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Study Design: Cross-sectional preoperative and intraoperative imaging study of L4-L5 lumbar degenerative spondylolisthesis (LDS)., Objective: To determine if alternate imaging modalities would identify LDS instability that did not meet the criteria for instability based on comparison of flexion and extension radiographs., Summary of Background Data: Pain may limit full flexion and extension maneuvers and thereby lead to underreporting of true dynamic translation and angulation in LDS. Alternate imaging pairs may identify instability missed by flexion-extension., Materials and Methods: Consecutive patients scheduled for surgery for single-level L4-L5 LDS had preoperative standing radiographs in the lateral, flexion, and extension positions, supine computed tomography (CT) scans, and intraoperative fluoroscopic images in the supine and prone positions after anesthesia but before incision. Instability was defined as translation ≥3.5 mm or angulation ≥11° between the following pairs of images: (1) flexion-extension; (2) CT-lateral; (3) lateral-intraoperative supine; (4) lateral-intraoperative prone; and (5) intraoperative supine-prone., Results: Of 240 patients (mean age 68 y, 54% women) 15 (6%) met the criteria for instability by flexion-extension, and 225 were classified as stable. Of these 225, another 84 patients (35% of total enrollment) were reclassified as unstable by comparison of CT-lateral images (21 patients) and by lateral-intraoperative images (63 patients). Nine of the 15 patients diagnosed with instability by flexion-extension had fusion (60%), and 68 of the 84 patients reclassified as unstable by other imaging pairs had fusion (81%) ( P =0.07). The 84 reclassified patients were more likely to undergo fusion compared with the 141 patients who persistently remained classified as stable (odds ratio=2.6, 95% CI: 1.4-4.9, P =0.004)., Conclusions: Our study provides evidence that flexion and extension radiographs underreport the dynamic extent of LDS and therefore should not be solely relied upon to ascertain instability. These findings have implications for how instability should be established and the extent of surgery that is indicated., Competing Interests: The authors report no conflicts of interest., (Copyright © 2022 Wolters Kluwer Health, Inc. All rights reserved.)