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Hydroxyapatite Formation in a Calcium Phosphate Cement Under Different Storage Conditions

Published

Author(s)

S Hirayama, Shozo Takagi, T Ikemi, Laurence Chow

Abstract

A water-activated self-hardening calcium phosphate cement (CPC) has been the subject of considerable interest in biomaterials research, especially as a bone graft material. A CPC derived from tetracalcium phosphate (TTCP) and dicalcium phosphate anhydrous (DCPA) hardens in 30 min with the formation of hydroxyapatite (HA) as the final product. In order to better understand how the strength development of CPC and conversion of the initial CPC components to HA as a function of time under two different storage conditions, namely, only in a mold or in a mold plus water immersion. Materials and Methods. TTCP was prepared by heating in a furnace an equimolar mixture of commercially obtained DCPA and CaCO3 at 1500 degrees} Celsius for 6 h followed by quenching at room temperature. The TTCP and the commercially obtained DCPA were ground in a planetary ball mill to obtain the desired median sized particles (17 micro υ and 1 micro υ, respectively). The CPC powder was prepared by mixing ground TTCP and DCPA powders to produce a Ca/P molar ratio of 1.67 . Cement samples were prepared by mixing the CPC powder with distilled water at a powder-to-liquid mass ratio of 4. The paste was placed into stainless steel molds (6 mm diameter x 3 mm height) and kept in the molds with 1 MPa of applied pressure for specific time periods up to 24 h in a 100 % relative humidity at 37 degrees} Celsius. In experiment 1, after removal from molds, the diametral tensile strength (DTS) of the samples was measured immediately; the fractured specimens were dried and ground for powder x-ray diffraction analysis (XRD) to determine the extent of HA formation. In experiment 2, after removal from molds, the samples were placed in water at 37 degrees} Celsius for the balance of a 24 h period from the time sample was prepared. The 24 h samples were then tested for DTS and were evaluated for HA formation by XRD as described in experiment 1. Results and Discussion. The mean DTS (n=5) and extent of HA formation (n=3) of the samples as a function of time in experiments 1 and 2 are shown in the figures (error bars denote standard deviations). In experiment 1, the DTS increased rapidly over the first 4 h and then more gradually; the extent of HA formation followed the same trend (Fig. 1). The strong correlation between DTS and HA content suggests that HA formation is primarily responsible for strength development. In experiment 2, the samples that were kept in the mold for a short time, e.g., 0.5 h, and then immersed in water for the balance of the 24 h period developed the same DTS and had the same HA content as the sample that was in the mold for 24 h (fig 2). The results suggest that under clinical conditions, where CPC is exposed to an aqueous environment during or immediately after hardening, the DTS and HA content of the cement at 24h would be comparable to those of the samples that were kept in the mold for the same length of time. Supported in part by NIST, ADAHF and NIH grant DE11789.
Citation
International Symposium on Advanced Materials With Biomedical Applications

Keywords

calcium phosphate cement, diametral tensile strength, dicalcium phosphate anhydrous, hydroxyapatite, storage conditions, tetracalcium phosphate, x-ray diffraction

Citation

Hirayama, S. , Takagi, S. , Ikemi, T. and Chow, L. (2021), Hydroxyapatite Formation in a Calcium Phosphate Cement Under Different Storage Conditions, International Symposium on Advanced Materials With Biomedical Applications (Accessed October 19, 2021)
Created October 12, 2021