Acquired spondylolisthesis: Which criterion on flexion-extension radiographs indicates its presence?

• A. Disk height change of more than 2 mm
• B. Static alignment
• C. Disc angle change greater than 10 degrees or translation greater than 3 mm
• D. No detectable change

Answer: (C)

The key idea is detecting dynamic instability on flexion-extension films. In acquired spondylolisthesis, the vertebra slips more with movement, so these images are used to see how much motion occurs between positions. If the segment shows angular change of more than about 10 degrees or forward translation of more than 3 mm from flexion to extension, that level of motion demonstrates instability and confirms the presence of spondylolisthesis. Disk height changes or static alignment don’t capture this dynamic slip—disk height change isn’t the criterion, static alignment just shows position in a single view, and no detectable change would imply no dynamic instability. The combination of a translational increase or an angular change beyond these thresholds on the dynamic views is what indicates acquired spondylolisthesis.