Surface Areas of Synthetic Calcium Phosphates and Bone Mineral 1

Abstract

The surface or interface of bone mineral is an important parameter in bone metabolism. In particular, this surface must play a role in the interchange of ions between the body fluids and bone mineral. This is a report on a study of the specific surfaces obtained on a series of synthetic hydroxyapatites. synthetic amorphous calcium phosphates, and bone samples, treated in different ways. The surface measurements have been made with gas adsorption and small angle X-ray scattering techniques. A comparison of the results obtained by these contrasting methods adds to our knowledge of the interface between bone mineral and the remainder of bone which is essentially water and collagen. In addition, this study gives a comparison of surface measurements made by these fundamentally different methods.

Materials and Methods. 1. Synthetics. Large batches of a series of samples of poorly crystallized hydroxyapatite and amorphous calcium phosphate were prepared by the method described by Eanes et al. (1). Surface area measurments were obtained using low temperature adsorption of N₂ and the application of the BET method (2). Concomitant surface measurements by small angle X-ray scattering were performed.

It was ascertained by wide angle X-ray diffraction (3), that the hydroxyapatites were 100% crystalline (i.e., contained no amorphous fraction) and the amorphous calcium phosphates were free of any crystalline apatite. This assay was necessary because the amorphous phase is a precursor in the chemical precipitation of hydroxyapatite (1) and one may contaminate the other.

The precipitated samples were separated from the solution by freeze-drying and then stored in stoppered vessels at room temperature. Prior to the adsorption measurements the samples were evacuated to 10^-6 Torr at 300° for periods of 18–36 hr. Weight losses on these samples due to loss of water varied from 10–20% as determined by weighing the samples prior to and after evacuation.