There are a couple different ways you could sign spine. A good way
is to indicate your "back" by using the common "A"-hand gesture.
Then use a couple of "G"-hands to show the spine.
It would be okay to use "F"-hands too. The "F"-handshape would
work very nicely since the back is composed of round disks.
You can learn
American Sign Language (ASL) online at American Sign Language University ™
ASL resources by Lifeprint.com © Dr. William Vicars