MXPA99009748A - An electrosurgical instrument - Google Patents

Abstract

En un instrumento electroquirúrgico susceptible a la impedancia de carga variable, particularmente la reactancia de carga variable, un generador de radiofrecuencia con frecuencia fija tiene una estación de salida (Q1, Q2, TR2-TR4, C1, VC2, 42) que consiste de por lo menos un dispositivo de potencia de salida y, acoplado al dispositivo de salida, una red de salida con una condición resonante que depende de la carga. Para compensar las variaciones en la reactancia de carga, la red de salida tiene un capacitor dinámicamente variable (VC2) que mantiene la sintonización de la red sustancialmente constante. La variación del capacitor (VC2) se realiza por medio de un comparador de fases y servo-dispositivo (44) que responden a variaciones en la diferencia de fases que se presenta en la etapa de salida debido a los cambios en reactancia de carga.

Description

AN ELECTRO-SURGICAL INSTRUMENT This invention relates to a radio frequency generator for use with a variable impedance load, and in particular to an electrosurgical instrument having a monopolar electrode unit incorporating such a generator, the frequency of operation of the generator that usually has a 5 MHz excess. An electrosurgical system comprises a handpiece, a monopolar electrode unit having a simple treatment electrode projecting from the handpiece, a generator unit, and a cable that couples the generator unit to the hand piece. Such systems are commonly used for various types of electrosurgery. Typically, a conductive pad is applied to the patient's body and connected to a return terminal of the generator unit to provide a return path for electrosurgical currents. The disadvantages of this process include the location of electrosurgical currents in tissues in the region of the return pad and, particularly at higher frequencies, not being able to predict the reactive components created by the cable between the generator and the handpiece, leading to unpredictable voltage levels at the electrode. The disadvantages are overcome, at least in part, by an instrument described in Applicant's International Application No. PCT / GB96 / 02577, which describes an electrosurgical instrument comprising a handpiece, a monopolar electrode unit having a simple treatment electrode projecting from the handpiece, and a radiofrequency generator inside the handpiece, the generator having a direct radio frequency output connection, said output connection is coupled to the electrode. On the other hand, the generator is isolated from the external elements. In particular, the generator has no other direct radio frequency output connection to, for example, a grounding element or a return pad. Providing the generator inside the handpiece, unpredictable impedance changes due to the effects of supply of radio frequency currents through a cable are avoided. The radiofrequency return currents pass between the patient and the generator by the capacitive coupling diverted via a conductive cover located around the generator. Preferably, the operating frequency of the generator is 5 MHz or greater. The higher the frequency, the higher the current level obtainable due to the reactance of the return path at high frequencies. The generator can be operated from a battery inside the handpiece. This minimizes radiated interference. The presence of an electrically conductive cover around the generator minimizes the variation in deflected capacitance caused by the user holding the handpiece in different ways. The cover is preferably insulated from the generator and can form a tubular handpiece body, e.g., in the form of a metal shell, or the body of the handpiece can be formed of an electrically insulating material which is metallized to provide the conductive cover. The metallization layer is on the outside of the body of the handpiece, or the body of the handpiece is metal by itself, the outer metal surface preferably being covered by an electrically isolated outer layer. The provision of the cover reduces the variations in deviated capacitance because the capacitance between the relevant generator leads and the cover is constant, and the cover provides a conductive body of constant area capacitively coupled to the patient. Although the cover reduces the variations in deviated capacitance, the variable impedance load which results from it and the unavoidable variations in the load caused by the change of conditions in the interface of the tissue to the electrode, presents considerable difficulties to maintain energy efficiency. To a lesser degree, this also presents difficulties to avoid breaking the output device due to temporary inequalities. Taking into account these difficulties, according to the first aspect of the present invention, a radio frequency generator (rf) capable of operating at a substantially fixed frequency under variable load conditions is provided, the generator having an output station comprising a output power device and, coupled to the power device, an output network that includes an output node of the treatment electrode, wherein the output network has a load-dependent resonant condition and includes a variable reactance element arranged for At least partially offset the effects of poor tuning of the output network due to variations in the load impedance. According to another aspect, the invention provides a r.f. capable of operating at a substantially fixed frequency under varying load conditions, the generator output station including a resonant output circuit which has a variable reactance element such as a variable capacitor arranged to compensate, at least partially, the changes in load impedance. The capacitor varies automatically in response to load impedance changes captured, for example, by monitoring phase changes in the output station and driving the capacitor, preferably mechanically, so that its capacitance is altered in a which leads to a phase difference captured at a preferred value or within a preferred range. Preferably the variable capacitor is coupled through the secondary winding of a generator output transformer (which may be an isolated transformer or an autotransformer) that is part of the output circuit tuned in parallel, the tuning which is affected by the impedance of the transformer. load. In this form, a change in load reactance can be compensated for maintaining the tuning of the output resonant circuit as far as practical so that in this way the output device or output devices are present (usually a pair of power transistor switching devices such as power MOSFET) with at least an approximately real load impedance. Next, it will be appreciated that variations in generator load impedance can be accommodated within a wide range without altering the operating frequency. This self tuning effect allows the use of a high Q output station for efficient coupling of the generator output, a requirement that is important in the case of a manual electrosurgical instrument powered by self-contained battery, as described above, in which the efficient operation is based on the good coupling between the instrument, the user and the patient. The phase information, which represents the load reactance, for controlling the variable capacitor can be obtained by comparing the output phase with the phase of a drive signal, e.g., the signal of r.f. an output transformer of an impulse signal applied to the base or connecting connection of an output device is supplied to the primary. The phase difference signal obtained afterwards, is amplified to provide a capacitor drive signal of a polarity such that the impedance decreases, the variable capacitance also decreases and is such that, in the case of the variable capacitor being connected through the Output impedance (eg, secondary winding of the transformer), a greater proportion of the current appropriate to the load is supplied. It should be noted that the output circuit of the generator can also include a coupled output capacitor, (ie, on the generator output line) and that the variable capacitor can constitute this coupled capacitor given that it is also part of the output resonant circuit . In this case, a decreased load impedance, although used to decrease the variable capacitance value, also decreases the available output current. This variation of the coupled capacitor can be used to limit the load on the generator output devices.

The variable capacitor, by itself, may comprise parallel plates of the capacitor, and means for varying the separation between the plates such as a piezo-electric actuator. Such a device has the advantage of being appropriate for the high voltages associated with the electrosurgical treatment and operating rapidly, usually within 10 ms. Much faster response times are possible depending on the nature of the piezo-electric actuator used. Generally, a high dielectric constant layer is interposed between the capacitor plates. The invention includes an electrosurgical instrument incorporating a generator as described above without a handpiece. The instrument has a monopolar treatment electrode projecting from the handpiece, and the variable reactance element of the generator is preferably located at or adjacent to the electrode entry location in the handpiece. The invention will now be described by way of example with reference to the drawings in which: Figure 1 is a diagram of an electrosurgical instrument, shown in use; and Figure 2 is a longitudinally diagrammatic cross section of the instrument; Figure 3 is an electrical circuit diagram equivalent to the instrument in use; Figure 4 is a simplified circuit diagram of a generator according to the invention and forming part of the instrument of Figures 1 and 2; Figure 5 is a circuit diagram of a phase comparator; and Figure 6 is a simplified circuit diagram of an alternative generator according to the invention. Referring to Figure 1, a self-contained electrosurgical instrument comprises an elongated cylindrical hand piece 10 which can be held "as a pencil" in the manner in which it is shown. An end portion 10A of the handpiece is tapered and an electrode unit in the form of a simple treatment electrode 12 projects axially from that end so that it can be brought into contact with the body 14 of a patient. An activator button 16 is provided on the tied end portion 10A. The body 10 of the handpiece can be formed from a sheet of metal, and is provided with an insulating cover made of, for example, a film material. Alternatively, the body of the handpiece 10 can be molded of an electrically insulating plastic material, and further metallized on the external or internal surface. If the metallization is on the external surface, an electrically insulating cover is provided to isolate the metallization from the user's hand. In the diagrammatic cross-section of Figure 2, the body of the handpiece 10 is shown comprising the conductive cover 10S and an insulation box 10C. An internal electronic unit 18 comprises a radio frequency generator and a battery is contained within the cover 10S. Although it is not essential that the electronic unit 18 be completely enclosed by the cover 10S as shown, it is preferable that at least the part of the generator be placed extended longitudinally with the cover. The cover 10S has many useful properties. The internal electronic unit 18 has a non-uniform mass and distribution within the box with different potentials relative to the ground. The 10S cover provides a uniform surface of the same or uniform potential, forming the insulation layer 101 with minimum size and thickness, the size of the cover can be formed maximally, and the capacitive coupling for both the patient, the user and the connected objects to external ground can be formed to the maximum.

By forming circumferentially continuous covers, the internal electronics are also effectively sieved against the levels of RF radiation potential interference. Forming the metal box to provide the cover provides a uniform heat distribution and also improves the dissipation of power generated within the electronic unit due to inefficiencies. Referring to Figure 3, the equivalent circuit of the instrument is considered when it is in use. Within the body of the handpiece, a radio frequency (RF) 18G generator operable at a frequency within one of the industrial, scientific or medical bands above 5 MHz is provided (switch 16 and cover 10S are not shown) in figure 3). In the preferred instrument, the operating frequency is 40.65 MHz. Other possible frequencies include 6.79 MHz, 13.65 MHz, 27.1 MHz and 915 MHz. The generator has an output connection coupled to the electrode 12 (Figure 1), and has no other Output connection for direct current flow to the patient. The conductive elements of the generator 18G (the elements are figuratively shown by reference 20 in Figure 3), act as an antenna 22 and are capacitively coupled, indirectly via the conductive cover 10S of the body of the handpiece 10 (see figures 1). and 2) via capacitance 24 to patient 14, represented as a second antenna in figure 3. The interface of the electrode to the tissue is represented by line 26. Therefore, when in use, the active output connection of the generator 18G is connected to patient 14 through the tissue being operated, the resistance of this tissue being represented by resistance 29 in Figure 3. The value of this resistance is usually 1 kO, and can drop down to 100O. The radiation conductors 20 of the generator 18G are also capacitively connected to the user by capacitance 30, which is the series combination of the generator to cover and the capacitance cover of the user, the user 32 being, in turn, capacitively coupled to a ground connection as represented by the capacitor 34. Because the patient 14 is also capacitively coupled to the ground connection (as represented by the capacitor 36 in FIG. 3), there is a capacitive path both indirect and direct between the user 32 and the patient 14. Similarly, there is an indirect path of the conductive elements of the generator 20 through the capacitance 38 of the handpiece body 10 (specifically the cover 10S) connected to ground and the capacitance in series 36 between patient 14 and the ground connection. The total capacitance between the generator 18G and the patient 14 resulting from the direct capacitance 24 from generator to patient, the capacitance 30 of the hand-to-handpiece body, user capacitances to ground connection, from body to ground connection and from patient to ground, 34, 38 and 36, respectively, at least 15 pF.

A battery that is not shown in the drawings which is also located inside the body of the handpiece 10. Preferably a nickel-cadmium or lithium battery, rechargeable via terminals at the opposite end of the body 10 of the electrode 12. These instruments mainly, but not exclusively, are intended for fine surgical work, such as spinal cord, neurological, plastic, ear, nose and throat and dental surgery, and office procedures. Referring now to Figure 4 of the drawings, an instrument generator comprises a fixed frequency oscillatory crystal based on a transistor Q4, feeding a driving station based on a transistor Q3 and having a TRI driving transformer, and a station of equilibrium output based on transistors Q1 and Q2. The transistors Q1, Q2 are MOSFET devices coupled between a high voltage supply (usually 50 volts) and having a common output connection 40 from which a radio frequency power signal is fed to the oscillator frequency via the coupled capacitor C5 to the primary coil of an output transformer TR2. The primary coil of the transformer TR2 is coupled to the output of the generator 42 via a coupled capacitor C1. The variable capacitor VC2 acts as a tuning element, together with the capacitance Cstray which represents the deviated capacitance to ground the electrode and the conductors connected to it. Coupled in series with the primary and secondary coils of the transformer TR2 respectively are the sensor transformers TR3 and TR4, the secondary coils from which a phase comparator and servo drive circuit 44 are fed to drive the variable capacitor VC2. It will be noted that the output station has two resonant circuits. A series circuit comprising a capacitor C5, the inductance dispersion of transformer TR2, and TR3, and a series circuit comprising the secondary coil of the transformer TR2 (the main element), the primary coil of TR4 (single-level) signal), together with the set of capacitors formed by a variable capacitor VC2, the coupled capacitor C1 and the deviated capacitance Cstray- The phase relationship between these two resonant circuits varies with both the load and deviated capacitance. The series resonant circuit is comparatively not affected, but the parallel circuit, connected to the output 42, is affected by both the load and the deviated capacitance. Accordingly, by deriving the pickup signals, using the transformers TR3 and TR4, from the output circuit, in this case, the circuit associated with the primary coil of the output of the transformer TR2, it is possible to derive a signal difference phase. , which can be used to alter the capacitance of capacitor VC2 and thereby compensate for variations that are not in tuning in the load and deviated capacitance. At this point it should be mentioned that any fixed value of the impedance matching circuit necessarily appears to be inductive or capacitive for the output switching devices Q1, Q2, which impairs the efficiency of the switches. However, because such changes in impedance are accompanied by phase changes, an output station employing a resonant tank circuit can be used to minimize harmonization, to derive the phase sensing signals for the phase comparator. 44. The phase of the comparator is illustrated in more detail in Figure 5, together with the transformers TR3 and TR4. The signaling levels for the phase comparator 44 are provided by the secondary coils of the transformers TR3 and TR4 as described above. These provide the phase information of each of the circuits associated with the transformer TR2. Using a slip coil of the transformer that receives the parallel signal of phases, the average wave rectification independent of alternate cycles is presented by virtue of diodes D11, D12 ,. By mixing both phases of the phase signal in series, the phase comparator is corrected. The error voltage developed at the output of the medium wave rectifier D11, D12 only reaches a value of zero, however, when the two sensor signals have a phase relationship offset by 90 ° with respect to each other. A preset phase change is provided by the combination of the variable resistor RV1 and the capacitor C13 connected through the secondary coil of the transformer TR4, as shown in Figure 5, to establish the preferred phase difference between the primary and secondary sides. side of the output of transformer TR2 (Figure 4). The uniformity of the output voltages of the rectifier is provided by the resistor-capacitor combinations R11, C11 and R12, C12 in FIG. 5, and the output error voltage derived from the output 46 is amplified by means of a comparator ( not shown) and passed to a suitable drive circuit (not shown). This drive circuit incorporates a dominant pole to compensate for potential instability in the feedback loop due to, for example, mechanical resonance in the assembly associated with the variable capacitor VC2. Referring again to Figure 4, the variable capacitor VC2, the coupling capacitor C1, and the deviated capacitance Cstray of the output capacitor arrangement all have an effect on the tuning of the output resonant circuit. By using the variable capacitor element as the tank capacitor, the maximum current supply can be obtained. In particular, the comparator and the impeller are arranged in such a way that the load impedance decreases and a corresponding change in the phase difference occurs through the output transformer TR2, the capacitance of the variable capacitor VC2 decreases, and a greater proportion of the available current to the load. It should be noted that the variable capacitance can, as an alternative, be used in place of the output coupling capacitor (C1). In this case, by decreasing the load impedance, both the variable coupled capacitance and the available current are decreased, which is useful in cases where the r.f. Q1, Q2 need to be protected against heavy loads. The variable capacitor VC2 can be of different shapes. It is preferred that it be constructed as a capacitor of parallel plates, the separation of the plates being controlled by a piezo-ceramic actuator. The actuator can be manufactured in two different ways: (a) a bending strip which acts in a similar way to a bimetallic strip, or (b) a longitudinal solid piezo-electric element (which can be of one or multiple layers) . Actuators of this type are available from Morgan Matroc, Inc. of Bedford, Ohio, USA. The type of bending strip of the actuator produces mechanical movement in response to a variable applied voltage creating differential expansion between two bonded materials. The bending can be improved by means of piezo-ceramic material attached to a metal substrate. Since the piezo-ceramic can be held by compression for better expansion, movement can be done in both directions. It is also possible to cover both sides of a metal substrate with piezo-material so that each coating is supported in the opposite direction to increase the degree of movement. When a voltage is applied to the strip, bending occurs. The applied voltage is usually between 50 and 100 volts to produce the proper movement in order to vary the capacitance of the variable capacitor. Usually, a capacitance change of 8 to 24 pF can be achieved. With the piezo-electric longitudinal actuator, the piezo-movement is used directly. The actuator is formed of a sandwich of devices to carry out sufficient movement in response to applied voltages. In general, the longitudinal piezo-actuator has a faster response time than the flex strip variety. It is also possible to alter the capacitance of the variable capacitor VC2 by an electromechanical or electromagnetic technique, such as by the use of a moving coil device. The capacitor itself (not shown in the drawings) comprises parallel plates with a thin intervening dielectric layer having a high dielectric constant. The preferred material is mica, which is available in the form of a thin sheet. The material has a dielectric strength of between 40 to 200 kV per mm. Because the capacitor is used as a capacitor or capacitor coupling capacitor circuit capacitor capacitor, as described above, the voltages across the capacitor plates can reach 1 kV. Usually, the thickness of the dielectric layer is in the region of 25 μm to 50 μm. With a relative dielectric constant of 6, it is possible to build a parallel plate capacitor with a capacitance of 2 pF per mmsq. With a total deflection of 50μm, the minimum capacitance, therefore, is 0.16 pF / mmsq.

With respect to the physical position of the variable capacitor VC2 within the hand piece shown in FIGS. 1 and 2, it is advantageous to mount it in the region of the entrance of the electrode 2 in the handpiece in order to minimize the allowed length between the output 42 (FIG. 4) of the generator and the exposed electrode 12. Although the response speed of the variable capacitor VC2 and its associated control circuitry is fast, the nature of the electrosurgical action is such that a temporary inequality will still be present. to very fast changes in the load impedance (for example, due to arcing and collapsing). In order to minimize the momentary voltage effect on the output devices Q1, Q2, the Schottky inverted diodes D1, D2 are provided in each of the MOSFET outputs Q1, Q2, as shown. The excess voltage across the devices Q1, Q2 at high rates of change is somewhat limited by the conduction of the diodes D1, D2, rather than by the behavior of the reactance diode. Although these diodes D1, D2 increase the switching capacitance, this can be compensated for by driving the output station as a partial inductive load so that the net effect on the output devices is purely resistive except when extreme inequalities are present. The generator output station shown in Figure 4 is a class C output station for configuration of totem poles connected from rail to rail. An alternative output station is shown in Figure 6. In this case, the crystal oscillator and the impeller based on transistors Q4 and Q3, respectively, remain in the same way as the clamping Zener diodes D3 through D6 connected through the gate connections of the output devices Q11, Q12 ,. However, the output devices Q11, Q12 are arranged in a half bridge configuration with additional Schottky diodes, as shown in Figure 6, and with the output devices having the primary coil of the output transformer TR5 coupled in series among them. The output resonant circuit remains the same as in the embodiment of FIG. 4. For simplicity, the sensitive circuits are not shown in FIG. 6. However, it should be noted that the phase comparator of FIG. input transformer coupled in rf gate drive circuits for the output devices Q11, Q12 and the other coupled, as before, in series with the secondary coil of the output transformer TR1. In summary, the circuitry described above provides an extremely fast capacitance change through the use of a fast responding linear actuator operating on a small movement scale, and phase comparison by means of a variable reactance element in the resonant circuit of

Claims (21)

1. departure. An impeller is provided for the variable reactance element in response to the phase comparison. CLAIMS 1. A radio frequency electrosurgical generator capable of operating at a substantially fixed frequency under variable load conditions, the generator having an output station comprising an output power device and, coupled to the power device, an output network that includes a treatment electrode output node, wherein the output network has a resonant condition that is load-dependent and includes a variable reactance element to at least partially compensate for the effects of poor tuning of the output network due to variations in load impedance.
2. 2 A generator according to claim 1, wherein the variable reactance element is associated with a signal path between the power device and said node.
3. A generator according to claim 1, which includes a sensor circuit having an output coupled to the variable reactance element, wherein the sensing circuit is processed to pick up changes in load impedance and to produce a load impedance of adjustment signal that responds to said output, and wherein the reactance element is arranged in such a way that its reactance is adjustable in response to the adjustment signal by which it performs said compensation.
4. 4. A generator according to claim 3, wherein the sensor circuit is processed to capture changes in load reactance.
5. A generator according to claim 3, wherein the sensor circuit comprises a phase comparator having inputs coupled to different respective parts of the output station and processed to generate an adjustment signal at the output of the sensor circuit. which causes the reactance element to be adjusted so as to make the phase difference between the signals at the inputs of the phase comparator to a predetermined value or within a predetermined scale.
6. 6. A generator according to claim 1, wherein the variable reactance element is a variable capacitor.
7. 7. A generator according to claim 1, wherein the variable reactance element includes a servo device.
8. A generator according to claim 1, wherein the output station includes an output transformer and the variable reactance element is coupled to a secondary coil of the transformer.
9. 9. A generator according to claim 1, wherein the variable reactance element forms part of an output network tuned in parallel.
10. A generator according to claim 9, wherein the variable reactance element is a capacitor and the tuned circuit includes a transformer coil.
11. 11. A generator according to claim 1, wherein the output network includes a generator output line, and wherein the variable reactance element is connected in series in the output line.
12. 12. A generator according to claim 11, wherein the variable reactance element is a capacitor.
13. A generator according to claim 1, wherein the variable reactance element forms part of an output circuit between the power device and a generator output node.
14. 14. A generator according to any preceding claim, wherein the generator has a conductive element that acts as an antenna.
15. 15. A generator according to any preceding claim, having an operating frequency in excess of 5. MHz.
16. 16. An electrosurgical instrument that includes a handpiece, a treatment electrode projecting from the handpiece, and a generator as claimed in any preceding claim, with the node of the output of the generator treatment electrode. connected to a treatment electrode.
17. 17. An instrument according to claim 16, having a simple treatment electrode.
18. 18. An instrument according to claim 17, is processed such that the radio frequency return path of the generator during use occurs by electromagnetic field transmission through air between the patient and the instrument.
19. 19. An instrument according to any of claims 16 to 18, including a conductive body processed to provide deviated capacitance by coupling the patient during use of the instrument as a return path by radiofrequency currents.
20. 20. An instrument according to claim 16, having a single direct radio frequency output connection, the return path being provided by a conductive body capacitively coupled to the patient by deflected capacitance.
21. 21. An instrument according to any of claims 16 to 20, which can operate a frequency in excess of 5 MHz.