Cortical Bone Mechanics and Composition: Effects of Age and Gender - Skeletal Aging and Osteoporosis: Biomechanics and Mechanobiology - page 57

Fig. 4 Molecular packing
models of mineralized
collagen fibrils and estimated
mechanical properties
presented by Jäger and
Fratzl [ 46 ]
Transmission electron micrographs of individual mineralized collagen fibrils
show that hydroxyapatite crystals are located mainly at the level of the gap regions
within the fibrils and more or less uniformly stacked across the fibril diameter [ 40 ].
In addition, high-voltage-electron stereomicroscopy reveals that bone mineral
crystals are located within collagen fibrils, suggesting that there exists a local
''bulging'' along the fibrils corresponding to a 68 nm periodicity, which contains
additional mass of minerals [ 44 ]. Moreover, atomic force microscopy (AFM)
analysis verifies that the gap regions in collagen fibrils are indeed filled with mineral
crystals [ 45 ]. In addition to the experimental observations, there are several attempts
in modeling mineralized collagen fibrils [ 41 , 46 ]. For example, a model with a
staggered array of mineral crystal platelets embedded in a collagen matrix (Fig. 4 )is
proposed to predict the molecular packing in collagen fibrils, which can be used to
predict both elastic modulus and fracture stress as a function of the amount of mineral
in the fibril [ 46 ]. Another model of mineralized collagen fibrils based on the data of
neutron diffraction, electron microscopy, crosslinking, and composition-density
predicts that three quarters of the mineral in bone is disposed within the fibrils [ 41 ].
Beyond the mineralized collagen fibril level, several models are proposed to
describe the interaction between the fibrils. Early studies propose a simple fiber-
reinforced composite to model the elastic properties of osteonal bone, assuming
(1) bone collagen fibrils are not principally aligned along the long axis of the bone,
but demonstrate an alignment of 30 with respect to the long axis, (2) 75% of
mineral crystals reside outside of collagen fibrils, and (3) mineral crystals outside
of collagen fibrils have their c-axis in the longitudinal direction [ 47 ]. Recently,
a multiscale modeling of bone as a fiber reinforced composite material is
proposed to describe the elastic properties of bone considering both mineralized
collagen fibrils and the extrafibrillar minerals, which are mechanically equivalent