Bone hydroxyapatite

Kokubo et al. [16,17] showed that the hydroxyapatite formation on the surfaces of bioactive materials in the living body can be reproduced even in an acellular protein-free simulated body fluid (SB F) with ion concentrations nearly equal to those of human blood plasma. This indicates that the hydroxyapatite layer is formed through chemical reaction of the bioactive glass with the surrounding body fluids. The formed layer consists of carbonated hydroxyapatite with small crystallites and low crystallinity, which is similar to bone hydroxyapatite. Hence the bioactivity of a material can be evaluated even in vitro by examining the hydroxyapatite formation on its surface in SBF. 

Fig. 11.17 XRD patterns of bone hydroxyapatite and sintered stoichiometric hydroxyapatite. (Bone hydroxyapatite was obtained by heating bone of pig at 600°C).

The bisphosphonates have a common structure, P-C-P, which is similar to the structure of the native pyrophosphate P-O-P found in bone hydroxyapatite. The different compounds in clinical use vary by the attachments to the R component of the native molecule. 

Mechanism of Action Abisphosphonate that binds to bone hydroxyapatite (part of the mineral matrix of bone) and inhibits osteoclast activity. Therapeutic Effect Reduces rate of bone f urnover and bone resorption, resulting in a net gain in bone mass. Pharmacokinetics Absorbed in the upper GI tract. Extent of absorption impaired by food or beverages (ofher than plain water). Rapidly binds to bone. Unabsorbed portion is eliminated in urine. Protein binding 90%. Half-life 10-60 hr. 

Mechanism of Action Abisphosphonate that binds to bone hydroxyapatite and inhibits osteoclasts. Therapeutic Effect Reduces bone turnover (the number of sites at... 

As a reactant, solvents frequently react with solntes. Snbstances that are insoluble in one solvent can dissolve in another by reacting with it. Thus, bone (hydroxyapatite, CasCOHXPOds) is insoluble in most solvents bnt dissolves in 100% sulfuric acid, with protonation of the phosphate. When any solnte is dissolved in a solvent, possible solute-solvent reactions must always be considered. Solvents can be used to modify the properties of solutes. Nitric acid dissolved in water behaves quiet differently to nitric acid dissolved in concentrated snlfnric acid. 

Garcia, R., and Doremus, R. H., Electron microscopy of the bone-hydroxyapatite interface from a human dental implant. /. Mat. Sci. Materials in Medicine 3,154-156 (1992). 

Bone is an anisotropic and viscoelastic ceramic matrix composite and is distinct from conventional ceramics. Its mechanical properties depend on its porosity, degree of mineralization, collagen fiber orientation, and other structural details. The data in Table 18.1 may be used to compare the physical and mechanical properties of bone, hydroxyapatite (the major mineral in bone, and hence, the most relevant material as a bioceramic), and CBPCs. 

Recently, PCL containing bovine bone hydroxyapatite (HA) and hydroxyapa-tite/Ag (HA-Ag) composite nanoflbers were prepared via an electrospinning process [43]. The morphology, structure and thermal properties of the PCL, PCL/HA, and PCL/HA-Ag composite nanoflbers before and after immersion in SBF were characterized. SEM images revealed that the nanoflbers were well-oriented and incorporated the HA-Ag nanoparticles well. Mechanical study revealed that the yield stress of PCL/HA-Ag composite nanoflbers showed a higher value than that of PCL/HA composite, possibly due to the addition of metallic Ag nanoparticles [43]. 

Phosphorus is part of bone hydroxyapatite [3Ca3(P04)2(0H)2]. Normally the body has sufficient phosphorus and bone resorption is not required. Insufficient intake (e.g., carbonated beverages replacing miUc) or increased phosphorous-bound calcium complexes (e.g.,... 

Syn. 6, 16 (1960) from Ca(N01)1.4H10 and (NH4)1P04 plus NH4OH Hayek. Newesely, ibid. 7, 63 (1963). Formation and structure of synthetic bone hydroxyapatites A. S. 

PiatteUi A. and Trisi P. 1994. A light and laser scanning microscopy study of bone/hydroxyapatite-coated titanium implants interface histochemical evidence of imminerahzed material in humans. /. Biomed. Mater. Res. 28 529-536. 

Calcium-based composites rely on their similarity to the mineral component of natural bone— hydroxyapatite (HA). The theory behind their use is that the body will see these materials as tissues that need to be remodeled, allowing them to be integrated with and then replaced by bone. Tricalcium phosphate [TCP, Ca3(P04)2], calcium sulfate [plaster of paris, CaS04], and hydroxyapatite [Ca,o(P04)s(OH)2] are all currently being used to fill bony deflects and stimulate or direct bone formation. Calcium suifate has been used for over a century due to its ready availability and bio-... 

The a form can be prepared by heating a dry mix of calcium pyrophosphate and chalk (5.65), while the P form (whitlockite) is obtained from aqueous Ca(OH>2 and H3PO4 with pH > 6.0. The P form is also obtained by heating bone hydroxyapatite at 700 C. 

Haversian canal Compact bone Spongy bone Hydroxyapatite... 

---