Biomedical Engineering Reference
(consisting mainly of proteoglycans, matrix proteins, and water), and carbonated
hydroxyapatite mineral particles [ 36 , 117 ]. The organic bone matrix imparts tough-
ness (i.e., energy absorption), while the mineral particles provide stiffness and
resistance to compressive loads. However, unlike the highly aligned parallel colla-
gen fibrils found within cartilage, bone's organic matrix is less well organized.
Material within the ZCC forms a similar biocomposite. In bone and calcified
cartilage, anisotropy is typically observed by a preferential orientation of
mineralized collagen fibrils and an increased modulus in the principal loading
directions [ 66 ].
Cells within bone and cartilage both deposit and subsequently regulate the
extracellular matrix. In bone, osteocytes trapped within the mineralized matrix
regulate mineralization, while osteoblasts and osteoclasts form new bone and
remove extant bone, respectively. Chondrocytes produce and maintain the cartilag-
inous matrix and align with the parallel-aligned collagen fibrils within hyaline
articular cartilage [ 118 ].
5.3.2 The Mineral Phase
The volume fraction of mineral generally predicts the stiffness of the bone composite.
Elastic properties follow particulate composite bounds at macroscopic and micro-
scopic length scales [ 119 , 120 ]. Other factors that contribute to the properties of
bone are mineral platelet size [ 117 , 121 , 122 ], particle orientation with respect to
collagen fibrils [ 50 ], and hydration of the tissue [ 123 ]. However, the mineral
composition, organization, and ability to interact with the organic matrix also
contribute to the resulting mechanical properties.
The mineral phase of bone consists primarily of an impure form of hydroxy-
apatite, Ca 10 (PO 4 ) 6 (OH) 2 [ 124 , 125 ], with substantial substitutions by carbonate
(CO 3 2 )intotheOH or PO 4+ regions of the apatite lattice [ 124 ]. Most bone
mineral(~98%)existsassmallplate-likecrystalsmeasuring~1 10 15 nm
[ 126 - 128 ] with some larger crystals measuring ~40 60 90 nm [ 127 ].
The size and shape of bone mineral may follow the initial crystal formation
within the gaps between collagen fibrils in the hole region [ 129 ]. However, larger
crystals have been observed, via atomic force microscopy (AFM) and transmis-
sion electron microscopy (TEM), to exist in the interfibrillar region [ 127 , 130 ,
131 ]. The bone mineral surface has been shown, via nuclear magnetic resonance,
to interact directly with the collagen fibrils and GAGs within bone's ground
substance [ 121 ].
The mineral phase of articular calcified cartilage possesses a similar crystal
structure to that of bone, with similar dimensional measurements [ 50 ]. However,