Which radiographic sign would indicate an unstable spondylolisthesis?

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Unstable spondylolisthesis shows a dynamic problem at the slipped level, where abnormal motion makes the lumbosacral junction contour change on plain radiographs. Rounding or doming of the sacral base happens when forward slipping at L5-S1 alters the way the sacrum meets the vertebral column, creating a curved, domed appearance rather than a smooth, straight base. This change signals that the slip is not well restrained by the surrounding bony and soft-tissue structures, i.e., it is mobile and unstable. In contrast, an increased disc space at L4-L5 can occur with degenerative changes and isn’t a direct marker of instability; hyperlordosis is more a compensatory posture, and a posterior element fracture shows structural disruption but doesn’t by itself confirm dynamic instability on a static image.