Abstract: A cannula has a shank and a tip which is elliptical in cross-section having radially extending crescent-shaped fins for engaging the tissue lining the tunnel of an incision. The cannula is thus retained in position and is not forced out with fluid pressure or by traction on attached tubing with eye movements or inadvertently hooking instruments. The cannula is rotated to disengage the fins from the incision by stretching the incision with the major axis of the elliptical tip. The cannula of the present invention is suitable for penetration of organs or vessels under fluid pressure for surgical procedures, in particular for ophthalmic surgical procedures.