Britney Spears underwent arthroscopic knee surgery Wednesday after injuring herself during a video shoot in Manhattan. The 22-year-old lip-synching superstar was doing choreography for her "Outrageous" video when her bum left knee gave out. An MRI scan later found floating cartilage and a dollop of silicone in her knee. There's no word on how this will impact the second leg of her Onyx Hotel Tour, slated to resume June 22 in Hartford, Conn.