Incidence and Characteristics of Clinical L5–S1 Adjacent Segment Degeneration after L5 Floating Lumbar Fusion: A Multicenter Study

STUDY DESIGN: Retrospective study. PURPOSE: This study aimed to evaluate the incidence, characteristics, and risk factors for clinical L5–S1 adjacent segment degeneration (ASD) after L5 floating lumbar fusion. OVERVIEW OF LITERATURE: ASD is known to occur after lumbar spine fusion at a certain frequency. Several studies on radiological L5–S1 ASD have been reported. However, there are only a few studies on L5–S1 ASD with clinical symptoms, including back pain and/or radiculopathy. METHODS: In total, 306 patients who received L5 floating lumbar fusion were included in this study. Clinical L5–S1 ASD was defined as newly developed radiculopathy in relation to the L5–S1 segment. Patients’ medical records and imaging data were retrospectively analyzed. The risk factors for clinical ASD were assessed by an inverse probability of treatment weighting-adjusted logistic regression analysis. RESULTS: Clinical L5–S1 ASD occurred in 17 patients (5.6%). The mean onset time of L5–S1 ASD was 12.9±7.5 months after the primary surgery. Among these patients, 10 (58.8%) presented with clinical L5–S1 ASD within 12 months. Reoperation was performed in three patients (1.0%). The severity of L5–S1 disk degeneration did not affect the occurrence of L5–S1 ASD. Logistic regression analysis showed that the number of fusion levels was a significant risk factor for clinical L5–S1 ASD. CONCLUSIONS: The incidence and characteristics of clinical L5–S1 ASD after L5 floating lumbar fusion were retrospectively investigated. This study established that the number of fusion levels was a significant candidate factor for clinical L5–S1 ASD. Careful clinical follow-up is deemed necessary after L5 floating lumbar fusion surgery, especially for patients who received multiple-level fusions.