Journal of Materials Science: Materials in Medicine, 1999
The fatigue of bone, in particular the associated modulus degradation and accumulation of permane... more The fatigue of bone, in particular the associated modulus degradation and accumulation of permanent strain, has been implicated as the cause of femoral neck fractures and the migration of total joint replacements. The objective of this study was to develop a technique to simulate the tensile fatigue behavior of human cortical bone. A combined continuum damage mechanics (CDM) and ®nite element analysis (FEA) approach was used to predict the number of cycles to failure, modulus degradation and accumulation of permanent strain of human cortical bone specimens. The simulation of fatigue testing of eight dumb-bell specimens of cortical bone were performed and the predictions compared with existing experimental data. The predictions from the ®nite element models were in close agreement with the experimental data. The models predicted similar development of modulus degradation and permanent strain as observed in the experimental tests. The technique is capable of predicting the accumulation of permanent strain without the need for simulating every single load step. These ®ndings suggest that the complex fatigue behavior of human cortical bone can be simulated using the described approach and forms the ®rst step for simulating the more complex mechanisms associated with femoral neck fractures and implant migration.
Cotton, JR, Zioupos, P., Winwood, K., Anderson, P., Davies, GR and Taylor, M. (2001) Static FEA o... more Cotton, JR, Zioupos, P., Winwood, K., Anderson, P., Davies, GR and Taylor, M. (2001) Static FEA of cortical bone tensile test with variations in modulus identified from micro-CT scans. In, Proceedings of the 16th Conference of the European Society of Biomaterials. 16th Conference of the European Society of Biomaterials , European Society of Biomaterials, 125. ... Full text not available from this repository.
Zioupos, P., Winwood, K., Cotton, J. and Taylor, M. (2002) The development of elastic and 'p... more Zioupos, P., Winwood, K., Cotton, J. and Taylor, M. (2002) The development of elastic and 'plastic' strains during fatigue damage accumulation of human cortical bone. At 13th Conference of the European Society of Biomechanics, Wroclaw, Poland, 01 - 04 Sep 2002. 2pp. ... Full text not available from this repository. ... RDF+N-Triples, RDF+N3, RDF+XML, Browse.
The migration, subsidence and loosening of prosthetic implants are a major concern for both the c... more The migration, subsidence and loosening of prosthetic implants are a major concern for both the clinical surgeon and for the morbidity and mortality of the patient. Such effects are frequently reported post-operatively within the first six months and progressively later on. Subsidence can be due to a number of factors including the quality of the cancellous/cortical bone, the method of fixation and design of the implant. In recent studies of ours we have utilised FEA codes and considered the build up of yield and plastic strains around the ...
Osteoporotic fractures present a significant social and economic burden, which is set to rise com... more Osteoporotic fractures present a significant social and economic burden, which is set to rise commensurately with the aging population. Greater understanding of the physicochemical differences between osteoporotic and normal conditions will facilitate the development of diagnostic technologies with increased performance and treatments with increased efficacy. Using coherent X-ray scattering we have evaluated a population of 108 ex vivo human bone samples comprised of non-fracture and fracture groups. Principal component fed linear discriminant analysis was used to develop a classification model to discern each condition resulting in a sensitivity and specificity of 93% and 91%, respectively. Evaluating the coherent X-ray scatter differences from each condition supports the hypothesis that a causal physicochemical change has occurred in the fracture group. This work is a critical step along the path towards developing an in vivo diagnostic tool for fracture risk prediction.
Duchenne muscular dystrophy (DMD) is an inherited, lethal disorder characterised by progressive m... more Duchenne muscular dystrophy (DMD) is an inherited, lethal disorder characterised by progressive muscle degeneration and associated bone abnormalities. We have previously demonstrated that P2RX7 purinergic receptors contribute to the pathogenesis of DMD, and found that P2RX7 ablation alleviated the severity of the disease. In this work we have used a dystrophic mdx mouse crossed with the global P2RX7 receptor to generate a knockout mouse (mdx/P2X7-/-), and compared its morphometric, mechanical and tissue properties against those of mdx, as well as the wild type (WT) and the P2RX7 knockout (P2X7-/-). Micro-computed tomography (µCT), three-point bending testing, scanning electron microscopy (SEM) and nanoindentation were utilised in the study. The bones were analysed at approximately 4 weeks of age to examine the impact of P2RX7 ablation on the bone properties during the acute disease phase, before muscle wasting is fully developed. The results show that P2RX7 purinoceptor ablation has produced improvement or significant improvement in some of the morphological, the mechanical and the tissue properties of the dystrophic bones examined. Specifically, although the ablation produced smaller bones with significantly lower total cross-section area (Tt.Ar) and Second Moment of Area (SMA), significantly higher cortical bone area (Ct.Ar), cortical bone area ratio (Ct.Ar/Tt.Ar) and trabecular bone volume fraction (BV/TV) are found in the mdx/P2X7-/-mice than in any other types. Further, the mdx/P2X7-/bones have relatively higher average flexural strength, work-to-fracture and significantly higher strain to failure compared with those of mdx, suggesting greater resistance to fracture. Indentation modulus, elasticity and creep are also significantly improved in the knockout cortical bones over those of mdx. These findings seem to suggest that specific pharmacological blockade of P2RX7 may improve dystrophic bones, with a potential for therapeutic application in the treatment of the disease.
Osteoporosis is clinically assessed from bone mineral density measurements using dual energy X-ra... more Osteoporosis is clinically assessed from bone mineral density measurements using dual energy X-ray absorption (DXA). However, these measurements do not always provide an accurate fracture prediction, arguably because DXA does not grapple with 'bone quality', which is a combined result of microarchitecture, texture, bone tissue properties, past loading history, material chemistry and bone physiology in reaction to disease. Studies addressing bone quality are comparatively few if one considers the potential importance of this factor. They suffer due to low number of human osteoporotic specimens, use of animal proxies and/or the lack of differentiation between confounding parameters such as gender and state of diseased bone. The present study considers bone samples donated from patients (n = 37) who suffered a femoral neck fracture and in this very well defined cohort we have produced in previous work fracture toughness measurements (FT) which quantify its ability to resist crack growth which reflects directly the structural integrity of the cancellous bone tissue. We investigated correlations between BV/TV and other microarchitectural parameters; we examined effects that may suggest differences in bone remodelling between males and females and compared the relationships with the FT properties. The data crucially has shown that TbTh, TbSp, SMI and TbN may provide a proxy or surrogate for BV/TV. Correlations between FT critical stress intensity values and microarchitecture parameters (BV/TV, BS/TV, TbN, BS/BV and SMI) for osteoporotic cancellous tissue were observed and are for the first time reported in this study. Overall, this study has not only highlighted that the fracture model based upon BMD could potentially be improved with inclusion of other microarchitecture parameters, but has also given us clear clues as to which of them are more influential in this role.
Uniaxial tensile tests were performed on glutaraldehyde fixed bovine pericardial strips prepared ... more Uniaxial tensile tests were performed on glutaraldehyde fixed bovine pericardial strips prepared from chemically modified pericardial samples. These samples originated from an area which demonstrated anisotropic mechanical properties in the native material and which is suitable for the construction of leaflets for pericardial bioprostheses. After glutaraldehyde fixation the tissue had retained its anisotropicity in stiffness and strength in two orthogonal directions. In the range of the functional stresses for a heart valve leaflet (< 1 MPa) the unconstrained fixation regime had modified the initial anisotropic elastic behavior into a more isotropic one. The implications of these findings are that leaflets manufactured from bovine pericardium can be made to resemble, to a degree, the well known anisotropy found in two orthogonal directions in natural human heart valve leaflets, or porcine bioprosthetic heart valve leaflets.
Cancellous bone from patients with osteoarthritis (OA) has a reduced material density and appears... more Cancellous bone from patients with osteoarthritis (OA) has a reduced material density and appears to be undermineralized. It is hypothesized that this will result in a reduction in the mechanical stiffness and strength of the bone matrix. In this study, bone was obtained from superior and inferior sites, subjected to relatively high and low loads, respectively, from human femoral heads retrieved after surgery for osteoporotic hip fracture (OP), or for hip arthroplasty due to OA. Microindentation testing was used to measure the hardness of cancellous bone at various depths from the subchondral bone plate. The elemental composition from immediately adjacent microscopic sites was determined using electron probe microanalysis (EPMA). Overall, OA bone was found to have hardness values that were 7% lower than those from OP bone. Bone from the inferior site was harder than that from the superior in both diseases except in female OP patients. There was no variation with depth below the subchondral plate and no difference between sexes. No difference was found in the composition of the bone from the different disease groups and no correlation was found between hardness and any of the composition measurements. Though only an indirect measurement of stiffness, the reduction in hardness values supports the hypothesis that OA bone has a reduced elastic modulus.
Acta Obstetricia et Gynecologica Scandinavica, 2007
... LETTER TO THE EDITOR: REPLY. Pedro Valenzuela,; R. Guijarro. Article first published online: ... more ... LETTER TO THE EDITOR: REPLY. Pedro Valenzuela,; R. Guijarro. Article first published online: 31 DEC 2010. DOI: 10.1080/00016340701212005. Acta Obstet Gynecol Scand 2007. Issue. Acta Obstetricia et Gynecologica Scandinavica. Volume 86, Issue 5, page 639, May 2007 ...
Journal of Biomedical Materials Research Part A, 2006
It is a common theme in basic bone biomechanics and in biomechanical applications that much of th... more It is a common theme in basic bone biomechanics and in biomechanical applications that much of the behaviour can be determined and is dictated by the level of strain, whether this pertains to bone physiology, bone remodelling, osseinduction, osseointegration or the development of damage. The development of damage, demonstrated by stiffness loss measurements, has already been reported in detail in the literature. However, the systematic study of the development of 'plastic' (irrecoverable) strains, which are associated with the inelastic mechanical behaviour of bone tissue has generally been overlooked. The present study compares the rates at which the elastic (εa) and plastic components (εp) of strain developed during tensile, compressive, and shear fatigue in human cortical bone of six individuals aged between 53 and 79 yrs of age. The overall hypothesis of this investigation is that there is a common underlying factor in the damage-related behaviour of bone, which may allow us to link together the various aspects of the damage related behaviour of bone. The rate of development of plastic strain (έp) and the rate of damage (έa) are described as a function of the cycle number, the stress (), and/or stress normalised by the modulus of elasticity (). The implications of our findings are discussed with respect to simple models/mechanisms, which may underlie the observed behaviour.
Journal of Musculoskeletal and Neuronal Interactions - JMNI
The longevity, success, or failure of an orthopaedic implant is dependent on its osseointegration... more The longevity, success, or failure of an orthopaedic implant is dependent on its osseointegration especially within the initial six months of the initial surgery. The development of strains plays a crucial role in both bone modelling and remodelling. For remodelling, in particular, strains of substantial values are required to activate the osteoblastic and osteoclastic activity for the osseointegration of the implant. Bone, however, is subject to "damage" when strain levels exceed a certain threshold level. Damage is manifested in the form of microcracks; it is linked to increased elastic strain amplitudes and is accompanied by the development of "plastic" (irrecoverable, residual) strains. Such strains increase the likelihood for the implant to subside or loosen. The present study examines the rates (per cycle) by which these two components of strain (elastic and "plastic") develop during fatigue cycling in two loading modes, tension and compression. T...
The equine radius is a useful subject for examining the adaptation of bone histology to loading b... more The equine radius is a useful subject for examining the adaptation of bone histology to loading because in life the anterior cortex is loaded almost entirely in tension, the posterior cortex in compression. The histology of the two cortices is correspondingly different, the osteones and the interstitial lamellae in the posterior cortex having a more transversely oriented fibre arrangement than those in the anterior cortex. Presumably as a result of this histological difference, the posterior cortex is stronger in compression than the anterior cortex; the anterior cortex is stronger in tension than the posterior cortex. We here use nanoindentation to examine how the Young's modulus of elasticity of secondary osteones and interstitial lamellae in the anterior and posterior cortices varied as a function of angle. The anterior osteones were stiffer than the posterior osteones when tested in the direction parallel to the bone's long axis, but became progressively relatively less ...
The Influence of New Technology on Medical Practice, 1988
The replacement of diseased heart valves with prosthetic devices became possible with the develop... more The replacement of diseased heart valves with prosthetic devices became possible with the development of methods for cardiopulmonary bypass in the late 1950s. The first ball and cage mechanical valve was implanted in 1960 (Starr and Edwards, 1961) and the first tissue valve in 1962 (Ross, 1962). During 1984, the last year for which there is complete data, 4935 open heart procedures were carried out for valve surgery and a similar number of mechanical and tissue valves were implanted.
The recently developed Reference Point Indentation (RPI) allows the measurements of bone properti... more The recently developed Reference Point Indentation (RPI) allows the measurements of bone properties at the tissue level in vivo. The goal of this study was to compare the local anisotropic behaviour of bovine plexiform bone measured with depth sensing micro-indentation tests and with RPI. Fifteen plexiform bone specimens were extracted from a bovine femur and polished down to 0.05 mm alumina paste for indentations along the axial, radial and circumferential directions (N¼5 per group). Twenty-four micro-indentations (2.5 mm in depth, 10% of them were excluded for testing problems) and four RPI-indentations ($ 50 mm in depth) were performed on each sample. The local indentation modulus E ind was found to be highest for the axial direction (24.372.5 GPa) compared to the one for the circumferential indentations (19% less stiff) and for the radial direction (30% less stiff). RPI measurements were also found to be dependent on indentation direction (po0.001) with the exception of the Indentation Distance Increase (IDI) (p¼0.173). In particular, the unloading slope US1 followed similar trends compared to the E ind : 0.4770.03 N/mm for axial, 11% lower for circumferential and 17% lower for radial. Significant correlations were found between US1 and E ind (p¼ 0.001; R 2 ¼0.58), while no significant relationship was found between IDI and any of the micro-indentation measurements (p40.157). In conclusion some of the RPI measurements can provide information about local anisotropy but IDI cannot. Moreover, there is a linear relationship between most local mechanical properties measured with RPI and with micro-indentations, but IDI does not correlate with any micro-indentation measurements.
Proceedings of the Royal Society B: Biological Sciences, 2004
The toughness of bone is an important feature in preventing it from fracturing. We consider the n... more The toughness of bone is an important feature in preventing it from fracturing. We consider the notch sensitivity in impact, and the associations between brittleness, notch sensitivity and post-yield energy absorption of mammalian mineralized tissues. Specimens of bone-like tissues covering a wide range of mineralization were broken, either notched or un-notched, in impact. The greater the mineral content, the greater was the notch sensitivity. Also, the more brittle tissues dissipated the least post-yield energy and were the most notch sensitive. It is suggested that since antler bone, the least mineralized of all known mammalian mineralized tissues, seems to be notch insensitive in impact, no adaptive purpose would be served by having mineralized tissues of a lower mineralization than antler. This may explain the lower cutoff in mineralization seen in mammals.
We examined the microcracking (damage) in the vicinity of a circular hole in bovine femoral bone ... more We examined the microcracking (damage) in the vicinity of a circular hole in bovine femoral bone specimens. The stresses near the hole were derived by a finite element analysis model using the bone’s elastic constants and yield stresses, which were determined from a series of mechanical tests specifically for the type of bone under examination. The spatial occurrence and distribution of microcracking was compared to the patterns of the predicted maximum principal stress, the von Mises stress, and the strain energy density function (all implicated by various workers as stimuli for bone remodelling) and to the predictions derived by the use of two engineering criteria for anisotropic yield under mixed mode of stress. The predictions for stresses and the strain energy density were all very similar, making it impossible to claim that any of them is superior to the others. However, empirical examination of the results of the Hencky-von Mises and Tsai-Wu anisotropic yield criteria showed that the Tsai-Wu criterion approximated reasonably the pattern of microcracking around the hole. We suggest that, in the light of the considerable damage observed in the vicinity of stress concentrators, similar damage in irregular material interfaces (i.e. near orthopaedic implants) would require the re-examination of the theories concerning bone remodelling so as to account for the possibility of occurrence of damage and the quantification of its magnitude and likely effect. The presence of considerable microdamage in bone long before it fails suggests that damage-based criteria are more likely to be successful predictors of bone remodelling behaviour than would stress or strain-based criteria.
During fatigue tests of cortical bone specimens, at the unload portion of the cycle (zero stress)... more During fatigue tests of cortical bone specimens, at the unload portion of the cycle (zero stress) non-zero strains occur and progressively accumulate as the test progresses. This non-zero strain is hypothesised to be mostly, if not entirely, describable as creep. This work examines the rate of accumulation of this strain and quantifies its stress dependency. A published relationship determined from creep tests of cortical bone (Journal of Biomechanics 21 (1988) 623) is combined with knowledge of the stress history during ...
Journal of Materials Science: Materials in Medicine, 1999
The fatigue of bone, in particular the associated modulus degradation and accumulation of permane... more The fatigue of bone, in particular the associated modulus degradation and accumulation of permanent strain, has been implicated as the cause of femoral neck fractures and the migration of total joint replacements. The objective of this study was to develop a technique to simulate the tensile fatigue behavior of human cortical bone. A combined continuum damage mechanics (CDM) and ®nite element analysis (FEA) approach was used to predict the number of cycles to failure, modulus degradation and accumulation of permanent strain of human cortical bone specimens. The simulation of fatigue testing of eight dumb-bell specimens of cortical bone were performed and the predictions compared with existing experimental data. The predictions from the ®nite element models were in close agreement with the experimental data. The models predicted similar development of modulus degradation and permanent strain as observed in the experimental tests. The technique is capable of predicting the accumulation of permanent strain without the need for simulating every single load step. These ®ndings suggest that the complex fatigue behavior of human cortical bone can be simulated using the described approach and forms the ®rst step for simulating the more complex mechanisms associated with femoral neck fractures and implant migration.
Cotton, JR, Zioupos, P., Winwood, K., Anderson, P., Davies, GR and Taylor, M. (2001) Static FEA o... more Cotton, JR, Zioupos, P., Winwood, K., Anderson, P., Davies, GR and Taylor, M. (2001) Static FEA of cortical bone tensile test with variations in modulus identified from micro-CT scans. In, Proceedings of the 16th Conference of the European Society of Biomaterials. 16th Conference of the European Society of Biomaterials , European Society of Biomaterials, 125. ... Full text not available from this repository.
Zioupos, P., Winwood, K., Cotton, J. and Taylor, M. (2002) The development of elastic and 'p... more Zioupos, P., Winwood, K., Cotton, J. and Taylor, M. (2002) The development of elastic and 'plastic' strains during fatigue damage accumulation of human cortical bone. At 13th Conference of the European Society of Biomechanics, Wroclaw, Poland, 01 - 04 Sep 2002. 2pp. ... Full text not available from this repository. ... RDF+N-Triples, RDF+N3, RDF+XML, Browse.
The migration, subsidence and loosening of prosthetic implants are a major concern for both the c... more The migration, subsidence and loosening of prosthetic implants are a major concern for both the clinical surgeon and for the morbidity and mortality of the patient. Such effects are frequently reported post-operatively within the first six months and progressively later on. Subsidence can be due to a number of factors including the quality of the cancellous/cortical bone, the method of fixation and design of the implant. In recent studies of ours we have utilised FEA codes and considered the build up of yield and plastic strains around the ...
Osteoporotic fractures present a significant social and economic burden, which is set to rise com... more Osteoporotic fractures present a significant social and economic burden, which is set to rise commensurately with the aging population. Greater understanding of the physicochemical differences between osteoporotic and normal conditions will facilitate the development of diagnostic technologies with increased performance and treatments with increased efficacy. Using coherent X-ray scattering we have evaluated a population of 108 ex vivo human bone samples comprised of non-fracture and fracture groups. Principal component fed linear discriminant analysis was used to develop a classification model to discern each condition resulting in a sensitivity and specificity of 93% and 91%, respectively. Evaluating the coherent X-ray scatter differences from each condition supports the hypothesis that a causal physicochemical change has occurred in the fracture group. This work is a critical step along the path towards developing an in vivo diagnostic tool for fracture risk prediction.
Duchenne muscular dystrophy (DMD) is an inherited, lethal disorder characterised by progressive m... more Duchenne muscular dystrophy (DMD) is an inherited, lethal disorder characterised by progressive muscle degeneration and associated bone abnormalities. We have previously demonstrated that P2RX7 purinergic receptors contribute to the pathogenesis of DMD, and found that P2RX7 ablation alleviated the severity of the disease. In this work we have used a dystrophic mdx mouse crossed with the global P2RX7 receptor to generate a knockout mouse (mdx/P2X7-/-), and compared its morphometric, mechanical and tissue properties against those of mdx, as well as the wild type (WT) and the P2RX7 knockout (P2X7-/-). Micro-computed tomography (µCT), three-point bending testing, scanning electron microscopy (SEM) and nanoindentation were utilised in the study. The bones were analysed at approximately 4 weeks of age to examine the impact of P2RX7 ablation on the bone properties during the acute disease phase, before muscle wasting is fully developed. The results show that P2RX7 purinoceptor ablation has produced improvement or significant improvement in some of the morphological, the mechanical and the tissue properties of the dystrophic bones examined. Specifically, although the ablation produced smaller bones with significantly lower total cross-section area (Tt.Ar) and Second Moment of Area (SMA), significantly higher cortical bone area (Ct.Ar), cortical bone area ratio (Ct.Ar/Tt.Ar) and trabecular bone volume fraction (BV/TV) are found in the mdx/P2X7-/-mice than in any other types. Further, the mdx/P2X7-/bones have relatively higher average flexural strength, work-to-fracture and significantly higher strain to failure compared with those of mdx, suggesting greater resistance to fracture. Indentation modulus, elasticity and creep are also significantly improved in the knockout cortical bones over those of mdx. These findings seem to suggest that specific pharmacological blockade of P2RX7 may improve dystrophic bones, with a potential for therapeutic application in the treatment of the disease.
Osteoporosis is clinically assessed from bone mineral density measurements using dual energy X-ra... more Osteoporosis is clinically assessed from bone mineral density measurements using dual energy X-ray absorption (DXA). However, these measurements do not always provide an accurate fracture prediction, arguably because DXA does not grapple with 'bone quality', which is a combined result of microarchitecture, texture, bone tissue properties, past loading history, material chemistry and bone physiology in reaction to disease. Studies addressing bone quality are comparatively few if one considers the potential importance of this factor. They suffer due to low number of human osteoporotic specimens, use of animal proxies and/or the lack of differentiation between confounding parameters such as gender and state of diseased bone. The present study considers bone samples donated from patients (n = 37) who suffered a femoral neck fracture and in this very well defined cohort we have produced in previous work fracture toughness measurements (FT) which quantify its ability to resist crack growth which reflects directly the structural integrity of the cancellous bone tissue. We investigated correlations between BV/TV and other microarchitectural parameters; we examined effects that may suggest differences in bone remodelling between males and females and compared the relationships with the FT properties. The data crucially has shown that TbTh, TbSp, SMI and TbN may provide a proxy or surrogate for BV/TV. Correlations between FT critical stress intensity values and microarchitecture parameters (BV/TV, BS/TV, TbN, BS/BV and SMI) for osteoporotic cancellous tissue were observed and are for the first time reported in this study. Overall, this study has not only highlighted that the fracture model based upon BMD could potentially be improved with inclusion of other microarchitecture parameters, but has also given us clear clues as to which of them are more influential in this role.
Uniaxial tensile tests were performed on glutaraldehyde fixed bovine pericardial strips prepared ... more Uniaxial tensile tests were performed on glutaraldehyde fixed bovine pericardial strips prepared from chemically modified pericardial samples. These samples originated from an area which demonstrated anisotropic mechanical properties in the native material and which is suitable for the construction of leaflets for pericardial bioprostheses. After glutaraldehyde fixation the tissue had retained its anisotropicity in stiffness and strength in two orthogonal directions. In the range of the functional stresses for a heart valve leaflet (< 1 MPa) the unconstrained fixation regime had modified the initial anisotropic elastic behavior into a more isotropic one. The implications of these findings are that leaflets manufactured from bovine pericardium can be made to resemble, to a degree, the well known anisotropy found in two orthogonal directions in natural human heart valve leaflets, or porcine bioprosthetic heart valve leaflets.
Cancellous bone from patients with osteoarthritis (OA) has a reduced material density and appears... more Cancellous bone from patients with osteoarthritis (OA) has a reduced material density and appears to be undermineralized. It is hypothesized that this will result in a reduction in the mechanical stiffness and strength of the bone matrix. In this study, bone was obtained from superior and inferior sites, subjected to relatively high and low loads, respectively, from human femoral heads retrieved after surgery for osteoporotic hip fracture (OP), or for hip arthroplasty due to OA. Microindentation testing was used to measure the hardness of cancellous bone at various depths from the subchondral bone plate. The elemental composition from immediately adjacent microscopic sites was determined using electron probe microanalysis (EPMA). Overall, OA bone was found to have hardness values that were 7% lower than those from OP bone. Bone from the inferior site was harder than that from the superior in both diseases except in female OP patients. There was no variation with depth below the subchondral plate and no difference between sexes. No difference was found in the composition of the bone from the different disease groups and no correlation was found between hardness and any of the composition measurements. Though only an indirect measurement of stiffness, the reduction in hardness values supports the hypothesis that OA bone has a reduced elastic modulus.
Acta Obstetricia et Gynecologica Scandinavica, 2007
... LETTER TO THE EDITOR: REPLY. Pedro Valenzuela,; R. Guijarro. Article first published online: ... more ... LETTER TO THE EDITOR: REPLY. Pedro Valenzuela,; R. Guijarro. Article first published online: 31 DEC 2010. DOI: 10.1080/00016340701212005. Acta Obstet Gynecol Scand 2007. Issue. Acta Obstetricia et Gynecologica Scandinavica. Volume 86, Issue 5, page 639, May 2007 ...
Journal of Biomedical Materials Research Part A, 2006
It is a common theme in basic bone biomechanics and in biomechanical applications that much of th... more It is a common theme in basic bone biomechanics and in biomechanical applications that much of the behaviour can be determined and is dictated by the level of strain, whether this pertains to bone physiology, bone remodelling, osseinduction, osseointegration or the development of damage. The development of damage, demonstrated by stiffness loss measurements, has already been reported in detail in the literature. However, the systematic study of the development of 'plastic' (irrecoverable) strains, which are associated with the inelastic mechanical behaviour of bone tissue has generally been overlooked. The present study compares the rates at which the elastic (εa) and plastic components (εp) of strain developed during tensile, compressive, and shear fatigue in human cortical bone of six individuals aged between 53 and 79 yrs of age. The overall hypothesis of this investigation is that there is a common underlying factor in the damage-related behaviour of bone, which may allow us to link together the various aspects of the damage related behaviour of bone. The rate of development of plastic strain (έp) and the rate of damage (έa) are described as a function of the cycle number, the stress (), and/or stress normalised by the modulus of elasticity (). The implications of our findings are discussed with respect to simple models/mechanisms, which may underlie the observed behaviour.
Journal of Musculoskeletal and Neuronal Interactions - JMNI
The longevity, success, or failure of an orthopaedic implant is dependent on its osseointegration... more The longevity, success, or failure of an orthopaedic implant is dependent on its osseointegration especially within the initial six months of the initial surgery. The development of strains plays a crucial role in both bone modelling and remodelling. For remodelling, in particular, strains of substantial values are required to activate the osteoblastic and osteoclastic activity for the osseointegration of the implant. Bone, however, is subject to "damage" when strain levels exceed a certain threshold level. Damage is manifested in the form of microcracks; it is linked to increased elastic strain amplitudes and is accompanied by the development of "plastic" (irrecoverable, residual) strains. Such strains increase the likelihood for the implant to subside or loosen. The present study examines the rates (per cycle) by which these two components of strain (elastic and "plastic") develop during fatigue cycling in two loading modes, tension and compression. T...
The equine radius is a useful subject for examining the adaptation of bone histology to loading b... more The equine radius is a useful subject for examining the adaptation of bone histology to loading because in life the anterior cortex is loaded almost entirely in tension, the posterior cortex in compression. The histology of the two cortices is correspondingly different, the osteones and the interstitial lamellae in the posterior cortex having a more transversely oriented fibre arrangement than those in the anterior cortex. Presumably as a result of this histological difference, the posterior cortex is stronger in compression than the anterior cortex; the anterior cortex is stronger in tension than the posterior cortex. We here use nanoindentation to examine how the Young's modulus of elasticity of secondary osteones and interstitial lamellae in the anterior and posterior cortices varied as a function of angle. The anterior osteones were stiffer than the posterior osteones when tested in the direction parallel to the bone's long axis, but became progressively relatively less ...
The Influence of New Technology on Medical Practice, 1988
The replacement of diseased heart valves with prosthetic devices became possible with the develop... more The replacement of diseased heart valves with prosthetic devices became possible with the development of methods for cardiopulmonary bypass in the late 1950s. The first ball and cage mechanical valve was implanted in 1960 (Starr and Edwards, 1961) and the first tissue valve in 1962 (Ross, 1962). During 1984, the last year for which there is complete data, 4935 open heart procedures were carried out for valve surgery and a similar number of mechanical and tissue valves were implanted.
The recently developed Reference Point Indentation (RPI) allows the measurements of bone properti... more The recently developed Reference Point Indentation (RPI) allows the measurements of bone properties at the tissue level in vivo. The goal of this study was to compare the local anisotropic behaviour of bovine plexiform bone measured with depth sensing micro-indentation tests and with RPI. Fifteen plexiform bone specimens were extracted from a bovine femur and polished down to 0.05 mm alumina paste for indentations along the axial, radial and circumferential directions (N¼5 per group). Twenty-four micro-indentations (2.5 mm in depth, 10% of them were excluded for testing problems) and four RPI-indentations ($ 50 mm in depth) were performed on each sample. The local indentation modulus E ind was found to be highest for the axial direction (24.372.5 GPa) compared to the one for the circumferential indentations (19% less stiff) and for the radial direction (30% less stiff). RPI measurements were also found to be dependent on indentation direction (po0.001) with the exception of the Indentation Distance Increase (IDI) (p¼0.173). In particular, the unloading slope US1 followed similar trends compared to the E ind : 0.4770.03 N/mm for axial, 11% lower for circumferential and 17% lower for radial. Significant correlations were found between US1 and E ind (p¼ 0.001; R 2 ¼0.58), while no significant relationship was found between IDI and any of the micro-indentation measurements (p40.157). In conclusion some of the RPI measurements can provide information about local anisotropy but IDI cannot. Moreover, there is a linear relationship between most local mechanical properties measured with RPI and with micro-indentations, but IDI does not correlate with any micro-indentation measurements.
Proceedings of the Royal Society B: Biological Sciences, 2004
The toughness of bone is an important feature in preventing it from fracturing. We consider the n... more The toughness of bone is an important feature in preventing it from fracturing. We consider the notch sensitivity in impact, and the associations between brittleness, notch sensitivity and post-yield energy absorption of mammalian mineralized tissues. Specimens of bone-like tissues covering a wide range of mineralization were broken, either notched or un-notched, in impact. The greater the mineral content, the greater was the notch sensitivity. Also, the more brittle tissues dissipated the least post-yield energy and were the most notch sensitive. It is suggested that since antler bone, the least mineralized of all known mammalian mineralized tissues, seems to be notch insensitive in impact, no adaptive purpose would be served by having mineralized tissues of a lower mineralization than antler. This may explain the lower cutoff in mineralization seen in mammals.
We examined the microcracking (damage) in the vicinity of a circular hole in bovine femoral bone ... more We examined the microcracking (damage) in the vicinity of a circular hole in bovine femoral bone specimens. The stresses near the hole were derived by a finite element analysis model using the bone’s elastic constants and yield stresses, which were determined from a series of mechanical tests specifically for the type of bone under examination. The spatial occurrence and distribution of microcracking was compared to the patterns of the predicted maximum principal stress, the von Mises stress, and the strain energy density function (all implicated by various workers as stimuli for bone remodelling) and to the predictions derived by the use of two engineering criteria for anisotropic yield under mixed mode of stress. The predictions for stresses and the strain energy density were all very similar, making it impossible to claim that any of them is superior to the others. However, empirical examination of the results of the Hencky-von Mises and Tsai-Wu anisotropic yield criteria showed that the Tsai-Wu criterion approximated reasonably the pattern of microcracking around the hole. We suggest that, in the light of the considerable damage observed in the vicinity of stress concentrators, similar damage in irregular material interfaces (i.e. near orthopaedic implants) would require the re-examination of the theories concerning bone remodelling so as to account for the possibility of occurrence of damage and the quantification of its magnitude and likely effect. The presence of considerable microdamage in bone long before it fails suggests that damage-based criteria are more likely to be successful predictors of bone remodelling behaviour than would stress or strain-based criteria.
During fatigue tests of cortical bone specimens, at the unload portion of the cycle (zero stress)... more During fatigue tests of cortical bone specimens, at the unload portion of the cycle (zero stress) non-zero strains occur and progressively accumulate as the test progresses. This non-zero strain is hypothesised to be mostly, if not entirely, describable as creep. This work examines the rate of accumulation of this strain and quantifies its stress dependency. A published relationship determined from creep tests of cortical bone (Journal of Biomechanics 21 (1988) 623) is combined with knowledge of the stress history during ...
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