What happens at the L5/S1 facet joint when implants are placed across the sacroiliac joint?

Background: Injuries to the posterior pelvic ring are often stabilized with fixation across the sacroiliac joint (SIJ). However, the compensatory changes at the neighboring L5/S1 facet joint are unknown. The objective of this study was to determine the compensatory change in pelvic kinematics and contact forces at the L5/S1 facet joint after fixation across the sacroiliac joint (SIJ) using a cadaveric model. Methods: Five fresh-frozen cadaveric pelvis specimens were dissected to remove non-structural soft tissue. Retroreflective markers were fixed to the L5 body, S1 body and bilateral anterior superior iliac spines to represent the motion of L5, S1 and the ileum, respectively. Pressure sensors were inserted in both L5/S1 facet joints. Testing was performed using a robotic system that applied load to mimic ambulation. Testing was performed prior to SIJ fixation, after unilateral SIJ fixation and bilateral fixation. Results: Contact force at the L5/S1 facet joint significantly increased by 55% from 48.4 N to 75.2 N following unilateral fixation ( p = 0.0161) and increased by 100% to 96.9 N after bilateral fixation ( p = 0.0038). Unilateral SIJ fixation increased flexion of the ilium relative to L5 from 1.2 degrees to 2.0 degrees ( p = 0.01) and increased axial rotation of L5 relative to S1 from 0.7 degrees to 1.6 degrees ( p = 0.001). Bilateral fixation increased flexion of the ilium relative to L5 to 2.0 degrees from 1.2 degrees prior to fixation ( p = 0.001), increased axial rotation of L5 relative to S1 to 1.2 degrees from 0.7 degrees prior to fixation ( p = 0.002) and increased flexion of L5 relative to S1 to 2.4 degrees from 1.5 degrees prior to fixation ( p = 0.04). Conclusion: The L5/S1 facet joint experiences compensatory increased motion under increased contact force after unilateral and bilateral SIJ fixation, possibly predisposing it to adjacent segment arthritis. Level of Evidence: V, cadaveric study