30. Influence of toggling on pullout force and screw diameter increase in spine fixation

Abstract

<h3>BACKGROUND CONTEXT</h3> Pedicle screw fixation provides spinal stabilization and maintains normal vertebral motions in degenerative spondylolisthesis and scoliosis. Pedicle screw fixation requires several parameters to be optimized at the screw-bone interface to minimize clinical and biomechanical failure, including screw trajectory, bone density and screw sizing. Although prior studies have demonstrated screw loosening as an effect of toggling, they ultimately focus on the magnitude of the pullout force after cyclic loading rather than screw-bone interface throughout cycling. <h3>PURPOSE</h3> A stiffness decay model may help to better predict stability and investigate the effects of pedicle screw diameter to pedicle width ratio on overall screw stability following cyclic loading in the thoracolumbar spine. <h3>OUTCOME MEASURES</h3> Pullout force, toggling forces, decay functions. <h3>METHODS</h3> Thoracolumbar vertebrae (T1-L5) were harvested from two cadaveric spines. Pedicle screws were placed using a standard surgical technique. Five-hundred cycles toggle testing was conducted with a uniaxial load frame followed by axial pullout tests. Exponential decay models with a stiffness coefficient were computed to examine toggling force over the number of toggle cycles for each screw. Linear regression modeling and Pearson's correlation tests were used to compare the decay coefficient with the ratio of the screw diameter to the pedicle diameter, the pullout strength, and pedicle loosening. Subgroup analysis was performed between under-sized (70% pedicle diameter) screws. <h3>RESULTS</h3> Sixty-eight pedicles were harvested. Of the 44 viable pedicles, 33 (75%) were from the thoracic spine and 11 (25%) were from the lumbar spine. There were 23 pedicle screws in the undersized group and 21 pedicle screws in the normal-sized group. The average screw diameter was 53% (SD 5%) of the pedicle diameter in the undersized group and 77% (SD 8%) of the pedicle diameter in the normal-sized group (p < 0.001). A significant association was found between the screw to pedicle diameter ratio and the natural log of the stiffness decay coefficient (p = 0.035). The average pullout force was 170 N (SD 75) in the under-sized pedicle group and 194 N (SD 109) in the normal-sized group (p=0.40). Higher pullout force was associated with a lower stiffness decay coefficient (p = 0.005). Both pullout force and the natural log of the stiffness decay coefficients were associated with the drop in maximum toggling force after 500 cycles (p = 0.026 and p < 0.001, respectively). Utilizing the stiffness decay coefficient provided a better fit model for the predicted drop in maximum toggling force with a lower Akaike information criterion. <h3>CONCLUSIONS</h3> Pedicle screws with a high stiffness decay coefficient during toggle testing had lower pullout strengths. While higher pedicle screw to bone diameter ratios were associated with increased stiffness decay coefficient, there was no relationship between pedicle screw size and pullout strength. In subgroup analysis, however, increasing pedicle screw to bone diameter ratios were associated with lower pullout strength in the normal-sized group. Clinically, selecting a screw size that maximizes the pedicle diameter-screw ratio may not always lead to better fixation in all spine levels or regions. <h3>FDA DEVICE/DRUG STATUS</h3> This abstract does not discuss or include any applicable devices or drugs.