Lumbar instability on flexion-extension radiographs can be indicated by which of the following?

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Dynamic stability of the lumbar spine is judged on flexion-extension films by looking for excessive motion between adjacent vertebrae. There are two ways to quantify that motion: translation, which is how far one vertebra slides forward relative to the one below, and angular motion, which is the change in angle between the two vertebrae from flexion to extension. If translation exceeds about 4 mm, or angular motion exceeds about 11 degrees, that level of motion is considered unstable. So instability can be indicated by either measurement when it crosses its threshold, which is why the best answer reflects that either translation beyond 4 mm or angular motion beyond 11 degrees signals instability. Looking at only one type of motion could miss instability detected by the other, and if neither threshold is reached, instability isn’t indicated on these dynamic radiographs.