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 mechanic al behavior of motion segments at time 0 and at 3, 6, and 12 months an d (b) the effect of the interface between the spacer and vertebral bon e on implant stability and bone ingrowth. A polymeric spacer was desig ned with compressive and torsional properties similar to those of the isolated canine lumbar disc. Implantation of the spacer in canine cada ver motion segments permitted in situ biomechanical evaluation at time 0. An in vivo study permitted continuous neurological monitoring of a nimals, with evaluation of mechanical behavior, stability, and ingrowt h at 3, 6, and 12 months. Mechanical testing of cadaver motion segment s with the spacer in situ resulted in decreased compressive and torsio nal stiffnesses, averaging 25 and 42%, respectively. This decrease was due to a combination of the surgical insult to the annulus and decort ication 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 sp acer-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 bon e interface. Early migration of five of the 12 spacers resulted in ecc entric loading patterns with consistent reactive osteophyte formation.