Four-time Pro Bowl center Matt Birk, a mainstay of the Minnesota Vikings' offensive line, underwent hip surgery Tuesday that could sideline him all of summer training camp and perhaps jeopardize his availability for the start of the regular season. The surgery, to repair a torn labrum in Birk's right hip, is the fourth procedure to which the seven-year veteran has been subjected over the past nine months. The three earlier operations were all to repair sports hernias. Birk believes the hip injury resulted from the sports hernias. "This is all a function of having tried to compensate for [the hernias]," said Birk, who has missed all of the offseason conditioning program. "When they checked out the hernia, to see how it was progressing, they found this thing. Hopefully, this will take care of it."