmural pressure, and deformed shape due to the change in the displacement of the plate. The analysis is based on a microcapsule model comprising an axi-symmetric elastic membrane with a constant capsule volume during deformation. Young’s modulus of the membrane, which is used in the analysis, was determined by applying the Hertz contact theory to the experimental results obtained by using an atomic force microscope. The bending stiffness and permeability of the membrane as well as the frictional force between the membrane and the plate were neglected in the analysis. A non-dimensional relationship between the applied force and the displacement of the plate was shown. The theoretically predicted compression force was smaller than the experimentally measured force in the small displacement region, whereas both forces were in good agreement in the large displacement region. The effects of change in Poisson’s ratio, which varied from 0 to 0.5, on the force, transmural pressure, and deformed shape were shown. Under the same displacement conditions, it was observed that the calculated deformed shape was almost coincident with the shape observed in the experiment.     

Orientation of the photosynthetic flagellate,Peridinium gatunense,in hypergravity

The photosynthetic freshwater flagellate,Peridinium gatunense, uses both positive phototaxis and negative gravitaxis to move upwards in the water column. At higher fluence rates approaching those at the surface of their habitat, the cells tend to become unoriented and thus stop their upward movement. Orientation and motility ofPeridinium gatunense has been studied in the slow rotating centrifuge microscope (NIZEMI), which allows observation of swimming behavior during centrifugation acceleration between 1g and 5g. The movement vectors were analyzed by real time image analysis capable of tracking many cells simultaneously. At 1g the orientation was not very precise, but the degree of orientation increased significantly at higher acceleration forces up to about 3g. Most cells were capable of swimming even against an acceleration vector of 3.8g; at higher acceleration forces the cells were not able to cope with the centrifugal force. The linear velocity of cells swimming against 1g was about 20% lower than that of cells moving in other directions. The velocity decreased even more in cells swimming against higher acceleration forces.     

A predictive model of fatigue in human skeletal muscles.

Fatigue is a major limitation to the clinical application of functional electrical stimulation. The activation pattern used during electrical stimulation affects force and fatigue. Identifying the activation pattern that produces the greatest force and least fatigue for each patient is, therefore, of great importance. Mathematical models that predict muscle forces and fatigue produced by a wide range of stimulation patterns would facilitate the search for optimal patterns. Previously, we developed a mathematical isometric force model that successfully identified the stimulation patterns that produced the greatest forces from healthy subjects under nonfatigue and fatigue conditions. The present study introduces a four-parameter fatigue model, coupled with the force model that predicts the fatigue induced by different stimulation patterns on different days during isometric contractions. This fatigue model accounted for 90% of the variability in forces produced by different fatigue tests. The predicted forces at the end of fatigue testing differed from those observed by only 9%. This model demonstrates the potential for predicting muscle fatigue in response to a wide range of stimulation patterns.     

Force–displacement measurements of earlywood bordered pits using a mesomechanical tester

The elastic properties of pit membranes are reported to have important implications in understanding air‐seeding phenomena in gymnosperms, and pit aspiration plays a large role in wood technological applications such as wood drying and preservative treatment. Here we present force–displacement measurements for pit membranes of circular bordered pits, collected on a mesomechanical testing system. The system consists of a quartz microprobe attached to a microforce sensor that is positioned and advanced with a micromanipulator mounted on an inverted microscope. Membrane displacement is measured from digital image analysis. Unaspirated pits from earlywood of never‐dried wood of Larix and Pinus and aspirated pits from earlywood of dried wood of Larix were tested to generate force–displacement curves up to the point of membrane failure. Two failure modes were observed: rupture or tearing of the pit membrane by the microprobe tip, and the stretching of the pit membrane until the torus was forced out of the pit chamber through the pit aperture without rupture, a condition we refer to as torus prolapse.     

Evaluating methods to quantify spatial variation in the velocity of biological invasions

Invading species rarely spread homogeneously through a landscape and invasion patterns typically display irregular frontal boundaries as the invasion progresses through space. Those irregular patterns are generally produced by local environmental factors that may slow or accelerate movement of the frontal boundary. While there is an abundant literature on species distribution modelling methods that quantify local suitability for species establishment, comparatively few studies have examined methods for measuring the local velocity of invasions that can then be statistically analysed in relation to spatially variable environmental factors. Previous studies have used simulations to compare different methods for estimating the overall rate of spread of an invasion. We adopted a similar approach of simulating invasions that resemble two real case‐studies, both in terms of their spatial resolution (i.e. considering the size of one cell as one km) and their spatial extent (> 600 000 km²). Simulations were sampled to compare how different methods used to measure local spread rate, namely the neighbouring, nearest distance and Delaunay methods, perform for spatio‐temporal comparisons. We varied the assessment using three levels of complexity of the spatio‐temporal pattern of invasion, three sample sizes (500, 1000 and 2000 points), three different spatial sampling patterns (stratified, random, aggregated), three interpolation methods (generalized linear model, kriging, thin plate spline regression) and two spatio‐temporal modelling structures (trend surface analysis and boundary displacement), resulting in a total of 486 different scenarios. The thin plate spline regression interpolation method, in combination with trend surface analysis, was found to provide the most robust local spread rate quantification as it was able to reliably accommodate different sampling conditions and invasion patterns. This best approach was successfully applied to two case‐studies, the invasion of France by the horse‐chestnut leafminer Cameraria ohridella and by the bluetongue virus, generally in agreement with previously published values of spread rates. Potential avenues for further research are discussed.     

The effects of the metal ions Mn2+ and Co2+ on muscle contraction in the Norway lobster Nephrops norvegicus (L.)

The effects of the metal ions manganese and cobalt on force production by the abdominal superficial flexor muscle of the Norway lobster, Nephrops norvegicus, have been studied in response to both neuronal stimulation and electrical field stimulation applied to an isolated neuromuscular preparation, and by selectively blocking synaptic transmission with ivermectin. In response to both forms of stimulation, low concentrations of manganese added to the standard N. norvegicus saline increased the contractile force produced by the muscle, whereas higher concentrations of manganese inhibited both responses in a dose-dependent manner, until force was completely abolished at concentrations above 2.9 mM manganese. Cobalt ions produced similar effects, and no significant difference was found between the concentration of the two ions at 50% force inhibition (K_m) or between the two stimulation methods (manganese: 1.22 mM; cobalt: 1.29 mM, P = 0.86). This suggests that they have a similar mode of action, and a postsynaptic site of inhibition. These K_m values are considerably higher than the concentrations of these ions known to accumulate in the haemolymph of N. norvegicus under eutrophic conditions, and it therefore seems unlikely that accumulations of manganese or cobalt ions under such conditions would cause any significant inhibition of muscle contraction force. Accepted: 28 April 1999     

A comparison of two centrifuge techniques for constructing vulnerability curves: insight into the ‘open‐vessel’ artifact

A vulnerability curve (VC) describes the extent of xylem cavitation resistance. Centrifuges have been used to generate VCs for decades via static‐ and flow‐centrifuge methods. Recently, the validity of the centrifuge techniques has been questioned. Researchers have hypothesized that the centrifuge techniques might yield unreliable VCs due to the open‐vessel artifact. However, other researchers reject this hypothesis. The focus of the dispute is centered on whether exponential VCs are more reliable when the static‐centrifuge method is used rather than the flow‐centrifuge method. To further test the reliability of the centrifuge technique, two centrifuges were manufactured to simulate the static‐ and flow‐centrifuge methods. VCs of three species with open vessels of known lengths were constructed using the two centrifuges. The results showed that both centrifuge techniques produced invalid VCs for Robinia because the water flow through stems under mild tension in centrifuges led to an increasing loss of water conductivity. In addition, the injection of water in the flow‐centrifuge exacerbated the loss of water conductivity. However, both centrifuge techniques yielded reliable VCs for Prunus, regardless of the presence of open vessels in the tested samples. We conclude that centrifuge techniques can be used in species with open vessels only when the centrifuge produces a VC that matches the bench‐dehydration VC.     

Cardiovascular responses of semi-arboreal snakes to chronic,intermittent hypergravity

Cardiovascular functions were studied in semi-arboreal rat snakes (Elaphe obsoleta) following long-term, intermittent exposure to +1.5G _z (head-totail acceleration) on a centrifuge. Snakes were held in a nearly straight position within horizontal plastic tubes during periods of centrifugation. Centrifugal acceleration, therefore, subjected snakes to a linear force gradient with the maximal force being experienced at the tail. Compared to non-centrifuged controls,G _z-acceimated snakes showed greater increases of heart rate during head-up tilt or acceleration, greater sensitivity of arterial pressure to circulating catecholamines, higher blood levels of corticosterone, and higher blood ratios of prostaglandin F₂/prostaglandin E₂. Cardiovascular tolerance to increased gravity during gradedG _z acceleration was measured as the maximum (caudal) acceleration force at which carotid arterial blood flow became null. When such tolerances were adjusted for effects of body size and other continuous variables incorporated into an analysis of covariance, the difference between the adjusted mean values of control and acelimated snakes (2.37 and 2.84G _z, respectively) corresponded closely to the 0.5G difference between the acelimationG (1.5) and Earth gravity (1.0). As in other vertebrates, cardiovascular tolerance toG _z stress tended to be increased by acclimation, short body length, high arterial pressure, and comparatively large blood volume. Voluntary body movements were important for promoting carotid blood flow at the higher levels ofG _z stress.Abbreviations bpm heat beats per minute - FV fluid volume - G gravitational or acceleration force - G _z gravitational or acceleration force in the head-to-tail direction - Hct hematocrit - HIP hydrostatic indifferent point - PGE prostaglandin E₂ - PGF prostaglandin F_2a - PGFM stable metabolite of PGF - RCV red cell volume - RIA radioimmunoassay - SAS statistical analysis system - TBV total blood volume     

Vulnerability to cavitation in Olea europaea current‐year shoots: further evidence of an open‐vessel artifact associated with centrifuge and air‐injection techniques

Different methods have been devised to analyze vulnerability to cavitation of plants. Although a good agreement between them is usually found, some discrepancies have been reported when measuring samples from long‐vesseled species. The aim of this study was to evaluate possible artifacts derived from different methods and sample sizes. Current‐year shoot segments of mature olive trees (Olea europaea), a long‐vesseled species, were used to generate vulnerability curves (VCs) by bench dehydration, pressure collar and both static‐ and flow‐centrifuge methods. For the latter, two different rotors were used to test possible effects of the rotor design on the curves. Indeed, high‐resolution computed tomography (HRCT) images were used to evaluate the functional status of xylem at different water potentials. Measurements of native embolism were used to validate the methods used. The pressure collar and the two centrifugal methods showed greater vulnerability to cavitation than the dehydration method. The shift in vulnerability thresholds in centrifuge methods was more pronounced in shorter samples, supporting the open‐vessel artifact hypothesis as a higher proportion of vessels were open in short samples. The two different rotor designs used for the flow‐centrifuge method revealed similar vulnerability to cavitation. Only the bench dehydration or HRCT methods produced VCs that agreed with native levels of embolism and water potential values measured in the field.     

Modeling the active process of the cochlea: phase relations, amplification, and spontaneous oscillation.

The high sensitivity and sharp frequency selectivity of acoustical signal transduction in the cochlea suggest that an active process pumps energy into the basilar membrane's oscillations. This function is generally attributed to outer hair cells, but its exact mechanism remains uncertain. Several classical models of amplification represent the load upon the basilar membrane as a single mass. Such models encounter a fundamental difficulty, however: the phase difference between basilar-membrane movement and the force generated by outer hair cells inhibits, rather than amplifies, the modeled basilar-membrane oscillations. For this reason, modelers must introduce artificially either negative impedance or an appropriate phase shift, neither of which is justified by physical analysis of the system. We consider here a physical model based upon the recent demonstration that the basilar membrane and reticular lamina can move independently, albeit with elastic coupling through outer hair cells. The mechanical model comprises two resonant masses, representing the basilar membrane and the reticular lamina, coupled through an intermediate spring, the outer hair cells. The spring's set point changes in response to displacement of the reticular lamina, which causes deflection of the hair bundles, variation of outer hair cell length and, hence, force production. Depending upon the frequency of the acoustical input, the basilar membrane and reticular lamina can oscillate either in phase or in counterphase. In the latter instance, the force produced by hair cells leads basilar-membrane oscillation, energy is pumped into basilar-membrane movement, and an external input can be strongly amplified. The model is also capable of producing spontaneous oscillation. In agreement with experimental observations, the model describes mechanical relaxation of the basilar membrane after electrical stimulation causes outer hair cells to change their length.     

Effects of linear acceleration on fish behavior and eye movements.

Behavioral responses and eye movements of fish during linear acceleration were reviewed. It is known that displacement of otoliths in the inner ear leads to body movements and/or eye movements. On the ground, the utriculus of the vestibular system is stimulated by otolith displacement caused by gravitational and inertial forces during horizontal acceleration of whole body. When the acceleration is imposed on the fish's longitudinal axis, the fish showed nose-down and nose-up posture for tailward and noseward displacement of otolith respectively. These responses were understood that the fish aligned his longitudinal body axis in a plane perpendicular to the direction of resultant force vector acting on the otoliths. When the acceleration was sideward, the fish rolled around his longitudinal body axis so that his back was tilted against the direction in which the inertial force acted on the otoliths. Linear acceleration applied to fish's longitudinal body axis evoked torsional eye movement. Direction of torsion coincided with the direction of acceleration, which compensate the change of resultant force vector produced by linear acceleration and gravity. Torsional movement of left and right eye coordinated with each other. In normal fish, both sinusoidal and rectangular acceleration of 0.1G could evoke clear eye torsion. Though the amplitude of response increased with increasing magnitude of acceleration up to 0.5 G, the torsion angle did not fully compensate the angle calculated from gravity and linear acceleration. Removal of the otolith on one side reduced the response amplitude of both eyes. The torsion angle evoked by rectangular acceleration was smaller than that evoked by sinusoidal acceleration in both normal and unilaterally labyrinthectomized fish. These results suggest that eye torsion of fish include both static and dynamic components.     

Shearing in a Biomimetic Apatite-Protein Composite: Molecular Dynamics of Slip Zone Formation,Plastic Flow and Backcreep Mechanisms

We report molecular dynamics simulations of shear in a biomimetic hydroxyapatite-collagen composite. Our model exhibits elastic properties fully dominated by the inorganic component. However, beyond the elastic regime the biomolecules along with the hierarchical nature of the composite account for the formation of structure-inherent slip zones. These accommodate shear without compromising the overall structure and lead to the sliding of intrinsically defined rods at roughly constant restoring force. Upon releasing load, rod displacement is reversible and backcreep is observed as gradual ionic rearrangement in the slip zone, subjected to an activation barrier.     

Biphasic poroviscoelastic simulation of the unconfined compression of articular cartilage: I--Simultaneous prediction of reaction force and lateral displacement

This study investigated the ability of the linear biphasic poroelastic (BPE) model and the linear biphasic poroviscoelastic (BPVE) model to simultaneously predict the reaction force and lateral displacement exhibited by articular cartilage during stress relaxation in unconfined compression. Both models consider articular cartilage as a binary mixture of a porous incompressible solid phase and an incompressible inviscid fluid phase. The BPE model assumes the solid phase is elastic, while the BPVE model assumes the solid phase is viscoelastic. In addition, the efficacy of two additional models was also examined, i.e., the transversely isotropic BPE (TIBPE) model, which considers transverse isotropy of the solid matrix within the framework of the linear BPE model assumptions, and a linear viscoelastic solid (LVE) model, which assumes that the viscoelastic behavior of articular cartilage is solely governed by the intrinsic viscoelastic nature of the solid matrix, independent of the interstitial fluid flow. It was found that the BPE model was able to accurately account for the lateral displacement, but unable to fit the short-term reaction force data of all specimens tested. The TIBPE model was able to account for either the lateral displacement or the reaction force, but not both simultaneously. The LVE model was able to account for the complete reaction force, but unable to fit the lateral displacement measured experimentally. The BPVE model was able to completely account for both lateral displacement and reaction force for all specimens tested. These results suggest that both the fluid flow-dependent and fluid flow-independent viscoelastic mechanisms are essential for a complete simulation of the viscoelastic phenomena of articular cartilage.     

Viral capsid equilibrium dynamics reveals nonuniform elastic properties

The long wavelength, low-frequency modes of motion are the relevant motions for understanding the continuum mechanical properties of biomolecules. By examining these low-frequency modes, in the context of a spherical harmonic basis set, we identify four elastic moduli that are required to describe the two-dimensional elastic behavior of capsids. This is in contrast to previous modeling and theoretical studies on elastic shells, which use only the two-dimensional Young's modulus (Y) and the bending modulus (κ) to describe the system. Presumably, the heterogeneity of the structure and the anisotropy of the biomolecular interactions lead to a deviation from the homogeneous, isotropic, linear elastic shell theory. We assign functional relevance of the various moduli governing different deformation modes, including a mode primarily sensed in atomic force microscopy nanoindentation experiments. We have performed our analysis on the T = 3 cowpea chlorotic mottle virus and our estimate for the nanoindentation modulus is in accord with experimental measurements.     

Coupling between rib cage and abdominal compartments of the relaxed chest wall

The volume displacements of the rib cage and abdomen of relaxed seated subjects were measured as functions of pleural pressure with the chest wall expanded by airway pressure and with the chest wall distorted by an external force applied to the rib cage. From the measured displacements for the two independent loads, the three compliances that describe the mechanical properties of the relaxed chest wall modeled as a linear elastic system with two degrees of freedom were obtained. The cross compliance that describes the coupling between the rib cage and abdomen was found to be small and positive, 0.01-0.02 1/cmH2O. The displacement of the rib cage by the external force was consistent with the displacement predicted by use of standard methods for calculating the mechanical advantage of the force.     

Large-scale perfusion culture process for suspended mammalian cells that uses a centrifuge with multiple settling zones

A high-cell-density perfusion culture process, using a novel centrifuge, was developed. The centrifuge has spiral multiple settling zones to separate cells from culture medium. Because of the multiple zones, the separation area can be efficiently increased without enlarging the diameter of the centrifuge. The centrifuge used in this study had a separation capacity of 2600 ml culture medium min^–1 at 100g of the centrifugal force. A new cell separation and withdrawal method was also developed. The cells separated in the centrifuge can be withdrawn easily from the centrifuge with no cell clogging by feeding a liquid carrier such as a perfluorocarbon into the centrifuge and pushing the cells out with the liquid carrier. By this culture process, monoclonal antibodies were produced with mouse-human hybridoma X87X at a cell density of about 8 × 10⁶ cells ml^–1 for 25 days. This centrifuge culture shows promise as a large-scale perfusion culture process.     

Two-step, predictive, isometric force model tested on data from human and rat muscles

Functionalelectrical stimulation can assist paralyzed individuals to performfunctional movements, but muscle fatigue is a major limitation to itspractical use. An accurate and predictive mathematical model canfacilitate the design of stimulation patterns that optimize aspects ofthe force transient while minimizing fatigue. Solution nonuniqueness, amajor shortcoming in previous work, was overcome with a simpler model.The model was tested on data collected during isometric contractions ofrat gastrocnemius muscles and human quadriceps femoris muscles undervarious physiological conditions. For each condition tested, parametervalues were identified using the force response to one or twostimulation trains. The parameterized model was then used to predictforces in response to other stimulation patterns. The predicted forcesclosely matched the measured forces. The model was not sensitive toinitial parameter estimates, demonstrating solution uniqueness. Bypredicting the force that develops in response to an arbitrary patternof stimulation, we envision the present model helping identify optimalstimulation patterns for activation of skeletal muscle duringfunctional electrical stimulation.