Controlled motion with the XL-TDR lateral-approach lumbar total disk replacement: in vitro kinematic investigation.

OBJECTIVE

Anterior-approach total disk replacement (TDR) devices are thought to retain close to so-called normal range of motion (ROM); however, they are also inherently unstable due to resection of the anterior longitudinal ligament and annulus. This instability/laxity is manifested as increased neutral zone (NZ) motion. The XL-TDR device (NuVasive, Inc., San Diego, California, United States) is implanted through a lateral approach that preserves the anterior ligamentous and annular structures. This potentially makes the XL-TDR device more stable than those delivered anteriorly. This study investigates XL-TDR biomechanical features in a cadaveric model.

METHODS

Biomechanical evaluation consisting of nondestructive multidirectional testing was performed with the hybrid protocol on six fresh-frozen cadaveric specimens (L2-S1). Motion segment kinematics were obtained using an optoelectronic system. Test conditions were (1) intact spine, (2) XL-TDR at L4-L5, and (3) XL-TDR at L4-L5 with anterior annulus/ligament resected. ROM and NZ were calculated for each condition in each loading direction (flexion-extension total, flexion alone, extension alone, lateral bending, and axial rotation).

RESULTS

Insertion of the XL-TDR device decreased ROM with respect to intact in all directions. NZ in all directions was not statistically different from intact (p < 0.05), although there was a trend toward decreased NZ in flexion (p = 0.078). Removing the anterior ligament/annulus increased ROM significantly with respect to the XL-TDR condition in all directions (p < 0.003). NZ also increased, with the most significant changes in extension, lateral bending, and axial rotation (p < 0.002).

CONCLUSIONS

The kinematics of XL-TDR have demonstrated that the retention of the anterior ligament/annulus has a significant stabilizing effect, diminishing ROM to less than intact, with a more controlled motion (more natural NZ). Removing the anterior ligament/annulus illustrated its stabilizing role, with ROM and NZ increasing significantly. Future studies will investigate the potential benefit of controlled XL-TDR motion on facet kinematics that may have clinical implications related to limiting facet degeneration.