Fusion has been the historic gold standard for the treatment of the degenerated disc in the spine. Fusion surgeries constrain the motion at the implanted level there by transferring additional load to the adjacent levels. Many clinical studies have shown that adjacent segment degeneration was observed in patients over time. In order to overcome the drawbacks of the fusion procedures, the artificial discs are being explored to treat disc degeneration related problems.
Artificial discs are designed to preserve motion at the index level and lessen effects on the adjacent segments. More recently, the need to preserve the sagittal balance following disc replacement has been stressed. Existing devices provide motion, but may not actively preserve or correct sagittal balance. A new disc design was compared with the existing disc designs, analyzing the effects of design parameters on Cervical Disc Arthroplasty Performance and Sagittal Balance, using a finite element model (FEM).
An experimentally validated ligamentous intact C3-C7 FEM was modified to simulate four disc designs: Synergy 0 degree (S0), Synergy 6 degree (S6), a ball and socket (BS) and a ball and trough (BT) at C5-C6 level. S0 has parallel metal endplates whereas S6 has 6 degrees built-in lordosis between the endplates. S0 and S6 both have a polymer core. BS has an inferior polymeric ball and a superior metal socket. BT has a metal ball on superior endplate with an elongated inferior metal trough. Appropriate material properties, contacts and boundary conditions were defined for all the models. A compressive follower load of 73.5N and incremental moments up to 1.5 Nm were applied to define the neutral posture and to simulate physiologic motions respectively. The follower load resulted in less than 1 degree of extension for intact, S0 and S6, extension around 3 degrees for BS and flexion over 1 degrees for BT. Flexion-extension Center Of Rotation (COR) was closer to intact for all the discs except for BT, where it was significantly superior. COR in lateral bending for BS was furthest from intact. The inherent device COR and device placement according to anatomical COR appears to affect the device behavior under NP loading. The S0, S6 and BT had index level angular-response curves similar to intact. BS had excessive and rapid motion compared to intact in the NP zone. The built-in lordosis of S6 shifted the response curve accordingly near the neutral zone-NP (+/- 0.5 Nm). S0 and S6 had comparable motions with the intact (~15% increase). In all of the cases, the motions increased as compared to intact, except in extension for BT design. The adjacent segment motions were not significantly affected in all cases.
The overall body of evidence from this FEM study supports that the Synergy disc design has kinematics closer to the intact spine and imparts a relatively stable neutral posture in the sagittal plane whereas BS and BT devices move freely through their range of motion.