In vivo tibial stiffness is maintained by whole bone morphology and cross-sectional geometry in growing female mice

Whole bone morphology, cortical geometry, and tissue material properties modulate skeletal stresses and strains that in turn influence skeletal physiology and remodeling. Understanding how bone stiffness, the relationship between applied load and tissue strain, is regulated by developmental changes in bone structure and tissue material properties is important in implementing biophysical strategies for promoting healthy bone growth and preventing bone loss. The goal of this study was to relate developmental patterns of in vivo whole bone stiffness to whole bone morphology, cross-sectional geometry, and tissue properties using a mouse axial loading model. We measured in vivo tibial stiffness in three age groups (6, 10, 16 wk old) of female C57Bl/6 mice during cyclic tibial compression. Tibial stiffness was then related to cortical geometry, longitudinal bone curvature, and tissue mineral density using microcomputed tomography (microCT). Tibial stiffness and the stresses induced by axial compression were generally maintained from 6 to 16 wks of age. Growth-related increases in cortical cross-sectional geometry and longitudinal bone curvature had counteracting effects on induced bone stresses and, therefore, maintained tibial stiffness similarly with growth. Tissue mineral density increased slightly from 6 to 16 wks of age, and although the effects of this increase on tibial stiffness were not directly measured, its role in the modulation of whole bone stiffness was likely minor over the age range examined. Thus, whole bone morphology, as characterized by longitudinal curvature, along with cortical geometry, plays an important role in modulating bone stiffness during development and should be considered when evaluating and designing in vivo loading studies and biophysical skeletal therapies.     

An expanded overview of postmenopausal osteoporosis

Why is the incidence of osteoporotic fracture so much higher in women than in men? The dominant medical view holds that the exaggerated skeletal fragility and fracture risk of postmenopausal women solely reflects the loss of bone following withdrawal of endogenous estrogen. Indeed, an enormous amount of research in this area has attempted to understand the rise in fractures after menopause in terms of the impact of estrogen lack on bone remodeling. Recent insights suggest that this simple view does not offer an adequate explanation for the greater susceptibility of older women to fracture compared to that of men. It seems more reasonable to view bone health as a lifelong process, reflecting the contributions and influences of myriad events occurring throughout life to skeletal acquisition and maintenance. Only recently has the medical community recognized that the amount of bone present at skeletal maturity makes a powerful contribution to lifelong skeletal status. A second area that must be incorporated into discussions of this topic relates to bone size and geometry. Women's bones are inherently smaller than those of men. A bone's strength is determined by its size as well as by its material properties. In boys, pubertal increases in the cortical thickness of long bones are achieved by (testosterone-dependent) periosteal apposition. By contrast, increased cortical thickness in girls reflects bone expansion into the medullary space, with little or no periosteal apposition, suggesting an inhibitory effect of estrogen on the latter process. Consequently, at skeletal maturity, men have wider bones of greater mechanical competence. Although estrogen is generally held to be skeletally protective, this aspect of its actions may actually render women more susceptible to some fractures. In later life, men may lose even more bone from appendicular sites than do women, but men show much greater concomitant increases in periosteal apposition than women, permitting them to maintain a relatively favorable mechanical profile. These several findings are based on cross-sectional observations of relatively few individuals and therefore require confirmation in prospective longitudinal studies. The degree to which gender-related differences in later life skeletal adaptation reflects a bone's mechanical or metabolic environment has been frequently discussed but still awaits experimental confirmation.     

Influence of geometrical personalization on the simulation of clavicle fractures

Finite element body models enable the evaluation of car occupant protection. In general, these models represent average males and inter-individual geometry variability is not taken into account. As the most frequent shoulder injury during car lateral accidents is a clavicle fracture, the purpose of this study is to investigate whether clavicle geometry has an influence on bone response until failure, and whether geometrical personalization of clavicle models is required. Eighteen clavicles from 9 subjects (5 males and 4 females, mean age: 76 +/- 12 years) were harvested. Six clavicles were scanned, enabling the development of subject-specific models and the quantification of geometrical features defining shape and cortical thickness. Bone mineral densities (BMD) were measured through double X-ray absorptiometry. Then, the general clavicle responses to dynamic compression until failure were studied. Simulations of the compression tests were carried out with the subject-specific models to assess the sensitivity of force-deflection clavicle responses to geometrical features. Clavicle fractures occurred at an average velocity of 1.41 +/- 0.4 ms(-1), with a fracture force of 1.48 +/- 0.46 kN and a deflection of 5.4 +/- 1.1 mm. A significant difference was found between male and female clavicle force values at rupture although their BMDs were not significantly different. Simulations with subject-specific models led to the conclusion that cortical bone thickness and bone shape have large effects on bone responses until failure and on fracture location. This study highlights the need for a geometrical personalization of clavicle models in order to take into account both gender discrepancies concerning clavicle shape and aging effects affecting cortical thickness.     

Allometry in the distribution of material properties and geometry of the felid skull: why larger species may need to change and how they may achieve it

Extant members of the cat family (Felidae) have been considered behaviourally and morphologically conservative, i.e., despite great differences in size, there is relatively little variation in either the shape of the felid skull and dentition across species, or in the way in which these structures are used to kill and dismember prey. Consequently felids have been considered an appropriate focus for a number of investigations into the influence of allometry on craniomandibular mechanics and morphology. However, although previous treatments have considered the role of shape, they have not investigated the influence of differences in the distribution of relatively stiff cortical and more compliant cancellous bone on performance. Here, using models that incorporate material properties for both cortical and cancellous bone, we apply three-dimensional (3D) finite element analysis (FEA) to models representing the skulls of seven extant felid species. Our objectives being to determine allometric trends regarding both overall geometry and the relative distributions of cortical and cancellous bone tissue. We also more comprehensively assess variation in the efficiency with which muscular force is converted to bite force and the capacity to resist associated stresses. Our results show that the cheetah (Acinonyx jubatus) may be exceptional regarding both the efficiency with which muscular force is converted to bite force and the distribution of stress. We found a negative allometric trend between cortical bone volume and total skull bone volume, and positive allometry between the total skull bone volume and skull surface area. Results gained from mathematical modelling of beam analogies suggest that these trends reflect a need for larger species to respond to physical challenges associated with increased size, and, that changes in skull shape, bone composition, or a combination of both may be required to accommodate these challenges. With geometrical scaling stress increases by the same factor, and displacement by the same factor squared, but the ultimate failure stress of the material is invariant. We find that as species become larger, overall skull bone volume relative to surface area increases by adding a higher proportion of less dense and more compliant cancellous bone. This results in an increased cross-sectional area and second moment of inertia, which acts to reduce the overall stresses. An overall saving in mass is a likely additional consequence. Although we do find evidence that skull stiffness does diminish with size, we also argue that this is at least in part mitigated through the influence of these allometric trends. We further suggest that these trends and the explanations for them may be universal for vertebrates.     

Material heterogeneity,microstructure, and microcracks demonstrate differential influence on crack initiation and propagation in cortical bone

The recent studies have shown that long-term bisphosphonate use may result in a number of mechanical alterations in the bone tissue including a reduction in compositional heterogeneity and an increase in microcrack density. There are limited number of experimental and computational studies in the literature that evaluated how these modifications affect crack initiation and propagation in cortical bone. Therefore, in this study, the entire crack growth process including initiation and propagation was simulated at the microscale by using the cohesive extended finite element method. Models with homogeneous and heterogeneous material properties (represented at the microscale capturing the variability in material property values and their distribution) as well as different microcrack density and microstructure were compared. The results showed that initiation fracture resistance was higher in models with homogeneous material properties compared to heterogeneous ones, whereas an opposite trend was observed in propagation fracture resistance. The increase in material heterogeneity level up to 10 different material property sets increased the propagation fracture resistance beyond which a decrease was observed while still remaining higher than the homogeneous material distribution. The simulation results also showed that the total osteonal area influenced crack propagation and the local osteonal area near the initial crack affected the crack initiation behavior. In addition, the initiation fracture resistance was higher in models representing bisphosphonate treated bone (low material heterogeneity, high microcrack density) compared to untreated bone models (high material heterogeneity, low microcrack density), whereas an opposite trend was observed at later stages of crack growth. In summary, the results demonstrated that tissue material heterogeneity, microstructure, and microcrack density influenced crack initiation and propagation differently. The findings also elucidate how possible modifications in material heterogeneity and microcrack density due to bisphosphonate treatment may influence the initiation and propagation fracture resistance of cortical bone.     

Atypical subtrochanteric femoral shaft fractures: role for mechanics and bone quality

Bisphosphonates are highly effective agents for reducing osteoporotic fractures in women and men, decreasing fracture incidence at the hip and spine up to 50%. In a small subset of patients, however, these agents have recently been associated with ''atypical femoral fractures'' (AFFs) in the subtrochanteric region or the diaphysis. These fractures have several atypical characteristics, including occurrence with minimal trauma; younger age than typical osteoporotic fractures; occurrence at cortical, rather than cancellous sites; early radiographic appearance similar to that of a stress fracture; transverse fracture pattern rather than the familiar spiral or transverse-oblique morphologies; initiation on the lateral cortex; and high risk of fracture on the contralateral side, at the same location as the initial fracture. Fracture is a mechanical phenomenon that occurs when the loads applied to a structure such as a long bone exceed its load-bearing capacity, either due to a single catastrophic overload (traumatic failure) or as a result of accumulated damage and crack propagation at sub-failure loads (fatigue failure). The association of AFFs with no or minimal trauma suggests a fatigue-based mechanism that depends on cortical cross-sectional geometry and tissue material properties. In the case of AFFs, bisphosphonate treatment may alter cortical tissue properties, as these agents are known to alter bone remodeling. This review discusses the use of bisphosphonates, their effects on bone remodeling, mechanics and tissue composition, their significance as an effective therapy for osteoporosis, and why these agents may increase fracture risk in a small population of patients.     

Bone density and risk of hip fracture in men and women: cross sectional analysis.

OBJECTIVE: To determine the relative contribution of decline in bone density to the increase in risk of hip fracture with age in men and women. DESIGN: Incidence data of hip fracture from the general population were combined with the bone density distribution in a sample from the same population and with a risk estimate of low bone density known from literature. SETTING: The Netherlands. SUBJECTS: All people with a hospital admission for a hip fracture in 1993, and bone density measured in a sample of 581.4 men and women aged 55 years and over in a district of Rotterdam. MAIN OUTCOME MEASURE: One year cumulative risk of hip fracture by age, sex, and bone density measured at the femoral neck. RESULTS: A quarter of all hip fractures occurred in men. Men reached the same incidence as women at five years older. Controlled for age, the risk of hip fracture by bone density was similar in men and women. The risk of hip fracture increased 13-fold from age 60 to 80; decrease in bone density associated with age contributed 1.9 (95% confidence interval 1.5 to 2.4) in women and 1.6 (1.3 to 1.8) in men. CONCLUSIONS: The risk of hip fracture by age and bone density is similar in men and women. The decrease in bone density associated with age makes a limited contribution to the exponential increase of the risk of hip fracture with age.     

A biomechanical study of periacetabular defects and cement filling

Periacetabular bone metastases cause severe pain and functional disability in cancer patients. Percutaneous acetabuloplasty (PCA) is a minimally invasive, image-guided procedure whereby cement is injected into lesion sites. Pain relief and functional restoration have been observed clinically; however, neither the biomechanical consequences of the lesions nor the effectiveness of the PCA technique are well understood. The objective of this study was to investigate how periacetabular lesion size, cortex involvement, and cement modulus affect pelvic bone stresses and strains under single-legged stance loading. Experiments were performed on a male cadaver pelvis under conditions of intact, periacetabular defect, and cement-filling with surface strains recorded at three strain gage locations. The experimental data were then employed to validate three-dimensional finite element models of the same pelvis, developed using computed tomography data. The models demonstrated that increases in cortical stresses were highest along the posterior column of the acetabulum, adjacent to the defect. Cortical stresses were more profoundly affected in the presence of transcortical defects, as compared to those involving only trabecular bone. Cement filling with a modulus of 2.2 GPa was shown to restore cortical stresses to near intact values, while a decrease in cement modulus due to inclusion of BaSO(4) reduced the restorative effect. Peak acetabular contact pressures increased less than 15% for all simulated defect conditions; however, the contact stresses were reduced to levels below intact in the presence of either cement filling. These results suggest that periacetabular defects may increase the vulnerability of the pelvis to fracture depending on size and cortical involvement and that PCA filling may lower the risk of periacetabular fractures.     

Experimental and probabilistic analysis of distal femoral periprosthetic fracture: a comparison of locking plate and intramedullary nail fixation. Part B: probabilistic investigation

The following is Part B of a two-part study. Part A evaluated, biomechanically, intramedullary (IM) nails versus locking plates for fixation of an extra-articular, metaphyseal wedge fracture in synthetic osteoporotic bone. Part B of this study introduces deterministic finite element (FE) models of each construct type in synthetic osteoporotic bone and investigates the probability of periprosthetic fracture of the locking plate compared with the retrograde IM nail using Monte Carlo simulation. Deterministic FE models of the fractured femur implanted with IM nail and locking plate, respectively, were developed and validated using experimental data presented in Part A of this study. The models were validated by comparing the load-displacement curve of the experimental data with the load-displacement curve of the FE simulation with a root-mean square error of less than 3?mm. The validated FE models were then modified by defining the cortical and cancellous bone modulus of elasticity as uncertain variables that could be assumed to vary randomly. Monte Carlo simulation was used to evaluate the probability of fracture (POF) of each fixation. The POF represents the cumulative probability that the predicted shear stresses in the cortical bone will exceed the expected shear strength of the cortical bone. This investigation provides information regarding the significance of post-operative damage accumulation on the POF of the implanted bones when the two fixations are used. The probabilistic analysis found the locking plate fixation to have a higher POF than the IM nail fixation under the applied loading conditions (locking plate 21.8% versus IM nail 0.019%).     

Fracture toughness and fatigue crack propagation rate of short fiber reinforced epoxy composites for analogue cortical bone

Third-generation mechanical analogue bone models and synthetic analogue cortical bone materials manufactured by Pacific Research Laboratories, Inc. (PRL) are popular tools for use in mechanical testing of various orthopedic implants and biomaterials. A major issue with these models is that the current third-generation epoxy-short fiberglass based composite used as the cortical bone substitute is prone to crack formation and failure in fatigue or repeated quasistatic loading of the model. The purpose of the present study was to compare the tensile and fracture mechanics properties of the current baseline (established PRL "third-generation" E-glass-fiber-epoxy) composite analogue for cortical bone to a new composite material formulation proposed for use as an enhanced fourth-generation cortical bone analogue material. Standard tensile, plane strain fracture toughness, and fatigue crack propagation rate tests were performed on both the third- and fourth-generation composite material formulations using standard ASTM test techniques. Injection molding techniques were used to create random fiber orientation in all test specimens. Standard dog-bone style tensile specimens were tested to obtain ultimate tensile strength and stiffness. Compact tension fracture toughness specimens were utilized to determine plane strain fracture toughness values. Reduced thickness compact tension specimens were also used to determine fatigue crack propagation rate behavior for the two material groups. Literature values for the same parameters for human cortical bone were compared to results from the third- and fourth-generation cortical analogue bone materials. Tensile properties of the fourth-generation material were closer to that of average human cortical bone than the third-generation material. Fracture toughness was significantly increased by 48% in the fourth-generation composite as compared to the third-generation analogue bone. The threshold stress intensity to propagate the crack was much higher for the fourth-generation material than for the third-generation composite. Even at the higher stress intensity threshold, the fatigue crack propagation rate was significantly decreased in the fourth-generation composite compared to the third-generation composite. These results indicate that the bone analogue models made from the fourth-generation analogue cortical bone material may exhibit better performance in fracture and longer fatigue lives than similar models made of third-generation analogue cortical bone material. Further fatigue testing of the new composite material in clinically relevant use of bone models is still required for verification of these results. Biomechanical test models using the superior fourth-generation cortical analogue material are currently in development.     

Skeletal development of non-institutionalized children with low Intelligence Quotients

Children of both sexes with mild mental retardation show delays in skeletal maturation and cortical bone growth, but boys in general are more affected than girls. Skeletal age for both sexes is below normal in the younger age groups, but approaches normality at adolescence. None of the measures of IQ is significantly correlated with any of the regional measures of skeletal age in the hand-wrist. The approach toward normality of skeletal development in this population at adolescence is reflected in their normal time of first appearance of the adductor sesamoid. Anomalies of the hand-wrist reflect irregularities in comparing normal and low-IQ children. Only brachymesophalangy V displays significantly higher frequencies in the low-IQ groups. Due to the cross-sectional nature of the study and the different methods of classifying anomalies, little can be said about the other comparisons. Growth in cortical bone reflects a consistent trend for both the humerus and second metacarpal. While measures of total diameter and total cross-sectional area do appear to be below normal standards, the measures of the marrow do not. This finding accounts for a decreased cortical bone thickness and cross-sectional area in the low-IQ sample. All measures of the humerus correlate significantly with two IQ scores in the boys and all three scores in the girls. Measures of the total metacarpal and marrow area show significant correlations with the three IQ scores among low-IQ boys, while only measures of total metacarpal area show significant correlations with verbal IQ in low-IQ girls. The degree of mental deficiency, then, is only irregularly associated with deficits of bone size for age.     

Clinical Significance and Pathogenesis of Osteoporosis

The development of osteoporosis with advancing age in man is a widespread if not a universal phenomenon. The average loss between youth and old age amounts to about 15% of the skeleton but involves a much larger proportion of trabecular than of cortical bone.The principal clinical manifestation of osteoporosis is fracture, and three osteoporotic fracture syndromes can be defined: the lower forearm fracture, which predominantly affects women between the ages of 50 and 65; the fracture of the proximal femur, which affects both sexes over the age of 70; and the relatively rare vertebral crush fracture syndrome, which may present at any age but is most common in elderly women.The lower forearm fracture rate is inversely related to the mean normal lower forearm x-ray “density” of the wrist, which falls by about 30% in the 15 years following the menopause. This process, which is associated with corresponding trabecular bone loss elsewhere in the skeleton, is associated with a corresponding rise in the fasting urinary calcium excretion. Some degree of negative calcium balance, and consequent bone resorption, probably occurs in everyone during the later part of the night because calcium absorption is completed within about three to five hours of a meal. In postmenopausal women, however, the sensitivity of the bone to parathyroid hormone appears to be increased, and their nocturnal negative calcium balance therefore comes to exceed the positive balance which can be achieved during the waking hours.Femoral neck fractures in old people reflect the further progression of osteoporosis with advancing age since the fracture rate is inversely correlated with the mean thickness of the metacarpal cortex in the normal population. This progressive osteoporosis is associated with and could well result from a steady decline in calcium absorption which is at least partially attributable to vitamin-D deficiency and reversible on vitamin-D treatment.The vertebral crush fracture syndrome represents a severe degree of spinal osteoporosis which may be associated with relatively normal peripheral bones. It probably results from an accelerated negative calcium balance which mobilizes trabecular bone preferentially. Some of the factors which may contribute to this accelerated negative balance have been identified and include a reduced rate of bone turnover, impaired calcium absorption, and low oestrogen activity as judged by vaginal smears, but there may well be others as yet unidentified.     

Age-dependent cortical bone loss in women from 18th and early 19th century London

Age-dependent cortical bone loss was investigated in an earlier British population. The study sample comprised female skeletons from the 18th/19th century crypt at Christ Church, Spitalfields, London. Bone loss was monitored using metacarpal radiogrammetry. Age at death was known exactly from coffin plates. Results indicated that peak cortical thickness was less than in modern subjects. Continuing periosteal apposition was evident throughout adulthood, and the rate of increase in metacarpal diameter resembled that in modern subjects. Bone loss from the endosteal surface was evident from the fifth decade onwards, and this outstripped the rate of subperiosteal gain so that there was a net loss of cortical bone with age. Cortical bone loss occurred at a similar rate to that in modern subjects. In contrast to modern populations, there was no evidence that loss of cortical bone was associated with increased propensity to fracture. The present results, together with those previously published for a British medieval skeletal assemblage, suggest that patterns of cortical bone loss in women have remained unchanged over at least the last millennium in Britain. Given the great changes in lifestyle which have occurred during this period, this suggests that lifestyle factors may be rather less important than is sometimes asserted in influencing the severity of osteoporosis, at least as far as loss of cortical bone is concerned.     

France Verus Wales

Assessment of roentgenographic measurements of cortical bone of the radius in 196 elderly females, including 63 diabetics, revealed that: (1) in the non-diabetic group there was a significant loss of cortical bone relative to the number of years after the menopause and to body weight; (2) although there was a significant loss of cortical bone relative to years postmenopausal in a group of diabetic patients the cortex in the diabetic group was better preserved than in those non-diabetic controls in whom no vertebral compressions were diagnosed in the roentgenograms; no correlation between bone loss and body weight was found among the diabetics; (3) the thinnest cortical bone and the lowest average body weight was found in the 34 non-diabetics with vertebral compression deformities. It thus appears that involutional osteoporosis will be less prevalent among old women suffering from diabetes mellitus than in comparable non-diabetic subjects, and more prevalent among non-diabetics of low body weight than in old women who are obese or of normal weight.     

Cortical bone thickness can adapt locally to muscular loading while changing with age

Mechanical loading of muscle action is concentrated at muscle attachment sites; thus there may be a potential for site-specific variation in cortical bone thickness. Humeri from an early 20th-century Finnish (Helsinki) and two medieval English (Newcastle, Blackgate and York, Barbican) populations were subjected to pQCT scanning to calculate site-specific cross-sectional cortical bone area (CA) for four locations and to measure cortical thickness at muscle attachment sites and non-attachment sites. We found that CA at 80% of humerus length was significantly reduced compared to more distal cross-sections, which can be due to reduced stresses at the proximal shaft. The principal direction of loading at 80% humerus length was towards mediolateral plane, likely due to fixing the humerus close to the torso. At 35% the main direction of loading was towards anteroposterior plane, reflecting elbow flexing forces. The principal direction of loading varied between populations, sides and sexes at 50% humerus length due to preference between elbow and shoulder joint; thus this location might be useful when trying to infer differences in activity. These changes are likely due to overall shaft adaptation to forces acting at the humerus. In addition, we found a potential for site-specific variation in cortical thickness; cortical bone at muscle attachment sites was significantly thicker compared to non-attachment sites. Lastly, CA at 35% of humerus length and cortical thickness at non-attachment sites decreased with age. These results underline the importance of muscle loading for bone mass preservation as well as indicate that a site-specific variation of bone mass is possible.     

Invited Review: Pathogenesis of osteoporosis.

Patients with fragility fractures may have abnormalities in bone structural and material properties such as larger or smaller bone size, fewer and thinner trabeculae, thinned and porous cortices, and tissue mineral content that is either too high or too low. Bone models and remodels throughout life; however, with advancing age, less bone is replaced than was resorbed within each remodeling site. Estrogen deficiency at menopause increases remodeling intensity: a greater proportion of bone is remodeled on its endosteal (inner) surface, and within each of the many sites even more bone is lost as more bone is resorbed while less is replaced, accelerating architectural decay. In men, there is no midlife increase in remodeling. Bone loss within each remodeling site proceeds by reduced bone formation, producing trabecular and cortical thinning. Hypogonadism in 20-30% of elderly men contributes to bone loss. In both sexes, calcium malabsorption and secondary hyperparathyroidism increase remodeling: more bone is removed from an ever-diminishing bone mass. As bone is removed from the endosteal envelope, concurrent bone formation on the periosteal (outer) bone surface during aging partly offsets bone loss and increases bone's cross-sectional area. Periosteal apposition is less in women than in men; therefore, women have more net bone loss because they gain less on the periosteal surface, not because they resorb more on the endosteal surface. More women than men experience fractures because their smaller skeleton incurs greater architectural damage and adapts less by periosteal apposition.     

Age-related changes in porosity and mineralization and in-service damage accumulation

It has been proposed that bone damageability (i.e. bone's susceptibility to formation of damage) increases with the elevation or suppression of bone turnover. Suppression of turnover via bisphosphonates increases local bone mineralization, which theoretically should increase the susceptibility of bone to microcrack formation. Elevation of bone turnover has also been proposed to increase bone microdamage through an increase in bone intracortical porosity and local stresses and strains. The goal of this paper was to investigate the above proposals, i.e., whether or not increases to mineral content and porosity increase bone in-service damageability. To do this, we measured in vivo diffuse damage area (Df.Dm.Ar, %) and microcrack density (Cr.Dn) (cracks/mm(2)) in the same specimen from human cortical bone of the midshaft of the proximal femur obtained from cadavers with an age range of eight decades and examined their relationships with porosity, mineralization and age. Results of this study showed that Cr.Dn and Df.Dm.Ar increased with a decrease in bulk mineralization. This finding does not appear to support the proposal that damage accumulation increases with low bone turnover that results in increases mineralization. It was proposed however that the negative correlation between damage accumulation and mineralization may be attributed to highly mineralized regions of bone existing with under-mineralized regions resulting in an overall decrease in average bone mineralization. It was also found that microdamage accumulates with increasing porosity which does appear to support the proposal that elevated bone turnover that results in increased porosity can accelerate microdamage accumulation. Finally, it was shown that linear microcracks and Df.Dm.Ar accumulate with age differently, but because they correlate with each other, one may be the precursor for the other.     

Modeling of dynamic fracture and damage in two-dimensional trabecular bone microstructures using the cohesive finite element method

Trabecular bone fracture is closely related to the trabecular architecture, microdamage accumulation, and bone tissue properties. Micro-finite-element models have been used to investigate the elastic and yield properties of trabecular bone but have only seen limited application in modeling the microstructure dependent fracture of trabecular bone. In this research, dynamic fracture in two-dimensional (2D) micrographs of ovine (sheep) trabecular bone is modeled using the cohesive finite element method. For this purpose, the bone tissue is modeled as an orthotropic material with the cohesive parameters calculated from the experimental fracture properties of the human cortical bone. Crack propagation analyses are carried out in two different 2D orthogonal sections cut from a three-dimensional 8 mm diameter cylindrical trabecular bone sample. The two sections differ in microstructural features such as area fraction (ratio of the 2D space occupied by bone tissue to the total 2D space), mean trabecula thickness, and connectivity. Analyses focus on understanding the effect of the rate of loading as well as on how the rate variation interacts with the microstructural features to cause anisotropy in microdamage accumulation and in the fracture resistance. Results are analyzed in terms of the dependence of fracture energy dissipation on the microstructural features as well as in terms of the changes in damage and stresses associated with the bone architecture variation. Besides the obvious dependence of the fracture behavior on the rate of loading, it is found that the microstructure strongly influences the fracture properties. The orthogonal section with lesser area fraction, low connectivity, and higher mean trabecula thickness is more resistant to fracture than the section with high area fraction, high connectivity, and lower mean trabecula thickness. In addition, it is found that the trabecular architecture leads to inhomogeneous distribution of damage, irrespective of the symmetry in the applied loading with the fracture of the entire bone section rapidly progressing to bone fragmentation once the accumulated damage in any trabeculae reaches a critical limit.     

Age-related properties at the microscale affect crack propagation in cortical bone

The increased risk for fracture with age is associated not only with reduced bone mass but also with impaired bone quality. At the microscale, bone quality is related to porosity, microstructural organization, accumulated microdamage and intrinsic material properties. However, the link between these characteristics and fracture behavior is still missing. Bone tissue has a complex structure and as age-related compositional and structural changes occur at all hierarchical length scales it is difficult to experimentally identify and discriminate the effect of each mechanism. The aim of this study was therefore to use computational models to analyze how microscale characteristics in terms of porosity, intrinsic toughness properties and microstructural organization affect the mechanical behavior of cortical bone. Tensile tests were simulated using realistic microstructural geometries based on microscopy images of human cortical bone. Crack propagation was modelled using the extended finite element method where cement lines surrounding osteons were modelled with an interface damage law to capture crack deflections along osteon boundaries. Both increased porosity and impaired material integrity resulted in straighter crack paths with cracks penetrating osteons, similar to what is seen experimentally for old cortical bone. However, only the latter predicted a more brittle failure behavior. Furthermore, the local porosity influenced the crack path more than the macroscopic porosity. In conclusion, age-related changes in cortical bone affect the crack path and the mechanical response. However, increased porosity alone was not driving damage in old bone, but instead impaired tissue integrity was required to capture brittle failure in aging bone.     

Cortical bone maintenance and geometry of the tibia in prehistoric children from Nubia's Batn el Hajar

The relationship between advancing age in adults and patterns of cortical bone maintenance has been extensively documented for archaeological populations (Dewey, et al., 1969; Van Gerven et al., 1969; Perzigian, 1973). Most recently, this research has been expanded to include a more thorough consideration of the geometric properties of bone in relationship to adult age changes (Martin and Atkinsin, 1977; Ruff and Hayes, 1983). To date, however, few studies have documented subadult patterns of cortical bone maintenance in archaeological populations and none have incorporated the relationship between patterns of cortical bone loss and gain and the changing geometric properties of growing bone. Using a sample of 172 tibias from children excavated from the Medieval Christian site of Kulubnarti, located in Nubia's Batn el Hajar, the present research examines the relationship between percent cortical area, bone mineral content, and cross-sectional moments of inertia. Among these children, bone mineral content increases steadily from birth in spite of a reduction in percent cortical area during early and late childhood. It appears, therefore, that tissue quality of the bone is not adversely affected by the reduction. Furthermore, the reduction in percent cortical area in later childhood corresponds to a dramatic increase in bending strength measured by cross-sectional moments of inertia. Thus, whether this cortical reduction is due entirely or in part to either normal modeling or nutritional stress, the tissue and organ quality of the bone is not adversely affected.