Shear Stress Distribution in the Trabeculae of Human Vertebral Bone

The statistical distribution of von Mises stress in the trabeculae of human vertebral cancellous bone was estimated using large-scale finite element models. The goal was to test the hypothesis that average trabecular von Mises stress is correlated to the maximum trabecular level von Mises stress. The hypothesis was proposed to explain the close experimental correlation between apparent strength and stiffness of human cancellous bone tissue. A three-parameter Weibull function described the probability distribution of the estimated von Mises stress (r²) > 0.99 for each of 23 cases). The mean von Mises stress was linearly related to the standard deviation (r²=0.63) supporting the hypothesis that average and maximum magnitude stress would be correlated. The coefficient of variation (COV) of the von Mises stress was nonlinearly related to apparent compressive strength, apparent stiffness, and bone volume fraction (adjusted r²=0.66, 0.56, 0.54, respectively) by a saturating exponential function [COV=A+B exp(–x/C)]. The COV of the stress was higher for low volume fraction tissue (<0.12) consistent with the weakness of low volume fraction tissue and suggesting that stress variation is better controlled in higher volume fraction tissue. We propose that the average stress and standard deviation of the stress are both controlled by bone remodeling in response to applied loading. © 2000 Biomedical Engineering Society. PAC00: 8719Rr     

Prestress Due to Dimensional Changes Caused by Demineralization: A Potential Mechanism for Microcracking in Bone

Microcracking in bone due to internal strains caused by mineralization is a possible mechanism of damage. Similar damage can be seen in other biological composites such as trees experiencing growth-related prestresses. Dimensional changes in cortical bone due to demineralization and experimental glycation were studied to test whether mineralization-related prestrains are consistent with observed microcracking patterns in bone. A microscopy technique that enables wet measurements of length and angle of milled bone specimens was used. Demineralization of bovine and human bones caused significant anisotropic changes in tissue size. Dimensional changes due to demineralization in bovine bone were prevented or reduced when collagen cross linking was increased by glycation. The dimensional changes of bone caused by demineralization are consistent with the hypothesis that mineralization-caused stresses in remodeling tissue can cause microcracks. © 2002 Biomedical Engineering Society. PAC2002: 8719Rr     

Structural and Radiological Parameters for the Nondestructive Characterization of Trabecular Bone

Trabecular bone is characterized by compositional and organizational factors. The former include porosity at microlevel and mineralization. The latter refer to the trabecular architecture. Both determine the mechanical properties of the trabecular bone. The aim of this study is to investigate the relationship between the mechanical properties and the local HU value, the bone mineral density, the in vitro histomorphometric properties assessed by means of microcomputed tomography, and the Young's modulus determined by ultrasound measurement. Also the correlation between local HU values based on CT data of the full bone and HU values based on CT data of excised trabecular bone cylinders is investigated. Therefore density and strength related parameters of 22 trabecular bone cylinders retrieved from a fresh cadaver femur were measured by using different techniques. The mean HU value of the excised bone samples is very highly correlated with the pQCT density (R²=0.95) and the CT-based morphometric parameter BV/TV (R²=0.95). The mean HU values, determined from the CT images of the planned and excised bone samples, are less highly correlated (R²=0.75). The Young's modulus E_US determined from the ultrasound measurement is highly correlated with the maximal stress _max (R²=0.88) but not with the mechanically determined Young's modulus E_mech (R²=0.67). The maximal stress _max correlates well with the density parameters (R² varies between 0.76 and 0.86). On the contrary the mechanically determined Young's modulus E_mech does not correlate well with the density parameters (R² varies between 0.52 and 0.56). The absorbed energy E_abs during the deformation is only highly correlated with the maximal stress _max (R²=0.83). The inclusion of structural parameters besides a density related parameter did improve the prediction of the Young's modulus and the maximal stress. In conclusion, it seems that the HU value from clinical CT scanning is a good predictor of the local bone density and volume fraction. A combination of local density and a measure of the structural anisotropy is clearly needed to achieve good predictions of bone mechanics. © 2001 Biomedical Engineering Society. PAC01: 8719Rr, 8759Ls, 4380Ev, 8759Fm, 4335Zc, 8170Cv     

The Influence of Mineralization on Intratrabecular Stress and Strain Distribution in Developing Trabecular Bone

The load-transfer pathway in trabecular bone is largely determined by its architecture. However, the influence of variations in mineralization is not known. The goal of this study was to examine the influence of inhomogeneously distributed degrees of mineralization (DMB) on intratrabecular stresses and strains. Cubic mandibular condylar bone specimens from fetal and newborn pigs were used. Finite element models were constructed, in which the element tissue moduli were scaled to the local DMB. Disregarding the observed distribution of mineralization was associated with an overestimation of average equivalent strain and underestimation of von Mises equivalent stress. From the surface of trabecular elements towards their core the strain decreased irrespective of tissue stiffness distribution. This indicates that the trabecular elements were bent during the compression experiment. Inhomogeneously distributed tissue stiffness resulted in a low stress at the surface that increased towards the core. In contrast, disregarding this tissue stiffness distribution resulted in high stress at the surface which decreased towards the core. It was concluded that the increased DMB, together with concurring alterations in architecture, during development leads to a structure which is able to resist increasing loads without an increase in average deformation, which may lead to damage.     

Effects of a Fixation Screw on Trabecular Structural Changes in a Vertebral Body Predicted by Remodeling Simulation

Computational simulation of trabecular surface remodeling was conducted to investigate the effects of a spinal fixation screw on trabecular structural changes in a vertebral body. By using voxel-based finite elements, computational models of the bone and screw were constructed in two structural scales of a vertebral body with an implanted screw and a bone–screw interface. In the vertebral body, the implantation of the fixation screw caused changes in the mechanical environment in cancellous bone, leading to trabecular structural changes at the cancellous level. The effects of the screw on trabecular orientation were greater in the regions above and below the screw than in those in front of the screw. In the case of the bone–screw interface, trabecular structural changes depended on the direction of load applied to the screw. It was suggested that the bone resorption predicted in the pull-out loading case is a candidate cause of the loosening of the screw. The results indicate that the effects of the implanted screw on trabecular structural changes are more important for longer-term fixation. © 2003 Biomedical Engineering Society. PAC2003: 8718Bb, 8719Rr, 8780Rb, 8710+e     

Lumbar Vertebral Body Bone Microstructural Scaling in Small to Medium‐Sized Strepsirhines

Bone mass, architecture, and tissue mineral density contribute to bone strength. As body mass (BM) increases any one or combination of these properties could change to maintain structural integrity. To better understand the structural origins of vertebral fragility and gain insight into the mechanisms that govern bone adaptation, we conducted an integrative analysis of bone mass and microarchitecture in the last lumbar vertebral body from nine strepsirhine species, ranging in size from 42 g (Microcebus rufus) to 2,440 g (Eulemur macaco). Bone mass and architecture were assessed via µCT for the whole body and spherical volumes of interest (VOI). Allometric equations were estimated and compared with predictions for geometric scaling, assuming axial compression as the dominant loading regime. Bone mass, microarchitectural, and vertebral body geometric variables predominantly scaled isometrically. Among structural variables, the degree of anisotropy (Tb.DA) was the only parameter independent of BM and other trabecular architectural variables. Tb.DA was related to positional behavior. Orthograde primates had higher average Tb.DA (1.60) and more craniocaudally oriented trabeculae while lorisines had the lowest Tb.DA (1.25), as well as variably oriented trabeculae. Finally, lorisines had the highest ratio of trabecular bone volume to cortical shell volume (～3x) and while there appears to be flexibility in this ratio, the total bone volume (trabecular + cortical) scales isometrically (BM^1.23, r² = 0.93) and appears tightly constrained. The common pattern of isometry in our measurements leaves open the question of how vertebral bodies in strepsirhine species compensate for increased BM. Anat Rec, 2013. © 2013 Wiley Periodicals, Inc.     

Optimizing bone cement stiffness for vertebroplasty through biomechanical effects analysis based on patient-specific three-dimensional finite element modeling

Vertebroplasty is a common and effective treatment for symptomatic osteoporotic vertebral compression fractures. However, the cemented and adjacent vertebras have a risk of recollapse due to largely unassured mechanisms, among which excessive stiffness of bone cement may be an important risk factor. This study aimed to find the most appropriate range of bone cement stiffness by analyzing its biomechanical effects on the augmented and adjacent vertebras of individual patient after vertebroplasty. A three-dimensional finite element model of T11-L1 osteoligamentous vertebras was reconstructed according to individual computed tomography data and validated by post mortem human subject experiment in literatures. Bone cement of varying stiffness was injected into the trabecular core of the T12 vertebra simulatively. The maximum von Mises stresses on cancellous and cortical bones of T11-L1 vertebras were analyzed under the loading conditions of flexion, extension, bending, and torsion. For the adjacent T11 and L1 vertebras, the stepwise elevation of the bone cement elastic modulus increased the maximum von Mises stress on the cancellous bone, but its effect on cortical bone was negligible. For the augmented T12 vertebra, the stresses on cancellous bone increased slightly under the loading condition of lateral bending and remained no impact on cortical bone. The linear interpolation revealed that the most suitable range of cement elastic modulus is 833.1 and 1408.1 Mpa for this patient. Increased elastic modulus of bone cement may lead to a growing risk of recollapse for the cemented vertebra as well as the adjacent vertebras. Our study provides a fresh perspective in clinical optimization of individual therapy in vertebroplasty.

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Prediction of hip fracture can be significantly improved by a single biomedical indicator

Femoral neck fractures are a relevant clinical and social problem. The aim of this study was to improve the prediction of patients at-risk of femoral neck fracture with respect to the current densitometric-based methods. In particular, finite element models were used to assess the prediction accuracy obtained by combining together data from the bone density distribution, the proximal femur anatomy, and the fall-related loading conditions. Two-dimensional finite element models were developed based on dual energy x-ray absorptiometry data. A population of 93 elder Caucasian women (half of them reporting a femoral neck fracture) were retrospectively classified both using the standard clinical protocol and Bayes' linear classifiers. This study showed that the bone mineral density in the femoral neck region dominated the fracture event (65% accuracy). Adding the subject's height and the neck-shaft angle to the bone density increased the accuracy to 77%. The classification accuracy was further improved to 82% by including the peak principal tensile strain obtained from the finite element analyses. This research demonstrated that adding one single biomechanical indicator to the standard clinical measurements improves the identification of patients at-risk of femoral neck fracture. © 2002 Biomedical Engineering Society. PAC2002: 8759Ls, 8719Rr     

Assessment of the Directional Elastic Moduli of Ewe Vertebral Cancellous Bone by Vibrational Testing

The ovariectomized ewe is being used as an animal model for postmenopausal osteoporosis. Data on the mechanical properties of ewe vertebral cancellous bone is needed to assess its effectiveness as a model for vertebral osteoporosis. This study utilized traditional compression testing and a novel nondestructive vibrational testing method to assess the directional mechanical properties of ewe vertebral cancellous bone. Composition and density properties were also assessed. It was hypothesized that vibrational testing would have utility in that it would allow for the anisotropic stiffness of cancellous bone to be assessed nondestructively. The present study has found that ewe vertebral cancellous bone has similar physical and mechanical properties to humans. The vibrational testing method described was able to nondestructively provide a valid measure of stiffness that was correlated with stiffness estimates from traditional compression testing. Furthermore, the stiffness measure from the vibration test was found to be sensitive to the architecture of cancellous bone. These results suggest the promise of this testing method for the nondestructive mechanical assessment of skeletal tissue. © 1999 Biomedical Engineering Society. PAC99: 8719Rr, 8170Bt, 8780-y, 0710-h     

Ultrasound velocity and attenuation in relation to maximum trabecula stress in the patella

The ultrasound velocity and attenuation were examined in 16 sets of human patellae. The average ultrasound velocity of patella was shown to be greater in the superior/inferior direction than in the anterior/posterior and medial/lateral directions. The distribution of bone mineral density (BMD) was also examined. The BMD of the patella varied with location. BMD values were largest at the superior and lateral regions and decreased inferiorly and medially. A two-dimensional finite element analysis was performed on each patella. The maximum von Mises stress occurred along the cortical shell on the non-articular surface. The trabecular von Mises stress existed in the posterior region of the patella. Correlation study showed that patellar BMD was significantly associated with each of three directional ultrasound velocities. The relationship between BMD and ultrasound attenuation was not significant. It was also found that the ultrasound velocity and attenuation were not significantly correlated with the maximum von Mises stress.     

Surgical and Morphological Factors that Affect Internal Mechanical Loads in Soft Tissues of the Transtibial Residuum

The residual limb of transtibial amputation (TTA) prosthetic users is threatened daily by pressure ulcers (PU) and deep tissue injury (DTI) caused mainly by sustained mechanical strains and stresses. Several risk factors dominate the extent of internal tissue loads in the residuum. In this study, we developed a set of three-dimensional finite element (FE) models that were variants of a patient-specific FE model, built from magnetic resonance imaging scans. The set of FE modes was utilized to assess the impact of the following risk factors on the strain/stress distribution in the muscle flap: (i) the tibial length, (ii) the tibial bevelment, (iii) a fibular osteophyte, (iv) the mechanical properties of the muscle, and (v) scarring in different locations and depths. A total of 12 nonlinear FE model configurations, representing variations in these factors, were built and solved. We present herein calculations of compression, tension and shear strains and stresses, von Mises stresses, and strain energy density averaged in critical locations in the muscle flap as well as volumes of concentration of elevated stresses in these areas. Our results overall show higher stresses accumulating in the bone proximity rather than in outlying soft tissues. The longer bone configurations spread the loads toward the external surfaces of the muscle flap. When shortening the truncated bones from 11.2 to 9.2 cm, the von Mises stresses at the distal edges of the bones were relieved considerably (by up to 80%), which indicates a predicted decreased risk for DTI. Decreasing the tibial bevelment mildly, from 52.3° to 37.7° caused propagation of internal stresses from the bone proximity toward the more superficial soft tissues of the residuum, thereby also theoretically reducing the risk for DTI. An osteophyte at the distal fibular end increased the strain and stress distributions directly under the fibula but had little effect (<1%) on stresses at other sites, e.g., under the tibia. Elevation of muscle stiffness (instantaneous shear modulus increase from 8.5 to 16.2 kPa), simulating variation between patients, and muscle flap contraction or spasm, showed the most substantial effect by an acute rise of the von Mises stresses at the bone proximity. The mean von Mises stresses at the bone proximity were approximately twofold higher in the contracted/spastic muscle when compared to the flaccid muscle. Locating a surgical scar in different sites and depths of the residuum had the least influence on the overall loading of the muscle flap (where stresses changed by <7%). Pending further validation by epidemiological PU and DTI risk factor studies, the conclusions of this study can be incorporated as guidelines for TTA surgeons, physical therapists, prosthetists, and the TTA patients themselves to minimize the onset of PU and DTI in this population. Additionally, the present analyses can be used to guide or focus epidemiological research of PU and DTI risk factors in the TTA population.     

Growth‐dependent phenotype in FasL‐deficient mandibular/alveolar bone

FASL (CD178) is known for its role in triggering apoptosis, mostly in relation with immune cells but additional functions have been reported more recently, including those in bone development. Examination of postnatal FasL‐deficient mice (gld) showed an increased bone deposition in adult mice when compared with wild types. However, a different phenotype was observed prenatally, when the gld bone was underdeveloped. The aim of the following investigation was to evaluate this indication for an growth‐dependent bone phenotype of gld mice and to search for the ‘switch point’. This study focused on the mandibular/alveolar bone as an important structure for tooth anchorage. In vivo micro‐computed tomography (CT) analysis was performed at different stages during the first month (6, 12 and 24 days) of postnatal bone development. In 6‐day‐old gld mice, a decrease in bone volume/tissue volume (BV/TV), trabecular thickness and trabecular number was revealed. In contrast, the 12‐day‐old gld mice showed an increased BV/TV and trabecular thickness in the alveolar bone. The same observation applied for bone status in 24‐day‐old gld mice. Therefore, changes in the bone phenotype occurred between day 6 and 12 of the postnatal development. The switch point is likely related to the changing proportion of bone cells at these stages of development, when the number of osteocytes increases. Indeed, the immunohistochemical analysis of FASL localized this protein in osteoblasts, whereas osteocytes were mostly negative at examined stages. The impact of FASL particularly on osteoblasts would agree with an earlier in vivo observed effect of FASL deficiency on expression of Mmp2, typical for osteoblasts, in the gld mandibular/alveolar bone. Notably, an age‐dependent bone phenotype was reported in Mmp2‐deficient mice.     

How to select the elastic modulus for cancellous bone in patient-specific continuum models of the spine

Patient-specific finite element (FE) modelling is a promising technology that is expected to support clinical assessment of the spine in the near future. To allow rapid, robust and economic patient-specific modelling of the whole spine or of large spine segments, it is practicable to consider vertebral cancellous bone in the spine as a continuum material, but the elastic modulus of that continuum material must reflect the quality of the individual vertebral bone. A numerical parametric model of lattice trabecular architecture has been developed for determining the apparent elastic modulus of cancellous bone E_cb in vertebrae. The model inputs were apparent morphological parameters (trabecular thickness Tb_Th and trabecular separation Tb_Sp) and the bone mineral density (BMD), which can all be measuredin vivo, using the spatial resolution of current clinical quantitative computed tomography (QCT) commercial whole-body scanners. The model predicted that E_cb values between 30 and 110 MPa represent normal morphology and BMD of human spinal cancellous bone. The present E_cb to Tb_Th, Tb_Sp and BMD relationships pave the way for automatic generation of patientspecific continuum FE spine models that consider the individual's osteoporotic or other degenerative condition of cancellous bone.     

Estimation of the Shear Stress on the Surface of an Aortic Valve Leaflet

The limited durability of xenograft heart valves and the limited supply of allografts have sparked interest in tissue engineered replacement valves. A bioreactor for tissue engineered valves must operate at conditions that optimize the biosynthetic abilities of seeded cells while promoting their adherence to the leaflet matrix. An important parameter is shear stress, which is known to influence cellular behavior and may thus be crucial in bioreactor optimization. Therefore, an accurate estimate of the shear stress on the leaflet surface would not only improve our understanding of the mechanical environment of aortic valve leaflets, but it would also aid in bioreactor design. To estimate the shear stress on the leaflet surface, two-component laser-Doppler velocimetry measurements have been conducted inside a transparent polyurethane valve with a trileaflet structure similar to the native aortic valve. Steady flow rates of 7.5, 15.0, and 22.5 L/min were examined to cover the complete range possible during the cardiac cycle. The laminar shear stresses were calculated by linear regression of four axial velocity measurements near the surface of the leaflet. The maximum shear stress recorded was 79 dyne/cm², in agreement with boundary layer theory and previous experimental and computational studies. This study has provided a range of shear stresses to be explored in bioreactor design and has defined a maximum shear stress at which cells must remain adherent upon a tissue engineered construct. © 1999 Biomedical Engineering Society. PAC99: 8719Rr, 8768+z, 8719Hh, 4262Be, 4727Nz, 0630Gv     

Ultrasonic wave propagation in trabecular bone predicted by the stratified model

The objective of this study was to investigate ultrasound propagation in trabecular bone by considering the wave reflection and transmission in a multilayered medium. The use of ultrasound to identify those at risk of osteoporosis is a promising diagnostic method providing a measure of bone mineral density (BMD). A stratified model was proposed to study the effect of transmission and reflection of ultrasound wave within the trabecular architecture on the relationship between ultrasound and BMD. The results demonstrated that ultrasound velocity in trabecular bone was highly correlated with the bone apparent density (r=0.97). Moreover, a consistent pattern of the frequency dependence of ultrasound attenuation coefficient has been observed between simulation using this model and experimental measurement of trabecular bone. The normalized broadband ultrasound attenuation (nBUA) derived from the simulation results revealed that nBUA was nonlinear with respect to trabecular porosity and BMD. The curve of the relationship between nBUA and BMD was parabolic in shape, and the peak magnitude of nBUA was observed at 60% of bone porosity. These results agreed with the published experimental data and demonstrated that according to the stratified model, reflection and transmission were important factors in the ultrasonic propagation through the trabecular bone. © 2001 Biomedical Engineering Society. PAC01: 4380Vj, 4380Qf, 8763Df, 8710+e, 4325Ed, 8719-j     

Short Term In Vivo Precision of Proximal Femoral Finite Element Modeling

As more therapies are introduced to treat osteoporosis, precise in vivo methods are needed to monitor response to therapy and to estimate the gains in bone strength that result from treatment. A method for evaluating the strength of the proximal femur was developed and its short term reproducibility, or precision, was determined in vivo. Ten volunteer subjects aged 51–62 years (mean 55.6 years), eight women and two men, were examined using a quantitative computed tomography (QCT) protocol. They were positioned, scanned, re-positioned and re-scanned. The QCT images were registered in three-dimensional space, and finite element (FE) models were generated and processed to simulate a stance phase load configuration. Stiffness was computed from each FE model, and strength was computed using a regression equation between FE stiffness and fracture load for a small set n=6 of experimental specimens. The coefficients of variation (COV) and repeatability (COR=2.23* 2*COV) were determined. The COV for the FE fracture load computed was 1.85%, and the detectable limit (coefficient of repeatability) for serial measurements was 5.85%. That is, if a change of 5.85% or more in computed FE fracture load is observed, it will be too large to be consistent with measurement variation, but instead can be interpreted as a real change in the strength of the bone. The detectable limit of this method makes it suitable for serial research studies on changes in femoral bone strength in vivo. © 2000 Biomedical Engineering Society. PAC00: 8719Rr, 8759Fm, 8710+e     

Regional Variations in the Apparent and Tissue-Level Mechanical Parameters of Vertebral Trabecular Bone with Aging Using Micro-Finite Element Analysis

The aim of this study was to obtain the apparent and tissue-level mechanical parameters of vertebral cancellous bones using micro-finite element analysis, and to identify the regional variations and their relative differences with respect to aging. Ninety trabecular specimens were obtained from six normal L4 vertebral bodies of six male cadavers in two age groups, three aged 62 years and three aged 69 years, and then were scanned using a high-resolution micro-Computed Tomography (micro-CT) system. The obtained micro-CT reconstruction models were then converted to micro-finite element models. Micro-finite element analyses were done to determine the apparent Young’s moduli and tissue-level Von Mises stress distribution for each trabecular specimen on the longitudinal direction, and medial–lateral and anterior–posterior directions (transverse directions), respectively. Regional variations about the mechanical parameters at both apparent and tissue levels in different transverse layers and vertical columns within and between the two age groups were then analyzed. The results showed significant decreases in the apparent Young’s moduli in each direction with aging, and those in the two transverse directions decreased more with aging compared with the longitudinal direction. Furthermore, there were no statistically significant differences between the mechanical parameters in the two transverse directions in both age groups. This study offered an insight into the distributions and variations of mechanical properties within a vertebral body. The mechanical parameters calculated from this study may help in a better understanding of regional fracture risks and the vertebral fracture mechanism in the prevention of osteoporotic fracture in elder individuals.     

Effects of Creep and Cyclic Loading on the Mechanical Properties and Failure of Human Achilles Tendons

The Achilles tendon is one of the most frequently injured tendons in humans, and yet the mechanisms underlying its injury are not well understood. This study examines the ex vivo mechanical behavior of excised human Achilles tendons to elucidate the relationships between mechanical loading and Achilles tendon injury. Eighteen tendons underwent creep testing at constant stresses from 35 to 75 MPa. Another 25 tendons underwent sinusoidal cyclic loading at 1 Hz between a minimum stress of 10 MPa and maximum stresses of 30–80 MPa. For the creep specimens, there was no significant relationship between applied stress and time to failure, but time to failure decreased exponentially with increasing initial strain (strain when target stress is first reached) and decreasing failure strain. For the cyclically loaded specimens, secant modulus decreased and cyclic energy dissipation increased over time. Time and cycles to failure decreased exponentially with increasing applied stress, increasing initial strain (peak strain from first loading cycle), and decreasing failure strain. For both creep and cyclic loading, initial strain was the best predictor of time or cycles to failure, supporting the hypothesis that strain is the primary mechanical parameter governing tendon damage accumulation and injury. The cyclically loaded specimens failed faster than would be expected if only time-dependent damage occurred, suggesting that repetitive loading also contributes to Achilles tendon injuries. © 2003 Biomedical Engineering Society. PAC2003: 8719Rr     

An investigation of the inelastic behaviour of trabecular bone during the press-fit implantation of a tibial component in total knee arthroplasty

The stress distribution and plastic deformation of peri-prosthetic trabecular bone during press-fit tibial component implantation in total knee arthroplasty is investigated using experimental and finite element techniques. It is revealed that the computed stress distribution, implantation force and plastic deformation in the trabecular bone is highly dependent on the plasticity formulation implemented. By incorporating pressure dependent yielding using a crushable foam plasticity formulation to simulate the trabecular bone during implantation, highly localised stress concentrations and plastic deformation are computed at the bone–implant interface. If the pressure dependent yield is neglected using a traditional von Mises plasticity formulation, a significantly different stress distribution and implantation force is computed in the peri-prosthetic trabecular bone. The results of the study highlight the importance of: (i) simulating the insertion process of press-fit stem implantation; (ii) implementing a pressure dependent plasticity formulation, such as the crushable foam plasticity formulation, for the trabecular bone; (iii) incorporating friction at the implant–bone interface during stem insertion. Simulation of the press-fit implantation process with an appropriate pressure dependent plasticity formulation should be implemented in the design and assessment of arthroplasty prostheses.     

Predicting time-dependent remodeling of bone around immediately loaded dental implants with different designs

The purpose of this study was to predict time-dependent biomechanics of bone around cylindrical screw dental implants with different macrogeometric designs under simulated immediate loading condition. The remodeling of bone around a parallel-sided and a tapered dental implant of same length was studied under 100 N oblique load by implementing the Stanford theory into three-dimensional finite element models. The results of the analyses were examined in five time intervals consisting loading immediately after implant placement, and after 1, 2, 3 and 4 weeks following implantation. Maximum principal stress, minimum principal stress, and strain energy density in peri-implant bone and displacement in x-(implant lateral direction with a projection of the oblique force) and y-(implant longitudinal direction) axes of the implant were evaluated. The highest value of the maximum and minimum principal stresses around both implants increased in cortical bone and decreased in trabecular bone. The maximum and minimum principal stresses in cortical bone were higher around the tapered cylindrical implant, but stresses in the trabecular bone were higher around the parallel-sided cylindrical implant. Strain energy density around both implants increased in cortical bone, slightly decreased in trabecular bone, and higher values were obtained for the parallel-sided cylindrical implant. Displacement values slightly decreased in time in x-axis, and an initial decrease followed by a slight increase was observed in the y-axis. Bone responded differently in remodeling for the two implant designs under immediate loading, where the cortical bone carried the highest load. Application of oblique loading resulted in increase of stiffness in the peri-implant bone.