You will need to find away out of the morgue. Search the nearby cupboards until you find the rib spreader. Go up the ramp and use it on the doors to the lift / elevator.
You then have a decision to make.
*** Spoiler - click to reveal ***You will pass out and when you wake up you need to decide whether to cut off your arm or not. It doesn't make much of a difference to the story which ever way you choose.
Go back to the doors and finish opening them. Go up the ladder and after a brief conversation on the roof of the hospital, you will get the achievement.