Biomechanical Comparison of Unicortical Versus Bicortical C1 Lateral Mass Screw Fixation

Abstract

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.

Biomechanical comparison of the pullout strengths of C1 lateral mass screws and C1 posterior arch screws

“…The present study confirms the results presented by Eck et al [9] that unicortical C1 lateral mass screws are biomechanically stable for atlantoaxial fixation. We evaluated the biomechanical strength of C1 posterior arch screws and have shown that unicortical posterior arch screws have superior resistance to pullout via axial load than do unicortical lateral mass screws in the atlas.…”

“…In the present study, the mean pullout strength for unicortical lateral mass screws was 821 N (range 387À1,645 N), confirming the results presented by Eck et al [9] and exceeding those of bicortical subaxial spine lateral mass screws, implying unicortical C1 lateral mass screws are a viable biomechanical option. All results required more than 355 N before failure.…”

“…Screws that can tolerate a load of 355 N or more are thought to be stable against physiological load. Eck et al [9] showed a mean pullout strength of 588 N (range 212À1,234 N) for unicortical C1 lateral mass screws. Considering that force is greater than 355 N, unicortical lateral mass screws are considered to be biomechanically stable against physiological loads.…”

“…That technique was later shown to be biomechanically equivalent to the use of transarticular screws in terms of resistance to C1ÀC2 motion; however, bicortical placement can put the internal carotid artery and hypoglossal nerve at risk [8]. The use of unicortical lateral mass screws is a safer option, with less risk to the aforementioned neurovascular structures, and it has been shown by Eck et al [9] to be biomechanically stable.…”

Modified C1 lateral mass screw insertion using a high entry point to avoid postoperative occipital neuralgia

“…8 In recent cadaver studies, the biomechanical stability of a unicortical C1 pedicle screw combined with a unicortical C2 pedicle screw-rod fixation technique was found to be equal to that of bicortical C1 LM screws combined with a unicortical C2 pedicle screw-rod fixation technique. 19,23 …”