Preparation of resorbable carbonate-substituted hollow hydroxyapatite microspheres and their evaluation in osseous defects in vivo

Hollow hydroxyapatite (HA) microspheres, with a high-surface-area mesoporous shell, can provide a unique bioactive and osteoconductive carrier for proteins to stimulate bone regeneration. However, synthetic HA has a slow resorption rate and a limited ability to remodel into bone. In the present study, hollow HA microspheres with controllable amounts of carbonate substitution (0-12 wt.%) were created using a novel glass conversion route and evaluated in vitro and in vivo. Hollow HA microspheres with ~ 12 wt.% of carbonate (designated CHA12) showed a higher surface area (236 m2 g− 1) than conventional hollow HA microspheres (179 m2 g− 1) and a faster degradation rate in a potassium acetate buffer solution. When implanted for 12 weeks in rat calvarial defects, the CHA12 and HA microspheres showed a limited capacity to regenerate bone but the CHA12 microspheres resorbed faster than the HA microspheres. Loading the microspheres with bone morphogenetic protein-2 (BMP2) (1 μg per defect) stimulated bone regeneration and accelerated resorption of the CHA12 microspheres. At 12 weeks, the amount of new bone in the defects implanted with the CHA12 microspheres (73 ± 8%) was significantly higher than the HA microspheres (59 ± 2%) while the amount of residual CHA12 microspheres (7 ± 2% of the total defect area) was significantly lower than the HA microspheres (21 ± 3%). The combination of these carbonate-substituted HA microspheres with clinically safe doses of BMP2 could provide promising implants for healing non-loaded bone defects.