Interrogation does not require torture.
You want this :
It has important information
Arguably the most effective technique for extracting useful information is skilled interview by a trained specialist. No knifes, fire, electrodes, pain, messing with their sanity. Just talk.
It's what the police do. It's what the British did extremely effectively ...
A tree can grow around fences, incooporating the obsticle into itself.. if you have antiseptic bars (silver) such a thing could happen to a long time prisoner. The bars have grown into his back.. over the aeons..
Electric shocks propel people back. If your prisoner is throw against the wall from the first one, they could continuously be given electric shocks that keep them thrown against the wall, unable to move away.
The electricity wouldn't necessarily even have to touch the prisoner.
According to Wikipedia:
...physical contact with energized ...