Properties of calcium phosphate cements with different tetracalcium phosphate and dicalcium phosphate anhydrous molar ratios.

Calcium phosphate cements (CPCs) were prepared using mixtures of tetracalcium phosphate (TTCP) and dicalcium phosphate anhydrous (DCPA), with TTCP/DCPA molar ratios of 1/1, 1/2, or 1/3, with the powder and water as the liquid. Diametral tensile strength (DTS), porosity, and phase composition (powder x-ray diffraction) were determined after the set specimens have been immersed in a physiological-like solution (PLS) for 1 d, 5 d, and 10 d. Cement dissolution rates in an acidified PLS were measured using a dual constant composition method. Setting times ((30 [+ or -] 1) min) were the same for all cements. DTS decreased with decreasing TTCP/DCPA ratio and, in some cases, also decreased with PLS immersion time. Porosity and hydroxyapatite (HA) formation increased with PLS immersion time. Cements with TTCP/DCPA ratios of 1/2 and 1/3, which formed calcium-deficient HA, dissolved more rapidly than the cement with a ratio of 1/1. In conclusion, cements may be prepared with a range of TTCP/DCPA ratios, and those with lower ratio had lower strengths but dissolved more rapidly in acidified PLS.

Key words: absorption; calcium phosphate; calcium phosphate cements; hydroxyapatite; mechanical properties; porosity.

Accepted: September 19, 2008

Available online: http://www.nist.gov/jres

1. Introduction

Water-activated self-hardening calcium phosphate cements (CPCs) have been the subject of considerable interest in biomaterial research, especially as bone graft materials. A CPC that consisted of equimolar amounts of tetracalcium phosphate (TTCP), [Ca.sub.4][([PO.sub.4]).sub.2]O, and dicalcium phosphate anhydrous (DCPA), [CaHPO.sub.4], was the first such material in clinical use. It was shown to harden in about 30 min when water was used as the cement liquid and formed hydroxyapatitie (HA) as the product. A previous study showed that the TTCP/DCPA cements can harden satisfactorily and develop reasonable strengths when the TTCP/DCPA molar ratio was as low as 1/2 (1). Another study (2) investigated properties of cements with TTCP/DCPA ratios ranging from 0.25 to 2.0. This study reported the pH of cement slurries during setting, and the strengths and HA conversion of 1-d cement samples.

Calcium-deficient HA with a Ca/P molar ratio lower than 1.67, the ratio for stoichiometric HA, were formed in cements with a TTCP/DCPA ratio less than 1.0 (2). Calcium-deficient HA is reported to be more soluble and therefore may resorb more rapidly in vivo than stoichiometric HA (3). Dissolution rate measurements under constant-composition conditions were previously proposed (4) as an in vitro model for estimating relative in vivo resorption rates for calcium phosphate bone graft materials. The present study investigated the dissolution rate of 1-d cement samples with TTCP/DCPA molar ratios of 1/1, 1/2, and 1/3, yielding overall Ca/P molar ratios of 1.67, 1.50, and 1.33, respectively. Extent of HA conversion, diametral tensile strength (DTS), and porosity of cement samples of varying TTCP/DCPA ratios that had been immersed in a physiological-like solution (PLS) for 1 d, 5 d, and 10 d were also determined.