A biomechanical comparison of modern anterior and posterior plate fixationof the cervical spine

Study Design. A biomechanical study was designed to assess relative rigidit y provided by anterior, posterior, or combined cervical fixation using cada veric cervical spine models for flexion-distraction injury and burst fractu re. Objectives. To compare the construct stability provided by anterior plating with locked fixation screws, posterior plating with lateral mass screws, a nd combined anterior-posterior fixation in clinically simulated 3-column in jury or corpectomy models. Summary of Background Data, Anterior plating with locked fixation screws is the most recent design and is found to provide better stability than the c onventional unlocked anterior plating. However, th ere are few data on the direct comparison of biomechanical stability provided by anterior plating w ith locked fixation screws versus posterior plating with lateral mass screw s. Biomechanical advantages of using combined anterior-posterior fixation c ompared with that of using either anterior or posterior fixation alone also have not been well investigated yet. Methods. Biomechanical flexibility tests were per formed using cervical spi nes (C2-T1) obtained from 10 fresh human cadavers. In group I (5 specimens) , one-level, 3-column injury was created at C4-C5 by removing the ligamentu m flavum and bilateral facet capsules, the posterior longitudinal ligament, and the posterior half of the intervertebral disc. In group II (5 specimen s), complete corpectomy of C5 was performed to simulate burst injury. In ea ch specimen, the intact spine underwent flexibility tests, and the followin g constructs were tested: (1) posterior lateral mass screw fixation (Axis p late) after injury; (2) polymethylmethacrylate anterior fusion block plus p osterior fixation; (3) polymethylmethacrylate block plus anterior (Orion pl ate) and posterior plate fixation; and (4) polymethylmethacrylate block plu s anterior fixation. Rotational angles of the C4-C5 (or C4-C6) segment were measured and normalized by the corresponding angles of the intact specimen to study the overall stabilizing effects. Results. Posterior plating with an interbody graft showed effective stabili zation of the unstable cervical segments in all loading modes in all cases, There was no significant stability improvement by the use of combined fixa tion compared with the posterior fixation with interbody grafting, although combined anterior-posterior fixation tended to provide greater stability t han both anterior and posterior fixation alone. Anterior fixation alone was found to fail in stabilizing the cervical spine, particularly in the flexi on-distraction injury model in which no contribution of posterior ligaments is available. Anterior plating fixation provided much greater fixation in the corpectomy model than in the flexion-distraction injury model. This fin ding suggests that preservation of the posterior ligaments may be an import ant factor in anterior plating fixation. Conclusions. This study showed that the posterior plating with interbody gr afting is biomechanically superior to anterior plating with locked fixation screws for stabilizing the one-level flexion-distraction injury or burst i njury. More rigid postoperative external orthoses should be considered if t he anterior plating is used alone for the treatment of unstable cervical in juries. It was also found that combined anterior and posterior fixation may not improve the stability significantly as compared with posterior graftin g with lateral mass screws and interbody grafting.