Chief of Spine Surgery University of Minnesota University of Minnesota Minneapolis, MN, US
Disclosure(s):
David W. Polly, MD, Jr.: No relevant disclosure to display
Introduction: Sacroiliac joint (SIJ) fusion is a common procedure for SIJ dysfunction and pain. Historically, standard screw thread forms were used; however, a novel screw fusion device has emerged using a porous surface and dynamic thread pitch to induce bony ingrowth. While primary insertional and pullout characteristics have been documented, no studies have examined engagement characteristics for revision. Therefore, this study investigated the re-insertional torque and pullout strength across various screw dimensions and conditions.
Methods: Mechanical testing (ASTM F543) was conducted to evaluate insertional torque and pullout strength, using 10 and 20pcf (lbs/ft3) polyurethane bone-foam blocks and TORQ iFuse Implants. A 10x35mm screw was inserted and served as the control. Subsequently, the 10x35mm screw was replaced with varying diameters (10, 11.5, 12.5mm) and lengths (35, 40, 45mm). Peak torque was measured for each test. The maximum axial (tensile) load required for pullout was recorded. Reinsertion followed two conditions: direct and after taping. ANOVA with pairwise comparisons (Bonferroni; α=0.05) and Pearson correlation tests were performed.
Results: Peak insertional torque was significantly less than the control for the same size (control vs 10x35 | 477.7 vs 322.16±14.5N-cm; p< 0.05) and higher for large screws (11.5x40, 11.5x45, 12.5x35, 12.5x40, 12.5x45 | 566.1, 662.6, 725.1, 789.3, 838.5±14.5N-cm; p< 0.05). Pullout force was significantly less than the control for short lengths (control vs 10x35, 11.5x35, 12.5x35 | 945.5 vs 594.5, 782.1, 757.8±32.6N-cm; p< 0.05) and higher for long screws (11.5x45 | 1193.7±32.6N-cm; p< 0.05), however, larger diameter showed diminishing returns. Insertional torque was dependent on bone-foam density (10 vs 20pcf | 232.6 vs 929.1±6.5N-cm; p< 0.05) but only dependent on tapping for the denser bone-foam (20pcf ‘tap’ vs ‘no tap’ | 1063.6 vs 794.5±9.2N-cm; p< 0.05). Pullout force was influenced by tapping (‘tap’ vs ‘no tap’) for both 10pcf (399.7 vs 514.9±20.6N; p< 0.05) and 20pcf (1799.1 vs 863.4±20.9N; p< 0.05) bone-foam. Insertional torque and pullout force were correlated across all conditions (r=0.83; p< 0.01).
Conclusion : These findings underscore the importance of proper implant size and technique to ensure SIJ stability and prevent failure.
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