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

Background contextConditions of the atlantoaxial complex requiring internal stabilization can result from trauma, malignancy, inflammatory diseases, and congenital malformation. Several techniques have been used for stabilization and fusion. Posterior wiring is biomechanically inferior to screw fixation. C1 lateral mass screws and C1 posterior arch screws are used for instrumentation of the atlas. Previous studies have shown that unicortical C1 lateral mass screws are biomechanically stable for fixation. No study has evaluated the biomechanical stability of C1 posterior arch screws or compared the two techniques.PurposeThe purpose of the study was to assess the differences in the pullout strength between C1 lateral mass screws and C1 posterior arch screws.Study designBiomechanical testing of pullout strengths of the two atlantal screw fixation techniques.MethodsThirteen fresh human cadaveric C1 vertebrae were harvested, stripped of soft tissues, evaluated with computed tomography for anomalies, and instrumented with unicortical C1 lateral mass screws on one side and unicortical C1 posterior arch screws on the other. Screw placement was confirmed with postinstrumentation fluoroscopy. Specimens were divided in the sagittal plane and potted in polymethylmethacrylate. Axial load to failure was applied with a material testing device. Load displacement curves were obtained, and the results were compared with Student t test. DePuy Spine, Inc. (Raynham, MA, USA) provided the hardware used in this study.ResultsMean pullout strength of the C1 lateral mass screws was 821 N (range 387−1,645 N±standard deviation [SD] 364). Mean pullout strength of the posterior arch screws was 1,403 N (range 483−2,200 N±SD 609 N). The difference was significant (p=.009). Five samples (38%) in the posterior arch group experienced bone failure before screw pullout.ConclusionsBoth unicortical lateral mass screws and unicortical posterior arch screws are viable options for fixation in the atlas. Unicortical posterior arch screws have superior resistance to pullout via axial load compared with unicortical lateral mass screws in the atlas.