Poster AA7 - Improving Bone Screw Flexion-Extension Testing: Test Fixture Setup Evaluation

Introduction: Bone screws are widely used within the field of orthopedics, and within spine. They are a supplemental fixation method used to align, immobilize, and stabilize the vertebrae, allowing for fusion to occur [1]. Medical device companies are continually working to enhance their products and provide surgeons with the market leading bone screw. To get these bone screws into surgeons’ hands and into patients’ bodies, FDA clearance is required, proving that the devices are safe and effective. Static flexion-extension moment (My) testing of the bone screws per ASTM F1798 is one of the required testing modalities for FDA clearance [2]. A common feature for bone screws is their ability to be locked down at different angles, optimizing rod capture for multi-level cases. The standard test does not account for this use case and thus, it is important to investigate bone screws at their maximum angulation in addition to what is required by the standard. Since there are no standard guidelines for testing these bone screws at maximum angulation, this study sought to evaluate the most optimal test fixture setup for bone screws at maximum angulation that can produce the highest gripping capacity, is easy to setup, and produces the smoothest data during modified ASTM F1798 static flexion-extension moment (My) testing.

Materials and

Methods: Four different test fixture setups (identified as 1 to 4) were tested with a sample size of 5 (n = 5). A Ø4.5mm Mariner polyaxial bone screws (Orthofix Medical Inc, Carlsbad, CA) at maximum angulation (60 degrees of conical range of motion) was used to evaluate these test fixture setups on the modified static flexion-extension moment (My) testing as shown in Figure 1. Modification of the ASTM standard places the screw at maximum angulation rather than parallel to the set screw axis. After constraining each test construct in the respective test fixtures, axial load was applied to the screw shank at 25mm from the center of the rod and at a rate of 25 mm/min. Gripping capacity, ultimate torque, angular displacement, torsional stiffness, and set screw locking torque were recorded. A single factor ANOVA (p < 0.05) and Tukey’s HSD (p < 0.05) tests were performed on the gripping capacity values for each test fixture setup to determine the significance between them.

Results, Conclusions, and Discussions: The ANOVA, for gripping capacity showed significant differences across all test fixture setups (Figure 2). Further analysis using Tukey’s HSD multiple comparison test was done to determine if the relationship between various test fixture setups had resulting gripping capacity data that was statistically significant (p < 0.05). Results indicated that there were between group differences across all groups except when comparing test fixture setups 1 and 2, and test fixture setups 3 and 4. In all cases, the failure mode was rotation of the screw shank within the tulip and bending of the screw shank. Overall, the test fixture setup 3 test constructs achieved the highest gripping capacity and test fixture setup 2 achieved the lowest.

With regards to ease of setup, test fixture setup 1 and test fixture setup 3 proved to be easier than test fixture setups 2 and 4. Test fixture setups 1 and 3 were more universal such that any screw type could be tested in these fixtures and were able to fully immobilize the screw during testing. Data smoothness was evaluated off moment vs angular displacement curves. These curves demonstrated that test fixture setups 3 and 4 had the smoothest data, minimizing actuator slippage by placing the screw shank parallel to the applied load at the start of the test.

Overall, test fixture setup 3 produced the highest gripping capacity, was easy to setup, had a universal test fixture setup which could be used with multiple screw designs, and produced the smoothest data.

Based on the results above, the use of test fixture setup 3 is recommended to be used when conducting modified ASTM F1798 static flexion-extension moment (My) testing when the screw is locked down at maximum angulation. More testing with a larger screw shank diameter is needed to determine whether bending of the screw shank had a significant effect on the gripping capacity. Testing with a screw from a different product line is needed to confirm that the results hold true in that test fixture setup 3 remains the best testing setup for this application.

Acknowledgements (Optional):

References:
[1] Park D K. Spinal Fusion - OrthoInfo - AAOS. https://orthoinfo.aaos.org/en/treatment/spinal-fusion/#:~:text=Spinal%20fusion%20eliminates%20motion%20between,or%20the%20normal%20aging%20process. Published February 2022. Accessed April 30, 2024.
[2] ASTM F1798-21 Standard Test Methods for Evaluating the Static and Fatigue Properties of Interconnection Mechanisms and Subassemblies Used in Spinal Arthrodesis Implants.