Interlaminar fixation using the atlantoaxial posterior fixation system (3XS system) for atlantoaxial instability: surgical results and biomechanical evaluation.

This study evaluated the surgical results for patients with atlantoaxial instability due to various lesions treated using the atlantoaxial posterior fixation system (3XS system; Kisco DIR, Paris, France), together with a biomechanical study of this system. The strength of the 3XS system during torsion was examined using a biomechanical simulation model. The 3XS system consists of a transverse unit, hooks, and rods. The lower part of the biomechanical simulation machine was rigidly fixed and the upper part was movable, allowing torsion to be applied until the point of failure. The test was started at 1.5 newton-meters, thought to be the maximum load on the upper cervical spine. The 3XS system tolerated torsion of up to 20 newton-meters, but became deformed. The instrument was fractured at 30 newton-meters. Fifteen patients, four with atlantoaxial instability, seven with os odontoideum, and four with odontoid fractures, underwent surgery using the 3XS system and an iliac bone fragment inserted between the C-1 and C-2 laminae. Postoperative rigid fixation of the lesion and optimal cervical alignment was obtained in all patients, and the patients were discharged within 2 weeks after surgery. Follow-up radiography showed bony fusion between C-1 and C-2 in all patients. Posterior fixations between C-1 and C-2 using the 3XS system were easy to perform and no surgical complications were encountered. The biomechanical study showed the 3XS system can tolerate torsions unlikely to occur during rotation movements in the atlantoaxial region in humans. The surgical use of the 3XS system for the treatment of atlantoaxial instability has several advantages.