Hydroxyapatite bone substance

Hydroxyapatite (HA) coating on the surface of the hip stem and the acetabular cup is the most recent advancement in artificial hip joint implant technology. This substance is a form of calcium phosphate, which is sprayed onto the hip implant. It is a material found in combination with calcium carbonate in bone tissue, and bones can easily adapt to it. When bone tissue does grow into HA, the tissue then fixes the hip joint implant permanently in position. These HA coatings are only used in press-fit, noncemented implants. 

The cations Mg and Ca are major components of bones. Calcium occurs as hydroxyapatite, a complicated substance whose chemical formula is Cas (P04)3 (OH). The structural form of magnesium in bones is not fully understood. In addition to being essential ingredients of bone, these two cations also play key roles in various biochemical reactions, including photosynthesis, the transmission of nerve impulses, and the formation of blood clots. 

It was obvious to early researchers on synthetic bone material that a pure calcium phosphate bioceramic would be the optimum replacement for human and mammalian bone. The calcium phosphate in human bone is called hydroxyapatite (Fig. 1). It is an ionic substance having the formula Ca5(0H)(P04)3. 

Phosphates play an enormously important role in biological systems. The genetic substances deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are phosphate esters (see Figure 9.11). Bones and teeth are constructed from collagen (fibrous protein) and single crystals of hydroxyapatite, Ca5(0H)(P04)j. Tooth decay involves acid attack on the phosphate, but the addition of fluoride ion to water supplies facilitates the formation of fluoroapatite, which is more resistant to decay. 

Coral is a natural substance made by marine invertebrates. According to Holmes et al. [1984], the marine invertebrates live in the limestone exostructure, or coral. The porous structure of the coral is unique for each species of marine invertebrate [Holmes et al., 1984]. Corals for use as bone implants are selected on the basis of structural similarity to bone [Holmes et al., 1984]. Coral provides an excellent structure for the ingrowth of bone, and the main component, calcium carbonate, is gradually resorbed by the body [Khavari and Bajpai, 1993]. Corals can also be converted to hydroxyapatite by a hydrothermal exchange process. Interpore 200, a coral hydroxyapatite, resembles cancellous bone [Sartoriset al., 1986]. Both pure coral (Biocoral) and coral transformed to hydroxyapatite are currently used to repair traumatized bone, replace diseased bone, and correct various bone defects. 

The tooth root is covered with a substance called cementum. This is a specialized tissue that shows some resemblance to bone [2] and has the approximate composition 455 hydroxyapatite, 33% protein (collagen) and 22% water by mass. Cementum occurs in two forms, acellular, which covers about two-thirds to the root, and cellular. 

Bone Char. Bone charcoal is a carbonaceous substance derived from the carbonization of selected grades of animal bones by heating dry bones in an airtight iron retort at 500-700°C for about four to six hours. Comparing the capacity of metal ions removal with aetivated carbon, bone charcoal provides not only a porous carbon surface for physical adsorption, but also a hydroxyapatite lattice—Ca,o(P04)g(OH)2 for ion exchange of metal ions. Based on these properties, this sorbent should have excellent adsorption capacities for metal ions. The charaeteristies of typieal bone char are shown in Table 15.9. 

Difficulties arise, not so much in explaining why mineralization readily and regularly occurs in certain tissues, but rather why other tissues, which resemble them in many ways, do not normally mineralize. Thus it is relatively easy to explain how crystals of a very sparingly soluble substance such as hydroxyapatite can be formed in bone, on the basis of the concentrations of calcium and phosphate ions present in blood these are sufficiently high to permit small crystals of biological apatite to grow at the expense of ions in solution. It is more difficult to appreciate why, under apparently similar conditions, a tissue such as skin which, like bone, contains... 

Bones and teeth contain a mineral substance called hydroxyapatite, Caio (PO4) e (OH) 2, a solid. What polyatomic ions are contained in the mineral substance of bone and teeth ... 

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