Risk Factors Affecting Cage Retropulsion into the Spinal Canal Following Posterior Lumbar Interbody Fusion: Association with Diffuse Idiopathic Skeletal Hyperostosis

Study Design: This was a retrospective observational study. Purpose: We identify risk factors, including physical and surgical factors, and comorbidities affecting cage retropulsion following posterior lumbar interbody fusion (PLIF). Overview of Literature: Diffuse idiopathic skeletal hyperostosis (DISH) is considered a risk factor for reoperation after PLIF. We evaluated the effect of DISH on cage retropulsion into the spinal canal, which may require surgical revision for severe neurological disorders. Methods: A total of 400 patients (175 men, 225 women) who underwent PLIF were observed for >1 year. Factors investigated includ-ed the frequency of cage retropulsion and surgical revision. In addition, physical (age, sex, disease), surgical (fusion and PLIF levels, cage number, grade 2 osteotomy), and comorbid (DISH, existing vertebral fracture) factors were compared between patients with and without cage retropulsion. Factors related to surgical revision during the observation period were also considered. Results: Cage retropulsion occurred in 15 patients and surgical revision was performed in 11. Revisions included the replacement of pedicle screws (PSs) with larger screws in all patients and supplementary implants in 10. Among the patients with cage retropul-sion, the average PLIF level was 2.7, with DISH present in nine patients and existing vertebral fractures in six. Factors affecting cage retropulsion were diagnoses of osteoporotic vertebral fracture, multilevel fusion, single-cage insertion, grade 2 osteotomy, presence of DISH, and existing vertebral fracture. Multivariable analysis indicated that retropulsion of a fusion cage occurred significantly more frequently in patients with DISH and multilevel PLIF. Conclusions: DISH and multilevel PLIF were significant risk factors affecting cage retropulsion.