Design and evaluation of the FlexiCore metal-on-metal intervertebral disc prosthesis.

Abstract

The technical difficulties associated with the development of an intervertebral disc prosthesis include endurance demands on the device, lack of consensus concerning the biomechanical principles governing the articulation of the spinal joint and the performance of materials available for implantation. Although biologically based disc prostheses and augmentations may be the endpoint of spinal disc replacements, these devices and associated technologies will still require decades of work in order to achieve fruition. The more immediate solution will require a durable, biocompatible device capable of restoring range of motion. The evaluation of such a device must include failure testing of critical components as well as a series of fatigue experiments under overloaded conditions. Recent literature citing adjacent level degeneration associated with segmental mobilization and a lack of correlation between successful fusion and clinical success has prompted the need for a dynamic intervertebral disc prosthesis. Combined with a better understanding of the biomechanics and the prospect of an increasing percentage of more elderly patients, the future of spinal fusion for pain and instability may need to be reexamined. The authors propose a novel metal-on-metal design for an intervertebral device that features a fixed center of rotation, a mechanical torsional limit, a unique feature to allow for the location of the device in a patient-specific manner, and means by which the device may be implanted directly anterior or anterolaterally. Physiological ranges of motion are retained by the prosthesis. In addition, initial and long-term fixations are achieved through spines and bone ingrowth coating. The device comprises a retained ball and socket positioned between two baseplates. The ball and socket joint permits articulation through the appropriate physiological range of motion and center of rotation as the baseplates provide a stable platform for implantation. An intervertebral disc prosthesis has been designed and has demonstrated mechanical performance beyond what is required physiologically under preliminary testing.