(I-734) Threaded Screw-based implant designs provide superior Biomechanics across the Sacroiliac Joint Compared to the Triangular implant designs: A Finite Element Study

Introduction: Sacro-iliac implants are often used for the treatment of unstable SIJ and alleviation of the pain originating from the joint area. While clinical studies have investigated the effect of such procedures, there is no data on the effect of the shape of the fixation implant on the biomechanics of the joint. This study utilizes finite element analysis to compare biomechanical responses of unilaterally instrumented SIJ using commercially available triangular implants versus circular threaded implants.

Methods: A cadaver-validated ligamentous finite element (FE) model of L1-pelvis was used. SIJ stabilization was simulated using three implants instrumented across the left SI joint, using two different implants design made of titanium: a fully threaded screw (SI-Restore screw) and a triangular implant (Si-Bone). The implant lengths were 65mm(superior), 50mm (middle), and 50mm (inferior). The Pelvic was fixed at the hip joint and the spine was subjected a 400N compressive follower load followed by a 10Nm moment applied at L1 to simulate anatomical motions of Flexion, extension, lateral bending and axial rotation. The range of motion across the SIJ along with the peak stress on the implants were reported.

Results: The motion at fused and contralateral SIJ decreased by 88% and 68% (Flex-Ext), 94% and 52% (LB) & 85% and 32% (AR) in screw vs. intact; the corresponding numbers in triangular implant were 83% and 54% (Flex-Ext), 62% and 37% (LB) and 50% and 21% (AR), respectively. The Peak stress on implants was 170 MPa (Flex-Ext) in the screw versus 130 MPa (Flex-Ext) in the triangular implant.

Conclusion : The screw shape implant was able to provide more stability to SIJ compared to the same-length Triangular shape implant under all loading conditions. The differences were more noticeable on the fused side where the threaded implant was able to provide up to 10-30% more additional stability to the joint. The peak stress created on the implants was lower in the triangular implant construct. However, they are lower than the fracture criteria in titanium implants.

How to Improve Patient Care: Threaded screw-based SIJ implant designs can yield superior biomechanical stability compared to commercially available triangular implants