Biomechanical comparison of posterior cervical fixation

Study Design. Biomechanical stability using four different posterior cervic al fixation techniques was evaluated in human cadaveric spine. Objectives. To introduce an alternative interspinous fixation technique usi ng wavy-shaped rods, and to compare its in vitro biomechanical stability wi th that of other posterior cervical fixation techniques. Summary of Background Data. Fixation of the posterior cervical spine with i nterspinous wiring is well known as Rogers' or Bohlman's technique. Recentl y, several plate fixation techniques have been used for posterior cervical stabilization. Since 1983, the authors have developed the wavy-shaped rod s ystem as an alternative to the interspinous fixation technique. This unique technique has been proven clinically useful in Japan. However, the authors are not aware of any prior biomechanical studies. Methods. Seven fresh frozen cervical human spines were tested at the C5-C6 motion segment. Nondestructive static biomechanical testing was performed w ith flexion-extension, lateral bending, and axial rotation for the followin g stabilization techniques: intact spine, creation of a Stage 3 distractive -flexion injury followed by fixation with the wavy-shaped rods bounded by t hree multistrand cables, interspinous wiring with a multistrand cable, trip le wiring technique using multistrand cables with a pair of unicortical gra fts from the ilium, and lateral mass plate fixation with Magerl's screw tec hnique. Testing was performed on a material testing machine (MTS 858 Bionix test system, MTS, Minneapolis, MN), and load displacement curves were obta ined using four linear extensometers and one rotatory extensometer across t he C5-C6 motion segment. Results. In axial compression loading, the reconstructed specimens showed s ignificant differences in range of motion measured at the anterior and post erior positions, and statistical analysis was performed using one-way analy sis of variance. In a comparison of the four fixation techniques, the const ruct with the wavy-shaped rod indicated significantly less motion both ante riorly and posteriorly than with the other fixation techniques. Also in fle xion-extension loading, all the techniques significantly limited the interv ertebral motion below the level of the intact motion segment. Particularly, the construct with the wavy-shaped rod showed the smallest mobility, 49.9% anteriorly and 9.3% posteriorly, compared with that of the intact spine. I n lateral bending, the lateral mass plate provided the greatest stability, which was superior to the intact segment, but the difference was not statis tically significant. In axial rotation, all the reconstruction techniques l imited the angular motion below the intact level (wavy rod, 68.0%; Rogers' wiring, 75.2%; Bohlman's triple wiring, 59.8%; lateral mass plate, 71.7%), but no significant differences were observed using one-way analysis of vari ance, as compared with the intact segment. Conclusions. All four reconstruction techniques restored the stability of t he cervical motion segment to at least the level of the intact motion segme nt before destabilization. An alternative cervical posterior fixation techn ique, the Wavy Rod system, was considered the most effective technique in s tabilizing a cervical motion segment, particularly in axial compression and flexion extension loading.