WO2024134211A1

WO2024134211A1 - Surgical instrument with interlocking end effector elements

Info

Publication number: WO2024134211A1
Application number: PCT/GB2023/053354
Authority: WO
Inventors: Oliver Charles SPARE; Colin BROOK; Alexander DOPPELSTEIN; Erik Schwarz
Original assignee: Cmr Surgical Limited
Priority date: 2022-12-22
Filing date: 2023-12-21
Publication date: 2024-06-27
Also published as: GB2625760A; GB202219578D0

Abstract

A surgical robotic instrument comprising: an end effector mount; and a pair of interacting end effector elements, each end effector element having a mounting structure whereby it is rotatably mounted to the end effector mount to allow rotation relative to the end effector mount about an axis; wherein the mounting structures of the end effector elements are interlocked so as to inhibit relative movement therebetween in a direction parallel to the axis.

Description

SURGICAL INSTRUMENT WITH INTERLOCKING END EFFECTOR ELEMENTS FIELD OF THE INVENTION This invention relates to robotic surgery, in particular to a surgical instrument for performing a surgical procedure. BACKGROUND It is known to use robots for assisting and performing surgery. Figure 1 illustrates a typical surgical robot 100 which comprises a base 108, an arm 102, and an instrument 105. The base supports the robot, and is itself attached rigidly to, for example, the operating theatre floor, the operating theatre ceiling or a trolley. The arm extends between the base and the instrument. The arm is articulated by means of multiple flexible joints 103 along its length, which are used to locate the surgical instrument in a desired location relative to the patient. The surgical instrument is attached to the distal end 104 of the robot arm. The surgical instrument penetrates the body of the patient 101 at a port 107 to access the surgical site. At its distal end, the instrument comprises an end effector 106 for engaging in a medical procedure. Figure 2 illustrates a typical surgical instrument 200 for performing robotic laparoscopic surgery. The surgical instrument comprises a base 201 by means of which the surgical instrument connects to the robot arm. A shaft 202 extends between base 201 and articulation 203. Articulation 203 terminates in an end effector 204. In figure 2, a pair of serrated jaws are illustrated as the end effector 204. The articulation 203 permits the end effector 204 to move relative to the shaft 202. It is desirable for at least two degrees of freedom to be provided to the motion of the end effector 204 by means of the articulation. Figure 3 illustrates an example of a known surgical instrument 300 in which end effector 204 is permitted to move relative to shaft 202 by means of pitch joint 301 and two yaw joints 302. Joint 301 enables the end effector 204 to rotate about pitch axis 303. Joints 302 enable each jaw of the end effector 204 to rotate about yaw axis 304. The joints are driven by cables 306, 307 and 308. Pulley 305 is used to direct cables 307 and 308 from their passage over the pitch joint to the yaw joints. Pulley 305 is offset from the central axis of the articulation 203. In the example shown in Figure 3, the end effector is a pair of graspers. However, there are many end effectors commonly used in which the end effector comprises two end effector elements which are moveable relative to one another about a joint, such as surgical scissors. Such instruments may optionally be electrosurgically (for example, monopolar) connected. In the case of surgical scissors, the scissor blades are generally shaped such that there is only one point of contact between each blade at any given aperture (for example, spread) angle. This is a high pressure contact point which is advantageous for effective cutting. The contact point will move along the profile of the blade from the base to tip as the scissors close. In conventional designs, in order for the force to be exerted against each blade, there may be two spring cups, one for each blade, sandwiched between the inner surface of a supporting structure, such as a clevis of a yoke, and the surface of the blade’s base. In other words, the spring cup sits in the yaw pin, about which the scissor blades rotate, on the outside of the blades and exerting force against the yoke’s clevis, which is the supporting structure. With usage, these spring cups can fatigue and the blades may no longer exert the same force against each other, which can degrade the cutting performance. Furthermore, end effector elements should preferably be aligned to optimise their performance. For such small assemblies used in surgery, this alignment is difficult to keep consistent across different instruments, as it becomes increasingly difficult to produce parts with tight tolerances at smaller scales. It is desirable to develop a solution for overcoming at least some of the above issues. SUMMARY According to a first aspect, there is provided a surgical robotic instrument comprising: an end effector mount; and a pair of interacting end effector elements, each end effector element having a mounting structure whereby that end effector element is rotatably mounted to the end effector mount to allow rotation relative to the end effector mount about an axis; wherein the mounting structures of the end effector elements are interlocked so as to inhibit relative movement therebetween in a direction parallel to the axis. The mounting structures of the end effector elements may be interlocked so as to inhibit relative movement therebetween in one or more directions normal to the axis. Each end effector element may have a respective tip. The tip may be located distally of the respective mounting structure of the end effector element. The distal end of the tip may be at an opposite end of the end effector element to the mounting structure. The mounting structure may be located proximally of the tip. The mounting structures of the end effector elements may be interlocked so as to urge respective tips of each of the pair of end effector elements together in a direction parallel to the axis. The mounting structures of the end effector elements may be interlocked so as to urge respective distal ends of the tips of each of the pair of end effector elements together in a direction parallel to the axis. The first and second end effector elements may be rotatable relative to each other about the axis. At least a part of each of the pair of end effector elements between the respective mounting structure and the distal end of the respective tip may be curved. The curvature may be in a plane normal to the axis. The pair of end effector elements may be configured such that for any rotation angle of the end effector elements about the axis, there is a single point of contact between parts of the end effector elements between the mounting structures and the distal ends of the tips. The single point of contact between the end effector elements may be different for different opening angles. Therefore, there may be a respective single point of contact for any given rotation angle and/or opening angle. At least a part of each of the pair of end effector elements between the respective mounting structure and the distal end of the respective tip may be configured to exert a force against the other of the pair of end effector elements in a direction parallel to the axis. Each of the pair of end effector elements may be configured to exert a force against the other of the pair of end effector elements in a direction parallel to the axis at the single point of contact for a given opening angle. When the pair of interacting end effector elements are in an open position, the distal end of the respective tip of one or more of the pair of end effector elements may be deflected relative to the respective mounting structure in a direction parallel to the axis. The mounting structures may each comprise one or more interlocking features. The one or more interlocking features of the mounting structure of one of the pair of end effector elements may be configured to engage with the one or more interlocking features of the mounting structure of the other of the pair of end effector elements. For example, the mounting structures may each comprise one or more protruding features and one or more recesses. The protruding feature(s) of a first one of the pair of end effector elements may be configured to engage with the recess(es) of a second one of the pair of effector elements. The protruding feature(s) of a first one of the pair of end effector elements may be configured to be received in the recess(es) of a second one of the pair of effector elements. The or each protruding feature may protrude in a direction normal to the axis. The or each protruding feature of the mounting structure of one of the pair of end effector elements may have side walls that abut corresponding side walls of a recess of the mounting structure of the other of the pair of end effector elements. The mounting structures of the end effector elements may each comprise a hooked member, wherein the hooked member of one of the pair of end effector elements is configured to interlock with the hooked member of the other of the pair of end effector elements. Each hooked member may comprise a protrusion and a recess. The protrusion of the hooked member of one of the end effector elements may be configured to be received in the recess of the hooked member of the other of the pair of end effector elements. The mounting structures may be interlocked across the axis (for example, on more than one side of the axis). The mounting structures may be interlocked on one side of the axis. The mounting structures may be interlocked distally of the axis. The mounting structure of one of the pair of end effector elements may comprise an outer wall extending parallel to an outer wall of the mounting structure of the other of the pair of end effector elements in a direction perpendicular to the axis. The end effector mount may comprise a supporting structure. The supporting structure may comprise two arms extending on either side of the mounting structures of the pair of end effector elements. The mounting structures of the end effector elements may be interlocked between the two arms of the supporting structure. The respective outer wall of the mounting structure of each of the end effector elements may abut a respective one of the two arms of the supporting structure. The respective outer wall of the mounting structure of each of the end effector elements may be rotatable relative to the respective one of the two arms of the supporting structure. The end effector mount may comprise a shaft about which the mounting structures are configured to rotate. The shaft may extend along the axis. The arms of the supporting structure may support the shaft. This shaft may be in the form of a pin. At least one of the pair of end effector elements may comprise a cutting blade. The cutting blade may be between the mounting structure and the distal end of the tip of an end effector element. The cutting blade may be flexible. At least part of at least one of the end effector elements may be flexible. At least part of each of the end effector elements may be flexible. The end effector element(s) may be flexible at its/their respective tip(s). Each of the pair of end effector elements may comprise a cutting blade. For example, the end effector may comprise a pair of scissors. One or more of the cutting blades may be flexible. The cutting blade may be more flexible than the mounting structure. The mounting structure may be more rigid than the cutting blade. This may allow the respective tips of the end effector elements to be urged together in a direction parallel to the axis when the mounting structures are interlocked. This may improve the cutting performance of the instrument. This may also allow spring cups which are traditionally placed between the outer faces of the mounting structures and the inner faces of the yoke to be removed, which may result in a more compact design of instrument with fewer components. The flexible cutting blade(s) may be flexible along its/their entire length(s). The blade(s) may be configured to flex throughout the length of the respective blade. This may consistently keep a single point of contact or pressure between the blades. This point may travel from the tips towards the mounting structures as the opening angle of the blades increases in use. The end effector elements may be biased to exert a force on each other in a direction parallel to the axis. The tips of the end effector elements may be biased to exert a force on each other in a direction parallel to the axis. The force exerted by the distal ends of the tips of the end effector elements on each other parallel to the axis may be greater than the force exerted by the end effector elements on each other parallel to the axis at a position proximal of the distal end of the tips. This may be the case when the end effector elements are closed (i.e. when the opening angle is zero). Each of the pair of end effector elements may comprise a jaw. The end effector may comprise a pair of jaws. For example, the end effector may be a surgical grasper. The pair of end effector elements may be interlocked by an additional interlocking member. The mounting structures of the pair of end effector elements may be interlocked by the interlocking member. The interlocking member may be a component. The component may be a separate component to the mounting structures. At least part of the component may have a frustoconical shape. The interlocking member may be a component having a double frustoconical form. Respective surfaces of the mounting structures may abut each other at a plane. The plane may be perpendicular to the axis. The component may have a smaller radius in the plane where the surfaces of the mounting structures of the end effector elements abut than in planes outboard of this plane along the axis. The interlocking member may be a component that is symmetrical about the plane at which the surfaces of the mounting structures of the end effector elements abut each other. The interlocking member may be a component having a cylindrical form. The component may lock with the mounting structure of each end effector element to inhibit the end effector elements from moving relative to one another in a direction parallel to the axis. The component may be received in a respective locking recess of each mounting structure to inhibit relative movement between the end effector elements in a direction parallel to the axis and/or in one or more directions perpendicular to the axis. The recess may have an at least partially frustoconical profile. The component may have a hole therethrough for receiving a supporting shaft about which the end effector elements are configured to rotate. The interlocking member may be enveloped by the mounting structures of the end effector elements. The interlocking member may be located between the mounting structures of the end effector elements. The mounting structures of the pair of end effector elements may be rotationally symmetric with respect to each other. They may have 180 degree rotational symmetry. The mounting structures may be electrically insulating components of an electrosurgical instrument (for example, a monopolar or bipolar electrosurgical instrument). The mounting structures may be at a proximal end of the end effector elements, relative to the distal end of the tips. According to a further aspect, there is provided a surgical robotic instrument comprising: an end effector mount; and a pair of interacting end effector elements, each end effector element having a mounting structure whereby that end effector element is rotatably mounted to the end effector mount to allow rotation relative to the end effector mount about an axis; wherein the mounting structures of the end effector elements are interlocked by an additional interlocking member so as to inhibit relative movement therebetween in one or more directions perpendicular to the axis. The interlocking member may be a component that is symmetrical about a plane at which parallel surfaces of the mounting structures of the end effector elements abut each other. The interlocking member may be a component having a cylindrical form. The component may lock with the mounting structure of each end effector element to inhibit the end effector elements from moving relative to one another in the one or more directions perpendicular to the axis. The component may be received in a respective locking recess of each mounting structure to inhibit relative movement between the end effector elements in one or more directions perpendicular to the axis. The recess may have a cylindrical profile. The component may have a hole therethrough for receiving a supporting shaft about which the end effector elements are configured to rotate. The interlocking member may be enveloped by the mounting structures of the end effector elements. The interlocking member may be located between the mounting structures of the end effector elements. BRIEF DESCRIPTION OF THE FIGURES The present disclosure will now be described by way of example with reference to the accompanying drawings. In the drawings: Figure 1 schematically illustrates an example of a known surgical robot. Figure 2 schematically illustrates an example of a known surgical instrument. Figure 3 schematically illustrates another example of a known surgical instrument. Figure 4 schematically illustrates an example of a surgical robot. Figures 5a and 5b schematically illustrate a surgical instrument with the distal end effector elements in a closed position. Figure 6 schematically illustrates a surgical instrument with the distal end effector elements in an open position. Figures 7a, 7b and 7c schematically illustrate examples of distal end effector elements. Figures 8a, 8b and 8c schematically illustrate examples of interlocked first and second distal end effector elements. Figure 9 schematically illustrates a further example of interlocked first and second distal end effector elements. Figure 10 schematically illustrates an example where the end effector comprises a pair of jaws. Figure 11 schematically illustrates an example where the end effector comprises a fenestrated grasper. Figures 12a and 12b schematically illustrate a further example of interlocked first and second distal end effector elements. Figures 13a, 13b, 13c and 13d schematically illustrate components of an example of an end effector. Figures 14a, 14b and 14c schematically illustrate components of another example of an end effector. Figures 15a, 15b and 15c schematically illustrate components of a further example of an end effector. Figures 16a, 16b and 16c schematically illustrate components of a further example of an end effector. DETAILED DESCRIPTION Figure 4 illustrates a surgical robot having an arm 400 which extends from a base 401. The arm comprises a number of rigid limbs 402. The limbs are coupled by revolute joints 403. The most proximal limb 402a is coupled to the base by joint 403a. It and the other limbs are coupled in series by further ones of the joints 403. A wrist 404 is made up of four individual revolute joints. The wrist 404 couples one limb (402b) to the most distal limb (402c) of the arm. The most distal limb 402c carries an attachment 405 for a surgical instrument 406. Each joint 403 of the arm has one or more motors 407 which can be operated to cause rotational motion at the respective joint, and one or more position and/or torque sensors 408 which provide information regarding the current configuration and/or load at that joint. The motors may be arranged proximally of the joints whose motion they drive, so as to improve weight distribution. For clarity, only some of the motors and sensors are shown in figure 4. The arm may be generally as described in our granted patent GB2523224B. The arm terminates in an attachment 405 for interfacing with the instrument 406. The attachment 405 comprises a drive assembly for driving articulation of the instrument, and a drive assembly interface for engaging an instrument interface of the instrument 406. Movable interface elements of the drive assembly interface mechanically engage corresponding movable interface elements of the instrument interface in order to transfer drive from the robot arm to the instrument. One instrument may be exchanged for another several times during a typical operation. Thus, the instrument is attachable and detachable from the robot arm during the operation. Features of the drive assembly interface and the instrument interface may aid their alignment when brought into engagement with each other, so as to reduce the accuracy with which they need to be aligned by the user. The instrument 406 comprises an end effector for performing an operation. The end effector may take any suitable form. For example, the end effector may be smooth jaws, serrated jaws, a gripper, a pair of scissors, or a needle holder. As described with respect to figure 2, the instrument comprises an articulation between the instrument shaft and the end effector. The articulation may comprise one or more joints which permit the end effector to move relative to the shaft of the instrument. The one or more joints in the articulation can be actuated by driving elements, such as cables. These driving elements are secured at the other end of the instrument shaft to the interface elements of the instrument interface. Thus, the robot arm transfers drive to the end effector as follows: movement of a drive assembly interface element moves an instrument interface element which moves a driving element which moves a joint of the articulation which moves the end effector. Controllers for the motors, torque sensors and encoders are distributed with the robot arm. The controllers are connected via a communication bus to control unit 409. A control unit 409 comprises a processor 410 and a memory 411. Memory 411 stores in a non-transient way software that is executable by the processor to control the operation of the motors 407 to cause the arm 400 to operate in the manner described herein. In particular, the software can control the processor 410 to cause the motors (for example via distributed controllers) to drive in dependence on inputs from the sensors 408 and from a surgeon command interface 412. The control unit 409 is coupled to the motors 407 for driving them in accordance with outputs generated by execution of the software. The control unit 409 is coupled to the sensors 408 for receiving sensed input from the sensors, and to the command interface 412 for receiving input from it. The respective couplings may, for example, each be electrical or optical cables, or may be provided by a wireless connection. The command interface 412 comprises one or more input devices whereby a user can request motion of the end effector in a desired way. The input devices could, for example, be manually operable mechanical input devices such as control handles or joysticks, or contactless input devices such as optical gesture sensors. The software stored in memory 411 is configured to respond to those inputs and cause the joints of the arm and instrument to move accordingly, in compliance with a pre-determined control strategy. The control strategy may include safety features which moderate the motion of the arm and instrument in response to command inputs. Thus, in summary, a surgeon at the command interface 412 can control the instrument 406 to move in such a way as to perform a desired surgical procedure. The control unit 409 and/or the command interface 412 may be remote from the arm 400. An attachment enables the surgical instrument 406 to be releasably attached to the distal end of the robot arm. The surgical instrument may be configured to extend linearly parallel with the rotation axis of the terminal joint of the arm. For example, the surgical instrument may extend along an axis coincident with the rotation axis of the terminal joint of the arm. The robot arm therefore comprises a series of arm links interspersed with joints. These joints may be revolute joints. The end of the robot arm distal to the base can be articulated relative to a base of the robot by movement of one or more of the joints. The surgical instrument is supported by the robot arm. The surgical instrument attaches to a drive assembly at the distal end of the robot arm. This attachment point is external to the patient. The surgical instrument has an elongate profile, with a shaft spanning between its proximal end which attaches to the robot arm and its distal end which accesses the surgical site within the patient body. The end effector is at the distal end of the shaft. The proximal end of the surgical instrument and the instrument shaft may be rigid with respect to each other and rigid with respect to the distal end of the robot arm when attached to it. An incision is made into the patient body, through which a port is inserted. The surgical instrument may penetrate the patient body through the port to access the surgical site. Alternatively, the surgical instrument may penetrate the body through a natural orifice of the body to access the surgical site. At the proximal end of the instrument, the shaft is connected to an instrument interface. The instrument interface engages with the drive assembly at the distal end of the robot arm. Specifically, individual instrument interface elements of the instrument interface each engage a respective individual drive assembly interface element of the drive assembly. The instrument interface is releasably engageable with the drive assembly. The instrument can be detached from the robot arm manually without requiring any tools. This enables the instrument to be detached from the drive assembly quickly and another instrument attached during an operation. At the distal end of the surgical instrument, the distal end of the shaft is connected to an end effector by an articulated coupling. The end effector engages in a surgical procedure at the surgical site. Figures 5a and 5b illustrate the distal end of an exemplary instrument which has a pair of scissors as the end effector 501. However, the end effector elements may be any type of end effector elements, such as opposing jaws. The end effector 501 comprises a pair of interacting distal end effector elements 509, 510 (which are interacting fingers, relative to the arm of the robot). In this example, the blade of each end effector element is curved. Figures 5a and 5b depict a straight configuration of the surgical instrument in which the end effector is aligned with the shaft 502. In this orientation, the longitudinal axis of the shaft 506 is coincident with the longitudinal axis of the articulation and the longitudinal axis of the end effector. Articulation of the joints of the instrument enable the end effector to take a range of attitudes relative to the shaft 502. In other configurations of the distal end of the instrument, articulation about the joints can be driven relative to the straight configuration of Figures 5a and 5b. The shaft 502 is connected to the end effector 501 by articulated coupling 503. The articulated coupling 503 comprises several joints. These joints enable the pose of the end effector to be altered relative to the direction of the instrument shaft. Although not shown in figures 5a and 5b, the end effector may also comprise joint(s). In the example of figures 5a and 5b, the articulated coupling 503 comprises a pitch joint 504. The pitch joint 504 rotates about pitch axis 505, which is perpendicular to the longitudinal axis 506 of the shaft 502. The pitch joint 504 permits a supporting body 513 (described below) and hence the end effector 501 to rotate about the pitch axis 505 relative to the shaft 502. In the example of figures 5a and 5b, the articulated coupling also comprises a first yaw joint 507 and a second yaw joint 511. First yaw joint 507 rotates about first yaw axis 508. Second yaw joint 511 rotates about second yaw axis 512. Both yaw axes 508 and 512 are perpendicular to pitch axis 505. Yaw axes 508 and 512 may be parallel. Yaw axes 508 and 512 may be collinear. Therefore, the end effector elements 509, 510 may rotate about a common rotation axis. The articulated coupling 503 comprises a supporting body 513. A supporting shaft may be supported by the supporting body. The end effector elements 509, 510 can rotate about this supporting shaft, which extends along the rotation axes 508, 512. The supporting body, and/or the supporting shaft that it supports, provide an end effector mount that the end effector elements 509, 510 are configured to rotate relative to. At one end, the supporting body 513 is connected to the shaft 502 by pitch joint 504. At its other end, the supporting body 513 is connected to the end effector 501 by the yaw joints 507 and 511. This supporting body is omitted from figure 5a for ease of illustration so as to enable the other structure of the articulated coupling to be more easily seen. The first yaw joint 507 is fast with the first end effector element 509 and permits the first end effector element 509 to rotate about the first yaw axis 508 relative to the supporting body 513 and the pitch joint 504. The second yaw joint 511 is fast with the second end effector element 510 and permits the second end effector element 510 to rotate about the second yaw axis 512 relative to the supporting body 513 and the pitch joint 504. In figure 5a, the end effector elements 509, 510 are shown in a closed configuration in which the blades abut at their distal tips. Generally speaking, the end effector elements 509, 510 are separated by an opening angle. Not all of the end effector elements may be separated. For example, when in an open configuration according to some opening angle, the end effector elements may still be in contact at their bases, as shown in Figure 6. The maximum opening angle may be, for example, between 40 to 45 degrees. There may be one or more stops on the first end effector element and/or the second end effector element to prevent the element from opening beyond the maximum opening angle and from closing beyond a maximum closing angle. The maximum closing angle may allow a maximum force (for example, a gripping force) to be exerted by one end effector element on the other end effector element. Suitably, the first end effector element 509 and the second end effector element 510 are independently rotatable about the axes 508, 512 by the first and second yaw joints 507, 511. The end effector elements may be rotated in the same direction or different directions by the first and second yaw joints. The first end effector element 509 may be rotated about the first yaw axis 508, whilst the second end effector element 510 is not rotated about the second yaw axis 512. The second end effector element 510 may be rotated about the second yaw axis 512, whilst the first end effector element 509 is not rotated about the first yaw axis 508. In this example, axes 508, 512 are a common rotation axis. In this example, the axes 508, 512 are perpendicular to the shaft of the end effector when the end effector element is aligned with the longitudinal axis of the shaft. The supporting structure 513 of the end effector mount may comprise two arms extending on either side of the bases of the first and second end effector elements. The supporting shaft about which the end effector elements can rotate may be supported by the two arms. The supporting structure 513 may be in the form of a clevis unit. The two arms may be arms of the clevis unit. The end effector mount may comprise a yoke. Figure 6 shows the end effector 501 in any open configuration where the end effector elements 509 and 510 are separated by an opening angle of approximately 45 degrees. The joints illustrated in figures 5a, 5b and 6 are driven by pairs of driving elements. The driving elements are elongate. They are flexible transverse to their longitudinal extent. They resist compression and tension forces along their longitudinal extent. The driving elements extend from the joints in the articulation through the shaft to the instrument interface. The driving elements have a high modulus. The driving elements remain taut in operation. They are not permitted to become slack. Thus, the driving elements are able to transfer drive from the instrument interface to the joints. Suitably, each joint of the articulation is driven by a pair of driving elements. Each pair of driving elements is secured at the other end of the instrument shaft to a respective instrument interface element of the instrument interface. Thus, the robot arm transfers drive to the end effector as follows: movement of a drive assembly interface element moves an instrument interface element which moves a driving element which moves one or more joints of the articulation and/or end effector which moves the end effector. The driving elements may be cables. The driving elements may comprise flexible portions and a rigid portion. Flexible portions engage the components of the instrument interface and the articulated coupling, and the rigid portion extends through all or part of the instrument shaft. For example, the flexible portion may be a cable, and the rigid portion may be a spoke. Other rigid portion(s) may be in the instrument interface or articulated coupling of the instrument. For example, rack and pinions may be in the instrument interface or articulated coupling of the instrument. Figures 5a, 5b and 6 illustrate a first pair of driving elements A1, A2 which are constrained to move around the first yaw joint 507. Driving elements A1, A2 drive rotation of the first end effector element 509 about the first yaw axis 508. Figures 5a, 5b and 6 illustrate a second pair of driving elements B1, B2 which are constrained to move around the second yaw joint 511. Driving elements B1, B2 drive rotation of the second end effector element 510 about the second yaw axis 512. Figures 5a and 5b also illustrate a third pair of driving elements C1, C2 (of which C2 is not visible) which are constrained to move around pitch joint 504. Driving elements C1, C2 drive rotation of the end effector 501 about the pitch axis 505. The end effector 501 can be rotated about the pitch axis 505 by applying tension to driving elements C1 and/or C2. The pitch joint 504 and yaw joints 507, 511 are independently driven by their respective driving elements. The end effector elements 509 and 510 are independently rotatable. The end effector elements can be rotated in opposing rotational directions. For example, the end effector elements can be rotated in opposing rotational directions towards each other by applying tension to driving elements A1 and B2. The end effector elements can be rotated in opposing rotational directions away from each other by applying tension to driving elements A2 and B1. Both end effector elements can be rotated in the same rotational direction, by applying tension to driving elements A1 and B1 or alternatively A2 and B2. This causes the end effector elements to yaw about the pivot axes 508 and 512. Alternatively, one end effector element can be rotated (in either rotational direction) whilst the other end effector element is maintained in position, by applying tension to only one of driving elements A1, A2, B1, B2. In Figures 5a, 5b and 6, the first yaw joint 507 and the second yaw joint 511 both permit rotation about the same axis (parallel to axes 508 and 512, which in this example are the same axis). However, the first and second yaw joints may alternatively permit rotation of the end effector elements about different axes. The axis of rotation of one of the end effector elements may be offset in the longitudinal direction of the shaft 506 from the axis of rotation of the other end effector element. The axis of rotation of one of the end effector elements may be offset in a direction transverse to the longitudinal direction of the shaft 506 from the axis of rotation of the other end effector element. The axis of rotation of one of the end effector elements may not be parallel to the axis of rotation of the other end effector element. The axes of rotation of the end effector elements 509, 510 may be offset in the longitudinal direction of the shaft and/or offset in a direction perpendicular to the longitudinal direction of the shaft and/or angled with respect to each other. This may be desirable as a result of the end effector elements being asymmetric. For example, in an electrosurgical element, a first end effector element may be powered and a second end effector element not powered and insulated from the first end effector element. To aid this, the axes of rotation of the two end effector elements may be offset in the direction perpendicular to the longitudinal direction of the shaft. In another example, a first end effector element may be a blade and a second end effector element a flat cutting surface. To aid use of the blade, the axes of rotation of the two end effector elements may be angled to one another. As described above, the end effector 501 comprises a pair of end effector elements 509, 510, which in Figures 5a, 5b and 6 are depicted as a pair of scissor blades. It will be understood that this is for illustrative purposes only. The end effector may take any suitable form. For example, the end effector may be a grasper, where the first and second end effector elements are first and second jaws. In one example, the first and second end effector elements each comprise one or more protruding features and one or more recesses. The protruding feature(s) of one of the first and second end effector elements are configured to engage with the recess(es) of the other of the first and second end effector elements. Figures 7a, 7b and 7c show examples of the individual elements 509, 510 where the end effector comprises a pair of scissor blades. Figure 7a shows a first view of the first end effector element 509. Figure 7b shows a first view of the second end effector element 510. Figure 7c shows a second view of the first end effector element 509, showing the back surface of the end effector element. Each end effector element comprises a blade. Each end effector element has a base at its proximal end and a tip at its distal end. The base provides a mounting structure for rotatably mounting the end effector element to the end effector mount. The mounting structure of each end effector element can rotate relative to the end effector mount. Each end effector element extends between the base and the tip. Each end effector element may extend between the base and the tip along a straight path or a curved path. In Figures 7a, 7b and 7c the end effector elements are curved. The curvature may be in a plane containing the rotation axes 508, 512. In this example, the base and the blade are integrally formed (i.e. are a single component). Thus, the mounting structure of each end effector element is integrally formed with the tip (including its distal end) of that end effector element. As shown in Figure 7a, the first end effector element 509 has a base indicated generally at 551. The base 551 has a hole 552 for receiving a supporting shaft which the end effector element 509 can rotate about. The supporting shaft may be part of the end effector mount. The supporting shaft may be in the form of a pin. When the instrument is assembled, the longitudinal axis of the supporting shaft is aligned with axes 508, 512. The tip of the first end effector element 509 is shown at 559. The base 551 comprises protruding features 553, 554. The protruding features 553, 554 each extend in a plane perpendicular to the axis of the hole 552 (and therefore extend in a plane perpendicular to the rotation axis 508 shown in Figures 5a, 5b and 6). The base 551 also comprises a guide 555 for the driving elements that cause that end effector element 509 to rotate about its axis. The recess between protruding features 553 and 554 can receive protruding feature 563 of the base 561 of the second end effector element 510. The side walls of protruding feature 563 abut the inner side walls of protruding features 553 and 554. As shown in Figure 7b, the second end effector element 510 has a base indicated generally at 561. The base 561 has a hole 562 for receiving a supporting shaft which the end effector element 510 can rotate about. The tip of the second end effector element 510 is shown at 569. The base 561 comprises protruding features 563, 564. The protruding features 563, 564 each extend in a plane perpendicular to the axis of the hole 562 (and therefore extend in a plane perpendicular to the rotation axis 512 shown in Figures 5a, 5b and 6). The base 561 also comprises a guide 565 for the driving elements that cause that end effector element 510 to rotate about its axis. Guide 565 also protrudes in a direction perpendicular to the rotation axis of the end effector element 510. The recess between protruding features 563 and 564 can receive protruding feature 563 of the base 561 of the second end effector element 510. The side walls of protruding feature 553 abut the inner side walls of protruding features 563 and 564. Figure 7c shows an alternative view of the first end effector element 509. The first end effector element 509 and the second end effector element 510 are configured so as to allow them to be interlocked at their respective mounting structures (i.e. at their bases, which are proximal to the tips of the end effector elements) when they are engaged with each other, so as to inhibit movement between them parallel to the rotation axes 508, 512, when they are installed on the instrument. In some embodiments, the interlocking of the first 509 and second 510 end effector elements inhibits relative movement between the first and second end effector elements in one or more directions normal to the axes. In some embodiments, the interlocking of the first and second end effector elements inhibits relative movement between the first and second end effector elements in all directions normal to the rotation axes 508, 512. When interlocked, the first and second end effector elements can move relative to each other by rotation about their respective rotation axes 508, 512. As mentioned above, the axes 508, 512 may be parallel and/or collinear. Therefore, the axes 508, 512 may define a single rotation axis. This interlocking of the mounting structures inhibits relative movement as described above. The interlocking may be so as to prevent movement completely, or restrict movement, compared to if the mounting structures were not interlocked. The bases may be configured to be engaged (and interlocked) before the supporting shaft is inserted through the bases and the coupled end effector elements are installed on the supporting structure of the end effector mount. That is, they are engaged as described above and the shaft is then inserted into the hole in each of the end effector elements. The shaft can then be installed between the arms of the supporting structure 513. In some examples, the interlocking of the first and second end effector elements at their bases may be so as to allow the end effector elements to move relative to one another in one or more directions perpendicular to the axes 508, 512. This allows the first end effector element 509 and the second end effector element 510 to rotate relative to one another and relative to the end effector mount but inhibits the first end effector element 509 from moving relative to the second end effector element 510 in a direction parallel to the rotation axes 508, 512. In other words, this allows the first end effector element 509 to rotate about the axis 508 and the second end effector element 510 to rotate about the axis 512. In some embodiments, this interlocking can urge the tip of the first end effector element towards the tip of the second end effector element in a direction parallel to the axes 508, 512. In the case where the end effector elements each comprise a blade, when the mounting structures of the end effector elements are interlocked, the blades can deflect away from the mounting structures towards each other. The blades can exert force against one another in a direction parallel to the axes 508, 512. Where part of the blades are curved, when the bases are interlocked, the blades may engage each other in a direction parallel to the axes 508, 512 due to their curvature forcing one of the blades, or both blades, to be deflected. One blade may be deflected more than the other, for example if one of the blades is stiffer than the other due to material and/or thickness differences. The mounting structure and the remainder of the end effector element may be integrally formed. The interlocking features of each end effector element may be integral with the remainder of the end effector element. In the examples shown in Figures 7a, 7b and 7c, the respective mounting structures of the first end effector element and the second end effector element are rotationally symmetric with respect to each other. In this example, the bases 551 and 561 have 180 degree rotational symmetry. However, in other examples this may not be the case. The articulation 503 described above comprises a supporting shaft extending along the first axis and through the respective bases 551, 561 of the first and second end effector elements. The shaft extends through holes 552, 562 in the first and second end effector elements and they are configured to rotate about the supporting shaft. In this example, the supporting shaft is cylindrical and the holes 552, 562 have a circular cross-section. The bases 551, 561 of each of the end effector elements therefore each have a hole 552, 562 which can receive the supporting shaft (which may be a yaw pin). The end effector elements rotate about this supporting shaft when the instrument is assembled. The centre of the hole defines the axis or axes about which the end effector elements rotate. In the examples shown in Figures 5a, 5b and 6, the first and second end effector elements are interlocked with each other across a common rotation axis. Therefore, the end effector elements may be engaged and interlocked on either side of the rotation axis. As shown in more detail in the different views of the end effector 501 in Figures 8a, 8b and 8c, in this example, a protrusion of one of the end effector element is configured to engage a recess of the other end effector element, and vice versa. The protrusion is received in the recess. A side wall of a protrusion of one end effector element may abut a side wall of a protrusion of the other end effector element. In the examples shows in Figures 7a-7c and 8a-8c, each end effector element comprises two protrusions. A recess is formed between the two protrusions. The side walls of a recess correspond to the side walls of the two protrusions on either side of that recess. The side walls of the recess and the protrusions extend in a plane normal to the longitudinal axis of the hole through the end effector element and normal to the yaw axis or axes about which the end effector element(s) rotate(s). A recess of one end effector elements may receive a protrusion of the other end effector elements. Each protrusion has side walls which abut side walls of the recess to inhibit movement between the bases of the first and second end effector elements parallel to their rotation axes. Each protrusion may be in the form of a ridge or fin. Each recess may be in the form of a channel or a slot. Therefore, a part of one of the end effector elements is received in a part of the other of the end effector elements. The interlocking features of the mounting structures (for example, the recesses and protrusions), when interlocked, can also prevent rotation of the end effector elements about unwanted axes of rotation (which may be rotation about any axis except the yaw axis (axes 508, 512)). This unwanted rotation can be referred to as "jaw rocking" and may result due to crimps of the drive cables being attached to the outer face of the jaw elements. This can cause unwanted movement about an axis that is perpendicular to the yaw axes. Figure 9 shows a further view of the first and second end effector elements 509, 510 when they are interlocked, with the walls of the second end effector element 510 being transparent so that the engagement between the two end effector elements can be more easily seen. A protruding feature on one of the end effector elements can therefore engage with a corresponding recess on the other end effector elements. When engaged, movement between the first and second end effector elements in the direction parallel to the first axis can be prevented. The protruding feature has side walls that abut corresponding side walls of a recess of the base of the other of the first and second end effector elements. In this embodiment, the protruding feature protrudes in a direction normal to the rotation axis or axes. However, the protruding features may protrude in a direction that is not normal to the rotation axis or axes. In this example, the scissor blades are shaped such that there is only one point of contact between each blade at any given aperture (e.g. spread angle). This is a high pressure contact point which allows for effective cutting. The contact point moves along the profile of the blade from base to tip as the blades close. The mounting structures of the end effector elements may be symmetrical or opposing, but need not be. The mounting structures of the end effector elements could have different features from those described above that allow them to prevent relative movement of the end effector elements. When the bases of the end effector elements are engaged, the outer side walls and/or the side walls of the protrusions of the bases of the end effector elements may be parallel, but the blade, which may be curved, may not be parallel to the other blade. As mentioned above, the end effector could have other forms. Figure 10 shows an example where the end effector comprises a pair of jaws. Figure 11 shows an example where the end effector is a fenestrated grasper. In these examples, the first and second end effector elements 509, 510 are interlocked at their bases and can rotate relative to one another about a common yaw axis, as described above. In the exemplary instruments described above, the instrument is a non-electrosurgical instrument. In other examples, the instrument may be an electrosurgical instrument. Generally, there are two types of electrosurgical instruments: monopolar and bipolar. For example, the instrument may comprise a pair of monopolar electrosurgical scissors. The monopolar electrosurgical instrument may further comprise a sleeve covering the distal end of the instrument leaving only the tips of the end effector elements exposed. In this instrument type, there are no electrosurgical cables routed from the proximal end of the instrument to the end effector elements and the entire distal end of the instrument is live. Hence, monopolar instruments can use the same interlocking mechanism as non-electrosurgical instruments. In bipolar electrosurgical instruments, the current is directed to the end effector elements by means of electrical cables routed to each end effector element individually. For the current to pass from one end effector element to the other at the tips of the elements through the tissue, instead of shorting at the base, the end effector elements are insulated from each other. The interlocking mechanism used in “cold” instruments can be modified for bipolar instruments. One way in which this could be done is shown in Figures 12a and 12b. This configuration may also be used for non- electrosurgical instruments. In the example shown in Figures 12a and 12b, the mounting structures of each end effector element 1201, 1202 comprise a hooked member 1203, 1204 that engages a corresponding hooked member on the other end effector element to interlock the first and second end effector elements 1201, 1202 so as to inhibit movement therebetween in a direction parallel to their yaw rotation axes (which in this example both extend along a common axis). The engagement of the hooked member inhibits movement of the first and second end effector elements in a direction parallel to the yaw axes, but allows them to rotate relative to one another about those axes. Each hooked member may engage the other hooked member in a plane parallel to the axis. The hooked member of each mounting structure comprises a recess and a protrusion. The protrusion of one of the end effector elements is received in the recess of the other end effector element, and vice versa. In this example, the recess and protrusion of each end effector element are formed in an insulative component of the mounting structure, which may be an over moulded component. In this example, the interlocking features (e.g. the one or more protrusions and the one or more recesses) of each of the mounting structures are not interlocked across the axis of rotation of the end effector elements relative to the mount, as in the embodiments described above. Therefore, the mounting structures of the end effector elements may be interlocked in a portion of the end effector that is located distally of the rotation axis. The solutions described herein interlock the end effector elements at their base (which may also be referred to as the root) in such a way that when the end effector elements are mounted on the rotation supporting shaft, the end effector elements, which may be blades in the examples of the curved scissor described above, are forced to bend and exert a force against each other. This can improve the lifetime of the instrument and reduces the components required. It can also make the end effector more compact. The solution described herein can also improve end effector misalignment. This interlocking end effector design can be applied to any other instrument type including rotating jaw-like end effectors, such that the interlocking mating faces provide support to each other, straightening the tip of the jaws with respect to one another, which in some applications is more important than the jaws being straight with respect to other parts of the instrument, such as the yoke of the supporting structure of the articulation. For example, this may be more important in instruments which have complementary features, such as complimentary electrodes, for which it is desirable to space the electrodes by a given amount for optimal electric effect. Further concepts will now be described which aim to further reduce misalignment between the first end effector element and the second end effector element when the instrument is assembled. These features may be used in conjunction with the interlocking embodiments described above, or with a standard end effector elements. The end effectors described below may be articulated by the structure described above with reference to Figures 5a, 5b and 6 comprising pulleys and pairs of driving elements for articulating the joints of the end effector. Figures 13a, 13b, 13c and 13d show an example where the end effector 1300 comprises an additional interlocking member in the form of a component 1350 for maintaining the alignment between the mounting structures (e.g. bases) of the first and second end effector elements 1301, 1302. In this example, the component 1350 is a separate component to the end effector elements 1301, 1302 that is located between the elements 1301, 1302 when the instrument is assembled. In this example, the component 1350 has a non-uniform cross-section. The component 1350 may be a bearing. The component may have a double frustoconical form. In this example, the component 1350 is symmetrical about a plane where the surfaces of the end effector elements engage each other. Therefore, at least part of the component has a frustoconical shape. The component 1350 locks with the mounting structure (e.g. base) of each end effector element to inhibit the end effector elements from moving relative to one another in a direction parallel to the rotation axis when the end effector is assembled. The component 1350 has a hole therethrough for receiving a supporting shaft about which the end effector elements 1301, 1302 can rotate. The hole is aligned with corresponding holes in the bases of the first and second end effector elements when the instrument is assembled. The component 1350 is received in a respective locking recess 1360, 1370 of each mounting structure to inhibit relative movement between the end effector elements in a direction parallel to the axis. The recess 1360, 1370 has a frustoconical profile to receive one of the frustums of the component 1350. This locks the mounting structures together and inhibits movement between them parallel to the axis and/or in one or more directions perpendicular to the axis. The component 1350 is enveloped by the mounting structures of the end effector elements. The component 1350 is located between the mounting structures of the end effector elements. Figures 14a, 14b and 14c show an example where the end effector 1400 comprises a component 1450 for maintaining the alignment between the bases of the first and second end effector elements 1401, 1402. In this example, the end effector elements 1401 and 1402 are identical. As shown in Figure 14b for end effector element 1401, the end effector elements comprise a recess 1403. In this example, the recess 1403 has a circular cross section and the component 1450 circular, has a uniform cross section. The component 1450 has a cylindrical form with parallel side walls. The component 1450 is received in the recesses of the end effector elements. The component 1450 inhibits relative movement between the mounting structures of the end effector elements in directions perpendicular to the yaw rotation axes when the end effector element is assembled. The component 1450 has a hole therethrough for receiving a supporting shaft about which the end effector elements 1401, 1402 can rotate. The hole is aligned with corresponding holes in the bases of the first and second end effector elements when the instrument is assembled. The component 1450 is enveloped by the mounting structures of the end effector elements. The component 1450 is located between the mounting structures of the end effector elements. Figures 15a, 15b and 15c show an example where the end effector 1500 comprises first and second end effector elements 1501, 1502. The mounting structure (base) of one of the end effector elements 1501 comprises a protrusion 1503 and the mounting structure (base) of the other end effector element 1502 comprises a recess 1504 configured to receive the protrusion 1503. In this example, the protrusion 1503 and the recess 1504 have circular cross sections. This can inhibit movement of the first and second end effector elements perpendicular to their respective rotation axes. Figures 16a, 16b and 16c show an example where the end effector 1600 comprises first and second end effector elements 1601, 1602. In this example, the mounting structure (base) of the first end effector element 1601 has a recess, indicated at 1603, and the mounting structure (base) of the second end effector element 1602 has a protrusion, indicated at 1604. In this example, the recess 1603 and the protrusion 1604 each have an alignment feature, shown at 1605 and 1606 respectively, that allows the first and second end effector elements to be engaged with each other in one orientation only. Therefore, in this example, when the instrument is assembled, the first and second end effector elements 1601, 1602 are configured to engage in a direction parallel to the respective axes of the holes through the mounting structures of the end effector elements (or in a direction normal to the outer walls of the mounting structures) in one orientation only. Each end effector element 1601, 1602 has a hole 1607, 1608 configured to receive a supporting shaft. The supporting shaft extends along the rotation axes for the first and second end effector elements. Once engaged in this orientation, the first and second end effector elements 1601, 1602 can be rotated relative to each other about their axes (or common axis). When the end effector elements are mounted on the shaft, the walls of the recess 1603 and the protrusion 1604 can help to maintain alignment of the outer walls of the mounting structures. This can further help to improve alignment and, once engaged, can prevent movement between the first and second end effector elements parallel to the yaw axis or axes. The alignment features 1605, 1606 can also act as a stop to prevent the first and second end effector from rotating relative to each other beyond their maximum opening angle. This example can inhibit relative movement between the mounting structures (bases) of the first and second end effector elements in a direction parallel to and/or perpendicular to the first axis. These methods of reducing misalignment have an advantage that the engaging parts of the end effector elements and/or their components for maintaining alignment have a large surface that is easier to machine or produce with surface tolerance that is used as a butting surface. Therefore, the first and second end effector elements may be engaged so as to inhibit (for example, prevent or reduce) movement between the mounting structures (bases) of the first end effector element and the second end effector element in a direction parallel to the rotation axis and/or perpendicular to their rotation axis. The respective bases of the first and second end effector elements may be engaged with each other and may also be engaged with an additional component (such as components 1350 and 1450 described above) to improve the alignment of the end effector elements. The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

CLAIMS 1. A surgical robotic instrument comprising: an end effector mount; and a pair of interacting end effector elements, each end effector element having a mounting structure whereby it is rotatably mounted to the end effector mount to allow rotation relative to the end effector mount about an axis; wherein the mounting structures of the end effector elements are interlocked so as to inhibit relative movement therebetween in a direction parallel to the axis.

2. The surgical robotic instrument as claimed in claim 1, wherein the mounting structures of the end effector elements are interlocked so as to inhibit relative movement therebetween in one or more directions normal to the axis.

3. The surgical robotic instrument as claimed in any preceding claim, wherein each of the pair of end effector elements comprises a respective tip located distally of the respective mounting structure.

4. The surgical robotic instrument as claimed in claim 3, wherein the mounting structures of the end effector elements are interlocked so as to urge the tips of the end effector elements together in a direction parallel to the axis.

5. The surgical robotic instrument as claimed in claim 3 or claim 4, wherein at least a part of each of the pair of end effector elements between the respective mounting structure and the distal end of the respective tip is curved.

6. The surgical robotic instrument as claimed in any of claims 3 to 5, wherein the pair of end effector elements are configured such that for any rotation angle of the end effector elements about the axis, there is a single point of contact between parts of the end effector elements between the mounting structures and the distal ends of the tips.

7. The surgical robotic instrument as claimed in any of claims 3 to 6, wherein at least a part of each of the pair of end effector elements between the respective mounting structure and the distal end of the respective tip is configured to exert a force against the other of the pair of end effector elements in a direction parallel to the axis.

8. The surgical robotic instrument as claimed in any preceding claim, wherein, when the pair of interacting end effector elements are in an open position, the distal end of the respective tip of one or more of the pair of end effector elements is deflected relative to the respective mounting structure in a direction parallel to the axis.

9. The surgical robotic instrument as claimed in any preceding claim, wherein the mounting structures each comprise one or more protruding features and one or more recesses, the protruding feature(s) of a first one of the pair of end effector elements being configured to engage with the recess(es) of a second one of the pair of effector elements.

10. The surgical robotic instrument as claimed in any preceding claim, wherein the mounting structures of the end effector elements each comprise a hooked member, wherein the hooked member of one of the pair of end effector elements is configured to interlock with the hooked member of the other of the pair of end effector elements.

11. The surgical robotic instrument as claimed in any preceding claim, wherein the mounting structure of one of the pair of end effector elements comprises an outer wall extending parallel to an outer wall of the mounting structure of the other of the pair of end effector elements in a direction perpendicular to the axis.

12. The surgical robotic instrument as claimed in any preceding claim, wherein the end effector mount comprises a shaft about which the mounting structures are configured to rotate, the shaft extending along the axis.

13. The surgical robotic instrument as claimed in preceding claim, wherein at least one of the pair of end effector elements comprises a cutting blade.

14. The surgical robotic instrument as claimed in preceding claim, wherein at least part of one or more of the end effector elements is flexible.

15. The surgical robotic instrument as claimed in claim 14 as dependent on claim 13, wherein the blade(s) of the at least one of the pair of end effector elements is/are flexible.

16. The surgical robotic instrument as claimed in claim 15, wherein the mounting structures are more rigid than the cutting blade(s).

17. The surgical robotic instrument as claimed in any of claims 1 to 12, wherein each of the pair of end effector elements comprises a jaw.

18. The surgical robotic instrument as claimed in any preceding claim, wherein the mounting structures of the pair of end effector elements are rotationally symmetric with respect to each other.

19. The surgical robotic instrument as claimed in any preceding claim, wherein the mounting structures are electrically insulating components of an electrosurgical instrument.

20. The surgical robotic instrument as claimed in any preceding claim, wherein the pair of end effector elements are interlocked by an additional interlocking member.

21. The surgical robotic instrument as claimed in claim 20, wherein the interlocking member is a component wherein at least part of the component has a frustoconical shape.

22. The surgical robotic instrument as claimed in claim 20 or claim 21, wherein the interlocking member is a component having a double frustoconical form.

23. The surgical robotic instrument as claimed in claim 20, wherein the interlocking member is a component having a cylindrical form.

24. The surgical robotic instrument as claimed in any of claims 20 to 23, wherein the interlocking member is a component that is symmetrical about a plane at which the surfaces of the mounting structures of the end effector elements engage each other.

25. The surgical robotic instrument as claimed in any of claims 20 to 24, wherein the interlocking member is configured to lock with the mounting structure of each end effector element to inhibit the end effector elements from moving relative to one another in a direction parallel to the axis.

26. The surgical robotic instrument as claimed in any preceding claim, wherein the force exerted by one end effector element on the other end effector element in a direction parallel to the axis is greater at the tips of the end effector elements than at the mounting structures.

27. A surgical robotic instrument comprising: an end effector mount; and a pair of interacting end effector elements, each end effector element having a mounting structure whereby that end effector element is rotatably mounted to the end effector mount to allow rotation relative to the end effector mount about an axis; wherein the mounting structures of the end effector elements are interlocked by an additional interlocking member so as to inhibit relative movement therebetween in one or more directions perpendicular to the axis.

28. The surgical robotic instrument as claimed in claim 27, wherein the interlocking member is a component that is symmetrical about a plane at which parallel surfaces of the mounting structures of the end effector elements abut each other.

29. The surgical robotic instrument as claimed in claim 27 or claim 28, wherein the interlocking member is a component having a cylindrical form.

30. The surgical robotic instrument as claimed in any of claims 20 to 25 or claims 27 to 29, wherein the interlocking member has a hole therethrough for receiving a supporting shaft about which the end effector elements are configured to rotate.

31. The surgical robotic instrument as claimed in any of claims 20 to 25 or claims 27 to 30, wherein the additional interlocking member is enveloped by the mounting structures of the end effector elements.