The combination of self-setting and biocompatibility makes calcium phosphate cements potentially useful materials for a variety of dental applications. The objective of this study was to investigate the setting and hardening mechanisms of a cement-type reaction leading to the formation of calcium-deficient hydroxyapatite at low temperature. Reactants used were alpha-tricalcium phosphate containing 17 wt% beta-tricalcium phosphate, and 2 wt% of precipitated hydroxyapatite as solid phase and an aqueous solution 2.5 wt% of disodium hydrogen phosphate as liquid phase. The transformation of the mixture was stopped at selected times by a freeze-drying techniques, so that the cement properties at various stages could be studied by means of x-ray diffraction, infrared spectroscopy, and scanning electron microscopy. Also, the compressive strength of the cement was measured as a function of time. The results showed that: (1) the cement setting was the result of the alpha-tricalcium phosphate hydrolysis, giving as a product calcium-deficient hydroxyapatite, while beta-tricalcium phosphate did not participate in the reaction; (2) the extent of conversion of alpha-TCP was nearly 80% after 24 hr; (3) both the extent of conversion and the compressive strength increased initially linearly with time, subsequently reaching a saturation level, with a strong correlation observed between them, indicating that the microstructural changes taking place as the setting reaction proceeded were responsible for the mechanical behavior of the cement; and (4) the microstructure of the set cement consisted of clusters of big plates with radial or parallel orientations in a matrix of small plate-like crystals.

Calcium phosphate bone cements (CPBCs) are osteotransductive, i.e. after implantation in bone they are transformed into new bone tissue. Furthermore, due to the fact that they are mouldable, their osteointegration is immediate. Their chemistry has been established previously. Some CPBCs contain amorphous calcium phosphate (ACP) and set by a sol-gel transition. The others are crystalline and can give as the reaction product dicalcium phosphate dihydrate (DCPD), calcium-deficient hydroxyapatite (CDHA), carbonated apatite (CA) or hydroxyapatite (HA). Mixed-type gypsum-DCPD cements are also described. In vivo rates of osteotransduction vary as follows: gypsum-DCPD > DCPD > CDHA approximately CA > HA. The osteotransduction of CDHA-type cements may be increased by adding dicalcium phosphate anhydrous (DCP) and/or CaCO3 to the cement powder. CPBCs can be used for healing of bone defects, bone augmentation and bone reconstruction. Incorporation of drugs like antibiotics and bone morphogenetic protein is envisaged. Load-bearing applications are allowed for CHDA-type, CA-type and HA-type CPBCs as they have a higher compressive strength than human trabecular bone (10 MPa).

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The setting reaction of a calcium phosphate bone cement consisting of a mixture of 63.2 wt % alpha-tertiary calcium phosphate (TCP)[alpha-Ca3(PO4)2], 27.7 wt % dicalcium phosphate (DCP) (CaHPO4), and 9.1 wt % of precipitated hydroxyapatite [(PHA) used as seed material] was investigated. The cement samples were prepared at a liquid-to-powder ratio of: L/P = 0.30 ml/g. Bi-distilled water was used as liquid solution. After mixing the powder and liquid, some samples were molded and aged in Ringer's solution at 37 degrees C. At fixed time intervals they were unmolded and then immediately frozen in liquid nitrogen at a temperature of TN = -196 degrees C, lyofilized, and examined by X-ray diffraction as powder samples. The compressive strength versus time was also measured in setting samples of this calcium phosphate bone cement. The crystal entanglement morphology was examined by scanning electron microscopy. The results showed that: 1) alpha-TCP reacted to a calcium-deficient hydroxyapatite (CDHA), Ca9(HPO4)(PO4)5O H, whereas DCP did not react significantly; 2) the reaction was nearly finished within 32 h, during which both the reaction percentage and the compressive strength increased versus time, with a strong correlation between them; and 3) the calcium phosphate bone cement showed in general a structure of groups of interconnected large plates distributed among agglomerations of small crystal plates arranged in very dense packings.

In the system CaO-P,O,-H,O 13 different solids with varying Ca/P ratios are known. In addition calcium phosphates containing other biocompatible constituents like Na, or K, or Mg or Cl or carbonate, are known. Therefore, a large number of combinations of such compounds is possible which might result in the formation of calcium phosphate cements upon mixing with water. However, the number of calcium phosphates possibly formed by precipitation at room or body temperatures is limited to 12, which should limit the number of suitable combinations.In this study more than 450 different combinations of reactants have been investigated.The results were evaluated on the basis of the following criteria: (a) was the intended reaction product formed? (b) was the final setting time shorter than 60 min? (c) was the compressive strength after soaking for 1 day in Ringer's solution at 37 "C higher than 2 MPa? We found that 15 formulations satisfied all of these criteria. The distribution of cements synthesized in this way was 3 DCPD type, 3 CMP type, 6 OCP type and 3 CDHA type cements. The DCPD type cements were acidic during setting and remained that for a long time afterwards. CDHA type cements were neutral or basic during setting, and remained neutral after completion of the reaction. The OCP type cements were neutral both during and after setting. Two CMP type cements were basic both during and after setting. In this study compressive strengths were found up to 90 MPa. Also, in the literature values up to 90 MPa have been reported for this type of cement. Taking into account the excellent biocompatibility and the good osteoconductivity of calcium phosphates and the fact that these calcium phosphate cements can be injected into the site of operation, it may be expected that these materials will become the materials of choice for bone replacement and augmentation. Their suitability for the fixation of metal endoprostheses for joint replacement should be investigated as well.

Calcium orthophosphates which either can. be formed by precipitation at room or body temperatures or by reactions at higher temperatures have been reviewed. Most formulations of cements contain at least one or more acidic components and one or more basic components which react when the powder is mixed with water. Several combinations of reactants are possible in the systems of calcium phosphates and even combinations with calcium phosphates containing sodium, potassium, magnesium, zinc, carbonate or chloride are thought to be useful. Furthermore, other compounds can be added as accelerators or retarders of the setting reaction or as promoters of bone ingrowth. In this study, over 100 formulations have been tested on their ability to set upon mixing with water. The initial and final setting times were measured with Gilmore needles.

Clinical requirements for calcium phosphate bone cements were formulated in terms of the initial setting time, the final setting time, the cohesion time, and the ultimate compressive strength. Two cement formulations were tested. Biocement F was made of a powder containing alpha-tertiary calcium phosphate, precipitated hydroxyapatite, and monetite. Biocement D powder also contained CaCO3. The liquid/powder (L/P) ratio of the cement paste and the accelerator concentrations (% Na2HPO4) in the cement liquid were varied. For Biocement F there was a small area of combinations of L/P ratio and percent Na2HPO4 for which all clinical requirements were satisfied. This area covered only pastes that could be applied as doughs. However, Biocement D showed a much larger area of full compliance and it covered both doughlike and injectable pastes.

The effect of additives and temperature on setting time, swelling time and compressive strength of a previously developed apatitic calcium phosphate cement was investigated. Setting was faster at body temperature than at room temperature. Early contact with aqueous solutions resembling blood and other body fluids had no effect. Deliberate additions of soluble carbonates, pyrophosphate or magnesium salts to the cement powder retarded or even inhibited setting. However, additions of calcium pyrophosphate, 13-tertiary calcium phosphate or sintered hydroxyapatite to the cement powder in amounts up to 10% had no effect on the cement properties. Several organic substances were used as additives. They all retarded the setting and decreased the strength of the cement considerably.