C1 posterior arch crossing screw fixation for atlantoaxial joint instability.

STUDY DESIGN: Anatomical measurements and in vitro biomechanical testing were performed to evaluate a new method for posterior C1 fixation.

OBJECTIVE: This study sought to assess C1 posterior arch crossing screw fixation for posterior C1-C2 fixation, using anatomical measurements and biomechanical testing with traditional C1 pedicle screws (PS) in a cadaveric model.

SUMMARY OF BACKGROUND DATA: Atlantoaxial instability often requires surgery, and the current methods for atlas fixation incur some risk to the vascular and neurological tissues. Thus, new, effective, and safe methods are needed for salvage operations.

METHODS: Morphometric analysis of the C1 posterior arch was performed using 3-dimensional computed tomography. Six fresh ligamentous human cervical spines (C0-C4) were evaluated for their biomechanics. The specimens were tested in their intact condition and after stabilization (C1-C2 PS, C1 posterior arch screws [PAS] with C2 PS) and injury due to 1.5 N·m of pure moment in flexion, extension, lateral bending, and axial rotation. During testing, 3-dimensional angular motion was measured with a motion capture platform (Vicon Nexus). Data for all scenarios were recorded, and statistical analysis was performed.

RESULTS: Anatomical assessment indicated that 91.51% of C1 posterior tubercles exceeded 7 mm in thickness, 93.40% had a width of the posterior arch of greater than 3.5 mm, and 65.57% had a unilateral screw length of greater than 15 mm, indicating that the posterior arch fixation could be achieved by two 3.5 × 15-mm screws placed in a crossed manner. Twenty-two cases (11%) were not suitable for crossing screw placement because the posterior arch was flat and the entry point was present on the same side. Biomechanical testing showed that the PS and PAS rod-screw systems significantly reduced flexibility in flexion, extension, and rotation compared with the intact position. For lateral bending, there was a trend for the C1 PS and PAS systems toward decreased flexibility in comparison with the intact position. At the same time, C1 PAS decreased C1-C2 movement by 33.0% in left bending (P = 0.171) and 24.4% in right bending (P = 0.095); however, no significant difference was observed for left bending with C1 PAS compared with C1 PS, and the C1 PS and PAS systems significantly reduced the flexibility more than destabilization.

CONCLUSION: Crossing screw fixation of the C1 posterior arch is straightforward and imposes little risk of injury to the neural and vascular structures as long as the implants remain intraosseous. According to the results of our anatomical and biomechanical study, C1 posterior arch crossing screw fixation may constitute an alternative method for posterior atlantoaxial fixation.

LEVEL OF EVIDENCE: 3.