Influence of porosity on the mechanical resistance of hydroxyapatite ceramics under compressive stress.

Calcium phosphate ceramics are biocompatible and may develop interactions with human living bone tissues. They are used clinically on the surface of orthopaedic implants to improve primary fixation or in the form of porous blocks. Their brittleness is often advanced as a limitation of their common clinical use. In order to study the influence of porosity on the mechanical strength of calcium phosphate ceramics, we have tested 150 cylindrical hydroxyapatite samples with open porosity. The total porous volume of the ceramics has been varied from 20% to 60% and the pore size from 5 microns to 400 microns. The result indicates that not only total porosity but also pore size can influence compressive strength, which is in good agreement with theoretical work. After mathematical treatment of the results, the experiments have been modelled in the form of a polynomial equation which can be used to predict and optimize mechanical strength. Moreover, this work supports the fact that compressive strength of controlled open porosity implants can be comparable with that of cancellous or cortical human bone, and suggests that porosity should be fitted to clinical application.