1449. Posterior atlantoaxial fixation with customized variable screw placement (VSP) plate and screws with a new sub facetal axis screw trajectory enhancing safety and biomechanics of fixation.

Authors: Sushil Vasant Patkar, MD MS (PUNE, India)

Introduction :Posterior atlantoaxial fixation with polyaxial screw and rods is the present prevalent method for treatment of atlantoaxial instability . A new trajectory for passing the axis screw under the superior articular facet downward and medially into the axis body bypassing the vertebral artery is described. Customized variable screw placement (VSP) plates and screws permit manipulation of the atlantoaxial joints for reduction and realignment and rigid fixation . Methods: Since March 2015 up to September 2018, twelve cases of atlantoaxial instability were treated using the new VSP plate screw construct. The atlantoaxial joints were exposed in usual manner, endplates were decorticated and autologous bone from the spinous processes grafted into the joints. The atlas screw was passed as described in previous literature. The entry point for the axis screw was 2-3 mm below the midpoint of the joint under the articular surface of the superior facet and directed downwards and medially for 18 mm to 20 mm into the body of the axis. The VSP plates were passed over the screw tops and with customized screw holder the joints were manipulated into reduction an realignment and fixed with hexagonal top nuts. Results: Adequate repositioning and rigid fixation on dynamic radiology of the joints was possible in all cases . Follow-up till 6 months shows maintenance of realignment. No case of vertebral artery injury or implant failure was encountered Conclusion: Vertebral artery injury is the most feared complication of posterior atlantoaxial fixation. This new axis screw trajectory bypasses the vertebral artery altogether. Polyaxial screw rod construct is biomechanically weaker than the VSP plate screw construct. The VSP design permits easy manipulation and repositioning of the atlantoaxial joint.