Biomechanical study of the lumbar spine using a unilateral pedicle screw fixation system.

Lumbar fusion combined with unilateral pedicle screw fixation has received favourable clinical reports. However, there are very few reports about the biomechanical properties of this system. The purpose of this study was to compare the biomechanics of a unilateral pedicle screw system with a bilateral system. Two fresh lumbar vertebral columns from human cadavers were used. Seven models were prepared by the sequential damage and spinal instrumentation of each specimen. Bending and rotation tests were performed to clarify the range of motion for each model using a 6-axis material tester that we have developed. We showed that the unilateral pedicle screw system offers only uneven fixation. This results in dispersion of rigidity depending on the direction of bending and rotation. The bilateral pedicle screw system, however, allows excellent fixation in all directions.