Resorbable Calcium Phosphate Ceramics

The major difference between the structure of a-TCP and P-TCP is the absence of cation-cation columns in the latter case. The results of the Kohn-Sham DFT calculations (Yin et al., 2003) indicates that P-TCP would be much more stable than a-TCP, thus confirming experimental results (see, e.g., Berger et al., 1995a). This situation is thought to be related to the different distributions of Ca atoms, that have a pronounced effect on the stabihty and the electronic properties of the different modifications of TCP. In particular, the uniformly distributed Ca vacancies stabilize the P-TCP structure, so that its solubihty is much less than that of the a-TCP modification. 

This reaction describes the formation of the self-setting bone cement that is used ubiquitously in dentistry (Brown and Chow, 1987 Posset et al., 1998). 

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The stoichiometric HAP, characterized by an atomic ratio Ca/P = 1.67 and a hexagonal structure, is the nearest relative of biologic apatite ciystals. Moreover, the HAP is the least soluble and the least resorbable calciirm phosphate. When an HAP ceramic is implanted in a bone site, the bone tissue formation is observed on its contact (osteoconduction) (see Figure 12.1). Besides, in certain conditions, calcium phosphate ceramics can induce the formation of bone tissue in ectopic sites. HAP implants appear in the form of dense ceramics or with variable porosity or again, in the case of prostheses, as thin coatings deposited by plasma sprayed on a metal. 

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). 

Calcium phosphate, also called tricalcium phosphate (TCP), serves as a model for a whole group of calcium phosphates which end with hydroxyapatite (HA). Another name for these materials, which resemble the mineral part of the bone chemically seen, is resorbable ceramics. 

Ceramics used in fabricating implants can be classified as nonabsorbable (relatively inert), bioactive or surface reactive (semi-inert) [Hench, 1991,1993] and biodegradable or resorbable (non-inert) [Hentrich et al., 1971 Graves et al., 1972]. Alumina, zirconia, silicone nitrides, and carbons are inert bioceramics. Certain glass ceramics and dense hydroxyapatites are semi-inert (bioreactive) and calcium phosphates and calcium aluminates are resorbable ceramics [Park and Lakes, 1992]. 

Although Plaster of Paris was used inl892asabone substitute [Peltier, 1961], the concept of using synthetic resorbable ceramics as bone substitutes was introduced in 1969 [Hentrich et al., 1969 Graves et al., 1972]. Resorbable ceramics, as the name implies, degrade upon implantation in the host. The resorbed material is replaced by endogenous tissues. The rate of degradation varies from material to material. Almost all bioresorbable ceramics except Biocoral and Plaster of Paris (calcium sulfate dihydrate) are variations of calcium phosphate (Table 39.8). Examples of resorbable ceramics are aluminum calcium phosphate, coralline. Plaster of Paris, hydroxyapatite, and tricalcium phosphate (Table 39.8). 

Yamamuro, T., Hench, L.L., and Wilson, J. (Eds.) (1990) Handbook of Bioactive Ceramics, Volume I Bioactive Glasses and Glass Ceramics, Volume II Calcium Phosphate and Hydroxylapatite Ceramics, CRC Press, Boca Raton, FL. A collection of articles on bioactive and resorbable bioceramics. 

All the resorbibles ceramics, except plaster (CaS04 /2H20), are based on calcium phosphates, vaiying their biodegradability in the sense ... 

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