Biomechanical comparison of unicortical versus bicortical C1 lateral mass screw fixation.
METHODS
Biomechanical study of pullout strength of unicortical versus bicortical C1 lateral mass screws using a cadaveric cervical spine model.


OBJECTIVE
To compare pullout strength of unicortical versus bicortical C1 lateral mass screws.


BACKGROUND
The internal carotid artery and hypoglossal nerve lie over the anterior aspect of the lateral mass of the atlas and are at risk from bicortical C1 lateral mass screws. Unicortical screws would reduce the risk of injury to these neurovascular structures; however, no data are available on the relative strength of unicortical versus bicortical C1 lateral mass screws.


METHODS
Fifteen cadaveric cervical spine specimens underwent axial pullout testing of C1 lateral mass screws. A unicortical C1 lateral mass screw was placed on 1 side with a contralateral bicortical screw.


RESULTS
The mean pullout strengths of the unicortical screws and bicortical screws were 588 N (range, 212 to 1234 N) and 807 N (range, 163 to 1460 N), respectively (P=0.008).


CONCLUSIONS
Bicortical C1 lateral mass screws were significantly stronger than unicortical screws; however, the mean pullout strength of both the unicortical and bicortical C1 screws were greater than previously reported values for subaxial lateral mass screws. On the basis of these data, the clinical necessity for using bicortical screw fixation in all patients must be questioned. If similar strength can be achieved using unicortical C1 lateral mass screw to that currently accepted in the subaxial spine, bicortical screws might not be justified for the C1 lateral mass. However, the ability to extrapolate C1-C2 data to subaxial spine data is uncertain because of the difference in normal physiologic loading at these levels.
Eck
Jason C
JC
Department of Orthopaedic Sugery, Mayo Clinic College of Medicine, Rochester, MN 55905, USA.
Walker
Matt P
MP
Currier
Bradford L
BL
Chen
Qingshan
Q
Yaszemski
Michael J
MJ
An
Kai-Nan
KN
D001863:Bone Screws; D002102:Cadaver; D001270:Cervical Atlas / Q000601:surgery*; D004867:Equipment Design; D005592:Fracture Fixation / Q000295:instrumentation*; D006801:Humans; D008422:Materials Testing; D013314:Stress, Mechanical; D013718:Tensile Strength