Poster AA8 - Improving Bone Screw Pullout Testing: Investigating the Impact of Clamping Distance

Introduction: Within orthopedics, bone screws are an essential tool in the surgeon’s kit1. They are used to connect bone to bone, implants to bone, and soft tissue to bone. One of the primary requirements of bone screws is the ability to withstand being pulled out of bone. Thus, the FDA requires that bone screws are tested in part using the axial pullout load test described in the standard ASTM F543, Annex A32. This test ensures that screws cleared for the market demonstrate the required fixation strength. For this test, clamps are used to keep the bone foam block attached to the load cell as the bone screw is pulled from the bone foam block. However, the standard does not prescribe a set clamping distance. This study investigates the effect of clamping distance on the load and stiffness results of axial screw pullout testing.

Materials and

Methods: 20 PCF solid rigid polyurethane foam (Pacific Research Laboratories; Vashon, WA), compliant with ASTM F18393, was used with a Ø3.50x22 mm NorthStar polyaxial pedicle screw (SeaSpine; Carlsbad, CA) for axial screw pullout testing per ASTM F543, Annex A3. The bone foam was cut into 180x40x40 mm sized blocks and pilot holes were created using a 24 gauge drill bit (slightly smaller than the minor of the screw). Pilot holes were drilled such that clamping distances of 50, 70, 110, 145, and 180 mm could be tested, with a minimum of 17.5 mm between pilot holes. The bone screws were inserted into the foam to a depth of 20 mm, per ASTM F543. The overall testing setup is shown in Figure 1(A). Pullout testing occurred at a rate of 5 mm/min until failure or 2.5 mm of displacement. Ultimate axial pullout load and stiffness were recorded. A single factor ANOVA (p < 0.05) and Tukey’s HSD (p < 0.05) tests were performed to determine significance between clamping distances.

Results, Conclusions, and Discussions: The ANOVA and Tukey’s HSD tests comparing the stiffness results of all clamping distances showed significant differences between 50 mm and 145 mm, between 70 mm and 145 mm, and between 110 mm and 180 mm clamping distances. On the other hand, the ANOVA comparing the ultimate axial pullout load results of all clamping distances showed no significant differences. The stiffness and ultimate axial pullout load averages are shown in Figure 1(B). Additionally, for clamping distances of 145 mm and 180 mm, the bone foam occasionally failed via fracture through the block, breaking the block into two at the screw insertion point. This occurred on 2 out of 6 blocks for the 145 mm clamping distance and 4 out of 6 blocks for the 180 mm clamping distance. All other blocks failed at a circle at the major of the screw, allowing the screw to pull out of the bone foam without otherwise damaging the block.
Based on the results above, clamping distances for ASTM F543 Annex A3 should be kept at or below 110 mm for a Ø3.50 screw to guarantee accurate stiffness and consistent ultimate axial pullout load results. Future testing with a variety of screw diameters is needed to determine if ideal clamping distance scales with screw diameter.

References:
[1] Bone Screws in Orthopaedic Surgery. (June, 2013). Orthopaedic Principles.
[2] ASTM F543-23 – Standard Specification and Test Methods for Metallic Medical Bone Screws.
[3] ASTM F1839-08 (2021) – Standard Specification for Rigid Polyurethane Foam for Use as a Standard Material for Testing Orthopaedic Devices and Instruments.

Acknowledgements (Optional): This study was funded by SeaSpine/OrthoFix Medical Inc.