Osteoconduction in large macroporous hydroxyapatite ceramic implants: evidence for a complementary integration and disintegration mechanism

Large, cylindrical implants of a porous calcium phosphate ceramic ("hydroxyapatite" starting material, HAC) were used to replace far greater than critical-sized sections of the midshaft of sheep tibiae and retrieved at 2 and 9 months; external fixation was used in the first 5 months. Excellent clinical function of these implants was reported in a previous study. The material retrieved was embedded in PMMA, and blocks were sectioned and surfaces were polished and carbon coated prior to study using digital backscattered electron (BSE) imaging. Detailed scanning electron microscopy study of the pattern of osseointegration of the implanted material at early (2 months) and late (9 months) timepoints revealed a previously unrecognized pattern of integration/disintegration of this implant material in tandem with bone growth. We conclude that bone adaptation to the HAC leads to its fracture and that the newly generated surfaces are equally osteoconductive. This leads to a self-propagating, self-annealing system in which defects in the HAC are mended by intercalation of bone.