A surgical apparatus comprises a body, an ultrasonic transducer, a shaft, an acoustic waveguide, an articulation section, and an end effector. The ultrasonic transducer is operable to convert electrical power into ultrasonic vibrations. The shaft couples the end effector and the body together. The acoustic waveguide is coupled with the transducer. The end effector comprises a clamp arm and an ultrasonic blade in acoustic communication with the ultrasonic transducer. The clamp arm is configured to pivot toward and away from the ultrasonic blade. The shaft assembly comprises an inner tube configured to longitudinally translate to thereby pivot the clamp arm toward and away from the ultrasonic blade. A distal portion of the inner tube is operable to flex to thereby accommodate pivoting of the clamp arm.