INTRODUCTION: Micro level finite element (μFE) models of trabecular bone have a variety of possible applications that include understanding fracture mechanism in normal and osteoporotic bone and examining the mechanical response of bone... more
INTRODUCTION: Micro level finite element (μFE) models of trabecular bone have a variety of possible applications that include understanding fracture mechanism in normal and osteoporotic bone and examining the mechanical response of bone to implants. Previous investigators used images or CT attenuations of real bone samples to develop μFE models. This involves: generating images of a bone sample using, for example, μ-CT; converting CT attenuations to elastic moduli; and mapping elastic moduli to 3D finite elements. The computational models created using these methods can only represent the samples used in their construction and any possible variations due to factors such as anatomical site, sex, age or degree of osteoporosity cannot be included without additional sample collection and processing. To authenticate any hypothesis it is essential to consider a wide range of samples with different characteristics. It is a colossal task to generate 3D μFE models from real samples for this ...
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References: [1] Kolipaka, Proc Joint Annual Meeting ISMRM-ESMRMB 2010 [2] Holzapfel, Ann Biomed Eng 2002; 30: 753-767 Summary: Hooke’s law states that the strain in a linearly elastic material is proportional to the applied load. Magnetic... more
References: [1] Kolipaka, Proc Joint Annual Meeting ISMRM-ESMRMB 2010 [2] Holzapfel, Ann Biomed Eng 2002; 30: 753-767 Summary: Hooke’s law states that the strain in a linearly elastic material is proportional to the applied load. Magnetic Resonance Elastography (MRE) uses this principle to deduce tissue stiffness by measuring the deformation created by externally generated waves, albeit using more complicated algorithms. Computational modelling can be used to demonstrate the feasibility of arterial MRE by showing the disruption of shear waves through a healthy vessel and an idealised fibrous and lipid plaque.
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Apparent (macro level) stiffness directionality (anisotropy) of trabecular bone has received considerable attention in previous reseach, which has also identified ‘bone response to mechanical forces’ as the cause. Anisotropy of cortical... more
Apparent (macro level) stiffness directionality (anisotropy) of trabecular bone has received considerable attention in previous reseach, which has also identified ‘bone response to mechanical forces’ as the cause. Anisotropy of cortical bone stiffness has received relatively little attention. The effect of increased porosity (due to age or disease) on the directionality of cortical bone stiffness has not been considered before. To address these issues the present study evaluated the variation of orthotropic elastic constants of cortical bone from the female anterior femoral midshaft. Micro-finite-element (μFE) analysis of bone samples from 27 female donors (age 53.4 ±23.6) were conducted to quantify the elastic constants at the periosteal and endosteal aspects. The ratio of canal volume to tissue volume (Ca.V/TV), analogous to porosity, was found to be the most significant predictor (r = 0.958) of the elastic constants. As would be expected stiffness decreases with increasing Ca.V/T...
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Assessment of the rupture risk of plaque is commonly made via imaging of the lumen reduction. However it is known that this is an imperfect criterion and that other features, such as plaque stiffness, may be more relevant. Magnetic... more
Assessment of the rupture risk of plaque is commonly made via imaging of the lumen reduction. However it is known that this is an imperfect criterion and that other features, such as plaque stiffness, may be more relevant. Magnetic Resonance Elastography (MRE) estimates the stiffness of tissues using inversion algorithms applied to the displacement of the tissue, measured in response to the induction of a shear wave from an external actuator. An idealised diseased vessel, embedded in an arbitrary tissue, was created to simulate the application of MRE to atherosclerotic plaque. The behaviour of shear waves propagating through an atherosclerotic plaque with development of the stenosis and lipid pool size was investigated. There is a local increase in wave displacement and decrease in wavelength, through a plaque containing a lipid pool compared to a purely fibrotic plaque. The amplitude of the wave through the lipid increases with lipid pool volume. This suggests that shear waves used...
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The clinical assessment of the rupture risk of atherosclerotic plaque is made by imaging the reduction of the lumen; via ultrasound or angiography. It is known that this is an imperfect criterion and that other characteristics of the... more
The clinical assessment of the rupture risk of atherosclerotic plaque is made by imaging the reduction of the lumen; via ultrasound or angiography. It is known that this is an imperfect criterion and that other characteristics of the plaque, such as the change in mechanical properties, may be more relevant. Magnetic Resonance Elastography (MRE) is a novel imaging technique that measures tissue stiffness. Magnetic resonance imaging measures the tissue displacement in response to harmonic shear waves excited on the surface of the body by a vibrating actuator. The images of wave propagation are transformed into an image of stiffness using an inversion algorithm. A steady state analysis was conducted on a simulation of MRE shear waves propagating through an idealised atherosclerotic plaque. The variation of the 2D complex wave response was investigated with regards to stenosis size, lipid pool size and excitation frequency. The detectability of small lipid pools was shown to increase wi...
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INTRODUCTION Computational modelling in orthopaedic biomechanics often uses continuum homogenised finite element models of the bone which require accurate definitions of its elastic properties. In this study the variation of the... more
INTRODUCTION Computational modelling in orthopaedic biomechanics often uses continuum homogenised finite element models of the bone which require accurate definitions of its elastic properties. In this study the variation of the orthotropic elastic constants of cortical bone from the female anterior femoral midshaft were evaulated. Micro-finite-element (μFE) analysis of bone samples from 27 female donors aged between 20 – 87years were conducted to quantify the elastic constants at the periosteal and endosteal aspects. The variation of properties with age and canal volume to total volume (Ca.V/TV) were evaluated.
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Ab stract— The clinical assessment of the rupture risk of atherosclerotic plaque is made by imaging the reduction of the lumen; via ultrasound or angiography. It is known that this is an imperfect criterion and that other characteristics... more
Ab stract— The clinical assessment of the rupture risk of atherosclerotic plaque is made by imaging the reduction of the lumen; via ultrasound or angiography. It is known that this is an imperfect criterion and that other characteristics of the plaque, such as the change in mechanical properties, may be more relevant. Magnetic Resonance Elastography (MRE) is a novel imaging technique that measures tissue stiffness. Magnetic resonance imaging measures the tissue displacement in response to harmonic shear waves excited on the surface of the body by a vibrating actuator. The images of wave propagation are transformed into an image of stiffness using an inversion algorithm. A steady state analysis was conducted on a simulation of MRE shear waves propagating through an idealised atherosclerotic plaque. The variation of the 2D complex wave response was investigated with regards to stenosis size, lipid pool size and excitation frequency. The detectability of small lipid pools was shown to ...
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The aim of the study is to investigate the biomechanical effects on the pelvis of the anterolateral and posterolateral approaches at the time of hip arthroplasty. In particular the study investigates the change in stress distribution, and... more
The aim of the study is to investigate the biomechanical effects on the pelvis of the anterolateral and posterolateral approaches at the time of hip arthroplasty. In particular the study investigates the change in stress distribution, and the change in muscle recruitment pattern following surgery. The study uses an advanced finite element model of the pelvis, in which the role of muscles and ligaments in determining the stress distribution in the pelvis is included. The model is altered for the posterolateral approach by excision of the external rotators. Different levels of gluteal damage for the anterolateral approach are modelled by excising in turn the anterior third, half, and two-thirds of the gluteus medius and minimus. Although attempt is generally made to repair gluteal damage at the time of surgery, it is clear the muscle volume will be compromised immediately after surgery. In support of previous clinical studies indicating an increased risk of limp, and pelvic tilt follo...
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At the time of medial opening wedge high tibial osteotomy (HTO) to realign the lower limb and offload medial compartment knee osteoarthritis, unwanted fractures can propagate from the osteotomy apex. The aim of this study was to use... more
At the time of medial opening wedge high tibial osteotomy (HTO) to realign the lower limb and offload medial compartment knee osteoarthritis, unwanted fractures can propagate from the osteotomy apex. The aim of this study was to use finite element (FE) analysis to determine the effect of hinge location and apical drill holes on cortical stresses and strains in HTO. A monoplanar medial opening wedge HTO was created above the tibial tuberosity in a composite tibia. Using the FE method, intact lateral hinges of different widths were considered (5, 7.5, and 10 mm). Additional apical drill holes (2, 4, and 6 mm diameters) were then incorporated into the 10 mm hinge model. The primary outcome measure was the maximum principal strain in the cortical bone surrounding the hinge axis. Secondary outcomes included the force required for osteotomy opening, minimum principal strain, and mean cortical bone stresses (maximum principal/minimum principal/von Mises). Larger intact hinges (10 mm) were associated with higher cortical bone maximum principal strain and stress, lower minimum principal strain/stress, and required greater force to open. Lateral cortex strain concentrations were present in all scenarios, but extended to the joint surface with the 10 mm hinge. Apical drill holes reduced the mean cortical bone maximum principal strain adjacent to the hinge axis: 2 mm hole 6% reduction; 4 mm 35% reduction; and 6 mm 55% reduction. Incorporating a 4‐mm apical drill hole centered 10 mm from the intact lateral cortex maintains a cortical bone hinge, minimizes cortical bone strains and reduces the force required to open the HTO; thus improving control.
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Aims Loosening is a well-known complication in the fixation of fractures using devices such as locking plates or unilateral fixators. It is believed that high strains in the bone at the bone-screw interface can initiate loosening, which... more
Aims Loosening is a well-known complication in the fixation of fractures using devices such as locking plates or unilateral fixators. It is believed that high strains in the bone at the bone-screw interface can initiate loosening, which can result in infection, and further loosening. Here, we present a new theory of loosening of implants. The time-dependent response of bone subjected to loads results in interfacial deformations in the bone which accumulate with cyclical loading and thus accentuates loosening. Methods We used an ‘ideal’ bone-screw system, in which the screw is subjected to cyclical lateral loads and trabecular bone is modelled as non-linear viscoelastic and non-linear viscoelastic-viscoplastic material, based on recent experiments, which we conducted. Results We found that the interfacial deformation in the bone increases with the number of cycles, and the use of a non-linear viscoelastic-viscoplastic model results in larger deformations, some of which are irrecovera...
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The deformation of bone when subjected to loads is not instantaneous but varies with time. To investigate this time-dependent behaviour sixteen bovine trabecular bone specimens were subjected to compressive loading, creep, unloading and... more
The deformation of bone when subjected to loads is not instantaneous but varies with time. To investigate this time-dependent behaviour sixteen bovine trabecular bone specimens were subjected to compressive loading, creep, unloading and recovery at multiple load levels corresponding to apparent strains of 2000-25,000 με. We found that: the time-dependent response of trabecular bone comprises of both recoverable and irrecoverable strains; the strain response is nonlinearly related to applied load levels; and the response is linked to bone volume fraction. Although majority of strain is recovered after the load-creep-unload-recovery cycle some residual strain always exists. The analysis of results indicates that trabecular bone becomes stiffer initially and then experiences stiffness degradation with the increasing load levels. Steady state creep rate was found to be dependent on applied stress level and bone volume fraction with a power law relationship.
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Trabecular bone is a cellular composite material comprising primarily of mineral and organic phases with their content ratio known to change with age. Therefore, the contribution of bone constituents on bone's mechanical behaviour, in... more
Trabecular bone is a cellular composite material comprising primarily of mineral and organic phases with their content ratio known to change with age. Therefore, the contribution of bone constituents on bone's mechanical behaviour, in tension and compression, at varying load levels and with changing porosity (which increases with age) is of great interest, but remains unknown. We investigated the mechanical response of demineralised bone by subjecting a set of bone samples to fully reversed cyclic tension-compression loads with varying magnitudes. We show that the tension to compression response of the organic phase of trabecular bone is asymmetric; it stiffens in tension and undergoes stiffness reduction in compression. Our results indicate that demineralised trabecular bone struts experience inelastic buckling under compression which causes irreversible damage, while irreversible strains due to microcracking are less visible in tension. We also identified that the values of th...
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Osteoporosis resulting in a reduction in bone stiffness and thinning of the cortex is almost universal in older patients. In this study a novel method to generate computational models of the distal femur which incorporate the effects of... more
Osteoporosis resulting in a reduction in bone stiffness and thinning of the cortex is almost universal in older patients. In this study a novel method to generate computational models of the distal femur which incorporate the effects of ageing and endosteal trabecularisation are presented. Application of this method to pre- and post-knee arthroplasty scenarios is then considered. These computational methods are found to provide a simple yet effective tool for assessing the post-arthroplasty mechanical environment in the knee for different patient types and can help evaluate vulnerability to supracondylar periprosthetic fracture following implantation. Our results show that the stresses in the periprosthetic region increase dramatically with ageing; this is particularly true for higher flexion angles. Stresses in the anterior region of the femoral cortex were also found to increase significantly post-implantation. The most dramatic increases in stresses and strains at these locations...
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Due to age-related changes to the material properties and thinning of the cortical bone structure, older patients with osteoporosis may be at greater risk of femoral fracture following total knee arthroplasty. This study investigates... more
Due to age-related changes to the material properties and thinning of the cortical bone structure, older patients with osteoporosis may be at greater risk of femoral fracture following total knee arthroplasty. This study investigates whether there is a potential role for stemmed prostheses in such scenarios to help mitigate peri-implant fracture risk, and if so what should the optimum stem length be to balance surgical bone loss with reduced fracture risk. Finite element models of the distal femur implanted with four different implant types: a posterior stabilising implant, a total stabilising implant with short stem (12 mm × 50 mm), a TS implant with medium stem (12 mm × 75 mm), and a TS implant with long stem (12 mm × 100 mm), were developed and analysed in this study. Osteoporotic properties were applied to the implanted femurs and the periprosthetic stresses and strains of each were recorded. All stem lengths examined were found to lead to a reduction in periprosthetic stress in...
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Motion at the bone-implant interface, following primary or revision knee arthroplasty, can be detrimental to the long-term survival of the implant. This study employs experimentally verified computational models of the distal femur to... more
Motion at the bone-implant interface, following primary or revision knee arthroplasty, can be detrimental to the long-term survival of the implant. This study employs experimentally verified computational models of the distal femur to characterize the relative motion at the bone-implant interface for three different implant types; a posterior stabilizing implant (PS), a total stabilizing implant (TS) with short stem (12 mm × 50 mm), and a total stabilizing implant (TS) with long offset stem (19 mm × 150 mm with a 4 mm lateral offset). Relative motion was investigated for both cemented and uncemented interface conditions. Monitoring relative motion about a single reference point, though useful for discerning global differences between implant types, was found to not be representative of the true pattern and distribution of motions which occur at the interface. The contribution of elastic deformation to apparent reference point motion varied based on implant type, with the PS and TSSS...
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Aseptic loosening, osteolysis, and infection are the most commonly reported reasons for revision total knee arthroplasty (TKA). This study examined the role of implant design features (e.g. condylar box, pegs) and stems in resisting... more
Aseptic loosening, osteolysis, and infection are the most commonly reported reasons for revision total knee arthroplasty (TKA). This study examined the role of implant design features (e.g. condylar box, pegs) and stems in resisting loosening, and also explored the sensitivity of the implants to a loose surgical fit due to saw blade oscillation. Finite element models of the distal femur implanted with four different implant types: cruciate retaining (CR), posterior stabilising (PS), total stabilising (TS) with short stem (12mm×50mm), and a total stabilising (TS) with long stem (19mm×150mm) were developed and analysed in this study. Two different fit conditions were considered: a normal fit, where the resections on the bone exactly match the internal profile of the implant, and a loose fit due to saw blade oscillation, characterised by removal of one millimetre of bone from the anterior and posterior surfaces of the distal femur. Frictional interfaces were employed at the bone-implan...
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The time-independent elastic properties of trabecular bone have been extensively investigated, and several stiffness-density relations have been proposed. Although it is recognized that trabecular bone exhibits time-dependent mechanical... more
The time-independent elastic properties of trabecular bone have been extensively investigated, and several stiffness-density relations have been proposed. Although it is recognized that trabecular bone exhibits time-dependent mechanical behaviour, a property of viscoelastic materials, the characterization of this behaviour has received limited attention. The objective of the present study was to investigate the time-dependent behaviour of bovine trabecular bone through a series of compressive creep-recovery experiments and to identify its nonlinear constitutive viscoelastic material parameters. Uniaxial compressive creep and recovery experiments at multiple loads were performed on cylindrical bovine trabecular bone samples ([Formula: see text]). Creep response was found to be significant and always comprised of recoverable and irrecoverable strains, even at low stress/strain levels. This response was also found to vary nonlinearly with applied stress. A systematic methodology was de...
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Fine-featured elastograms may provide additional information of radiological interest in the context of in vivo elastography. Here a new image processing pipeline called ESP (Elastography Software Pipeline) is developed to create Magnetic... more
Fine-featured elastograms may provide additional information of radiological interest in the context of in vivo elastography. Here a new image processing pipeline called ESP (Elastography Software Pipeline) is developed to create Magnetic Resonance Elastography (MRE) maps of viscoelastic parameters (complex modulus magnitude |G(*)| and loss angle ϕ) that preserve fine-scale information through nonlinear, multi-scale extensions of typical MRE post-processing techniques. A new MRE image processing pipeline was developed that incorporates wavelet-domain denoising, image-driven noise estimation, and feature detection. ESP was first validated using simulated data, including viscoelastic Finite Element Method (FEM) simulations, at multiple noise levels. ESP images were compared with MDEV pipeline images, both in the FEM models and in three ten-subject cohorts of brain, thigh, and liver acquisitions. ESP and MDEV mean values were compared to 2D local frequency estimation (LFE) mean values ...
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Load induced thermal strain (LITS) is an integral part of the behaviour of concrete in fire. The existence of LITS has been well documented and modelled by different researchers. It is vital that this strain development is correctly... more
Load induced thermal strain (LITS) is an integral part of the behaviour of concrete in fire. The existence of LITS has been well documented and modelled by different researchers. It is vital that this strain development is correctly represented in structural models, as the locked in ...