Bioactive ceramic coatings have been available for some 15 years and have been proved to have many advantages with regard to the short and mid-term behaviour compared to cemented implants. Nevertheless, the useful lifetime for this kind of prostheses is comparable to conventional implants, and requires further investigation. We present a combined study relating the defect structure in plasma sprayed HAp coatings to their microstructure and internal stress state with regard to biochemical changes in a simulated biological environment.