Other mistake: In the scene where Ripley throws the basketball at Johner, Christie says to Ripley, "Ok, I've got a new game, tag .." He hits her with the handles straight in the face once, he misses the next shot, misses again, and Ripley hits him to the ground, but you neither see or hear the handles fall. (00:23:15)
Other mistake: At the end of the movie, as the ship is approaching Earth, the Earth is backwards. Africa is obviously pointing in the wrong direction, as if the image has been reversed.
Other mistake: When Ripley pulls the knife out of her hand the blade is smoking as her acid blood is melting the steel, however shortly after she touches the girls face and hands, and the acid blood has no effect.
Other mistake: The remaining crew of the Betty, plus 2 others, have to swim 90 feet underwater. This they do, but once they are out of the water, the only one who is wet is Winona Ryder.
Answer: This is just speculation (and I haven't watched the movie for ages) but the operation is done with medical equipment in a facility designed for studying Aliens, which the military knows a little about. Maybe it is made of futuristic acid proof material.
If the surgical tools were acid proof, surely the floors of the cells that contain the grown alien specimens would also be acid proof. But that is how they escape: by sacrificing one of their own in order to spill acid blood onto the floor.
Well, you can imagine any acid-proof metal is probably very expensive. They can't make the entire station out of that stuff. Surgical equipment is probably necessary for their research, so they make an exception.
lionhead