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PubMed2002

Mechanical behavior of chemically treated ostrich pericardium subjected to uniaxial tensile testing: influence of the suture.

The mechanical behavior of sutured ostrich pericardium was studied by uniaxial tensile testing. One hundred forty-four tissue specimens were assessed: 96 sutured samples (48 in which a centrally located suture was placed at an angle of 90 degrees with respect to the longitudinal axis, whereas in the remaining 48, a centrally located suture was placed at a 45 degrees angle to the longitudinal axis, in sets of 12 samples each, sewn with sutures made of Gore-Tex, nylon, Prolene, or silk), and 48 unsutured controls. Each group of 24 samples sewn at one angle or the other with the different suture materials was assayed together with a corresponding control group of 12 unsutured samples. The mean tensile strengths in the unsutured controls ranged between 30.16 MPa and 43.42 MPa, whereas those of the sutured sets ranged from 14.68 MPa to 21.91 MPa. The latter presented a statistically significant loss of resistance (p < 0.01) when compared with the unsutured tissue samples. The angle of the suture with respect to the longitudinal axis influenced the degree of shear stress produced by the suture, as well as the behavior of the different suture materials used. The set of samples sewn with Prolene appeared to be that most sensitive to changes in the angle of the suture, whereas tissue sewn at a 45 degrees angle with Gore-Tex presented lower shear stress values in comparison with samples in which the other three materials were used. A method of tissue selection based on morphological and mechanical criteria was used to ensure the homogeneity of the results in such a way that the coefficients of determination (R2) for the stress/strain curve fitting equation ranged between 0.888 and 0.995. This excellent fit made it possible, applying regression analysis, to predict the mechanical behavior of a specimen by determining that of a contiguous tissue sample. Thus, it should be possible, at least theoretically, to characterize the behavior of a specific region or zone of the biomaterial. In conclusion, ostrich pericardium exhibits strong resistance to rupture, even when sutured. The selection method used ensures the homogeneity of the samples and, thus, of the results. The angle of the suture with respect to the longitudinal axis, where the load is centered, determines the shear stress produced by the suture and the mechanical behavior of each suture material.

Animals↗

PubMed2000

Influence of mechanical behavior on the wear of 4 clinically relevant polymeric biomaterials in a hip simulator.

The elastic and large-deformation mechanical behavior of 4 materials with known clinical performance was examined and correlated with the wear behavior in a hip simulator. Acetabular liners of a commercially available design were machined from ultra-high molecular weight polyethylene (UHMWPE), high-density polyethylene (HDPE), polytetrafluoroethylene (PTFE), and polyacetal and wear tested in a multidirectional hip joint simulator. Elastic and large-deformation mechanical behavior was directly measured from the wear-tested liners using the small punch test. The finite element method was used to compute elastic modulus from the measured small punch test initial stiffness, and the contact stress for the liners was calculated using the theory of elasticity solution. Positive, statistically significant correlations were observed between the hip simulator wear rate and the initial peak load, ultimate load, and work to failure from the small punch test. Negative correlations were observed between the wear rate and the elastic modulus and contact stress. The results of this study support the hypothesis that the large-deformation mechanical behavior of a polymer plays a greater role in the wear mechanisms prevalent in total hip replacements than the elastic behavior.

Biocompatible Materials↗

PubMed2003

Sutured calf pericardium: influence of the type and angle of the suture on mechanical behavior.

Careful selection of the biological tissue to be used in cardiac bioprostheses and a thorough knowledge of its mechanical behavior, facilitating both the prediction of this behavior and the interactions between the tissue and the other materials employed, is the best approach to designing a durable implant. For this purpose, a study involving uniaxial tensile testing of calf pericardium was carried out. Two sets of three contiguous strips of tissue were obtained from each pericardial membrane, to perform a total of 144 trials. Two samples were sewn with one of four commercially available suture materials: Gore-Tex, nylon, Prolene and silk. In each set of three samples, the center strip remained intact and unsutured to serve as a control, while the left-hand strip was sutured at a 45 degrees angle with respect to the longitudinal axis and the right-hand strip was sewn at a 90 degrees angle. All the samples were tested until rupture. The results demonstrated a significant loss of mean load (p<0.01) in the sutured samples at rupture. The angle of the suture had no influence on these results, although the stress/strain curves showed that, as the tensile stress increased, the mechanical behaviors were less uniform. The rupture of the collagen fibers could explain this phenomenon. The pericardium sutured with Gore-Tex presented a greater strain, or deformation (elongation), at lower levels of stress, regardless of the angle of the suture. The tissue selection criteria, based on the use of paired samples, enabled a correct prediction of the mechanical behavior of the tissue, with excellent correlation coefficients (>0.98) and a high degree of homogeneity in the results.

Journal Article↗

PubMed1994

Mechanical behavior of damaged trabecular bone.

The mechanical behavior of damaged trabecular bone may play a role in the etiology of age-related spine fractures since damaged bone exists in and may weaken the elderly vertebral body. To describe some characteristics of damaged trabecular bone, we measured the changes in modulus and strength that occur when bovine trabecular bone is loaded in compression to various strains beyond its elastic range. Twenty-three reduced-section specimens, taken from 17 different bones, were loaded from 0-X-0-9% strain, where X was one of four strains: 1.0% (n = 7), 2.5% (n = 6), 4.0% (n = 5), or 5.5% (n = 5). We found that modulus was reduced for all applied strains, whereas strength was reduced only for strain levels > or = 2.5%; the percentage changes in modulus and strength were independent of Young's modulus but were highly dependent on the magnitude of the applied strains; modulus was always reduced more than strength; and simple statistical models, using knowledge of only the applied strains, predicted well the percentage reductions in modulus (r2 = 0.97) and strength (r2 = 0.74). The modulus reductions reported here are in qualitative agreement with those for cortical bone in tensile loading, supporting the concept that the damage behaviors of cortical and trabecular bone are similar for low strains (< or = 4.0%). In addition, because modulus was always reduced more than strength, damaged trabecular bone may be stress protected in vivo by redistribution of stresses to undamaged bone.

Algorithms↗

PubMed1993

Passive mechanical behavior of human neutrophils: power-law fluid.

The mechanical behavior of the neutrophil plays an important role in both the microcirculation and the immune system. Several laboratories in the past have developed mechanical models to describe different aspects of neutrophil deformability. In this study, the passive mechanical properties of normal human neutrophils have been further characterized. The cellular mechanical properties were assessed by single cell micropipette aspiration at fixed aspiration pressures. A numerical simulation was developed to interpret the experiments in terms of cell mechanical properties based on the Newtonian liquid drop model (Yeung and Evans, Biophys. J., 56: 139-149, 1989). The cytoplasmic viscosity was determined as a function of the ratio of the initial cell size to the pipette radius, the cortical tension, aspiration pressure, and the whole cell aspiration time. The cortical tension of passive neutrophils was measured to be about 2.7 x 10(-5) N/m. The apparent viscosity of neutrophil cytoplasm was found to depend on aspiration pressure, and ranged from approximately 500 Pa.s at an aspiration pressure of 98 Pa (1.0 cm H2O) to approximately 50 Pa.s at 882 Pa (9.0 cm H2O) when tested with a 4.0-micron pipette. These data provide the first documentation that the neutrophil cytoplasm exhibits non-Newtonian behavior. To further characterize the non-Newtonian behavior of human neutrophils, a mean shear rate gamma m was estimated based on the numerical simulation. The apparent cytoplasmic viscosity appears to decrease as the mean shear rate increases. The dependence of cytoplasmic viscosity on the mean shear rate can be approximated as a power-law relationship described by mu = mu c(gamma m/gamma c)-b, where mu is the cytoplasmic viscosity, gamma m is the mean shear rate, mu c is the characteristic viscosity at characteristic shear rate gamma c, and b is a material coefficient. When gamma c was set to 1 s-1, the material coefficients for passive neutrophils were determined to be mu c = 130 +/- 23 Pa.s and b = 0.52 +/- 0.09 for normal neutrophils. The power-law approximation has a remarkable ability to reconcile discrepancies among published values of the cytoplasmic viscosity measured using different techniques, even though these values differ by nearly two orders of magnitude. Thus, the power-law fluid model is a promising candidate for describing the passive mechanical behavior of human neutrophils in large deformation. It can also account for some discrepancies between cellular behavior in single-cell micromechanical experiments and predictions based on the assumption that the cytoplasm is a simple Newtonian fluid.

Biomechanical Phenomena↗

PubMed2001

Mechanical behavior in living cells consistent with the tensegrity model.

Alternative models of cell mechanics depict the living cell as a simple mechanical continuum, porous filament gel, tensed cortical membrane, or tensegrity network that maintains a stabilizing prestress through incorporation of discrete structural elements that bear compression. Real-time microscopic analysis of cells containing GFP-labeled microtubules and associated mitochondria revealed that living cells behave like discrete structures composed of an interconnected network of actin microfilaments and microtubules when mechanical stresses are applied to cell surface integrin receptors. Quantitation of cell tractional forces and cellular prestress by using traction force microscopy confirmed that microtubules bear compression and are responsible for a significant portion of the cytoskeletal prestress that determines cell shape stability under conditions in which myosin light chain phosphorylation and intracellular calcium remained unchanged. Quantitative measurements of both static and dynamic mechanical behaviors in cells also were consistent with specific a priori predictions of the tensegrity model. These findings suggest that tensegrity represents a unified model of cell mechanics that may help to explain how mechanical behaviors emerge through collective interactions among different cytoskeletal filaments and extracellular adhesions in living cells.

Animals↗

PubMed2004

Remeshed smoothed particle hydrodynamics simulation of the mechanical behavior of human organs.

In computer aided surgery the accurate simulation of the mechanical behavior of human organs is essential for the development of surgical simulators. In this paper we introduce particle based simulations of two different human organ materials modeled as linear viscoelastic solids. The constitutive equations for the material behavior are discretized using a particle approach based on the Smoothed Particle Hydrodynamics (SPH) method while the body surface is tracked using level sets. A key aspect of this approach is its flexibility which allows the simulation of complex time varying topologies with large deformations. The accuracy of the original formulation is significantly enhanced by using a particle reinitialization technique resulting in remeshed Smoothed Particle Hydrodynamics (rSPH). The mechanical parameters of the systems used in the simulations are derived from experimental measurements on human cadaver organs. We compare the mechanical behavior of liver- and kidney-like materials based on the dynamic simulations of a tensile test case. Moreover, we present a particle based reconstruction of the liver topology and its strain distribution under a small local load. Finally, we demonstrate a unified formulation of fluid structure interaction based on particle methods.

Biomechanical Phenomena↗

PubMed2006

Mechanical behavior of bulk amorphous alloys reinforced by ductile particles at cryogenic temperatures.

The mechanical behavior of Zr-based bulk amorphous alloy composites (BAACs) was investigated at 77 K. The 5 vol. % Ta-BAAC maintained large plastic strains of approximately 13% with a 16% strength increase, when compared with that at 298 K. The interaction between shear bands and particles shows that shear extension in particles has limited penetration, and shear bands build up around particles. In addition to on the failure surface of the amorphous matrix, molten characteristics were also found on the surface of sheared particles. Pair distribution function studies were performed to understand the mechanical behavior.

Journal Article↗

PubMed1995

Effect of abnormal mineralization on the mechanical behavior of X-linked hypophosphatemic mice femora.

The Hyp mouse is an established animal model of X-linked hypophosphatemia, one of the most common genetic forms of metabolic bone disease in humans. This study describes the first determination of whole bone mechanical behavior in the heterozygous male and female Hyp mouse. Femora from 12-week-old mice were tested in torsion. The contribution of structural and material properties to mechanical behavior was determined by geometrical evaluation prior to testing and by analysis of the diaphyseal mineral after testing. The male and female Hyp femora were found to undergo significantly more angular deformation at failure than the same sex normal femora (82.49 +/- 24.37 vs. 22.63 +/- 8.02 rad/m [corrected] for the females and 128.90 +/- 37.05 vs. 22.79 +/- 7.24 rad/m [corrected] for the males) and to have a significantly lower structural stiffness (0.373 +/- 0.130 x 10(-3) vs. 1.33 +/- 0.380 x 10(-3) [corrected] [N-m/(rad/m)] for the females and 0.167 +/- 0.104 x 10(-3) vs. 1.60 +/- 0.502 x 10(-3) [corrected] [N-m/(rad/m)] for the males). The male Hyp femora had a significantly lower failure torque than male normal femora (1.58 +/- 0.62 x 10(-2) vs. 3.44 +/- 1.57 x 10(-2) N-m). Because the polar movement of inertia, a geometrical property that affects torsional behavior, was not significantly different between the Hyp femora and the same sex normals, differences in mechanical behavior were attributed to material properties.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1996

Characterization of the mechanical behavior of human knee ligaments: a numerical-experimental approach.

During knee-joint motions, the fiber bundles of the knee ligaments are nonuniformly loaded in a recruitment pattern, which depends on successive relative orientations of the insertion sites. These fiber bundles vary with respect to length, orientation and mechanical properties. As a result, the stiffness characteristics of the ligaments as a whole are variable during knee-joint motion. The purpose of the present study is to characterize this variable mechanical behavior. It is hypothesized that for this purpose it is essential to consider the ligaments mechanically as multi-bundle structures in which the variability in fiber bundle characteristics is accounted for, rather than as one-dimensional structures. To verify this hypothesis, bone-ligament-bone preparations of the ligaments were subjected to series of unidirectional subfailure tensile tests in which the relative insertion orientations were varied. For each individual test specimen, this series of tensile tests was simulated with a mathematical ligament model. Geometrically, this model consists of multiple line elements, of which the insertions and orientations are anatomically based. In a mathematical optimization process, the unknown stiffness and recruitment parameters of the line elements are identified by fitting the variable stiffness characteristics of the model to those of the test series. Thus, lumped parameters are obtained which describe the mechanical behavior of the ligament as a function of the relative insertion orientation. This method of identification was applied to all four knee ligaments. In all cases, a satisfactory fit between experimental results and computer simulation was obtained, although the residual errors were lower for the cruciate ligaments (1.0-2.4%) than for the collateral ligaments (3.7-8.1%). It was found that models with three or less line elements were very sensitive to geometrical parameters, whereas models with more than 7 line elements suffered from mathematical redundancy. Between 4 and 7 line elements little difference was found. It is concluded that the present ligament models can realistically simulate the variable tensile behavior of human knee ligaments. Hereby the hypothesis is verified that it is essential to consider the ligaments of the knee as multi-bundle structures in order to characterize fully their mechanical behavior.

Aged↗

PubMed1999

The effects of morphine on responding under variable-interval schedules: rate-related effects, behavioral mechanisms and Herrnstein's hyperbola.

The study was designed to integrate rate-related principles and Herrnstein's hyperbolic matching equation (1970) as they apply to drug effects. It was also designed to assess the validity of an interpretation of Herrnstein's equation used to account for the behavioral mechanisms of drug action. Eight rats were trained to press a lever under a range of variable-interval (VI) schedules of reinforcement using sucrose solution (0.1 M) as a reinforcer. After 90 baseline sessions the effect of morphine sulfate, 3, 6, 12 and 17 mg/kg, on responding was tested. Response rates were suppressed in a dose-dependent manner after morphine administration. Herrnstein's equation, which states that responding is a function of relative reinforcement rate, provided a good description of the relationship between responding and reinforcement under most conditions. Therefore, the traditional variable used to predict the drug effect in rate-dependency functions, baseline response rate, was compared to a new variable, baseline relative reinforcement rate. Relative reinforcement rate performed as well as response rate in predicting the drug effect. In order to test if Herrnstein's hyperbola can identify the behavioral mechanisms of the action of morphine, the concentration of the sucrose solution was increased (0.32 M). The y-asymptote of Herrnstein's equation, k, increased as sucrose concentration increased. This calls into question one common interpretation of Herrnstein's equation used to identify the behavioral mechanisms of drug action.

Algorithms↗

PubMed1991

Respiratory system mechanical behavior in the chicken.

We evaluated whether the avian respiratory system displays the same fundamental mechanical behavior during external forcing as found in mammals. We measured airway flow and pressures in the trachea, air sacs and thoracoabdominal cavity in 4 anesthetized-paralyzed roosters during sinusoidal volume oscillations at the trachea in the normal range of euthermic breathing frequency, f(0.2 to 1.0 Hz), and tidal volume, VT (10-50 ml). From the pressure and flow waveforms, we calculated resistance (R) and elastance (E) of the total respiratory system and its major compartments (lungs, air sacs and chest wall). E of the chest wall was minimum (147 cmH2O.L-1 +/- 7 SE) at 0.2 Hz-50 ml and was consistently, slightly lower than E of the total respiratory system over the entire range studied. Both elastances showed the same dependence on f and VT, increasing slightly with increasing f and decreasing with increasing VT. R of the chest wall was maximum (35.6 cmH2O.L- 1.sec-1 +/- 2.2 SE) at 0.2 Hz-10 ml and decreased with increasing f and VT, although the VT effect diminished at the higher f. E and R of the air sacs were much smaller than those of the chest wall, but showed similar f and VT dependencies. R of the lungs, due to resistance of the airways, was minimum (6.8 cmH2O.L-1.sec-1 +/- 1.5 SE) at 0.2 Hz-10 ml and increased with both f and VT. Total respiratory R reflected R of the air sacs and chest wall at low f and R of the lungs at high f. The f and VT dependencies of E and R in the chicken were strikingly similar to those measured in various types of mammalian respiratory tissues (Stamenović et al. (1990) J. Appl. Physiol. 69: 973-988. We conclude that, despite important anatomical differences between species, avian and mammalian respiratory tissues exhibit fundamentally similar mechanical behavior.

Airway Resistance↗

PubMed2002

Modeling the tensile mechanical behavior of bone along the longitudinal direction.

The tensile stress-strain behavior of bone along its longitudinal axis is modeled by using a simple shear-lag theory, wherein, stresses and strains in a unit cell consisting of an organic matrix reinforced by overlapped mineral platelets are computed. It is assumed that loads are transferred between overlapped mineral-platelets by shear in the organic matrix. The mechanical behavior of bone in which the matrix partially or completely debonds from the sides of the overlapped mineral platelets (after an ultimate interfacial shear stress value is exceeded) is modeled. It is shown that the tensile mechanical behavior of bone can be modeled only by assuming little or no debonding of the organic from the mineral. A physical phenomenon that explains the tensile behavior of bone is, after the interfacial shear stress has reached a constant value over the length of the mineral platelets, the collagen molecules/microfibrils (with the associated mineral platelets) move relative to one another. The tensile stress-strain curve of bovine bone is modeled using this model. The theory predicts the mechanical behavior of the tissue in the elastic, yield and post-yield region. The ultimate strain and strengths are not predicted in the present model.

Bone and Bones↗

PubMed2004

Quantitative analyses of methamphetamine's effects on self-control choices: implications for elucidating behavioral mechanisms of drug action.

The purpose of the present research was to utilize quantitative methods to identify behavioral mechanisms involved in the effects of stimulant drugs on choice in a self-control procedure. A logarithmic equation based upon a combination of the matching law and hyperbolic discounting was used to separate drug-induced changes in sensitivity to reinforcement delay from drug-induced changes in sensitivity to reinforcement amount. Pigeons responded under a concurrent-chains schedule. In the initial link, two keys were illuminated simultaneously and access to the terminal link was controlled by a single random-interval (RI) schedule; pecks on one or the other key lead to its terminal link with a 0.5 probability. In the terminal links, one alternative provided 1-s access to food (the smaller reinforcer) and the other alternative provided 4-s access to food (the larger reinforcer). The signaled delay to the smaller reinforcer always was 2s, whereas the signaled delay to the larger reinforcer increased from 2 to 40s within each session, across 10-min blocks. In general, intermediate doses of methamphetamine increased preference for the larger more delayed reinforcer. Quantitative analyses indicated that, in most cases, methamphetamine decreased sensitivity to reinforcement delay. In a few instances, concomitant decreases in sensitivity to reinforcement amount also occurred. These results suggest that a reduced sensitivity to reinforcement delay may be important behavioral mechanism of the effects of stimulants on self-control choices, and that this effect sometimes can be accompanied by a decreased sensitivity to reinforcement amount.

Animals↗

PubMed2000

Mechanical behavior of calcified plaques: a summary of compression and stress-relaxation experiments.

This paper summarizes the results from mechanical testing of atherosclerotic plaques performed in the Cardiovascular Mechanics Laboratory and the Laboratory for Implantable Materials at UMBC. The motivation for our work is that balloon angioplasty, stenting, and roto-ablation are mechanical processes that are designed to permanently alter the shape of an occluded arterial lumen. The mechanisms of permanent plaque deformation are not known. Therefore, to study the mechanical behavior of plaques, we performed mechanical tests on atherosclerotic lesions with different compositions and investigated differences in the materials' mechanical responses. Atherosclerotic plaque specimens were subjected to two main types of loading: multiple cyclic compression and stress-relaxation. The multiple-cycle test protocol was two fifteen-cycle loading phases that were separated by a 10-15 minute unloaded "rest" period. The compressive stress-relaxation test protocol was a series of three consecutive loadings (called phases I, II, and III). Each phase consisted of a 25% compression that was achieved in less than 1 second, a 10 minute relaxation period, and a 10 minute unloaded "rest" period between loadings. In the multiple cycle compressive loading, plaques exhibited three distinct types of behavior, which corresponded to the plaque compositions. Calcified plaques showed behaviors distinct from other plaque types and healthy vessels. In contrast to the cyclic compression results, plaque types could not be distinguished solely on the basis of stress relaxation behavior. Calcified and fibrous plaques had similar behavior, and therefore histology was used for definite identification. Calcified plaques have unique mechanical properties, and therefore interventions like angioplasty, roto-ablation, and stenting may require protocols specific for calcified lesions. The optimum protocols for calcified plaques may be quite different from plaques with other compositions. It is essential to learn more about the mechanical behavior of all plaque types to increase the success rate of occlusive atherosclerosis treatments.

Angioplasty, Balloon↗

PubMed2005

Dependence of mechanical behavior of the murine tail disc on regional material properties: a parametric finite element study.

In vivo rodent tail models are becoming more widely used for exploring the role of mechanical loading on the initiation and progression of intervertebral disc degeneration. Historically, finite element models (FEMs) have been useful for predicting disc mechanics in humans. However, differences in geometry and tissue properties may limit the predictive utility of these models for rodent discs. Clearly, models that are specific for rodent tail discs and accurately simulate the disc's transient mechanical behavior would serve as important tools for clarifying disc mechanics in these animal models. An FEM was developed based on the structure, geometry, and scale of the mouse tail disc. Importantly, two sources of time-dependent mechanical behavior were incorporated: viscoelasticity of the matrix, and fluid permeation. In addition, a novel strain-dependent swelling pressure was implemented through the introduction of a dilatational stress in nuclear elements. The model was then validated against data from quasi-static tension-compression and compressive creep experiments performed previously using mouse tail discs. Finally, sensitivity analyses were performed in which material parameters of each disc subregion were individually varied. During disc compression, matrix consolidation was observed to occur preferentially at the periphery of the nucleus pulposus. Sensitivity analyses revealed that disc mechanics was greatly influenced by changes in nucleus pulposus material properties, but rather insensitive to variations in any of the endplate properties. Moreover, three key features of the model-nuclear swelling pressure, lamellar collagen viscoelasticity, and interstitial fluid permeation-were found to be critical for accurate simulation of disc mechanics. In particular, collagen viscoelasticity dominated the transient behavior of the disc during the initial 2200 s of creep loading, while fluid permeation governed disc deformation thereafter. The FEM developed in this study exhibited excellent agreement with transient creep behavior of intact mouse tail motion segments. Notably, the model was able to produce spatial variations in nucleus pulposus matrix consolidation that are consistent with previous observations in nuclear cell morphology made in mouse discs using confocal microscopy. Results of this study emphasize the need for including nucleus swelling pressure, collagen viscoelasticity, and fluid permeation when simulating transient changes in matrix and fluid stress/strain. Sensitivity analyses suggest that further characterization of nucleus pulposus material properties should be pursued, due to its significance in steady-state and transient disc mechanical response.

Animals↗

PubMed2001

Prediction of mechanical behaviors at interfaces between bone and two interbody cages of lumbar spine segments.

STUDY DESIGN: The effects of mechanical parameters at bone-implant interfaces of the lumbar spine segments were investigated under various combined loadings using the finite element method. OBJECTIVE: To investigate the mechanical behaviors at bone-cage interfaces of lumbar spine segments with two interbody cages (two thread inserts). SUMMARY OF BACKGROUND DATA: It is known that among many factors, relative micromotion at bone-implant interfaces can hinder bone growth into the surface pores of an implant. Loading conditions, mechanical properties of the materials, friction coefficients at the interfaces, and geometry of spinal segments would affect relative micromotion and spinal stability. In particular, relative micromotion is related closely to friction at bone-implant interfaces after arthroplasty. METHODS: A finite element model of human L3-L4 lumbar segments with two titanium interbody cages was constructed. This finite element model was used to investigate mechanical behavior at the bone-cage interface. Relative micromotion (slip distance on the contact surfaces), posterior axial displacement, and stress were predicted for changes of friction coefficients, loading conditions, and age-related material-geometric properties of the spinal segments. RESULTS: Relative micromotion (slip distance) at the interfaces was obvious at their edges under axial compression. The slip occurred primarily at the anterior edges under torsion with preload, whereas it occurred primarily at the edges of the left cage under lateral bending with preload. Relative micromotion at the interfaces increased significantly as the apparent density of cancellous bone or the friction coefficient of the interfaces decreased. A significant increase in slip distance at the anterior anulus occurred with an addition of torsion to the compressive preload. CONCLUSIONS: Relative micromotion is sensitive to the friction coefficient of the interfaces, the bone density, and the loading conditions. A reduction in age-related bone density is less likely to allow bone growth into surface pores of the cage. However, it is likely that the larger the disc area or pedicle diameter, the more stable the interbody fusion of the spinal segments.

Age Factors↗

PubMed1978

Physical and mechanical behavior of polyurethane elastomer formulations used for facial prostheses.

The initial results of this study of the physical and mechanical behavior of polyurethanes used for facial prostheses are presented. The parameters studied were surface hardness, modulus of elasticity (M), strength (S), percentage of elongation, and S/M ratio. Results showed that the physical and mechanical behavior of the polyurethanes can be altered by variations in the basic composition of the material accomplished through changes in the ratio of Part A to Part B and by the addition of catalysts. Properties of compositions with low quantities of Part B (isocyanate) and no catalyst reached or approximated those parameters proposed as ideal goals to simulate living tissue. When preparing polyurethane material, proper mixing is essential to avoid entrapment of air and phase separation of the catalyst. These conditions, which may render the prostheses less durable, are masked by the addition of colorants in the prearation of a sample for clinical application.

Chemical Phenomena↗