During an escort back to his cell an I/M turned and elbowed the CO while they were walking on the stairway. The CO grabbed ahold of the I/M and they both fell down the flight of stairs. The CO was able to control the I/M at the bottom of the stairs and apply mechanical restraints. The CO was treated at an outside hospital for injuries sustained to his arm as well as suffering a concussion. He did not return to duty.