The chemical laboratory at a commercial coating company is responsible for characterizing and testing the quality of hydroxyapatite (HA) for both incoming raw material and the applied coatings prior to acceptance of a manufactured lot. Quality control tests of the raw material include measurement of crystallinity of the HA using X-ray diffraction (XRD) and Fourier Transform Infrared spectroscopic (FTIR) methods and chemical analysis of the HA for the proper calcium to phosphorus ratio and for the presence of impurities. The finished coatings undergo all the same tests as the raw material plus an evaluation of adhesion.
Evaluated raw materials have varied widely in composition and properties. Chemical compositions ranged from stoichiometric HA to tricalcium phosphate. Impurities ranged from a probably biologically insignificant presence of magnesium to the presence of over four weight percent of toxic barium and included up to 10 weight percent soluble calcium carbonate. The crystallinity varied from greater than 90% to less than 5%. Variations in crystallinity, the presence of calcium oxide and calcium carbonate, and substitutions of cations other than calcium have the potential of causing variations in the final coating. The reported crystallinity variation was between two different lots from the same supplier. Therefore, it is necessary to continue Quality Control (QC) testing, even for previously approved vendors. The samples with high levels of barium were submitted for use as a less expensive source of HA. This indicates the need to examine carefully all sources of raw materials.
Since the coating used by our company on implants is applied in an inert gas plasma arc, no introduction of foreign cations occurs. However, changes in crystallinity and the transformation from HA to other calcium phosphatephases can occur. The early detection of such changes allows for rapid correction of operator parameters.
In the process of testing HA, a variety of techniques have been developed which may be of general use in the field of HA studies. These include a rapid screening for the presence of carbonate by use of an FTIR, examination of the presence of gross levels of contaminants and simultaneous measurement of grain size and shape by use of a scanning electron microscope with an energy dispersive spectrometer, and working methods to determine the concentration of foreign cations using an atomic absorption spectrophotometer. Additionally, the use of a common vocabulary for HA properties and standardized methods will allow comparison of coatings, not of sales hype.