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

Large, cylindrical implants of a porous calcium phosphate ceramic ("hydroxy apatite" starting material, HAC) were used to replace far greater than crit ical-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 clini cal function of these implants was reported in a previous study. The materi al retrieved was embedded in PMMA, and blocks were sectioned and surfaces w ere polished and carbon coated prior to study using digital backscattered e lectron (BSE) imaging. Detailed scanning electron microscopy study of the p attern of osseointegration of the implanted material at early (2 months) an d late (9 months) timepoints revealed a previously unrecognized pattern of integration/disintegration of this implant material in tandem with bone gro wth. We conclude that bone adaptation to the HAC leads to its fracture and that the newly generated surfaces are equally osteoconductive, This leads t o a self-propagating, self-annealing system in which defects in the HAC are mended by intercalation of bone. (Bone 24:579-589; 1999) (C) 1999 by Elsev ier Science Inc. All rights reserved.