Novel unilateral C1 double screw and ipsilateral C2 pedicle screw placement combined with contralateral laminar screw-rod fixation for atlantoaxial instability

Purpose: To investigate the anatomical and biomechanical feasibility of the unilateral C1 double screw [pedicle screw (PS) + lateral mass screw (LMS)] and ipsilateral C2 PS combined with contralateral C2 laminar screw (LS)-rod fixation for atlantoaxial instability by comparison with traditional posterior fixation methods.

Methods: Fifteen sets of complete dry bony specimens of atlas were used for morphometric analysis. The working length, width and thickness of the C1 PSs and LMSs were manually measured. Ten fresh-frozen cervical spines (C0-C7) were used to complete the range of motion (ROM) testing in their intact condition, under destabilization and after stabilization by the following procedures: unilateral C1-C2 PS rod fixation (Group A), bilateral C1-C2 PS rod fixation (Group B), and unilateral C1 double screw and ipsilateral C2 PS combined with contralateral C2 LS rod fixation (Group C).

Results: The working thickness of the C1 PS was ≤ 3.5 mm in only one (1/15 = 6.7%) specimen. The other parameters were > 3.5 mm in all specimens. In the ROM test, all fixation groups showed significantly reduced flexibility in all directions compared with both the intact and destabilization groups. Further, Groups B and C showed better stability in all directions than Group A. However, no significant differences were observed between Groups B and C.

Conclusion: The C1 unilateral lateral mass could mostly contain two screws(PS + LMS) with diameters ≤ 3.5 mm. The novel technique of unilateral C1 double screw and ipsilateral C2 PS combined with contralateral C2 LS rod fixation provided better stability than unilateral PS rod fixation and similar as bilateral PS rod fixation. Therefore, it is a feasible salvage method that provides a new insight into atlantoaxial instability. These slides can be retrieved under Electronic Supplementary Material.