Biomechanical comparison between C-7 lateral mass and pedicle screws in subaxial cervical constructs. Presented at the 2009 Joint Spine Meeting. Laboratory investigation.
OBJECTIVE
The aim of this study was to conduct the first in vitro biomechanical comparison of immediate and postcyclical rigidities of C-7 lateral mass versus C-7 pedicle screws in posterior C4-7 constructs.


METHODS
Ten human cadaveric spines were treated with C4-6 lateral mass screw and C-7 lateral mass (5 specimens) versus pedicle (5 specimens) screw fixation. Spines were potted in polymethylmethacrylate bone cement and placed on a materials testing machine. Rotation about the axis of bending was measured using passive retroreflective markers and infrared motion capture cameras. The motion of C-4 relative to C-7 in flexion-extension and lateral bending was assessed uninstrumented, immediately after instrumentation, and following 40,000 cycles of 4 Nm of flexion-extension and lateral bending moments at 1 Hz. The effect of instrumentation and cyclical loading on rotational motion across C4-7 was analyzed for significance.


RESULTS
Preinstrumented spines for the 2 cohorts were comparable in bone mineral density and range of motion in both flexion-extension (p = 0.33) and lateral bending (p = 0.16). Lateral mass and pedicle screw constructs significantly reduced motion during flexion-extension (11.3°-0.26° for lateral mass screws, p = 0.002; 10.51°-0.30° for pedicle screws, p = 0.008) and lateral bending (7.38°-0.27° for lateral mass screws, p = 0.003; 11.65°-0.49° for pedicle screws, p = 0.03). After cyclical loading in both cohorts, rotational motion over C4-7 was increased during flexion-extension (0.26°-0.68° for lateral mass screws; 0.30°-1.31° for pedicle screws) and lateral bending (0.27°-0.39° and 0.49°-0.80°, respectively), although the increase was not statistically significant (p > 0.05). There was no statistical difference in postcyclical flexion-extension (p = 0.20) and lateral bending (0.10) between lateral mass and pedicle screws.


CONCLUSIONS
Both C-7 lateral mass and C-7 pedicle screws allow equally rigid fixation of subaxial lateral mass constructs ending at C-7. Immediately and within a simulated 6-week postfixation period, C-7 lateral mass screws may be as effective as C-7 pedicle screws in biomechanically stabilizing long subaxial lateral mass constructs.
Medical Scientist Training Program, Johns Hopkins School of Medicine, Department of Neurosurgery, Johns Hopkins Hospital, Baltimore, Maryland 21287, USA.
Risheng
R
Xu
Matthew J
MJ
McGirt
Edward G
EG
Sutter
Daniel M
DM
Sciubba
Jean-Paul
JP
Wolinsky
Timothy F
TF
Witham
Ziya L
ZL
Gokaslan
Ali
A
Bydon
D001696:Biomechanical Phenomena; D001863:Bone Screws; D002574:Cervical Vertebrae / Q000601:surgery*; D006801:Humans; D016268:Internal Fixators; D016059:Range of Motion, Articular; D013123:Spinal Fusion / Q000295:instrumentation*; D016474:Weight-Bearing