I know nothing about this, but know how to bring it up. Here is a site you can go to. It sounds to deep for me.
[BOOK] Clinical Neurology - Google Books Resultby Graeme J. Hankey, Joanna M. Wardlaw - 2002
... adherence of the roots to the thecal membranes, and obliteration of the thecal sac. ... easily appreciated sign again being bunching of the nerve roots. ...
books.google.com/books?isbn=1888799676...
Or go to google and type in: nerve bunching of the thecal sac