Due to the violent events which occurred during SHODAN's takeover of Citadel, by the time the Hacker awakens from his healing coma there are only three functional Surgery Machines remaining on the entire station. It is therefore a very good idea to memorize their location and/or place markers on the map.
Additionally, while seemingly invulnerable, they can be destroyed if they take enough damage from explosives or weapons! Watch your aim around these invaluable machines!
They are found on the following Levels:
There's also a Surgery Machine on Level 2 - Research, but it's broken.