A surgical assembly includes a cannula and an obturator. The cannula has open proximal and distal ends defining a lumen therebetween. The lumen defines an inner surface of the cannula having a mating structure. The obturator is generally tubular and has an outer surface including a corresponding mating structure. The respective mating structures of the obturator and the cannula engage each other so as prevent relative rotation of the obturator and cannula, and to prevent inadvertent disconnection of an upper valve housing attached to the cannula.