What angular motion on flexion-extension views indicates lumbar instability?

• A. <5 degrees
• B. >11 degrees
• C. >15 degrees
• D. >20 degrees

Answer: (B)

Excessive angular motion between adjacent lumbar vertebrae on flexion-extension radiographs signals instability. To assess it, measure the angle between the endplates of the two involved vertebrae on the neutral view, then measure the same angle on full flexion and full extension. The difference between the flexion and extension angles is the angular motion. When this change exceeds about 11 degrees, it indicates segmental instability because it reflects laxity beyond normal physiological motion. This threshold comes from established criteria for lumbar instability and helps distinguish truly unstable motion from normal variability. Keep in mind that another indicator is translation between positions (often looked at as a separate measure, such as about 4 mm of displacement), and measurements can vary with level and technique.