Novel hydroxyapatite ceramics with an interconnective porous structure exhibit superior osteoconduction in vivo

Calcium hydroxyapatite ceramics (CHA) are nontoxic materials, provoke little reaction from tissues, and by virtue of these properties represent a good starting point for creating bone substitutes. Although several porous CHAs have been used clinically, there have been few reports that CHA is fully replaced by newly formed bone, which may be due to its structure and the limited connectivity between pores. We recently developed a fully interconnected porous CHA (IP-CHA) by adopting a "foam-gel" technique. Structural analysis by scanning electron microscopy revealed that IP-CHA had spherical pores of uniform size that were interconnected by window-like holes. The surface of the wall structure was smooth, and hydroxyapatite particles were bound tightly to one another. Most of the interpore connections of IP-CHA ranged from 10 to 80 microm in diameter (average, 40 microm). When the cylindrical IP-CHA (diameter, 6 mm; height, 15 mm) was implanted into a rabbit femoral condyle, bone, and bone marrow with abundant vessels formed deep in the pores through the interpore connections. Within a period of 6 weeks, new bone had formed and penetrated to a distance of 3 mm from the surface of the IP-CHA implant. Furthermore, a compression test at 9 weeks revealed that the implanted IP-CHA steadily increased in strength to more than double the value of the initial test. These results indicate that the IP-CHA may have clinical utility as a superior bone substitute.