Effects of Lumbosacral Arthrodesis on the Biomechanics of the Sacroiliac Joint

BACKGROUND: It is unclear whether the sacroiliac joint is vulnerable to adjacent segment disease. Clinical studies have suggested that many patients who have undergone lumbar arthrodesis will develop adjacent segment disease, which may contribute to sacroiliac joint degeneration. The purpose of the present study was to examine whether arthrodesis in the lumbar spine results in altered biomechanics at the sacroiliac joint that could contribute to adjacent segment disease within the joint. METHODS: With use of human cadavers in a biomechanical laboratory study, the effects of lower-lumbar arthrodesis and sacroiliac screws on the biomechanics of the sacroiliac joint were assessed. Human cadaveric pelves with lumbar spines were biomechanically tested in flexion-extension, rotation about the vertical axis, and compression along the vertical axis with single and double-leg support. Four conditions were compared: (1) intact, (2) L4-L5 arthrodesis, (3) L4-S1 arthrodesis, and (4) left sacroiliac screw. Construct vertical and horizontal motions at the anterior and posterior surfaces of the sacroiliac joint were measured. RESULTS: Significant measurable increases in motion of the sacroiliac joint related to arthrodesis of the lumbar spine occurred with flexion-extension loading (p < 0.05). No significant changes were observed for rotation about the vertical axis or compression along the vertical axis with single and double-leg support. CONCLUSIONS: After 360°, 1 or 2-level lumbosacral spine arthrodesis, the sacroiliac joint showed a significant increase in rotational motion with flexion-extension loading. Increases in horizontal translation with axial rotation loading and vertical translation with axial compression loading were not significant. CLINICAL RELEVANCE: The risk of significant alteration of normal sacroiliac kinematics should be considered in all patients undergoing 360° lumbosacral arthrodesis.