Abstract: BACKGROUND: The addition of traditional rod-rod fixation for atlantoaxial joint disease to C1-C2 pedicle screw-rod fixation (C1-C2 PSR) can provide stronger anti-rotation stability for screw/rod fixation, but there is a risk of installation difficulties, impact on bone graft bed, and spinal cord injury. The new horizontal screw-screw crosslink (hS-S CL) designed by the authors can effectively overcome the above shortcomings, but its biomechanical properties are unclear.  
OBJECTIVE: To analyze biomechanical properties of new horizontal screw-screw crosslink in C1-C2 PSR by three-dimensional finite element analysis. 
METHODS: CT thin layer scanning data were collected from the occipital base to the axis (C0-2) of one adult healthy male volunteer. The atlantoaxial finite element models were established respectively: the normal group, the unstable group, the non-crosslink group (unstable+C1-C2 PSR), and the crosslink group (C1-C2 PSR+hS-S CL). Range of motion and Von Miss Stresses in flexion and extension, lateral flexion and rotation of the four groups were calculated by applying 1.5 Nm torque to each finite element model, and the stress cloud was extracted. 
RESULTS AND CONCLUSION: (1) Range of motion of the unstable group was increased by 43.8%-78.7% compared with the normal group, and the range of motion of the internal fixation groups was 90.2%-98.7% lower than that of the unstable group under six conditions. The range of motion of the crosslink group and the non-crosslink group was basically the same in flexion and extension states, but in lateral flexion and rotation states, the range of motion of the crosslink group decreased 34.3%-43.8% and 78.6%-79.1%, respectively, compared with the non-crosslink group, and range of motion decreased most obviously in rotation state. (2) The stress peak of the internal plant model: The maximum stress of the crosslink group was generally smaller than that of the non-crosslink group, and the stress peak value of all the internal fixation groups was the lowest when the extension was carried out. (3) The stress cloud of internal plants showed that there was no obvious stress concentration phenomenon in the internal fixation, and the main stress distribution areas were the screw root and bone joint, and the crosslink ends were the screw tail groove or the joint rod joint. (4) The new horizontal screw-screw crosslink can obviously improve the anti-rotation stability of internal fixation and it can share part of the pressure in the three-dimensional motion direction of the internal fixation system and reduce the maximum stress of the internal plants. However, the stress distribution is obvious at both ends of the crosslink, and this part may be prone to fracture of the crosslink. 

Key words: crosslink, posterior, atlanto-axial, three-dimensional finite element, cervical vertebra, internal fixation