A surgical system comprising: a rigid curved cannula; and a surgical instrument having: a proximal end transmission mechanism, wherein the transmission mechanism comprises a force transmission interface to a robotic manipulator of a surgical system; a distal end surgical end effector; a shaft that extends from the transmission mechanism to the surgical end effector, wherein the shaft is passively flexible and configured to extend along a channel of the rigid curved cannula, the cannula for constraining movement of the shaft to support the surgical end effector in a position beyond a distal end of the cannula to perform a surgical procedure; a drive element extending along the shaft, the drive element being configured to transmit force from the proximal end transmission mechanism to the end effector; and a force isolation element routed through the shaft, the force isolation element coupled to and extending between the proximal end transmission mechanism and the distal end surgical end effector, the force isolation element being configured to isolate the drive element from movement in response to bending of the shaft during translation of the shaft along the cannula; characterised in that the force isolation element comprises one or more rods, the or each rod being coupled to and extending between the proximal end transmission mechanism and the end effector; wherein the drive element does not extend through any of said one or more rods. a rigid curved cannula; and a surgical instrument having: a proximal end transmission mechanism, wherein the transmission mechanism comprises a force transmission interface to a robotic manipulator of a surgical system; a distal end surgical end effector; a shaft that extends from the transmission mechanism to the surgical end effector, wherein the shaft is passively flexible and configured to extend along a channel of the rigid curved cannula, the cannula for constraining movement of the shaft to support the surgical end effector in a position beyond a distal end of the cannula to perform a surgical procedure; a drive element extending along the shaft, the drive element being configured to transmit force from the proximal end transmission mechanism to the end effector; and a force isolation element routed through the shaft, the force isolation element coupled to and extending between the proximal end transmission mechanism and the distal end surgical end effector, the force isolation element being configured to isolate the drive element from movement in response to bending of the shaft during translation of the shaft along the cannula; characterised in that the force isolation element comprises one or more rods, the or each rod being coupled to and extending between the proximal end transmission mechanism and the end effector; wherein the drive element does not extend through any of said one or more rods. a proximal end transmission mechanism, wherein the transmission mechanism comprises a force transmission interface to a robotic manipulator of a surgical system; a distal end surgical end effector; a shaft that extends from the transmission mechanism to the surgical end effector, wherein the shaft is passively flexible and configured to extend along a channel of the rigid curved cannula, the cannula for constraining movement of the shaft to support the surgical end effector in a position beyond a distal end of the cannula to perform a surgical procedure; a drive element extending along the shaft, the drive element being configured to transmit force from the proximal end transmission mechanism to the end effector; and a force isolation element routed through the shaft, the force isolation element coupled to and extending between the proximal end transmission mechanism and the distal end surgical end effector, the force isolation element being configured to isolate the drive element from movement in response to bending of the shaft during translation of the shaft along the cannula; characterised in that the force isolation element comprises one or more rods, the or each rod being coupled to and extending between the proximal end transmission mechanism and the end effector; wherein the drive element does not extend through any of said one or more rods.