Cervical stability with lateral mass plating: unicortical versus bicortical screw purchase.

STUDY DESIGN

A biomechanical study using human cadaveric cervical spines was conducted.

OBJECTIVE

To determine whether the cervical stability achieved with lateral mass fixation using unicortical screw purchase is comparable with that obtained with lateral mass fixation using bicortical screw purchase.

SUMMARY OF BACKGROUND DATA

Cervical lateral mass fixation has traditionally required bicortical screw fixation, which is associated with potential injury to the vertebral vessels and cervical nerve roots. Unicortical fixation eliminates these possible complications. Limited stability studies suggest that unicortical fixation in the lateral masses provides stability comparable with that of bicortical fixation. However, no comprehensive biomechanical study has compared the cervical stability between bicortical and unicortical lateral mass fixations.

METHODS

A total of 11 human cadaveric cervical spinal sections between C3 and C5 were tested in flexion-extension, torsion, and lateral bending modes, both with and without laminectomy, on an MTS machine. The lateral masses were plated and tested with bicortical screws in all 11 specimens. Long (14 mm) unicortical screws were used in 8, and short (10 mm) screws were used in 6 of the 11 specimens. Displacement then was determined in millimeters (of flexion-extension and lateral bending) and degrees (of torsion) for each value of applied torque (0.45, 0.9, 1.35, 1.8 Nm). Two-way analysis of variance using unequal data sets then was applied.

RESULTS

Analysis demonstrates that bicortical constructs were, on the average, stiffer than unicortical constructs in most bending modes. No significant differences were found, however, between bicortical constructs and long unicortical constructs without laminectomy. In the presence of a destabilizing laminectomy, a difference was found in the lateral bending mode. More differences were found between bicortical and short unicortical constructs both with and without laminectomy.

CONCLUSIONS

Cervical lateral mass fixation with "long" (up to but not through the anterior lateral mass cortex) unicortical Magerl screws may provide an acceptable and safe alternative to bicortical screw fixation.