Apparatus for percutaneous access to a patient's body comprising a first steerable tube (12), shaped to define a first lumen, and a first coupling (152) at a longitudinal site of the first tube； and a second steerable tube (14), shaped to define a second lumen and a second coupling (154), the second coupling being intracorporeally couplable to the first coupling, the apparatus having (A) an unlocked state in which the second tube is rotatable within the first lumen, and (B) a locked state in which the second coupling is coupled to the first coupling, and rotation of the second tube is inhibited. The apparatus is configured such that when the second coupling becomes disposed at the longitudinal site in a given rotational orientation of the second tube, the apparatus moves into the locked state by the second coupling automatically coupling to the first coupling. Other embodiments are also described.
