Amorphous calcium phosphate

Amorphous calcium phosphate (ACP or ATCP) is a glassy precipitate of variable composition that is formed in double decomposition reactions involving a soluble phosphate and calcium salts (e.g. (NH4)2HPO4 + Ca(NO3)2)[1] performed under carefully controlled pH conditions. Such a mixture is also known as calcium phosphate cement.[2] The precipitate will either be "amorphous tricalcium phosphate", ATCP, or calcium deficient hydroxyapatite, CDHA, Ca9(HPO4)(PO4)5(OH), (note that CDHA is sometimes termed apatitic calcium triphosphate).[1][3][4] The composition of amorphous calcium phosphate is CaxHy(PO4)z·nH2O, where n is between 3 and 4.5.[4] Precipitation from the moderately supersaturated and basic solution containing magnesium produces amorphous magnesium calcium phosphate (AMCP) in which magnesium incorporated into the ACP structure.[5]

A commercial preparation of ACP is casein phosphopeptide-amorphous calcium phosphate (CPP-ACP), derived from cow milk. It is sold under various brand names including Recaldent and Tooth Mousse, intended to be applied directly on teeth. Its clinical usefulness is unknown.[6]

Biogenic ACP

Biogenic ACP has been found in the inner ear of embryonic sharks, mammalian milk and dental enamel. However, whilst the unequivocal presence of ACP in bones and teeth is the subject of debate, there is evidence that transient amorphous precursors are involved in the development of bone and teeth.[4] The ACP in bovine milk (CPP-ACP) is believed to involve calcium phosphate nanoclusters covered in a shell of casein phosphopeptides. A typical casein micelle of radius 100 nm contains around 10,000 casein molecules and 800 nanoclusters of ACP, each of an approximate diameter of 4.8 nm. The concentration of calcium phosphate is higher in milk than in serum, but it rarely forms deposits of insoluble phosphates.[7] Unfolded phosphopeptides are believed to sequester ACP nanoclusters,[8] and form stable complexes in other biofluids such as urine and blood serum, thus preventing deposition of insoluble calcium phosphates and calcification of soft tissue. In the laboratory, stored samples of formulations of artificial blood, serum, urine and milk (which approximate the pH of the naturally occurring fluid) deposit insoluble phosphates. The addition of suitable phosphopeptides prevents precipitation.[7]

Posner's clusters

Following investigations into the composition of amorphous calcium phosphates precipitated under different conditions, Posner and Betts suggested in the mid 1970s that the structural unit of ACP was a neutral cluster Ca9(PO4)6.[4] Calculations support the description of a cluster with central Ca2+ ion surrounded by six phosphate PO43− anions, which in turn are surrounded by eight further calcium ions.[9] The resulting cluster is estimated to have a diameter of around 950 pm (0.95 nm). These are now generally referred to as Posner's clusters. Precipitated ACP is believed to be made up of particles containing a number of Posner's clusters with water in the intervening spaces. While plasma spray coated ACP may contain Posner's clusters, there cannot be any water present.[4]

Use in dental treatment

Amorphous calcium phosphate in combination with casein phosphopeptide has been used as a dental treatment to treat incipient dental decay. ACP sees its main use as an occluding agent, which aids in reducing sensitivity. Studies have shown that it does form a remineralized phase of hydroxyapatite consistent with the natural enamel. In addition, clinical studies have shown that patients who whiten their teeth have reduced sensitivity after treatment.[10] It is believed that ACP hydrolyzes under physiological temperatures and pH to form octacalcium phosphate as an intermediate, and then surface apatite.

Method of mineralization

ACP lacks the long-range, periodic atomic-scale order of crystalline calcium phosphates. The X-ray diffraction pattern is broad and diffuse with a maximum at 25 degree 2 theta, and no other different features compared with well-crystallized hydroxyapatite. Under electron microscopy, its morphological form is shown as small spheroidal particles in the scale of tenths nanometer. In aqueous media, ACP is easily transformed into crystalline phases such as octacalcium phosphate and apatite due to the growing of microcrystallites. It has been demonstrated that ACP has better osteoconductivity and biodegradability than tricalcium phosphate and hydroxyapatite in vivo.[11]

Moreover, it can increase alkaline phosphatase activities of mesoblasts, enhance cell proliferation and promote cell adhesion. The unique role of ACP during the formation of mineralized tissues makes it a promising candidate material for tissue repair and regeneration. ACP may also be a potential remineralizing agent in dental applications. Recently developed ACP-filled bioactive composites are believed to be effective anti-demineralizing/remineralizing agents for the preservation and repair of tooth structures.[11]

See also

References
  1. Destainville, A., Champion, E., Bernache-Assollant, D., Laborde, E. (2003). "Synthesis, characterization and thermal behavior of apatitic tricalcium phosphate". Materials Chemistry and Physics. 80 (1): 269–277. doi:10.1016/S0254-0584(02)00466-2. ISSN 1742-7061.CS1 maint: uses authors parameter (link)  – via ScienceDirect (Subscription may be required or content may be available in libraries.)
  2. Al-Sanabani, JS; Madfa, AA; Al-Sanabani, FA (2013). "Application of calcium phosphate materials in dentistry". International Journal of Biomaterials. 2013: 876132. doi:10.1155/2013/876132. PMC 3710628. PMID 23878541.
  3. Rey, C.; Combes, C.; Drouet, C.; Grossin, D. (2011). "1.111. Bioactive Ceramics: Physical Chemistry". In Ducheyne, Paul (ed.). Comprehensive Biomaterials. 1. Elsevier. pp. 187–281. doi:10.1016/B978-0-08-055294-1.00178-1. ISBN 978-0-08-055294-1.  – via ScienceDirect (Subscription may be required or content may be available in libraries.)
  4. Dorozhkin, Sergey V. (December 2012). "Amorphous calcium (ortho)phosphates". Acta Biomaterialia. 6 (12): 4457–4475. doi:10.1016/j.actbio.2010.06.031. ISSN 1742-7061. PMID 20609395.CS1 maint: uses authors parameter (link)  – via ScienceDirect (Subscription may be required or content may be available in libraries.)
  5. Babaie, Elham; Zhou, Huan; Lin, Boren; Bhaduri, Sarit B. (August 2015). "Influence of ethanol content in the precipitation medium on the composition, structure and reactivity of magnesium–calcium phosphate". Materials Science and Engineering: C. 53: 204–211. doi:10.1016/j.msec.2015.04.011. PMID 26042708.
  6. Hani, Thikrayat Bani; O'Connell, Anne C.; Duane, Brett (24 June 2016). "Casein phosphopeptide-amorphous calcium phosphate products in caries prevention". Evidence-Based Dentistry. 17 (2): 46–47. doi:10.1038/sj.ebd.6401168. PMID 27339237.
  7. Holt, Carl (June 2013). "Unfolded phosphopolypeptides enable soft and hard tissues to coexist in the same organism with relative ease". Current Opinion in Structural Biology. 23 (3): 420–425. doi:10.1016/j.sbi.2013.02.010. ISSN 0959-440X. PMID 23622834.CS1 maint: uses authors parameter (link)  – via ScienceDirect (Subscription may be required or content may be available in libraries.)
  8. Holt, Carl; Sørensen, Esben S.; Clegg, Roger A. (2009). "Role of calcium phosphate nanoclusters in the control of calcification". FEBS Journal. 276 (8): 2308–2323. doi:10.1111/j.1742-4658.2009.06958.x. ISSN 1742-464X. PMID 19292864.
  9. Kanzaki, Noriko; Treboux, Gabin; Onuma, Kazuo; Tsutsumi, Sadao; Ito, Atsuo (November 2011). "Calcium phosphate clusters". Biomaterials. 22 (21): 2921–2929. doi:10.1016/S0142-9612(01)00039-4. ISSN 0142-9612. PMID 11561898.CS1 maint: uses authors parameter (link)  – via ScienceDirect (Subscription may be required or content may be available in libraries.)
  10. Van Haywood, B (2002). "Dentine hypersensitivity: bleaching and restorative considerations for successful management". International Dental Journal. 52: 376–384. doi:10.1002/j.1875-595x.2002.tb00937.x.
  11. Zhao, J.; Liu, Y.; Sun, W. B.; Zhang, H. (2011). "Amorphous calcium phosphate and its application in dentistry". Chem. Cent. J. 5: 40. doi:10.1186/1752-153X-5-40. PMC 3143077. PMID 21740535.
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