Choosing a cervical disc replacement.

BACKGROUND CONTENT: Three important basic scientific studies are presented that measured the volumetric density of longitudinal bony columns within the cervical vertebra. The most solid bone is lateral, adjacent to the uncovertebral joints in a radial pattern.
PURPOSE: To characterize the best footprint, profile and biomaterials to construct a cervical disc replacement. STUDY
DESIGN: A compilation of biomechanical and anatomical basic scientific studies.
METHODS: Microcomputed tomographic imaging, trabecular density and mineral distribution were quantitated from human cervical vertebra.
RESULTS: The lateral portions of the cervical vertebra are subjected to higher bending loads than the lumbar vertebral bodies. Therefore, the optimal prosthesis needs to be anchored in the lateral uncovertebral bone. To reduce the incidence of cervical subsidence, the prosthesis needs to be more rectangular than round to take advantage of the radially oriented lateral trabeculae. TiCaP (titanium/calcium phosphate) (Cervitech, Inc., Rockaway, NJ) bony ingrowth coating leads to 10% to 15% greater bony integration than plasma-sprayed titanium. TiCaP causes a supersaturated solution of CaP at the metal-bone interface, which enables reprecipitation of hydroxyapatite and superior bony integration. The optimum pore size of the ingrowth coating of the lumbar spine is 75 to 300 microns, whereas in the cervical spine the optimal ingrowth coating is 20 to 30 microns. This is an order of magnitude lower in pore size to match the smaller cervical trabecular architecture.
CONCLUSIONS: Kinematic considerations for the cervical spine show the load is 1/9th the load carried by the lumbar spine or 50 N per segment. Knowing the sliding distance and wear characteristics of conventional biomaterials (ultrahigh molecular weight polyethylene and cobalt chrome) demonstrates that the generation of particulate debris should be a very minor consideration with cervical arthroplasty.