Fig. 4

Qualitative LSS severity grade classification according to Schizas et al. (2010). Normal: The roots lie dorsally and occupy less than half of the dural sac area. Grade A: (a) Cerebro-spinal fluid (CSF) is clearly visible within the dural sac and the distribution of the roots is irregular. Grade B: (b) The roots are distributed through the entire cross section of the thecal sac but they can still be individualized. Some CSF is still present, giving the sac a grainy appearance. Grade C: (c) single roots cannot be recognized anymore. They appear as one gray mass that completely fills the narrowed thecal sac. There is an epidural triangle of fat (2) between the arch (1) and thecal sac. Grade D: (d) In contrast to grade C, the triangle of fat has been completely squeezed out