An ultrasonic surgical instrument, comprising: (a) an end effector, wherein the end effector comprises: (i) an ultrasonic blade, and (ii) a clamp arm configured to move relative to the ultrasonic blade between an open position and a closed position; (b) a shaft assembly, comprising: (i) a proximal shaft portion extending along a longitudinal axis, (ii) an acoustic waveguide extending proximally from the ultrasonic blade, wherein the acoustic waveguide comprises a flexible portion, (iii) a distal shaft portion extending along a distal axis, and (iv) an articulation section interposed between the proximal shaft portion and the distal shaft portion, wherein the flexible portion of the acoustic waveguide extends along the articulation section, wherein the articulation section is configured to deflect the distal shaft portion and the end effector relative to the longitudinal axis between a straight configuration and an articulated configuration; and (c) a waveguide grounding feature associated with the distal shaft portion, wherein the waveguide grounding feature is configured to inhibit the ultrasonic blade from shifting relative to the clamp arm along the distal axis as the end effector is driven between the straight configuration and the articulated configuration; wherein the waveguide grounding feature comprises a pin extending through the acoustic waveguide; wherein the acoustic waveguide comprises a distal flange housed within the distal shaft portion, wherein the pin extends from the distal flange; characterised in that the waveguide grounding feature comprises a flange sleeve surrounding the distal flange. (a) an end effector, wherein the end effector comprises: (i) an ultrasonic blade, and (ii) a clamp arm configured to move relative to the ultrasonic blade between an open position and a closed position; (i) an ultrasonic blade, and (ii) a clamp arm configured to move relative to the ultrasonic blade between an open position and a closed position; (b) a shaft assembly, comprising: (i) a proximal shaft portion extending along a longitudinal axis, (ii) an acoustic waveguide extending proximally from the ultrasonic blade, wherein the acoustic waveguide comprises a flexible portion, (iii) a distal shaft portion extending along a distal axis, and (iv) an articulation section interposed between the proximal shaft portion and the distal shaft portion, wherein the flexible portion of the acoustic waveguide extends along the articulation section, wherein the articulation section is configured to deflect the distal shaft portion and the end effector relative to the longitudinal axis between a straight configuration and an articulated configuration; and (i) a proximal shaft portion extending along a longitudinal axis, (ii) an acoustic waveguide extending proximally from the ultrasonic blade, wherein the acoustic waveguide comprises a flexible portion, (iii) a distal shaft portion extending along a distal axis, and (iv) an articulation section interposed between the proximal shaft portion and the distal shaft portion, wherein the flexible portion of the acoustic waveguide extends along the articulation section, wherein the articulation section is configured to deflect the distal shaft portion and the end effector relative to the longitudinal axis between a straight configuration and an articulated configuration; and (c) a waveguide grounding feature associated with the distal shaft portion, wherein the waveguide grounding feature is configured to inhibit the ultrasonic blade from shifting relative to the clamp arm along the distal axis as the end effector is driven between the straight configuration and the articulated configuration; wherein the waveguide grounding feature comprises a pin extending through the acoustic waveguide; wherein the acoustic waveguide comprises a distal flange housed within the distal shaft portion, wherein the pin extends from the distal flange; characterised in that the waveguide grounding feature comprises a flange sleeve surrounding the distal flange. wherein the waveguide grounding feature comprises a pin extending through the acoustic waveguide; wherein the acoustic waveguide comprises a distal flange housed within the distal shaft portion, wherein the pin extends from the distal flange; characterised in that the waveguide grounding feature comprises a flange sleeve surrounding the distal flange.