Effect of heat treatment on setting behavior and compressive strength of tetracalcium phosphate cement

Abstract

Due to its superior biocompatibility and osteoconductivity, calcium phosphate cement (CPC) has been suggested for use as a filling material for dental and orthopedic applications [1–4]. The primary reaction product of most CPCs, after being implanted for a while, is various kinds of apatite [5], which is the main inorganic component of human bones and teeth [6, 7]. Tetracalcium phosphate (TTCP), a member of the calcium phosphate family, is known to have a high chemical activity and reacts with aqueous solution at room temperature [8]. The obtained hardened material has a high affinity for the living body and is therefore frequently used as a constituent in CPC powder [9–11]. Despite many advantages, there are problems for CPC during practical application, including working/setting time being too short or too long and dispersion upon early contact with blood or aqueous media. Appropriate working/setting times are critical for surgical applications. In the laboratory, a monolithic TTCP powder with nano-sized whiskers grown on surface was recently developed [12], which demonstrated excellent mechanical properties and biological responses with a relatively short working/setting time. The present work provides a simple heat treatment method for prolonging the working/setting time, while maintaining the strength, of the TTCP cement. The TTCP powder used for the study was fabricated in-house from the reaction of dicalcium pyrophosphate (Ca2P2O7) (Sigma Chem. Co., St. Louis, MO, USA) and calcium carbonate (CaCO3) (Katayama Chem. Co., Tokyo, Japan) by a weight ratio of 1:1.27. The powders were mixed uniformly in ethanol for 12 hr, followed by heating in an oven to let the powders dry. The dried powder mixture was then heated to 1400 ◦C and allowed to react and form TTCP [13]. Since a “whisker treatment” [12] has been very effective in enhancing the properties of TTCP cement, the same treatment was used to treat the TTCP powder for the present study. The whisker-treated TTCP powder was heat-treated in an air furnace (N 7/H, Nabertherm©R, Germany). Different heat-treatment temperatures (140–400 ◦C) and times (30 and 120 min) were used for the study. To form a CPC paste, the TTCP powder was mixed with a diammonium hydrogenphosphate ((NH4)2HPO4) hardening solution with a pH value of 8.6 and liquid/powder ratio of 0.3 ml/g. After mixing for 1 min, the cement