A handle shaft is disposed at a proximal end of a catheter. A wire extends from a distal part of the catheter, proximally through the catheter to the handle shaft. A knob is disposed outside of the handle shaft. A threaded mechanism provides operative coupling between the knob and the handle shaft such that (i) rotation of the knob with respect to the handle shaft alters tension in the wire via the threaded mechanism functioning as a linear actuator; and (ii) non-rotational axial movement of the knob with respect to the handle shaft alters tension in the wire without the threaded mechanism functioning as a linear actuator. Other embodiments are also described.