Mechanical evaluation of a canine intervertebral disc spacer: in situ and in vivo studies

An elastomeric intervertebral disc spacer with hydroxyapatite ingrowth surfaces was implanted in a canine model. We studied (a) the mechanical behavior of motion segments at time 0 and at 3, 6, and 12 months and (b) the effect of the interface between the spacer and vertebral bone on implant stability and bone ingrowth. A polymeric spacer was designed with compressive and torsional properties similar to those of the isolated canine lumbar disc. Implantation of the spacer in canine cadaver motion segments permitted in situ biomechanical evaluation at time 0. An in vivo study permitted continuous neurological monitoring of animals, with evaluation of mechanical behavior, stability, and ingrowth at 3, 6, and 12 months. Mechanical testing of cadaver motion segments with the spacer in situ resulted in decreased compressive and torsional stiffnesses, averaging 25 and 42%, respectively. This decrease was due to a combination of the surgical insult to the annulus and decortication of adjacent vertebral endplates. In the in vivo study, all 12 animals tolerated the surgery well and none had permanent neurological impairment. The measured parameters indicated that behavior of the spacer-motion segment composite appeared to return to normal within 3-6 months. However, despite use of a porous hydroxyapatite on the implant surface, there was no significant bone ingrowth. Instead, a layer of dense fibrous connective tissue was formed at the spacer-vertebral bone interface. Early migration of five of the 12 spacers resulted in eccentric loading patterns with consistent reactive osteophyte formation.