A comparative biomechanical study of a non-threaded triangular titanium implant versus a fully threaded screw: assessing pullout strength of two sacroiliac joint fixation implant designs.

Background: Considering that implant backout is a recognized mode of failure, evaluating the pullout strength is critical for assessing anchoring efficacy. The Sacrix^® fully threaded screw (TS) was designed specifically for Less Exposure Spine Surgery (LESS) to reduce incision size, surgical time, and blood loss, using two implants for sacroiliac joint (SIJ) fixation. This study compares the Sacrix^® design with the SI-Bone iFuse non-threaded triangular titanium implant (TTI) design, which is widely regarded as the industry standard, and represents the first comparative biomechanical pullout strength study of these implant designs currently used in SIJ fusions.

Methods: We conducted mechanical static axial pullout tests on three 7.0 mm × 45 mm iFuse non-threaded TTIs and six 8.0 mm × 40 mm Sacrix^® fully TS embedded in polyurethane foam blocks. An INSTRON 8874 Bi-Axial Tabletop Servohydraulic Dynamic Testing System was used to perform the tests by applying a 2.5 kN axial load.

Results: The effective surface areas of the iFuse non-threaded TTI and Sacrix^® fully threaded TS were comparable, measuring 294.15 and 289.81 mm², respectively. The TS exhibited a significantly higher mean static axial pullout strength of 814.90 N [standard deviation (SD), ±99.428 N] compared to the TTI 200.14 N (SD, ±14.428 N). Statistical analyses, including Welch's t-test and Mann-Whitney U test, revealed significant differences in pullout strength between the two implants (P<0.05). Variance analysis confirmed the differences in pullout strength variances between the implants (P=0.040), suggesting that the variability in pullout strength was distinct for each implant.

Conclusions: The Sacrix^® fully threaded TS demonstrated a threefold increase in pullout strength compared with the SI-Bone iFuse non-threaded TTI, suggesting that future SIJ fusion designs should favor threaded over non-threaded implants for improved anchoring capability.