A Cadaver-Based Biomechanical Evaluation of a Novel Posterior Approach to Sacroiliac Joint Fusion: Analysis of the Fixation and Center of the Instantaneous Axis of Rotation.

Purpose: The purpose of this study was to assess the stabilizing effect of a posterior joint fixation technique using a novel cortical allograft implant in unilateral and bilateral fixation constructs. We hypothesize that fixation would reduce the joint's range of motion during flexion-extension, axial rotation, and lateral bending loads. We also hypothesize that fixation would shift the center of the instantaneous axis of rotation during the predominant flexion-extension motions towards the implant's location, and that this shift would be correlated with the reduction in flexion-extension range of motion.

Materials and methods: Six cadaveric sacroiliac joint specimens were tested under intact, unilateral fixation, and bilateral fixation conditions. The total range of motion (ROM) of the sacroiliac joint in flexion-extension, lateral bending, and axial rotation were evaluated by an optical tracking system, in a multidirectional flexibility pure moment model, between ± 7.5 Nm applied moment loads. The centers of the instantaneous axis of rotation (cIAR) of the sacroiliac joint were evaluated during flexion-extension loading. A correlation analysis was performed between the ROM reduction in flexion-extension upon implantation and shift of the cIAR to the graft implantation site.

Results: Unilateral and bilateral fixations generated sacroiliac joint ROM reductions in flexion-extension, lateral bending, and axial rotation motions. Fixation shifted the cIAR to the graft implantation site. Reduction in the total range of motion had a moderate correlation with the shift of the cIAR.

Conclusion: Our novel posterior approach presents a multifaceted mechanism for stabilizing the joint: first, by the reduction of the total range of motion in all planes of motion; second, by shifting the centers of the instantaneous axis of rotation towards the implant's location in the predominant plane of motion, ensuring little to no motion at the implantation site, thus promoting fusion in this region.