A comparison of biomechanical stability and pullout strength of two C1-C2 fixation constructs

Background context: Several fusion techniques are used to treat atlantoaxial instability. Recent literature suggests that intralaminar screw (LS) fixation and pedicle screw (PS) fixation offer similar stability and comparable pullout strength. No studies have compared these characteristics after cyclic loading. Purpose: To compare the stability and pullout strength of intra-LSs and PSs in a C1-C2 instability model after 1,000 cycles of axial loading. Study design: In vitro biomechanical study. Outcome measures: Stability in axial rotation and screw pullout strength after cyclic loading. Methods: Six fresh-frozen human cadaveric cervical spines (C1-C2) were used in this study. C1-C2 instability was mimicked via odontoidotomy at its base and posterior soft-tissue release, including the supraspinous ligaments and facet joint capsules. Specimens were tested to 1,000 cycles after stabilization with two fixation constructs: C1 lateral mass (LM) screws and C2 intra-LSs (C1LM-C2LS) and C1 LM screws and C2 PSs (C1LM-C2PS). Angular motion was recorded for right and left axial rotation using an Optotrak 3020 system (Northern Digital, Waterloo, Ontario, Canada). Tensile loading to failure was then performed collinear to the longitudinal axis of the screw, and the data were recorded as peak pullout strength in newtons. Results: There was no statistically significant difference in stability (measured in degrees of rotation) between the intra-LS and PS constructs at 250, 500, 750, and 1,000 cycles of axial rotation. Furthermore, there was no significant difference in stability at 250 cycles versus 1,000 cycles for the LS (1.30 vs. 1.49, p=.80) or PS (0.84 vs. 0.85, p=.96). Pedicle screws had higher pullout strength when compared with the intra-LSs (757.5±239 vs. 583.4±472 N); however, high standard deviation precluded statistical significance (p=.44). Conclusions: Our data suggest that a C1LM and C2LS construct has similar biomechanical stability when compared with a C1LM and C2PS construct after 1,000 cycles of axial rotation. Furthermore, PSs had higher pullout strength when compared with LSs; however, this result was not statistically significant.