The cross section of a curved portion of a curved cannula is oval shaped. The oval is oriented such that a major axis of the oval is generally aligned with the curved portion's bend radius. In one aspect, the cannula tube is ovalized so that the radius of curvature of an interior wall approaches the outer radius of an instrument component that contacts the curved portion during instrument insertion. In this first aspect, a wider contact patch area between the instrument component and the cannula results, which reduces friction and stick-slip during insertion. In another aspect, the cannula tube is ovalized so that the radius of curvature of an interior wall is less than the outer radius of an instrument component that contacts the curved portion during instrument insertion. In this second aspect, two contact patches are established between the instrument component and cannula, which also reduces friction and stick-slip during instrument insertion. Also disclosed is a cannula orientation marking that changes circumferential orientation along a length the curved cannula to help a person who is inserting the cannula to properly twist the cannula during insertion.
