Radiological evidence of spontaneous spinal arthrodesis in patients with lower lumbar spondylolisthesis

Study Design: Retrospective review of a consecutive series of patients with radiological evidence of spondylolisthesis. Objective: To establish the incidence and characteristics of spontaneous spinal arthrodesis in the setting of lower lumbar spondylolisthesis. Summary of Background Data: Spontaneous spinal arthrodesis of lumbar spondylolisthesis is a finding that is not discussed in the literature outside of isolated case reports. Identifying spontaneous spinal arthrodesis may impact the surgical plan, as patients with existing fusion may not require instrumentation and might require more work to reduce their spondylolisthesis. Methods: We reviewed a consecutive series of 1490 lumbar spine computed tomography scans from the year 2010 for radiological evidence of spondylolisthesis at either L4-L5 or L5-S1. Patients were excluded if they had undergone previous lumbar surgery. Scans were assessed for the presence of spontaneous fusion based on the following criteria: (1) a solid bridging anterior or posterior vertebral body osteophyte, (2) contiguous bone formation from one vertebral body to another, or (3) contiguous bone across the facet joints bilaterally. Patients were characterized by demographic variables, radiological characteristics including type of spondylolisthesis, and presenting symptomology. Differences between patients in the fused and nonfused cohorts were compared with univariate analysis. Results: A total of 86 separate instances of spondylolisthesis were identified, of which 18 (20.9%) had radiological evidence of spontaneous fusion. The most common site of fusion was in the bilateral facets, followed by directly in the intervertebral disc space, and bridging osteophytes adjoining the vertebral bodies. There were significant differences between patients in the fused and nonfused cohorts in terms of average age (fused: 74.3 ± 10.7 yr vs. nonfused: 63.3 ± 18.6 yr, P = 0.019), sex (fused: 88.9% female vs. nonfused: 57.4% female, P = 0.013), and rate of pars defects (fused: 11.1% vs. nonfused: 35.3%, P = 0.047). Conclusion: In this study, 20.9% of patients with lumbar spondylolisthesis have radiological signs of spontaneous fusion. Further work is needed to better characterize the natural history and clinical-radiological correlation of spontaneous fusion in spondylolisthesis.