Bone as a Composite Material - Biomechanics of Hard Tissues - page 103

Ca 2 + ion vacancy in apatite often contains sodium, potassium, magnesium, and
zinc substitutions [12].
Composite models of bone material are complicated by what is not known
about the exact physical characteristics of the constituent phases. The exact shape
and size of the nanometer-sized crystals that mineralize the underlying collagen
matrix remain unclear. A variety of techniques used to examine bone mineral have
quantified bone crystals over a wide range of shapes and sizes [14-16].
Atomic force microscopy (AFM) measurements of bone mineral revealed
plate-shaped crystals of 30-200 nm length and width and 1.5-4 nm thickness
[14]. Such measurements are certainly complicated by the source and type
of bone used for such analysis. Moreover, a composites model is profoundly
affected by assumptions of a platelike and needlelike mineral structure or one
that is 50 versus 200 nm in length. A survey of the literature shows that the
majority of bone mineral ( ∼ 98%) exists as small platelike crystals measuring
≈ 1nm × 10 nm × 15 nm [16-18]. AFM techniques have determined that a few
larger crystals exist that measure ≈ 40 nm × 60 nm × 90 nm [17] and platelike
structures of < 10 nm thickness × 30-200 nm length and width [14]. Platelike crys-
tals were also observed via transmission electron microscopy (TEM) to be 6-9 nm
thick, 20-60 nm wide, and 30-120 nm long [15]. However, the exact structure of
these crystals is brought into question by X-ray diffraction measurements that esti-
mate the average length of crystals along their c -axis to possibly extend to lengths
as large as several hundreds of nanometers [16]. Individual mineral platelets are
physically bonded together with ''bridging crystallites'' such that the bone that
has been completely deproteinated resembles a porous ceramic with substantial
compressive strength [15].
3.2.3
Physical Structure of Bone Material
The interactions between collagen and mineral are also of considerable interest
and debate. The initial formation of crystals between the collagen fibrils in the hole
region or ''Hodge-Petruska'' gaps, originally proposed by Petruska and Hodge in
1964, may drive the resulting shape and size of bone minerals [19]. Ultimately, the
fibrillar collagen structure is interrupted and, disrupted by the presence of mineral
forming within the gaps [20]. Crystals exist that are too large to fit within the fibril
or hole region and have been observed, via TEM, to exist in the interfibrillar region
[17, 20, 21]. TEM observations have shown that the majority of bone crystals lie
within and on the surface of the collagen fibers [15, 22] where the long axis of the
crystals lies parallel to the long axis of the collagen fibrils.
3.2.4
Water
Interestingly, the water in bone has received relatively little attention from a
mechanical perspective, especially in the context considering bone as a composite