An instrument (11, 111, 211, 311, 411, 511, 611, 711) comprising: a longitudinally extending support defining an axis, the support having an uncompressed state of comparatively greater length and a compressed state of comparatively lesser length, the support comprising a flexible distal section, the flexible distal section comprising a coiled filament of round cross-sectional shape and defining a first path relative to the axis when in the compressed state, the flexible distal section having a tensile strength, a filamentary cross-sectional area, and a break load, wherein the break load of the flexible distal section is equal to the tensile strength of the flexible distal section multiplied by the filamentary cross-sectional area of the flexible distal section; a core-wire extending along the axis and anchored to the flexible distal section of the support, the core-wire having a relaxed state of comparatively lesser length and a tensioned state of comparatively greater length, the core-wire comprising a proximal portion terminating at a proximal end of the core-wire and a distal portion terminating at a distal end of the core-wire, the distal portion of the core-wire defining a second path relative to the axis when in the relaxed state, wherein the second path differs from the first path and includes a looped shape, wherein the distal portion of the core-wire has a filamentary diameter (d 2) and a filamentary cross-sectional area, wherein the distal portion of the core-wire also has an upper plateau stress in response to a tensile force applied thereto, wherein a plateau force of the distal portion of the core-wire is equal to the upper plateau stress multiplied by the filamentary cross-sectional area of the distal portion of the core-wire, wherein the distal portion of the core-wire has a length, and wherein the filamentary diameter (d 2) of the distal portion of the core-wire is substantially uniform over the length of the distal portion of the core-wire in the relaxed state; and an actuator secured to a proximal end of the core-wire and to a proximal end of the support to selectively apply both a tensile force to the core-wire and a compressive force to the support, the tensile force causing the core-wire to transition from its relaxed state to its tensioned state, the compressive force causing the support to transition from its uncompressed state to its compressed state; characterized in that the break load of the flexible distal section of the support is greater than the plateau force of the distal portion of the core-wire. a longitudinally extending support defining an axis, the support having an uncompressed state of comparatively greater length and a compressed state of comparatively lesser length, the support comprising a flexible distal section, the flexible distal section comprising a coiled filament of round cross-sectional shape and defining a first path relative to the axis when in the compressed state, the flexible distal section having a tensile strength, a filamentary cross-sectional area, and a break load, wherein the break load of the flexible distal section is equal to the tensile strength of the flexible distal section multiplied by the filamentary cross-sectional area of the flexible distal section; a core-wire extending along the axis and anchored to the flexible distal section of the support, the core-wire having a relaxed state of comparatively lesser length and a tensioned state of comparatively greater length, the core-wire comprising a proximal portion terminating at a proximal end of the core-wire and a distal portion terminating at a distal end of the core-wire, the distal portion of the core-wire defining a second path relative to the axis when in the relaxed state, wherein the second path differs from the first path and includes a looped shape, wherein the distal portion of the core-wire has a filamentary diameter (d 2) and a filamentary cross-sectional area, wherein the distal portion of the core-wire also has an upper plateau stress in response to a tensile force applied thereto, wherein a plateau force of the distal portion of the core-wire is equal to the upper plateau stress multiplied by the filamentary cross-sectional area of the distal portion of the core-wire, wherein the distal portion of the core-wire has a length, and wherein the filamentary diameter (d 2) of the distal portion of the core-wire is substantially uniform over the length of the distal portion of the core-wire in the relaxed state; and an actuator secured to a proximal end of the core-wire and to a proximal end of the support to selectively apply both a tensile force to the core-wire and a compressive force to the support, the tensile force causing the core-wire to transition from its relaxed state to its tensioned state, the compressive force causing the support to transition from its uncompressed state to its compressed state; characterized in that the break load of the flexible distal section of the support is greater than the plateau force of the distal portion of the core-wire.