599 - Biomechanical Assessment of Stresses, Weight and Movement Within the Sacroiliac Joint to Better Understand Fixation Device Placement; A Finite Element Analysis (FEA) Study

Purpose: The sacroiliac (SI) joints, located between the iliac bones and sacrum, provide stability and absorb impact during movement. Over time, these joints may become painful due to arthritis, ligament stiffness, or injuries from falls, accidents, or surgery. SI joint fusion is often performed to alleviate pain. This FEA study examines the biomechanical performance of the sacroiliac joint under various natural loading conditions, aiming to determine the location of the movement and stressors within the joint. This information expects to identify the best location and trajectory for surgical intervention of the sacroiliac joint for improved technique and placement ultimately leading to better patient outcomes.

Materials and Methods: Finite element analysis (FEA) simulated the biomechanical performance of pelvic models under clinical loading conditions. This analysis was conducted using modelling and simulation tools developed within the ANSYS environment.

Load scenarios included flexion, extension and rotation set to mimic a single leg stance, ramp movement to 7.5Nm. Weight mimicked a double leg stance, ramp force 880N. All testing included both ligament strain and bone stress. Boundary conditions were the nodes on the left of the pelvis and the right ends of the pelvis were completely fixed.The weight condition describes the vertical downward force of 880N that was applied to the sacral plate. This condition serves as a baseline before movement (e.g., flexion-extension lateral bending, axial rotation) is applied.

Results: Greatest load-bearing region lies inferior near S3 where you can see direct migration of stress at a peak value of 20 MPa from the sacral plate where the loading condition is being applied.



Thus, the best location and posterior trajectory for surgical intervention of the SIJ with the intention of bone fusion is a posterior inline trajectory, positioned inferiorly near pelvic anatomical landmark S3.
Furthermore, this location in the SIJ space is the most direct and precise, shown in the image d of the axial slices figure.

Conclusion: These FEA results clearly demonstrated and illustrated the SIJ hot-spots. These hot-spots represent the optimum area to place SIJ fixation devices. Using the identified posterior trajectory to target this area, practitioners may more accurately place these fusion devices within the SIJ to improve patient outcomes.