Increasing pelvic tilt after lumbosacral fusion predicts higher frequency of posttreatment sacroiliac joint fusion.

Objective: Sacroiliac joint dysfunction is increasingly recognized as a source of low back pain. Sacroiliac joint dysfunction after lumbosacral fusion is a potential cause of persistent disability and may be difficult to predict. The authors sought to investigate the understudied relationship between spinopelvic parameters and frequency of sacroiliac joint fusion after lumbosacral fusion.

Methods: The authors performed a retrospective analysis of patients who underwent lumbar fusion surgery between 2016 and 2022. Patients with < 2 years of follow-up data, lower instrumented vertebra (LIV) above S1, prior spinopelvic fixation, inadequate postoperative standing radiographs, S2-alar-iliac pelvic fixation technique, prior or concomitant sacroiliac joint dysfunction or fusion surgery at the time of the index surgery were excluded. Patients with fusion including S1 with or without iliac fixation were included. Demographic and surgical data were collected. Spinopelvic parameters were measured on standing radiographs. Summary statistics and univariate and multivariable analyses were performed.

Results: Ninety patients met the inclusion and exclusion criteria. The patient mean age was 61.33 ± 10.72 years; 61.1% of patients were female. The mean BMI was 30.80 ± 6.80. There were 49 patients (54.4%) with fusion to S1 and 41 patients (45.6%) with fusion to S1 plus iliac fixation. Among the entire cohort, 30 (33.3%) had a positive response to postoperative sacroiliac joint injections and 23 (25.6%) patients underwent sacroiliac joint fusion after a diagnosis of sacroiliac joint dysfunction was confirmed by injection. A head-to-head comparison of patients who did and did not have sacroiliac joint fusion demonstrated no significant differences in age, sex, BMI, smoking status, bone radiodensity (mean Hounsfield units at S1), number of spinal fusion levels, interbody technique at L5-S1, change between preoperative and postoperative spinopelvic parameters, and postoperative pelvic incidence (PI), lumbar lordosis (LL), and sacral slope. There was no head-to-head difference in frequency of sacroiliac joint fusion between patients with and those without iliac fixation. Patients who had sacroiliac joint fusion had a greater postoperative pelvic tilt (PT) (p = 0.002) and higher postoperative PI-LL mismatch (p = 0.004). On multivariable analysis after forward selection, higher postoperative PT (OR 1.102, 95% CI 1.033-1.186, p = 0.003) was an independent and significant predictor of having sacroiliac joint fusion after index lumbosacral fusion surgery.

Conclusions: Lumbosacral fusion patients with higher postoperative PT were more likely to have sacroiliac joint fusion after lumbosacral fusion surgery for treatment of sacroiliac joint dysfunction.