Apatite crystallization

Chitosan membranes can also be superficially modified, for instance with 3-isocyanatopropyl triethoxysilane. Silanol groups and calcium salt acted as nucleation sites and accelerator, respectively, for the formation of apatite crystals therefore, this chitosan membrane is a bioactive guided bone-regeneration material thanks to its apatite-forming ability [341]. 

By far the most abundant phosphate mineral is apatite, which accounts for more than 95% of all P in the Earth s crust. The basic composition of apatite is listed in Table 14-2. Apatite exhibits a hexagonal crystal structure with long open channels parallel to the c-axis. In its pure form, F , OH , or Cl occupies sites along this axis to form fluorapatite, hydroxyapatite, or chlor-apatite, respectively. However, because of the "open" nature of the apatite crystal lattice, many minor substitutions are possible and "pure" forms of apatite as depicted by the general formula in Table 14-2 are rarely found. 

The properties described above have important consequences for the way in which these skeletal tissues are subsequently preserved, and hence their usefulness or otherwise as recorders of dietary signals. Several points from the discussion above are relevant here. It is useful to ask what are the most important mechanisms or routes for change in buried bones and teeth One could divide these processes into those with simple addition of new non-apatitic material (various minerals such as pyrites, silicates and simple carbonates) in pores and spaces (Hassan and Ortner 1977), and those related to change within the apatite crystals, usually in the form of recrystallization and crystal growth. The first kind of process has severe implications for alteration of bone and dentine, partly because they are porous materials with high surface area initially and because the approximately 20-30% by volume occupied by collagen is subsequently lost by hydrolysis and/or consumption by bacteria and the void filled by new minerals. Enamel is much denser and contains no pores or Haversian canals and there is very, little organic material to lose and replace with extraneous material. Cracks are the only interstices available for deposition of material. 

The estimation of the crystallinity index (Cl) of bone is based on one of the four vibrational modes associated with the apatite phosphate group. In amorphous calcium phosphate, the absorption band at 550-600 cm-1 appears as a single broad peak, whilst in hydroxyapatite it is split into bands of unequal intensity by the apatite crystal field (Sillen and Parkington 1996). Based on the splitting factor introduced by Termine and Posner (1966), Weiner and Bar-Yosef (1990) proposed the use of a crystallinity index to measure the crystallinity of bone mineral. As illustrated in Fig. 4.7, the Cl is estimated by drawing a base line from 750 to 495 cm 1 and measuring the heights of the absorption peaks at 603 cm-1 (measurement a), 565 cm 1 (measurement b) and the distance from the base line to the lowest point between the two peaks (c). Cl is calculated from the formula ... 

The most important organic components of bone are collagens (mainly type 1 see p.344) and proteoglycans (see p. 346). These form the extracellular matrix into which the apatite crystals are deposited (biomineralization). Various proteins are involved in this not yet fully understood process of bone formation, including collagens and phosphatases. Alkaline phosphatase is found in osteoblasts and add phosphatase in osteoclasts. Both of these enzymes serve as marker enzymes for bone cells. 

Banes and Hailer [81] studied the effect of fluoride addition on the size and morphology of apatite crystals in close-to-physiological conditions. These authors in particular reported that fluoride uptake was accompanied by some anisotropic growth of the apatite crystals the width and/or thickness of the crystals increased with F uptake while no noticeable change in length was observed. In addition, LeGeros et al. [66] pointed out the decrease in calcium deficiency linked to a progressive fluoride incorporation. 

Control of the pH and temperature of the precipitating solution is important to provide optimised conditions for stoichiometric, homogeneous, fluorhydroxyapatite formation. Similar conditions and set-up can be used for the synthesis of fluoride-substituted apatite crystals with varying size, crystallinity and morphology depending on the preparation temperature [124] a purge of the synthesis system with nitrogen gas ensures the preparation of carbonate-free fluorhydroxyapatite at ambient temperature [125]. 

Fluoridated apatite crystals can grow using the dual membrane system involving on the one hand a calcium acetate solution and on the other hand a phosphate solution at physiological temperature with a pH of 6.5. lijima et al. showed that the combination of fluoride ions, added to the phosphate solution, and amelogenin (a major protein in the enamel extracellular matrix), present in the reaction space between the two membranes, controlled the transformation of octacalcium phosphate (OCP) into fine rod-like fluoridated apatite crystals with habit, size... 

M. lijima, D.G.A. Nelson, Y. Pan, A.T. Kreinbrink, M. Adachi, T. Goto, Y. Mori waki, Fluoride analysis of apatite crystals with a central planar OCP inclusion Concerning the role of F" ions on apatite/OCP/apatite structure formation, Calcif. Tissue Int. 59 (1996) 377-384. 

C. Robinson, K. Yamamoto, S.D. Connell, J. Kirkham, H. Nakagaki, A.D. Smith, The effect of fluoride on the nanostructure and surface pK of enamel crystals An atomic force microscopy study of human and rat enamel, Eur. J. Oral Sci. 114 (2006) 99-104. E.D. Eanes, A.W. Mailer, The effect of fluoride on the size and morphology of apatite crystals grown from physiological solutions, Calcif. Tissue Int. 63 (1998) 250-257. 

M. lijima, J. Moradian-Oldak, Control of apatite crystal growth in a fluoride containing amelogenin-rich matrix, Biomaterials 26 (2005) 1595-1603. 

Under neutral conditions, fluoride is also able to induce nucleation and growth of apatite crystals without the involvement of OCP [72]. This requires fluoride concentrations of 0.5 ppm or higher, which are rarely achieved in vivo except in cases where fluorosis may result. It is significant that in severe cases of fluorotic enamel, ultra-structural studies [73] have shown the occurrence of a proliferation of apatite nuclei, suggesting that the presence of fluoride may act to encourage precipitation of crystals of fluorapatite. 

Y. Miaka, S. Shimoda, M. Fukase, T. Aoba, Epitaxial overgrowth of apatite crystals on the thin-ribbon precursor at early stages of porcine enamel mineralisation, Calcif. Tissue Int. 53 (1993) 257-261. 

Fig. 5. SEM photograph showing a secondary apatite crystal within a rim of altered basalt around a salt inclusion. The other minerals contained in the rim are sheet silicates and hematite.

M. lizima and Y. Moriwaki, In vitro study of the formation mechanism of tooth enamel apatite crystals - Effects of organic matrices on crystal growth of octacalcium phosphate(OCP),/.Japan. Assoc. Crystal Growth, 26,1999,175-83 (in Japanese with English abstract)... 

The dimensions of bone crystallites can be as small as 96 10 A, with the order of magnitude even a factor of two or three smaller56. The mean size of the apatite crystals in dentine and cementum is of the same order as that found in bone. Enamel crystals, however, are at least an order of magnitude larger in all dimensions. There appears to be good agreement that the smallest dimension of the bone apatite crystal is about 50 A. On the other hand, there is a discrepancy in the reported size of the... 

In these tissues the distribution of vesicles correspond closely to the patterns of matrix mineralization. Furthermore, it has been suggested that crystals of HA are deposited within the vesicles. Subsequently apatite is deposited within the vesicles and upon their surfaces to produce typical modular clusters of mineral. Such morphological observations strongly implicate the matrix vesicles in the formation of apatite crystals (Fig. 9). Once the first crystals are formed, they mineralize further by epitactic crystal growth458. ... 

However, methane-diphosphonate could not prevent the growth of apatite crystals in vitro on prepared sinews of rats tail out of a metastable solution with calcium and phosphate ions. On the contrary, the precipitated crystalline particles were bigger and better crystallized than those from control solutions. This is in surprising contrast to most of the information from the literature. No other calcium phosphate minerals besides apatite have been found by X-ray diffraction, whereas under comparable conditions brushite and octacalcium phosphate grow on collagenous sinews549. ... 

Collagen fibres (a) and hydroxyl apatite crystals (b) together form larger units, the so-called micro-fibrils (c). It should be pointed out here that the collagen fibres are helically intertwined and consequently... 

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. 

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