38 research outputs found
A comparison of the physical and chemical differences between cancellous and cortical bovine bone mineral at two ages
To assess possible differences between the mineral phases of cortical and cancellous bone, the structure and composition of isolated bovine mineral crystals from young (1–3 months) and old (4–5 years) postnatal bovine animals were analyzed by a variety of complementary techniques: chemical analyses, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and 31P solid-state magic angle spinning nuclear magnetic resonance spectroscopy (NMR). This combination of methods represents the most complete physicochemical characterization of cancellous and cortical bone mineral completed thus far. Spectra obtained from XRD, FTIR, and 31P NMR all confirmed that the mineral was calcium phosphate in the form of carbonated apatite; however, a crystal maturation process was evident between the young and old and between cancellous and cortical mineral crystals. Two-way analyses of variance showed larger
increases of crystal size and Ca/P ratio for the cortical vs. cancellous bone of 1–3 month than the 4–5 year animals.
The Ca/(P + CO3) remained nearly constant within a given
bone type and in both bone types at 4–5 years. The carbonate
and phosphate FTIR band ratios revealed a decrease of labile ions with age and in cortical, relative to cancellous, bone. Overall, the same aging or maturation trends were observed for young vs. old and cancellous vs. cortical. Based on the larger proportion of newly formed bone in cancellous bone relative to cortical bone, the major differences between the cancellous and cortical mineral crystals must be ascribed to differences in average age of the crystals
A Model for the Ultrastructure of Bone Based on Electron Microscopy of Ion-Milled Sections
The relationship between the mineral component of bone and associated collagen has been a matter of continued dispute. We use transmission electron microscopy (TEM) of cryogenically ion milled sections of fully-mineralized cortical bone to study the spatial and topological relationship between mineral and collagen. We observe that hydroxyapatite (HA) occurs largely as elongated plate-like structures which are external to and oriented parallel to the collagen fibrils. Dark field images suggest that the structures (“mineral structures”) are polycrystalline. They are approximately 5 nm thick, 70 nm wide and several hundred nm long. Using energy-dispersive X-ray analysis we show that approximately 70% of the HA occurs as mineral structures external to the fibrils. The remainder is found constrained to the gap zones. Comparative studies of other species suggest that this structural motif is ubiquitous in all vertebrates
Molecular mechanics of mineralized collagen fibrils in bone
Bone is a natural composite of collagen protein and the mineral hydroxyapatite. The structure of bone is known to be important to its load-bearing characteristics, but relatively little is known about this structure or the mechanism that govern deformation at the molecular scale. Here we perform full-atomistic calculations of the three-dimensional molecular structure of a mineralized collagen protein matrix to try to better understand its mechanical characteristics under tensile loading at various mineral densities. We find that as the mineral density increases, the tensile modulus of the network increases monotonically and well beyond that of pure collagen fibrils. Our results suggest that the mineral crystals within this network bears up to four times the stress of the collagen fibrils, whereas the collagen is predominantly responsible for the material’s deformation response. These findings reveal the mechanism by which bone is able to achieve superior energy dissipation and fracture resistance characteristics beyond its individual constituents.United States. Office of Naval Research (N000141010562)United States. Army Research Office (W991NF-09-1-0541)United States. Army Research Office (W911NF-10-1-0127)National Science Foundation (U.S.) (CMMI-0642545
Intermolecular channels direct crystal orientation in mineralized collagen
The mineralized collagen fibril is the basic building block of bone, and is commonly pictured as a parallel array of ultrathin carbonated hydroxyapatite (HAp) platelets distributed throughout the collagen. This orientation is often attributed to an epitaxial relationship between the HAp and collagen molecules inside 2D voids within the fibril. Although recent studies have questioned this model, the structural relationship between the collagen matrix and HAp, and the mechanisms by which collagen directs mineralization remain unclear. Here, we use XRD to reveal that the voids in the collagen are in fact cylindrical pores with diameters of ~2 nm, while electron microscopy shows that the HAp crystals in bone are only uniaxially oriented with respect to the collagen. From in vitro mineralization studies with HAp, CaCO3 and γ-FeOOH we conclude that confinement within these pores, together with the anisotropic growth of HAp, dictates the orientation of HAp crystals within the collagen fibril
Characterisation of time-dependent mechanical behaviour of trabecular bone and its constituents
Trabecular bone is a porous composite material which consists of a mineral
phase (mainly hydroxyapatite), organic phase (mostly type I collagen) and water
assembled into a complex, hierarchical structure. In biomechanical modelling,
its mechanical response to loads is generally assumed to be instantaneous,
i.e. it is treated as a time-independent material. It is, however, recognised
that the response of trabecular bone to loads is time-dependent. Study
of this time-dependent behaviour is important in several contexts such as: to
understand energy dissipation ability of bone; to understand the age-related
non-traumatic fractures; to predict implant loosening due to cyclic loading; to
understand progressive vertebral deformity; and for pre-clinical evaluation of
total joint replacement.
To investigate time-dependent behaviour, bovine trabecular bone samples
were subjected to compressive loading, creep, unloading and recovery at multiple
load levels (corresponding to apparent strain of 2,000-25,000 με). The
results show that: the time-dependent behaviour of trabecular bone comprises
of both recoverable and irrecoverable strains; the strain response is nonlinearly
related to applied load levels; and the response is associated with bone volume
fraction. It was found that bone with low porosity demonstrates elastic
stiffening followed by elastic softening, while elastic softening is demonstrated
by porous bone at relatively low loads. Linear, nonlinear viscoelastic and nonlinear
viscoelastic-viscoplastic constitutive models were developed to predict
trabecular bone’s time-dependent behaviour. Nonlinear viscoelastic constitutive model was found to predict the recovery behaviour well, while nonlinear
viscoelastic-viscoplastic model predicts the full creep-recovery behaviour reasonably
well. Depending on the requirements all these models can be used to
incorporate time-dependent behaviour in finite element models.
To evaluate the contribution of the key constituents of trabecular bone and
its microstructure, tests were conducted on demineralised and deproteinised
samples. Reversed cyclic loading experiments (tension to compression) were
conducted on demineralised trabecular bone samples. It was found that demineralised
bone exhibits asymmetric mechanical response - elastic stiffening
in tension and softening in compression. This tension to compression transition
was found to be smooth. Tensile multiple-load-creep-unload-recovery experiments
on demineralised trabecular samples show irrecoverable strain (or
residual strain) even at the low stress levels. Demineralised trabecular bone
samples demonstrate elastic stiffening with increasing load levels in tension,
and their time-dependent behaviour is nonlinear with respect to applied loads .
Nonlinear viscoelastic constitutive model was developed which can predict its
recovery behaviour well. Experiments on deproteinised samples showed that
their modulus and strength are reasonably well related to bone volume fraction.
The study considers an application of time-dependent behaviour of trabecular
bone. Time-dependent properties are assigned to trabecular bone in a
bone-screw system, in which the screw is subjected to cyclic loading. It is
found that separation between bone and the screw at the interface can increase
with increasing number of cycles which can accentuate loosening. The
relative larger deformation occurs when this system to be loaded at the higher
loading frequency. The deformation at the bone-screw interface is related to
trabecular bone’s bone volume fraction; screws in a more porous bone are at
a higher risk of loosening
A new era for understanding amyloid structures and disease
The aggregation of proteins into amyloid fibrils and their deposition into plaques and intracellular inclusions is the hallmark of amyloid disease. The accumulation and deposition of amyloid fibrils, collectively known as amyloidosis, is associated with many pathological conditions that can be associated with ageing, such as Alzheimer disease, Parkinson disease, type II diabetes and dialysis-related amyloidosis. However, elucidation of the atomic structure of amyloid fibrils formed from their intact protein precursors and how fibril formation relates to disease has remained elusive. Recent advances in structural biology techniques, including cryo-electron microscopy and solid-state NMR spectroscopy, have finally broken this impasse. The first near-atomic-resolution structures of amyloid fibrils formed in vitro, seeded from plaque material and analysed directly ex vivo are now available. The results reveal cross-β structures that are far more intricate than anticipated. Here, we describe these structures, highlighting their similarities and differences, and the basis for their toxicity. We discuss how amyloid structure may affect the ability of fibrils to spread to different sites in the cell and between organisms in a prion-like manner, along with their roles in disease. These molecular insights will aid in understanding the development and spread of amyloid diseases and are inspiring new strategies for therapeutic intervention