Factors of Adjacent Segment Disease Onset After Microsurgical Decompression for Lumbar Spinal Canal Stenosis.

OBJECTIVE: Few studies have examined the underlying cause of adjacent segment disease (ASD) after decompression surgery for lumbar spinal stenosis. The goal of this study is to investigate factors related to the onset of ASD after decompression surgery based on the imaging results. METHODS: We examined 95 patients who underwent single-level decompression for lumbar spinal stenosis (L3/4, L4/5) and follow-up for 5 or more years. Radiographic images were performed preoperatively and at each year of follow-up. We then examined image parameters by focusing on the level operated on and adjacent segments in relation to the postoperative onset of symptomatic ASD. RESULTS: During the mean observation period of 7.5 years, 39 of 95 patients developed symptomatic ASD. Patients with a high preoperative sagittal rotation angle in adjacent segments possibly developed postoperative ASD (P = 0.0006). Furthermore, postoperative ASD tended to be unlikely in patients who exhibited postoperative slip progression at the operated level (P = 0.025). Based on receiver operating characteristic analysis, ASD developed with a probability of 91.3% in patients with a preoperative sagittal rotation angle of ≥7.5° in adjacent segments when there was no postoperative slip progression at the operated level. However, ASD developed in only 16.7% of patients with a preoperative adjacent segment sagittal rotation angle of 7.5° or less when there was postoperative slip progression at the operated level. CONCLUSIONS: Biomechanical changes at the operated level and adjacent segments contribute to the onset of ASD after lumbar decompression. Preoperative high sagittal rotation angle of adjacent segments and negative postoperative slip progression at the operated level are risk factors of ASD.