Slippage of a spinous process hook during flexion in a flexible fixation system for the lumbar spine

This study was undertaken to measure the amount of slippage of a spinous process hook (that forms part of a flexible fixation system) during flexion. Human cadaveric lumbar spines (10) were fitted with the device. A rig was designed to apply flexural displacements to a spine using a materials testing machine. Spherical markers were attached to the spine and hook. As a spine was flexed a digital video camera was used to record the positions of the markers. The movements of the markers were measured using interactive computer software to assess any slippage of the spinous process hook. During flexion the overall mean hook slippage was measured to be 0.10 mm (standard deviation 0.04 mm). The mean hook slippage, for each of the 10 specimens, was in the range of 0.05–0.14 mm. The results imply that slippage of a spinous process hook during flexion is small.