The Utility of Diagnosing Lumbar Rotational Instability from Twist CT Scans

The twist CT has been advocated as a useful diagnostic tool for determining lumbar segment instability. According to this test, facet joints showing wide separation on lumbar spine rotation are believed to be pathological. However, no reports have presented the reliability of measuring facet joint separation from clinical twist CT films, nor tested for an association between tendency to separate and anatomical joint orientation.Forty-five patients, referred for mechanical low back pain, were scanned first in neutral, then right and left trunk twisted positions. Images were video-digitized onto computer for quantitative measurement of facet joint morphology. The induced facet joint separation was contrasted with the neutral scan joint approximation and any difference was expressed in millimeters.The coefficient of variability (c.v.) of defining right and left transverse plane facet joint orien-tation, and of joint separation, from repeated assessment in ten images was acceptable (c.v. = 2% and 3.5%, respectively). The average joint angle increased from L2–3 to L5–S1 and the mean facet joint separation of 2.6 mm was greatest at the L4–5 level. Forty-three percent of all segments showed little additional separation (<0.2 mm) when in a rotated posture. In the remainder, a slight relationship existed between facet geometry and separation; joints with wider angles were associated with greater separation than those with a more sagittal configuration (r=0.36, p<0.01).No conclusive trend was recorded in this patient series for facet joints to separate when the subject adopted an end-range rotated posture. A mild association between physiological separation of facet joints and their morphological angle was determined. This trend alone should not afford confidence in attributing such separation as having pathological significance.