A needle assembly for guided extension through tissue in a surgical procedure, comprising: a stiff, straight outer first cannula, terminating distally in a distal-facing aperture; a second cannula adapted to be passed through and extended out of the distal-facing aperture of said stiff straight outer first cannula, said second cannula being configured to bend to define a known angle of curvature after extending from the distal-facing aperture; and a self-straightening inner element disposed within said second cannula, said self-straightening inner element having a flexibility such that it passes through said second cannula even when said second cannula is bent, but adopts its straight form after exiting said second cannula; wherein, relative to the first cannula, the known angle of curvature of the second cannula and the straight form of the self-straightening inner element together allow a tip position of the self-straightening inner element to be known from the linear extension of the second cannula and the self-straightening inner element; characterised in that the second cannula has a slotted wall, slotted by a plurality of lateral slots along at least part of the slotted wall, said slots extending circumferentially across a majority of the wall's circumference; a wire is disposed within the second cannula, along the slotted wall of the second cannula, and attached to a distal end region of said second cannula; wherein the wire is under tension to bend said second cannula, when the slotted wall is extended from the straight outer first cannula, until adjacent sides of said slots in said slotted wall abut each other, said abutting of the sides of the slots defining the known angle of curvature of said second cannula; and said wire is tensioned by means of a spring connected between said wire and an anchor point on a proximal point of said second cannula. a stiff, straight outer first cannula, terminating distally in a distal-facing aperture; a second cannula adapted to be passed through and extended out of the distal-facing aperture of said stiff straight outer first cannula, said second cannula being configured to bend to define a known angle of curvature after extending from the distal-facing aperture; and a self-straightening inner element disposed within said second cannula, said self-straightening inner element having a flexibility such that it passes through said second cannula even when said second cannula is bent, but adopts its straight form after exiting said second cannula; wherein, relative to the first cannula, the known angle of curvature of the second cannula and the straight form of the self-straightening inner element together allow a tip position of the self-straightening inner element to be known from the linear extension of the second cannula and the self-straightening inner element; characterised in that the second cannula has a slotted wall, slotted by a plurality of lateral slots along at least part of the slotted wall, said slots extending circumferentially across a majority of the wall's circumference; a wire is disposed within the second cannula, along the slotted wall of the second cannula, and attached to a distal end region of said second cannula; wherein the wire is under tension to bend said second cannula, when the slotted wall is extended from the straight outer first cannula, until adjacent sides of said slots in said slotted wall abut each other, said abutting of the sides of the slots defining the known angle of curvature of said second cannula; and said wire is tensioned by means of a spring connected between said wire and an anchor point on a proximal point of said second cannula.