NANOINDENTATION OF BONE - Nanoindentation with Biological Applications - page 186

common Ca 2+ ion vacancy which often contains sodium, potassium,
magnesium, and zinc substitutions. 16 The mineral phase of bone has
been widely studied by a variety of techniques: x-ray diffraction, infrared
and Raman spectroscopy, neutron diffraction, electron microscopy,
nuclear magnetic resonance (NMR), and atomic force microscopy
(AFM). 16-23 Despite this wide range of analysis techniques, the collagen-
mineral relationship is not fully understood. The nano-sized minerals in
bone are smaller and contain more substitutions that those in enamel. 16
These extremely small crystals have the majority of ions occupying
surface or near surface positions in the lattice, leading to further
uncertainties about the structure of bone mineral. Most bone mineral
(~98%) appears to exist as small plate-like crystals that measure

10 × 15 nm. 23-25 Additionally, AFM techniques have determined that a
few larger crystals exist that measure 40 × 60 × 90 nm. 23 The size and
shape of bone minerals may be a product of the initial formation of
crystals between the collagen fibrils, in the hole region, as originally
proposed by Petruska and Hodge in 1964. 26 However, the larger crystals
are too large to fit within the fibril or hole region and have been
observed, via AFM and transmission electron microscopy (TEM), to
exist in the interfibrillar region. 23,27,28 Further, TEM observations have
shown that the majority of bone crystal exists within and on the surface
of the collagen fibers. 29

1 ×

1.3.2. The organic phase

The organic phase of bone is primarily composed of Type I collagen
fibers (90%) and ground substance (proteoglycans, glycoproteins, small
proteings such as osteopontin, and lipids). The collagen preferentially
aligns and creates a three dimensional network, or scaffold, into and
onto which mineral crystals form. 26 The collagen fibrils self-assemble
from individual tropocollagen molecules (triple helices) and exhibit
characteristic banding every 67 nm due to overlap ( Fig. 7-1 ) . Fibrils then
form larger collagen fibers that are highly organized and cross-linked. 30
The geometrically complex collagen fibers contribute to the viscoelastic
response seen in bone and provide toughness to the brittle mineral
phase. 31 The non-fibrillar organic phase is suggested to act as a glue that