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Biomedical subjects

J D Currey

Publications and source records attributed to J D Currey.

At least 37 records · Page 2Linked to original sources
PubMed1999

The design of mineralised hard tissues for their mechanical functions.

Most hard tissues have as their primary purpose to be stiff. Outside the arthropods, mineralisation of a soft organic matrix is the almost universal method of producing high stiffness. However, stiffening brings with it the undesirable mechanical result of brittleness (lack of toughness). The mineralisation of some tissues, such as bone and dentine, can be modified rather easily, in evolutionary terms, to produce the optimum mix of stiffness with bending strength (which, except at the highest mineralisations, go together) on one hand and toughness on the other hand. However, in most other tissues, such as mollusc shell, echinoderm skeleton, brachiopod shell, barnacle shell and enamel, mineralisation is almost all-or-none, and no subtle gradations seem possible. In such cases, other features, such as architecture, must be modified to produce a useful skeleton. Not only the mechanical properties of the skeletal tissue, but its cost, mass and time taken for production will, biologists tend to assume, be balanced by natural selection to produce a satisfactory result. However, such complexity makes it difficult to be sure that we understand the extent to which mineralised skeletal materials are the best possible solution to the problems facing the animals and that we are not just telling 'Just-So' stories. Furthermore, there are some skeletal materials that do not seem to make much sense at the moment, although no doubt all will become clear eventually.

Adaptation, Physiological↗
PubMed1999

The development of microcracking and failure in bone depends on the loading mode to which it is adapted.

During locomotion, the anterior cortex of the equine radius is loaded predominantly in tension, the posterior predominantly in compression. The anterior cortex is relatively strong in tension, the posterior in compression. We investigated the pattern of failure of specimens from the two cortices using laser scanning confocal microscopy. All specimens were loaded in four-point bending to increasingly higher loads. We quantified the amount of diffuse microcracking on the tensile side of these specimens by observing the amount of light emitted under laser illumination. The amount of light emitted agreed well with subjective estimates of the amount of microcracking. Tensile microcracks first appeared at a strain of approximately 0.004, and all specimens showed considerable growth in microcrack density once the tensile strain had passed approximately 0.008. In specimens from the posterior cortex, there was little compressive microcracking, and such cracks as were present were small and diffuse. These specimens failed on the tensile side first. In specimens from the anterior cortex, compression cracks were more numerous, longer and less diffuse, and specimens failed initially in compression. The patterns of failure in the bone tissues of the two cortices are what would be expected assuming they were adapted to the mode of loading to which they are usually subjected.

Animals↗
PubMed1998

Changes in the stiffness, strength, and toughness of human cortical bone with age.

Aging adversely affects the elastic and ultimate properties of human cortical bone as seen in uniaxial tests in quasi static loading, high strain rate impact or fatigue. Little is known about the full effects of aging on toughness and its relationship with strength. In the present article the elastic modulus (E), strength (sigma f), fracture toughness (KC and J-integral), and work of fracture (Wf) were determined in specimens of male human femoral bone aged between 35-92 years. In this way we investigated whether fracture of bone in three situations, allowing various amounts of damage prior to fracture, can provide a better insight into the fracture process and also the relative importance of these experimental methods for assessing the soundness of bone material. We found a steady and significant decrease with age for all these mechanical measures. E fell by 2.3%, from its value of 15.2 GPa at 35 years of age, per decade of later life; sigma f fell similarly from 170 MPa by 3.7%; KC from 6.4 MPa m1/2 by 4.1%; J-integral from 1.2 kJ m-2 by 3%, and the Wf from 3.4 kJ m-2 by 8.7%. In aging bone there was a deterioration in the elastic properties of the material. This reduced the (elastically calculated) critical stress intensity level (KC) required to initiate a macrocrack, or the nonlinear energy associated with the onset of fracture (J). The macrocrack was preceded by less damage, and once created needed less energy to drive through the tissue (Wf).

Adult↗
PubMed1997

Effects of ionizing radiation on the mechanical properties of human bone.

Allogeneic bone grafts are frequently sterilized by means of ionizing radiation. We investigated the effects of ionizing radiation on both quasistatic and impact mechanical properties of human bone. Specimens from four paired femora of four donors received doses of 29.5 kGy ("standard," frequently used by tissue banks), 94.7 kGy ("high"), or 17 kGy ("low") of ionizing radiation. Young's modulus was unchanged by any level of radiation. Radiation significantly reduced bending strength, work to fracture, and impact energy absorption; in each case, the severity of the effect increased from low to standard to high doses of radiation. Work to fracture was particularly severely degraded; specimens irradiated with the high dose absorbed only 5% of the energy of the controls. Radiation, even at relatively low doses, makes the bone more brittle and thereby reduces its energy-absorbing capacity. We suggest that because the level of radiation required to produce an acceptable level of viral inactivation (90 kGy) produces an unacceptable reduction in the mechanical integrity of the bone, low levels of radiation, sufficient to produce bacterial safety, should be used in conjunction with biological tests to ensure viral safety.

Adult↗
PubMed1997

Exercise of young thoroughbred horses increases impact strength of the third metacarpal bone.

Exercise can have a profound effect on bone mass, but little is known of its effect on bone's material properties. In this experiment, our hypothesis was that a large difference in the training regimen of young thoroughbreds would produce a measurable difference in the mechanical properties of their bone material. When they were about 19 months old, eight thoroughbred racehorses were given one of two exercise regimens that lasted for 19 weeks: four horses (controls) were walked for 40 minutes a day but had no other exercise, and the remaining four (exercised) were additionally trotted for 20 minutes a day and given progressively intensive exercise on a treadmill. Mechanical testing to failure was performed on longitudinal beam specimens of the mid-diaphysis of the metacarpal. There was no difference in Young's modulus or bending strength between the two groups, although these properties varied somewhat depending on the position within the cortex from which the specimens had come. The specimens from the exercised horses had a slightly higher toughness, as measured by work (area under the load-deformation curve). They had a considerably higher impact strength. The impact strength of specimens from the outer cortex was also higher than that of those from the inner cortex in both groups. Impact strength correlated positively with the amount of microcracking produced during testing. Microcracking is related to structural and microstructural features in the bone. Increased loading caused the bone to respond in a way that enhanced its ability to microcrack and hence its toughness.

Analysis of Variance↗
PubMed1997

The ability of ultrasound velocity to predict the stiffness of cancellous bone in vitro.

The mechanical status of bones is an important consideration in skeletal pathological conditions such as osteoporosis, which result in fracture at predominantly cancellous bone sites. Density is a good predictor of the stiffness and strength of cancellous bone. However, these mechanical properties are also dependent on the cancellous bone's architecture. The objective of this work was to investigate the ability of ultrasound velocity to predict the Young's modulus of elasticity of cancellous bone. The cancellous bone specimens were 20 mm cubes from bovine femur and 21 mm diameter mediolateral cylinders cored from human calcaneus. Ultrasound velocity (V) and Young's modulus (E) were determined in three orthogonal directions for the bovine cubes [anteroposterior (AP), mediolateral (ML), and proximodistal (PD)], and mediolaterally in the calcaneus. Apparent density (p) was determined after the other tests. Density alone explains 87.6% of the variance of Young's modulus in human calcaneal and bovine femoral bone tested in the PD direction only. Velocity, however, explains 95% and a combination of density and velocity 97%. Velocity and stiffness are not random with respect to the three directions in the bovine specimens. Further, for each cube we obtained the mean of the three values of E and of V, and characterized each value of E and V by their deviation from their mean. There is an extremely strong positive correlation (r = 0.80) showing that the degree of deviation is consistent for E and V, and of the same sign. These results demonstrate that the velocity of ultrasound in cubes of cancellous bone can give structure-specific information. In particular, knowledge of both density and velocity allows better predictions of stiffness than do density or ultrasound velocity on their own. Because there are noninvasive methods of measuring density that do not depend on ultrasonic measurement the combination of these two measurements promises, eventually, to give improved assessment of a bone's weakness and liability to fracture.

Analysis of Variance↗
PubMed1996

Experimental and theoretical quantification of the development of damage in fatigue tests of bone and antler.

This study concerns the development of damage (as measured by a reduction in elastic modulus) in two kinds of bones differing considerably in their degrees of mineralisation: laminar bone from bovine femur and osteonal bone from red deer antler. Antler bone is much tougher than 'ordinary' bone and its failure properties have been investigated in: (i) monotonic tensile tests and (ii) creep rupture experiments. Tensile fatigue is another way of examining how damage develops in bone. The development of damage in the present fatigue tests was non-linear with the cycle number, the degree of non-linearity was dependent on the level of stress and followed a clearly different course for bone and antler. Antler was a more damage-tolerant material, being able to achieve a reduction in the final modulus of elasticity, just prior to failure, three times greater than 'ordinary' bone. The evolution of damage is quantified by an empirical and a graphical method and by the use of Continuum Damage Mechanics (CDM) expressions. The CDM method shows important conditions, found in antler, but not in bone, that seen necessary for achieving stable fractures and consequently producing very tough materials.

Animals↗
PubMed1996

The accumulation of fatigue microdamage in human cortical bone of two different ages in vitro.

OBJECTIVE: To analyse the development of damage during fatigue cycling of human bone. DESIGN: Changes in compliance and the cycles to failure were monitored in cortical bone samples subjected to oscillating stress in vitro. BACKGROUND: Previous studies produced mainly the relationship between the applied stress and the final cycles to failure (sigma-N(f)plots). However, cyclic stressing increases the compliance of the bone continuously, and causes a progressive mechanical/structural degradation. Recording this accumulation of damage allows one to know how close bone is to the point of failure; more importantly, it allows a more comprehensive modelling of fatigue processes in cortical bone. METHODS: The occurrence of material damage was continuously monitored during the tests. The 20 specimens came from two female subjects, 27 and 56 years old. The range of the cyclic stresses was 58-130 MPa. RESULTS: The damage was quantified with a graphical and an empirical/numerical method, and we have also microscopically observed the generation of internal microcracks. The range of cycles to failure was from 1 to 210,000. CONCLUSIONS: It was observed that (i) the older tissue showed a lower fatigue strength than the younger one, (ii) both tissues sustained similar damage levels prior to failure, and (iii) they both showed a continuous accumulation of damage during the tests, the course of which depended on the level of stress.

Journal Article↗
PubMed1996

Orthogonal relationships between ultrasonic velocity and material properties of bovine cancellous bone.

Osteoporotic fractures follow a period of asymptomatic bone loss and hence bone strength, predominantly in cancellous bone. An effective management of osteoporosis requires an understanding of the mechanical behaviour of cancellous bone including the anisotropic dependence. Ultrasound velocity (V) and elasticity (Young's modulus, E) were measured in the three orthogonal directions in 20 mm cubes of bovine cancellous bone. Student paired t-test analysis showed significant variations in velocity and elasticity for the three orthogonal directions, the highest significance being between proximal-distal (PD) and antero-posterior (AP) directions with t = 5.63 and 4.09 for velocity and elasticity respectively, the lowest significance between medio-lateral (ML) and antero-posterior directions. Elasticity followed a power law relationship with apparent density (p) as reported in the literature, the exponent (b) being direction dependent (b = 1.98 +/- 0.21 for PD, 2.42 +/- 0.24 for AP and 2.03 +/- 0.17 for ML). The adjusted R2 values between elasticity and apparent density were highly significant (79.9% for PD, 81.9% for AP and 85.7% for ML). The relationship between velocity and apparent density is less significant in terms of the amount of variance explained (48.5% for PD, 63.3% for AP and 64.4% for ML). R2 values relating elasticity and velocity were again highly significant (79.4% for PD, 82.9% for AP and 80.5% for ML) and the coefficients, determined by regression analysis, independent of direction. Analysis of velocity, elasticity and density data for a range of reference materials demonstrated that experimentally measured longitudinal wave velocity could be reliably substituted into the bar wave equation (v = square root E/p). This implies that a combination of velocity and apparent density may be an improved indicator of bone fragility than density alone.

Animals↗
PubMed1996

Prediction of mechanical properties of the human calcaneus by broadband ultrasonic attenuation.

Broadband ultrasonic attenuation (dB MHz cm-1, nBUA) was determined for specimens from 20 human calcanei, along with apparent density, elasticity (Young's modulus), and compressive strength. The calcanei were modified to provide "whole" (only soft tissue removed), "core" (mediolateral cores corresponding to in vivo measurement region), "can" (cortical end plates removed from core), and "def" (core defatted) samples. The nBUA values for the various modifications were highly correlated. The presence of the cortical endplates creates a significant nBUA, probably due to complex phase interactions. nBUAcan was a good predictor of elasticity (R2 = 75.7%) and strength (R2 = 73.6%). Apparent density was a better predictor of the mechanical variables than nBUA, with R2 values of 88.5% for elasticity and 87.6% for strength. The morphological anisotropy defined by "fabric" for the specimens was extremely uniform. The coefficient of variation in nBUA (40.5%) and compressive strength (64.4%) was significantly greater than for apparent density (23.5%) and fabric (6.7%). It is well known that a power law relationship exists between apparent density and elasticity or strength in cancellous bone. An interesting finding in this work is that there also appears to be a power law relationship between nBUA and apparent density, with an exponent of approximately 2, which, in the light of clinical implications, warrants further investigation.

Aged↗
PubMed1995

Experimentally determined microcracking around a circular hole in a flat plate of bone: comparison with predicted stresses.

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.

Animals↗
PubMed1995

Effect of formaldehyde fixation on some mechanical properties of bovine bone.

The risk of infection of investigators working on the biomechanics of human bone from a variety of modern pathogens including the human immunodeficiency virus or the hepatitis B virus has increased recently. New safety procedures are needed to reduce that risk. The procedure we follow in our laboratory employs brief (< 3 h) fixation in formaldehyde, and we report here the effects it has on some mechanical properties of bovine bone. Results in quasistatic loading tests were almost unaffected by our fixation protocol, but a significant decrease in impact strength was found. These results indicate that there may be some interaction between fixation and strain rate dependent effects and, therefore, some caution is needed when using common biomechanical measurement methods on fixed bone material.

Animals↗
PubMed1994

Dependence of mechanical properties on fibre angle in narwhal tusk, a highly oriented biological composite.

The successful modelling of the mechanical properties of mineralized tissues depends critically on the knowledge of the off-axis behaviour of individual unidirectional lamellae. Information on this is lacking. In this work we attempt to rectify the situation. Young's modulus, measured in bending and tension, and the tensile strength and ultimate strain to failure of the dentine of the narwhal Monodon monoceros, were determined on specimens that had almost unidirectional fibres, whose direction differed considerably from specimen to specimen. Modulus and strength decreased steadily with the degree of off-angle loading, falling to about 45% of maximum for modulus, and 35% of maximum for strength. Ultimate strain showed a less uniform behaviour, and remained remarkably high at large angles. Differences in mechanical behaviour were not related to the very small differences in mineral content measured between specimens. These findings have strong implications for modelling the anisotropic behaviour of bone, because dentine is very much like bone in most important respects. Predictions using classical composite theory are reasonably satisfactory, as long as the mineral crystals are assumed to be platelets, not rods.

Animals↗
PubMed1994

An examination of the micromechanics of failure of bone and antler by acoustic emission tests and Laser Scanning Confocal Microscopy.

Bone fails as a result of damage accumulation in the form of microcracks. Antler is in essence less mineralized bone, but is much tougher than ordinary bone. Failure through damage accumulation also happens in antler, so it is clear that antler's microcracks must be pervasive and that natural selection has produced mechanisms for this material which make it more damage-tolerant. We have examined the development of damage in bone and antler by cyclic loading in three-point bending. We have also conducted acoustic emission studies on these two materials during monotonic tensile loading, and used Laser Scanning Confocal Microscopy (LSCM) to observe microcracks in samples which had been mechanically damaged. We found that diffuse microcracking coincides with the appearance of a macroscopic region of 'yield' in the stress/strain curve of bone and antler; it is related to the local stress (or strain) field that microcracks interact with the structure (particularly in antler). Both acoustic emissions studies and LSCM observations indicated that antler may possess additional modes of stable micromechanical failure compared with ordinary bone.

Animals↗
PubMed1992

Mechanical properties of microcallus in human cancellous bone.

Until now, the mechanical properties of the microcalluses that form in human cancellous bone have been unexplained. We measured the microhardnesses of microcalluses in cancellous bone, of the trabeculae within the microcalluses, of the trabeculae adjacent to microcalluses, and of trabeculae lacking microcalluses in a human tibia and femur. We observed no important differences between materials at the four different sites. Because the microhardness of bone is very closely related to its stiffness, this finding indicates that microcalluses are likely to stiffen the trabeculae in which they are formed, even though they may surround unhealed fractures of the cancellous trabeculae.

Adult↗