Surgery – Instruments – Electrical application
Reexamination Certificate
1999-12-20
2001-12-04
Dvorak, Linda C. M. (Department: 3739)
Surgery
Instruments
Electrical application
C606S034000, C606S035000, C606S038000
Reexamination Certificate
active
06325799
ABSTRACT:
BACKGROUND OF THE INVENTION
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 operating frequency of the generator being typically in excess of 5 MHz.
A known electrosurgical system comprises a handpiece, a monopolar electrode unit having a single treatment electrode projecting from the handpiece, a generator unit, and a cable coupling the generator unit to the handpiece. Such systems are commonly used for various types of electrosurgery. Normally, a conductive pad is applied to the body of the patient and connected to a return terminal of the generator unit to provide a return path for electrosurgical currents. Disadvantages of this arrangement include the localisation of electrosurgical currents in tissue in the region of the return pad and, particularly at higher frequencies, the unpredictability of reactive components created by the cable between the generator and the handpiece, leading to unpredictable voltage levels at the electrode.
These disadvantages are overcome at least in part by an instrument described in the applicant's International Application No. PCT/GB96/02577 which discloses an electrosurgical instrument comprising a handpiece, a monopolar electrode unit having a single treatment electrode projecting from the handpiece, and a radio-frequency generator within the handpiece, the generator having a single direct radio-frequency output connection, which output connection is coupled to the electrode. The generator is otherwise isolated from external elements. In particular, the generator has no other direct radio frequency output connection to, for example, an earthed element or to a return pad. By providing the generator within the handpiece, unpredictable impedance changes due to the effects of supplying radio-frequency currents through a cable are avoided. Radio-frequency return currents pass between the patient and the generator by stray capacitive coupling via a conductive shield located around the generator.
Preferably, the operating frequency of the generator is 5 MHz or greater. The higher the frequency, the greater the attainable current level due to the reduced reactance of the return path at raised frequencies. The generator may be powered from a battery within the handpiece. This minimises radiated interference.
The presence of an electrically conductive shield around the generator minimises the variation in stray capacitance caused by the user gripping the handpiece in different ways. The shield is preferably isolated from the generator and may form a tubular handpiece body, e.g. in the form of a metallic casing, or the handpiece body may be formed of an electrically insulative material which is metallised to provide the conductive shield. Where the metallisation layer is on the outside of the handpiece body, or the handpiece body is itself metallic, the outer metallic surface is preferably covered by an electrically insulating outer layer. Provision of the shield reduces stray capacitance variations because the capacitance between the relevant generator conductors and the shield is constant, and the shield provides a conductive body of constant area capacitively coupled to the patient.
Although the shield reduces variations in stray capacitance, the variable impedance load which results from this and from the inevitable variations in load caused by changing conditions at the tissue-to-electrode interface poses considerable difficulties in maintaining energy efficiency. To a lesser degree, also it poses difficulties in preventing output device breakdown due to transient mismatches.
BRIEF SUMMARY OF THE INVENTION
With these difficulties in mind, according to a first aspect of the present invention there is provided a radio frequency (r.f.) generator capable of operating at a substantially fixed frequency under varying load conditions, the generator having an output stage comprising an output power device and, coupled to the power device, an output network including a treatment electrode output node, wherein the output network has a load-dependent resonant condition and includes a variable reactance element arranged to compensate at least partially for output network mistuning effects due to variations in load impedance. According to another aspect, the invention provides a r.f. generator capable of operating at a substantially fixed frequency under varying load conditions, the output stage of the generator including a resonant output circuit which has a variable reactance element such as a variable capacitor arranged to compensate, at least partially, for changes in load impedance. The capacitor is automatically varied in response to sensed load impedance changes by, for instance, monitoring phase changes in the output stage and driving the capacitor, preferably mechanically, such that its capacitance is altered in a manner which brings a sensed phase difference to a preferred value or to within a preferred range. Preferably the variable capacitor is coupled across the secondary winding of an output transformer of the generator (which may be an isolating transformer or an auto-transformer), forming part of a parallel tuned output circuit, the tuning of which is affected by the impedance of the load.
In this way, a change in load reactance can be compensated for so as to maintain tuning of the output resonant circuit as far as practicable thereby to present the output device or devices (typically a pair of power transistor switching devices such as power MOSFETs) with an at least approximately real load impedance. It will be appreciated, then, that variations in generator load impedance can be accommodated within a wide range without altering the frequency of operation.
This auto-tuning effect allows the use of a high-Q output stage for efficient coupling of the generator output, a requirement that is important in the case of a self-contained battery-powered handheld electrosurgical instrument as described above in which efficient operation is assisted by good coupling between the instrument, the user and the patient.
Phase information, representing load reactance, for controlling the variable capacitor may be obtained by comparing output phase with the phase of a driving signal. e.g. the r.f. signal supplied to the primary of an output transformer or a driving signal applied to the base or gate connection of an output device. The phase difference signal obtained is then amplified to provide a capacitor drive signal of a polarity such that as load impedance decreases, the variable capacitance also decreases and such that, in the case of the variable capacitor being connected across the output impedance (e.g. the transformer secondary winding), a greater proportion of the available current is supplied to the load.
It should be noted that the output circuit of the generator may also include an output coupling capacitor (i.e. in series in the generator output line) and that the variable capacitor may constitute that coupling capacitor since it also forms part of the output resonant circuit. In that case a decreasing load impedance, while being used to decrease the variable capacitance value, also decreases the available output current. This coupling capacitor variation can be used to limit the load on the generator output device or devices.
The variable capacitor itself may comprise parallel capacitor plates, and means for varying the spacing between the plates such as a piezo-electric actuator. Such a device has the advantage of being able to withstand the high voltages associated with electrosurgical treatment and to operate quickly, typically 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 set out above within a handpiece. The instrument has a monopola
Dvorak Linda C. M.
Gyrus Medical Limited
Nixon & Vanderhye P.C.
Ruddy David M.
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