Cranial bone apposition and ingrowth in a porous nickel-titanium implant

A 5 x 5 x 1-mm uncoated porous nickel-titanium (nitinol) implant was placed 4 mm to either side of the midsection of the frontal bone and 4 mm anterior to the coronal suture of the cranial bone of New Zealand White rabbits. In the other frontal location, a 5 x 5 x 1-mm coralline hydroxyapatite (HA) (Interpore 200, a well-known craniofacial implant material) implant was fitted. Rabbits were killed at each of three postsurgical intervals (2, 6, and 12 weeks), and the implants were evaluated for gross biocompatibility, bony contact, and ingrowth. No adjacent macrophage cells were observed for either implant type, and overlaying soft tissues and connective tissues readily adhered to the implants even after 2 weeks. Both materials made bone contact with the surrounding cranial hard tissue, and percent ingrowth increased with surgical recovery time. Measurements of microhardness and bone histologic parameters indicated that bone in contact with and grown into the implants was similar in properties to the surrounding cranial bone. Porous nitinol implants therefore appear to allow for significant cranial bone ingrowth after as few as 12 weeks, and thus nitinol appears to be suitable for craniofacial applications. Compared to HA, the nitinol implants demonstrated a trend for less total apposition and more total ingrowth after 6 and 12 weeks of implantation.