Calcium orthophosphates (CaPO4): occurrence and properties

Sergey V Dorozhkin

doi:10.1007/s40204-015-0045-z

Abstract

The present overview is intended to point the readers’ attention to the important subject of calcium orthophosphates (CaPO4). This type of materials is of the special significance for the human beings because they represent the inorganic part of major normal (bones, teeth and antlers) and pathological (i.e., those appearing due to various diseases) calcified tissues of mammals. For example, atherosclerosis results in blood vessel blockage caused by a solid composite of cholesterol with CaPO4, while dental caries and osteoporosis mean a partial decalcification of teeth and bones, respectively, that results in replacement of a less soluble and harder biological apatite by more soluble and softer calcium hydrogenorthophosphates. Therefore, the processes of both normal and pathological calcifications are just an in vivo crystallization of CaPO4. Similarly, dental caries and osteoporosis might be considered as in vivo dissolution of CaPO4. In addition, natural CaPO4 are the major source of phosphorus, which is used to produce agricultural fertilizers, detergents and various phosphorus-containing chemicals. Thus, there is a great significance of CaPO4 for the humankind and, in this paper, an overview on the current knowledge on this subject is provided.

Highlights

Due to the abundance in nature and presence in living organisms, calcium phosphates are the inorganic compounds of a special interest for human being
One should note on the difficulties in mimicking the calcification process that occurs in bones and teeth
By the end of the XX-th century, it became clear that CaPO4-based biomaterials and bioceramics by themselves could not give a complete response to the clinical needs for artificial implants

Summary

Introduction

Due to the abundance in nature (as phosphate ores) and presence in living organisms (as bones, teeth, deer antlers and the majority of various pathological calcifications), calcium phosphates are the inorganic compounds of a special interest for human being. The easiest way to perform the crystallization would be mixing of aqueous solutions containing the ions of calcium and orthophosphate (LeGeros 1991; Elliott 1994; Amjad 1997) Such type of crystallization provides precipitates with the properties (chemical composition, Ca/P ratio, crystallinity level, particle size distribution, etc.) far different from those of biological apatite. Whenever possible this should be avoided because the application of condensed solutions of SBF leads to changes in the chemical composition of the precipitates; namely, the concentration of carbonates increases, while the concentration of orthophosphates decreases (Dorozhkina and Dorozhkin 2003b) To conclude this part, one should note on the difficulties in mimicking the calcification process that occurs in bones and teeth. Such biomimetically prepared CaPO4 precipitates are occasionally called ‘‘organoapatites’’ (Spoerke and Stupp 2003; Storrie and Stupp 2005)

Conclusions and outlook

Findings

Compliance with ethical standards