Reduced diameter steerable instrument

A longitudinal steerable tool (100) having a proximal (20) and distal (40) end comprising:

• a set of longitudinal members (110, LMs) arranged in a longitudinal direction around a fictive tube (180) maintained at an essentially constant circumferential and radial position with respect to the fictive tube (180) and being slidable relative to the fictive tube (180), and
• a bendable proximal part (130, BPP), bendable distal part (134, BDP), and a shaft region, SR (132) between the BDP (134) and BPP (130) wherein movements of the BPP (130) are transmitted to the BDP (134) along the SR (132) by the LMs (110)
• a set of LM guides (300) configured to maintain the LMs (110) at the essentially constant circumferential and radial position and slidable with respect to the fictive tube (180), wherein
• the BPP (130) and/or BDP (134) is each provided with two or more articulated LM guides (310a-c, 314a-c) of the set of LM guides,
• each articulated LM guide (300) in the BPP (130) and/or BDP (134) is provided with a set of discrete channels (350) each accommodating one or two LMs (110), and being open to a circumferential edge (356) of the LM guide (300), which channels are arranged around the fictive tube (180), wherein each channel is open-edged in a radial direction with respect to a circumferential edge of the body,
• a sheath unit (430) is disposed at least partially over each of the BPP (130) and/or BDP (134), the sheath unit (430) having less compliance in a radial direction compared with an axial direction, and comprises a first layer having little or no compliance in a radial direction, and a second layer having compliance at least in an axial direction, the first layer comprises a segmented tube (410) of a non-radially compliant material optionally heat shrink material, and the second layer comprises a compliant tube (420) and is disposed over the first layer, optionally wherein the first and second layers are not bonded together,
• a circumferential outer edge of the articulated LM guide (300) in the BPP (130) and/or BDP (134) is provided with a circumferential guide that defines a circumferential annular path in which a detached segment (414a) of the segmented tube (410) is retained.