Ectopic bone induction on and in porous hydroxyapatite combined with collagen and bone morphogenetic protein

Porous hydroxyapatite (HA-P) discs (5 mm in diameter; 1.5 mm thick; porosity, 80%; mean pore size, 200 micron) were impregnated with purified bovine skin collagen (1 mg/disc) and a small amount of semipurified bone morphogenetic protein (BMP) of sarcoma origin (100 micrograms/disc) and implanted into dorsal muscles of ddY mice. Within one week new ectopic cartilagenous tissue was consistently formed on the surface of the discs adjacent to the host tissue. The cartilage was resorbed and replaced by normal bone containing hematopoietic bone marrow four weeks after implantation and the discs became encased in the newly formed bone. HA-P discs impregnated with only collagen (HA-P/collagen) or only BMP (100 micrograms/disc; HA-P/BMP) did not evoke formation of new cartilage or bone. These results indicate that collagen is effective as a carrier of BMP for expression of the biologic activity of the latter in vivo and that it may be of practical use as a carrier of BMP with synthetic biomaterials.