Intermediate screws in short segment pedicular fixation for thoracic and lumbar fractures: a biomechanical study.

To determine the effect of adding pedicle screws at the level of a burst fracture (intermediate screws) on the stiffness of a short segment pedicle fixation, an in vitro biomechanical study was carried out. Six fresh-frozen pig lumbar spine specimens were used. The flexibility of the intact specimens was examined in flexion, extension, lateral bending, and torsion. An unstable burst fracture model was created by the dropped-mass technique. The unstable spine specimens were instrumented with pedicle screws. The flexibility was tested again with and without intermediate screws. The addition of intermediate screws provided a smaller range of motion in flexion-extension (P<0.001), torsion (P<0.001), and lateral bending (P=0.014). The slopes of the load displacement curves increased in flexion (P<0.001), extension (P=0.003), lateral bending (P=0.003), and torsion (P=0.006), signifying a decrease in flexibility. The addition of intermediate screws at the level of a burst fracture significantly increases the stiffness of a short segment pedicular fixation.