P-TCP

Other Ceramic Calcium Phosphate Materials. Other ceramic calcium phosphate materials for repairing bony defect iaclude p-tricalcium phosphate (P-TCP) [7758-87-4], P-Ca2(PO, and biphasic calcium phosphate (BCP) ceramics which consist of both P-TCP and HA. Unlike ceramic HA, P-TCP resorbs ia the tissue (293). The in vivo dissolution of BCP ceramic implants was shown (296) to iacrease with increasing P-TCP/HA ratio ia the implants. Both P-TCP and BCP can lead to new bone growth to various extents depending on the appHcations and the type of materials used (293,296). 

TCP, Ca3(PC>4)2, exists in two crystal forms, viz. a-TCP and p-TCP. Both forms can only be prepared by calcination at high temperature (>800 °C).70 a-TCP is meta stable at room temperature and is more soluble than p-TCP in water. The crystal structure of a-TCP shows that there are 16 inequivalent phosphorus sites.110 Its 31P MAS spectrum contains only 14 peaks, where the chemical shifts range from —3 to 5 ppm.111 Although there are only three crystallographically inequivalent phosphorus sites for (3-TCP,112,113 there are as many as 16 resolved peaks in the 31P MAS spectrum because of the presence of some calcium vacancies.114 The 31P chemical shifts of (S-TCP are in the same range as reported for a-TCP, but the overall patterns of their 31P MAS spectra are quite different. In addition, it has also been found that even a minor substitution of Ca2+ by Na+, Mg21 or Zn2+ ions will perturb the 31P MAS spectrum, revealing the substantial effect of the counter ions near the phosphorus sites.114,115... 

State-of-the-art materials for resorbable calcium orthophosphate bioceramics as bone graft substitutes are still TCPs, either as the low temperature modification (0-TCP) or one of the high temperature modifications (a-TCP, a -TCP). The P-TCP is stable below 1125 °C. The a-TCP exists up to 1475 °C, and above this temperature up to the melting point at 1756 °C the a -TCP modification is stable (Figure 4.22). 

Figure 4.21 Solubility isotherms of calcium phosphates at 25 °C. HAp, hydroxyapatite DCPD, brushite DCPA, monetite p-TCP, p-tricalcium phosphate OCP, octacalcium phosphate and TTCP, tetracalcium phosphate. (After De Groot et al. (1990).)...

Results of Kohn-Sham density functional calculations (Yin, Stott and Rubio, 2003) indicate that P-TCP is much more stable than a-TCP, confirming experimental results (see, for example, Berger, Gildenhaar and Ploska, 1995b). This is thought to be related to different distributions of Ca atoms that have a pronounced effect on the stability and the electronic properties of the different modifications of TCP. In particular, the uniformly distributed Ca vacancies stabilise the P-TCP structure so that its solubility is much lower than that of the a-TCP modification. 

There is still another stabilisation mechanisms of the structure of P-TCP by incorporation of Mg2+ ions forming a compound that is closely related to the mineral whitlockite, Ca18(Mg,Fe)2(HP04)2(P04)12. The Mg ions find their place in vacancy positions of the A-type cation-anion columns. Similarly, the structure of a-TCP can likewise be stabilised by incorporation of Mg2+ ions into all available vacancy sites of both types of the column to form a compound such as Ca7Mg9(Ca,Mg)2(P04)12 (Mathew and Takagi, 2001). 

Figure 4.24 Structure of p-TCP. (A) (0 01) projection. The numbers refer to Ca ions with different coordinations. (B) (a) In this (001) projection each A-type column (labeled "A") is surrounded by six B-type columns (unlabeled) and each B-type column...

Pulsed currents with a density of 15 mA cm-2 resulted in a homogenous Ca-deficient HAp coating that after an appropriate thermal treatment consisted of a biphasic mixture of 52% HAp and 48% P-TCP (Drevet and Benhayoune, 2012). Moreover, addition of 9% hydrogen peroxide to the electrolyte also modified the chemical composition of the electrodeposited coating. Under these conditions, a stoichiometric and fully crystallised HAp coating was obtained... 

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