Abstract: BACKGROUND: The effect of anterior controllable anteriodisplacement and fusion on the biomechanics of cervical spine is still unclear. Previous studies have majorly focused on surgical techniques, the medium- and long-term efficacy, and postoperative complications of anterior controllable anteriodisplacement and fusion. 
OBJECTIVE: To analyze the biomechanical effects of anterior controllable anteriodisplacement and fusion on vertebrae-ossification of posterior longitudinal ligament complex and implants of the cervical spine using finite element method.
METHODS: A healthy male volunteer was recruited for CT scanning of the entire cervical spine. Using the finite element analysis software, a normal whole cervical spine model was constructed and its validity was verified by comparison with the previous articles. Subsequently, a preoperative model of continuous posterior longitudinal ligament ossification involving C4, C5, and C6 was constructed. Based on the preoperative model, a three-dimensional finite element model of anterior controllable anteriodisplacement and fusion was created. After constrain of the lower surface of the C7 vertebral body of the two models, an axial force of 50 N and a moment of 1.0 N·m were applied to the upper surface of the C1 cone body. Under forward flexion, posterior extension, left/right bending, and left/right rotation conditions, the effects of anterior controllable anteriodisplacement and fusion on vertebrae-ossification of posterior longitudinal ligament complex and implants were further analyzed. 
RESULTS AND CONCLUSION: (1) From the preoperative model, it was found that the ossification stress was mainly concentrated in the C4/5 segment; the maximum stresses of vertebrae-ossification of posterior longitudinal ligament complex under the conditions of forward flexion, posterior extension, left bending, right bending, left rotation and right rotation were 10.1, 148.6, 68.9, 74.8, 83.8, and 85.1 MPa, respectively. (2) After anterior controllable anteriodisplacement and fusion, the distribution area of stress concentration at the vertebrae-ossification of posterior longitudinal ligament complex did not change significantly, but the values were decreased obviously; in addition to the increase of stress (+44.7%) in the anterior flexion at the surgical model of anterior controllable anteriodisplacement and fusion, when compared with the preoperative one, the anterior controllable anteriodisplacement and fusion stress was significantly lower than that in the preoperative model under the other five working conditions, in which the value was decreased by -74.1% at the posterior extension position. Under the left bending, right bending, left rotation and right rotation, the ossification stress was decreased by 62.2%, 63.3%, 66.4%, and 67.9%, respectively. (3) The stress of titanium plate and screw was mainly concentrated at the both ends; the largest posterior extension stress was 149.5 MPa while the smallest forward flexion stress was 43.3 MPa. The stress of the four intervertebral cages was mainly concentrated at the C3/4 and C6/7 ones; and the stress was mainly distributed around the upper and lower surfaces of the fusion device, its value ranging from 30.8 MPa (the largest extension stress) to 11.5 MPa (the lowest forward flexion stress). The stress of the implants (titanium plate, screw, and intervertebral cage) was mainly concentrated at the two ends with the largest values, which would lead to the fracture of the titanium plate screw and the loosening of the screws. (4) In conclusion, anterior controllable anteriodisplacement and fusion was able to significantly reduce the stress of vertebrae-ossification of posterior longitudinal ligament complex, and may help prevent excessive proliferation and compression of nerves. After surgery, much attention should be paid to the occurrence of loosening of the screws, or displacement and fracture of titanium plates at the both ends.

Key words: ossification of posterior longitudinal ligament, anterior controllable anteriodisplacement and fusion, finite element analysis, ligament ossification, internal implant, biomechanics