Epidural fat interposition - one more sign of spondylolysis

Sherif H & Mahfouz A from Qatar have published an elegant article in European Radiology on a new sign; epidural fat interposition between the dura and the spinous process at the level of spondylolysis.

The principle is the same as that of the "widened canal sign". Because of the break in the pars, there is separation of the anterior and posterior elements. This results in a widening of the canal even in the absence of spondylolisthesis. Simultaneously fat is seen between the spinous process and the dura to make up for the increased canal size.

Typically there is a general belief that MRI is not very good for the diagnosis of spondylolysis. A careful evaluation of the pars on parasagittal images and a high alert for the signs mentioned above actually allows pretty accurate diagnosis of lysis even in the absence of listhesis. It is rare for us to ask for thin-section CT to make this diagnosis.

One condition that may produce a false positive for the diagnosis of spondylolysis is mild spinal epidural lipomatosis, though given the clinical setting and additional findings seen in lysis, it seems highly unlikely that we could confuse the two diagnoses.