Background Context: Fusion typically consists of joint preparation, grafting, and rigid fixation. Fusion has been successfully used to treat symptomatic disruptions of the sacroiliac joint (SIJ) and degenerative sacroiliitis using purpose-specific, threaded implants. The biomechanical performance of these systems is important but has not been studied. Purpose: The objective of this study was to compare two techniques for placing primary (12.5 mm) and secondary (8.5 mm) implants across the SIJ. Study Design: This is a human cadaveric biomechanical study of SIJ fixation. Materials and Methods: Pure-moment testing was performed on 14 human SIJs in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) with motion measured across the SIJ. Specimens were tested intact, after destabilization (cutting the pubic symphysis), after decortication and implantation of a primary 12.5-mm implant at S1 plus an 8.5-mm secondary implant at either S1 (S1–S1, n=8) or S2 (S1–S2, n=8), after cyclic loading, and after removal of the secondary implant. Ranges of motion (ROMs) were calculated for each test. Bone density was assessed on computed tomography and correlated with age and ROM. This study was funded by Zyga Technology but was run at an independent biomechanics laboratory. Results: The mean±standard deviation intact ROM was 3.0±1.6° in FE, 1.5±1.0° in LB, and 2.0±1.0° in AR. Destabilization significantly increased the ROM by a mean 60%–150%. Implantation, in turn, significantly decreased ROM by 65%–71%, below the intact ROM. Cyclic loading did not impact ROM. Removing the secondary implant increased ROM by 46%–88% (non-significant). There was no difference between S1–S1 and S1–S2 constructs. Bone density was inversely correlated with age (R=0.69) and ROM (R=0.36–0.58). Conclusions: Fixation with two threaded rods significantly reduces SIJ motion even in the presence of joint preparation and after initial loading. The location of the secondary 8.5-mm implant does not affect construct performance. Low bone density significantly affects fixation and should be considered when planning fusion constructs. Findings should be interpreted in the context of ongoing clinical studies.