Vertebral Cancellous Bone Research Articles

A Biomechanical Analysis of the Effects of Resorption Cavities on Cancellous Bone Strength

We evaluated the effects of resorption cavities on cancellous bone strength using computational methods. Adding cavities to cancellous bone caused reductions in strength and stiffness that were greater than expected from the associated changes in bone volume and more pronounced when cavities were targeted to regions of high tissue strain. The amount of bone turnover in the skeleton has recently been implicated as a factor influencing bone strength. One mechanism proposed to explain this effect is that resorption cavities reduce the effective thickness of trabeculae and modify local stress distributions leading to reduced mechanical performance of the entire structure. In this study, we tested the plausibility of this mechanism. High-resolution finite element models were created from muCT images of 16 vertebral cancellous bone samples, as well as from images of the samples in which cavities had been added digitally-either at regions of high strain (targeted) or placed at random on the bone surface (nontargeted). The effect of resorption cavities on predicted bone strength and stiffness was evaluated by comparing the relationships between mechanical properties and bone volume fraction among the three groups (the original images, those with nontargeted cavities, and those with targeted cavities). Addition of resorption cavities modified the relationship between mechanical properties and bone volume fraction in the finite element models such that, for a given bone volume fraction, stiffness and yield strength were reduced compared with the original images (p < 0.05). The differences in yield strength-volume fraction relationships between the original models and those with targeted cavities were significantly greater than those between the original models and those with nontargeted cavities (p < 0.05). None of the differences in predicted mechanical properties per unit bone volume fraction could be accounted for by 3D measures of microarchitecture. Resorption cavities may influence cancellous bone strength and stiffness independent of their effect on bone volume. The effects of cavities on bone mechanical performance relative to bone volume are greater when cavities are targeted to regions of high strain and cannot be predicted using standard microarchitecture measures.

Strain-Dependent Variations in the Response of Cancellous Bone to Ovariectomy in Mice

The goal of this study was to characterize the skeletal response to ovariectomy in mice (129P3, C57BL/6, and B6129PF2) commonly used in gene manipulation studies to evaluate their potential as preclinical models of postmenopausal osteoporosis. The magnitude of cancellous bone loss and cellular indices of increased bone turnover in response to ovariectomy varied with mouse type and skeletal site, but in general, were less pronounced and less consistent than in Sprague-Dawley rats, the established preclinical model for postmenopausal bone loss. The ovariectomized (OVX) rat is the most widely used preclinical rodent model for postmenopausal osteoporosis. However, the underlying mechanisms of bone disorders, including osteoporosis, have been explored predominantly in the mouse. The purpose of this study was to evaluate mice (129P3 and C57BL/6 inbred strains and their F2 hybrid offspring, B6129PF2), commonly used for gene knockout and overexpression studies, for their potential as preclinical models of postmenopausal bone loss. The mice were OVX or sham-operated at 4 months of age and killed at 1 or 3 months after surgery. Lumbar vertebrae and distal femora were subjected to histomorphometric assessment. Mice in the two strains and the F2 hybrids (will be referred to as strain for the remainder of the abstract) lost vertebral cancellous bone after OVX; bone volume (BV/TV) was 20% and 27% lower at 1 and 3 months after surgery, respectively. The decreased cancellous BV/TV was associated with an increase in osteoclast surface at 1 month after OVX in the 129P3 strain only. Osteoblast surface was increased by 20% with OVX at both 1 and 3 months after surgery, irrespective of mouse strain. However, bone formation rate was not altered by OVX in any of the mouse strains. In contrast to the lumbar vertebrae, cancellous bone loss in response to OVX differed in the distal femur among the three mouse strains. OVX had no significant effect on distal femur BV/TV in the B6129PF2 mouse strain. In the C57BL/6 strain, cancellous BV/TV was reduced by OVX at 1 month after surgery but not at 3 months after surgery, whereas distal femur BV/TV in 129P3 mice was reduced at 3 months after surgery. Osteoclast surface was not affected by OVX at either time-point in the C57BL/6 strain, but was increased by 116% at 1 month after surgery in the 129P3 strain. Osteoblast surface was increased with OVX at 1 month after surgery, irrespective of strain, whereas bone formation rate was not altered by OVX at either time-point in any of the strains. The magnitude of cancellous bone loss and cellular indices of increased bone turnover in response to OVX varied with mouse strain and skeletal site, but in general, were less pronounced and less consistent than in the Sprague-Dawley rat. Although mouse models will continue to provide insights into genetic influences on bone mass and turnover, caution should be exercised when using 129P3 and C57BL/6 mice, and their F2 hybrids, as models for postmenopausal bone loss and preclinical testing of potential therapies for osteoporosis.

Vitamin D restores tibial and vertebral cancellous bone loss associated with binge alcohol exposure

Vitamin D restores tibial and vertebral cancellous bone loss associated with binge alcohol exposure

Lipids isolated from bone induce the migration of human breast cancer cells

Bone is the most common site to which breast cancer cells metastasize. We found that osteoblast-like MG63 cells and human bone tissue contain the bile acid salt sodium deoxycholate (DC). MG63 cells take up and accumulate DC from the medium, suggesting that the bone-derived DC originates from serum. DC released from MG63 cells or bone tissue promotes cell survival and induces the migration of metastatic human breast cancer MDA-MB-231 cells. The bile acid receptor farnesoid X receptor (FXR) antagonist Z-guggulsterone prevents the migration of these cells and induces apoptosis. DC increases the gene expression of FXR and induces its translocation to the nucleus of MDA-MB-231 cells. Nuclear translocation of FXR is concurrent with the increase of urokinase-type plasminogen activator (uPA) and the formation of F-actin, two factors critical for the migration of breast cancer cells. Our results suggest a novel mechanism by which DC-induced increase of uPA and binding to the uPA receptor of the same breast cancer cell self-propel its migration and metastasis to the bone.

THE CLASSIC: Quantitative Computed Tomography of Vertebral Spongiosa: A Sensitive Method for Detecting Early Bone Loss after Oophorectomy

THE CLASSIC: Quantitative Computed Tomography of Vertebral Spongiosa: A Sensitive Method for Detecting Early Bone Loss after Oophorectomy

Radiographic and Histologic Findings of Vertebral Augmentation Using Polymethylmethacrylate in the Primate Spine

Animal study. To investigate the gross behavior and the histologic effect(s) of polymethylmethacrylate (PMMA) in primate vertebral bodies after percutaneous vertebroplasty and kyphoplasty. PMMA is known to induce thermal osteonecrosis in exposed bone during tumor surgery and total joint arthroplasty, and barium sulfate as well as PMMA are known to induce a foreign body reaction, but the effects of PMMA on vertebral cancellous bone have not been well documented. Furthermore, little is known about the mechanisms of cement extravasation that occasionally occur during these procedures. Lastly, it is not known if differences exist between vertebroplasty and kyphoplasty with regards to the above occurrences. Six living, elderly, female baboons were used in this study. Seven vertebrae from T12 to L6 were investigated in each animal. Two levels underwent vertebroplasty, two underwent kyphoplasty, and one underwent cavity creation only without cement augmentation in each animal. Three animals were killed at 24 hours and three at 26 weeks after surgery. The specimens were examined macroscopically for cement migration, and histologically for evidence of thermal necrosis, foreign body reaction, cement migration, and intravascular extravasation. Small zones of osteonecrosis were observed histologically at the bone-cement interface in only two kyphoplasty (2 of 6) and two vertebroplasty specimens (2 of 5) at 26 weeks after surgery. All of these necrotic segments of bone were associated with new bone formation. Foreign body reactions were observed in all specimens at 26 weeks after surgery, yet none of them was associated with bone resorption. The incidence of cement leak into the spinal canal and adjacent soft tissues was 6 of 11 and 4 of 11 in vertebroplasty specimens and 5 of 12 and 3 of 12 in kyphoplasty specimens, respectively. There was no statistical difference in the incidence of cement leakage into the spinal canal (P = 0.54) and adjacent soft tissues (P = 0.55) between vertebroplasty and kyphoplasty specimens. The incidence of intravascular cement was significantly greater in vertebroplasty (11 of 12, 91%) than in kyphoplasty (5 of 12, 42%) (P = 0.013). PMMA caused relatively little necrotic exothermal effect. The presence of PMMA with barium sulfate induces a mild foreign body reaction. The use of a balloon tamp creates a void by displacing pieces of bone, which may result in a lower incidence of intravascular cement leak.

Prediction of the apparent elastic modulus and strength of vertebral cancellous bone based on patient-specific apparent morphology

4528 Citation: Journal of Biomechanics 2006; Vol. 39 Suppl. 1, page S5 Prediction of the apparent elastic modulus and strength of vertebral cancellous bone based on patient-specific apparent morphology I. Diamant1, R. Shahar3, Y. Masharawi2, A. Gefen1 1 Department of Biomedical Engineering and 2 Department of Physical Therapy, Tel Aviv University, Israel 3 Faculty of Agriculture, Koret School of Veterinary Medicine, The Hebrew University of Jerusalem, Israel In the context of osteoporotic fractures, the strength and stiffness of cancellous bone are key determinants of the fracture risk. However, standard imaging (DEXA, QCT) cannot measure these mechanical properties directly, and so, an estimation of the risk for fracture is made by correlating the relative density or mineral density at a skeletal site with statistics of fracture occurrence. This method provides limited and partial indications on the fracture risk because of the indirect assessment. Fortunately, this problem can be resolved by means of patient-specific finite element (FE) modeling of cancellous bone, which is expected to support clinical assessment of the fracture risk in the near future (Diamant et al., 2005). We developed an FE parametric model of lattice cancellous bone structures to determine its apparent elastic modulus and strength based on clinically measured morphological/physical parameters: trabecular thickness, trabecular separation and bone mineral density. We further conducted uniaxial compression tests on canine vertebral cancellous bone specimens in order to validate the model predictions of specimen-specific strength and stiffness. Our model predictions of strength and stiffness matched the experimental results from canine bone within errors of 17.7% and 12.8%, respectively. We then applied our model to predict human cancellous bone properties from individual specimens and obtained apparent elastic modulus of 190±50 MPa and strength of 1.7±0.4 MPa for L1 and L5 vertebrae. Overall, the model was shown to be useful for predicting the mechanical properties of individual specimens, which supports its clinical applicability as well as its utility as a research tool to reduce the large number of experiments usually needed for characterizing the mechanical properties of osteoporotic cancellous bone. Reference: Diamant I., Shahar R., Gefen A. (2005). Med Biol Eng Comput 43: 465-472.

Regional Variations in Microstructural Properties of Vertebral Trabeculae With Structural Groups

Micro-computed tomography (CT) scanning to investigate three-dimensional microstructural properties of L4 vertebral bodies. To identify the regional variations in the three-dimensional microstructural properties of vertebral cancellous bones with respect to structural types for the prediction of related regional fracture risks. The literature contains no reports on regional variations in morphologic properties of vertebral trabeculae with microstructural types, which may shed light on the patterns of osteoporotic fractures. Ninety cubic cancellous specimens were obtained from 6 normal L4 vertebral bodies of 6 male donors 62 to 70 years of age and were scanned using a high-resolution micro-CT system. These specimens were further divided into two groups according to the average structure model index (SMI) of the 15 trabecular specimens in each vertebral body. Adjustment for age differences was done for the microstructural parameters, i.e.-, bone volume fraction, trabecular number, trabecular thickness, structure model index, degree of architectural anisotropy, and connectivity density, to allow investigation on the regional variations in different transverse layers and vertical columns independent of age. Trabecular specimens with lower mass were liable to form high-SMI group and the differences in all parameters reached significance level either between columns or between layers from two groups. The anterior column in the high-SMI group is more susceptible to vertebral body wedge fracture; and in the low-SMI group, off-axis bone damage is most harmful to the central column of vertebral trabeculae. The data obtained may help to identify the most critical locations of fracture risks at an early stage and provide a microstructural basis for the repair and clinical treatment of vertebral fractures.

Relationship between CT intensity, micro-architecture and mechanical properties of porcine vertebral cancellous bone

In vivo assessment of bone density is insufficient for the evaluation of osteoporosis in patients. A more complete diagnostic tool for the determination of bone quality is needed. Micro-computed tomography imaging allows a non-destructive method for evaluating cancellous bone micro-architecture. However, lengthened exposure to ionizing radiation prevents patients to be imaged by such a system. The aim for this study was to elucidate the relationships between image intensity (of Hounsfield units), cancellous bone micro-architecture and mechanical properties. Using pig vertebral cancellous bone, the bone specimens were imaged using clinical and micro-computed tomography scanners and subsequently subjected to uniaxial compression testing. Results indicate that micro-architecture can be predicted using clinical image intensity. Micro-architectural parameters relevant to osteoporosis study, such as percent bone volume, trabecular bone pattern factor, structure model index, trabecular thickness and trabecular separation have shown significant correlation with R2 values of 0.83, 0.80, 0.70, 0.72, and 0.54, respectively, when correlated to Hounsfield units. In addition, the correlation of mechanical properties (E, sigma yield, and sigma ult) in the superior-inferior direction (the primary loading direction), to micro-architecture parameters has also been good (R2 > 0.5) for all except tissue volume, tissue surface and degree of anisotropy. This proves that the predictive power of bone strength and stiffness was improved with the combination of bone density and micro-architecture information. This work supports the prediction of micro-architecture using current clinical computed tomography imaging technology.

Vertebral cancellous bone turn-over: Microcallus and bridges in backscatter electron microscopy

Backscatter electron microscopy (BSE) is a powerful technique for investigating cancellous bone structure. Its main function is to offer information regarding the degree of mineralization of the tissue within individual trabeculae. To illustrate the qualitative information that can be drawn from BSE imaging technique, we present a study on human vertebral cancellous bone. This tissue is continuously remodeled through osteoclastic resorption and osteoblastic new bone apposition. It is thought that osteoclastic resorption pits are especially deleterious for vertebral bone architecture since they often perforate the thin trabeculae; the osteoblasts being unable to repair the gap. In addition, excessive stress may also disrupt the architecture in case of trabecular fracture or damage accumulation. Waves of new bone formation were easy to identify in BSE. Often these waves were connecting both edges of a perforation and called bridges. Additionally, we present a few images of microcallus formations. A microcallus is described as a small mass of woven bone that generally repairs a trabecula. The microstructural aspects of different microcalluses are presented and discussed. Both bridges and microcallus should be considered as examples of the repair porcess since they obviously preserve the connectivity of the trabeculae. However, bridges were much more frequent than microcallus (396 vs 15). Both mechanisms probably illustrate the normal response to different local stimuli.

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

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

Biomechanical impact of vertebroplasty

Objectives. – To examine the biomechanisms underlying adjacent fractures following vertebroplasty, an emerging procedure to stabilize fractured vertebrae. In this procedure, bone cement is injected percutaneously into the vertebral cancellous bone. Once hardened, the cement offers mechanical reinforcement to the weakened vertebra. Recent clinical and biomechanical reports suggest that this procedure may cause new fractures adjacent to the one augmented. The cause and extend is unclear yet. The focus here is on the biomechanical hypothesis resulting from the rigid cement augmentation. Methods. – A combination of experimental and numerical studies, in additional to a review of recent clinical reports. Results. – The broader finding suggests that vertebroplasty changes the mechanical loading in adjacent vertebrae. Specifically, an increase in adjacent loading in the range of 17% has been found. The mechanism underlying this increase seemed to stem from the excessive cement rigidity that reduced the endplate bulge of the augmented vertebra, thereby reducing the local spinal joint flexibility. The reduction in joint flexibility seeks to reverse itself by creating an increase in the inter-vertebral disc pressure. The increased disc pressure seeks to relieve itself by increasing the load on the adjacent vertebra. The increased load on the adjacent vertebra relates directly to an increased risk of fracture. Conclusions. – Although an increasing amount of evidence exists to support this theory of the origin of adjacent fractures, one must be cautious. Vertebroplasty is a relatively new procedure and further observations and, ultimately, prospective clinical studies are required to conclusively determine the cause and extend of adjacent fractures.

Trabecular Architecture of Lumbar Vertebral Pedicle

Investigation on architecture of lumbar pedicle. To determine morphological properties of pedicular cancellous bone. Many researchers have been stimulated to study trabecular architecture by improvements in stereological technology. Although the structure of vertebral cancellous bone has been well studied in the literature, no information is available about the architecture of pedicular cancellous bone. Eight cadaveric L3 lumbar vertebrae were harvested. After collecting the bone mineral density (BMD) data on the vertebrae, pedicle isthmuses were removed from the vertebral bodies using a reciprocal hand saw. The BMD measurements were done on the dissected pedicle isthmus specimens. All the specimens were then analyzed using a micro-computed tomography unit. Morphologic parameters of trabecular bone were calculated. Bone volume was found as 0.209 +/- 0.046, whereas Tb.Th, Tb.Sp, and Tb.N were found to be 0.201 +/- 0.035 mm, 0.930 +/- 0.123 mm, and 1.098 +/- 0.136 mm(-1), respectively. Connectivity density and structure model index were observed to be 3.135 +/- 0.918 mm(-3), 0.37, whereas degree of anisotropy value was 1.241 +/- 0.093. Vertebral BMD could explain 63% of variance in bone density of a pedicle isthmus. The structure of the pedicular cancellous bone is somewhat different from that of vertebral body. The trabecular architecture within the pedicle isthmus is isotropic and plate-like. The thickness and number of the trabeculae were greater than those of vertebral trabeculae. Decrease in the bone volume with age is mainly by thinning of the trabeculae and increasing in trabecular spacing, but not by loss of mass.

Weaning Initiates a Rapid and Powerful Anabolic Phase in the Rat Maternal Skeleton1

Maternal skeletal mineral lost during lactation is rapidly restored after weaning. The purposes of this study were to determine when increases of bone formation occur after weaning, whether the expanding osteoblast population is derived from proliferating progenitors, and to relate these skeletal changes to known endocrine events at weaning. Female rats were allowed to complete one reproductive cycle. Half of these rats were mated a second time and allowed to lactate for 20 days. The other half served as an age-matched, normal estrus cycling comparison group. One day after weaning, the dams and their comparison group were given four injections of bromodeoxyuridine (BrdU) at 8-h intervals. Indices of bone formation and the kinetics of BrdU-labeled cells were measured in lumbar vertebral cancellous bone. At 2 days after weaning, cancellous bone formation rates were substantially greater than those in the nonmated rats. Indices of bone formation more than doubled from the second to seventh day after weaning. At 25 h after the first BrdU injection in the postweaned rats, considerable numbers of labeled cells were observed on or near the bone surface, with about 17% of the osteoblast population labeled. Labeled osteoblasts peaked at 20%-24% compared with 4% in the normal estrus cycling group. Immediately following weaning, there is a profound increase in the osteoblast population in maternal cancellous bone. Many, if not most of these newly formed osteoblasts were derived from proliferating progenitors. It is possible that the endocrine milieu of lactation expands or primes the osteoprogenitor pool for this rapid anabolic phase.

Effects of Vitamin K 1 on fluoride-induced bone changes in growing rats: A histomorphometric and radiodensitometric study

The aim of this study was to investigate the potential effects of Vitamin K(1) supplementation on skeletal changes related to fluoride in growing rats. Forty male Wistar rats aged 4 weeks were assigned at random into three groups: high-dose fluoride (125 ppm) group; high-dose fluoride+Vitamin K(1) (0.2mg/(g day)) group; and a control group. The experimental period was 12 weeks. The L(3) vertebrae and the right tibiae were removed, and specimens were analysed by histologic and histomorphometric methods. Quantitative radiodensitometry was also employed to assess the differences in bone mineral density (BMD) between the groups. In the tibia, total tissue area was higher in the study groups than the control group (P<0.05). Cortical bone area was slightly higher in the fluoride+K(1) group than the fluoride group, and marrow cavity area was lower in the fluoride+K(1) group (P<0.05). In the L3 vertebral cancellous bone, bone volume, trabecular number and trabecular thickness were higher in the study groups than the control group (P<0.05). Trabecular separation was reduced in the study groups (P<0.05), and was lower in the fluoride+K(1) group than the fluoride group (P<0.05). The fluoride+K(1) group had a significantly higher BMD than the other groups (P<0.05), and the fluoride group had a significantly higher BMD than the control group (P<0.05). The present study found that fluoride administration increased bone mass in both vertebrae and tibiae in growing rats. Simultaneous administration of Vitamin K(1) and fluoride resulted in an additional increase in vertebral bone mass.

Regional variations in microstructural properties of vertebral trabeculae with aging

The aim of this study was to identify regional variations in the three-dimensional microstructure of vertebral cancellous bones, and their relative differences with respect to aging. Ninety trabecular specimens were obtained from six normal L4 vertebral bodies of six male cadaver donors in two age groups, three aged 62 years and three aged 69 years; (n=45, each). In each vertebral body, five trabecular columns, each of 8x8x25 mm3, were cut from the anterior, posterior, central, right, and left regions. These columns were scanned, using high-resolution micro-computed tomography (microCT), three times, to obtain superior, middle, and inferior layers. Fifteen regions were obtained for each vertebral body. For all 90 trabecular specimens the bone volume fraction (BV/TV), trabecular number (Tb.N), and trabecular thickness (Tb.Th), as well as the three radii of the mean intercept length (MIL) ellipsoid (H1, H2, and H3) were determined. Regional variations in different transverse layers and vertical columns within and between the two age groups were then analyzed. The results showed significant differences in BV/TV, Tb.N, DA, and H2/H3 between the two age groups. The BV/TV and Tb.N were decreased, while the anisotropic parameters were increased significantly with age, increasing from age 62 to 69. Change in Tb.Th was not statistically significant, although the average was slightly smaller in the 69-year group. Each microstructural parameter followed its own pattern of regional variation within each group, suggesting both mechanical and age-related adaptation. This is the first study that has provided microstructural data of the vertebral body in a Chinese sample. These data may help us to gain more insight into the mechanism of the occurrence of lumbar osteoporosis and the related regional fracture risks, and may provide a reference for better enhancement of fracture repair.

Sexual dimorphism and age dependence of osteocyte lacunar density for human vertebral cancellous bone

The sexual dimorphism in age-related loss of human vertebral cancellous bone is not fully understood and could be related to dimorphism in the bone cell populations. The objective of this study was to investigate age- and gender-related differences in the osteocyte population and its relationship with bone volume fraction for human vertebral cancellous bone. Histomorphometric techniques were used to quantify osteocyte lacunae (a measure of osteocyte population) and bone volume fraction in male and female human T12 vertebrae, the most common site of vertebral fracture. Two measures of osteocyte population [number of osteocytes per bone area (OtLcDn) and number of osteocytes per total area (OtLcN/TA)] and their relationships with age and bone volume fraction were found to be sexually dimorphic. Dimorphism in osteocyte density may explain the dimorphic patterns of bone loss in human vertebrae due to the sensory and signal communication functions that osteocytes perform.

Collagen Content and Organization Relate to Bone Nanomechanical Properties

AbstractCancellous bone plays an important load-bearing role in the skeleton, yet relatively little is known about the microstructure-mechanical property relationships of the tissue at the sub-10 [.proportional]m level. Cancellous tissue is characterized by a layered microstructure with variable proportions of collagen and mineral. The lamellar material is substantially stiffer than the interlamellar material at the nanomechanical level. However, the microstructural origin of the observed differences in mechanical properties of these structures has not been investigated. In this study, second harmonic generation microscopy was used to examine collagen in human vertebral cancellous bone. At the same location in the tissue, nanoindentation was used to assess the indentation modulus of lamellar and interlamellar bone. The stiff lamellae corresponded to areas of highly ordered, collagen-rich material, while the compliant interlamellar regions corresponded to areas of unoriented or collagen-poor material. The lamellar bone was approximately 30% stiffer and contained approximately 50% more oriented collagen than the interlamellar bone. These observed differences in the mechanical properties and collagen content and organization of lamellar and interlamellar tissue are consistent with previous scanning electron microscopy studies showing greater mineral and collagen content and organization in lamellar bone. Given the well-known coupling between collagen and mineral in bone tissue, the mineral distribution may mirror that of the aligned collagen. However, similar measurements of local variations in mineral content are needed to confirm this hypothesis and may provide additional insights into the tissue nanomechanical behavior.

Could the intraosseous fluid in cancellous bone bear external load significantly within the elastic range?

Cancellous bone is a two-phase material comprising a porous solid and a fluid. The intraosseous fluid fills the voids of the porous solid and occupies more than 85 per cent of the volume of cancellous bone. Cancellous bone undergoes various loadings; therefore could the intraosseous fluid in cancellous bone bear external load significantly? To answer this question, a specific experimental setup representing the most restrictive fluid flow boundaries around a bovine vertebral cancellous bone sample was designed. Then, a quasi-static loading was applied up to the strain of 0.6 per cent as the measured intraosseous pressure changed in the undrained and drained conditions. A significant intraosseous pressure was generated in the undrained condition, but no intraosseous pressure generation was generated in the drained condition. The maximum external load-bearing capability of the intraosseous fluid in bovine vertebral cancellous bone at the strain of 0.6 per cent was about 66 per cent of the total load in the experimental setup used in this study.

Microfracture and changes in energy absorption to fracture of young vertebral cancellous bone following physiological fatigue loading.

Fifty-five human thoracolumbar vertebrae were randomly fatigue loaded and analyzed. The purpose of this study was to explore the relationship between fatigue loading, trabecular microfracture, and energy absorption to fracture in human cadaveric thoracolumbar vertebrae. Although trabecular microfractures are found in vivo and have been produced by fatigue loading in vitro, the effect of the level of physiologic fatigue loading on microfracture and energy absorption has not been investigated. Fifty-five human thoracolumbar vertebrae (T11-L4) were randomly divided into 5 groups: 1) control (no loading, n = 6); 2) axial compression to yield (n = 7); and 3-5) 20,000 cycles of fatigue loading at 2 Hz (each n = 14). The level of fatigue loading was determined as a proportion of the yield load of Group 2 as follows: 10% (Group 3), 20% (Group 4), and 30% (Group 5). Half of the specimens in groups 3 to 5 were used for radiographic and histomorphometric analysis to determine microfracture density and distribution, whereas the other half were tested to determine the energy absorption to yield failure. No radiographic evidence of gross fracture was found in any of the groups following fatigue loading. A mean 7.5% increase in stiffness was found in specimens subject to cyclic loading at 10% of yield stress (Group 3). Fatigue at 20% (Group 4) and 30% of yield stress (Group 5) caused significantly higher (P < 0.05) increases in mean stiffness of 23.6% and 24.2%, respectively. Microfracture density increased from 0.46/mm in Group 3 to 0.66/mm in Group 4 and 0.94/mm in Group 5 (P < 0.05). The energy absorbed to failure decreased from 21.9 J in Group 3 to 18.1 J and 19.6 J in Groups 4 and 5, respectively (P < 0.05). Fatigue loading at physiologic levels produced microfractures that are not detectable by radiography. Increased fatigue load results in an increase in microfracture density and decrease energy absorbed to fracture, indicating a reduced resistance to further fatigue loading.