Many spinal conditions, including degenerative disc disease, can be treated by spinal fusion or artificial disc replacement (ADR). ADR has several advantages over spinal fusion. The most important advantage of ADR is the preservation of spinal motion. Spinal fusion eliminates motion across the fused segments of the spine. Consequently, the discs adjacent to the fused level are subjected to increased stress. The increased stress increases the changes of future surgery to treat the degeneration of the discs adjacent to the fusion. However, motion through an ADR also allows motion through the facet joints. Motion across arthritic facet joints could lead to pain following ADR. Some surgeons believe patients with degenerative disease and arthritis of the facet joints are not candidates for ADR.
Current ADR designs do not attempt to limit the pressure across the facet joints or facet joint motion. Indeed, prior art ADR generally do not restrict motion. For example, some ADR designs place bags of hydrogel into the disc space. Hydrogel bags do not limit motion in any direction. In fact, bags filled with hydrogels may not provide distraction across the disc space. ADR designs with metal plates and polyethylene spacers may restrict translation but they do not limit the other motions mentioned above. The articular surface of the poly spacer is generally convex in all directions. Some ADR designs limit motion translation by attaching the ADR halves at a hinge.
FIG. 1A is a lateral view of a prior-art artificial disc replacement (ADR) 30. FIG. 1B is an anterior view of the prior-art ADR 30. FIG. 1C is a drawing which shows the prior-art ADR 30 in flexion, and FIG. 1D is a drawing which shows the ADR device 30 in extension. Note that, due to impingement, left bending as permitted by the typical prior-art device, increases pressure on the left facet, whereas right bending increases pressure on the right facet. Rotation increases pressure on the right facet and the left facet, and vice versa.
The alignment of one Artificial Disc Replacement (ADR) endplate (EP) relative to the other ADR EP is critical to the function of articulating ADRs. Many ADRs rely on movement between a convexity on one ADR EP and a concavity on the other ADR EP. Alternatively, convex spacers are used between concavities on the ADR EPs. Improperly aligned ADR EPs risk excessive surface wear from incongruent opposing articulating surfaces. Furthermore, improper alignment will decrease ADR motion.
The endplates of prior art ADRs are inserted simultaneously to assure proper alignment. Most ADRs are held in the disc space by the fit of projections from the ADR EP into the vertebra above and below the ADR. Inserting ADR EPs simultaneously limits the length of these projections, however.