Via the Celtics press release:

The Boston Celtics announced today that forward Kevin Garnett underwent successful right knee arthroscopy and removal of posterior knee bone spurs today at the New England Baptist Hospital. The surgery was performed by Team Physician Dr. Brian McKeon and was assisted by Dr. Michael Belkin and Dr. John Richmond.

"After this successful surgery, Kevin can now begin to focus on rehabilitating his knee, returning to top physical shape and fully preparing for the 2009-10 season," Celtics President of Basketball Operations Danny Ainge remarked.

Update: Scott Souza provides this clarification.  Please read:

It is important to note that this surgery was not intended to correct the problem that sidelined him for all but four games from the end of February to the playoffs. The tendon issue requires rest and rehab, while the bone spurs were - according to Rivers - a lingering issue from the start of the season that needed to be taken care of eventually.