Q31 - Comparison of Test Directionality, Test Fixture Design, and Test Fixture Material for Static Axial Torque Testing per ASTM F1798.

Introduction: : As spinal fusion surgeries become more common in aging populations, the complexity of these procedures has increased in parallel. In particular, thoracolumbar spinal fusion surgeries show variable success rates, highlighting the complex nature of these procedures and the need for revisions. Approximately 9-45% of thoracolumbar spinal fusion surgeries require revisions, or the modification of previously implanted hardware to address complications in spinal support and alignment [1]. Standard hardware consists of screws inserted into vertebral bones, and rods attached to these screws to restore healthy spinal curvature. During revision procedures, variable parallel connectors (VPCs) are connected to rods in existing hardware to facilitate the addition of new components to extend structural support to adjacent vertebral levels (Figure 1) [2]. To gain FDA approval for use in spinal surgery, VPCs must undergo mechanical testing. One mechanical test that is performed is static axial torque testing per ASTM F1798, which measures torsional resistance to loosening of interconnection mechanisms of spinal implants [3]. This test evaluates whether spinal implants like VPCs can maintain their connections with rods under physiological torsional forces, ensuring their safety and efficacy for surgical use. However, variability in torsional force measurements during static axial torque testing may occur based on test directionality, test fixture material, and test fixture design. Thus, the objective of this study is to investigate these test parameters for static axial torque testing of VPCs to reduce variability in torsional force measurements.

Materials and

Methods: : Static axial torque testing was conducted with three different test fixtures in clockwise (CW) and counterclockwise (CCW) directions (Figure 2). A Mariner Outrigger VPC (Orthofix Medical Inc., Carlsbad, CA) was used to evaluate differences in torsional force measurements when tested in CW versus CCW directions, generic fit versus custom fit test fixtures, and stainless steel versus 3D-printed plastic test fixtures. A sample size of five (N=5) was tested for each group for a total of four groups. Each sample was tested on an axial-torsional load frame (MTS Bionix; Eden Prairie, MN) at a rate of 60°/min for 60° angular displacement per ASTM F1798. Gripping capacity and torsional stiffness values were recorded. Two-sample t-tests (P < 0.05) and a single-factor ANOVA (P < 0.05) were performed on the recorded values to determine statistically significant differences between test directionality, test fixture material, and test fixture design.

Results, Conclusions, and Discussions:: Two-sample t-tests showed no statistically significant differences in gripping capacity and torsional stiffness between samples tested in the CW versus CCW directions, and samples tested in stainless steel versus 3D-printed plastic test fixtures. Two-sample t-tests showed statistically significant differences in gripping capacity and torsional stiffness between samples tested in generic fit versus custom fit test fixtures. The ANOVA showed statistically significant differences in gripping capacity and torsional stiffness for samples tested in test fixtures with different designs and materials (Figure 3).

Based on the above results, static axial torque testing per ASTM F1798 may be tested in either CW or CCW directions, and in stainless steel or 3D-printed plastic test fixtures for VPCs. The test fixture design, however, may provide more consistent results when designed to be form-fitting to the VPC. Further testing with VPCs of different designs is needed to confirm the impact of test directionality, test fixture design, and test fixture material on torsional force measurements.

Acknowledgements and/or References (Optional)::

References:

[1] Kelly, M. P., Lenke, L. G., Bridwell, K. H., Agarwal, R., Godzik, J., & Koester, L. (2013). Fate of the Adult Revision Spinal Deformity Patient. Spine, 38(19), E1196–E1200. https://doi.org/10.1097/brs.0b013e31829e764b

[2] Connector System - Orthofix. (2025, January 20). Orthofix. https://orthofix.com/products/spine-solutions/spine-procedures/posterior-thoracolumbar-fixation/connector-system/

[3] ASTM F1798 – Standard Test Method for Evaluating the Static and Fatigue Properties of Interconnection Mechanisms and Subassemblies Used in Spinal Arthrodesis Implants.