This study determined the bone formation in porous calcium carbonate (CC) and porous hydroxyapatite (HA) in ectopic sites. The bone formation stimulus was derived from bone marrow cells. CC and HA in the shape of disks were implanted with or without rat marrow cells into subcutaneous sites of syngeneic rats. The CC and HA had identical microstructure: pore size was 190-230 pm, porosity was 50-60% and they were fully interconnected. Bone did not form in any implants without marrow cells (disks themselves), whereas bone consistently formed in the pores of all implants with marrow cells after 4 weeks. The bone formation of both CC and HA occurred initially on surface of the pore regions and progressed toward the center of the pore. Scanning electron microscopy and electron-probe microanalysis revealed a continuum of calcium at the interfaces of both bone/ CC and bone/HA implants. These results indicate that the bone formation in calcium carbonate derived from marine corals is comparable to the bioactive hydroxyapatite.
IN T RODUC T ION
Hydroxyapatite ceramics (HA) are known to be biocompatible, osteoconductive and osteophilic with bone directly bonding to the surface of the HA.'-* Therefore, these ceramics are currently used as bone graft s~bstitutes.~-~ When porous HA is placed next to viable bone, ingrowth of bone forming cells and bone formation are observed in the pores?-' The clinical application of HA may be limited, however, because HA is considered to be essentially nonre-~orbable."'~ An ideal bone grafting material should be replaceable by the host bone. Therefore, the implant needs to be both biodegradable and osteoconductive. Among the biodegradable materials, the calcium carbonate (CC) skeleton of marine corals has been reported to be rapidly biodegradable and *To whom correspondence should be addressed. Though the materials used in this study were supplied by Interpore International and Dr. Edwin C. Shors is an employee of the company, no benefit of any kind will be received either directly or indirectly by the authors.