Age-related bone fractures are a major health concern to the elderly population. In addition to the loss of bone mass, the deterioration of bone quality is another major reason for such fractures. The decline of bone quality is manifested with the accumulation of microdamage in bone with age [1]. Two major types of microdamage have been observed in bone tissue: linear microcracks and diffuse damage [2]. Linear microcracks are individual cracks at a size of microns or larger and are usually visible under an optical microscope. On the other hand, diffuse damage is detectable only by staining and consists of an extensive network of fine, ultrastructural-level defects. The mechanisms for the formation of these two distinct types of microdamage in bone are still unclear. In this study, probabilistic finite element models of mineral-collagen composites were used to study the progression of microdamage in bone, thereby exploring the conditions under which linear microcracks and diffuse damage are produced in bone.

The objective of the present study is to determine to the three-dimensional segment-specific coupling factors from healthy, normal, non-degenerated human cervical spinal columns under bending. Using pure moment loading protocols, cadaver subaxial columns were fixed at the two ends, off-axis forces and moments were recorded using a distal six-axis load cell, and primary and secondary rotations were obtained as a function of applied moments, up to 2.0 Nm. Segmental bone mineral densities were determined using quantitated computed tomography (QCT). Coupling factors were the smallest at the cervico-thoracic spinal segment. Cranial and caudal decreases of the coupling factors occurred from the mid-cervical segment at all applied moment levels, from 65 to 72% at the C4-C5 segment and 59 to 75% at the C5-C6 segment. On average, 55% of the primary rotation was coupled with the secondary rotation across the entire column. Peak coupling occurring in these segments may indicate a greater propensity for age-related changes to these vertebral levels, and offer support to the commonly observed lower cervical degeneration in clinical studies.

Current fracture risk estimates for the spine are based on average measures of bone mineral density (BMD) in the vertebral centrum. However, these measures of BMD explain only ∼60% of the variance in vertebral strength [1] and do not discriminate well between fracture and non-fracture cohorts [2]. These limitations of average BMD are likely due to the heterogeneous distribution of bone tissue throughout the vertebra. The density and architecture of vertebral trabecular bone vary as a function of position in the centrum [3]; moreover, these spatial variations have been shown to vary substantially among individuals and with age [3].

Osteoporosis is an age-related skeletal condition characterized by low bone mineral density and deterioration of the trabecular architecture leading to increased susceptibility to fracture [1]. Wolff hypothesized that trabecular architecture adapts to have its principal material axes aligned with the principal loading directions. Regions of experimentally labeled trabecular microdamage correspond to areas of high stress and strain calculated from FEA [2]. Studying the morphology of numerically predicted regions of tissue level yielding might provide insight into the role of trabecular architecture in the strength of trabecular bone.

The stiffness and strength of a healing bone fracture depend on the geometry of the fracture callus as well as the mechanical properties of the callus tissues. In the clinical setting, the mechanical stability of a healing fracture is often estimated qualitatively based on the radioopacity of the callus tissue. However, a quantitative association between tissue mineralization and mechanical properties has yet to be established for these tissues. Quantifying callus tissue material properties and mineral content may therefore enable improved non-invasive assessments of bone healing. In addition, elucidating relationships between callus tissue composition and function will provide a means of understanding the mechanism by which the injured bone recovers its mechanical integrity.

Bone, a key part of the paleontological and archeological records, can provide insight into the biology, ecology and the environment of ancient vertebrates. Bone is a composite material in which the nanomechanical properties are dependent on the local organic content, mineral content, and microstructural organization. However it is unclear as to how these properties are affected by burial, environmental influences, temperature, or time. The acidity of volcanic soils causes resorption of the bone mineral and may result in demineralization of the bone. As such, very few bone remains are found in volcanic soils and this rare sample can provide insight into the preservation under such extreme conditions. While the effects of volcanic soils on bone are unknown, exposure to hostile environmental conditions increases the potential for dramatic alteration of the mechanical behavior. In this study, a human long bone from around 600 A.D. and a modern human femur were studied using nanoindentation. Testing, performed in both longitudinal and transverse directions, revealed preservation of bone’s natural anisotropy. Additionally, the preserved bone’s lower modulus values suggest the dissolution of bone mineral.

A number of factors across multiple length scales (Table 1) may contribute to the mechanical competence of bone ( i.e. , the resistance to fracture). This structural complexity hinders a complete mechanistic explanation (model) for elderly bone fragility. Although age-related loss of bone mass has been viewed as the reason for the rising incidence of bone fractures in the elderly,( 2 ) bone quality is more meaningful to provide insights into why individuals with ‘normal’ bone mineral density (BMD) suffer fractures. Bone is a hierarchical and hydrated composite of mineral and collagen phases,( 3 , 4 ) showing a rather complex response to the mechanical loading.( 5 , 6 ) While previous research has investigated the resistance of bone to crack propagation,( 7 , 8 ) fatigue behavior,( 9–11 ) and mechanical changes to load-induced damage,( 12–14 ) little is known about the mechanical behavior of bone as a function of progressive deformation (strain). In this study, by employing a previously developed progressive loading scheme,( 15 ) we intend to investigate the progressive stress-strain behavior of bone with increasing strain levels, so as to gain more information on the underlying mechanisms for post-yield behavior of bone. It is hypothesized that the post-yield behavior of bone varies with the increasing strain, which plays a significant role in sustaining the toughness or quality of bone.

Previous studies have shown that bone fracture toughness is also significantly correlated to changes in porosity, microarchitecture, osteonal morphology, collagen integrity, microdamage, and the interactions of water with collagen and mineral phases, all of which are measures of bone quality. Currently, the influence of water removal on the strength and toughness of cortical bone has studied by Nyman et al [1]. The results have shown that loss of water in the collagen phase decreases the toughness of bone, whereas loss of water associated with the mineral phase decreases both bone strength and toughness. However, in that paper the loss of water was used the dehydration method for such mechanism studies. Here, we hypothesize that the NMR relaxation technique can non-destructively and non-invasively assess the porosity, bound water (water bound to collage and mineral phases) and mobile water (water within the pores).

Musculoskeletal injuries are a common cause of pain and disability, and result in significant health care costs. Rotator cuff injuries, which make up the majority of soft tissue injuries to the upper extremity, commonly require surgical repair to the humeral head. At the rotator cuff, for example, the recurrence of tears to surgically repaired tendons has been reported to be as high as 94% [1]. The most dramatic feature of the failed tendon-to-bone healing scenarios is the lack of a transition zone between the tendon and bone that existed before the injury [2].

Bone is a living tissue which is continually adapting to its biological environment via continuous formation and resorption. It is generally accepted that bone remodeling occurs in response to daily mechanical loading. The remodeling process enables various functions, such as damage repair, adaptation to mechanical loads, and mineral homeostasis [1]. The cells that are responsible for the bone remodeling process are the bone resorbing osteoclasts and the bone forming osteoblasts. These cells closely coordinate their actions in a basic multicellular unit to renew “packets” of bone.

Bone mineral density (BMD) has been identified as a major factor in spine construct strength, with failures resulting in pedicle screw loosening and pullout 2 . Computed tomography (CT) scans have been shown to effectively measure BMD 1,4 . Previous research has utilized this linear correlation of CT Hounsfield Units (HU) to BMD in order to determine BMD as a function of anatomic location within cervical vertebrae 1 ; however, the lumbar spine has not yet been reported on. The goal of this study was to describe BMD of anatomical regions within lumbar vertebrae using the correlation between HU and BMD. It was hypothesized that posterior elements of the spine would exhibit significantly different BMD than the vertebral body. This was tested through means comparison of BMD for each anatomical region.

Hip fractures are among the most common types of osteoporotic fractures that affect one in three women and one in six men over the age of 50 [1]. It is well known that loss of bone mass, quantified by bone mineral density using Dual-energy X-ray Absorptiometry (DXA), is associated with the increasing risk of bone fractures. However, bone mineral density (BMD) alone cannot fully explain changes in fracture risks [2, 3]. In particular, BMD is not able to predict fracture risks for women with osteopenia, in which a BMD T-score is between −1.0 and −2.5. This suggests additional factors (i.e., bone quality) should be considered in predicting fracture risks [4].

Bone mineral density gained during adolescent years is a significant determinant of bone quality maintained during adult-hood [1]. The ability to enhance the structural integrity of bone at a young age reduces the risk of bone fracture as well as the development of musculoskeletal diseases, such as osteoporosis [1].

Conditions such as diabetes and obesity have been found to affect the mechanical integrity of bone. Studies have shown that diabetic rodent models exhibit lower levels of new bone formation during fracture healing 1 , lower bone mineral density (BMD) 2 , and increased risk of fracture 3 . There are differences, however, in the bone integrity of bone samples from type I and type II diabetics, which is most likely due to obesity 2 . Findings from research on obesity’s effects on bone integrity have been controversial; although there is an increase in bone mineral density (BMD) with increasing body mass index (BMI) and a decrease in fracture incidence in the central body regions in obese women compared to healthy weight women due to soft tissue padding, there is an increase in fracture incidence at extremeties 4 . Other studies have shown that while cortical bone strength may not be adversely affected by high-fat diets, cancellous bone BMD and mechanical strength was significantly lower in high-fat diet mice than low-fat diet mice 5 . In addition, extreme obesity has been associated with lower BMD despite the general trend of increased BMD with higher BMI 6 .

Tough, graded interfaces are a broadly conserved feature of biological attachment of two dissimilar materials, but the mechanisms by which toughening occurs are not fully known. In biological systems, graded structures are commonly found in the attachments of materials with very different properties such as bone, which is very stiff, and tendon, which is compliant 1 . Previous research has shown that the interface between these two materials is composed mostly of collagen, proteoglycans, and carbonated apatite crystals (mineral). Our working hypothesis is that gradients in mineral content and collagen orientation combine to provide a compliant energy absorbing region between the tendon and bone 1 .

Tendons are connective tissues that transmit loads between bone and muscle. The biological solution to the problem of connecting relatively compliant tendon to stiffer (∼2 orders of magnitude) bone is a gradient interface zone ∼100μm wide. Over the tendon-bone-junction (TBJ) a linear transition takes place in the ECM inorganic:organic (mineral:collagen) ratio as well as mineral crystallinity from that of tendon to bone. While small TBJ injuries can heal via regeneration, severe defects undergo repair-mediated healing characterized by fibrocartilagenous scar tissue with inferior biomechanical and functional properties. Severe TBJ injuries are common in athletes, the elderly, and following severe craniofacial and extremity trauma. Many tendon injuries (i.e. supraspinatus injuries), particularly those associated with acute trauma, are prone to occur at the TBJ due to high levels of region-specific stress concentrations; rotator cuff tendons injuries, one of the most common TBJ injuries, exhibit re-tears at rates as high as 94%. The scale of such defects and current poor clinical results suggest the need for a biomaterial solution that can mimic the dynamic heterogeneities of the native insertion and tendon body to induce rapid, functional regeneration. Three-dimensional collagen-GAG (CG) scaffolds have been successfully used clinically to regenerate large soft tissue defects (skin, peripheral nerves); they act by mimicking the native extracellular matrix (ECM) of the damaged tissue to prevent wound contraction and scar tissue synthesis. However these scaffolds have not traditionally been used for orthopedics due to an inability to recapitulate two critical features of orthopedic tissues: multiscale structural complexity, biomechanical properties.

Bone qualities that are measurable via clinically available modalities and that can explain fracture risk beyond what is explainable by bone mineral density (BMD) are of significant interest. Evidence from literature suggests that the heterogeneity of BMD within a vertebra, in addition to the average BMD, may be an important determinant of the mechanical properties of a vertebra 1–3 and risk of a clinical vertebral fracture 4 . Much of the experimental evidence comes from tests, in which vertebrae are monotonically loaded and relates BMD heterogeneity to the quasi-static properties of a vertebra 1, 3 . The appearance of clinical vertebral fractures is in the form of progressive deformities indicating that fatigue processes are involved. However, the relationships between BMD heterogeneity and fatigue properties of a vertebra are not well-understood.

Mechanical unloading has deleterious effects on the musculoskeletal system, and exercise offers a way to reduce or reverse these effects. Studies of crew members from the International Space Station have documented bone losses that do not fully recover even years after returning to Earth [1,2], and this raises concerns with repeated missions. We have used the adult hindlimb unloaded (HU) rat model [3] to simulate repeated exposure to microgravity, and to study densitometric and mechanical properties at multiple bone sites. This study focuses on mixed bone sites including proximal tibia metaphysis (PTM), and femoral neck (FN) region which also has a significant clinical relevance. Surprisingly, losses for the 2nd HU were milder than those for the 1st HU for bone mineral content (BMC) and volumetric bone mineral density (vBMD), suggesting a possible protective effect of the 1st HU [4]. Comparison to a separate group exposed to a single period of HU initiated at the same age as the 2 nd HU group ruled out age effects contributing to the smaller deficit. BMC and vBMD values returned to baseline but remained below aging cage control (CC) values, raising the question of whether the milder losses for the 2nd HU might be attributable, wholly or partially, to lower levels at the start of the 2nd HU. The goal of the current study was to determine if adding a resistance exercise regimen during recovery from the 1st HU would restore bone properties to CC levels and/or significantly affect the response to the 2nd HU.

In this study, human cortical bone was tested in compression to determine the in situ behavior of mineral crystals using a synergistic approach combining a progressive loading scheme and synchrotron X-ray scattering techniques. By quantifying the orientation distribution of mineral crystals, the average strain tensor of each subset of mineral crystals in the same orientation was determined based on its lattice deformation in three distinct crystallographic directions. The stress tensor of the crystals was determined based on the Hooke’s law using the stiffness tensor well derived in the literature ( 1 ). By examining the concurrent changes in the in situ and bulk behaviors, the contribution of mineral crystals to the bulk behavior of bone was discussed.

Senile osteoporosis has been defined as a skeletal disorder characterized by a deterioration of bone matrix with an increase in susceptibility to fracture with increasing age (1). Studies have shown that over 25 million Americans are currently afflicted with osteoporosis and/or osteopoenia, with this number doubling by 2020. (2) (3) With the propensity for fracture being large in older osteoporotic patients, quantification of bone mineral density (BMD) is essential for proper implant selection in the event of a fracture. Dual energy x-ray absorptiometry (DEXA) is the clinical gold standard for BMD assessment; however, the hardware associated with this technique is neither portable nor appropriate for intraoperative use, and both of these qualities are occasionally necessary clinically.