BackgroundTransforaminal lumbar interbody fusion (TLIF) is a common surgical procedure for lumbar spondylolisthesis and intervertebral foraminal stenosis. Sacroiliac joint ankylosis is also known to occur in patients without axial spondyloarthritis. When sacroiliac joint bony ankylosis occurs and sacroiliac joint mobility is lost, stresses from the lower extremities to the lumbar spine are no longer buffered and are expected to be concentrated between the fifth lumbar (L5) and the first sacral (S1) vertebrae. We hypothesized that sacroiliac joint bony ankylosis could adversely affect L5/S1 intervertebral fusion and investigated the postoperative intervertebral fusion rate in single intervertebral TLIF on L5/S1 among patients with bony ankylosis of the sacroiliac joint.MethodsSeventy-two patients who had undergone TLIF in the L5/S1 single intervertebral segment since 2014 and had a follow-up of at least one year after surgery were included in the study. Seventy-two patients were divided into the following two groups for comparison: group A consisted of 17 patients with bony ankylosis of the sacroiliac joint on either side on preoperative CT, and group N consisted of 55 patients without ankylosis. We investigated the intervertebral segment fusion rate one year postoperatively. Fisher's exact tests were used for statistical analysis, with a significance level ofP< 0.05.ResultsTwelve patients (71%) in group A and 50 patients (91%) in group N had a fusion of the L5/S1 intervertebral segment one year after TLIF surgery, with a significantly lower rate in group A (P= 0.049).ConclusionsWe conclude that the presence of preoperative sacroiliac joint bony ankylosis is a risk factor for postoperative intervertebral fusion failure after single-segment TLIF at L5/S1.