Structural arrangements at the interface between plasma sprayed calcium phosphates and bone.

Plasma sprayed coatings of tetracalcium phosphate, magnesium whitlockite and three types of hydroxyapatite, varying in degree of crystallinity, were evaluated with light microscopy, scanning electron microscopy and backscatter electron microscopy (BSE) after implantation periods of 1, 2 and 4 wk in rat femora. BSE revealed that both tetracalcium phosphate and semi-crystalline hydroxyapatite underwent distinct bulk degradation and loss of relatively large particles. Amorphous hydroxyapatite showed a gradual surface degradation, indicated by a transition zone varying in grey level between that of the coating and bone tissue, while degradation was negligible with the highly crystalline material and magnesium whitlockite. Degradation appeared to be related to bone apposition, since more bone seemed to be present on amorphous hydroxyapatite and tetracalcium phosphate, as compared to highly crystalline hydroxyapatite and magnesium whitlockite coatings. At the interface between bone and magnesium whitlockite, a seam of unmineralized bone-like tissue was frequently seen with light microscopy, while few areas with bone contact were present. X-ray microanalysis revealed that both the magnesium whitlockite coating and the unmineralized bone-like tissue contained substantial amounts of aluminium which, in addition to possible influences of magnesium, may have caused the impaired mineralization. The results of this preliminary study indicate that, with regard to early bone formation, amorphous hydroxyapatite coatings seem to be beneficial over highly crystalline coatings. However, further experiments should be performed to give conclusive data on (i) the statistical significance of the differences in bone apposition rate, and (ii) the long-term behaviour of both amorphous and highly crystalline coatings in bone and their relation to implant performance.