The prevalence of redundant nerve roots in standing positional MRI decreases by half in supine and almost to zero in flexed seated position: a retrospective cross-sectional cohort study

PURPOSE: This retrospective cross-sectional cohort study investigated the influence of posture on lordosis (LL), length of the spinal canal (LSC), anteroposterior diameter (APD L1-L5), dural cross-sectional area (DCSA) of the lumbar spinal canal, and the prevalence of redundant nerve roots (RNR) using positional magnetic resonance imaging (MRI) (0.6 T). METHODS: Sixty-eight patients with single-level degenerative central lumbar spinal stenosis (cLSS) presenting with RNR in the standing position (STA) were also investigated in supine (SUP) or neutral seated (SIT) and flexed seated (FLEX) positions. Additionally, 45 patients complaining of back pain and without MRI evidence of LSS were evaluated. Statistical significance was set at p < 0.05. RESULTS: Controls (A) and patients with cLSS (B) were comparable in terms of mean age (p = 0.88) and sex (p = 0.22). The progressive transition from STA to FLEX led to a comparable decrease in LL (p = 0.97), an increase in LSC (p = 0.80), and an increase in APD L1-L5 (p = 0.78). The APD of the stenotic level increased disproportionally between the different postures, up to 67% in FLEX compared to 29% in adjacent non-stenotic levels (p < 0.001). Therefore, the prevalence of RNR decreased to 49, 26, and 4% in SUP, SIT, and FLEX, respectively. CONCLUSION: The prevalence of RNR in standing position was underestimated by half in supine position. Body postures modified LL, LSC, and APD similarly in patients and controls. Stenotic levels compensated for insufficient intraspinal volume with a disproportionate enlargement when switching from the STA to FLEX.