Purpose: The current gold-standard for atlanto-axial fixation is C1-C2 Transarticular Screw (TS) fixation. In certain cases, the complicated nature of vertebral artery injury could make the application of bilateral transar-ticular screws impossible. This study biomechanically compares three atlantoaxial transarticular salvaging fixation techniques.

Method: Nine Fresh ligamentous human cervical spine specimens (C0-C4) were thawed and the tissue surrounding the spine, except the ligaments and discs, was carefully removed. Pure moments were applied to skull in increments of 0.5 Nm from 0 Nm to 2.0 Nm with the help of loading arms, nylon strings and pulleys. The specimens were tested in extension (EXT), flexion (FLEX), left lateral bending (LB), right lateral bending (RB), left axial rotation (LR) and right axial rotation (RR) for all the cases. The positions of the LEDs were recorded using an Optotrak Motion Measurement System (Northern Digital, Waterloo, Ontario, Canada) and was converted into three rotations (flexion/extension, lateral bending and axial rotation) using rigid body kinematic principles in relation to the fixed base. The specimens were tested intact and after type II odontoid fracture, were instrumented and tested with three fixation constructs:

C1-C2 TS on right side and C1LMS-C2PS on contralateral side

Results: All of the three instrumented cases significantly reduced motion across C1-C2 segment in all the modes when compared to intact (P< 0.005, two-tailed unpaired t-test at confidence interval of ninety-five percent) except in extension. TS+C1lM+C2PS is significantly stiffer than TS+ Wire only in axial rotation (P< 0.05) and equivalent in flexion/extension (P=0.75/P=0.51) and left/right bending (P=0.22/P=0.58). TS+C1LM +C2PS is equivalent to TS+C1LM+C2IL in all the loading modes (P> 0.05). TS+C1LM+C2IL is significantly stiffer than TS+Wire in axial rotation (P < 0.05) and equivalent in flexion/extension (P=0.93) and left/right bending (P=0.69/P=0.84).

Conclusion: This study showed that TS+C1LMS+C2PS fixation is equivalent to TS+C1LMS+C2ILS fixation in all the rotation modes and superior to TS+Wire fixation in axial rotation averaged over all ranges of motion. Also, TS+C1LMS+C2ILS fixation is superior to TS+Wire fixation in axial rotation averaged over all ranges of motion.