INTERVERTEBRAL SPACER AS AN ADJUNCT TO ANTERIOR LUMBAR FUSION - PART II - 6-MONTH IMPLANTATION IN BABOONS

Failure of interbody fusions in the lumbar spine an common due to reli ance on the graft for structural support during healing by creeping su bstitution. Support of the interspace with an implant should result in improved fusion success. The objective of this study was to evaluate the stability of the implant in vivo and its potential as an adjunct t o promote interbody arthrodesis, Prototype 3, a porous coated interver tebral spacer with extension lugs made of Ti-6Al-4V, was implanted ver tically between adjacent lumbar vertebrae anteriorly in four baboons u ndergoing anterior interbody fusion. The animals were allowed freedom of activity for 6 months before being killed. A transperitoneal approa ch was made exposing the L4-L5 or L5-L6 interspace. At time of killing , clinical evaluation of the implant-vertebral body construct showed s tability to manual stresses applied in extension, flexion, and rotatio n, Serial radiographs taken during the 6 months of implantation showed no change in position or displacement of the implants. Axial and tors ional cyclic loads were applied to each spine at 1 cycle/s for 20,000 cycles. Statistical analysis of the motion profiles for intact and imp lanted spines demonstrated no significant difference in axial or rotat ional displacements at the arthrodesis level or adjacent unoperated le vels, L1 and L4. The in vivo 6-month study in baboons confirmed implan t stability and maintenance of disc space height. Variable osseous hea ling was noted. Release of plasma spray beading may have resulted from improper application on the implant or micromotion within the constru ct. A better method to mechanically interlock the plungers is being st udied. Clinical trials based on this work appear justified.