One Direction Star Niall Horan said he has undergone knee surgery and asked his fans to respect his privacy as he recovers. Horan, 20, had been spotted in a London airport earlier in the week wearing a cast and using crutches, the Mirror reported. "Hey guys I know most of you have seen those pics today! Yes I've had major surgery, a much larger surgery than first anticipated, but I would like to tell you, it was a complete success!" he wrote on Twitter Friday. "So all I ask of you guys is that you would respect my privacy as I try to recover." Horan's surgery was the result of a dislocation problem he has had since childhood. He is resting up for the group's tour in April, the Mirror said.