ABSTRACT
OBJECTIVE: It is not clear from the studies reported in the literature whether the addition of cross-links to bilateral pedicle screws increases the stiffness of the construct significantly to fix multilevel vertebral bodies. The current study addresses the question of how cross-links change the rotational stiffness of constructs with 6, 12, and 20 pedicle screws. METHODS: Seven thoracolumbar 10-level spine segments (T4-L1), from individuals of average age 72 years (range 46-83 years), were instrumented with pedicle screws (6, 12, and 20) and tested in torsion with and without cross-links. As the cadaver specimens had signs of osteopenia, constructs consisting of 10 wood blocks 35 x 35 x 78 mm (weight x height x length) were also tested following an identical protocol as in the cadavers to prove the concept. RESULTS: For both wood blocks and cadavers, the rotational stiffness increased linearly with number of screws with and without cross-links. Two cross-links increased rotational stiffness significantly for both 12 and 20 screws (nonsignificant for 6 screws) for wood block constructs. However, the rotational stiffness values of the cadaver constructs with or without cross-links failed to show any significant differences. CONCLUSIONS: This work has raised the need for developing a suitable animal model for the study of multilevel vertebral bodies to simulate the fixation of scoliosis corrections.
