Cortical bone collagen

The stress-strain curves for cortical bones at various strain rates are shown in Figure 5.130. The mechanical behavior is as expected from a composite of linear elastic ceramic reinforcement (HA) and a compliant, ductile polymer matrix (collagen). In fact, the tensile modulus values for bone can be modeled to within a factor of two by a rule-of-mixtures calculation on the basis of a 0.5 volume fraction HA-reinforced... 

A follow-up study showed the relation between the composition and the redistribution of stress [47], Cortical bone was loaded with external compression and tension, and the stress stored within the apatite crystals was assessed via the shift in phosphate Vi band in two regions a collagen-rich area and an apatite-rich area. In the collagen-rich areas, stress was released under external tension, but localized stress intensification occurred under external. In apatite-rich areas, both tensile and compressive stresses were observed. 

Typically, bone has a solid outer portion called cortical bone and a porous inner part called cancellous bone. The amounts of each vary with location in the body. The cortical bone is a ceramic containing calcium compounds and viscous liquids, a protein called collagen , and an organic polymer. In addition to HAP, bone consists of calcium carbonate and calcium phosphate. HAP is 69 wt.% of total calcium phosphate compounds [4]. Part of the Ca in these compounds is substituted by Na, K, Mg, and Sr. Hydroxyl ions in the HAP are also substituted by F, CO3, or Cl, which makes the apatite a fluoroapatite, dahllite or chloroapatite, respectively. These substitutions are considered to play significant roles in the structure and mechanical properties of bones. 

Fig. 25 NMR spectra of human cortical bone and collagen type I (100 MHz for MAS...

Pezzotti and coworkers monitored the spectral shift of Raman bands with mechanical loading to examine the in situ response of femoral and cortical bone to external stress [58, 59]. It has been proposed that collagen operates by a... 

Figure 4.7 (a) The isovolumetric collagen fraction In bovine cortical bone is shown in the center, and maps of stress stored in the apatite crystals within this area are shown at the side. The values on each individual stress map Indicate the remote uniaxial stress field... 

In contrast to trabecular bone, in the cortical bone of ovx monkeys the mineral content was significantly increased in endosteal regions, while the crystallinity and collagen crosslink ratio remained constant in both periosteal and endosteal tissue [66]. Because these parameters displayed different trends in cortical and trabecular bone, the compositional adaptations may be site-specific. 

Bone composition, such as mineralizahon, carbonate accumulation, crystallinity and collagen crosslinking, has been shown to change with age [41], and is correlated with the bone s mechanical properhes [42, 43]. In microdamaged bone, the collagen crosslinking is altered but the mineralization and crystallinity are unaffected [44] (Figure 14.6). Alterahons in bone composition have been observed in diseases such as osteoporosis [45], osteopetrosis [46] and osteoarthritis [47]. The treatment of osteoporosis with nandrolone decanoate, an anabolic steroid, was shown to alter the cortical bone composition [48], whereas treatment with bisphos-phonates had little effect [49]. 

Figure 3.2 Schematics of the hierarchical architecture of cortical bone. (A) Longitudinal section of femur. (B) Enlarged cross section of cortical bone showing cylindrical osteons. (C) Enlargement of an osteon showing the central Haversian canal with a blood vessel, the concentric lamellae and the radial canaliculi (see also J). A more detailed view of an osteon is shown in the inset in the bottom right. (D) Collagen fibre composed of hundreds of fibrils. The evenly spaced dark spirals are...

The composition of bone depends on a large number of factors the species, which bone, the location from which the sample is taken, and the age, sex, and type of bone tissue, for example, woven, cancellous, cortical. However, a rough estimate for overall composition by volume is one-third Ap, one-third collagen and other organic components, and one-third H2O. Some data in the literature for the composition of adult human and bovine cortical bone are given in Table 47.1. 

Bone is a natural composite of apatite-collagen. So a composite of polymer matrix containing bioactive particulate filler is a natural choice for substituting cortical bone. Bone substitutes can be easily made by using hydroxyapatite (HA) particles as the bioactive component because of its close similarity with bone apatite and its excellent bioactivity. So a composite made of HA with polymer matrix provides... 

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