Inability to prove instability. A critical appraisal of clinical-radiological flexion-extension studies in lumbar disc degeneration.

In two groups of patients with low back pain radiological flexion-extension studies were made (one in the standing position, the other in the lateral decubitus position), and the degree of mobility and location of instantaneous centres of motion determined between L2 and the sacrum. The study failed to demonstrate distinctly abnormal patterns of motion ('instability'). The main reason is that such problems, as shown in the biomechanical laboratory, are not reproduced in the clinical setting. Smaller abnormalities will remain undetected because of measurement errors of the method, making differentiation between normal and abnormal motion virtually impossible.