Every portrait in this practice has a spine. The spine is not a theme. It is not a thesis statement. It is not the thing the piece is "about." The spine is the single image or fact that holds the entire structure in place — the one thing that, if you removed it, would cause the whole portrait to collapse into a pile of well-written sentences with nothing connecting them.
In "The Grenade She Was Always Carrying," the spine is the Pacinian corpuscle — the vibrotactile receptor in the chest wall that responds to frequencies between 200 and 300 hertz. That is the contralto range. The entire portrait hangs from that single piece of physiology. Every sentence either extends from it or returns to it. Remove it, and you have a nice essay about a singer in Pennsylvania. Leave it in, and you have a portrait where the reader's own chest becomes part of the argument.
Finding the spine is not the same as choosing a topic. The spine is usually the thing you discover on the third or fourth day of writing, when the piece has already been going in one direction and suddenly reveals that it was actually going in another direction the whole time. The spine was there before you found it. Your job is to notice.