Subsidence of THA stems due to acrylic cement creep is extremely sensitive to interface friction

Acrylic cement, used to fixate total hip arthroplasty (THA), creeps under dynamic and static loading conditions. As a result, THA stems, which are debonded from the cement, may gradually subside, depending on their shape and surface roughness. The purpose of this study was to evaluate the relationship among dynamic load, creep characteristics, interface friction, and subsidence patterns.

A laboratory model consisting of a metal tapered cone, surrounded by a cement mantle, was developed. The cone was gradually compressed in the cement by a dynamic, sinusoidal axial force, cycling between 0 and 7 kN for 1.7 million cycles at a frequency of 1 Hz. Subsidence and cement strain were monitored. Two tapers were tested in this way. The relationships among subsidence, creep properties and interface friction were evaluated from a finite element (FE) model, used to simulate the experiments. In this model, the creep properties obtained in dynamic and static, tension and compression experiments measured earlier, were used.

The subsidence patterns of both tapers were similar, but one subsided more than the other (380 vs 630 μm). Both subsided stepwise instead of continuous, with a frequency much smaller than that of the applied load. The characteristics of the subsidence and cement-strain patterns could be reproduced by the FE model, but not with great numerical precision. The stepwise subsidence could be explained by slip-stick mechanisms at the interface starting distally and gradually working towards proximal. Variations in friction from 0.25 to 0.50 reduced the total subsidence and the step frequency by about 50%.

It was concluded that FE-models used to simulate the mechanical endurance characteristics of THA reconstructions, extended to incorporate cement creep, produce realistic results. These results showed that prosthetic subsidence under dynamic loads occurs due to cement creep. The extent of the subsidence is extremely sensitive to interface friction, hence to small variations in surface roughness and cement constitution. This may explain the relatively large variation of in vivo prosthetic subsidence rates reported in the literature.     

Experimental and Numerical Analyses of the Pull-out Response of a Steel Post/Bovine Bone Cementless Fixation

Effect of initial interference fit on pull-out strength in cementless fixation between bovine tibia and smooth stainless steel post was investigated in this study.Compressive behavior of bovine spongious bone was studied using mechanical testing in order to evaluate the elastic-plastic properties in different regions of the proximal tibia.Friction tests were carried out in the aim to evaluate the friction behavior of the contact between bovine spongious bone and stainless steel.A cylindrical stainless steel post inserted in a pre-drilled bovine tibia with an initial interference fit was taken as an in vitro model to assess the contribution of post fixation to the initial stability of the Total Knee Arthroplasty (TKA) tibial component.Pull-out experiments were carried out for different initial interference fits.Finite Element Models (FEM) using local elastic-plastic properties of the bovine bone were developed for the analysis of the experimental ultimate pull-out force results.At the post/bone interface,Coulomb friction was considered in the FEM calculations with pressure-dependent friction coefficient.It was found that the FEM results of the ultimate force are in good agreement with the experimental results.The analysis of the FEM interfacial stresses indicates that the micro-slip initiation depends on the local bone properties.     

Numerical simulation of the insertion process of an uncemented hip prosthesis in order to evaluate the influence of residual stress and contact distribution on the stem initial stability

The long-term success of a cementless total hip arthroplasty depends on the implant geometry and interface bonding characteristics (fit, coating and ingrowth) and on stem stiffness. This study evaluates the influence of stem geometry and fitting conditions on the evolution and distribution of the bone–stem contact, stress and strain during and after the hip stem insertion, by means of dynamic finite element techniques. Next, the influence of the mechanical state (bone–stem contact, stress and strain) resulted from the insertion process on the stem initial resistance to subsidence is investigated. In addition, a study on the influence of bone–stem interface conditions (friction) on the insertion process and on the initial stem stability under physiological loading is performed. The results indicate that for a stem with tapered shape the contact in the proximal part of the stem was improved, but contact in the calcar region was achieved only when extra press-fit conditions were considered. Changes in stem geometry towards a more tapered shape and extra press fit and variation in the bone–stem interface conditions (contact amount and high friction) led to a raise in the total insertion force. A direct positive relationship was found between the stem resistance to subsidence and stem geometry (tapering and press fit), bone–stem interface conditions (bone–stem contact and friction interface) and the mechanical status at the end of the insertion (residual stress and strain). Therefore, further studies on evaluating the initial performance of different stem types should consider the parameters describing the bone–stem interface conditions and the mechanical state resulted from the insertion process.     

Push Force Analysis of Anchor Block of the Oil and Gas Pipeline in a Single-Slope Tunnel Based on the Energy Balance Method

In this paper, a single-slope tunnel pipeline was analysed considering the effects of vertical earth pressure, horizontal soil pressure, inner pressure, thermal expansion force and pipeline—soil friction. The concept of stagnation point for the pipeline was proposed. Considering the deformation compatibility condition of the pipeline elbow, the push force of anchor blocks of a single-slope tunnel pipeline was derived based on an energy method. Then, the theoretical formula for this force is thus generated. Using the analytical equation, the push force of the anchor block of an X80 large-diameter pipeline from the West—East Gas Transmission Project was determined. Meanwhile, to verify the results of the analytical method, and the finite element method, four categories of finite element codes were introduced to calculate the push force, including CAESARII, ANSYS, AutoPIPE and ALGOR. The results show that the analytical results agree well with the numerical results, and the maximum relative error is only 4.1%. Therefore, the results obtained with the analytical method can satisfy engineering requirements.     

Can loaded interface characteristics influence strain distributions in muscle adjacent to bony prominences?

Pressure distributions at the interface between skin and supporting tissues are used in design of supporting surfaces like beds, wheel chairs, prostheses and in sales brochures to support commercial products. The reasoning behind this is, that equal pressure distributions in the absence of high pressure gradients is assumed to minimise the risk of developing pressure sores. Notwithstanding the difficulty in performing reproducible and accurate pressure measurements, the question arises if the interface pressure distribution is representative of the internal mechanical state of the soft tissues involved. The paper describes a study of the mechanical condition of a supported buttock contact, depending on cushion properties, relative properties of tissue layers and friction. Numerical, mechanical simulations of a buttock on a supporting cushion are described. The ischial tuberosity is modelled as a rigid body, whereas the overlying muscle, fat and skin layers are modelled as a non-linear Ogden material. Material parameters and thickness of the fat layer are varied. Coulomb friction between buttock and cushion is modelled with different values of the friction coefficient. Moreover, the thickness and properties of the cushion are varied. High shear strains are found in the muscle near the bony prominence and the fat layer near the symmetry line. The performed parameter variations lead to large differences in shear strain in the fat layer but relatively small variations in the skeletal muscle. Even with a soft cushion, leading to a high reduction of the interface pressure the deformation of the skeletal muscle near the bone is high enough to form a risk, which is a clear argument that interface pressures alone are not sufficient to evaluate supporting surfaces.     

Stem surface roughness alters creep induced subsidence and 'taper-lock' in a cemented femoral hip prosthesis

The clinical success of polished tapered stems has been widely reported in numerous long term studies. The mechanical environment that exists for polished tapered stems, however, is not fully understood. In this investigation, a collarless, tapered femoral total hip stem with an unsupported distal tip was evaluated using a 'physiological' three-dimensional (3D) finite element analysis. It was hypothesized that stem-cement interface friction, which alters the magnitude and orientation of the cement mantle stress, would subsequently influence stem 'taper-lock' and viscoelastic relaxation of bone cement stresses. The hypothesis that creep-induced subsidence would result in increases to stem-cement normal (radial) interface stresses was also examined. Utilizing a viscoelastic material model for the bone cement in the analysis, three different stem-cement interface conditions were considered: debonded stem with zero friction coefficient (mu=0) (frictionless), debonded stem with stem-cement interface friction (mu=0.22) ('smooth' or polished) and a completely bonded stem ('rough'). Stem roughness had a profound influence on cement mantle stress, stem subsidence and cement mantle stress relaxation over the 24-h test period. The frictionless and smooth tapered stems generated compressive normal stress at the stem-cement interface creating a mechanical environment indicative of 'taper-lock'. The normal stress increased with decreasing stem-cement interface friction but decreased proximally with time and stem subsidence. Stem subsidence also increased with decreasing stem-cement interface friction. We conclude that polished stems have a greater potential to develop 'taper-lock' fixation than do rough stems. However, subsidence is not an important determinant of the maintenance of 'taper-lock'. Rather subsidence is a function of stem-cement interface friction and bone cement creep.     

Fluid-structure interaction modeling of aneurysmal arteries under steady-state and pulsatile blood flow: a stability analysis

Tortuous aneurysmal arteries are often associated with a higher risk of rupture but the mechanism remains unclear. The goal of this study was to analyze the buckling and post-buckling behaviors of aneurysmal arteries under pulsatile flow. To accomplish this goal, we analyzed the buckling behavior of model carotid and abdominal aorta with aneurysms by utilizing fluid-structure interaction (FSI) method with realistic waveforms boundary conditions. FSI simulations were done under steady-state and pulsatile flow for normal (1.5) and reduced (1.3) axial stretch ratios to investigate the influence of aneurysm, pulsatile lumen pressure and axial tension on stability. Our results indicated that aneurysmal artery buckled at the critical buckling pressure and its deflection nonlinearly increased with increasing lumen pressure. Buckling elevates the peak stress (up to 118%). The maximum aneurysm wall stress at pulsatile FSI flow was (29%) higher than under static pressure at the peak lumen pressure of 130 mmHg. Buckling results show an increase in lumen shear stress at the inner side of the maximum deflection. Vortex flow was dramatically enlarged with increasing lumen pressure and artery diameter. Aneurysmal arteries are more susceptible than normal arteries to mechanical instability which causes high stresses in the aneurysm wall that could lead to aneurysm rupture.     

Loss of stability: a new look at the physics of cell wall behavior during plant cell growth

In this article we investigate aspects of turgor-driven plant cell growth within the framework of a model derived from the Eulerian concept of instability. In particular we explore the relationship between cell geometry and cell turgor pressure by extending loss of stability theory to encompass cylindrical cells. Beginning with an analysis of the three-dimensional stress and strain of a cylindrical pressure vessel, we demonstrate that loss of stability is the inevitable result of gradually increasing internal pressure in a cylindrical cell. The turgor pressure predictions based on this model differ from the more traditional viscoelastic or creep-based models in that they incorporate both cell geometry and wall mechanical properties in a single term. To confirm our predicted working turgor pressures, we obtained wall dimensions, elastic moduli, and turgor pressures of sequential internodal cells of intact Chara corallina plants by direct measurement. The results show that turgor pressure predictions based on loss of stability theory fall within the expected physiological range of turgor pressures for this plant. We also studied the effect of varying wall Poisson's ratio nu on extension growth in living cells, showing that while increasing elastic modulus has an understandably negative effect on wall expansion, increasing Poisson's ratio would be expected to accelerate wall expansion.     

Assessment of fiber strength in a urinary bladder by using experimental pressure volume curves: an analytical method

In the present study, an analytical method is developed to deduce the constitutive equations of fibers embedded in a thick shell from the time-variant pressure volume curves obtained by experimental procedures. It is assumed that the spherical shell under consideration is composed of a fiber reinforced material and undergoes radial deflection, modeling the behavior of some biological shells such as urinary bladder. The fiber stress is expressed as a function of fiber strain, rate of strain and the degree of biochemical activation. The function form is chosen such that equations of mechanical equilibrium can be integrated analytically to yield chamber pressure as a function of chamber volume, time rate of change of volume and activation. Arbitrary coefficients appearing in the fiber stress-equation are also present in the resultant time-variant pressure-volume relation. These coefficients can be determined by curve-fitting commonly used clinical data such as cystometry measurements.     

Increasing strain and strain rate strengthen transient stiffness but weaken the response to subsequent compression for articular cartilage in unconfined compression

Strain amplitude and strain rate dependent nonlinear behavior and load-induced mechanical property alterations of full-thickness bovine articular cartilage attached to bone were investigated in unconfined compression. A sequence of test compressions of finite deformation (ranging from 0.9% to 34.5% nominal strain) was performed at strain rates ranging from approximately 0.053%/s to 5.8%/s. Peak and equilibrium loads were analyzed to determine strain amplitude and strain rate dependence of linear versus nonlinear responses. The test protocol was designed to reveal changes in mechanical properties due to these finite deformations by interspersing small-amplitude witness ramps of approximately 1.1% deformation and approximately 0.44%/s strain rate between the test ramps ("witness" meaning to assess any mechanical property changes). We found that peak loads displayed high nonlinearity, stiffening with both increasing compression amplitude and more so with increasing strain rate. The response to witness ramps suggested that mechanical weakening occurred when compression amplitude reached 1.9-2.9% strain and beyond, and that weakening was much more significant at higher strain rate. These findings delineate regimes of linear versus nonlinear behavior of cartilage, and indicate the types of loads which can cause mechanical property alterations. Biological implications of this study are that strain amplitude and strain rate dependent stiffening may be essential to bear physiological loads and to protect cells and matrix from mechanical damage. Structural changes reflected by mechanical weakening at small compression could also initiate remodeling or disease processes.     

Depth-dependent Compressive Equilibrium Properties of Articular Cartilage Explained by its Composition

For this study, we hypothesized that the depth-dependent compressive equilibrium properties of articular cartilage are the inherent consequence of its depth-dependent composition, and not the result of depth-dependent material properties. To test this hypothesis, our recently developed fibril-reinforced poroviscoelastic swelling model was expanded to include the influence of intra- and extra-fibrillar water content, and the influence of the solid fraction on the compressive properties of the tissue. With this model, the depth-dependent compressive equilibrium properties of articular cartilage were determined, and compared with experimental data from the literature. The typical depth-dependent behavior of articular cartilage was predicted by this model. The effective aggregate modulus was highly strain-dependent. It decreased with increasing strain for low strains, and increases with increasing strain for high strains. This effect was more pronounced with increasing distance from the articular surface. The main insight from this study is that the depth-dependent material behavior of articular cartilage can be obtained from its depth-dependent composition only. This eliminates the need for the assumption that the material properties of the different constituents themselves vary with depth. Such insights are important for understanding cartilage mechanical behavior, cartilage damage mechanisms and tissue engineering studies.     

Mechanical instability of normal and aneurysmal arteries

Tortuous arteries associated with aneurysms have been observed in aged patients with atherosclerosis and hypertension. However, the underlying mechanism is poorly understood. The objective of this study was to determine the effect of aneurysms on arterial buckling instability and the effect of buckling on aneurysm wall stress. We investigated the mechanical buckling and post-buckling behavior of normal and aneurysmal carotid arteries and aorta’s using computational simulations and experimental measurements to elucidate the interrelationship between artery buckling and aneurysms. Buckling tests were done in porcine carotid arteries with small aneurysms created using elastase treatment. Parametric studies were done for model aneurysms with orthotropic nonlinear elastic walls using finite element simulations. Our results demonstrated that arteries buckled at a critical buckling pressure and the post-buckling deflection increased nonlinearly with increasing pressure. The presence of an aneurysm can reduce the critical buckling pressure of arteries, although the effect depends on the aneurysm’s dimensions. Buckled aneurysms demonstrated a higher peak wall stress compared to unbuckled aneurysms under the same lumen pressure. We conclude that aneurysmal arteries are vulnerable to mechanical buckling and mechanical buckling could lead to high stresses in the aneurysm wall. Buckling could be a possible mechanism for the development of tortuous aneurysmal arteries such as in the Loeys–Dietz syndrome.     

Frictional properties of regenerated cartilage in vitro

Although tribological function is the most important mechanical property of articular cartilage, few studies have examined this function in tissue-engineered cartilage. We investigated changes in the frictional properties of cartilage regenerated from the inoculation of rabbit chondrocytes into fibroin sponge. A reciprocating friction-testing apparatus was used to measure the friction coefficient of the regenerated cartilage under a small load. The specimen was slid against a stainless steel plate in a water vessel filled with physiological saline. The applied load was 0.03 N, the stroke length was 20 mm, and the mean sliding velocity was 0.8 mm/s. The friction coefficient of the regenerated cartilage decreased with increasing cultivation time, because a hydrophilic layer of synthesized extracellular matrix was formed on the fibroin sponge surface. The friction coefficient of the regenerated cartilage was as low as that of natural cartilage in the early stages of the sliding tests, but it increased with increasing duration of sliding owing to exudation of interstitial water from the surface layer.     

The role of human red blood cell membrane skeletal proteins in repetitive membrane strain and rupture

The influence of the membrane skeleton on cell membrane deformability, elasticity, and rupture after repetitive cycles of membrane strain, release and rupture was investigated. Human red blood cells were electrofused to doublets, which showed the post-fusion oscillation-movement. Geometrical developments of heat-treated cells were measured and compared to control cells. Alterations of cluster length and fusion zone diameter during repetitive colloidosmotic swelling period grow with heat treatment, and the number of precedent swell phases has minor influence on these values. Irrespective of the treatment, the geometrical doublet configuration at which a membrane rupture is initiated has an almost constant roundness index of 0.89. Increasing heat treatment temperature was shown to affect both deformability and elasticity of the membrane, such that doublets start each swell phase of the oscillation cycle from decreased roundness values. Evidence is given that there is a difference in the mechanical properties between the membrane at the fusion zone and the membrane of native red blood cells.     

The role of human red blood cell membrane skeletal proteins in repetitive membrane strain and rupture

The influence of the membrane skeleton on cell membrane deformability, elasticity, and rupture after repetitive cycles of membrane strain, release and rupture was investigated. Human red blood cells were electrofused to doublets, which showed the post-fusion oscillation-movement. Geometrical developments of heat-treated cells were measured and compared to control cells. Alterations of cluster length and fusion zone diameter during repetitive colloidosmotic swelling period grow with heat treatment, and the number of precedent swell phases has minor influence on these values. Irrespective of the treatment, the geometrical doublet configuration at which a membrane rupture is initiated has an almost constant roundness index of 0.89. Increasing heat treatment temperature was shown to affect both deformability and elasticity of the membrane, such that doublets start each swell phase of the oscillation cycle from decreased roundness values. Evidence is given that there is a difference in the mechanical properties between the membrane at the fusion zone and the membrane of native red blood cells.     

The role of chromium and nickel on the thermal and mechanical properties of FeNiCr austenitic stainless steels under high pressure and temperature: a molecular dynamics study

The effect of Cr and Ni content on thermo-mechanical properties of FeNiCr austenitic stainless steel under ambient and high pressure and temperature were investigated by MD simulations. The FCC structure was selected as optimum structure for FeNiCr system based on obtained MD results from Bonny EAM potential and valid experimental results. The structural and mechanical properties of pure Fe, Ni, and Cr were also estimated based on this potential, indicating good agreement with experimental results. These properties were computed for four experimental case studies which showed less than 10% error. Moreover, the elastic constants of the Fe–(8–18)Ni–(18–25)Cr systems were estimated. Results showed that bulk modulus increases by increasing the Ni and Cr contents, which can be connected to the changes in bonding electrons. The thermal properties of FeNiCr were calculated in ambient and high pressure. Although thermo-mechanical properties confirm good agreement with experimental results at the ambient condition, however, they indicate that FeNiCr Bonny potential is not applicable at high pressure. In order to tackle this issue, a hybrid potential was used at high Pressure/Temperature. The results illustrate enhanced mechanical properties, increase of melting point and reduction of LTE in high pressure and deteriorated mechanical properties at high temperature.     

Structural dynamics and resonance in plants with nonlinear stiffness

Although most biomaterials are characterized by strong stiffness nonlinearities, the majority of studies of plant biomechanics and structural dynamics focus on the linear elastic range of their behavior. In this paper, the effects of hardening (elastic modulus increases with strain) and softening (elastic modulus decreases with strain) nonlinearities on the structural dynamics of plant stems are investigated. A number of recent studies suggest that trees, crops, and other plants often uproot or snap when they are forced by gusting winds or waves at their natural frequency. This can be attributed to the fact that the deflections of the plant, and hence mechanical stresses along the stem and root system, are greatest during resonance. To better understand the effect of nonlinear stiffness on the resonant behavior of plants, plant stems have been modeled here as forced Duffing oscillators with softening or hardening nonlinearities. The results of this study suggest that the resonant behavior of plants with nonlinear stiffness is substantially different from that predicted by linear models of plant structural dynamics. Parameter values were considered over a range relevant to most plants. The maximum amplitudes of deflection of the plant stem were calculated numerically for forcing frequencies ranging from zero to twice the natural frequency. For hardening nonlinearities, the resonant behavior was 'pushed' to higher frequencies, and the maximum deflection amplitudes were lower than for the linear case. For softening nonlinearities, the resonant behavior was pushed to lower frequencies, and the maximum deflection amplitudes were higher than for the linear case. These nonlinearities could be beneficial or detrimental to the stability of the plant, depending on the environment. Damping had the effect of drastically decreasing deflection amplitudes and reducing the effect of the nonlinearities.     

添加秸秆后黑土力学行为特征——秸秆含量的影响

为探明秸秆还田量对黑土力学特性的影响,以典型黑土区未经开垦(高有机质低黏粒含量)以及耕作40年(低有机质高黏粒含量)的土壤为研究对象,在秸秆长度为15 mm条件下,采用室内固结和三轴剪切相结合的方法,通过压缩指数、回弹指数、黏聚力及内摩擦角的测定与分析,模拟研究了黑土压缩-回弹及抗剪切行为对不同小麦秸秆还田比例(0%、25%、50%、75%、100%)的响应规律.结果表明: 添加秸秆后黑土力学性能明显改善,以开垦多年有机质含量低的黑土改善效果更为显著.秸秆全量还田时,最不易被压缩且压实后恢复能力最强;秸秆50%还田时,土体抗剪强度最大;当秸秆还田比例为50%～100%时,黑土抗压与抗蚀效果较好.     

The Effect of Oil Contamination on the Geotechnical Properties of Fine-Grained Soils

Every day, petrochemical activities, oil spills, and pipeline or reservoir leakage contaminate the ground. In addition to environmental concerns, such as groundwater pollution, the alteration of geotechnical properties of the contaminated soil is also cause for worry. Contamination has been proven to alter the geotechnical properties of soil, and researchers have extensively studied the properties of contaminated granular soils. However, the effect of oil contamination on the geotechnical properties of fine-grained soils has not yet been well evaluated. Therefore, a comprehensive set of laboratory tests has been conducted on both uncontaminated and contaminated fine-grained soils containing different amounts of crude oil. The soil samples were taken from the lands in the vicinity of the Tehran oil refinery site where there is a vast area subjected to this problem.

The results of this study indicated that an increase in the angle of internal friction, maximum dry density, compression index, and Atterberg limits as well as a decrease in optimum water content and cohesion occur as the oil content increases. Moreover, aging caused a further decrease in cohesion but had no specific effect on the internal friction angle. These effects should be taken into consideration in the oil refinery site development programs. In addition, because treatment technologies for site clean-up are expensive, by investigating the geotechnical properties of contaminated soil, we are planning to develop methods of utilizing the soil as construction material.     

Soil Penetration by Earthworms and Plant Roots—Mechanical Energetics of Bioturbation of Compacted Soils

We quantify mechanical processes common to soil penetration by earthworms and growing plant roots, including the energetic requirements for soil plastic displacement. The basic mechanical model considers cavity expansion into a plastic wet soil involving wedging by root tips or earthworms via cone-like penetration followed by cavity expansion due to pressurized earthworm hydroskeleton or root radial growth. The mechanical stresses and resulting soil strains determine the mechanical energy required for bioturbation under different soil hydro-mechanical conditions for a realistic range of root/earthworm geometries. Modeling results suggest that higher soil water content and reduced clay content reduce the strain energy required for soil penetration. The critical earthworm or root pressure increases with increased diameter of root or earthworm, however, results are insensitive to the cone apex (shape of the tip). The invested mechanical energy per unit length increase with increasing earthworm and plant root diameters, whereas mechanical energy per unit of displaced soil volume decreases with larger diameters. The study provides a quantitative framework for estimating energy requirements for soil penetration work done by earthworms and plant roots, and delineates intrinsic and external mechanical limits for bioturbation processes. Estimated energy requirements for earthworm biopore networks are linked to consumption of soil organic matter and suggest that earthworm populations are likely to consume a significant fraction of ecosystem net primary production to sustain their subterranean activities.