A Finite Element Investigation And A Clinical Analysis Of Three Hybrid Posterior C1-2 Fixation Techniques

1. Objective:The aim of this subject is to investigate the stability and mechanics features of three hybrid posterior C1-2 fixation techniques by means of finite element and to offer basic evidences for their clinical application. Further, by evaluating the clinical outcome of 22 cases adopting the C1-2 fixation techniques in long term follow-up, we aim at verifying their safety and efficacy in practical work.2. Materials and MethodsThree-dimension finite element analysisTwo three-dimension finite element models were built: normal human upper cervical spine, human upper cervical spine of C1-2 dislocation. Further, three different hybrid posterior C1-2 fixation techniques including C1-C2 transarticular screw + C1 lateral mass screw+ C2 pedicle screw (TS+ C1LMS+ C2PS), C1-C2 transarticular screw+ C1 lateral mass screw+ C2 intralaminar screw (TS+ C1LMS+ C2ILS) and C1-C2 transarticular screw+ wiring (TS+Gallie) were loaded on the model. In addition, stability of the three techniques and the biomechanics features were evaluated through three-dimension finite element method.Clinical investigation22 patients with instable C1-2 who adopted variable hybrid posterior C1-2 fixation techniques were followed up for mean 14 months. the medical record, graphic materials, treatments, neurological defect improvements, and postoperative fusion status are analyzed. JOA score system is used to evaluate the nerve improvement and to evaluate the postoperative fusion, we performed imaging methods including X-rays and 3-D CT.3. Results3-D finite element analysis Establishment of 3-D finite element model and biomechanical analysis of three hybrid posterior C1-2 fixation techniques3-D finite element model of anterior C1-2 dislocation with a broken ligamentum transversum and without odontoid process fracture assumes a ideal fidelity in appearance and the geometric similarity is satifactory. Rang of motion (ROM) of C1-2 in all modes increases significantly with a 17.8. increase in anterior flexion and a 13.7。increase in posterior extension.Three hybrid posterior C1-2 fixation techniques can all reduce the ROM of C1-2 in which TS+C1LMS+C2PS shows the best stability in the modes of anterior flexion, posterior extension, and rotation. On the other hand TS+Gallie shows the poorest stability and TS+C1LMS+C2PS assumes approximate stability with TS+C1LMS+C2PS but shows slightly inferior in rotation. In the aspect of internal forces, unilateral TS bears highest internal stress especially in the anterior flexion, lateral curvature and rotation.Clinical case follow-upThe operation duration ranged 120-280 minutes (mean 210 minutes), intraoperative bleeding was about 350 ml. in the mean 14 months'follow-up, all the patients get neurological improvement to some extent. JOA score increased from preoperative mean 6.9 to 14.7 at postoperative 3 months. the improve rate was 77.8%.In the mean 14 months'follow-up, no internal fixation related complications such as screw broken, second dislocation was detected. The imaging investigation showed no C1-2 instability. In all the 22 patients, 21 patients achieved bony fusion in 3 months and they all achieved fusion in posteroperative 6 months.4. conclusionsHybrid posterior C1-2 fixation techniques (TS+C1LMS+C2PS, TS+C1LMS+C2ILS, and TS+Gallie) can limit ROM of C1-2 in all the directions. TS+C1LMS+C2PS shows the best stability in the modes of anterior flexion, posterior extension, and rotation. On the other hand TS+Gallie shows the poorest stability and TS+C1LMS+C2PS assumes approximate stability with TS+C1LMS+C2PS but shows slightly inferior in rotation. In the aspect of internal forces, unilateral TS bears highest internal stress especially in the anterior flexion, lateral curvature and rotation. The three hybrid posterior C1-2 fixation techniques are safe, solid, effective and can be clinical applied to selected case according individual conditions.