Living Surgery

Eren is captured by human scientists who go to extremes to study titan shifters. They vivisect him (live dissection), force feed him human parts while he's in human form, and test his pain endurance.

Eren never transforms because they tell him innocents are living in the town the scientists have taken him to, and Eren knows if he transformed then with all his anger he'd kill innocents and wouldn't be able to control himself.

Levi finds him eventually and rescues him.

Original work found here: snkkinks.dreamwidth.org/2124.html?thread=1614924
Shingeki no Kyojin belongs to Hajime Isayama.