Object: 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.