Niall managed to have the knee surgery in the US under the radar but was photographed on his way back:
According to an update account: Niall was in Colorado for the knee surgery from January 4th - 15th. His actual surgery was on the 6th
Pic of him coming back:
https://twitter.com/HPussies/status/.../photo/1/large
Some fans seem to have reacted badly to him "lying" about it (As if he owes them every details of his life!

), maybe that's why he wrote this tweet:
Hey guys I know most of you have seen those pics today! Yes I've had major knee surgery , a much larger surgery than first anticipated ,
But i would like to tell u , it was a complete success ! Considering the scale of it! So all I ask of you guys is that you would respect my
Privacy as I try to recover and get for the where we are tour!
Basically, it seems there was some sort of meltdown in the usual "drama over nothing" 1D fans tradition.
He also called out one update account because it posted wrong info -simply tweeting them "wrong !", nothing more (not sure about what exactly, I only saw the commotion after the fact)...
I don't really understand why he went out of his way to do that, but I can understand the boys' annoyance at having their lives investigated and dissected for everyone to see. I suppose having update account constantly supply information about their whereabouts & lives is annoying: it means more fans stalking if it's accurate, and giving more wind to the rumour mill when it isn't accurate.
However, those update accounts & numerous fans on twitter are also what helped them being successfull, so I don't think turning on them is a great idea either -not that I am sure that was his intention. I am not saying I agree with the stalking, but the big crowds following their every move is what helped promote 1D...
It's a fine line to walk for all of the boys who probably want to protect whatever privacy they have, good luck to them...
I am just glad that the surgery went well & I hope that Niall will get the rest & peace that he needs