Abstract
The evaluation of the lumbar spine by CT or MR in supine position provides excellent information about the morphology of the spinal structures (inside and outside the spinal canal) but of course cannot give dynamic information. By Newton’s Third Law we know that every axial vector acting on a functional spine unit corresponds a second vector of equal intensity but of opposite direction that counterbalances the first one and brings all the structures in a dynamic equilibrium. The definition of spinal instability by White and Panjabi is well known and X-ray plain film with dynamic testing is not always satisfactory for diagnosis.
From April 2003 through May 2004 we evaluated 45 patients with with a dedicated device able to produce an axial load on the patient with a dynamometer to measure the load applied. The study can be performed in all patients in whom spinal instability is clinically suspected, even if the patients underwent spine surgery. The study was performed by CT or MR but always before in rest and stress conditions. The applied load is 60–70 % of body weight, based on literature data regarding orthostatic load at the L3 S1 level. A spiral multi-slice acquisition technique must be used to obtain thin scans with which excellent 2D multiplanar (MPR) and 3D reconstructions can then be achieved.
Under normal conditions, under an axial loader, we do not see significant changes in spine morphology except for pseudo-protrusion of the disks. In pathologic conditions (38 patients) we must pay attention to the following: disc modifications (appearance or disappearance of vacuum phenomenon, volumetric increasing/appearance of protrusions or herniations); intersomatic findings (increment of the lysthesis, narrowing or widening of disc space, rotational movements, posterior translation of the vertebral body); interarticular joints modifications and neural foramina narrowing; and thickening of the ligamenta flava and narrowing of interspinous spaces and hypermobility of the spinous process.
We believe the CT/MR axial loader currently represents the only way to gain dynamic information in patients in whom spinal instability is clinically suspected.
