Abstract:
A bipolar electrosurgical instrument has opposable seal surfaces on its jaws for grasping and sealing vessels and vascular tissue. Inner and outer instrument members allow arcuate motion of the seal surfaces. An open lockbox provides a pivot with lateral support to maintain alignment of the lateral surfaces. Ratchets on the instrument members hold a constant closure force on the tissue during the seal process. A shank portion on each member is tuned to provide an appropriate spring force to hold the seal surfaces together. During surgery, the instrument can be used to grasp and clamp vascular tissue and apply bipolar electrosurgical current through the clamped tissue. In one embodiment, the seal surfaces are partially insulated to prevent a short circuit when the instrument jaws are closed together. In another embodiment, the seal surfaces are removably mounted on the jaws.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS: 
   This application is a continuation of U.S. patent application Ser. No. 10/090,081 (now U.S. Pat. No. 6,743,229) filed on Mar. 1, 2002, which is a continuation of U.S. patent application Ser. No. 09/502,933 (now U.S. Pat. No. 6,352,536) filed on Feb. 11, 2000, which is a continuation of U.S. patent application Ser. No. 08/968,779 (now U.S. Pat. No. 6,187,003) filed on Nov. 12, 1997, the entire contents of all of which being incorporated by reference herein. 

   FIELD OF THE INVENTION 
   This invention relates to an electrosurgical instrument for permanently closing vessels in a human or animal, and more particularly to a bipolar electrosurgical instrument that seals vessels and vascular tissue by applying a combination of pressure and electrosurgical current. 
   BACKGROUND OF THE DISCLOSURE 
   A hemostat is commonly used in surgical procedures to grasp, dissect and clamp tissue. It is typically a simple pliers-like tool that uses mechanical action between its jaws to constrict vessels without cutting them. It is also typical for hemostats to have an interlocking ratchet between the handles so that the device can be clamped and locked in place. 
   Many hemostats are used in a typical open-surgical procedure. Once vascular tissue has been clamped with a hemostat, it is common for a surgeon to tie a suture around the tissue to close it off permanently prior to removing the hemostat. Several hemostats may be left in the surgical field until the surgeon has the opportunity to tie a suture around each section of clamped tissue. 
   Small blood vessels have been closed using electrosurgical instruments without the need for sutures. For example, neurosurgeons have used bipolar instruments to coagulate vessels in the brain that are smaller than two millimeters in diameter. These bipolar instruments are typically tweezers-like devices with two arms that can be deflected toward each other to grasp tissue. However, it has been found that these instruments are not capable of sealing blood vessels with diameters larger than about two millimeters. There has been a long-felt need for an easy way to seal larger vessels and vascular tissue bundles without the need for sutures. 
   It is thought that the process of coagulating small vessels is fundamentally different than vessel sealing. Coagulation is defined as a process of desiccating tissue wherein the tissue cells are ruptured and dried. Vessel sealing is defined as the process of liquefying the collagen in the tissue so that it crosslinks and reforms into a fused mass. Thus, coagulation of small vessels is sufficient to permanently close them. Larger vessels need to be sealed to assure permanent closure. 
   A number of bipolar electrosurgical forceps and clamps are known in the field. However, these instruments are not designed to apply the correct pressure to a blood vessel to achieve a lasting seal. All of these instrument also suffer from the drawback that they do not combine the simplicity and familiarity of a hemostat with a bipolar electrosurgical circuit. 
   An example of a bipolar electrosurgical power curve for vessel sealing is disclosed in a U.S. patent application entitled, “Energy Delivery System for Vessel Sealing,” Ser. No. 08/530,495, filed Sep. 19, 1995, and is hereby incorporated by reference and made a part of this disclosure. 
   A U.S. patent application entitled, “Vascular Tissue Sealing Pressure Control and Method,” Ser. No. 08/530,450, filed on Sep. 19, 1995, discloses another surgical tool for sealing vessels, and is hereby incorporated by reference and made a part of this disclosure. 
   One of the important advances of the present system is that it can effectively seal larger vessels of a patient without leaving any foreign material in the body of the patient. The present system is capable of sealing vessels as large as ten millimeters in diameter. Another advantage of the present system is that the surgeon can visually inspect the integrity of the seal. 
   This invention works with a combination of pressure and controlled application of electrosurgical energy to achieve the desired result. Therefore, the system requires a tool to grasp and apply an appropriate amount of pressure to the tissue of the patient. The term “pressure” refers to the closure force on the vessels or other tissue that is applied by the end effectors of the tool. The tool must also be capable of conducting electrosurgical energy to the tissue concurrently with the application of pressure. 
   An electrosurgical generator is used to generate the electrosurgical energy. The electrosurgical energy is preferably applied in a specified manner by using an automatic control system. The control system regulates the output current and the output voltage of the electrosurgical generator in a manner that provides optimal vessel sealing. 
   One of the advances of the present invention is that a high current is applied to the tissue in order to melt the proteins. The high current is important for its effect on the tissue. Similarly, the output voltage is regulated to reduce sparking and localized tissue heating. The voltage is preferably kept below one hundred sixty volts RMS, and in the preferred embodiment is kept below one hundred twenty volts RMS. 
   Earlier attempts to seal vessels with electrosurgery were unsuccessful in part because a relatively low current was applied. The present invention may draw a maximum current in excess of two amperes RMS through the tissue. This level of output current is higher than the design capabilities of many presently available electrosurgical generators. 
   Charring of the tissue can be avoided by terminating the flow of electrosurgical energy to the tissue at an appropriate time. There are several techniques for determining when to terminate the electrosurgical energy. One technique is to monitor the impedance of the output load on the electrosurgical generator. When the impedance reaches a certain level, preferably above one thousand ohms, the electrosurgical energy should be terminated. 
   Another technique is to monitor the phase angle between the output voltage and the output current. Energy delivery to the surgical tool should be terminated preferably when the output current leads the output voltage by an angle greater than approximately fifty degrees. 
   A third technique for determining when to terminate the electrosurgical energy is to monitor the output current. As the tissue desiccates, the amount of electrical current flowing through the tissue decreases. The generator may terminate the energy delivery to the surgical tool when the output current drops below approximately 200 milliamperes RMS. 
   It is preferable to maintain pressure on the vessels or tissue of the patient for a short time after the electrosurgical energy has been substantially terminated. This allows the tissue to cool in its newly sealed state. An audible tone indicator in the generator is preferably available to indicate to the surgeon when it is appropriate to release the pressure on the tissue. The time delay may be up to five seconds after terminating the energy delivery to the surgical tool. 
   In the preferred embodiment there are four main steps for using the tissue sealing system. The first step may include applying and maintaining pressure on the tissue. The second step may include rapidly heating the tissue with electrosurgical energy. The third step may include lowering the energy which is delivered to the tissue so that the tissue will desiccate without charring. The final step may include terminating the electrosurgical energy delivery to the tissue so that the tissue is allowed to cool while still under pressure. 
   An automatic control system is preferably located within the electrosurgical generator and has, as one of its functions, the ability to automatically transition through the different levels of electrosurgical energy delivery. In an alternative embodiment, the power delivery to the surgical tool may not have discrete, step-wise levels. Instead, the power delivery may be a smooth function which initially delivers a high current, and then transitions to a lower power lever to desiccate the tissue, followed by termination of the power delivery when the impedance of the tissue rises above approximately one thousand ohms. 
   What follows is a summary of the various embodiments of the invention. The preferred embodiment of the electrosurgical energy delivery system is used for sealing vessels and other tissues of a patient. The system comprises a generator, a surgical tool, and means for controlling the level of electrosurgical energy which is delivered to the tissue. 
   The generator is preferably capable of delivering a controlled level of high frequency electrosurgical energy. The output of the generator may be characterized as having an output voltage and an output current which are each regulated in the preferred embodiment. The generator in the present system could limit the output voltage to a value below one hundred sixty volts RMS, and most preferably would be limited below one hundred twenty volts RMS. One of the reasons for limiting the output voltage is to avoid sparks and arcing which cause local high temperature zones in the tissue, and can also result in the tissue sticking to the electrodes. Another disadvantage of arcing is that it may result in transection of the vessel. 
   The surgical tool is most preferably connected to the generator output for receiving the electrosurgical energy. The surgical tool may take the form of forceps, clamps, or any instrument with articulating members for grasping tissue. 
   In a bipolar configuration, one member of the surgical tool will be electrically connected to be an active electrode, and another member of the surgical tool will be electrically connected to be a return electrode. Alternatively, in a monopolar arrangement, the surgical tool may be electrically connected to only one electrical pole of the generator, while the patient is electrically connected to the other electrical pole. While the members are grasping tissue, electrosurgical energy from the generator will flow in circuit through the tissue. 
   In the preferred embodiment, there are means for controlling the level of electrosurgical energy delivered to the surgical tool. The level of electrosurgical energy is controlled such that the vessels and other tissues are sealed as they are grasped by the members of the surgical tool. The level of electrosurgical power may refer to the RMS power output of the generator, which may be a function of output voltage, output current, frequency, and duty cycle. 
   The surgical tool may also have means for applying pressure to the vessels and other tissues between the members concurrently with the application of the electrosurgical energy. The pressure application means can take of the form of a latch or indent which holds a known spring force against the members of the tool. There may be several selectable levels of pressure available from the surgical tool. For example, it may be desirable to apply a high level of pressure to arteries and vascular tissue, and a lower level of pressure to veins. 
   During an operation, the surgeon may grasp a vessel with the surgical tool and operate the mechanisms on the tool to apply the desired level of pressure to the vessel. Once the pressure has been applied to the vessel, the surgeon may activate the electrosurgical energy. The generator applies the appropriate amount of electrosurgical energy according to a specified power curve. 
   There are several methods for feedback control to the generator. Feedback control is important because the transition points in the power curve are scheduled to occur according to the state of the tissue. In addition, it would be undesirable to apply too much energy to the tissue and thus cause charring and sticking. Several parameters may be monitored for purposes of feedback control. These parameters include the impedance of the tissue, the phase angle between the output voltage and output current, the level of output current flowing through the tissue, and the temperature of the tissue. 
   It is preferable for the generator to have means for at least approximating impedance of the vessels and other tissues of the patient as they are grasped by the members of the surgical tool. For example, one way to approximate the impedance of the tissue is to assume that the impedance is mostly resistive, and thus make the approximation by dividing the output voltage by the output current. Other numerical techniques for approximating impedance are available so that a long division need not be performed. One such approximation technique is to scale the output voltage and output current appropriately so that a range of impedance may be estimated by mere comparison and bit shifting in a digital circuit. 
   Impedance of the tissue is a good indicator of the state of desiccation of the tissue. One reason for having an estimate of the impedance is to control the level of electrosurgical energy so that it is substantially terminated when the impedance of the vessels and other tissues rises above approximately one thousand ohms. In certain embodiments of the invention, it may be convenient to terminate the energy delivery to the surgical tool when the estimate of impedance rises above 2048 ohms. 
   The preferred means for controlling the level of electrosurgical energy comprises several stages. The first stage is a rapid power delivery function for rapidly increasing the power delivery to the vessels and other tissues until a first impedance breakpoint is reached. The second stage is a constant power delivery function for maintaining a constant power delivery to the vessels and other tissues until proteins in the vessels and other tissues have melted. The third stage is a low power delivery function for maintaining a low power delivery to the vessels and other tissues until a second impedance breakpoint is reached. In the preferred embodiment, the transitions between the stages are executed automatically in the generator without further input from the surgeon. The impedance breakpoints are preferably 16 ohms for the first breakpoint, and 2048 ohms for the second breakpoint. 
   A method for sealing vessels and other tissues of a patient is also claimed. The method comprises the steps of: applying pressure to the vessels and other tissues of the patient; applying a first level of electrosurgical energy to the vessels and other tissue sufficient to melt proteins in the tissue; applying a second level of electrosurgical energy to the vessels and other tissue sufficient to cause desiccation without charring; reducing the electrosurgical energy substantially for a length of time sufficient to allow the vessels and other tissues to cool into a new compressed form; and relieving the pressure on the tissue. The step of relieving the pressure on the tissue may occur after a delay of less than five seconds. Additionally, there may be a step of creating an audible indication after the delay is over. 
   An additional step in the method may be approximating impedance of the vessels and other tissues. If this step is carried out, there may be another step of terminating the second level of electrosurgical energy after the impedance of the vessels and other tissues rises above approximately one thousand ohms. 
   FIG.  1 ′ is a schematic diagram of an electrosurgical vessel sealing system. 
   FIG.  2 ′ is a set of power curves which represent the electrosurgical power delivered to the tissue as a function of the tissue impedance. 
   An electrosurgical energy delivery system  10 ′ is shown in FIG.  1 ′. The system  10 ′ is used for sealing vessels and other tissues of a patient  13 ′, including ducts, veins, arteries, and vascular tissue. The system  10 ′ comprises an electrosurgical generator  11 ′, a surgical tool  12 ′, and means to control the output of the electrosurgical generator  11 ′ such that it works cooperatively with the surgical tool  12 ′ to effectively seal vessels and other tissues of a patient  13 ′. 
   The electrosurgical generator  11 ′ must be capable of delivering a controlled level of electrosurgical output power. The output power may be controlled by adjusting the output current and the output voltage. The surgical tool  12 ′ is electrically connected to the generator  11 ′ for receiving the electrosurgical power. The surgical tool  12 ′ has members  14 ′, or end effectors, capable of grasping the vessels and other tissues of the patient  13 ′. The members  14 ′ are also capable of applying and maintaining a relatively constant level of pressure to the vessel. 
   The electrosurgical generator  11 ′ must have means for automatically controlling the level of electrosurgical power delivered to the surgical tool  12 ′. This can be in the form of a feedback control system. In the preferred embodiment, there are also circuits for limiting the output current and the output voltage. In one embodiment, an adjustable high voltage power supply is used to adjust an RF output stage for controlling the electrosurgical output. 
   The power output of the generator  11 ′ is described in terms of a power curve, and a preferred embodiment is shown in FIG.  2 ′. The power curve may be described in terms of several stages. The stages may be discrete, or may be approximated by a smooth continuous function. In the first stage of the power curve, the electrosurgical generator  11 ′ delivers output power even at impedances below approximately sixteen ohms, and holds a high power lever until the proteins in the tissue have sufficiently melted. During this first stage, the output current is allowed to increase to a maximum amplitude which is typically greater than two amperes RMS. It has been found that a high current is important for effective vessel sealing. 
   After the first stage, the electrosurgical power is lowered to a level sufficient to desiccate the vessels and other tissues. The lower power enables the desiccation to occur without charring the tissue. 
   A final stage involves allowing the tissue to cool into its new sealed form. During this final stage, the application of electrosurgical power to the tissue is substantially terminated. After the tissue has cooled, the closure force is released. The length of time for cooling is typically less than five seconds. In the preferred embodiment, a audible tone would indicate to the surgeon that the sealing process was complete. The surgeon would thereafter release the vessel from the surgical tool  12 ′. 
   It is thought that the initial high current causes proteins in the tissue to melt. The subsequent lower power delivery to the tissue allows the proteins to cross link. As the tissue cools, the new cross linked tissue will form a permanent seal of the vessel. 
   The surgical tool  12 ′ may further comprise an index for selectively applying multiple levels of closure force between the members  14 ′. For example, arteries will require a greater closure force than veins. It has been found that a closure force of greater than 1500 grams is effective for sealing arteries. A closure force of less than 500 grams is effective for sealing veins. 
   In the preferred embodiment, the surgical tool  12 ′ will have a spring that compresses to hold a closure force on the members  14 ′. The index is mechanically linked to the spring such that each successive stop on the index holds a higher compression on the spring. The spring will not begin to compress until the members  14 ′ encounter resistance to closure. 
   In the preferred embodiment, the generator  11 ′ further comprises means for approximating impedance of the vessels and other tissues of the patient  13 ′ as they are grasped by the members  14 ′ of the surgical tool  12 ′. The calculation of impedance can require long division and other lengthy mathematical manipulations. There are a variety of techniques for making a quick approximation of impedance which would be sufficient for purposes of controlling the power output of the electrosurgical generator  11 ′. For example, comparison of the output voltage with the output current can yield an estimate of the impedance without resorting to long division. 
   The impedance of the tissue gives an indication of the state of desiccation of the tissue. By monitoring impedance, the generator  11 ′ can provide the appropriate amount of electrosurgical energy without charring the tissue. For example, the power control circuit includes a power cutoff function for substantially terminating the power delivery to the surgical tool  12 ′ when the impedance of the vessels and other tissues rises above approximately one thousand ohms. 
   The power control curves shown in FIG.  2 ′ represent the electrosurgical output of the generator  11 ′ as a function of tissue impedance. At low impedances, the electrosurgical power is increased by rapidly increasing the output current, as shown by the segment labeled A. The increase in electrosurgical power is terminated after a first impedance breakpoint is reached. The first impedance breakpoint is shown as Point  1  in FIG.  2 ′. In the preferred embodiment, this point is typically below 20 ohms. 
   Next, the electrosurgical power is held approximately constant until proteins in the vessels and other tissues have melted. The impedance at which this segment ends will vary in accordance with the magnitude of the RMS power. Thus, where the maximum RMS power is approximately 125 Watts, this segment will end at approximately 128 ohms. This is shown as the segment labeled B in FIG.  2 ′. Where a lower power is used, such as 75 Watts, the segment may end at 256 ohms. This is shown as the segment labeled C in FIG.  2 ′. 
   Next, the output power is lowered to less than half of its maximum value. The low power delivery is terminated when a second impedance breakpoint is reached. In the preferred embodiment, the second breakpoint is approximately at 2048 ohms. 
   As an alternative to using impedance to determine the second breakpoint, the phase angle between current and voltage may be used. In this alternative embodiment, the generator  11 ′ includes means for substantially terminating the power delivery to the surgical tool  12 ′ when the output current leads the output voltage by an angle greater than approximately fifty degrees. 
   In yet another alternative embodiment, the generator  11 ′ will terminate the power delivery to the surgical tool  12 ′ when the output current drops below approximately 200 milliamperes RMS. 
   It is desirable to have the generator  11 ′ limit its output voltage at all times to less than one hundred sixty volts RMS. The reason for keeping the output voltage low is to prevent arcing and the resulting localized tissue burn spots which might cause the tissue seal to fail. 
   A method for sealing vessels and other tissues of a patient  13 ′ comprises the following steps. First, apply a closure force to the vessels and other tissues of the patient  13 ′ sufficient to substantially close off the interior passages of the vessels or tissue. Second, apply a first level of electrosurgical power to the vessels and other tissues, wherein the peak output current is greater than two amperes and the peak output voltage is less than one hundred sixty volts RMS. Third, reduce the electrosurgical power to a second level which is less than half of the first level. Fourth, apply the second level of electrosurgical power to the vessels and other tissue of the patient  13 ′ for a length of time sufficient to cause desiccation without charring. Fifth, reduce the electrosurgical power substantially for a length of time sufficient to allow the vessels and other tissues to cool into a new compressed form. Sixth, relieve the closure force on the tissue. 
   The fifth step of reducing the electrosurgical power can be accomplished either by terminating the power to the surgical tool  12 ′, or by reducing the power to the surgical tool  12 ′ to a very low level. In one embodiment, the electrosurgical energy would be terminated completely so that the tissue  13 ′ would cool in the fastest time possible. In an alternative embodiment, the generator  11 ′ would continue to output approximately one watt of power for the purpose of maintaining a closed circuit with the tissue  13 ′ until the tissue has cooled into its compressed form. 
   In the preferred embodiment, the method for sealing vessels and other tissues will have the additional step of periodically approximating the impedance of the vessels and other tissues. This step will enable a control system in the generator  11 ′ to adjust the output power in accordance with the impedance of the tissue. For example, the step of applying a second level of electrosurgical power would be terminated after the impedance of the vessels and other tissues rises above approximately one thousand ohms. 
   Alternatively, the step of substantially terminating the power delivery to the surgical tool  12 ′ can occur when the output current leads the output voltage by an angle greater than approximately fifty degrees. An additional alternative is to terminate the power delivery to the surgical tool  12 ′ when the output current drops below approximately 200 milliamperes RMS. 
   In the preferred embodiment, there are additional steps of limiting the output voltage to a value below approximately one hundred sixty volts RMS, and audibly indicating when the closure force on the vessels and other tissues should be removed. The audible indication occurs after substantially reducing the level of electrosurgical power, and after a further delay of less than five seconds. 
   U.S. Pat. No. 371,664 discloses a pair of electric forceps with positive and negative electric poles located on the jaws. 
   U.S. Pat. No. 728,883 discloses an electrothermic instrument in which electricity is used to heat one of the jaws of the instrument. 
   U.S. Pat. No. 1,586,645 discloses a bipolar instrument for coagulating tissue. 
   U.S. Pat. No. 2,002,594 discloses a bipolar laparoscopic instrument for treating tissue, whereby coagulation and cutting of tissue can be performed with the same instrument. 
   U.S. Pat. No. 2,176,479 discloses an instrument for finding and removing metal particles. The jaws of the instrument are designed to complete an electrical circuit when conductive material is placed therebetween. An insulated pivot and an insulated ratchet are used to prevent a short circuit. 
   U.S. Pat. No. 3,651,811 discloses a bipolar electrosurgical instrument for cutting and coagulating tissue. 
   U.S. Pat. No. 4,005,714 discloses bipolar coagulation forceps with jaws that open and close by way of an actuating sleeve. 
   U.S. Pat. Nos. 4,370,980 and 5,116,332 disclose an electrocautery hemostats wherein the hemostatic clamping function and the electrocautery function may be accomplished with a single instrument. Monopolar electrosurgical designs are shown and described. 
   U.S. Pat. No. 4,552,143 discloses a family of removable switch electrocautery instruments, including an electrocautery hemostat. Monopolar electrosurgical designs are shown and described. 
   U.S. Pat. No. 5,026,370 discloses an electrocautery forceps instrument having an enclosed electrical switching mechanism. Monopolar electrosurgical designs are shown and described. 
   U.S. Pat. No. 5,443,463 discloses coagulating forceps having a plurality of electrodes. 
   U.S. Pat. No. 5,484,436 discloses bipolar electrosurgical instruments for simultaneously cutting and coagulating tissue. 
   The article, “The Mechanism of Blood Vessel Closure by High Frequency Electrocoagulation” discloses experiments upon the blood vessels of dogs. The sentence starting on the last line of page 823 describes “an electrode forceps, each of the blades being insulated form the other and each connected to a terminal of the high frequency generator.” 
   The article, “Studies on coagulation and development of an automatic computerized bipolar coagulator” discloses on page 150 that, “It was not possible to coagulate safely arteries with a diameter larger than 2 to 2.5 mm.” On page 151, line 5, it is noted that “Veins can be coagulated safely up to a diameter of 3 to 4 mm.” 
   Russian Patent 401,367 discloses a bipolar instrument with a linkage that brings the working jaws together in a parallel manner. 
   Prior disclosures have not provided a design for a bipolar electrosurgical instrument capable of conveniently applying a constant pressure, from a calibrated spring-loaded source held by a ratchet, that is sufficient to seal vessels and vascular tissue. 
   SUMMARY OF THE INVENTION 
   It is the general objective of this invention to provide a bipolar electrosurgical instrument that can fuse tissue without the need for a suture or surgical clips. The instrument conducts electrosurgical current between two seal surfaces located on opposable jaws. The electrosurgical current passes through tissue clamped between the jaws and remolds the collagen to fuse the tissue and form a permanent seal. 
   One advantage of the invention is that blood vessels can be quickly fused and permanently sealed against passage of blood or other fluids. The instrument thereby reduces operating-room time, provides improved access to target tissues, and increases the efficiency of the surgical procedure. 
   Another advantage is that no sutures or staples are required to permanently seal blood vessels, and no foreign material is left in the body of the patient. 
   Yet another advantage is that vessels can be sealed as the instrument is applied, and then the instrument can be removed from the surgical field. This keeps the surgical field clear of extraneous tools that may hinder the surgeon&#39;s access to the surgical site. 
   Yet another advantage is that the proper amount of pressure can be applied by the instrument to the vessel or vessels, thereby increasing the likelihood of a successful surgical outcome. 
   The bipolar electrosurgical instrument of the present invention comprises inner and outer members connected by an open lockbox, interlocking ratchet teeth, and electrical terminals with conductive pathways leading to seal surfaces. The inner and outer members each have a ring handle near a proximal end and an opposable seal surface near a distal end. The proximal end is held and controlled by the surgeon, while the distal end is used to manipulate tissue. The open lockbox joins the inner and outer members to allow arcuate motion of each opposable seal surface. The open lockbox is generally designed to provide lateral support so that both seal surfaces move in approximately the same plane. The seal surfaces are preferably aligned opposite each other when the instrument jaws are closed together. To provide lateral support, the open lockbox comprises a pivot and at least one flange extending over the inner member and attached to the outer member. 
   The instrument is tuned to provide a proper closure force by adjusting the dimensions of a shank portion on each of the inner and outer members. The shank portion is defined as the portion of each member bounded by its respective ratchet stub and the open lockbox. During use, the surgeon squeezes the ring handles to compress tissue between the seal surfaces. The shank portion of each member flexes in the manner of a cantilever spring, and can be locked in a deflected position with the ratchet to hold a constant force. It is one of the objects of the invention to provide a range of ratchet stops that correspond to a range of appropriate closure forces on the seal surfaces of the instrument. 
   Ratchet teeth are located on each member near the ring handle. The ratchet teeth are generally designed to interlock against the spring force from the shanks. The spring force is thus transmitted through the pivot to hold the seal surfaces against each other. A range of closure forces is required in an instrument, depending on the type and thickness of the tissue to be sealed. It is thus desirable to have several ratchet stops, each providing a progressively larger force to the seal surfaces. 
   An electrical connector is located on each ring handle. The electrical connector may be a metal post that is integrally formed with the member and ring handle. Bipolar electrical cables from an electrosurgical generator are connected to the instrument at the electrical connectors. An electrically conductive path on each of the inner and outer members conducts the electrosurgical current to the seal surfaces. The electrically conductive path may be along the stainless steel members. An electrically insulative coating is preferably bonded to the outer surfaces of the members to protect the surgeon and patient against inadvertent electrical burns. 
   The following terms are herein defined as follows. The applied force of the instrument is the total force being applied to the tissue between the jaws. The jaws are the members near the distal end of the instrument, from the lockbox to the tip of the instrument. The electrodes are the metal surfaces that conduct electricity to the tissue. The seal surface is the feature on the electrode that comes in direct contact with the tissue. The shank is the portion of each member between the lockbox and the ratchet. The ring handles are the elements on the members, near the proximal end of the instrument, that are grasped by the surgeon. The lockbox is the structure that allows the members to pivot, including the pivot pin and other cooperating surfaces. The inner member is the member that is generally captured in the interior of the lockbox. The outer member is the member that is on the outside of the lockbox. Electrode pressure is calculated by dividing the applied force over the complete area of the seal surface. Tissue pressure is calculated by dividing the applied force over the area of tissue placed between the jaws. 
   It has been found through experimentation that an instrument for vessel fusion (also referred herein as vessel sealing) should compress the tissue with a proper amount of pressure between the instrument jaws. The pressure is preferably sufficient to close any blood-carrying lumen. The pressure is preferably low enough so that the tissue is not split apart within the instrument jaws. 
   The jaws of the instrument should not short-circuit during the procedure. The tissue will typically decrease in thickness when electrosurgical current is applied, thereby allowing the seal surfaces to move closer together. This decrease in thickness should not result in the electrodes making direct contact with each other. Otherwise, a short circuit could give the electrosurgical current a preferential path around the tissue and may result in a poor seal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a bipolar instrument for vessel fusion, shown partially exploded. 
     FIG.  1 ′ is a schematic diagram of an electrosurgical vessel sealing system. 
       FIG. 2  is a schematic plan view of a bipolar instrument for vessel fusion having a longer curved jaw. 
     FIG.  2 ′ is a set of power curves which represent the electrosurgical power delivered to the tissue as a function of the tissue impedance. 
       FIG. 3  is a side view of the instrument shown in  FIG. 2 . 
       FIG. 4  is a schematic plan view of an alternative embodiment of an instrument for vessel fusion having a shorter curved jaw. 
       FIG. 5  is side view of the instrument shown in  FIG. 4 . 
       FIG. 6  is a schematic plan view of an alternative embodiment of an instrument for vessel fusion having a straight jaw. 
       FIG. 7  is a side view of the instrument shown in  FIG. 7 . 
       FIG. 8  is a perspective view of a shoulder pin. 
       FIG. 9  is a side view of a shoulder pin. 
       FIG. 10  is a front view of a shoulder pin. 
       FIG. 11  is a top view each of a pair of seal surfaces showing conductive regions and insulative regions that prevent a short circuit when the seal surfaces are mated in opposition. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , the instrument  10  has an inner member  11  and an outer member  12 . The members  11  and  12  are connected through an open lockbox  13  which has a gap between flanges  33 . The terms “inner” and “outer” are used to distinguish the members  11  and  12 , and their component parts, according to the members&#39; respective positions at the open lockbox  13 . The inner member  11  is fitted generally within the inner surfaces of the open lockbox  13  and is captured by the flanges  33 . The outer member generally forms the outside surfaces of the open lockbox  13 . 
   The inner member  11  has an inner shank  14 , an inner jaw  16 , and an inner ring handle  20 . Similarly, the outer member  12  has an outer shank  15 , an outer jaw  17 , and an outer ring handle  21 . The ring handles,  20  and  21 , are designed for a surgeon to hold and manipulate the instrument  10 . The jaws,  16  and  17 , are designed to grasp tissue between the opposing seal surfaces  18  and  19 . 
   Each shank,  14  and  15 , has a respective ratchet stub  24  or  25 . Ratchet teeth,  26  and  27 , are designed to interlock in a manner that hold the members,  11  and  12 , in position. The shanks  14  and  15  are deflected in the manner of a cantilever spring when the jaws are forced together by the surgeon. The deflection of the shanks  14  and  15  produces a spring restoring force that can be opposed by interlocking the ratchet teeth,  26  and  27 . 
   The instrument  10  does not cause a short circuit when the ratchet teeth,  26  and  27 , are interlocked. This is accomplished by a suitable selection and placement of electrically insulating materials. In the preferred embodiment, the ratchet teeth  26  and  27  are composed of a polymeric material which is press-fit into the ratchet stubs  24  and  25 . A ratchet screw  28  is used in the preferred embodiment to secure the ratchet teeth  26  and  27  into the ratchet stubs  24  and  25 . During manufacture, the ratchet teeth  26  and  27  may be formed from a blank after the blank has been press fit into the ratchet stubs  24  and  25 . 
   In a second embodiment, one of the members,  11  or  12 , includes the ratchet stub and ratchet teeth as in integral part of the member, while the other member,  12  or  11 , has an insulative layer that prevents a short circuit between the members  11  and  12  when the ratchets are engaged. 
   The open lockbox  13  has the function of providing a pivoting joint for the members  11  and  12 . In addition, the flanges  33  provide lateral support to help maintain alignment of the jaws  16  and  17 . Closed lockbox designs are typically used in standard hemostat designs, wherein an inner member is completely captured through a slot in an outer member. The open lockbox  13  in present invention has a gap between the flanges  33  that is different from a closed lockbox design. The gap in the open lockbox  13  provides convenient access to install an electrically insulated pivot. 
   The electrically insulated pivot in the present invention comprises a shoulder washer  29  supporting a lockbox screw  30 . The shoulder washer  29  is composed of an electrically insulative material that prevents a short circuit between the members  11  and  12 . A large screw cap  31  fits over the head of the lockbox screw  30 . A small screw cap  32  fits over the threaded end of the lockbox screw  30 . 
   Each member  11  and  12  is connected to a pole of a bipolar electrosurgical generator. Electrical connectors  22  and  23  are located on the ring handles  20  and  21  to provide a convenient point of connection. The members  11  and  12  are formed of an electrically conductive material, such as stainless steel. The exposed surfaces of the members, except for the connectors  22  and  23  and the seal surfaces  18  and  19 , are preferably spray coated with an insulating material. 
   The characteristics of the bipolar electrosurgical current are determined by the design of the electrosurgical generator. In the preferred embodiment, the generator will have an output wherein the peak-to-peak voltage will not exceed 130 Volts. This is because higher voltages can cause sparking which results in localized burning of tissue which may result in a failure of the tissue weld. The preferred embodiment has the generator capable of producing high frequency output current of at least 2 Amps RMS. High electrical current is important because it heats the tissue sufficiently to melt the collagen. Lower electrical currents will often produce weak tissue welds with low bursting strength. 
   During operation, the instrument  10  is used to grasp tissue between the seal surfaces  18  and  19 . The surgeon squeezes the ring handles  20  and  21  together, causing pressure to be applied to the tissue. The ratchet teeth  26  and  27  are interlocked at the appropriate ratchet setting, depending on the tissue type and tissue thickness. Bipolar electrosurgical current is applied through the instrument and the tissue to cause the tissue to fuse. 
   The jaws  16  and  17  have a structure and cross-section that resist bending under load. Thus, for purposes of engineering analysis, the shank portions  14  and  15  act as a cantilever supported beam once the seal surfaces  18  and  19  have been mated. The length of this idealized cantilever beam extends from the lockbox screw  30  to the location of the respective ratchet subs  24  or  25 . It is possible to model each shank as a cantilever spring having a spring constant. Each ratchet position is designed to transmit a particular closure force to the jaws  16  and  17  against the action of the restoring force of the cantilever spring. 
   The spring constant is generally a function of Young&#39;s Modulus of the shank material, the moment of inertia of the shank, and the length of the shank portion  14  and  15 . When the jaws  16  and  17  of the instrument  10  are closed together, each shank  14  and  15  approximates a cantilever-supported beam. It is properly assumed that the deflection of each shank  14  and  15  remains within the linear range of its stress-strain curve. The behavior of such a beam is well known to materials engineers. A large spring constant will result in large closure forces between the seal surfaces  18  and  19 . Similarly, a small spring constant will result in a small closure forces between the seal surfaces  18  and  19 . The choice of a proper spring constant will depend on the length of the shank  14  or  15  and the distance between ratchet stops  26  and  27 . 
   Experimental results in animal studies suggest that the magnitude of pressure exerted on the tissue by the seal surfaces  18  and  19  is important in assuring a proper surgical outcome. Tissue pressures within a working range of 7 kg/cm 2  to 13 kg/cm 2  have been shown to be effective for sealing arteries and vascular bundles. It is desirable to tune the spring constant of the shank portions  14  and  15 , in conjunction with the placement of the ratchet teeth  26  and  27 , such that successive ratchet positions will yield pressures within the working range. In one embodiment, the successive ratchet positions are two millimeters apart. 
   Pressure on the tissue can be described in several ways. Engineers will recognize that the amount of pressure exerted on the tissue depends on the surface area of the tissue that is in contact with the seal surfaces. In the one embodiment, the width of each seal surface  18  and  19  is in the range of 2 to 5 millimeters, and preferably 4 millimeters width, while the length of each seal surface  18  and  19  is preferably in the range of 10 to 30 millimeters. It has been found through experimentation that at least one interlocking ratchet position preferably holds the closure force between approximately 400 and 650 grams per millimeter of seal surface width. For example, if the width of the seal surface  18  and  19  is 4 millimeters, the closure force is preferably in the range of 1600 grams to 2600 grams. In one embodiment, the closure force is 525 grams per millimeter of width, yielding a closure force of 2100 grams for a 4 millimeter width seal surface  18  and  19 . 
   It has been found experimentally that local current concentrations can result in an uneven tissue effect, and to reduce the possibility of this outcome, each seal surface  18  and  19  has a radiused edge in the preferred embodiment. In addition, a tapered seal surface  18  and  19  has been shown to be advantageous in certain embodiments because the taper allows for a relatively constant pressure on the tissue along the length of the seal surfaces  18  and  19 . The width of the seal surfaces  18  and  19  is adjusted, in certain embodiments, wherein the closure force divided by the width is approximately constant along the length. 
   In one embodiment, a stop  37 , made from insulative material, is located in the instrument to maintain a minimum separation of at least 0.3 millimeters between the seal surfaces  18  and  19 , as shown in  FIG. 1 . The stop  37  reduces the possibility of short circuits between the seal surfaces  18  and  19 . 
   In certain embodiments, as shown in  FIGS. 11A and 11B , the seal surfaces  18  and  19  comprise conductive regions  38  and insulative regions  39  arranged such that each conductive region  38  opposes an insulative region  39  when the opposable seal surfaces  18  and  19  are mated in opposition. The seal surfaces  18  and  19 , in certain embodiments, may be removable from its respective member  11  or  12  by standard mechanical interfaces, such as a pin and socket arrangement. 
     FIG. 2  shows an embodiment for a thirty-two millimeter curved seal surface.  FIG. 3  is a side view of  FIG. 2 . The members  11  and  12  in  FIG. 2  are formed from American Iron and Steel Institute (AISI)  410  stainless steel. The length and cross sectional area of the shank portions  14  and  15  are shown in  FIGS. 2 and 3  to provide a spring constant of twenty-five pounds per inch deflection. 
   The embodiment shown in  FIGS. 4 and 5  has a twenty millimeter curved seal surface. The embodiment shown in  FIGS. 6 and 7  has a thirty-two millimeter straight seal surface. Each embodiment in  FIGS. 2 through 7  is designed to have the look and feel of a standard hemostat. 
     FIGS. 8 ,  9  and  10  show three views of a shoulder pin  34  that can be used, in certain embodiments, instead of the lockbox screw  30  to connect the members  11  and  12 . The shoulder pin  34  has at least one ramp surface  35  that engages one of the members  11  or  12  to cause increasing mechanical interference as the jaws  16  and  17  move toward each other. In one embodiment, the shoulder pin  34  forms part of the open lockbox  13  to aid alignment of the seal surfaces  18  and  19 . In another embodiment, the shoulder pin  34  is used without an open-lockbox  13 , and movably pins the members  11  and  12  together without a flange  33 . The interference fit may require the calibration of the instrument  10  to insure that the applied force will be sufficient to provide the appropriate working pressure between the seal surfaces  18  and  19 . A slightly higher spring constant in the shank portions  14  and  15  is preferably used, depending on the level of interference caused by the shoulder pin. 
   A method of using the bipolar electrosurgical instrument comprises the following steps. A surgeon grasps the ring handles  20  and  21  on the instrument  10  to manipulate the jaws  16  and  17 . A vessel or vascular tissue is compressed between the opposable seal surfaces  18  and  19 . The opposable seal surfaces  18  and  19  preferably come together in aligned opposition due to the alignment action of the open-lockbox  13 , or in certain embodiments due to the alignment action of the shoulder pin  34 . The surgeon further deflects the shank portions  14  and  15  of the members  11  and  12  to engage the ratchet teeth  26  and  27 . The engagement of the ratchet teeth  26  and  27  hold the shank portions  14  and  15  in their deflected positions to provide a constant spring force that is transmitted as a closure force to the jaws  16  and  17 . An electrosurgical generator is connected to the instrument  10  through connectors  22  and  23  on the ring handles  20  and  21 . An electrical switch is used to close a circuit between the generator and the instrument  10 . The switch may be a footswitch such as Valleylab&#39;s catalog number E6009, available from Valleylab Inc., Boulder Colo. The electrosurgical current flows through an electrically conductive path on each of the inner and outer members  11  and  12  between its respective electrical connector,  22  or  23 , and its respective seal surface,  18  or  19 . An electrically insulative coating  36  substantially covers each member  11  and  12 , except for the seal surfaces  18  and  19 , to protect the surgeon against electrical arcs. 
   It is to be understood that the above described embodiments are only illustrative of the application of the principles of the present invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of the present invention. The appended claims are intended to cover such modifications and arrangements.