Biomechanical Testing of a Lumbar Motion Normalization Device

Introduction Two specific scenarios that may benefit from dynamic lumbar stabilization are single-level moderate instability, where the stabilizing tissues are relatively incompetent, and juxta-level to fusion, where the last instrumented level requires intermediate stiffness (“topping off”) to prevent transfer of high stresses from the stiffer fusion construct to the intact adjacent levels. Both scenarios were evaluated in vitro to determine the biomechanical response of a new pedicle screw-based dynamic stabilizer. Materials and Methods Seven human cadaveric L2-S1 segments were tested (1) intact, (2) after moderate destabilization, (3) after 2-level hybrid posterior fixation, consisting of bilateral dynamic pedicle screws at L4 interconnected with rigid rods to standard pedicle screws at L5 and S1, (4) after 2-level rigid fixation, (5) after 1-level (L4-L5) dynamic fixation, and (6) after 1-level rigid fixation. In each condition, flexion, extension, lateral bending, and axial rotation were induced with pure moments of 7.5 Nm, while angular motion was tracked optoelectronically. Range of motion (ROM) and sagittal instantaneous axis of rotation (IAR) were calculated. Results In 1-level constructs, dynamic hardware allowed 104% of intact ROM, whereas rigid hardware allowed 49% of intact ROM. Relative to the intact IAR location at L4-L5, the IAR was shifted significantly farther posterior by rigid 1-level instrumentation than dynamic 1-level instrumentation. In 2-level constructs, the dynamic level (L4-L5) allowed significantly greater ROM than the rigid level (L5-S1) in all directions but allowed significantly less ROM than the intact level (L3-L4) in all directions except axial rotation. Conclusion Dynamic instrumentation shifted the IAR less than rigid instrumentation, providing more favorable kinematics. This dynamic stabilizer provided 1-level ROM that was close to intact ROM during all loading modes in vitro. In the “topping off” construct, the dynamic segment allowed intermediate ROM to give balanced transitional flexibility. Disclosure of Interest J. Zucherman: Conflict with Spartek Medical, Inc. K. Hsu: Conflict with Spartek Medical, Inc. N. Crawford: Conflict with Spartek Medical, Inc. L. Perez-Orribo: None declared P. Reyes: None declared N. Rodriguez-Martinez: None declared References Bono CM, Kadaba M, Vaccaro AR. Posterior pedicle fixation-based dynamic stabilization devices for the treatment of degenerative diseases of the lumbar spine. J Spi