[00421 A medical device having a releasable retaining includes needle cannula connected to a hub having an open proximal end with a frusto-conically shaped cavity therein. The cavity is part of a passageway through the hub which is in fluid communication with the lumen of the needle cannula. A retaining element is releasably connected to the hub. The hub includes an aperture and at least one protuberance projecting into the aperture for engaging the frusto-conically shaped tip of a fluid delivery device. The protuberance is shaped to offer less resistance to hub movement in a direction of engagement than in a direction of disengagement with the tip. The retaining element is connected to the hub so that when the hub is engaged to the tip, the force required to disengage the retaining element from the tip is greater than the force required to disengage the retaining element from the hub.
