PHOSPHATE CEMENT SYSTEMS

A.K. Sarkar, Hydration/dehydration characteristics of struvite and dittmarite pertaining to magnesium ammonium phosphate cement system, J. Mater. Set, 26 (1991) 2514-2518. 

A few specific examples of phosphate cement systems are listed... 

The formation of magnesium ammonium phosphate monohydrate, MgNH4P04 H20 at low water contents was reported by Popovics, et al. t The use of excess water in magnesia-phosphate cement systems has been found to lead to increased porosity and strength reduction. 

Figure 28. (a) DTA curves of magnesia-phosphate cement systems (b) DTA curves of... 

Kuang, G.M., Yau, W.P., Chiu, K.Y., 2014. Lxrcal apphcation of strontium in a calcium phosphate cement system accelerates heahng of soft tissue grafts in anterior cruciate ligament reconstruction experiment using a rabbit model. American Journal of Sports Medicine 42 (12), 2996-3002. http //dx.doi.org/10.1177/0363546514549536. 

Polymeric Calcium Phosphate Cements. Aqueous solutions of polymers such as poly(acryHc acid), poly(vinyl alcohol), gelatin, etc, and/or autopolymerizable monomer systems, eg, 2-hydroxyethyl methacrylate, glycerol dimethacrylate, calcium dimethacrylate, etc, have been used as Hquid vehicles (41,42,76) for the self-setting calcium phosphate cement derived from tetracalcium phosphate and dicalcium phosphate [7757-93-9J. 

The biocompatible CBPC development has occurred only in the last few years, and the recent trend has been to evaluate them as biocompatible ceramics. After all, biological systems form bone and dentine at room temperature, and it is natural to expect that biocompatible ceramics should also be formed at ambient temperature, preferably in a biological environment when placed in a body as a paste. CBPCs allow such placement. We have discussed such calcium phosphate-based cements in Chapter 13. Calcium-based CBPCs, especially those constituting hydroxyapatite (HAP), are a natural choice. HAP is a primary mineral in bone [3], and hence calcium phosphate cements can mimic natural bone. Some of these ceramics with tailored composition and microstructure are already in use, yet there is ample room for improvement. This Chapter focuses on the most recent biocompatible CBPCs and their testing in a biological environment. To understand biocompatible material and its biological environment, it is first necessary to understand the structure of bone and how it is formed. 

The reaction between hard-burned MgO, water, and potassium dihydrogen phosphate produces a quick setting cementitious mass, KMgP04-6H20. This cement system was originally developed at Argonne National Laboratory for the stabilization and encapsulation of hazardous and radioactive wastes (Wagh et al., 1998). 

Phosphorylated chitosan (P-C) was prepared and used as an additive for calcium phosphate cement (CPC) to improve its mechanical properties and particularly its compressive strength. The CPC/P-C system was proposed for use as a bone filler. 

Huan, Z. Chang, J. (2009) Novel bioactive composite bone cements based on the beta-tricalcium phosphate-monocalcium phosphate monohydrate composite cement system. Acta Biomater, 5, 1253-64. 

Ultimately, the cement hardens to form dahllite, a carbonated apatite similar to that found in the mineral phase of bone [27, 47]. The cement has a micro-porous structure with pores smaller than 1 //m [86]. Calcium phosphate cement is available from multiple manufactures, as an example Norian Skeletal Repair System (SRS, Synthes West Chester, PA) consists of a liquid sodium phosphate solution and powder monocalcium phosphate monohydrate, tricalcium phosphate, and calcium carbonate. After mixing, the cement has a working time of approximately five minutes and hardens in approximately ten minutes. The material has a compressive strength of approximately 10 MPa at 10 minutes and maximum compressive strength of approximately 50 MPa within 24 hours. 

An interesting application concerns the chitosan- calcium phosphate cement. Chitosan or chitosan glycerophosphate mixed with calcium phosphate and citric acid forms an attractive injectable self-hardening system for bone repair or filling indications [134, 213, 214],... 

In some other binder systems a reaction between constituents of different basicity is responsible for setting and hardening, as in calcium phosphate cements for example ... 

Calcium aluminate phosphate cementitious systems are obtained by combining calcium aluminate cement with alkali or ammonium phosphates or polyphosphates (Sugama and Carciello, 1991 Ma and Brown, 1992, 1994 Walter and Odler, 1996). They are discussed in section 12.3. 

The presence of tetracalcium phosphate is essential for this reaction to take place, as this is the only phase in the Ca0-P205-H20 system that is more basie than hydroxyapatite. Moreover, the structure similarity of tetracalcium phosphate to hydroxyapatite may also stimulate the reaction. Of the ealcium phosphates that are more acid than hydroxyapatite, the best results are obtained with the dicalcium phosphates, and thus only these are commonly used in calcium phosphate cements. In pure water and at ambient temperature reactions (12.4) and (12.5) progress very slowly however, both the reaction rate and the resulting strength may be controlled by the experimental conditions employed. 

Magnesium oxysulfate cement pastes can be produced through reaction of MgO and an aqueous solution of MgS04 7H20.t Alternatively magnesium chloride solutions can be added to calciiun sulfates or calcium phosphate-sulfate mixtures. The presence of phosphates enhances the water resistance of this cement system. 

Calcium phosphate-based systems have wide applications in biomedical areas. Brown has outlined the similarities between the hydration of calcium silicates and calcium phosphates. The hydration products in both systems have high surface areas, variable composition, and poor crystallinity. Pozzolanic reactions and Hadley-like grains form in both systems. The primary cement-water reactions for C3S and tetracalcium phosphate are as follows ... 

Typical strength development data of air cured silico-phosphate cement indicate that this system develops about 30% of its 28 day compressive strength in 2 hours. The tensile bond strength, compared with conventional concrete substrates, is very poor.I ]... 

Abdelrazig and co-workers studied the hydration (at 22°C) of MgO (75 g) and monoammonium phosphate (56 g) a composition ratio not dissimilar to commercial phosphate cements.P Mortars (Systems 1 and 2) and pastes (System 4) were investigated. The mortars (containing quartz sand) were prepared at water/solid ratios of 0.62 and 0.125 the pastes had water/solid ratios of 0.125. Thermograms (DTA) of mortar and paste are shown in Fig. 28a. The hydration time is one week. 

A number of delivery systems have been developed for the controlled release of bone morphogenic protein-2 (BMP-2). For example, calcimn phosphate cement-based materials have shown the ability to deliver recombinant human bone morphogenic protein-2 (rhBMP-2) to increase alkaline phosphatase (ALP) activity in MC3T3-E1 cells in vitro,and enhance bone formation both ectopically and in an ulna osteotomy model. In addition to cement-based systems, polymeric materials have also been heavily researched as potential delivery vehicles of BMP-2. Saito et al. have developed a temperature sensitive poly(D,L-lactic acid-polyethylene glycol) (PLA-PEG) block copolymer as an... 

Ginebra, M.P., Traykova, T. and Planell, JA. (2006) Calcium phosphate cements as bone drug delivery systems a review. Journal of Controlled Release, 113,102 10. 

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