Age-related changes in the collagen network and toughness of bone

Diabetes Mellitus: Does it Affect Bone?

Both diabetes and fractures affect a large proportion of older adults. Recent cohort studies indicate that diabetes itself is associated with increased risk of fracture of the hip, proximal humerus, and foot. Observational studies and animal models suggest that decreased bone strength in diabetes may contribute to fracture risk but this remains a controversial issue. Type 1 diabetes is associated with modest reductions in bone mineral density (BMD) but type 2 diabetes is often characterized by elevated BMD. This paradox of higher BMD but increased fracture risk in type 2 diabetes may be explained by a combination of more frequent falls and poorer bone quality. Diabetes can impact bone through multiple pathways, some with contradictory effects, including obesity, changes in insulin levels, higher concentrations of advanced glycation end products in collagen, hypercalciuria associated with glycosuria, reduced renal function, lower insulin-like growth factor-I, microangiopathy, and inflammation. A better understanding of how diabetes metabolism and treatments affect bone would improve fracture prevention efforts in older diabetic adults.

Tensile damage and its effects on cortical bone

Plexiform bovine bone samples are repeatedly loaded in tension along their longitudinal axis. In order to induce damage in the bone tissue, bone samples are loaded past their yield point. Half of the bone samples from the damaged group were stored in saline to allow for viscoelastic recovery while the others were decalcified. Tensile tests were conducted on these samples to characterize the effects of damage on the mechanical behavior of the organic matrix (decalcified samples) as well as on bone tissue (stored in saline). The ultimate strain of the damaged decalcified bone is 29% higher compared to that of non-damaged decalcified (control) bone. The ultimate stresses as well as the elastic moduli are similar in both decalcified groups. This phenomenon is also observed in other collagenous tissue (tendon and ligament). This may suggest that damage in bone is caused by shear failure of the organic matrix; transverse separation of the collagen molecules or microfibrils from each other. In contrast, there is a trend towards lowered ultimate strains in damaged bone, which is soaked in saline, with respect to control bone samples (not damaged). The damaged bone tissue exhibits a bi-linear behavior in contrast to the mechanical behavior of non-damaged bone. The initial elastic modulus (below 55 MPa) and ultimate strength of damaged bone are similar to that in non-damaged bone.

Bone development and age-related bone loss in male C57BL/6J mice

The objective of this study was to examine changes in the long bones of male C57BL/6J mice with growth and aging, and to consider the applicability of this animal for use in studying Type II osteoporosis. Male C57BL/6J mice were aged in our colony between 4 and 104 weeks (n=9-15/group). The right femur and humeri were measured for length and subjected to mechanical testing (3-point flexure) and compositional analysis. The left femurs were embedded and thick slices at the mid-diaphysis were assessed for morphology, formation indices, and bone structure. In young mice, rapid growth was marked by substantial increases in bone size, mineral mass, and mechanical properties. Maturity occurred between 12 and 42 weeks of age with the maintenance of bone mass and mechanical properties. From peak levels, mice aged for 104 weeks experienced decreased whole femur mass (12.1 and 18.6% for dry and ash mass, respectively), percentage mineralization (7.4%), diminished whole bone stiffness (29.2%), energy to fracture (51.8%), and decreased cortical thickness (20.1%). Indices of surface-based formation decreased rapidly from the onset of the study. However, the periosteal perimeter and, consequently, the cross-sectional moments of inertia continued to increase through 104 weeks, thus maintaining structural properties. This compensated for cortical thinning and increased brittleness due to decreased mineralization and stiffness. The shape of the mid-diaphysis became increasingly less elliptical in aged mice, and endocortical resorption and evidence of subsequent formation were present in 20-50% of femurs aged > or =78 weeks. This, combined with the appearance of excessive endocortical resorption after 52 weeks, indicated a shift in normal mechanisms regulating bone shape and location, and was suggestive of remodeling. The pattern of bone loss at the femoral mid-diaphysis in this study is markedly similar to that seen in cortical bone in the human femoral neck in Type II osteoporosis. This study has thus demonstrated that the male C57BL/6J mouse is a novel and appropriate model for use in studying endogenous, aging-related osteopenia and may be a useful model for the study of Type II osteoporosis.

Bone mineral and collagen quality in humeri of ovariectomized cynomolgus monkeys given rhPTH(1-34) for 18 months

A recent study of ovariectomized monkeys, treated with recombinant human parathyroid hormone (rhPTH)(1-34) at 1 or 5 mg/kg/day for 18 months or for 12 months followed by 6 months withdrawal from treatment, showed significant differences in the geometry and histomorphometry of cortical bone of the midshaft humerus. To determine the extent to which the rapid bone turnover and cortical porosity induced by rhPTH(1-34) in ovariectomized monkeys modified mineral content, mineral crystal maturity and collagen maturity (cross-link distribution) in the cortical periosteal and endosteal regions, cross-sections of the cortical bone of the mid-humerus, were examined using Fourier transform infrared imaging (FTIRI). FTIRI analyses demonstrated that rhPTH(1-34) altered bone mineral and collagen properties in a dose-dependent manner. Mineral crystal maturity and collagen cross-link ratio (pyridinoline/dehydro-dihydroxylysinonorleucine) on both endosteal and periosteal surfaces decreased relative to ovariectomized animals, consistent with new bone formation. These changes were partially sustained after withdrawal of the higher dose of rhPTH(1-34), suggesting a prolonged after-effect on bone properties for at least two bone remodeling cycles. In conclusion, treatment of ovariectomized monkeys with rhPTH(1-34) had significant effects on cortical bone mineral-to-matrix ratio, mineral crystal maturity, and collagen cross-link ratio. These were fully reversible when the 1-microg rhPTH(1-34) treatment was withdrawn, but only partially reversed when the 5-microg rhPTH(1-34) dose was withdrawn.

Mechanistic fracture criteria for the failure of human cortical bone

A mechanistic understanding of fracture in human bone is critical to predicting fracture risk associated with age and disease. Despite extensive work, a mechanistic framework for describing how the microstructure affects the failure of bone is lacking. Although micromechanical models incorporating local failure criteria have been developed for metallic and ceramic materials, few such models exist for biological materials. In fact, there is no proof to support the widely held belief that fracture in bone is locally strain-controlled, as for example has been shown for ductile fracture in metallic materials. In the present study, we provide such evidence through a novel series of experiments involving a double-notch-bend geometry, designed to shed light on the nature of the critical failure events in bone. We examine how the propagating crack interacts with the bone microstructure to provide some mechanistic understanding of fracture and to define how properties vary with orientation. It was found that fracture in human cortical bone is consistent with strain-controlled failure, and the influence of microstructure can be described in terms of several toughening mechanisms. We provide estimates of the relative importance of these mechanisms, such as uncracked-ligament bridging.

The contribution of the organic matrix to bone's material properties

Bone is a two-phase porous composite material comprised primarily of collagen and mineral, which together provide its mechanical properties. The contribution of the mineral phase to bone's mechanical properties has dominated scientific thinking. Collagen's role has been underappreciated and not very well studied. However, there is evidence that changes in collagen content, or changes to inter- and intrafibrillar collagen cross-linking, can reduce the energy required to cause bone failure (toughness), and increase fracture risk. Although collagen may have less effect on bone's strength and stiffness than does mineral, it may have a profound effect on bone fragility. Collagen changes that occur with age and reduce bone's toughness may be an important factor in the risk of fracture in older women with low bone mass.