X42 - Characterization of Thoracolumbar Pedicle Screw Tulips: Investigating the Impact of Geometry and Material Differences on Rod Gripping Capacity

Introduction: : The global prevalence of spinal deformities and injuries drives the need for close to 313 million spinal surgeries annually1. Thoracolumbar pedicle screws are critical components in spinal fusion surgeries, providing stability and facilitating the correction of a wide range of these spinal deformities and injuries. The efficacy of these screws largely depends on their ability to securely grip the connecting rods, ensuring proper alignment and load distribution across the spinal construct. To assess the mechanical performance of these thoracolumbar pedicle screws, engineers use ASTM F17982, a widely recognized standard for evaluating the gripping capacity (Fz) of spinal implants. This standard provides guidelines for testing the gripping capacity of spinal devices, which is instrumental in ensuring their long-term stability in patients. The procedure outlined by the standard will allow us to consistently test tulips of varying geometry and material and isolate their effects on gripping capacity. The findings from this study are expected to provide valuable insights for both manufacturers in designing superior thoracolumbar pedicle screws and surgeons in selecting the most effective implants for their patients.

Materials and

Methods: : Static axial compression was conducted on Ø6.0x40 mm Mariner pedicle screws (SeaSpine; Carlsbad, CA) per ASTM F1798. Seven tulip groups of differing geometry and material were subjected to a load applied to their rods at a displacement controlled rate of 25mm per minute until failure or until 5mm of displacement occurred. Five samples for each tulip group were tested on an axial-torsional load frame (MTS Bionix; Eden Prairie, MN). During each test, displacement and force were collected at 50 Hz, which were used to calculate gripping capacity per ASTM F1798 for each test construct. A single factor ANOVA (p < 0.05) and Student’s T-test (p < 0.05) were performed on the analyzed values to determine assess the similarities or differences between gripping capacities for each tulip type.

Results, Conclusions, and Discussions:: An ANOVA test across all tulip groups revealed significant differences in mean gripping capacity between the different tulip groups. When the ANOVA was rerun, excluding the polyaxial and uniaxial tulips, the P-value was greater than 0.05, indicating that the gripping capacities for the uniplanar, deformity, trauma, polyaxial (CoCr), and closed Ti tulips are statistically similar. However, when either the polyaxial tulip, the uniaxial tulip, or both were included with the other tulip types, the P-value dropped below 0.05, suggesting significant differences in gripping capacity. Furthermore, a Student’s T-test confirmed that the gripping capacity for the polyaxial tulip was statistically different from all other tulip types in the dataset, and the uniaxial tulip's gripping capacity differed significantly from 4 out of the 6 other groups. The two that uniaxial was calculated to be statistically similar to were the uniplanar and deformity tulips.
Based on the results above, the polyaxial and uniaxial tulips have rod gripping capacities different than all the other tulip types. These two tulip types have up to 30% greater gripping capacities compared to the others. The gripping capacities of the remaining five tulip types were found to be statistically similar. Overall, these findings will be useful to surgeons correcting spinal deformities and injuries utilizing the Mariner pedicle screw system. This data will help surgeons make more informed decisions about proper tulip selection to correct different deformities and injuries. Hence, this should lead to a better and more predictable clinical outcome for patients undergoing surgery with this system.

Acknowledgements and/or References (Optional):: This project was supported by OrthoFix Medical Inc.