Hydroxyapatite continued

Once fluoride ions react with bone, they are not easily dissolved out or exchanged by other elements. If bone is buried for long periods of time, the relative amount of fluorine in the bone gradually increases as a function of time the "fluoridation" process continues until the maximum amount of fluorine (necessary to convert all the hydroxyapatite to fluorapatite) is reached. The total concentration of fluor in carbonated fluorapatite can reach levels as high as above 3%. There is ample room, therefore, for an increase in the relative amount of fluorine in buried bone. Determining the relative amount of fluorine in buried bone may thus serve as a tool for dating bone. 

The bisphosphonates are all analogues of pyrophosphate. They inhibit osteoclast resorption of bone and they are able to inhibit the formation and dissolution of hydroxyapatite crystals, however their exact mechanism is not well understood. Other effects which have relevance for bone homeostasis include inhibition of the activities of PTH, prostaglandins and 1,25-dihydroxy vitamin D. Bisphosphonates bind to bone with high affinity. They have therefore a duration of action that continues long after their use has been stopped. 

The various findings about fluoride and its interaction with the hydroxyapatite at the molecular level show that the relationship is complicated and multifaceted. The broad conclusion from the enormous volume of work that has led to our current understanding of the role of fluoride is that it is overwhelmingly beneficial. It promotes numerous desirable properties in tooth mineral, reducing solubility through action in both the saliva and in the mineral phase, it shifts the demineralisation/remineralisation equilibrium in favour of remineralisation, and through its actions in the solid state, ensures that the kinetically favoured OCP is transformed into the more thermodynamically stable hydroxyapatite. Research continues, and there is no doubt that there is still more to learn about the complexities of the interaction of fluoride with hydroxypatite under physiological conditions. 

It was stated that hydrated calcium monohydrogen phosphate in amorphous or cryptocrystalline form is a potential precursor in the formation of hydroxyapatite because the structural position of Ca2+ on (010) and (110) crystal planes of both minerals essentially correspond to one another492. These planes of calcium ions could easily serve as transition boundaries with little distortion of crystal structure the same holds true for octacalcium phosphate or defect apatites. Thus apatite may form from amorphous or microcrystalline calcium monohydrogen phosphate possible via octacalcium phosphate or defect apatites. This process may already start inside the matrix vesicles and continue during extravesicular activities. 

Recent examples include the batch synthesis of C0AI2O4 nanocrystals and the continuous synthesis of nano-hydroxyapatite. In the first example, the researchers wanted to prepare metal oxide particles that could be easily... 

Antibody-bound steroid was adsorbed to hydroxyapatite after addition to the incubation medium as a suspension or a solid powder. Free and antibody-bound steroid can also be separated in small columns of hy-droxyapaptite, eluting antibody-bound steroid with a continuous, increasing concentration of phosphate buffer, pH 7.0 (see Fig. 1). 

Pamidronate in the disodium form, a second-generation bisphosphonate, has an intermediate antiresorptive activity its continuous administration produces rapid suppression of bone resorption. Unlike etidronate, it does not impair bone mineralization at therapeutic dosages in patients with Paget s disease. Pamidronate inhibits osteoclast activity primarily by binding with hydroxyapatite crystals in the bone matrix, preventing the attachment of osteoclast precursor cells. Other mechanisms of action of matrix-bound pamidronate may include direct... 

Additionally, the pellicle modulates the process of mineral precipitation at the enamel surface. Saliva is supersaturated with respect to calcium and phosphate ions and provides the potential for remineralisation of the demineralised enamel surface. In the absence of the pellicle, calcium phosphate salts would continuously precipitate on the enamel surface [100]. Acidic PRPs reveal a high affinity for hydroxyapatite and are obligate components of the in vivo-formed pellicle layer [9, 10]. These pellicle proteins can act as inhibitors for surface-induced precipitation of calcium salts onto the enamel [101, 102], thus providing mineral homeostasis at the enamel-saliva interface. 

Onuma, K., Oyane, A., Kokubo, T., Treboux, G., Kanzaki, N., and Ito, A. (2000) Precipitation kinetics of hydroxyapatite revealed by the continuous-angle laser light-scattering technique./. Phys. Chem., B104, 11950-11956. 

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