from what I understand by reading the results of my MRI, and I am no expert at all, but it seems like l4 l5 and s1 verterbraes surround the thecal sac, and if the protrude out or are bulging, that would compress the thecal sac...That is on the anterior side...hope that helps someone
Thecal sac is also known as the lumbar cistern, and is the space below the termination of the spinal cord (L1/L2). It is covered by dura and arachnoid mater, and contains CSF (cerebrospinal fluid). A compression can result from vertebral disc herniation, edematous tissue in the surrounding epidural space, tumors, or anything that can invade the surrounding area (meningiomas, etc). If it's a disc herniation, since it's below the level of the spinal cord, it will compress nerve roots, typically compressing the transversing root below the level of the disc herniation. ie.-an L4/L5 herniation will compress nerve root L5 and you will get dysfunction according to that root and functional distribution. It can also impinge on lower roots, however this is uncommon. Hope that helps.