Bilateral L5 pedicle fracture with L5–S1 spondylolisthesis after single-level L4–5 posterior lumbar interbody fusion: illustrative case

BACKGROUND: Single-level posterior lumbar interbody fusion (PLIF) or transforaminal lumbar interbody fusion (TLIF) is a commonly performed surgical procedure for L4–5 isthmic spondylolisthesis. Postoperative L5 pedicle fracture with rapidly progressive spondylolisthesis at L5–S1 segment after L4–5 PLIF/TLIF is quite rare, and the etiology remains unclear. This report describes this rare complication and proposes a possible etiology focusing on the lumbosacral sagittal imbalance characterized by an anteriorly shifted lumbar loading axis. OBSERVATIONS: The authors report a case complicated by L5 bilateral pedicle fractures and rapidly progressive spondylolisthesis at the L5–S1 segment very early after a single-level PLIF for L4–5 isthmic spondylolisthesis. Meyerding grade III anterolisthesis was observed at L5–S1 segment by 3 months after the initial surgery. Additional surgery was performed, and the fixation was extended to L4–ilium. Fracture healing was observed at 6 months postoperatively. LESSONS: This complication may have been caused by abnormal local shear forces on the posterior neural arch of L5 vertebra and L5–S1 intervertebral disc, which were triggered by the fusion surgery for L4 shear-type spondylolisthesis. L4 sagittal vertical axis is considered a reasonable parameter representing lumbosacral sagittal imbalance with an anteriorly shifted loading axis and may be a candidate for the predictive parameters of this rare complication.