Shih et al;  Biomechanical Evaluation of Sacroiliac Joint Fixation with Decortication. The Spine Journal.
Fusion typically consists of joint preparation, grafting and rigid fixation. Fusion has been successfully used to treat symptomatic disruptions of the SIJ and degenerative sacroiliitis using purpose-specific, threaded implants. The biomechanical performance of these systems is important but has not been studied.
To compare two techniques for placing primary (12.5mm) and secondary (8.5mm) implants across the SIJ.
A human cadaveric biomechanical study of sacroiliac joint (SIJ) fixation.
Pure moment testing was performed on fourteen human sacroiliac joints 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 12.5mm implant at S1 plus an 8.5mm secondary implant at either S1 (S1/S1, n=8) or S2 (S1/S2, n=8); after Cyclic Loading; after removal of the secondary implant. Ranges of motion (ROM) were calculated for each test. Bone density was assessed on CT and correlated with age and ROM. This study was funded by Zyga Technology but was run at an independent biomechanics laboratory.
The mean ± SD intact ROM was 3.0±1.6 degrees in FE, 1.5±1.0 degrees 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% (nonsignificant). There was no difference between S1/S1 and S1/S2 constructs. Bone density was inversely correlated to age (R=.69) and ROM (R=.36-.58).
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, 8.5mm 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.