Study design:

Retrospective observational study.

Purpose:

In this study we identify risk factors, including patient demographics, sagittal parameters, and clinical examinations, affecting incomplete L5/S posterior lumbar interbody fusion (PLIF).

Overview of literature:

The lumbosacral spine is considered to have an interbody fusion rate lower than that of the lumbar spine, but few studies have investigated the cause, including investigating the pelvis. We believe that pelvic morphology can affect L5/S interbody fusion of the lumbosacral spine.

Methods:

We observed 141 patients (61 men, 80 women; average age, 65.8 years) who had undergone PLIF and checked for the presence of L5/S interbody fusion. We investigated factors such as age, gender, the presence of diffuse idiopathic skeletal hyperostosis (DISH), fusion level, and grade 2 osteotomy, as well as pre-, post-, and post-preoperative L5/S disk height and angle, lumbar lordosis, Visual Analog Scale (VAS) score, Japanese Orthopaedic Association (JOA) score, and pelvic incidence (PI), comparing those with and without L5/S interbody fusion. In addition, we analyzed the patients classified into short-level (n=111) and multi-level fusion groups (n=30).

Results:

Overall, the L5/S interbody fusion rate was 70% (short-level, 78%; multi-level, 40%). Age and pre- and post-preoperative L5/S disk angle were significantly different in each fusion level group. DISH presence, grade 2 osteotomy, and postoperative VAS and JOA scores were significantly different in the short-level fusion group, whereas PI was significantly different in the multi-level fusion group.

Conclusions:

Incomplete union after L5/S PLIF correlates with advanced age, many fusion levels, and a large value of preoperative and a small value of post-preoperative L5/S disk angles.