Electrosurgical instrument

Abstract

A robotic electrosurgical instrument 200 comprises a shaft 202 and an end effector 210 connected to a distal end of the shaft, the end effector comprising opposing first 211 and second 212 end effecto

Description

Field The present disclosure relates to a robotic surgical instrument for use in electrosurgical procedures. Background Electrosurgery involves applying a radio frequency current to tissue to achieve a particular effect. Electrosurgical instruments have several uses in surgery. For example, electrosurgical instruments can be used to cut tissue, desiccate tissue (dry it out), and seal vessels within tissue. Depending on the desired effect, different amounts of power per unit volume are delivered to the tissue. For example, to cut tissue using vaporisation, a relatively high power is delivered to the tissue to rapidly heat water in the cells of the tissue so that the water vaporises and the cells rupture. As another example, to desiccate tissue, a relatively low power is supplied to the tissue to slowly heat the water in the cells of the tissue which causes the tissue to dry out. Desiccated tissue can be cut fairly easily by mechanically tearing or pulling the desiccated tissue. Thus, tissue can be cut either from direct vaporisation of the tissue, or via desiccating and tearing the tissue. To seal a vessel in the tissue, a relatively low power is applied to heat internal collagen to around 70 - 100°C and then cool it down to form a seal. In monopolar electrosurgery the high frequency current passes through the patient from an active electrode of the monopolar electrosurgical instrument to a separate return electrode placed on the patient. In bipolar electrosurgery the active and return electrodes are both within the bipolar electrosurgical instrument. The current passes from the active electrode of the bipolar electrosurgical instrument to the return electrode of the bipolar electrosurgical instrument via the tissue between the active and return electrode. It is desirable to increase the effectiveness of the cutting action performed by an electrosurgical instrument. In particular, it is desirable to increase the effectiveness of an electrosurgical instrument when cutting into a tissue plane. A tissue plane is a large thin membrane. The edges of a tissue plane are not usually accessible during an operation. This makes tissue planes difficult to grasp between the jaws of the instrument. Tissue planes therefore need to be cut in a different manner to tissue that can be grasped between jaws. Tissue planes are usually cut using monopolar electrosurgery, in which a monopolar hook is used to make a hole in the tissue plane. Monopolar hooks can make holes in a tissue plane without having to physically contact the tissue by energising the monopolar tip and bringing the tip into close proximity with the tissue plane. The cutting performance can further be improved by forcing the monopolar hook into the tissue. Monopolar electrosurgery is not suitable for every operation. For example, monopolar electrosurgery cannot be used on a patient who has a pacemaker. It is desirable to provide a bipolar electrosurgical instrument that is suitable for cutting tissue planes. It is further desirable to increase the cutting performance of a bipolar electrosurgical instrument when cutting mesentery and other types of tough tissue. Whilst increasing the voltage of the cutting instrument can improve the cutting performance, increasing the voltage too much can cause sparking across electrodes to occur. It is further desirable to avoid the surgeon needing to switch between instruments during an operation to perform different kinds of cutting and sealing operations. It is desirable to have one robotic bipolar electrosurgical instrument that can operate in multiple types of cutting and sealing modes. Summary According to the present disclosure there is provided a robotic electrosurgical instrument comprising a shaft and an end effector connected to a distal end of the shaft, the end effector comprising opposing first and second end effector elements, the first end effector element comprising a primary electrode and at least one secondary electrode, wherein the primary electrode and the at least one secondary electrode protrude from a distal end of the first end effector element such that when the end effector approaches a plane head-on when the first and second end effector elements are in a closed configuration, the primary electrode and the at least one secondary electrode contact the plane before the remainder of the end effector. The primary electrode may protrude further from the distal end of the first end effector element than the secondary electrode such that when the end effector approaches the plane head-on when the first and second end effector elements are in a closed configuration, the primary electrode contacts the plane before the secondary electrode. The primary electrode may be spaced from the at least one secondary electrode such that when the first and second end effector elements are in a closed configuration, a plane parallel to the longitudinal axis of the first end effector element does not intersect both the primary electrode and the at least one secondary electrode. The instrument may be operable in a distal cutting mode in which the primary electrode is connected to a higher potential than the at least one secondary electrode such that current flows between the primary electrode and the at least one secondary electrode distally of the end effector. The primary electrode and the at least one secondary electrode may be fixed in position with respect to the first end effector element. The end effector may be connected to the distal end of the shaft via an articulation for articulating the end effector. The articulation may permit each end effector element to move relative to the shaft between the closed configuration and an open configuration. The primary electrode and the at least one secondary electrode may protrude from the distal end of the first end effector element in a direction parallel to the longitudinal axis of the first end effector element. The primary electrode and the at least one secondary electrode may each comprise a distal face for contacting the plane head-on. The distal face of the at least one secondary electrode may have a surface area that is greater than a surface area of the distal face of the primary electrode. The primary electrode may have an exposed surface area that is less than an exposed surface area of the at least one secondary electrode. The ratio of the exposed surface area of the at least one secondary electrode to the exposed surface area of the primary electrode may be greater than 5. The distal face of the at least one secondary electrode may have a width in a lateral direction that is greater than a width in the same direction of the distal face of the primary electrode. The first end effector element may comprise a first pair of secondary electrodes including the at least one secondary electrode. The primary electrode may be between the first pair of secondary electrodes. The first pair of secondary electrodes may protrude from the distal end of the end effector such that when the end effector approaches the plane head-on, when the first and second end effector elements are in a closed configuration, the primary electrode and the pair of secondary electrodes contact the plane before the remainder of the end effector. The first end effector element may comprise an internal face opposing the second end effector element. The first end effector element may comprise a first pair of lateral electrodes each extending longitudinally along the internal face of the first end effector element. The second end effector element may comprise an internal face opposing the first end effector element. The second end effector element may comprise a second pair of lateral electrodes each extending longitudinally along the internal face of the second end effector element. The instrument may be operable in a sealing mode in which the first pair of lateral electrodes is connected to a different potential than the second pair of lateral electrodes such that current flows between the first pair of lateral electrodes and the second pair of lateral electrodes. The first end effector element may comprise a first internal middle electrode between the first pair of lateral electrodes. The first internal middle electrode may extend longitudinally along the internal face of the first end effector element. The instrument may be operable in an internal cutting mode in which the first internal middle electrode is connected to a higher potential than at least one lateral electrode in the first or second pairs of lateral electrodes such that current flows between the primary electrode and that at least one lateral electrode internally of the end effector. The second end effector element may comprise a second internal middle electrode between the second pair of lateral electrodes. The second internal middle electrode may extend longitudinally along the internal face of the second end effector element. The instrument may be operable in an internal cutting mode in which the second internal middle electrode is connected to a higher potential than at least one lateral electrode in the first or second pairs of lateral electrodes such that current flows between the second internal middle electrode and that at least one lateral electrode internally of the end effector. The instrument may be operable in an internal double-cutting mode in which the first and second internal middle electrodes are connected to a higher potential than at least one lateral electrode in the first or second pairs of lateral electrodes such that current flows between the first and second internal middle electrodes and that at least one lateral electrode internally of the end effector. The first internal middle electrode may be part of the primary electrode. The first pair of lateral electrodes may be a part of the first pair of secondary electrodes. When operable in the internal cutting mode or internal double-cutting mode, the primary electrode may not be electrically activated. When operable in the internal cutting mode or internal double-cutting mode, the secondary electrodes may not be electrically activated. The first internal middle electrode may be recessed in the internal face of the first end effector element. Brief Description of the Drawings The present invention will now be described by way of example with reference to the accompanying drawings. In the drawings: Figure 1 shows an example of surgical robotic system. Figure 2a and 2b shows a side view of an example bipolar electrosurgical instrument. Figure 3 shows an example bipolar electrosurgical instrument. Figure 4 shows a plan view of an example bipolar electrosurgical instrument approaching a tissue plane. Figure 5a to 5c is a head-on view of the distal end of various example end effectors. Figure 6 is a cross-section of an end effector comprising internal electrodes. Figure 7 is an exploded view of each end effector element and its components. Detailed Description Figure 1 shows a typical surgical robotic system. A surgical robot 100 comprises a base 102, an arm 104, and a surgical instrument 106. The base supports the robot. The arm extends between the base and the instrument. The arm is articulated by flexible joints 108 along its length. The surgical instrument is attached to the distal end of the robot arm. The surgical instrument comprises a shaft connected to an end effector 110 at its distal end for engaging in a medical procedure. A surgeon controls the surgical robot 100 via a remote surgeon console 112. The surgeon console comprises one or more surgeon input devices 114. These may take the form of a hand controller or foot pedal. The surgeon console also comprises a display 116. A control system 118 connects the surgeon console 112 to the surgical robot 100. The control system receives inputs from the surgeon input device(s) 114 and converts these to control signals to move the joints of the robot arm 104 and instrument 106. The control system sends these control signals to the robot, where the corresponding joints are driven accordingly. Figures 2a and 2b illustrate an example electrosurgical instrument 200 for use in performing electrosurgery. For example, instrument 200 may be a robotic electrosurgical instrument for performing bipolar electrosurgery. The instrument may comprise a base 201 for attaching the instrument to the robot arm. A shaft 202 extends between the base 201 and an end effector 210. The end effector 210 is connected to the distal end of the shaft. In this example, the end effector is connected to the distal end of the shaft via an articulation 203. The articulation is at the distal end of the shaft for articulating the end effector 210. The articulation may permit the end effector to move relative to the shaft. For example, the articulation may move the end effector 210 with at least two degrees of freedom. The end effector 210 comprises a pair of end effector elements. Figures 2a and 2b show the end effector 210 comprising a first end effector element 211 and a second end effector element 212. The first and second end effector elements 211, 212 oppose each other to form a pair of jaws. The first and second end effector elements may be rotatable in opposing rotational directions. The first and second end effector elements may be individually rotatable. For example, the articulation may allow each end effector element to be independently movable. The articulation may comprise joints which permit the end effector 210 to move relative to the shaft 202. The joints of the articulation may be driven by driving elements, such as cables. Each end effector element may be rotated around a joint by movement of a respective driving element. The instrument 200 described herein may conveniently be articulated using driving elements because the instrument does not need to comprise components which are pushed across joints of the articulation to perform a cutting operation. In an example, the articulation may comprise a pitch joint which rotates about a pitch axis. The pitch axis is perpendicular to the longitudinal axis of the shaft. The pitch joint permits the end effector to rotate about the pitch axis relative to the shaft. The articulation may further comprise a first and second yaw joint which each rotate about a respective yaw axis. The first and second yaw joint are distal of the pitch joint. The respective yaw axes of the first and second yaw joints may be parallel. Each yaw axis is perpendicularto both the longitudinal axis of the shaft and the pitch axis, when the shaft and articulation are in a straight configuration. The first yaw joint may be fast with the first end effector element and permit the first end effector element to rotate about the yaw axis of the first yaw joint, relative to the pitch joint and shaft. The second yaw joint may be fast with the second end effector element and permit the second end effector element to rotate about the yaw axis of the second yaw joint, relative to the pitch joint and the shaft. Each joint may be articulated using the driving elements described above. In one example, each end effector element may not be independently moveable. Movement of one end effector element may cause a corresponding movement of the other end effector element. For example, the instrument 200 may comprise a push-pull rod which may move both end effector elements between an open and closed configuration in the same movement. The end effector elements may be articulated using a rack and pinion which is actuated by means of a rod. In another example, one end effector element may be fixed in position with respect to the shaft. The other end effector element may be moveable with respect to the fixed end effector element. For example, the articulation may be in the form of a hinge which allows movement of the moveable end effector element relative to the fixed end effector element. Other means for actuation of the end effector elements are possible. The first end effector element 211 comprises electrodes 207 for performing electrosurgery. Specifically, the first end effector element 211 comprises electrodes 207 for performing a distal cutting operation, as will be described in more detail below. The end effector 210 may comprise additional electrodes 208 on either or both the end effector elements 211, 212 for performing other types of electrosurgical operations, as will be described in more detail later. The electrodes 207 (and optionally 208) are powered by electrosurgical cables (not shown) that extend through the shaft 202 to the end effector 210. The electrosurgical cables may be constrained to pass from the end effector 210 internally through the articulation to the shaft 202. In an example, the shaft may comprise internal spokes or rods extending from the proximal end of the shaft (e.g. the base 201) through the centre of the shaft to reach the end effector 210 or articulation 203 at the distal end of the shaft. The electrosurgical cables may be fixed to the spokes of the shaft to keep the electrosurgical cables taut. If the instrument comprises an articulation 203 driven by driving elements (e.g. cables), the driving elements may be connected to the spokes of the shaft at each end, along with the electrosurgical cables. Once the electrosurgical cables have passed through the shaft, they may be routed externally to the articulation or end effector. At one end, the electrosurgical cables are connected to electrodes in the end effector. Each electrode is connected to an electrosurgical cable. There may be a separate electrosurgical cable for each electrode. For example, if there are six electrodes in the end effector (three on each jaw), there may be six electrosurgical cables extending through the shaft to the end effector. By powering each electrode with a separate electrosurgical cable, the electrodes can be activated independently of each other. Alternatively, one electrosurgical cable may be used to power multiple electrodes. For example, if the electrodes are activated in pairs, only one electrosurgical cable per pair may be needed to power each electrode in the pair. This can reduce the number of cables extending through the shaft. At the other end, the electrosurgical cables are connected to an electrosurgical generator (not shown). The electrosurgical generator provides electrosurgical energy to power the electrodes via the electrosurgical cables. The electrosurgical generator generates electrosurgical signals for driving the instrument. The electrosurgical generator may generate different current waveforms. The electrosurgical generator may be capable of generating multiple different current waveforms to achieve different surgical effects. For example, electrosurgical generator may be configured to generate COAG and CUT waveforms. The COAG waveform consists of bursts of radio frequency, which when used at a low power setting causes a desiccation effect, and when used at a high-power setting causes a fulguration effect. The CUT waveform is a continuous waveform at higher voltage than COAG, which causes the tissue to be cut. The CUT waveform is typically a constant RMS voltage / current waveform once the desired voltage / current has been reached. The electrosurgical generator may be configured by a user to generate a particular waveform. The electrosurgical generator comprises any suitable means for configuring the waveforms to be generated. Preferably, the maximum voltage delivered by electrosurgical generator to the electrosurgical cables is in the range 250V to 480V. This can reduce the chances of sparking, as described herein. Figure 2a depicts the instrument 200 when the first and second end effector elements are in a closed configuration. When the end effector elements are in a closed configuration, the internal faces of each end effector element directly oppose each other. In the example in figure 2a, in the closed configuration there is a gap between the first and second end effector elements. Generally, having a gap between the first and second end effector elements when in a closed configuration provides space for tissue to be held between the jaws when the instrument is in use, e.g. when performing a sealing operation. The width of the gap may vary depending on the tissue being operated on. In some examples, there may be little or no gap between the first and second end effector elements when they are in a closed configuration. For example, the internal faces of the first and second end effector elements may abut (e.g., be in contact with) one another when in a closed configuration. Thus, the arrangement of the first and second end effector elements in the closed configuration may be variable. As mentioned above, electrodes 208 may not be present in the end effector 210. Other electrodes may be present that are not visible in figures 2a and 2b. It will be appreciated there may be other gaps between electrodes that are not visible from the side view shown in figure 2a. The details of the arrangements of the electrodes within each end effector element will be described later. Figure 2b depicts the instrument 200 of figure 2a when the jaws of the end effector are in an open configuration. When the first and second end effector elements are in an open configuration, an opening angle a is formed between the first and second end effector elements. The opening angle a of the end effector is the angle between the longitudinal axis of the first end effector element and the longitudinal axis of the second end effector element. In an example, the first and second end effector elements rotate relative to each other about a common joint. In this example, the longitudinal axis of the first end effector element and the longitudinal axis of the second end effector element intersect at the common joint. In the example where one end effector element is fixed with respect to the shaft and one end effector element is moveable with respect to the fixed end effector element about a joint, the longitudinal axis of the fixed end effector element may intersect the longitudinal axis of the moveable end effector element at the joint. The longitudinal axis of the first end effector element is an axis parallel to the length of the first end effector element. The longitudinal axis of the second end effector element is an axis parallel to the length of the second end effector element. In an example, when the first and second end effector elements are in a closed configuration (e.g. as shown in figure 2a), the longitudinal axes of the first and second end effector elements are parallel with respect to each other. In the example shown in figure 2a, when in a closed configuration, the longitudinal axes of the first and second end effector elements are parallel with the longitudinal axis of the shaft 202. The longitudinal axis of the shaft (e.g. the axis C in figure 2b) is an axis parallel to the length of the shaft. The longitudinal axis of the shaft extends through the middle of the shaft from the base 201 to the end effector 210. In figure 4, the longitudinal axis of the shaft, C, is parallel with the longitudinal axis of the first end effector element. As mentioned above, the electrodes 207 of the first end effector element shown in figure 2a and 2b are designed for performing a distal cutting operation. A distal cutting operation cuts tissue distally of the end effector. This is in contrast to a cutting operation in which tissue is cut between jaws of an end effector using electrodes positioned on an internal face of each jaw. Performing a cutting operation on tissue between the jaws of the end effector using electrodes on the internal face of each jaw will be referred to herein as an internal cutting operation, to distinguish it from a distal cutting operation. Some surgical instruments are designed to perform both a distal cutting operation with a monopolar hook, and an internal cutting operation with separate electrodes in which tissue is cut between the jaws of the end effector. Such instruments often have a safety mechanism in which the monopolar hook used for the distal cutting operation can only be activated when the jaws of the end effector are open. By only activating the monopolar hook when the jaws are open, the chances of sparking occurring between the monopolar hook and any conductive elements within the jaws is reduced because the distance between the active electrode and the conductive elements in the jaws has been increased. Requiring the end effector elements to be open during a distal cutting operation limits the uses of the surgical instrument, particularly when performing electrosurgery in a restricted area. Furthermore, performing a distal cutting operation with a monopolar hook as opposed to using bipolar electrosurgery limits the uses of the instrument. The inventors have devised a bipolar electrosurgical instrument in which a protruding electrode designed for performing a distal cutting operation may be safely activated when the end effector elements are in an open or closed configuration, provided that a secondary electrode also protrudes distally from the instrument to provide a safe return electrode for the current to return to during the distal cutting operation. Figure 3 shows an example bipolar electrosurgical instrument 200 for performing a distal cutting operation. In figure 3, the first and second end effector elements 211, 212 are in a closed configuration. At its distal end, the first end effector element 211 comprises a primary electrode 301 and at least one secondary electrode 302. In the example instrument depicted in figure 3, the first end effector element 211 comprises a pair of secondary electrodes 302, 303. However, the first end effector element need only have one secondary electrode to perform a distal cutting operation as will be described. Insulation 320 is shown between the primary electrode 201 and the pair of secondary electrodes 302, 303. The primary electrode 301 and the secondary electrode(s) 302, 303 protrude from the distal end of the first end effector element 211. In other words, the primary and the secondary electrode(s) extend beyond the main profile of the instrument. Thus, the primary and secondary electrodes are electrically exposed with respect to the rest of the instrument when the first and second end effector elements are in a closed configuration. The primary and the at least one secondary electrode protrude from the distal end of the first end effector element such that when the end effector approaches a plane (e.g. a plane of material, ora tissue plane) head-on, when the first and second end effector elements are in a closed configuration, the primary electrode and the at least one secondary electrode contact the plane before the remainder of the end effector. The primary electrode and at least one secondary electrode protrude in a direction parallel to the longitudinal axis of the first end effector element. When the first end effector element is aligned with the shaft, as shown in figure 3, the primary and the at least one secondary electrode protrude in a direction parallel to the longitudinal axis of the shaft. For example, figure 4 shows a plan view of the end effector 210 approaching a plane 400 head-on. In this example, the plane 400 is a tissue plane. In this example, the first end effector element 211 is aligned with the shaft 202 so that the longitudinal axis of the shaft 202 is parallel to the longitudinal axis of the first end effector element 211. The end effector elements 211 and 212 are in a closed configuration in figure 4 such that the longitudinal axis of the second end effector element 212 (not visible in figure 4) is also parallel with the longitudinal axis of the shaft. It can be seen from figure 4 that the primary electrode 301 and the secondary electrodes 302, 303 will contact the plane 400 before the rest of the end effector 210. In this example, the primary electrode 301 will contact the plane 400 before the secondary electrodes 302, 303, although this may not always be the case. Approaching the plane "head-on" refers to advancing the end effector towards the plane in an orientation in which the distal face of the end effector is directly opposing the plane. The end effector elements are in a closed configuration when approaching the plane head-on, so that the distal end of the first end effector element and the distal end of the second end effector element are adjacent. As mentioned above, each end effector element may comprise additional electrodes 208 positioned on an inner face of each end effector element, as will be explained later. When the first and second end effector elements are in a closed configuration, the primary and the at least one secondary electrode are exposed relative to the inner face of each end effector elements. The primary and secondary electrode(s) may be fixed in position in the first end effector element. So, they can protrude from the first end effector element regardless of the relative positions of the first and second end effector elements (i.e. regardless of whether they are in the open or closed configuration). The primary electrode is configured for performing a distal cutting operation. In an example, the primary electrode is a cut electrode. In other words, in an example, the primary electrode is a dedicated electrode for performing a distal cutting operation. In this example, the primary electrode is not used for performing a sealing operation. The primary electrode may be configured for performing a cutting operation only. The primary electrode may extend internally into the first end effector element, wherein a separate section of the electrode may be used for performing an internal cutting operation, as will be explained later. The secondary electrodes are not configured for performing a distal cutting operation. In an example, the secondary electrodes are return electrodes. The secondary electrodes may extend internally into the first end effector element, wherein they may be used for performing a sealing operation, as will be explained later. The electrosurgical instrument of the present disclosure can operate in a distal cutting mode for performing a distal cutting operation. When operating in a distal cutting mode, the electrosurgical generator generates a current waveform suitable for performing a distal cutting operation. The current waveform for the distal cut operation may be a CUT waveform. In another example, the current waveform for the distal cut operation may be a modified COAG waveform. For example, the current waveform may be a COAG waveform with a higher voltage than in a standard COAG waveform, so as to cause a cutting effect rather than a coagulation effect. In an example, the current waveform may be a sine wave with a max RMS voltage of 280V. The current waveform is transmitted via an electrosurgical cable to the primary electrode of the first end effector element. When operating in the distal cutting mode, the primary electrode is connected to a higher potential than the secondary electrodes such that current flows between the primary electrode and the at least one secondary electrode. Since both the primary electrode and the at least one secondary electrode protrude from the distal end of the first end effector element, current flows distally of the first end effector element between the primary and secondary electrodes. During distal cutting, only the primary electrode receives the current waveform generated by the electrosurgical generator. Any internal electrodes such as electrodes 208 do not receive the current waveform generated by the electrosurgical generator during a distal cutting operation. To perform the distal cutting operation, the end effector is advanced towards the tissue to be cut, with the primary electrode being electrically activated as described above. Because the primary electrode and at least one secondary protrude from the distal end of the first end effector, they make contact with the tissue before the rest of the instrument. Once contact is made between the tissue and the primary electrode, and the tissue and the at least one secondary electrode, current flows between the primary electrode and the at least one secondary electrode through the tissue. In some examples, the primary electrode is not electrically activated when the end effector is advanced towards the tissue. In some examples, the primary electrode may only be electrically activated when it makes contact with the tissue. This may improve the safety of the instrument, as it can reduce the chances of accidentally cutting tissue before the end effector reaches the desired cutting target. Since the current used is alternating, it will be understood that the direction of current flow between the primary electrode and the secondary electrodes will alternate. The tissue is cut in the region where the tissue contacts the primary electrode. By having at least one secondary electrode protrude from the distal end of the first end effector element in addition to the protruding primary electrode, current is encouraged to flow between the primary electrode and the at least one secondary electrode, ratherthan to other conductive elements in the instrument. Thus, the protrusion of the at least one secondary electrode acts to control the flow of current from the primary electrode such that the current is confined to the region in which cutting is desired. In an example, the at least one secondary electrode 302, 303 may be flush with the primary electrode. In other words, the at least one secondary electrode may protrude from the distal end of the first end effector element to the same point as the primary electrode such when the end effector approaches the tissue head-on, when the first and second end effector elements are in a closed configuration, the primary electrode and the at least one secondary electrode contact a tissue plane at the same time. In some cases, having the at least one secondary electrode flush with the primary electrode can be less effective at cutting through thicker tissue planes. In an example, the at least one secondary is not flush with the primary electrode. The at least one secondary electrode may not protrude from the distal end of the first end effector element as much as the primary electrode. For example, as shown in figure 4, the primary electrode may protrude further from the distal end of the first end effector element than the secondary electrode such that when the end effector approaches tissue head-on, when the first and second end effector elements are in a closed configuration, the primary electrode contacts the tissue before the secondary electrode. In other words, the distance in a direction parallel to the longitudinal axis of the first end effector element from the distal end of the first end effector to a distal end of the primary electrode isgreaterthan the distance in the same direction from the distal end of the first end effector to a distal end of the at least one secondary electrode. In an example, the primary electrode may protrude from the distal end of the first end effector element by between 0.5mm to 1.5mm. For example, the primary electrode may protrude from the distal end of the first end effector element by 0.6mm. The secondary electrode(s) may protrude from the distal end of the first end effector element by between 0.1mm to 1mm. For example, the secondary electrode(s) may protrude from the distal end of the first end effector element by 0.5mm. The primary electrode may protrude further from the distal end of the first end effector element by between 0.1mm to 1.4mm when compared to the protrusion of the secondary electrode from the distal end of the first end effector element. For example, the primary electrode may protrude by 1.2mm from the distal end of the first end effector element, and the secondary electrode may protrude by 0.5mm from the distal end of the first end effector element. By staggering the protrusion of the primary electrode with respect to the secondary electrode so that the primary electrode protrudes more than the secondary electrode, an improved cutting effect can be seen when cutting into thicker, tougher tissue. This is because when the primary electrode protrudes more than the secondary electrode, the primary electrode can be forced into the thicker tissue before the secondary electrode to increase contact with the tissue at the primary electrode. In particular, the thicker tissue can be bent around the edges of the primary electrode, where the local electric field is likely to be more concentrated. If the secondary electrode protruded by as much as the primary electrode, the secondary electrode would also get forced into the thicker tissue, which may cause an unintended cutting effect at the edges of the secondary electrode. To ensure that the tissue is cut in the desired area (i.e. where the primary electrode contacts the tissue) rather than at the secondary electrode, the primary electrode is shaped and sized so as to maximise the local electric field at the primary electrode compared with at the secondary electrode. A high local electric field value at the primary electrode results in high local power density at that electrode. The greater the local power density at the primary electrode, the more energy is transferred to the tissue at the point of contact with the primary electrode. This results in faster and more effective vaporisation of the water in the cells within the tissue, thus increasing the cutting performance in that area. An example value of a high local electric field is 1.8KV / mm. The primary electrode and the at least one secondary electrode are designed to contact tissue at a distal face of each electrode. Figures 5a to 5c show the end effector when viewed head-on, in which the distal faces of the primary and secondary electrodes can be seen. The first and second end effector elements 211, 212 are in a closed configuration in figures 5a to 5c. Figure 5a shows the distal end of the first end effector element 211 comprising a pair of secondary electrodes 302, 303. In figures 5b and 5c, the first end effector element comprises only one secondary electrode 302. As can be seen in the examples shown in figures 5a to 5c, the distal face of the primary electrode 301 is narrower than the distal face of each secondary electrode 302, 303 of the first end effector element. In other words, the width of the distal face of the primary electrode is less than the width of the distal face of the at least one secondary electrode. The arrows in figure 5a depict the width of the distal face of the primary electrode 301 and the secondary electrodes 302, 303. The width of the distal face of each electrode is measured in a lateral direction with respect to the length of the end effector. For example, in figure 4, the longitudinal direction is parallel to the longitudinal axis of the shaft, and the lateral direction is perpendicular to the longitudinal axis of the shaft when the instrument is viewed in a plan view. In other words, the width of the distal face of each electrode is measured in a direction perpendicular to the plane bisecting each of the first and second end effector elements when the first and second end effector elements are in a closed configuration. In the context of the electrodes positioned in the first end effector element as shown in figures 5a to 5c, the width of an electrode refers to the largest distance from one edge of the electrode to the opposite edge of that electrode in a direction perpendicular to the plane bisecting each of the first and second end effector elements 211, 212 when in a closed configuration. The plane bisecting each of the first and second end effector elements is labelled as plane B in figures 5a to 5c. A narrow primary electrode creates a more concentrated electric field around the primary electrode which increases the power density at the primary electrode, thus improving its cutting performance. In an example, the width of the distal face of the primary electrode may be in the range 0.05mm to 1.5mm. In another example, the width of the distal face of the primary electrode may be in the range 0.1mm to 1mm. For example, the width of the distal face of the primary electrode may be 0.5mm. In another example, the width of the distal face of the primary electrode may be 0.1mm. In an example, the width of the distal face of each secondary electrode may be in the range 0.1mm to 2mm. In another example, the width of the distal face of each secondary electrode may be in the range 0.5mm to 1.5mm. For example, the width of the distal face of each secondary electrode may be 0.7mm. Preferably, the ratio in total width of the distal face of the secondary electrode(s) compared to the distal face of the primary electrode is greater than 1. For example, the distal face of the secondary electrode may have a width of 2mm, and the distal face of the primary electrode may have a width of 1.2mm. The ratio of the widths is therefore 1.667 (2mm divided by 1.2mm). The ratio in total width of the distal face of the secondary electrode(s) to the width of the distal face of the primary electrode may be greater than 10. For example, for an instrument having two secondary electrodes such as that depicted in figure 5a, each secondary electrode may have a distal face of width 0.7mm, and the primary electrode may have a distal face of width 0.1mm. The ratio of widths is therefore 14 (1.4mm divided by 0.1mm). The primary electrode may have a pointed or squared off tip. For example, the primary electrode shown in figure 4 has a pointed tip. By having a pointed or squared off tip, as opposed to a rounded tip, the electric field at the primary electrode can be concentrated around the edges of the electrode, which increases the maximum local electric field at those points. This can improve the cutting performance at the primary electrode. The secondary electrode(s) are shaped and sized to reduce the chances of there being regions of high-power density at the secondary electrode(s). If a secondary electrode makes poor contact with the tissue being cut, it can cause an unintentional cutting effect at the secondary electrode. In addition, if the secondary electrode makes contact with the tissue at its edges (e.g. where the local electric field is relatively strong), this may cause an unintentional cutting effect. Preferably, the at least one secondary electrode contacts a larger surface area of the tissue than the primary electrode. So, preferably the surface area of the distal face of the at least one secondary electrode is greater than the surface area of the distal face of the primary electrode. For example, in each of figures 5a to 5c, the distal faces of the secondary electrode(s) have a greater surface area than the distal face of the primary electrode. They are also wider than the primary electrode, as mentioned above. Both of these factors contribute to reducing the power density at the secondary electrodes and reducing the chances of there being regions at the secondary electrodes with a more concentrated electric field than the primary electrode. The secondary electrode(s) may also have rounded edges to lower the maximum local electric field at the edges of the secondary electrodes. Thus, the secondary electrode(s) is shaped and sized to as to reduce the chances of a cutting effect at the secondary electrodes when operating in a distal cutting operation. Preferably, the primary electrode has an exposed surface area that is less than an exposed surface area of the at least one secondary electrode. The exposed surface area of an electrode is the surface area of the electrode that is not covered by insulation. In other words, the exposed surface area of an electrode is the surface area of the electrode that is conductive (e.g., a conductive surface area). In an example, the exposed surface area of the primary electrode is in the range 0.5mm2 to 3mm2. In an example, the exposed surface area of the primary electrode is in the range 1mm2 to 2mm2. For example, the exposed surface area of the primary electrode may be 1.8mm2. In an example, the exposed surface area of the at least one secondary electrode is in the range 5mm2 to 15mm2. In an example, the exposed surface area of the at least one secondary electrode is in the range 7mm2 to 10mm2 For example, the exposed surface area of each secondary electrode may be 7.4mm2. The ratio of the exposed surface area of the primary electrode to the exposed surface area of the secondary electrode (or total exposed surface area of the secondary electrodes combined) affects the power density of the primary electrode. The ratio between the power density of the secondary electrode(s) <5Psecondary and the power density of the primary electrode <5Pprimary can be approximated as: 3Pprimary _ ^secondary ^Psecondary ^primary Where Asecondary is the total exposed surface area of the secondary electrode(s) and Aprimary is the exposed surface area of the primary electrode. The above approximation assumes a constant tissue conductivity. Thus, if there is a large ratio between the exposed surface area of the primary electrode and the exposed surface area of the secondary electrode(s), the power density of the primary electrode can be controlled to be higher than the power density of the secondary electrode(s). If the ratio of the exposed surface areas is too small, an unintended cutting effect can occur at the secondary electrode(s) instead of the primary electrode. Preferably, the ratio in total exposed surface area of the secondary electrode(s) compared to the exposed surface area of the primary electrode is greater than 1. For example, the total exposed surface area of the secondary electrode may be 5mm2, and the exposed surface area of the primary electrode may be 0.8mm2. The ratio of the exposed surface area is therefore 6.25. The ratio in total exposed surface area of the secondary electrode(s) to the exposed surface area of the primary electrode may be greater than 10. For example, for an instrument having two secondary electrodes such as that depicted in figure 5a, each secondary electrode may have an exposed surface area of 15mm2 (and so a total exposed surface area of 30mm2), and the primary electrode may have an exposed surface area of 2mm2. The ratio of exposed surface area is therefore 15. Sparking The primary electrode and secondary electrodes may be configured such that sparking between the electrodes is avoided. Sparking occurs when the path of least resistance for the current is via an air gap between electrodes, rather than through the tissue. This can happen if the tissue impedance increases, or if the electrodes are positioned too close to each other. Sparking depends on the dielectric strength of air, which is expressed in terms of volts per distance. Whilst increasing the voltage applied at the primary electrode improves cutting performance, it also increases the chances of sparking between electrodes which can burn the surrounding tissue. So, preferably, a high local electric field at the primary electrode is balanced with a low mean electric field between the primary electrode and the secondary electrode(s) to avoid sparking. The mean electric field (V / m) is the average electric field between an electrode pair. The lower the mean electric field between an electrode pair, the lower the chances of sparking. This metric depends on the distance between the electrodes and the voltage applied. If the electric field between two electrodes approaches 3 kV / mm, dielectric breakdown of air is expected, and sparks may appear. An example safe value forthe mean direct electric field is 0.7 kV / mm. To decrease the chances of sparking between the secondary electrode and primary electrode, the primary electrode may be laterally offset from the secondary electrode(s). In other words, the primary electrode may be spaced from the secondary electrode(s) in a direction parallel to a plane separating the first and second end effector elements when the first and second end effector elements are in a closed configuration (e.g. plane A in figure 5a). For example, in the head-on view of the distal end of the end effector in figure 5a, the secondary electrodes 302, 303 are laterally offset from the primary electrode 301, as indicated by the arrow I. The secondary electrode 302 of figure 5c is also laterally offset from the primary electrode 301. The lateral direction is a direction perpendicular to the plane bisecting each of the first and second end effector elements when they are in a closed configuration (e.g. plane B in figure 5a). Each secondary electrode may be laterally offset from the primary electrode by between 0.1mm to 2.5mm. In another example, each secondary electrode may be laterally offset from the primary electrode by between 0.5mm to 2mm. In another example, the lateral offset be 1.5mm. In other words, there may be a gap of 1.5mm between the closest edge of the primary electrode and the closest edge of the secondary electrode in a direction perpendicular to the plane bisecting each of the first and second end effector elements when the first and second end effector elements are in a closed configuration. To decrease the chances of sparking between the secondary electrode and primary electrode, the primary electrode may be vertically offset from the secondary electrode(s). In other words, the primary electrode may be spaced from the secondary electrode(s) in a direction parallel to the plane bisecting each of the first and second end effector elements when they are in a closed configuration (e.g. plane B in figure 5b). For example, in the head-on view of the example instrument shown in figure 5b and 5c, the primary electrode 301 is vertically offset from the secondary electrode 302, as indicated by the arrow v. The electrodes are offset vertically such that, when the first and second end effector elements are in a closed configuration, a plane parallel to the longitudinal axis of the first end effector element does not intersect both the primary electrode and the at least one secondary electrode (e.g. plane D in figure 5b and 5c). In an example, the vertical offset between the primary electrode and the secondary electrode(s) is in the range 0.1mm to 2mm. In another example, the vertical offset is in the range 0.5mm to 1mm. For example, the vertical offset may be 0.7mm. By increasing the distance between the primary electrode and the secondary electrodes(s) using vertical and / or lateral offsetting of the electrodes, higher voltages can be used at the primary electrode with reduced chances of sparking. In addition, spacing the electrodes from one another can improve the contact with each electrode on the tissue compared with when they are very close to each other. For example, if the primary electrode protrudes further than the secondary electrode, the tissue may bend around the primary electrode. If the secondary electrode is positioned too close to the primary electrode, the distal face of the secondary electrode may not make full contact with the tissue due to a tenting effect around the primary electrode. By spacing the secondary electrodes from the primary electrode, better contact can be achieved at the secondary electrode, even when cutting into thicker tissue. Multiple operational modes The bipolar electrosurgical instrument described above is configured to perform a distal cutting operation in which current passes between a primary electrode and at least one secondary electrode which each protrude distally from the first end effector element 311. As mentioned above, the end effector 210 may comprise additional electrodes 208 for performing other kinds of operations, as will now be described. A typical bipolar electrosurgical instrument is designed for performing an internal cutting operation in which tissue is clamped between jaws of the instrument. In such an instrument, electrodes are positioned on the internal face of each jaw. Electricity is passed between the internal electrodes within the jaws so that the tissue clamped between the jaws can be cut. Usually, one jaw comprises an electrode designed for performing a cutting action (referred to as a cut electrode) and the opposite jaw comprises at least one electrode designed to act as the return electrode. Often the blood vessels surrounding the cutting site need to be sealed to prevent blood flow from interfering with an internal cutting operation. Some bipolar electrosurgical instruments will therefore also include opposing electrodes on each jaw that are designed for sealing the vessels around the internal cutting site, prior to performing an internal cutting operation. Such electrodes are sometimes referred to as seal electrodes. Either or both of the pairs of electrodes used for the sealing operation may also be used as the return electrode(s) in the internal cutting operation. The bipolar electrosurgical instrument described herein may be configured to operate in a variety of internal cutting modes and in a sealing mode, in addition to the above-described distal cutting mode. This can avoid the need for the surgeon to change instruments part-way through an operation. In addition, by using the same electrodes in the end effector in different ways to provide the different operating modes, the utility of the bipolar instrument can be increased without increasing the size of the end effector. Thus, the diameter of the instrument can be kept small. Figure 6 shows a cross-sectional view of the end effector 210 of the bipolar electrosurgical instrument that is suitable for operating in a variety of modes. The distal end of the end effector cannot be seen in figure 6. The first end effector element 211 comprises an internal face which opposes the second end effector element 212. The second end effector element 212 comprises an internal face which opposes the first end effector element. When in a closed configuration, the internal faces of the first and second end effector element are directly facing each other, as shown in figure 6. The first end effector element 211 may comprise a first pair of lateral electrodes, 616, 617. The at least one secondary electrode 302 described above with reference to the distal cutting operation may be part of the first pair of lateral electrodes. That is, the at least one secondary electrode 302 described above may be one of the lateral electrodes in the first pair of lateral electrodes. For example, the secondary electrodes 302, 303 shown in figure 3 may be the same electrodes as lateral electrodes 616, 617 in the first pair of lateral electrodes. If this is the case, the secondary electrodes 302, 303 may extend inwardly from their protruding end to form the lateral electrodes 616, 617 shown in figure 6. The first pair of lateral electrodes, 616, 617, extend longitudinally along the internal face of the first end effector element 211. For example, in figure 2a, an electrode 208 can be seen extending longitudinally along the internal face of the first end effector element 211. The electrode 208 on the first end effector element 211 visible in figure 2a may be one of the lateral electrodes of the first pair of lateral electrodes. The second end effector element 212 comprises a second pair of lateral electrodes, 619, 620. The second pair of lateral electrodes extend longitudinally along the internal face of the second end effector element. For example, in figure 2a, an electrode 208 can be seen extending longitudinally along the internal face of the second end effector element 212. The electrode 208 on the second end effector element 212 visible in figure 2a may be one of the lateral electrodes of the second pair of lateral electrodes. As another example, figure 7 shows electrodes the first and second pair of lateral electrodes 616, 617 and 619, 620 extending longitudinally along the respective first and second end effector elements 211, 212. The first and second pair of lateral electrodes oppose each other when the first and second end effector elements are in a closed configuration. An electrosurgical cable (not shown) connects each of the lateral electrodes in each pair to the electrosurgical generator described above. The same electrosurgical cable may connect to both lateral electrodes within each pair of lateral electrodes. The first and second pair of lateral electrodes are configured for performing a sealing operation. In an example, the first and second pair of lateral electrodes are a first and second pair of seal electrodes. In other words, in an example, the first and second pair of lateral electrodes are dedicated electrodes for performing a sealing operation. In this example, the first and second pair of lateral electrodes are not used for performing a cutting operation. The first and second pair of lateral electrodes may be configured for performing a sealing operation only. The instrument may be operable in a sealing mode in which the first pair of lateral electrodes is connected to a different potential than the second pair of lateral electrodes such that current flows between the first pair of lateral electrodes and the second pair of lateral electrodes. To operate in the sealing mode, the electrosurgical generator generates a current waveform suitable for performing a sealing operation (e.g., a COAG waveform) and transmits this to the lateral electrodes via the electrosurgical cables. The tissue that is in contact with the lateral electrodes can then be sealed by the lateral electrodes. The first pair of lateral electrodes may be electrically exposed parts of the same conductive component. The one conductive component that forms both lateral electrodes may have a U-shaped cross-section. The tops of the U-shape may be the two exposed ends that form each lateral electrode. For example, the lateral electrodes 616, 617 visible in figure 7 are formed from one conductive element with a U-shaped cross-section. In the same way, the second pair of lateral electrodes 619, 620 may be electrically exposed parts of the same conductive component. For example, the lateral electrodes 619, 620 in figure 7 are formed from one conductive element with a U-shaped cross-section. Alternatively, each lateral electrode in the first and / or second pair of lateral electrodes may be formed form a separate conductive component, which may or may not be electrically connected together (for example, if each pair is activated by one electrosurgical cable). As mentioned above, the at least one secondary electrode may be one of the lateral electrodes in the first pair of lateral electrodes. The lateral electrodes in the first pair of lateral electrodes may be formed from the same conductive component as the secondary electrode(s) for performing a distal cutting operation. For example, figure 7 shows the lateral electrodes 616, 617 of the first end effector element 711 being formed from the same conductive component as the first pair of secondary electrodes 302, 303. In an example, the lateral electrodes 616, 617 and the secondary electrodes 302, 303 may be the same electrodes. In another example, the secondary electrodes 302, 303 may be formed from a separate conductive element which may or may not be electrically connected to the lateral electrodes 616, 617. The width of the lateral face of the lateral electrodes in the first pair of lateral electrodes may be greater than the width of the distal face of the secondary electrode. In an example, the width of the lateral face of the lateral electrodes 616, 617 is 1mm. The width of the distal face of the secondary electrode(s) 302, 303 may be 0.7mm - i.e. narrower than the width of the lateral face of the first pair of lateral electrodes. The first end effector element 211 may also comprise a first internal electrode 615 between the first pair of lateral electrodes 616, 617. That is, there is a lateral electrode either side of the first internal middle electrode 615. The first internal middle electrode 615 extends longitudinally along the internal face of the first end effector element. The first internal middle electrode cannot be seen in figure 2a because it is obscured by one of the lateral electrodes of the first pair of lateral electrodes. The first internal middle electrode 615 may be a part of the primary electrode 301. For example, the first internal middle electrode 615 may be formed from the same conductive element as the primary electrode 301. In other words, the first internal middle electrode and the primary electrode may be electrically exposed parts of the same conductive component. For example, in figure 7, the first internal middle electrode 615 is formed from the same conductive component as the primary electrode 301, which protrudes from the distal end of the end effector when the elements in figure 7 are assembled together. The first internal middle electrode may be electrically activated separately to the primary electrode, for example via a different electrosurgical cable. In another example, the primary electrode may be a separate electrode to the first internal middle electrode. The instrument may be operable in an internal cutting mode in which the first internal middle electrode is connected to a higher potential than at least one lateral electrode in the first or second pair of lateral electrodes such that current flows between the first internal middle electrode and that at least one lateral electrode. The current flow is internal to the end effector, rather than distally of the end effector. When operating in the internal cutting mode, the electrosurgical generator generates a current waveform suitable for performing a cutting operation (e.g., a CUT waveform) and transmits this to the first internal middle electrode via an electrosurgical cable. The tissue that is in contact with the first internal middle electrode can be cut by the first internal middle electrode. Any of the lateral electrodes in the first or second pair of lateral electrodes can act as the return electrode for the current when operating in an internal cutting mode. The second end effector element 212 may comprise a second internal middle electrode 618. The second internal middle electrode 618 is positioned between the second pair of lateral electrodes 619, 620. That is, there is a lateral electrode either side of the second internal middle electrode 618. The second internal middle electrode 618 extends longitudinally along the internal face of the second end effector element. The instrument may be operable in an internal cutting mode in which the second internal middle electrode is connected to a higher potential than at least one lateral electrode in the first or second pairs of lateral electrodes such that current flows between the second internal middle electrode and that at least one lateral electrode. The current flow is internal to the end effector, rather than distally of the end effector. When operating in the internal cutting mode, the electrosurgical generator generates a current waveform suitable for performing a cutting operation (e.g., a CUT waveform) and transmits this to the second internal middle electrode via an electrosurgical cable. The tissue that is in contact with the second internal middle electrode can be cut by the second internal middle electrode. Any of the lateral electrodes in the first or second pair of lateral electrodes can act as the return electrode for the current when operating in an internal cutting mode. The instrument is thus operable in two internal cutting modes in which either the first or second internal middle electrode can be electrically activated. To provide an instrument that can operate in an internal cutting mode, a sealing mode, and a distal cutting mode, only one internal middle electrode out of the first or second internal middle electrodes is required, in addition to the primary electrode, the at least one secondary electrode, the first pair of lateral electrodes (which may include the at least one secondary electrode), and the second pair of lateral electrodes. When operating in either internal cutting modes described above, the primary electrode is not electrically active. This can avoid the primary electrode causing an undesired cutting effect at the distal end of the instrument. When operating in either internal cutting modes, the secondary electrodes may not be electrically active. The cutting performance of the instrument when operating in an internal cutting mode can be improved by electrically activating both the first and second middle electrodes at the same time. This is because the tissue being cut is in electrical contact with an internal middle electrode on both sides of the tissue, as opposed to only one. This increased electrical contact on the tissue improves the transfer of energy to the tissue. This may result in faster vaporisation of the water in the cells within the tissue and a more homogenous cutting effect. Preferably, the instrument is operable in an internal double-cutting mode in which the first and second internal middle electrodes are connected to a higher potential than at least one lateral electrode in the first or second pairs of lateral electrodes such that current flows between the first and second internal middle electrodes and that at least one lateral electrode. As with either internal cutting modes above, any of the lateral electrodes in the first or second pair of lateral electrodes can act as the return electrode for the current when operating in the internal double-cutting mode. In some examples, it may be preferable for 5 one or both of the lateral electrodes of the second pair of lateral electrodes to act as the return electrode(s) when operating in the internal double-cut mode. For example, if the secondary electrodes are formed from the same conductive piece of material as the first pair of lateral electrodes, it may be preferable to use the lateral electrodes of the opposing end effector element (i.e. the second pair of lateral electrodes) as the return electrodes when 10 operating in the internal double-cut mode, to reduce the chances of any unintended warming effect at the secondary electrodes at the distal end of the instrument. When operating in the internal double-cutting mode described above, the primary electrode is not electrically active. This can avoid the primary electrode causing an undesired cutting effect at the distal end of the instrument. 15 When operating in internal double-cutting mode, the secondary electrodes may not be electrically active. Thus, the instrument described herein and depicted in figure 6 can operate in the following operational modes: a distal cutting mode, an internal cutting mode, an internal doublecutting mode, and a sealing mode. A summary of these modes is shown in table 1. Operational mode Active electrode Return electrode Distal cutting mode Primary electrode of the first end effector element At least one secondary electrode of the first end effector element Internal cutting mode First internal middle electrode or second internal middle electrode Any lateral electrode of the first or second pairs of lateral electrodes Internal double cutting mode First internal middle electrode and second internal middle electrode Any lateral electrode of the first or second pairs of lateral electrodes (but preferably at least one lateral electrode of the second pair of lateral electrodes). Sealing mode A lateral electrode in each of the first and second pairs of lateral electrodes The opposing lateral electrode in each of the first and second pairs of lateral electrodes Table 1 Figure 7 depicts an exploded view of the instrument 200, showing its possible construction. As described above, in figure 7, the primary electrode 301 is part of the first middle electrode 615, and the secondary electrodes 302, 303 are part of the first pair of lateral electrodes. In this example, each end effector element comprises a casing, 740, in which the respective electrodes (615 - 620) are housed. The secondary electrodes 302, 303 and the primary electrode 301 protrude from the casing and are the only electrically exposed electrodes once the parts shown in figure 7 are assembled together and the first and second end effector elements are in a closed configuration. Each end effector element also comprises insulation 710 surrounding the electrodes. Each end effector element 211, 212 may be hinged together at the articulation (not shown) to form the jaws of the end effector 210. The first and second internal middle electrodes are designed with the same considerations as the primary electrode in relation to producing regions of high power density at the first and second internal middle electrodes. The same design considerations also apply to the lateral electrodes of each pair of lateral electrodes compared with the secondary electrodes. For example, the first and second internal middle electrodes are preferably narrow and with a reduced exposed surface area compared with each lateral electrode so that regions of high power density are concentrated at the first and second internal middle electrodes. Overdose arrangement The first and second internal middle electrodes may be arranged such that when the first and second end effector elements are in a closed configuration, a point on the second internal middle electrode 618 (e.g. point 'X' in figure 6) is closer to both lateral electrodes of first pair of lateral electrodes 616, 617 than the second pair of lateral electrodes 619, 620. This can reduce the chances of sparking because there is a greater distance between the second internal middle electrode and the second pair of lateral electrodes, so current is less likely to jump across the air gap between them. The shortest path for current to flow between the second internal middle electrode is through the tissue to the lateral electrodes 616, 617 on the opposite jaw. The second internal middle electrode 618 may be shaped and sized such that when the first and second end effector elements are in a closed configuration, a plane intersects the second internal middle electrode and both lateral electrodes of the first end effector element 211. For example, the second internal middle electrode may extend from the second end effector element to intersect with a plane formed by the lateral faces of the lateral electrodes of the first pair of lateral electrodes. In figure 6, the plane formed by the lateral faces of the first pair of lateral electrodes of the first end effector element is labelled O. As shown in figure 6, the second internal middle electrode 618 extends from the second end effector element 612 to intersect the plane O. Thus, the second internal middle electrodes is shaped and sized so as to impinge on the volume defined by and enclosed by the lateral electrodes of first end effector element, when the end effector elements are in a closed configuration. The extension of the second internal middle electrode over the plane formed by the lateral faces of the first pair of lateral electrodes is termed "over-close". An effect of the over-close of the second internal middle electrode is that the tissue being cut is stretched around the second internal middle electrode when the end effector elements are in a closed configuration. This stretching of the tissue can encourage the tissue to tear along the length of the second internal middle electrode, thus improving the cutting performance of the instrument. Furthermore, the first end effector element 211 may form a gutter shape in which to receive the second internal middle electrode, as shown in figure 6. In other words, the first end effector element 211 may comprise a recess around the first internal middle electrode 615. So, the first internal middle electrode may be recessed in the internal face of the first end effector element. The gutter shape or recess provides space for the tissue to bend around the second internal middle electrode with over-close. The depth of the gutter and the protrusion of the second internal middle electrode from the base of the gutter varies the vertical gap between the first and second internal middle electrodes when the first and second end effector elements are in the closed configuration. In other words, the depth of the gutter and the protrusion of the first internal middle electrode from the base of the gutter dictates the minimum vertical gap between the first and second end effector elements. The vertical gap between the first and second internal middle electrodes is the gap between the first and second internal middle electrodes in a direction parallel to the plane bisecting each of the first and second end effector elements when the first and second end effector elements are in a closed configuration (e.g. plane B). If the gap is too large, then electrical contact may only be made at one of the internal middle electrodes, thus degrading the cutting performance of the instrument. On the other hand, if there is little or no gap then the tissue cannot be compressed when sealing is performed. In one example, the vertical gap lies in the range 0.01mm to 0.5mm. In another example, the vertical gap lies in the range 0.05mm to 0.3mm. In another example, the vertical gap lies in the range 0.1mm to 0.2mm. In an example, the vertical gap between the first and second internal middle electrodes is 0.15mm. Furthermore, when the primary electrode is formed from the same conductive element as the first internal middle electrode, by having the first internal middle electrode positioned in the recess of the first end effector element, the conductive component that forms the primary electrode and the first internal middle electrode is distanced from the electrodes on the second end effector element when the first and second end effector elements are in a closed configuration. This can help reduce the chances of sparking occurring between the primary electrode and the electrodes on the second end effector element when operating in a distal cutting mode. 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

1. A robotic electrosurgical instrument comprising:a shaft; andan end effector connected to a distal end of the shaft, the end effector comprising opposing first and second end effector elements,the first end effector element comprising a primary electrode and at least one secondary electrode,wherein the primary electrode and the at least one secondary electrode protrude from a distal end of the first end effector element such that when the end effector approaches a plane head-on when the first and second end effector elements are in a closed configuration, the primary electrode and the at least one secondary electrode contact the plane before the remainder of the end effector.

2. The robotic electrosurgical instrument of claim 1, wherein the primary electrode protrudes further from the distal end of the first end effector element than the secondary electrode such that when the end effector approaches the plane head-on when the first and second end effector elements are in a closed configuration, the primary electrode contacts the plane before the secondary electrode.

3. The robotic electrosurgical instrument of claim 1 or 2, wherein the primary electrode is spaced from the at least one secondary electrode such that when the first and second end effector elements are in a closed configuration, a plane parallel to the longitudinal axis of the first end effector element does not intersect both the primary electrode and the at least one secondary electrode.

4. The robotic electrosurgical instrument of any preceding claim, wherein the instrument is operable in a distal cutting mode in which the primary electrode is connected to a higher potential than the at least one secondary electrode such that current flows between the primary electrode and the at least one secondary electrode distally of the end effector.

5. The robotic electrosurgical instrument of any preceding claim, wherein the primary electrode and the at least one secondary electrode are fixed in position with respect to the first end effector element.

6. The robotic electrosurgical instrument of any preceding claim, wherein the end effector is connected to the distal end of the shaft via an articulation for articulating the end effector, the articulation permitting each end effector element to move relative to the shaft between the closed configuration and an open configuration.

7. The robotic electrosurgical instrument of any preceding claim wherein the primary electrode and the at least one secondary electrode protrude from the distal end of the first end effector element in a direction parallel to the longitudinal axis of the first end effector element.

8. The robotic electrosurgical instrument of any preceding claim, wherein the primary electrode and the at least one secondary electrode each comprise a distal face for contacting the plane head-on, the distal face of the at least one secondary electrode having a surface area that is greater than a surface area of the distal face of the primary electrode.

9. The robotic electrosurgical instrument of any preceding claim, wherein the primary electrode has an exposed surface area that is less than an exposed surface area of the at least one secondary electrode.

10. The robotic electrosurgical instrument of claim 9, wherein the ratio of the exposed surface area of the at least one secondary electrode to the exposed surface area of the primary electrode is greater than 5.

11. The robotic electrosurgical instrument of any preceding claim, wherein the primary electrode and the at least one secondary electrode each comprise a distal face for contacting the plane head-on, the distal face of the at least one secondary electrode having a width in a lateral direction that is greater than a width in the same direction of the distal face of the primary electrode.

12. The robotic electrosurgicaI instrument of any preceding claim, wherein the first end effector element comprises a first pair of secondary electrodes including the at least one secondary electrode, the primary electrode being between the first pair of secondary electrodes, wherein the first pair of secondary electrodes protrude from the distal end of the end effector such that when the end effector approaches the plane head-on, when the first and second end effector elements are in a closed configuration, the primary electrode and the pair of secondary electrodes contact the plane before the remainder of the end effector.

13. The robotic electrosurgicaI instrument of any preceding claim, wherein the first end effector element comprises an internal face opposing the second end effector element, and the first end effector element comprises a first pair of lateral electrodes each extending longitudinally along the internal face of the first end effector element.

14. The robotic electrosurgical instrument of any preceding claim, wherein the second end effector element comprises an internal face opposing the first end effector element, and the second end effector element comprises a second pair of lateral electrodes each extending longitudinally along the internal face of the second end effector element.

15. The robotic electrosurgical instrument of claim 14 when dependent on claim 13, wherein the instrument is operable in a sealing mode in which the first pair of lateral electrodes is connected to a different potential than the second pair of lateral electrodes such that current flows between the first pair of lateral electrodes and the second pair of lateral electrodes.

16. The robotic electrosurgical instrument of claim 15, wherein the first end effector element comprises a first internal middle electrode between the first pair of lateral electrodes, the first internal middle electrode extending longitudinally along the internal face of the first end effector element.

17. The robotic electrosurgicaI instrument of claim 16, wherein the instrument is operable in an internal cutting mode in which the first internal middle electrode is connected to a higher potential than at least one lateral electrode in the first or second pairs of lateral electrodes such that current flows between the primary electrode and that at least one lateral electrode internally of the end effector.

18. The robotic electrosurgical instrument of any of claims 15 to 17, wherein the second end effector element comprises a second internal middle electrode between the second pair of lateral electrodes, the second internal middle electrode extending longitudinally along the internal face of the second end effector element.

19. The robotic electrosurgical instrument of claim 18, wherein the instrument is operable in an internal cutting mode in which the second internal middle electrode is connected to a higher potential than at least one lateral electrode in the first or second pairs of lateral electrodes such that current flows between the second internal middle electrode and that at least one lateral electrode internally of the end effector.

20. The robotic electrosurgical instrument of claim 19 when dependent on claim 16, wherein the instrument is operable in an internal double-cutting mode in which the first and second internal middle electrodes are connected to a higher potential than at least one lateral electrode in the first or second pairs of lateral electrodes such that current flows between the first and second internal middle electrodes and that at least one lateral electrode internally of the end effector.

21. The robotic electrosurgical instrument of any of claims 16 or 20, wherein the first internal middle electrode is part of the primary electrode.

22. The robotic electrosurgical instrument of any of claims 17,19 or 20, wherein, when operable in the internal cutting mode or internal double-cutting mode, the primary electrode is not electrically activated.

23. The robotic electrosurgical instrument of any of claims 17,19, 20, or 21, wherein, when operable in the internal cutting mode or internal double-cutting mode, the secondary electrodes are not electrically activated.5 24. The robotic electrosurgical instrument of any of claims 16 to 23, wherein the firstinternal middle electrode is recessed in the internal face of the first end effector element.

25. The robotic electrosurgical instrument of claim 13 when dependent on claim 12, wherein the first pair of lateral electrodes are part of the first pair of secondary electrodes.04 10 24AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS:-CLAIMS1. A robotic electrosurgical instrument comprising:a shaft; andan end effector connected to a distal end of the shaft, the end effector comprising opposing first and second end effector elements,the first end effector element comprising a primary electrode and a first pair of secondary electrodes, the primary electrode being between the first pair of secondary electrodes,wherein the primary electrode and the first pair of secondary electrodes protrude from a distal end of the first end effector element such that when the end effector approaches a plane head-on when the first and second end effector elements are in a closed configuration, the primary electrode and the first pair of secondary electrodes contact the plane before the remainder of the end effector.

2. The robotic electrosurgical instrument of claim 1, wherein the primary electrode protrudes further from the distal end of the first end effector element than the first pair of secondary electrodes such that when the end effector approaches the plane head-on when the first and second end effector elements are in a closed configuration, the primary electrode contacts the plane before the first pair of secondary electrodes.

3. The robotic electrosurgical instrument of claim 1 or 2, wherein the primary electrode is spaced from at least one secondary electrode of the first pair of secondary electrodes such that when the first and second end effector elements are in a closed configuration, a plane parallel to the longitudinal axis of the first end effector element does not intersect both the primary electrode and the at least one secondary electrode.

4. The robotic electrosurgical instrument of any preceding claim, wherein the instrument is operable in a distal cutting mode in which the primary electrode is connected to a higher potential than at least one secondary electrode of the first pair of secondary electrodes such that current flows between the primary electrode and the at least one secondary electrode distally of the end effector.04 10 245. The robotic electrosurgical instrument of any preceding claim, wherein the primary electrode and at least one secondary electrode of the first pair of secondary electrodes are fixed in position with respect to the first end effector element.

6. The robotic electrosurgical instrument of any preceding claim, wherein the end effector is connected to the distal end of the shaft via an articulation for articulating the end effector, the articulation permitting each end effector element to move relative to the shaft between the closed configuration and an open configuration.

7. The robotic electrosurgical instrument of any preceding claim wherein the primary electrode and at least one secondary electrode of the first pair of secondary electrodes protrude from the distal end of the first end effector element in a direction parallel to the longitudinal axis of the first end effector element.

8. The robotic electrosurgical instrument of any preceding claim, wherein the primary electrode and at least one secondary electrode of the first pair of secondary electrodes each comprise a distal face for contacting the plane head-on, the distal face of each secondary electrode of the first pair of secondary electrodes having a surface area that is greater than a surface area of the distal face of the primary electrode.

9. The robotic electrosurgical instrument of any preceding claim, wherein the primary electrode has an exposed surface area that is less than an exposed surface area of at least one secondary electrode of the first pair of secondary electrodes.

10. The robotic electrosurgical instrument of claim 9, wherein the ratio of the exposed surface area of the at least one secondary electrode to the exposed surface area of the primary electrode is greater than 5.

11. The robotic electrosurgical instrument of any preceding claim, wherein the primary electrode and at least one secondary electrode of the first pair of secondary electrodes each comprise a distal face for contacting the plane head-on, the distal face of the at least one04 10 24secondary electrode having a width in a lateral direction that is greater than a width in the same direction of the distal face of the primary electrode.

12. The robotic electrosurgical instrument of any preceding claim, wherein the first end effector element comprises an internal face opposing the second end effector element, and the first end effector element comprises a first pair of lateral electrodes each extending longitudinally along the internal face of the first end effector element.

13. The robotic electrosurgical instrument of any preceding claim, wherein the second end effector element comprises an internal face opposing the first end effector element, and the second end effector element comprises a second pair of lateral electrodes each extending longitudinally along the internal face of the second end effector element.

14. The robotic electrosurgical instrument of claim 13 when dependent on claim 12, wherein the instrument is operable in a sealing mode in which the first pair of lateral electrodes is connected to a different potential than the second pair of lateral electrodes such that current flows between the first pair of lateral electrodes and the second pair of lateral electrodes.

15. The robotic electrosurgical instrument of claim 14, wherein the first end effector element comprises a first internal middle electrode between the first pair of lateral electrodes, the first internal middle electrode extending longitudinally along the internal face of the first end effector element.

16. The robotic electrosurgical instrument of claim 15, wherein the instrument is operable in an internal cutting mode in which the first internal middle electrode is connected to a higher potential than at least one lateral electrode in the first or second pairs of lateral electrodes such that current flows between the primary electrode and that at least one lateral electrode internally of the end effector.

17. The robotic electrosurgical instrument of any of claims 14 to 16, wherein the second end effector element comprises a second internal middle electrode between the second pair of04 10 24lateral electrodes, the second internal middle electrode extending longitudinally along the internal face of the second end effector element.

18. The robotic electrosurgical instrument of claim 17, wherein the instrument is operable in an internal cutting mode in which the second internal middle electrode is connected to a higher potential than at least one lateral electrode in the first or second pairs of lateral electrodes such that current flows between the second internal middle electrode and that at least one lateral electrode internally of the end effector.

19. The robotic electrosurgical instrument of claim 18 when dependent on claim 15, wherein the instrument is operable in an internal double-cutting mode in which the first and second internal middle electrodes are connected to a higher potential than at least one lateral electrode in the first or second pairs of lateral electrodes such that current flows between the first and second internal middle electrodes and that at least one lateral electrode internally of the end effector.

20. The robotic electrosurgical instrument of any of claims 15 or 19, wherein the first internal middle electrode is part of the primary electrode.

21. The robotic electrosurgical instrument of any of claims 16,18 or 19, wherein, when operable in the internal cutting mode or internal double-cutting mode, the primary electrode is not electrically activated.

22. The robotic electrosurgical instrument of any of claims 16,18,19, or 20, wherein, when operable in the internal cutting mode or internal double-cutting mode, the secondary electrodes are not electrically activated.

23. The robotic electrosurgical instrument of any of claims 15 to 22, wherein the first internal middle electrode is recessed in the internal face of the first end effector element.

24. The robotic electrosurgical instrument of claim 12, wherein the first pair of lateral electrodes are part of the first pair of secondary electrodes.

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