This is a review of the retrospective, single-center cohort study published in the North American Spine Society Journal by Waters et al in which the authors investigated the incidence, timing, and factors associated with contralateral sacroiliac joint (SIJ) fusion (SIJF) following unilateral SIJF. This study researched an interesting and clinically relevant topic that builds upon prior literature related to SIJFs.

The study identified 247 patients who underwent SIJF between 2011 and 2024. Patients were categorized into three groups: (1) SIJF patients who underwent only unilateral SIJF (n=195), (2) SIJF patients who presented with unilateral SIJ pain, later developed contralateral symptoms, and went on to have contralateral fusion (n=16), and (3) SIJF patients who presented with bilateral SIJ pain and went on to have contralateral fusion (n=36). Mean follow-up for cohort 1 was 59.0 +/- 40.1 months, whereas cohort 2 and 3 had a minimum follow-up of 18 months following the index SIJF.

Over the study period, 52 patients (21%) ended up undergoing SIJF contralateral to their index SIJF (groups 2 and 3 combined). The overall mean time (cohort 2 and 3) between the first and second SIJF was 9.9 ± 11.8 months, with >90% of contralateral fusions occurring within 18 months of the index surgery. Patients who presented with bilateral pain at baseline (cohort 3) progressed more quickly to contralateral surgery than those who initially presented with unilateral pain (cohort 2) (6.2 vs12.7 months).

The study went on to assess factors predictive of going on to have SIJF on the contralateral side. A prior single-level L5–S1 fusion emerged as a key risk factor, conferring more than twice the odds of subsequent contralateral SIJF (OR 2.55). However, multilevel fusion involving L5–S1 and superior levels was not associated with increased odds of bilateral SIJF. Interestingly, prior lumbar fusion excluding L5–S1 was protective against subsequent contralateral SIJF (OR 0.20).

Contrary to the present findings, previous studies (primarily biomechanical or small single-center cohorts) have suggested that longer fusions including L5–S1 increases SIJ stress, compounded with additional fused levels. The authors of the currently reviewed study propose that this discrepancy may reflect differences between physiologic in vivo range of motion and cadaveric biomechanical models in patients with multilevel fusion. They further suggest that multilevel fusion patients, who often have greater disability and reduced mobility, may place less mechanical stress on the SIJ as a whole. Another proposed explanation is that while multilevel fusions increase adjacent segment motion, they may also promote better end-fusion alignment, mitigating the propagation of adjacent segment disease despite greater SIJ mobility.

The currently reviewed study offers valuable insight into the natural history and progression of SIJFs progressing to have contralateral SIJF and identified predictors of contralateral intervention. The study includes a well-defined cohort, consistent diagnostic criteria, and meaningful follow-up. Limitations include the single-institution design, potential surgeon bias in operative decision-making, and lack of adjustment for nonclinical factors such as socioeconomic status, baseline activity level, or social support.

In summary, Waters et al provide an important contribution to the limited clinical literature on contralateral SIJF following index SIJF. Their findings suggest that over one in five patients may require subsequent contralateral fusion, most within 18 months, and that bilateral pain or prior L5–S1 fusion are key risk factors. These results highlight the importance of early risk stratification to guide preoperative counseling and postoperative monitoring, as well as the need for larger, multi-institutional studies to validate and expand upon these findings.