I found a great definition of my form of tethered cord syndrome at http://www.btinternet.com/~tetheredcordresources/
"The filum terminale is like a thin elastic band, about 8 inches long. At the top, it is formed from one of the layers of tissue surrounding your spinal cord, and extends from the bottom of your spinal cord to the tip of your tail bone. The outside of the "elastic band" has a few nerve fibres sticking to it.
The filum terminale works as an anchor for the spinal cord. For people with normal anatomy, the filum stretches when they bend over to allow the spinal cord to move up in the spinal column and then goes back to normal length when the person straightens up and gently pulls the spinal cord back to its normal position.
When the filum is fat-filled, fibrous and tight, it will not allow the spinal cord to move up and down within the spinal column, and so the spinal cord and the nerves end up being stretched instead of the filum. In most people this causes nerve damage".
Everybody has a filum terminale; it is a threadlike piece of tissue that connects the end of the spinal cord to the sacral end of the spinal canal in the pelvic area. In patients with a thickened filum (defined as more than 2 mm in diameter) [Yundt, 1997], the filum is shorter or lower-lying than normal and is thickened with fatty or fibrous tissue. This abnormality causes the filum to become relatively inelastic (a bit like a rope or cord, rather than an elastic band) and the spinal cord becomes tethered at an abnormally low level, thereby giving rise to the recognised signs and symptoms of a tethered spinal cord."