Biomechanics of a Posterior Lumbar Motion Stabilizing Device: In Vitro Comparison to Intact and Fused Conditions.
METHODS
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.


BACKGROUND
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.


CONCLUSIONS
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.


METHODS
N/A.
*Barrow Neurological Institute†St. Mary's Medical Center‡San Francisco Orthopaedic Surgeons.
Luis
L
Perez-Orribo
James F
JF
Zucherman
Kenneth Y
KY
Hsu
Phillip M
PM
Reyes
Nestor G
NG
Rodriguez-Martinez
Neil R
NR
Crawford
D001696:Biomechanical Phenomena; D001863:Bone Screws; D002102:Cadaver; D005260:Female; D006801:Humans; D007593:Joint Instability / Q000503:physiopathology / Q000601:surgery*; D008159:Lumbar Vertebrae / Q000503:physiopathology / Q000601:surgery*; D008297:Male; D008422:Materials Testing; D008875:Middle Aged; D016059:Range of Motion, Articular; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods; D013314:Stress, Mechanical