I don't think it was a dirty knee. It was Brown's knee that did
damage, yes, but the the point of contact was in line with Brown's
center of mass. In the replay, it seemed to me that Brown just ran
straight into him face-to-face, his knee driving through into the
inner thigh of the other player (Handzus, was it?). Brown did not
stick out his leg to trip or stop a player flying past, which is what
I think of as a flagrant use of the knee as a weapon.