A Study On Preparation And Properties Of Novel Calcium Phosphate Cement

High biocompatibility self –setting capability near perfect adaptation of thecement to the tissue surfaces in a defect, and an optimum resorption ratefollowed by new bone formation are some of the distinctive advantages ofcalcium phosphate cement (CPC). CPC is desirable in a broad range of clinicalapplication for repairing bone defects. In this dissertation, two series of CPC were mainly investigated. They wereused alpha tricalcium phosphate (α-TCP) as an essential constituent and mixedwith aqueous solution of disodium phosphate (Na2HPO4) or citric acidrespectively. In the first series of CPC, the relationship between the physical andchemical change, the paste microstructure induced by setting reaction and theirmechanism were studied systematically. The results showed that the chemicalprocesses of CPC hydration reaction were controlled by dissolution andprecipitation chemical reactions. The setting time of CPC was related to thedissolution of α-TCP and the formation rate of amorphous calcium phosphates.The reaction of α-TCP was finally converted to hydroxyapatite (HA), whilethe conversion of the reactant to HA can increase the compressive strength ofthe samples. Here the growth process was controlled via α-TCP's surfacechemistry and growth space at early stage, while there was diffusion controll atlate stage. In another series which used the solution of citric acid as liquid phase, theeffects of preparing conditions, e.g. liquid to powder ratio, particle size andadditive as zirconia on compressive strength and setting time of CPC wasdiscussed. The CPC microstructure induced by setting reaction and theirmechanical property were analyzed, that provided a basis for determination ofoptimal preparing conditions for high performance CPC. The results revealedthat the acid-base reaction of the citric acid and α-TCP occured during theinitial stages of cement mixing, a core structure consisting of unreactedα-TCPparticles embedded in an amorphous matrix made up of calcium citrate wasformed. The microstructure of CPC could be controlled by changing preparingconditions. Moreover, the improved compressive strength and the suitablesetting time of CPC would be obtained. The setting time could be shorten andcompressive strength of CPC could be improved in some degree by decrease α-TCP particle size and reduce the liquid phase properly. Declining particle sizecould accelerate the hydration reaction and reduce the liquid needed especially.The latter will be benefit to the injectability of CPC. In the preparation of theinjectable CPC, citric acid behaves as water-reducing agent, can enhance thefluidity of the cement paste. But it is clear that citric acid has been acting as aretarder for the dissolution ofα-TCP and the processes of nucleation andgrowth of apatite crystals because of its overuse. The addition of zirconia canshorten the setting time, furthermore, the formation of Zr-OH group on thesurface of zirconia particles is effective for apatite nucleation. Based on biomimicry, adjuvants of gelatin and chitosan were used, andtheir inferences on the property of CPC were investigated. The result showedthat the use of these adjuvants extended the setting time and reduces thecompressive strength of CPC, but microstructure analysis indicated that they areeffective for the growth of apatite crystalines. Variation of pH values in the slurry of CPC during its hydration wasstudied. The results show that the pH values in CPCs of two series close tophysiological range (6.8-7.2) after 24h, which is suitable for clinical use.