Osseointegration in cortical sheep bone of calcium phosphate implants evaluated by PIXE method and histology.

Osseointegration of porous calcium phosphate ceramics evolves in several stages once implanted. Histologic analysis has often been used to evaluate the mechanism of integration of this material. Histologic parameters can be completed by physical analysis to obtain a semiquantitative evaluation of the osseointegration process. The histologic observation of hydroxyapatite (HA)-ceramic-containing bone sections was associated with proton-induced X-ray emission (PIXE) analysis and the results obtained by both methods were compared. Porous HA-ceramic cylinders were implanted in cortical bone of sheep femurs for periods ranging from 2 to 36 weeks. Thick sections of the implant containing bone were made at the end of the implantation period. A scanning line with two proton impacts 0.5 mm apart was plotted from the edges of cortical bone across the implanted ceramic and the X-rays produced were determined. Calcium, phosphorus, zinc, strontium, and iron contents were measured. Following PIXE analysis, the sections were surface-stained and observed under a light microscope to define the osseointegration index. Two regions of the curves were identified for each element characterizing either the bone tissue or the ceramic. Zinc and strontium present in the bone tissue but absent from the ceramics appeared after the 8th and the 12th implantation weeks, respectively. Iron present in the implant decreased with time, and calcium and phosphorus contents tended to be the same at the end of the implantation period in both curve regions. Histologic observation showed that immature bone invaded the pores of the outer layer of the ceramic as early as 2 weeks after implantation. Ceramics were totally osseointegrated 20 weeks after implantation. Osseointegration was apparently still evolving as judged by the PIXE method when histologic integration was considered complete.