Subject hydroxyapatite

The nature of mineral phases present in bone, dentin, enamel and other phosphatic tissues, and their mode of formation have been subjects of lively discussions among health scientists and crystallographers. Bioscientists most commonly accept the viewpoint that the inorganic phase of bones or teeth is principally hydroxyapatite, Caio(P04)6(OH)2, and deviation in Ca/P ratio from common hydroxyapatite (Ca/P = 1.667) observed in mineralized tissues is explained by the presence of amorphous phosphates. In contrast, many crystallographers favor the idea of carbonate apatite, i.e. dahllite, as the major crystalline phase in biophosphates and they doubt the existence of amorphous phases. The topic has been reviewed14,15,22,28, 37,44,47,348-358) no common consent has yet been reached. In the following an attempt is made to at least coordinate the controversial findings. 

Fluoride is well established as effective for the prophylaxis of dental caries and has been under investigation for the treatment of osteoporosis. Both therapeutic applications originated from epidemiologic observations that subjects living in areas with naturally fluoridated water (1-2 ppm) had less dental caries and fewer vertebral compression fractures than subjects living in nonfluoridated water areas. Fluoride is accumulated by bones and teeth, where it may stabilize the hydroxyapatite crystal. Such a mechanism may explain the effectiveness of fluoride in increasing the resistance of teeth to dental caries, but it does not explain new bone growth. 

Tooth enamel is a hydroxyapatite, Ca5(P04)30H, which is subject to attack by acids produced when bacteria act on food residues. A more resistant coating is fonned when the OH ions are replaced by F ions. The resulting mineral is calledJluoroapatite ... 

To counteract the high corrosion rate of Mg alloy implants electrochemically deposited coatings of fluorine-doped hydroxyapatite (FHAp) and brushite (DCPD) were developed by Bakhsheshi-Rad et al. (2014). As shown in Table 5.2 potentiodynamic polarisation measurements of uncoated and coated Mg-Ca alloys subjected to electrochemical corrosion in Kokubo s SBF (Table 7.8) confirmed the corrosion-resistant nature of the coatings. The anodic polarisation curve of the uncoated specimen shows a breakdown immediately after the... 

The existence of OHAp as products of partial dehydroxylation of hydroxyapatite was postulated earlier (Bredig, Franck and Fiildner, 1933 Trombe and Montel, 1978) but the existence of OAp was subject to controversy for many years until it has now been well established thanks to modern analytical techniques (e.g., Gross and Pluduma, 2012 see Chapter 6.2.1.4). Liao etal. (1999) reported that (i) a small amount of OH- ions is always present in the structure of OAp and (ii) even a loss of 75% of the chemically bound water maintains the apatite channel structure. The limiting composition is then Ca10(PO4)6(OH)0 5O0 75 that corresponds to a water loss of 1.34% (Trombe and Montel, 1971). Hence, partially dehydroxylated hydroxyapatite (designated OHAp) could be described either as a non-stoichiometric... 

Figure 6.5 Schematic model of the thermal decomposition of a spherical hydroxyapatite particle subjected to high temperature in a plasma jet. (a) Mode of decomposition...

Porter, A.E., Taak, P., Hobbs, L.W., Coathup, M.J., Blunn, G.W., and Spector, M. (2004) Bone bonding to hydroxyapatite and titanium surfaces on femoral stems retrieved from human subjects at autopsy. Biomaterials, 25 (21), 5199-5208. 

Elliott (1964) has stated that the frequency of the O—H band at 3570 cm in hydroxyapatite is lowered if fluorine partially substitutes for the hydroxyl ions. The shift depends on the amount of substitution, and is rather small a 50% substitution causes a lowering of only about 20cm However, he believed that the determination of the frequency of the O—H band to measure the amount of replacement of hydroxyl by fluoride ions in hydroxyapatite could be as sensitive as X-ray diffraction techniques and is not subject to the same limitation, namely, that the apatite must be well crystallized. 

X 10 cm) of DEAE-Sepharose CL-6B and the column was eluted with a 300 ml linear gradient (0-0.4 M) of KCl. The active solution was concentrated in dialysis tubing with poly(ethylene glycol) 20,000 powder for 12 hr and subjected to a Sephadex G-15 column (1.1 x 15 cm) to remove KCl. Enzyme fractions were chromatographed on a hydroxyapatite column (1.6 x 10 cm) equilibrated with 10 mM potassium phosphate buffer, pH 7.8, and eluted with a 160 ml linear gradient (lO-rlOO mM) of the above phosphate buffer. The purified enzyme was collected and dialyzed for 5 hr against Tris-HCl buffer, pH. 8.3. 

The literature on hydroxyapatite is very extensive and numerous varieties have been prepared by different methods. Many of these have non-stoichiometric compositions and include calcium-deficient varieties, tricalcium phosphate hydrates and various precipitated products with Ca/P ratios between 1.3 and 2.00 (Ca/P = 1.67 theoretical). While the more impure products may contain quantities of Ca(OH)2, CaHP04 -2H20 or Ca3(P04)2, the non-stoichiometry can in many cases only be accounted for by lattice vacancies (denoted as ), or snbstitntions within the crystal lattice or on its surface. The non-stoichiometry of apatites ranains a subject of much investigation since it is related to physical as well as chanical properties. 

ADP-ribose) synthetase and the latter fraction involves ADP-ribosyltransferase. To clarify that the radioactive compound formed by the latter fraction was indeed the mono(ADP-ribose) molecule, the acid-insoluble reaction product was treated with alkali at 37°C for 2 h. The radioactive material solubilized was adjusted to pH 7.0 and subjected to high performance liquid chromatography with reverse phase column. The eluate was monitored by UV and fractionated and radioactivity of the fraction was measured. The retention time of the radioactive product coincided with that of authentic mono(ADP-ribose). Furthermore, by treatment with snake venom phosphodiesterase this radioactive peak, tentatively considered to be ADP-ribose, migrated to the position corresponding to the 5 -AMP. These results indicate that hen liver nuclei contain ADP-ribosyltransferase. We purified this enzyme to a homogeneous state through salt extraction, gel filtration, hydroxyapatite, phenyl-Sepharose, Cm-cellulose, and DNA-Sepharose [3]. 

Scheme 10.1 Thermal decomposition scheme of hydroxyapatite subjected to a hot plasma jet.

PARK are frequently used without surface modification. There are a significant number of papers and patents which describe PEEK modified with fillers such as hydroxyapatite (HA) or calcium phosphates, titanimn coatings or even biomimetic protein and peptide sequences. Some of these are described in reference [2]. However, in HA-filled PARK there is a trade-off between mechanical properties and modified biocompatibility. Various attempts have been made to overcome this limitation - for example by using HA coatings or HA whiskers [3, 4]. Biological modifications would be subject to extremely complex regulatory approval. In fact unmodified PEEK has been shown to be comparable in vitro with the bone forming capacity of titanium [5]. 

---