A comparison of pedicle and lateral mass screw construct stiffnesses at the cervicothoracic junction: a biomechanical study.

STUDY
DESIGN: Biomechanical comparison of five different posterior cervicothoracic junction (C7-T1) fixation constructs in a cadaveric model.
OBJECTIVES: To determine whether augmenting C7 lateral mass screws with spinous process wires or additional fixation in the C6 lateral mass can create constructs of similar normalized stiffness to that of C7 pedicle screws. SUMMARY OF BACKGROUND DATA: Cervical pedicle screws are known to provide excellent fixation but are potentially dangerous and technically demanding to insert. Lateral mass screws are safer and easier to insert but have less pullout strength and must often be short at C7.
METHODS: Twelve cadaveric cervicothoracic specimens (C5-T2) were randomly assigned to one of three experiments: Experiment A (Part 1 and Part 2), Experiment B, and Experiment C (Part 1 and Part 2) (n = 4 each for each experiment). First, the intact specimens were biomechanically tested according to a seven-part loading protocol. The specimens were then destabilized, and then restabilized with the following constructs in conjunction with bilateral T1 pedicle screws and biomechanically tested again using the same seven-part biomechanical protocol as was applied to the intact specimens. Experiment A: Part 1: lateral mass screw fixation at C7 (C7LM); then Part 2: retested after augmentation with triple wiring (C7LM+W). Experiment B: pedicle screw fixation at C7 (C7PS). Experiment C: Part 1: C6 and C7 lateral mass screws (C6C7LM); then Part 2: retested after augmentation with triple wiring (C6C7LM+W). Thus, five different constructs were biomechanically compared in these three experiments.
RESULTS: None of the lateral mass constructs demonstrated a significant increase in normalized stiffness when augmented with wiring in any mode of testing. In axial compression, the C7PS construct showed significantly higher (P < 0.001) normalized stiffness than any of the other four constructs. In extension, there were no significant differences among any of the five constructs. Inflexion, right/left lateral bending and right/left axial torsion, the C7PS construct again showed significantly higher normalized stiffness (P < 0.05) than lateral mass fixation at C7 alone. However, in these five modes of testing, the addition of a secondary point of lateral mass fixation at C6 (C6C7LM) produced a construct with a normalized stiffness similar to that of C7PS with no significant difference (P > 0.05).
CONCLUSION: C7 pedicle screw fixation provides the construct with the highest normalized stiffness for stabilizing the cervicothoracic junction. If C7 pedicle fixation is not possible, then performing two-level lateral mass fixation at C6 and C7 will achieve a construct with similar normalized stiffness except in axial compression. The addition of triple wiring to the spinous processes does not significantly increase lateral mass construct normalized stiffness.