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Study Design: Nondestructive flexibility tests were performed in vitro, comparing multiple conditions of fixation in a single group of specimens., Objective: To compare the biomechanical behavior of the lumbar spine in the intact condition, after implanting a novel motion stabilizer, and after implanting a rigid fixator., Summary of Background Data: 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., 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, angular range of motion (ROM) and sagittal instantaneous axis of rotation (IAR) were assessed., Results: In 1-level constructs, dynamic hardware allowed 104% of intact ROM, whereas rigid hardware allowed 49% of intact ROM. Relative to the intact, the IAR was shifted significantly farther posterior by rigid 1-level instrumentation than by dynamic 1-level instrumentation. In 2-level constructs, the dynamic level allowed significantly greater ROM than the rigid level in all directions but allowed significantly less ROM than the intact level 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., Level of Evidence: N/A.

Study Design: Seven different combinations of posterior screw fixation, with or without interbody support, were compared in vitro using nondestructive flexibility tests., Objective: To study the biomechanical behavior of a new cortical screw (CS) fixation construct relative to the traditional pedicle screw (PS) construct., Summary of Background Data: The CS is an alternative to the PS for posterior fixation of the lumbar spine. The CS trajectory is more sagittally and cranially oriented than the PS, being anchored in the pars interarticularis. Like PS fixation, CS fixation uses interconnecting rods fastened with top-locking connectors. Stability after bilateral CS fixation was compared with stability after bilateral PS fixation in the setting of intact disc and with direct lateral interbody fixation (DLIF) or transforaminal lateral interbody fixation (TLIF) support., Methods: Standard nondestructive flexibility tests were performed in cadaveric lumbar specimens, allowing non-paired comparisons of specific conditions from 28 specimens (4 groups of 7) within a larger experiment of multiple hardware configurations. Condition tested and group from which results originated were as follows: (1) intact (all groups); (2) with L3-L4 bilateral PS-rods (group 1); (3) with bilateral CS-rods (group 2); (4) with DLIF (group 3); (5) with DLIF + CS-rods (group 4); (6) with DLIF + PS-rods (group 3); (7) with TLIF + CS-rods (group 2), and (8) with TLIF + PS-rods (group 2). To assess spinal stability, the mean range of motion, lax zone, and stiff zone at L3-L4 were compared during flexion-extension, lateral bending, and axial rotation., Results: With intact disc, stability was equivalent after PS-rod and CS-rod fixation, except that PS-rod fixation was stiffer during axial rotation. With DLIF support, there was no significant difference in stability between PS-rod and CS-rod fixation. With TLIF support, PS-rod fixation was stiffer than CS-rod fixation during lateral bending., Conclusion: Bilateral CS-rod fixation provided about the same stability in cadaveric specimens as PS-rod fixation regardless of the presence of interbody, TLIF, or DLIF support.