Introduction: Sacroiliac joint pain (SIJP) after lumbar fusion surgery has recently gained attention as a source of low back pain after lumbar fusion. There are two risk factors for postoperative SIJP, i.e., fusion involving the sacrum and multiple-segment fusion. In this study, we examined whether SIJP could occur more frequently in patients with two risk factors (multiple-segment fusion to sacrum). Further, we examined SIJP after multiple-segment (≥3) lumbar fusion, focusing on the difference between floating fusion (non-fused sacrum) and fixed fusion (fused sacrum).

Methods: Ninety-one patients who underwent multiple-segment lumbar fusion were included. Patients without preoperative clinical SIJP were considered. Of these, 17 developed new-onset SIJP. We investigated postoperative SIJP development, duration from surgery to SIJP onset, and postoperative treatment outcomes of SIJP patients using Japanese Orthopaedic Association (JOA) scores. We compared the findings between floating fusion group and fixed fusion group.

Results: The incidence of SIJP was significantly higher with fixed fusion (32.1%) than with floating fusion (12.7%). The mean time of onset of sacroiliac joint pain was at 8.63 (2-13) months after surgery in the floating fusion group and 3.78 (1-10) months after surgery in the fixed fusion group, indicating that incidence occurred significantly earlier in the fixed fusion group. Our treatment outcome indicated that the mean JOA score significantly improved in the floating fusion group from 5.13 at the time of onset to 9.50 at the time of final follow-up; however, in the fixed fusion group, it improved from 5.78 at the time of onset to 7.33 at the time of final follow-up, indicating no significant improvement.

Conclusions: In multiple-segment lumbar fusion, fixed fusion (fused sacrum) has a very high risk of SIJP. In addition, the onset of SIJP in such cases may occur earlier. This aspect deserves consideration, given the difficulty of pain treatment.