X43 - Design and Execution of a Custom Test Fixture for Test Method Verification per ASTM F3631, Annex A2

Introduction: : The prevalence of spine surgery in the United States is growing exponentially every year. According to the American Spine Registry (ASR) in their 2025 annual report, 342,842 spine procedures occurred in 2023 alone [1]. Additional data reported to the ASR between 2015 and 2023 showed approximately 36.4% of 32,443 total reported cases were for lumbar spinal fusion procedures. With increasing prevalence of spinal fusion procedures, medical device manufacturers (MDMs) are working towards continuous improvement of their products to gain the largest market share. To get these products out into the field and into surgeon’s hands, mechanical testing is required to ensure device safety and efficacy. MDMs must test their products according to the standards that are established by ASTM, a technical standards committee. This committee works to provide the most up to date standard test methods. Recently, a new standard was introduced for assessment of intra-operative durability of intervertebral body fusion devices (IBFD) (ASTM F3631) [2]. When new standards are released, the MDM must either purchase or design test fixtures that can properly test their devices. This study aims to verify that testing can be properly executed in full compliance with the ASTM F3631, Annex A2: Impact Test Method Using Static Rigid Block using a custom-designed test fixture.

Materials and

Methods: : Testing was conducted using a custom-designed test fixture (Figure 1A) in accordance with ASTM F3631, Annex A2. Reef TO IBFDs and inserters (Orthofix Medical Inc, Carlsbad, CA) were tested under two distinct impact loading schemes (ILS) (Figure 2). A sample size of five (n=5) IBFDs were used per group and one inserter was used for all test constructs (IBFD and inserter combination) across groups. A custom stainless-steel test block, machined with a pocket matching the geometry of the Reef TO IBFD, with a depth of approximately 20% of the total interbody length was used in testing (Figure 1B). This test block acted as a rigid base to prevent movement of the test construct during impact loading.

For each test construct, the following data was collected: drop height at failure, number of impacts at failure, and failure mode. A two-sample t-test assuming equal variances was performed on the drop height at failure and the number of impacts at failure to evaluate the performance differences between the two ILSs.

The custom-designed test fixture was also evaluated based on its ability to maintain vertical alignment within ±3° of the vertical z-axis (monitored using a digital angle indicator), maintain the drop height within ±2mm of the target drop height, and its ability to provide a rigid base to prevent movement of the test constructs during impact loading. Meeting these criteria was used to confirm that testing can be executed in full compliance with the standard.

Results, Conclusions, and Discussions:: The two-sample t-test assuming equal variances for drop height at failure and number of impacts at failure showed that there were no significant differences between the two ILSs (p > 0.05) (Figure 3). In all cases, the failure mode was fracture of the Reef TO IBFD. No failure of the inserter component was observed.

The custom test fixture met most of the compliance criteria as outlined per ASTM F3631, Annex A2. vertical alignment to the central z-axis was maintained to ±0.68° for ILS 1 and ±0.66° for ILS 2. Drop height was maintained within ±2mm of the target drop height for ILS 1 using the guide rods fixed-height increments of 10mm. For ILS 2, maintaining consistent drop height within ±2mm was more difficult to maintain due to the loading scheme increasing drop height by 25mm increments rather than the 10mm fixed-height increments on the guide rod. The rigid base provided sufficient rigidity to maintain alignment and prevent movement before impact loading. However, during impact loading, it was observed that the test constructs would dislodge from the pocket upon impact from greater drop heights. Manual repositioning was necessary to continue testing when this occurred, which introduced minor variability in seating and alignment. While this did not appear to compromise vertical alignment or results of impact loading per ASTM F3631, Annex A2, it did highlight a limitation in the test fixture design that could be improved for better construct retention during testing.

The results above confirm that the test fixture performs consistently across ILSs, and meet compliance criteria per ASTM F3631, Annex A2. However, design adjustments are necessary for improved construct immobilization and drop height consistency. Further testing of additional product lines is needed to confirm that this custom-designed test fixture can test constructs of all shapes and sizes in compliance with the standard.

Acknowledgements and/or References (Optional)::

References:
[1] American Spine Registry (ASR): 2025 Annual Report. Rosemont, IL: American Academy of Orthopaedic Surgeons (AAOS) in partnership with American Association of Neurological Surgeons, 2025.
[2] ASTM F3631-24 Standard Test Method for Assessment of Intra-operative Durability of Intervertebral Body Fusion Devices.