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Study Design: Cadaveric biomechanics study. Purpose: This study investigated the effects of unilateral sacroiliac joint (SIJ) fixation for fusion with/without L5-S1 fixation on contralateral SIJ range of motion (ROM). Overview of Literature: SIJ fusion raises concerns that unilateral SIJ stabilization for fusion may increase contralateral SIJ mobility, leading to accelerated SIJ degeneration. Also, prior lumbosacral fixation may lead to accelerated SIJ degeneration, due to adjacent level effects. SIJ fixation biomechanics have been evaluated, showing a reduced-ROM, but SIJ fixation effects on contralateral nonfixated SIJ remain unknown. Methods: Seven human lumbopelvic spines were used, each affixed to six-degrees-of-freedom testing apparatus; 8.5-Nm pure unconstrained bending moments applied in flexion-extension, lateral bending, and axial rotation. The ROM of left and right SIJ was measured using a motion analysis system. Each specimen tested as (1) intact, (2) injury (left), (3) L5-S1 fixation, (4) unilateral stabilization (left), (5) unilateral stabilization+L5-S1 fixation, (6) bilateral stabilization, and (7) bilateral stabilization+L5-S1 fixation. Both left-sided iliosacral and posterior ligaments were cut for injury condition to model SIJ instability before surgery. Results: There were no statistical differences between fixated and contralateral nonfixated SIJ ROM following unilateral stabilization with/without L5-S1 fixation for all loading directions (p >0.930). Injured condition and L5-S1 fixation provided the largest motion increases across both joints; no significant differences were recorded between SIJs in any loading direction (p >0.850). Unilateral and bilateral stabilization with/without L5-S1 fixation reduced ROM compared with the injured condition for both SIJs, with bilateral stabilization providing maximum stability. Conclusions: In the cadaveric model, unilateral SIJ stabilization with/without lumbosacral fixation did not lead to significant contralateral SIJ hypermobility; long-term changes and in vivo response may differ. [ABSTRACT FROM AUTHOR]