INDIANAPOLIS — Indianapolis Colts safety Bob Sanders, already out with a sprained right ankle, has had arthroscopic surgery on a knee.

The NFL Defensive Player of the Year last season injured his ankle Sept. 14 at Minnesota and was initially expected to miss four to six weeks. Colts owner Jim Irsay said the recovery period for the sprain would likely be longer than that for the knee, so the surgery was scheduled during his absence.

The team, which announced the surgery on its Web site today, has not said which knee was operated on or the nature of the ailment.