Surgical instruments with articulating shafts

An articulatable surgical instrument comprising:

• an end effector to treat tissue, wherein the end effector comprises an ultrasonic blade;
• a hollow shaft extending proximally from the end effector along a longitudinal axis, the hollow shaft having an exterior wall and an interior wall, wherein the interior wall defines a groove within the hollow shaft, wherein the groove is parallel to a plane that intersects the longitudinal axis, and wherein the groove comprises a first position and a second position proximal from the first position;
• a waveguide acoustically coupled to the ultrasonic blade and extending proximally through the hollow shaft;
• a first interface member coupled to either the waveguide or the ultrasonic blade at a coupling point and extending proximally, wherein the first interface member comprises a first peg member positioned within the groove;
• and
• a second interface member coupled to either the waveguide or the ultrasonic blade at the coupling point and extending proximally, wherein the second interface member comprises a second peg member positioned within the groove, and wherein upon rotation of the hollow shaft relative to the waveguide, the first and second peg members translate within the groove, causing bending of the first and second interface members and deflection of the first and second interface members away from the longitudinal axis.