Design and biomechanical evaluation of atlantoaxial lateral mass fusion cage

Objective To design an atlantoaxial lateral mass fusion cage and evaluate its biomechanical stability when it is combined with atlantoaxial vertebral pedicle screw fixation. Methods Forty-six sets of CT 3D reconstruction pieces of the normal atlantoaxial junction were chosen to measure sagittal diameter and transverse diameter of atlantoaxial lateral mass joint, sagittal diameter and transverse diameter of epistropheus lateral mass and space height of atlantoaxial lateral mass joint. An atlantoaxial lateral mass fusion cage was designed on this basis. Six fresh human cadaveric cervical spines (C0-C4) were used as samples to measure 3D motion range of C1, and 2 segments under 1.5 N·m load. 3D motion range of samples under the following situations was measured at random: intact state, unstable state (ligament around odontoid process was cut off), fixation with atlantoaxial joint screw+Gallie steel wire, atlantoaxial pedicle screw, atlantoaxial lateral mass joint fusion cage+atlantoaxial vertebral pedicle screw. Results Corresponding width/length of fusion cage is 8/11, 9/12, 10/13 mm, respectively, and the height is designed to 3.5, 4.0, and 4.5 mm, respectively. The motion range of three internal fixation methods is less than that under intact state and unstable state. The difference has statistical significance. The C1+C2+cage fixation produces the least motion range in lateral bending and axial rotation directions and generates the highest motion range in flexion/extension direction. But, the difference has no statistical significance. Conclusion The C1+C2+cage internal fixation technique has similar stability with common atlantoaxial internal fixation method and can provide extra atlantoaxial fusion spots. Thus, it may be a feasible alternative for atlantoaxial fusion when the posterior arch of the atlas is absent. Key words: Biomechanics; Internal fixators; Atlanto-axial joint; Spinal fusion