Effects of physiological and manipulative loading conditions on the sacroiliac joint: a three-dimensional finite element analysis

Manual therapy is applied in the conservative management of sacroiliac joint (SIJ)-related symptoms, but whether its biomechanical effect is primarily related to joint mobilization or periarticular soft-tissue modulation remains unclear. This study compared the mechanical responses of the SIJ-iliolumbar ligament complex under four prescribed loading and constraint configurations: physiological compressive stress (PCS), traction manipulation stress (TMS), oblique-pulling manipulation (OPM), and counter-directional sacral thrust with iliac mobilization (CDST-IM) using three-dimensional finite element analysis. A CT-based lumbar spine-pelvis finite element model was reconstructed from computed tomography data of a healthy 31-year-old male volunteer. Four loading conditions were simulated: 300 N downward compression at L4, 300 N vertical traction at L4, 300 N traction combined with 10° posterior rotation of the right ilium, and 600 N posterior force applied to the right ilium along the SIJ direction. von Mises stress (VMS), Total Deformation (TD), and range of motion (ROM) of the SIJ, together with iliolumbar ligament VMS and TD, were evaluated. PCS and TMS showed little difference in the evaluated responses under bilateral iliac constraint. OPM generated the greatest cartilage VMS, cartilage TD, right SIJ ROM, and iliolumbar ligament TD, whereas CDST-IM produced a more moderate SIJ response with comparatively low ligament TD. On the right side, the surface showing greater cartilage TD changed from the sacral surface under PCS/TMS to the iliac surface under OPM/CDST-IM. These findings indicate limited SIJ micromotion and distinct ligament responses under the prescribed loading and constraint configurations.