The effects of damage and microcracking on the impact strength of bone

Microcracking has been shown to occur when bone is 'damaged' as shown by a loss of stiffness. The effect on bone's toughness of the types of damage produced at low losses of stiffness are not known. We loaded bovine bone specimens in bending and tension to stiffness losses of up to 27%, and examined the microcracking produced. The tensile specimens had diffuse arrays of microcracks of 2-20 microm in length, characteristic of tensile loading, on all surfaces. The bending specimens showed tensile microcracking on the tensile surface and characteristic long, straight, cross-hatched compression cracks on the compressive surface. Specimens were then broken in impact. Those that had been damaged in bending were divided into two groups, in one group the part of the specimen which had undergone compression damage was placed in tension, and in the other group the tensile damage was placed in tension. Tensile damage loaded in tension did not reduce the bone's energy-absorbing ability in impact until a modulus reduction of over 20%. However compression damage loaded in tension did severely reduce the bone's energy absorption capabilities (by an average of about 40%).     

Off-axis loads cause failure of the distal radius at lower magnitudes than axial loads: a finite element analysis

Distal radius (Colles') fractures are a common fall-related injury in older adults and frequently result in long-term pain and reduced ability to perform activities of daily living. Because the occurrence of a fracture during a fall depends on both the strength of the bone and upon the kinematics and kinetics of the impact itself, we sought to understand how changes in bone mineral density (BMD) and loading direction affect the fracture strength and fracture initiation location in the distal radius. A three-dimensional finite element model of the radius, scaphoid, and lunate was used to examine changes of +/-2% and +/-4% BMD, and both axial and physiologically relevant off-axis loads on the radius. Changes in BMD resulted in similar percent changes in fracture strength. However, modifying the applied load to include dorsal and lateral components (assuming a dorsal view of the wrist, rather than an anatomic view) resulted in a 47% decrease in fracture strength (axial failure load: 2752N, off-axis: 1448N). Loading direction also influenced the fracture initiation site. Axially loaded radii failed on the medial surface immediately proximal to the styloid process. In contrast, off-axis loads, containing dorsal and lateral components, caused failure on the dorsal-lateral surface. Because the radius appears to be very sensitive to loading direction, the results suggest that much of the variability in fracture strength seen in cadaver studies may be attributed to varying boundary conditions. The results further suggest that interventions focused on reducing the incidence of Colles' fractures when falls onto the upper extremities are unavoidable may benefit from increasing the extent to which the radius is loaded along its axis.     

Early cement damage around a femoral stem is concentrated at the cement/bone interface

This study aimed to improve understanding of the mechanical aspects of cemented implant loosening. After aggressive fatigue loading of stem/cement/femur constructs, micro-cracks and stem/bone micro-motions were quantified to answer three research questions: Are cracks preferentially associated with the stem/cement interface, the cement/bone interface or voids? Is cement damage dependent on axial position? Does cement damage correlate with micro-motion between the stem and the bone? Eight Charnley Cobra stems were implanted in cadaveric femora. Six stem/cement/femur constructs were subjected to "stair-climbing" loads for 300 kcycles at 2Hz. Loads were normalized by construct stiffness to avoid fracture. Two additional constructs were not loaded. Transverse sections were cut at 10mm intervals, stained with a fluorescent dye penetrant and examined using epi-fluorescence stereomicroscopy. Crack lengths and cement areas were recorded for 9 sections per specimen. Crack length-density was calculated by dividing summed crack length by cement mantle area. To isolate the effect of loading, length-density data were offset by the baseline length-density measured in the non-loaded specimens. Significantly more cracks were associated with the interdigitated area (35.1%+/-11.6%) and the cement/bone interface (31.0%+/-6.2%) than with the stem/cement interface (11.0%+/-5.2%) or voids (6.1%+/-4.8%) (p<0.05). Load-induced micro-crack length-density was significantly dependent on axial position, increasing proximally (p<0.001). Micro-motions were small, all stems rotated internally. Cement damage did not correlate with micro-motion.     

Mechanical damage to the intervertebral disc annulus fibrosus subjected to tensile loading

Damage of the annulus fibrosus is implicated in common spinal pathologies. The objective of this study was to obtain a quantitative relationship between both the number of cycles and the magnitude of tensile strain resulting in damage to the annulus fibrosus. Four rectangular tensile specimens oriented in the circumferential direction were harvested from the outer annulus of 8 bovine caudal discs (n = 32) and subjected to one of four tensile testing protocols: (i) ultimate tensile strain (UTS) test; (ii) baseline cyclic test with 4 series of 400 cycles of baseline cyclic loading (peak strain = 20% UTS); (iii & iv) acute and fatigue damage cyclic tests consisting of 4 x 400 cycles of baseline cyclic loading with intermittent loading to 1 and 100 cycles, respectively, with peak tensile strain of 40%, 60%, and 80% UTS. Normalized peak stress for all mechanically loaded specimens was reduced from 0.89 to 0.11 of the baseline control levels, and depended on the magnitude of damaging strain and number of cycles at that damaging strain. Baseline, acute, and fatigue protocols resulted in permanent deformation of 3.5%, 6.7% and 9.6% elongation, respectively. Damage to the laminate structure of the annulus in the absence of biochemical activity in this study was assessed using histology, transmission electron microscopy, and biochemical measurements and was most likely a result of separation of annulus layers (i.e., delamination). Permanent elongation and stress reduction in the annulus may manifest in the motion segment as sub-catastrophic damage including increased neutral zone, disc bulging, and loss of nucleus pulposus pressure. The preparation of rectangular tensile strip specimens required cutting of collagen fibers and may influence absolute values of results, however, it is not expected to affect the comparisons between loading groups or dose-response reported.     

Aging enhances a mechanically-induced reduction in tendon strength by an active process involving matrix metalloproteinase activity

Age-associated and degenerative loss of functional integrity in soft tissues develops from effects of cumulative and subtle changes in their extracellular matrix (ECM). The highly ordered tendon ECM provides the tissue with its tensile strength during loading. As age and exercise collide in the high incidence of tendinopathies, we hypothesized that aged tendons fail due to cumulative damage resulting from a combination of diminished matrix repair and fragmentation of ECM proteins induced by prolonged cyclical loading, and that this is an active cell-mediated process. We developed an equine tendon explant model to examine the effect of age on the influence of prolonged cyclical loading at physiologically relevant strain rates (5% strain, 1 Hz for 24 h) on tissue mechanical properties, loss of ECM protein and matrix metalloproteinase (MMP) expression. We show significantly diminished mechanical strength of cyclically loaded tissue compared to controls (39.7 +/- 12%, P 相似文献   

Damage accumulation in vertebral trabecular bone depends on loading mode and direction

Osteoporotic vertebral fractures constitute a major clinical problem in ageing societies. A third of all vertebral fractures is caused by falls, 15% by lifting heavy loads or traffic accidents and over 50% are not relatable to a traumatic event. In the latter case vertebrae show sinter processes which indicate the accumulation of damage and permanent deformation. Accumulated damage may not be visible on radiographs but increases the risk of fracture and could lead to vertebral collapse. Clear understanding of the accumulation of damage and residual strains and their dependence on loading mode and direction is important for understanding vertebral fractures. Altogether, 251 cylindrical samples (8×18-25mm) were obtained from 50 male and 54 female fresh frozen human vertebrae (T1-L3) of 65 (21-94) years. Vertebrae were randomly assigned to three groups cranial-caudal, anterior-posterior and latero-lateral. Specimens were mechanically loaded in compression, tension or torsion in five load steps at a strain rate of 0.2%/s. Three conditioning cycles were driven per load step. Stress-strain curves were reconstructed from the force-displacement or from the moment-twist angle curves. Damage accumulated from 0 to 86% in compression, from 0 to 76% in tension and from 0 to 86% in torsion through the five load steps. Residual strains accumulated from 0 to -0.008mm/mm in compression, 0 to 0.006mm/mm in tension and 0 to 0.026rad/rad in torsion. Significantly less damage (p<0.05) but not residual strains accumulated in transverse directions. This study provides detailed experimental insights into the damage behaviour of vertebral trabecular bone under various loads occurring in vivo. Damage but not residual strain evolution seems to be anisotropic. Both seem to evolve differently under different loading modes. The results could be of importance in understanding vertebral fractures.     

The role of the lamellar interface during torsional yielding of human cortical bone

Fragility fractures are a result of alterations in bone quantity, tissue properties, applied loads, or a combination of these factors. The current study addresses the contribution of cortical bone tissue properties to skeletal fragility by characterizing the shear damage accumulation processes which occur during torsional yielding in normal bone. Samples of human femoral cortical bone were loaded in torsion and damaged at a post-yield twist level. The number of microcracks within osteons, interstitial tissue, and along cement lines were assessed using basic fuchsin staining. Damage density measures (number of cracks/mm2) were correlated with stiffness degradation and changes in relaxation. Damaged samples exhibited a wide variation in total microcrack density, ranging from 1.1 to 43.3 cracks/mm2 with a mean density of 19.7 +/- 9.8 cracks/mm2. Lamellar interface cracks comprised more than 75% of the total damage, indicating that the lamellar interface is weak in shear and is a principal site of shear damage accumulation. Damage density was positively correlated with secant stiffness degradation, but only explained 22% of the variability in degradation. In contrast, damage density was uncorrelated with the changes in relaxation, indicating that a simple crack counting measure such as microcrack density was not an appropriate measure of relaxation degradation. Finally, a nonuniform microcrack density distribution was observed, suggesting that internal shear stresses were redistributed within the torsion samples during post-yield loading. The results suggested that the lamellar interface in human cortical bone plays an important role in torsional yielding by keeping cracks physically isolated from each other and delaying microcrack coalescence in order to postpone the inevitable formation of the fatal crack.     

Effect of boundary conditions,impact loading and hydraulic stiffening on femoral fracture strength

Patient specific quantitative CT (QCT) imaging data together with the finite element (FE) method may provide an accurate prediction of a patient's femoral strength and fracture risk. Although numerous FE models investigating femoral fracture strength have been published, there is little consent on the effect of boundary conditions, dynamic loading and hydraulic strengthening due to intra-medullary pressure on the predicted fracture strength. We developed a QCT-derived FE model of a proximal femur that included node-specific modulus assigned based on the local bone density. The effect of three commonly used boundary conditions published in literature were investigated by comparing the resulting strain field due to an applied fracture load. The models were also augmented with viscoelastic material properties and subject to a realistic impact load profile to determine the effect of dynamic loads on the strain field. Finally, the effect of hydraulic strengthening was investigated by including node specific permeability and performing a coupled pore diffusion and stress analysis of the FE model. Results showed that all boundary conditions yield the same strain field patterns, but peak strains were 22% lower and fracture load was 18% higher when loaded at the greater trochanter than when loaded at the femoral head. Comparison of the dynamic models showed that material viscoelasticity was important, but inertial effects (vibration and shock) were not. Finally, pore pressure changes did not cause significant hydraulic strengthening of bone under fall impact loading.     

Stress intensity variations in bone microcracks during the repair process

Microscopic cracks form and grow in compact bone in vivo due to cyclic loading. Their growth can cause stress fractures and has been implicated in the processes of remodelling and adaptation. These cracks are repaired by the actions of BMUs which are mobile resorption cavities. In this work, we studied the interaction between cracks and BMUs by making finite element models representing different stages in the repair process. The tendency of the crack to grow was measured by its stress intensity factor, K. We found that K changes in a complex manner during the repair process, both decreasing and increasing depending on the size of the crack and the type of loading applied. For loading conditions similar to those that exist in vivo, the presence of the BMU can cause K to rise significantly, in some cases by more than 20%, implying a substantial increase in crack growth rate. This information is important for our general understanding of the complexities of the repair process, and especially for the development of theoretical models to simulate damage and repair in bone.     

Application of circular statistics in the study of crack distribution around cemented femoral components

Cemented stem constructs were loaded in cyclic fatigue using stair climbing loading and the resulting fatigue damage to the cement mantle was determined in terms of angular position of crack and crack length. Techniques from circular statistics were used to determine if the distribution of micro-cracks was uniform. With a designated orientation of 0 degrees -90 degrees -180 degrees -270 degrees indicating lateral-anterior-medial-posterior anatomic directions, the overall distribution of cracks was not uniform (p<0.05) with a mean crack direction in the postero-medial (249 degrees) quadrant of the mantle. The crack angular distribution for proximal (postero-medial; 251 degrees) and distal (antero-medial; 112 degrees) regions of the cement mantle was also different (p<0.025). These findings suggest that the location of cement damage depends on anatomic position and appears to correspond with the tensile stress field in the cement mantle.     

Levels of cyclic GMP and cyclic AMP in regenerating forelimbs of adult newts following denervation

The effect of short-term denervation (0, 12, 24, and 72 hours) on the levels of cyclic 3'5'-guanosine monophosphate (cGMP) and cyclic 3'5'-adenosine monophosphate (cAMP) in adult newt (Notophthalmus viridescens) forelimbs at 15, 22, and 35 days of regeneration was investigated. Regenerate blastema and stump cyclic nucleotide levels were compared with those of the contralateral intact forelimb and hindlimb, with levels in the normally regenerating blastema, and with levels measured in the forelimbs of intact, nonoperated animals. Variations in cyclic nucleotide levels occurred according to regeneration stage and tissue type. Changes in level were noted immediately upon denervation and subsequently at other sample times in all regenerate and control series. Parallel fluctuations occurred in regenerate stump and contralateral intact forelimbs. Our results from nonamputated denervated and sham-denervated animals indicate that short-term, denervation-associated cyclic nucleotide fluctuations cannot be attributed solely to the loss of innervation.     

Abnormal limb regeneration in adult newts exposed to a pulsed electromagnetic field

This investigation examined two questions: 1) whether or not forelimb regeneration results in a faithful replacement of the distal skeletal pattern and 2) what effect exposure to a pulsed electromagnetic field (PEMF), the type reported to facilitate healing of human non-united bone fractures, would have on the process of limb regeneration. Of the native forelimbs, 98% (132 out 134) had a skeletal pattern that showed little difference with the only variation being the range of carpal bones (5-8). Following a 4-5 month postamputation period, the skeletal pattern of the normal regenerates was examined. While 72% (135 out of 188) of these forelimbs resembled the native group, 28% (53 out of 188) were abnormal. These abnormalities consisted of the loss of a digit, fused carpals, and long bone defects which occurred singly or in combination with one another. Exposure to a PEMF for the first 30 days postamputation, followed by a 3-4 month postamputation period, produced in addition to the normal (60%, 144 out of 240) and abnormal forelimbs (28%, 67 out of 240), a group of forelimbs with unique gross defects (12%, 28 out of 240). These defects, occurring singly or in combination, included the loss of two or more digits with associated loss of carpals, absence of the entire hand pattern, and abnormalities associated with the radius and ulna. We can offer no explanation for these observations.     

The influence of cyclic tension amplitude on chondrocyte matrix synthesis: experimental and finite element analyses

While not generally viewed as physiologically significant in articular cartilage, substantial tension can develop in fibrocartilage structures and in articular cartilage injuries. This study examined how different amplitudes of cyclic tension influence chondrocyte matrix synthesis. Bovine articular chondrocytes seeded in fibrin gels were loaded continuously for 48 hours at 1.0 Hz with displacements of 5%, 10%, or 20%. Protein and proteoglycan synthesis were measured by (3)H-proline and (35)S-sulfate incorporation, respectively. A poroelastic finite element model of the fibrin gel was developed to determine the strain distributions, hydrostatic pressures, and fluid velocities within the constructs at the various levels of displacement. Compared to unloaded controls, 10% and 20% displacements inhibited proteoglycan synthesis to the same extent, while 5% displacement had no effect. Tensile loading did not significantly affect protein synthesis. The finite element model predicted a wide range of strains and fluid velocities within the region of the gel analyzed for matrix synthesis, and the ranges overlapped for the different levels of displacement. These results indicate that the cyclic tension amplitude influences chondrocyte proteoglycan synthesis and that there may be a threshold in the response.     

Effect of cyclic loading on the nanoscale deformation of hydroxyapatite and collagen fibrils in bovine bone

Cyclic compressive loading tests were carried out on bovine femoral bones at body temperature $(37\,^{\circ }\hbox {C})$ , with varying mean stresses ( $-55$ to $-80$  MPa) and loading frequencies (0.5–5 Hz). At various times, the cyclic loading was interrupted to carry out high-energy X-ray scattering measurements of the internal strains developing in the hydroxyapatite (HAP) platelets and the collagen fibrils. The residual strains upon unloading were always tensile in the HAP and compressive in the fibrils, and each increases in magnitude with loading cycles, which can be explained from damage at the HAP–collagen interface and accumulation of plastic deformation within the collagen phase. The samples tested at a higher mean stress and stress amplitude, and at lower loading frequencies exhibit greater plastic deformation and damage accumulation, which is attributed to greater contribution of creep. Synchrotron microcomputed tomography of some of the specimens showed that cracks are produced during cyclic loading and that they mostly occur concentric with Haversian canals.     

Effect of spinal level and loading conditions on the production of vertebral burst fractures in a porcine model

Vertebral burst fractures are commonly studied with experimental animal models. There is however a lack of consensus as to what parameters are important to create an unstable burst fracture with a significant canal encroachment on such model. This study aims to assess the effect of the loading rate, flexion angle, spinal level, and their interactions on the production of a vertebral thoracolumbar burst fracture on a porcine model. Sixteen functional spinal units composed of three vertebrae were harvested from mature Yucatan minipigs. Two loading rates (0.01 and 500 mm/s), two flexion angles (0° and 15°), and two spinal levels (T11-T13 and T14-L2) were studied, following a full factorial experimental plan with one repetition. Compression was applied to each functional unit to create a vertebral fracture. The load-to-failure, loss of compressive stiffness, final canal encroachment, and fracture type were used as criteria to evaluate the resulting fracture. All specimens compressed without flexion resulted in burst fractures. Half of the specimens compressed with the 15° flexion angle resulted in compression fractures. Specimens positioned without flexion lost more of their compressive stiffness and had more significant canal encroachment. Fractured units compressed with a higher loading rate resulted in a greater loss of compressive stiffness. The spinal level had no significant effect on the resulting fractures. The main parameters which affect the resulting fracture are the loading rate and the flexion angle. A higher loading rate and the absence of flexion favors the production of burst fractures with a greater canal encroachment.     

Specimen-specific modeling of hip fracture pattern and repair

Hip fracture remains a major health problem for the elderly. Clinical studies have assessed fracture risk based on bone quality in the aging population and cadaveric testing has quantified bone strength and fracture loads. Prior modeling has primarily focused on quantifying the strain distribution in bone as an indicator of fracture risk. Recent advances in the extended finite element method (XFEM) enable prediction of the initiation and propagation of cracks without requiring a priori knowledge of the crack path. Accordingly, the objectives of this study were to predict femoral fracture in specimen-specific models using the XFEM approach, to perform one-to-one comparisons of predicted and in vitro fracture patterns, and to develop a framework to assess the mechanics and load transfer in the fractured femur when it is repaired with an osteosynthesis implant. Five specimen-specific femur models were developed from in vitro experiments under a simulated stance loading condition. Predicted fracture patterns closely matched the in vitro patterns; however, predictions of fracture load differed by approximately 50% due to sensitivity to local material properties. Specimen-specific intertrochanteric fractures were induced by subjecting the femur models to a sideways fall and repaired with a contemporary implant. Under a post-surgical stance loading, model-predicted load sharing between the implant and bone across the fracture surface varied from 59%:41% to 89%:11%, underscoring the importance of considering anatomic and fracture variability in the evaluation of implants. XFEM modeling shows potential as a macro-level analysis enabling fracture investigations of clinical cohorts, including at-risk groups, and the design of robust implants.     

Neuromuscular neutral zones response to cyclic lumbar flexion

The in vivo lumbar spine of the anaesthetized feline was subjected to passive cyclic anterior flexion-extension at 0.25 Hz and 40 N peak load for cumulative 60 min duration. Displacement (or displacement neuromuscular neutral zones-DNNZ) and tension (or tension neuromuscular neutral zones-TNNZ) at which reflexive EMG activity from the multifidi muscles was initiated and terminated were recorded, for single-test cycles, before and for 7h after cyclic loading. Displacement and tension NNZs increased significantly after loading. The displacement NNZs decreased exponentially to near baseline by the 7th hour of rest. The tension NNZs, however, decreased to below the baseline by the 2nd to 3rd hour after loading and continued decreasing into the 7th hour. Peak EMG significantly decreased (49-57%) to below the baseline immediately after loading and then exponentially increased, exceeding the baseline by the 2nd to 3rd hour and reaching 33-59% above baseline by the 7th hour. EMG median frequency decreased after loading and then exceeded the baseline after the 3rd hour, indicating initial de-recruitment, followed by recruitment of new motor units. These findings suggest that the lumbar spine was exposed to instability for 2-3h after cyclic loading, due to concurrent laxity of the viscoelastic tissues and deficient muscular activity. A delayed neuromuscular compensation mechanism was found to exist, triggering the musculature significantly earlier and at higher magnitude than baseline, while the viscoelastic tissues were still lax. Thus, it is suggested that prolonged cyclic loading may compromise lumbar stability during the immediate 2-3h post-loading, increasing the risk of injury.     

Development of an Ex Vivo Protocol to Model Bone Fracture in Laying Hens Resulting from Collisions

Fractures of the keel bone, a bone extending ventrally from the sternum, are a serious health and welfare problem in free range laying hens. Recent findings suggest that a major cause of keel damage within extensive systems is collisions with internal housing structures, though investigative efforts have been hindered by difficulties in examining mechanisms and likely influencing factors at the moment of fracture. The objectives of this study were to develop an ex vivo impact protocol to model bone fracture in hens caused by collision, to assess impact and bird-related factors influencing fracture occurrence and severity, and to identify correlations of mechanical and structural properties between different skeletal sites. We induced keel bone fractures in euthanized hens using a drop-weight impact tester able to generate a range of impact energies, producing fractures that replicate those commonly found in commercial settings. The results demonstrated that impact energies of a similar order to those expected in normal housing were able to produce fractures, and that greater collision energies resulted in an increased likelihood of fractures and of greater severity. Relationships were also seen with keel’s lateral surface bone mineral density, and the peak reactive force (strength) at the base of the manubrial spine. Correlations were also identified between the keel and long bones with respect to both strength and bone mineral density. This is the first study able to relate impact and bone characteristics with keel bone fracture at the moment of collision. Greater understanding of these relationships will provide means to reduce levels of breakage and severity in commercial systems.     

Fatigue fracture of mother-of-pearl and its significance for predatory techniques

Specimens of nacre from the pearl oyster Pinctada margaritifera were loaded in bending to various fractions of the load needed to cause fracture. For a small number of cycles, less than about 1000, no changes in compliance or strength were found if the load was less than about 60% of the load necessary to cause failure. Above this fraction the compliance increased, the strength usually decreased but occasionally increased.
We conclude that, because the loading of a shell by a predator is likely to be far less controlled than loading by a machine, causing a shell to break by fatigue fracture is unlikely to be a tactic employed by a predator, though it may happen fortuitously.     

The effect of cement creep and cement fatigue damage on the micromechanics of the cement–bone interface

The cement–bone interface provides fixation for the cement mantle within the bone. The cement–bone interface is affected by fatigue loading in terms of fatigue damage or microcracks and creep, both mostly in the cement. This study investigates how fatigue damage and cement creep separately affect the mechanical response of the cement–bone interface at various load levels in terms of plastic displacement and crack formation. Two FEA models were created, which were based on micro-computed tomography data of two physical cement–bone interface specimens. These models were subjected to tensile fatigue loads with four different magnitudes. Three deformation modes of the cement were considered: ‘only creep’, ‘only damage’ or ‘creep and damage’. The interfacial plastic deformation, the crack reduction as a result of creep and the interfacial stresses in the bone were monitored. The results demonstrate that, although some models failed early, the majority of plastic displacement was caused by fatigue damage, rather than cement creep. However, cement creep does decrease the crack formation in the cement up to 20%. Finally, while cement creep hardly influences the stress levels in the bone, fatigue damage of the cement considerably increases the stress levels in the bone. We conclude that at low load levels the plastic displacement is mainly caused by creep. At moderate to high load levels, however, the plastic displacement is dominated by fatigue damage and is hardly affected by creep, although creep reduced the number of cracks in moderate to high load region.