Carbonate substituted hydroxyapatite: Resorption by osteoclasts modifies the osteoblastic response

Carbonate substitution within the hydroxyapatite (HA) lattice improves osteoconduction, although the mechanism by which this occurs is unclear. Discs of dense, sintered, phase-pure HA and AB-type carbonate substituted hydroxyapatite (CHA) were cultured for 21 days with human CD14+ cells in the presence of macrophage-colony stimulating factor (M-CSF) and soluble receptor activator of nuclear factor (NF)-kappaB (sRANKL), during which time osteoclasts developed and resorbed the ceramic surface. Discs were then seeded with human osteoblasts (HOBs), and proliferation and collagen synthesis were measured. On some discs, the conditioned proteinaceous layer left behind by the osteoclasts was preserved. Proliferation of HOBs was increased on resorbed compared to control (unresorbed) surfaces on both materials, provided this osteoclast-conditioned layer was left intact. Collagen synthesis by HOBs was increased on previously resorbed surfaces compared to unresorbed surfaces. This effect was seen on both materials but was seen at an earlier time point on CHA. The results suggest that osteoclasts can condition synthetic bioceramic surfaces and alter the responses of osteoblasts that subsequently populate them. Carbonate substitution may enhance osteoconduction indirectly via effects on enhanced bioresorption.