SYSTEM FOR CUTTING BIOLOGICAL
TISSUE
This is a continuation of application Ser. No. 08/193,111, filed as PCT/DE92/00675 Aug. 2,1992 published as W093/ 5 03680 mar. 4, 1993, abandoned.
TECHNICAL FIELD
The present invention relates to a system for cutting biological tissue with high-frequency current. 1°
STATE OF THE ART
High-frequency currents are employed in surgery to cut biological tissue or to coagulate, i.e. stop the bleeding. During cutting, an almost continuous high-frequency power 15 is supplied. Aproblem in high-frequency surgery is the right dosage of the power during cutting. If the power is set too low, the tissue undergoes mechanical stress, it cannot be cut quickly or the cutting procedure comes to a complete halt. On the other hand, setting the high-frequency power too 20 high results in a strong electric arc between the surgical probe and the tissue. This electric arc causes major tissue necrosis which impairs the healing process. Too strong an electric arc has also other drawbacks. The essential disadvantage is a partial rectification of the high-frequency cur- 25 rent due to the electric arc, which leads to the danger of nerve and muscle stimulation in the patient. Such muscle and nerve stimulation may result in sudden, unexpected movements by the patient even if the patient is under full narcosis. In this event, the surgeon no longer has control, and 30 there is high risk that the patient will be injured by the surgical probe. Moreover, too strong an electric arc decomposes the tissue and in the case of underwater cutting, as e.g., in urology, the rinsing fluid may even be thermally dissociated. Both processes generate explosive gas mixtures 35 which can lead to dangerous explosions in the patient's body when operating in body cavities.
The power required for cutting and the size and intensity of the resulting electric arc are also influenced by many exterior parameters. The main influential factors are, e.g., the specific electric conductivity of the tissue being cut, dependent, on the one hand, on the type of tissue and, on the other hand, on the rate at which the tissue is desiccating,
the momentary cutting speed,
the momentary cutting depth,
the shape of the surgical probe,
the dimensions of the surgical probe,
the specific electric conductivity of a rinsing fluid that might be present: this conductivity can change even 50 during the cutting due to blood flow and electrolytes,
the configuration of the operation site, respectively the distribution of high-ohmic and low-ohmic tissue elements there,
the momentary current density distribution in the patient's body: this current density distribution can change extremely rapidly and drastically, in particular, if an electric arc is ignited between the surgical probe and the tissue to be cut. An adjustment of one of the characteristic values of the high frequency surgical generator, which are, e.g. the high-frequency current delivered to the patient, the high-frequency voltage applied to the patient, the high-frequency power input to the patient, and 65 the no-load voltage set at the generator, can compensate for only some of the influences due to the external
parameters. Thus, e.g. an adjustment proposed in DP-A-0285 962 of the output voltage to a constant value largely compensates for the influences: cutting depth and cutting speed. An altered specific electric conductivity of the tissue, e.g. due to desiccation of the tissue requires changing the output voltage, can therefore not be influenced, in particular, by such an adjustment.
The optimum setting of the high-frequency generator is when there is a small electric arc between the surgical probe and the tissue. On the one hand, the electric arc ensures a cutting-friendly dot-shaped transmission of the highfrequency current from the surgical probe to the tissue, but on the other hand does not lead to the described drawbacks of a strong electric arc.
German patent P 25 04 280, describes an apparatus for cutting and/or coagulating human tissue with highfrequency current having a indicator device which shows the size and intensity of the electric arc occurring between the probe and the tissue by means of an electric signal and which contains an adjustment device which regulates the strength of the current of the high-frequency current delivered to the patient and thereby also the high-frequency power input to the patient in such a manner that the size and intensity of the electric arc corresponds to a pre-set value.
Measurements during operations conducted with surgical generators whose output setting is carried out according to this adjustment principle show distinct advantages over operations with generators lacking such controls. Even if the paramenters also influencing the necessary generator setting change drastically, such as electric conductivity of the tissue, the degree at which the tissue is desiccating, cutting speed, cutting depth, shape and dimensions of the surgical probe, etc., one and the same setting of the desired value for the size and intensity of the electric arc can be operated with. As there is hardly any scab formation, the power input to the patient could be decreased in some cases to a third compared to similar operations with a generator without an electric arc adjustment.
Nonetheless, the adjustment has some drawbacks. They can be described if the physical effects connected with cutting at the operation site with a burning electric arc are examined more closely. The electric arc is not dependent solely on the power dosage. A number of other physical effects influence the size and intensity of the electric arc.
First of all, the electric voltage between the surgical probe and the tissue must be sufficiently high that an electric arc can even ignite. This requires, on the one hand, a suitably high no-load voltage of the generator, but also the presence, on the other hand, of a high-ohmic or insulating layer between the surgical probe and the tissue. If the surgical probe is covered with crust, this layer may, under circumstances, be formed by a coat of dried, coagulated blood and adhering remains of tissue. If there is a small gap between the surgical probe and the tissue, air or a only minimally conductive rinsing fluid forms the high-ohmic or insulating layer. If the surgical probe comes into contact with the tissue and it has a clean surface, this high-ohmic or insulating layer is formed by a vapor layer created when the cell fluid vaporizes. The thickness of the resulting vapor layer depends on the electric input power.
The thickness of the high-ohmic or insulating layer then itself influences the electric arc and its effects. The thicker the high-ohmic or insulating layer, the greater the sparking distance of the electric arc and the greater the amount of power is converted into energy at the arcing point of the electric arc. This causes some of the described drawbacks
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when a strong electric arc occurs. As the sparking distance of the electric arc increases, the interrelationship between the high-frequency current in the electric arc and the highfrequency voltage at the electric arc becomes more and more non-linear. This increases the non-linear signals, primarily 5 harmonious with the momentary generator frequency, caused by the high-frequency current and high-frequency voltage in the electric arc. These are, on the one hand, the harmonic 2nd, 3rd, 4th, and higher order, whose frequencies are the twofold, threefold, fourfold, ... of the momentary 10 frequency of the output signal and it is the harmonic 0 (zero) order which describes the rectifier effect of the electric arc. This rectifier component created in the electric arc is responsible for the nerve and muscle stimulation.
The thickness of the vapor layer as a thermal effect does 15 not immediately follow the momentary power input. Thus the adjustment system has a dead time. This is especially noticeable when starting to cut. Between the point of switching on the generator and the point when the electric arc first ignites, there is an not to be neglected interval; it sometimes 20 takes several seconds until cutting actually commences. It is a known fact in adjustment technology that adjustment systems which contain dead times are very difficult to stabilize.
Moreover, the electric arc does not burn evenly the whole 25 time on the surface of the surgical probe. The electric arc is, if the voltage is sufficiently high, ignited there where the vapor layer is the thinnest. The high concentration of energy generated by the electric arc at the arcing point of the high-frequency current vaporizes the cell fluid there, the 30 arcing point then quickly becomes the point with the thickest insulating layer. The electric arc then ignites at another point. In this way, the electric arc scans the entire surface of the surgical probe and ultimately vaporizes the cell fluid along its entire surface. The site and the sparking distance of 35 the electric arc is so random that the burning of the electric arc must be considered a stochastic process. This effects the spectrum of high-frequency current and high-frequency voltage. Thus, e.g. the spectral ranges created by the electric arc are not of constant amplitude, the speed of change 40 reaching the upper limit which is predetermined by the working frequency. As a result there is, in addition, a broadband noise in the frequency spectrum, used in EP-A0O 219 568 to detect the electric arc.
If such stochastic fluctuations influence the measured 45 values employed for adjustment, the incidental fluctuations have to be compensated by averaging. The measurement of the stochastic processes therefore requires a finite measuring period. This, on the other hand, means that the adjustment cannot occur at any desired rate. Due to this finite period, 50 which passes until there is an unequivocal control value, the electric arcs cannot be adjusted to a constant momentary value. An additional problem in adjusting electric arcs lies in the known physical fact that the non-linear interrelationship between the high-frequency voltage and the high-frequency 55 current in the electric arc has partially negative rises, i.e. it can happen that when raising the momentary voltage, the momentary current decreases and when lowering the momentary voltage the momentary current rises. It is known that such processes can excite oscillations and destabilize 60 adjustments.
DESCRIPTION OF THE INVENTION
The object of the present invention is thus to design the system for cutting biological tissue with high-frequency 65 currents in such a manner that a stable adjustment is obtained despite the afore-described dead times, the required
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averaging and the threat of destabilization of the adjustment by the physical effects of the electric arc.
Accordingly, in order to indicate the size and intensity of the electric arc, the system is combined with an adjustment of at least one of the characteristic values of the generator. At least one of the characteristic values of the generator is adjusted to a 1st desired value. In this way, the effect of one component of the external parameters on the cutting behavior is eliminated. Preferably, the characteristic value is adjusted to a desired value which influences the external parameter that has the most influence on the cutting process in the type of surgery just being conducted. If the type of tissue and the desiccation state only change slowly, but the cutting depth or cutting speed have to be varied continually, it is advantageous to adjust the output voltage.
In setting the high-frequency current or the highfrequency power to a 1st desired value, the influence of the specific electric conductivity on the cutting behavior is largely eliminated; in this event the influence of the cutting depth and cutting speed on the cutting behavior remain.
These uninfluenceable effects of external parameters by the respective setting of the characteristic value of the generator are compensated for in that the 1st desired value is not constant but is gained by a comparison of the electric signal of a indicator device for the size and intensity of the electric arc with a 2nd desired value. The gaining of the 1st desired value occurs in an evaluation unit to which, on the one hand, the electric output signal of the indicator device for the size and intensity of the electric arc is transmitted and, on the other hand, to which the 2nd desired value is transmitted. For stable adjustment, it is necessary that the 1st desired value generated in the evaluation unit for the adjustment device changes slower by at least one order of magnitude than the adjustment device which needs time to adjust the characteristic value to the desired value.
Short-term changes of the external parameters are thus regulated by quick operating adjustment of the characteristic value in its effect on the cutting behavior. Averaged over a longer period, the size and intensity of the electric arc are constant and determined by the 2nd desired value.
The 2nd value for the size and intensity of the electric arc are supplied by the desired-value transmitter. In the simplest case, the desired-value transmitter supplies a fixed desired value. Usually the surgeon can influence the 2nd desired value supplied by the desired value transmitter and adapt it to the goal of the operation. Very small 2nd desired values for the size and intensity of the electric arc lead to incisions with minimal necrosis and minimal muscle and nerve stimulation. This setting is selected if, e.g., cutting is in the vicinity of nerve centers and there is a danger that the patient will twitch because these nerves have been stimulated. Such sudden movements by the patient make surgery more difficult and present the risk that the surgeon may cut too deeply and thereby seriously injure the patient.
In surgery in which much tissue is to be removed, e.g., in the case of prostatectomy up to 100 g of prostate tissue, a higher setting of the desired value for the size and intensity of the electric arc permits quick cutting. As at the beginning of this type of surgery, the tissue is removed in several layers, a greater degree of necrosis in the top layers is no problem, because these necrotic sections of tissue will be removed in the course of the operation.
The invented combination of an adjustment device for a characteristic value of the generator component and the indicator device for the size and intensity of the electric arc yields further advantages for the design of the system for cutting biological tissue.
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