Does spinopelvic imbalance after minimally invasive lower lumbar fusion predispose patients to subsequent minimally invasive sacroiliac joint fusions? a case-control study.

Purpose: Sacroiliac joint (SIJ) dysfunction is a frequently overlooked source of persistent low back pain after lower lumbar surgery. The SIJ is an adjacent joint to the lower lumbar spine and can thus be subject to adjacent segment disease (ASD) after lower lumbar fusion. Spinopelvic imbalance can potentially predispose a patient to ASD in the SIJ, causing SIJ dysfunction. However, the impact of spinopelvic imbalance following minimally invasive (MIS) lower lumbar fusion on SIJ dysfunction and, thus, subsequent fusion remains inconclusive. We sought to determine whether spinopelvic imbalance after lower lumbar fusion predisposes patients to subsequent MIS SIJ fusion (SIJF).

Methods: We retrospectively reviewed consecutive elective lower lumbar fusion patients (ending levels L5/S1) to select our cases and controls in a 1:1 match. We included patients who underwent MIS lower lumbar fusion from 10/2005 to 05/2021 at a single institution with an ending level including L5 or S1, a starting level at or below T12, and had at least two years of follow-up. We excluded patients with inadequate or absent postoperative X-rays, those who underwent lumbar fusion for trauma or infection, pelvic fixation, revision lumbar surgery, and prior SIJF. We performed a pilot study to determine the sample size as 190 per group. Confounders, including the number of pregnancies, were collected. Consistent with prior literature, we defined spinopelvic imbalance as a PI-LL mismatch of <-10° or > 10° [1-5]. We compared the groups using univariate analysis. The odds of developing SIJF related to PI-LL mismatch were calculated using Chi-Square. We performed multivariable analysis modeling on SIJF to adjust for covariates.

Results: We included 488 patients (203 SIJF vs. 285 control). Between groups, the SIJF patients were significantly younger (52.55 ± 12.81 vs. 55.84 ± 14.71; P = 0.005), had a higher proportion of females (67.5% vs. 55.1%; P = 0.006), and increased levels fused (1.78 ± 0.97 vs. 1.38 ± 0.64; P < 0.001). Interestingly, number of pregnancies was not significantly different (P = 0.791). PI-LL mismatch demonstrated a 3.54 increased odds of requiring subsequent SIJF (p < 0.001). Multiple logistic regression adjusting for age at lumbar fusion, female sex, and number of levels demonstrated that PI-LL mismatch (OR 1.10; 95% CI 1.06-1.15; p < 0.001) and operative levels (OR 1.84; 95% CI 1.42-2.39; p < 0.001) were independently associated with significantly increased odds of requiring SIJF, while age (OR 0.98; 95% CI 0.96-0.99; p < 0.001) and female sex (OR 0.54; 95% CI 0.36-0.81; p = 0.003) were associated with significantly decreased odds of requiring SIJF.

Conclusion: PI-LL mismatch following MIS lower lumbar fusion was independently associated with significant odds of requiring subsequent SIJF. Optimizing spinopelvic imbalance can decrease the odds of requiring subsequent SIJF after MIS lower lumbar surgery. Given the biases associated with the retrospective and observational nature of our study design, further prospective studies are needed.