Influence of orthopedic implant structure on adjacent bone density and on stability

We evaluated the ability of a porous metallic interbody fusion implant made with porous nitinol (PNT) to achieve intervertebral fusion and the capacity of stabilization at the implantation site 3, 6, and 12 months after implantation. Sixteen sheep each received 1 PNT implant and 1 titanium (TiAIV) cage at intervertebral lumbar levels L2-L3 and L4-L5; 3 other sheep were used as untreated controls. The TiAIV cage was used as a control implant. After animal sacrifice, computed tomography was used to study peri-implant bone mineral density (BMD), and histologic slices were used to evaluate implant osseointegration. BMD around PNT implants was close to physiological (control value) BMD, whereas BMD around TiAIV cages was usually higher (sclerosis) than physiological BMD. Histologic analysis showed better osseointegration with PNT implants than with TiAIV cages. Sclerosis might result from bone acting to stabilize implants in their implantation sites. Compared with PNT implants, TiAIV cages seemed to be unstable in their implantation sites. For PNT implants, osseointegration was successful, and surrounding BMD was close to physiologic BMD.