Spinal Stenosis

Spinal stenosis: How do you measure it? 

A small canal diameter has been associated with an increased risk of injury; however, there is a lack of reliable normative data on spinal canal diameters and several different proposals have been reported by researchers.

Stenosis can be determined by several causes.

Classically, congenital, acquired (degenerative) or mixed forms are distinguished. Moreover, central stenosis is distinguished from lateral, foraminal or concentric ones.

Cervical stenosis is usually defined as a canal diameter of < 12 mm. An index often used on oplan X-ray film was the Torg ratio (canal vs body sagittal diameters. If less than 1= stenosis).

A more modern concept is the SAC (space available for the cord).

MRI permits to directly visualize the cord and the subarachnoid space. The mean diameter of the cervical cord is 7 mm and the SAC is therefore reduced when less than a 1 mm CSF layer is present around the cord, thus indicating a severe stenosis.

At the lumbar level, congenital stenosis shows short pedicles, while acquired forms have their maximal expression at the level of the endplates. A mid sagittal diameter inferior to 14 mm is considered below normal limits. If inferior to 10 mm, it is severely below.

Following both radiological and post-mortem studies, narrow canal appears to be very common. Therefore, radiographic finding should be closely correlated with clinical signs to make the diagnosis of stenosis and to make decision regarding treatment. Electromyography is strongly recommended to achieve the correct interpretation.