This invention relates to a surgical cannula that has a simple ball valve assembly comprising a ball, rotatably mounted on an axle extending between opposing legs of a pivotable yoke, which engages a seat to seal the instrument inlet to the cannula passageway when a trocar instrument is removed and moves...http://www.google.com/patents/US5397314?utm_source=gb-gplus-sharePatent US5397314 - Surgical cannula with ball valve
