Surgery – Instruments – Electrical application
Reexamination Certificate
2002-03-21
2004-12-21
Cohen, Lee S. (Department: 3739)
Surgery
Instruments
Electrical application
C606S034000, C606S050000
Reexamination Certificate
active
06832998
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a bipolar electrosurgical cutting device such as a scalpel blade, and to an electrosurgical system comprising an electrosurgical generator and a bipolar electrosurgical cutting device. Such systems are commonly used for the cutting of tissue in surgical intervention, most commonly in “keyhole” or minimally invasive surgery, but also in “open” surgery.
Electrosurgical cutting devices generally fall into two categories, monopolar and bipolar. In a monopolar device, a radio frequency signal is supplied to an active electrode which is used to cut tissue at the target site, an electrical circuit being completed by a grounding pad which is generally a large area pad attached to the patient at a location remote from the target site. In contrast, in a bipolar arrangement, both an active and a return electrode are present on the cutting device, and the current flows from the active electrode to the return electrode, often by way of an arc formed therebetween.
An early example of a bipolar RF cutting device is described in U.S. Pat. No. 4,706,667 issued to Roos, in which the return or “neutral” electrode is set back from the active electrode. Details for the areas of the cutting and neutral electrodes are given, and the neutral electrode is said to be perpendicularly spaced from the active electrode by between 5 and 15 mm. In a series of patents including U.S. Pat. Nos. 3,970,088, 3,987,795 and 4,043,342, Morrison describes a cutting/coagulation device which has “sesquipolar” electrode structures. These devices are said to be a cross between monopolar and bipolar devices, with return electrodes which are carried on the cutting instrument, but which are preferably between 3 and 50 times larger in area than the cutting electrode. In one example (U.S. Pat. No. 3,970,088), the active electrode is covered with a porous, electrically-insulating layer, separating the active electrode from the tissue to be treated and causing arcing between the electrode and the tissue. The insulating layer is said to be between 0.125 and 0.25 mm (0.005 and 0.01 inches) in thickness.
In another series of patents (including U.S. Pat. Nos. 4,674,498, 4,850,353, 4,862,890 and 4,958,539), Stasz proposed a variety of cutting blade designs. These were designed with relatively small gaps between two electrodes such that arcing would occur therebetween when an RF signal was applied to the blade, the arcing causing the cutting of the tissue. Because arcing was designed to occur between the electrodes, the typical thickness for the insulating material separating the electrodes was between 0.025 and 0.075 mm (0.001 and 0.003 inches).
SUMMARY OF THE INVENTION
The present invention seeks to provide a bipolar cutting blade which is an improvement over the prior art.
Accordingly, there is provided an electrosurgical system comprising a bipolar cutting blade, a handpiece to which the cutting blade is secured, and an electrosurgical generator for supplying a radio frequency voltage signal to the cutting blade, the cutting blade comprising first and second electrodes, and an electrical insulator spacing apart the electrodes, the spacing being between 0.25 mm and 3.0 mm, and the electrosurgical generator being adapted to supply a radio frequency voltage signal to the cutting blade which has a substantially constant peak voltage value, the relationship between the peak voltage value and the spacing between the electrodes being such that the electric field intensity between the electrodes is between 0.1 volts/&mgr;m and 2.0 volts/&mgr;m, the first electrode having a characteristic which is dissimilar from that of the second electrode such that the first electrode is encouraged to become an active electrode and the second electrode is encouraged to become a return electrode.
The term “blade” is herein meant to include all devices which are designed such that both the active cutting electrode and the return electrode are designed to enter the incision made by the instrument. It is not necessary that the cutting device is only capable of making an axial incision, and indeed it will be shown below that embodiments of the present invention are capable of removing tissue in a lateral direction.
The first important feature of the present invention is that the spacing between the electrodes and the electric field intensity therebetween is carefully controlled such that there is no direct arcing between the electrodes in the absence of tissue. The electric field intensity between the electrodes is preferably between 0.15 volts/&mgr;m and 1.1 volts/&mgr;m, and typically between 0.2 volts/&mgr;m and 1.1 volts/&mgr;m. In one preferred arrangement, the spacing between the first and second electrodes is between 0.25 mm and 1.0 mm, and the electric field intensity between the electrodes is between 0.33 volts/&mgr;m and 1.1 volts/&mgr;m. This ensures that the field intensity is sufficient for arcing to occur between the first electrode and the tissue, but not directly between the first and second electrodes.
However, even where direct arcing between the electrodes is prevented, there is still a potential problem if the two electrodes are similar in design. In a bipolar cutting device, only one of the electrodes will assume a high potential to tissue (and become the “active” electrode), with the remaining electrode assuming virtually the same potential as the tissue (becoming the “return” electrode). Where the first and second electrodes are similar, which electrode becomes the active can be a matter of circumstance. Usually, whichever electrode first contacts the tissue will become the return electrode, with the other electrode becoming the active electrode. This means that, in some circumstances, one electrode will be the active electrode, and at other times the other electrode will be the active electrode. Not only does this make the device difficult for the surgeon to control (as it will be uncertain as to exactly where the cutting action will occur), but as it is likely that any particular electrode will at some time have been active.
When an electrode is active, there is a build up of condensation products on the surface thereof. This is not a problem when the electrode continues to be the active electrode, but it does make the electrode unsuitable for use as a return electrode. Thus, in the instance where two similar electrodes are employed, it is likely that, as each will at some times become active electrode and at other times the return electrode, the build up of products on both electrodes will lead to a decrease in performance of the instrument. Therefore, the present invention provides that the first electrode has a characteristic which is dissimilar from that of the second electrode, in order to encourage one electrode to assume preferentially the role of the active electrode.
The characteristic of the first electrode which is dissimilar from that of the second electrode conveniently comprises the cross-sectional area of the electrode, the cross-sectional area of the first electrode being substantially smaller than that of the second electrode. This will help to ensure that the first electrode (being of a smaller cross-sectional area) will experience a relatively high initial impedance on contact with tissue, while the relatively larger area second electrode will experience a relatively lower initial impedance on contact with tissue. This arrangement will assist in encouraging the first electrode to become the active electrode and the second electrode to become the return electrode.
The characteristic of the first electrode which is dissimilar from that of the second electrode alternatively or additionally comprises the thermal conductivity of the electrode, the thermal conductivity of the first electrode being substantially lower than that of the second electrode. In addition to the initial impedance, the rate of rise of the impedance is a factor influencing which electrode will become active. The impedance will rise with desiccation of the tissue, and the rate of desiccation wil
Cohen Lee S.
Gyrus Group PLC
Nixon & Vanderhye P.C.
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