| Most calcium phosphates are classified as resorbable biomaterials. This means that under physiological conditions they will dissolve. The benefit of calcium phosphate biomaterials is that the dissolution products can be readily assimilated by the human body. Calcium Phosphates as Bone Defect Fillers Due the resorbable nature of calcium phosphate, with the general exception of hydroxyapatite, they have been proposed as potential bone defect fillers. In this application, they would fill the void and gradually dissolve away, being replaced by bone. However, the uncontrollable resorption rate has hindered their uptake in clinical applications. Calcium Phosphate Compounds Listed in the table below are some calcium phosphate compounds of biomaterials interest. Table 1. Some calcium phosphate compounds of biomaterials interest. | | | | | | | Amorphous calcium phosphate | ACP | - | - | - | | Dicalcium Phosphate | DCP | CaHPO4 | Monetite | 1.00 | | Tricalcium Phosphate | α-TCP | Ca3(PO4)2 | | 1.50 | | Tricalcium Phosphate | β-TCP | Ca3(PO4)2 | Whitlockite | 1.50 | | Pentacalcium Hydroxyl Apatite | HAp | Ca10(PO4)6(OH)2 | Hydroxyapatite | 1.67 | | Tetracalcium Phosphate Monoxide | TTCP | Ca4O(PO4)2 | Hilgenstockite | 2.00 | Amorphous calcium phosphate is a phase that is often formed during high temperature processing, such as is the case with plasma spraying of hydroxyapatite. It and other phases, may be associated with hydroxyapatite after high temperature processing and the subsequent decomposition when dealing with hydroxyapatite Solubility of Calcium Phosphate Compounds While the forming method and exact stoichiometry will have an effect on solubility, the generally accepted order of solubility is: ACP > DCP > TTCP > α-TCP > β-TCP >> HAp The relative insolubility of hydroxyapatite compared to the other calcium phosphate phases is not surprising as it is the only stable calcium phosphate compound at pH’s above 4.2. Below this, dicalcium phosphate dihydrate (CaHPO4.2H2O) is the stable compound. It is not uncommon for unstable calcium phosphates to dissolve and repreciptate as the stable compound at a given pH. Under normal physiological conditions of pH 7.2, hydroxyapatite is the stable calcium phosphate compound. This may drop to as low as pH 5.5 in the region of tissue damage, although this would eventually return to pH 7.2 over a period of time. Even under these conditions hydroxyapatite is still the stable phase. | 
