Histomorphometric assessment of the mechanisms for rapid ingrowth of bone to HA/TCP coated implants

The purpose of this work is to use dynamic histomorphometry to evaluate the basic biological mechanisms by which hydroxyapatite/tricalcium phosphate (HA/TCP) implant coatings accelerate bone formation rates. Twenty-five rabbits had an HA/TCP coated cylindrical titanium fiber metal mesh implant surgically placed in the subchondral bone of the proximal tibia and a noncoated implant placed in the contralateral tibia. Twenty-two of these animals had HA/TCP coated cylindrical solid titanium implants placed in the distal femur and an uncoated implant placed in the contralateral femur. The animals were double labeled with vital stains, and sacrificed at 3, 6, 16, or 26 weeks after surgery. Histomorphometric analyses were done of the bone implant interfaces. Both static and dynamic histomorphometric parameters indicate that HA/TCP coatings stimulate faster bone ingrowth to coated fiber metal implants through the early production of woven bone and by subsequent rapid lamellar bone formation rates. Coated fiber metal implants demonstrated significantly more bone ingrowth than noncoated implants through 16 weeks postimplantation, but not by 26 weeks. In solid implants, the differences between coated and noncoated implants are less pronounced and not statistically significant, although there is a trend toward increased bone apposition to the surface of the implants over the first 16 weeks following implantation. The clinical significance of these results is that coated implants may allow earlier return to normal weightbearing.