Patent Publication Number: US-6217531-B1

Title: Adjustable electrode and related method

Description:
BACKGROUND OF THE INVENTION 
     The present application claims foreign priority based on German application 197 46 972 filed on Oct. 24, 1997. 
     1. Field of the Invention 
     The present invention relates to the area of lithotripters; more particularly, a lithotripter electrode having an automatically adjusting spark gap. 
     2. Description of Related Art 
     Lithotripters exist for the contact-free destruction of concrements, e.g. kidney stones, in living bodies. Such devices are also used for the treatment of orthopedic ailments such as heal spurs and tennis elbow as well as non-union of bone problems. Lithotripters and related hardware are described in a number of patents; all of those mentioned below are hereby incorporated by reference. 
     Lithotripters use an electric underwater spark to generate the shock waves necessary to effect treatment. The spark is generated by an electrode usually mounted in a reflector that is used to focus the shock waves. Examples of these attempts may be found disclosed in U.S. Pat. Nos. 4,608,983 and 4,730,614. 
     In general, shock wave generation uses a spark produced by a discharge between electrodes. The discharge across the spark gap results from the discharge of an electrical capacitor. Varying the amount of the charging voltage of the capacitor regulates the shock wave energy. A larger or smaller voltage results in the formation of a stronger or weaker spark and thus modifies the strength of the shock wave and the size of the therapeutically active focus and thus in turn the applied shock wave energy. 
     It is desirable to provide a broad energy spectrum because of the various energy levels of shock waves used to treat different ailments. However, the voltage cannot be varied at will without replacing the electrode assembly because the spark gap, the gap between the electrodes, controls the discharge process. A wider gap requires a larger minimum voltage to bridge the distance between the two electrodes with a spark. 
     Early lithotripter electrodes used a fixed spark gap. One disadvantage to a fixed-gap electrode is that the electrodes slowly burn away after repeated use, thus increasing the spark gap distance and requiring a greater amount of voltage to generate a spark. But the larger gap and larger minimum voltage produces a stronger shock wave. One invention intended to resolve the electrode burn off issue is disclosed in U.S. Pat. No. 4,809,682. 
     Another disadvantage is that a low energy shock wave requires a low amount of voltage to be used with a relatively narrow spark gap while a high-energy shock wave requires a large amount of voltage to be used with a relatively wide spark gap. Accordingly, low energy shock waves could not be generated immediately following treatment using high-energy shock waves and vice versa without wholesale replacement of the electrode assembly. If an electrode assembly with a relatively small spark gap is used with a higher voltage, an energy-inefficient spark is produced because a portion of the energy bleeds off into the surroundings and is transformed into acoustic energy while another portion is transformed into heat energy and does not contribute to the formation of the shock wave. In other words, the proper voltage applied to the capacitor must be matched with a proper spark gap to produce an efficient shock wave of the desired energy level. 
     Another disadvantage with some lithotripter electrode assemblies is the inability to easily exchange one set of electrodes for another. For example, if the electrodes are to be reconditioned or refurbished, electrodes that are permanently attached cannot be removed and replaced. 
     Subsequent to the disclosure of fixed-gap electrode assemblies, adjustable gap assemblies were invented to overcome the difficulties associated with fixed-gap assemblies. One type, as disclosed by Patent EP 0.349.915 suffers from the disadvantage that it must be adjusted manually; another type, disclosed in U.S. Pat. No. 4,730,614 can only be adjusted in one direction. 
     Accordingly, there remains a need for an improved, self-adjusting lithotripter electrode assembly that allows a variety of energy levels to be employed, compensates for electrode bum-off, and increases the overall life of the electrode assembly. 
     SUMMARY OF THE INVENTION 
     The present invention relates to medical treatment using shock wave therapy and related method; more particularly, a self-adjusting lithotripter electrode assembly. The preferred embodiment of the electrode assembly includes an insulator assembly, an electrode arrangement, a charging system, a mechanism for measuring electrical voltages, a mechanism for adjusting the distance between inner and outer electrode tips, and a controller. The insulator assembly includes an insulator body having a hollow central portion with a threaded inner wall. The insulator assembly also includes inner and outer conductors that are electrically connected to the charging system and are physically connected to inner and outer electrodes, respectively. The electrodes are positioned such that their longitudinal axes are aligned and the tips of the electrodes are in relatively close physical proximity. The distance between the tips is defined as the spark gap. The charging system includes a capacitor and a voltage source. The electrical measuring mechanism includes a conventional meter device. The controller includes a microprocessor, microcomputer, or equivalent device. 
     The operation is as follows. A voltage is applied to the capacitor that is charged at a constant rate. When the voltage reaches a certain level, a spark is produced across the spark gap as the capacitor discharges. The electrical measuring device measures the actual discharge voltage and a corresponding signal is sent to the controller. The controller then compares the discharge voltage to an optimum, i.e., reference, discharge voltage. If the spark gap is correctly adjusted, the discharge of the second capacitor is at its maximum voltage and no correction is made. However, if the spark gap is too narrow, the discharge of the second capacitor occurs before the capacitor has achieved its maximum value. If the spark gap is too wide, there is either only a partial discharge after the capacitor has reached its maximum value or no discharge at all. In either case, the spark gap is not set to its optimum distance, resulting in an incomplete use of the energy stored in the capacitor. Accordingly, the controller issues a correction signal to initiate a spark gap adjustment, thus actuating the motor and associated components. The motor engages the gearbox that in turn moves the threaded element forward or rearward, thus positioning the inner conductor and the inner electrode such that the spark gap is of a distance capable of producing a spark at the optimum or reference voltage. 
     An alternate embodiment utilizes an additional capacitor and an inductor. The discharge of the first capacitor does not take place directly across the spark gap, but instead discharges to a second capacitor that is directly connected to the electrode conductors. When the voltage from the second capacitor reaches a sufficient value, a spark is then created across the spark gap. The controller compares the charge and discharge characteristics of the second capacitor. If a discrepancy exists between the actual discharge voltage and the reference discharge voltage, the controller computes the proper spark gap and issues a signal to the motor, which results in a spark gap adjustment as described above. 
     One advantage of the present invention includes a solution to the electrode burn-off problem by automatically maintaining a proper spark gap. 
     Another advantage of the present invention includes the ability to provide a wide spectrum of energy levels without the necessity of replacing the electrodes. 
     Still another advantage of the present invention includes the ability to easily replace the electrodes when needed. 
     Yet still another advantage of the present invention includes the elimination of manual adjustment of the spark gap. 
     Yet still another advantage of the present invention includes the ability to both widen and narrow the spark gap. 
     Additional advantages of the present invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, which exemplifies the best mode of carrying out the invention. 
     The invention itself, together with further objects and advantages, can be better understood by reference to the following detailed description and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a system diagram of the preferred embodiment of the present invention. 
     FIG. 1B is an enlarged side elevational view of the electrode assembly of the present invention. 
     FIG. 1C is a forward end view of the electrode assembly shown in FIG.  1 B. 
     FIG. 2 is an electrical schematic of an alternate embodiment of the present invention. 
     FIG. 3A is a graph of the voltage experienced over time of the first capacitor of the alternate embodiment shown in FIG.  2 . 
     FIG. 3B is a graph of the voltage experienced over time of the inductor of the alternate embodiment shown in FIG.  2 . 
     FIG. 3C is a graph of the voltage experienced over time of the second capacitor of the alternate embodiment shown in FIG.  2 . 
     FIG. 4A is a graph of the voltage experienced over time of the second capacitor of the alternate embodiment shown in FIG. 2, including a voltage offset. 
     FIG. 4B is a graph of the integral of the voltage experienced over time of the second capacitor of the alternate embodiment shown in FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PRESENT INVENTION 
     Referring now to FIGS. 1A-1C, the preferred embodiment of the electrode assembly  100 , which has a forward end  101  and a rearward end  102 , includes a insulator assembly  200 , an electrode arrangement  300 , a charging system  400 , a mechanism  500  for measuring electrical voltages, a mechanism  600  for adjusting the distance between inner and outer electrode tips, and a controller  700 . 
     The insulator assembly  200  includes an insulator body  201  that is cylindrical in construction having a hollow central portion  201   a . The insulator  201  has a threaded inner wall  202 . The insulator  201  is mounted in a focusing device  900 , the focus device  900  having an outer wall  901  with an opening  901   a  through which the insulator  201  is partially disposed. The insulator  201  also includes an outer locking ring  215 , an inner locking ring  220 , and a seal  225 , best shown in FIG.  1 B. 
     The insulator assembly  200  further includes an inner conductor  205  and an outer conductor  210 . The inner conductor  205  is a rod-like component that is slidably positioned within the central portion  201   a  of the insulator body  201  as shown in FIG.  1 B. In the preferred embodiment, the inner conductor  205  has a threaded forward end  206  for engaging an inner electrode  305 , described in further detail below. The inner conductor  205  is made of an electrically conductive metal or equivalent material. The outer conductor  210  surrounds the insulator body  201  and is made of a material similar to or identical to that of the inner conductor. 
     The electrode arrangement  300  includes an inner electrode  305  and an outer electrode  310 . The inner electrode  305  is a short, rod-like component and has a tapered tip  306  and a threaded rearward end  307 . It is coaxially affixed to the inner conductor  205  via the threaded end  307  engaging the threaded end  206  of the inner conductor  205  as shown in FIG.  1 B and is partially disposed within the insulator  201 . Alternately, the inner electrode  305  may be soldered to the inner conductor  205  or attached in a similar manner. The inner electrode  305  is made of an electrically conductive metal or equivalent material and is electrically connected to the inner conductor  205 . 
     The outer electrode  310  is a short, rod-like component and also has a tapered tip  311 . The outer electrode  310  is supported by the outer electrode cage members  312 , each of which includes a hook  313  that is formed at a generally right angle to the cage member  312 . The outer electrode cage members  312  are J-shaped at the forward end, best shown in FIG. 1B, to help alleviate the stress caused by the high voltage. The outer electrode  310  is mounted to the insulator  201  at the forward end  101  of the electrode assembly  100  as shown. The outer electrode  310  is usually attached to the outer electrode cage  312  by a soldering process or equivalent. The outer electrode  310  is positioned such that the longitudinal axes of the inner and outer electrodes  305  and  310  are aligned and the tips  306  and  311  of the electrodes  305  and  310  are in relatively close physical proximity. The distance D between the tip  306  of the inner electrode  305  and the tip  311  of the outer electrode  310  is defined as the spark gap  315 . 
     The charging system  400  includes a high voltage switch  401 , typically a thyratron in the preferred embodiment, and a capacitor  405  that is a high-voltage variety of standard construction. It is electrically connected to the inner and outer conductors  205  and  210 . The capacitor  405  is also electrically connected to a voltage source (not shown) and the controller  700  as shown in FIG.  1 A. 
     The device  500  for measuring electrical voltages is a conventional electrical meter (not shown) or equivalent. It may be an integral part of the controller  700 , described below, or may be a separate unit. 
     The mechanism  600  for adjusting the spark gap  315  includes a motor  605 , a gearbox  610 , and a threaded element  615  having threads  616 . The motor  605  is mechanically connected to the gearbox  610  that in turn is mechanically connected to the threaded element  615 . The threaded element  615  is partially disposed within the rearward end of the insulator  201  such that the threads  616  on the outer surface of the threaded element  615  engage the threaded inner wall  202  of the insulator  201  at the rearward end  102  of the electrode assembly  100 . Alternately, the inner conductor  205  and the threaded element  615  may be a formed as a single integral component. 
     The controller  700  typically includes a microprocessor, microcomputer, or other like device (not shown) capable of performing at least complex mathematical and comparative functions. The controller  700  is electrically connected to the motor  605  and the capacitor  405  and  410 . 
     One feature of the present invention includes the ability to quickly change electrodes for reconditioning or other maintenance. First, the outer locking ring  215  is moved in the rearward direction. The inner locking ring  220  is also moved in the same direction, thus allowing the outer electrode cage hooks  313  to disengage from the groove  210   a  in the insulator body  210 . The outer electrode  310  and cage  312  is then pulled away from the electrode assembly  100 . With the outer electrode  310  and cage  312  out of the way, the inner electrode  305  may be unscrewed from the inner conductor  205 . New electrodes may then be easily installed with the hooks  313  of the new cage  312  engaging the groove  210  and locking rings  215  and  220  and spacer  225  frictionally retaining the hooks  313  in place. 
     The operation of the electrode assembly  100  of the present invention is as follows. A voltage V is applied to the capacitor  405 , which is charged at a constant rate. When the voltage reaches a certain level V d , a spark is produced across the spark gap  315  as the capacitor  405  discharges. The actual discharge voltage V d  is measured by the electrical measuring device  500  and a corresponding signal is sent to the controller  700 . The controller  700  then compares the discharge voltage V d  to an optimum, i.e., reference, discharge voltage V dref . If a discrepancy exists between the actual discharge voltage V d  and the reference discharge voltage V dref , the controller  700  computes the proper spark gap  315  and issues a signal to the motor  605 . The motor  605  engages the gearbox  610  that in turn moves the threaded element  615  forward or rearward, thus positioning the inner conductor  205  and the inner electrode  305  such that the spark gap  315  is of distance capable of producing a spark at the optimum or reference voltage V dref . 
     In an alternate embodiment of the present invention, a second capacitor  410  is used that is electrically connected in series with the first capacitor  405  with an inductor  415  in between the two capacitors  405  and  410  as shown in the electrical schematic FIG.  2 . The high voltage switch  401  used is a thyratron or equivalent. The controller  700  is also connected to the second capacitor  410 . 
     FIGS. 3A-3C are voltage vs. time graphs that depict the operation, that is, the sequence of electrical events, during the formation of a spark in the alternate embodiment. A voltage V is applied to the capacitor  405  that is charged at a linear rate over time t 1 , depicted by curve portion  10 . The controller  700 , via line  498  as shown in FIG. 2, monitors the charging of the capacitor  405 . The maximum load of the capacitor  405  is reached at point  11  and remains constant, i.e., fully charged over time t 2 , depicted by curve portion  12 . At a time certain, point  11 , the switch  401  is actuated and a controlled discharge is initiated, depicted by curve portion  14 . 
     As the voltage from the first capacitor  405  is discharged, the voltage experienced by L 1  begins to increase, as depicted by curve portion  20  in FIG.  3 B and the second capacitor  410  begins to charge, depicted by curve portion  30  in FIG. 3C, both occurring over time period t 3 . At the end of time period t 3 , the voltage experienced by L 1  reaches its maximum, V C1max , shown as point  21  in FIG.  3 B and the curve portion  30  in FIG. 3C reaches a point of inflection  31 , i.e., the point where the slope of the curve  30  changes from positive to negative. 
     During time period t 4 , the voltage experienced by the inductor  415  drops off as shown by curve portion  22  in FIG. 3B; the capacitor  410  continues to charge as shown by curve portion  32 , although the rate of charge is decreasing. As the voltage experienced by the inductor  415  reaches zero at the end of time period t 4 , the voltage V C2max  of the second capacitor  410  reaches its maximum value as depicted by point  34  in FIG.  3 C and the second capacitor  410  is fully charged. It is at this point, ideally, that the second capacitor  410  should discharge and a spark should form, as depicted by curve portion  36 . A spark formed at this point in time indicates that the spark gap  315  is at its optimum distance D and that all the energy in the second capacitor  410  is being used to form the spark. A spark that is produced before point  34  in FIG. 3C is reached indicates that the spark gap  315  is too narrow; a spark that is produced after point  34  is reached indicates the spark gap  315  is too wide. Generally speaking, a spark that is produced at between 90% and 100% of the second capacitor&#39;s maximum voltage V C2max  is considered acceptable. In other words, a spark produced in the hatched region A between curve portions  37  and  38  is considered acceptable for the present invention, although acceptable error parameters can be varied. The controller  700 , via line  499  as shown in FIG. 2, monitors the charging and discharging of the capacitor  410 . 
     If the spark gap  315  is much narrower than optimum, then a spark will be formed prior to the voltage curve reaching 90% of the maximum, V C2(.9)  value, shown by point  33  in FIG.  3 C. In such a case, the controller  700  issues a correction signal to the motor  605  and the spark gap  315  would be adjusted (made wider) by the method described above. If, on the other hand, the spark gap  315  is much wider than optimum, then either a) a spark will be formed subsequent to the voltage curve dropping off 90% of the maximum, V C2(.9)  value, shown by point  35  in FIG. 3C, or b) no spark will be produced at all, as shown by curve portion  39  in FIG.  3 C. In such a case where the spark gap  315  is much too wide, the controller  700  issues a correction signal to the motor  605  and the spark gap  315  would be physically adjusted (made narrower) by the method described above. 
     To increase the accuracy of the correction process described above, it is possible to examine a series of charges and discharges before making a spark gap correction, as opposed to examining only one charge and discharge cycle prior to making a correction. The controller  700  is programmed to analyze a predetermined number of charges and discharges prior to making a determination. The series is then statistically analyzed and only then is a correction made, if necessary. Thus, a single false voltage measurement or other glitch would not result in an unnecessary correction that would ultimately have to be recorrected. 
     As discussed above, it is possible to determine the optimum spark gap  315  by examining the charge and discharge voltage characteristics of the second capacitor. However, an even more accurate method is available. The method is accomplished by adding a negative 50% of the reference voltage to the curve of the second capacitor  410  as shown in FIG. 4A, resulting in a new curve  30 ′/ 32 ′ that has a point of inflection  31 ′ intersecting with the time axis. The new charge/discharge curve is then integrated and inverted by the controller  700 , resulting in an integral curve shown in FIG.  4 B. The maximum integrated value, V imax , shown as point  41  in FIG. 4B, corresponds to the point of inflection  31 ′ in FIG.  4 A. If the discharge of the second capacitor  410  occurs in the acceptable range shown by hatched area A in FIG. 4A, such as is the depicted by point  34 ′, the discharge will appear in the acceptable range depicted by hatched area B in FIG. 4B as point  44 . A discharge that occurs too soon (which would appear along curve portion  32 ′ in FIG. 4A) because of a spark gap that is too narrow appears on the integral curve portion  42  above the upper reference value V ihi . Similarly, a discharge that occurs too late, or not at all (which would appear along curve portion  39 ′ in FIG.  4 A), because of a spark gap that is too wide will appear on the integral curve portion  49  below the lower reference value V ilo . In either case, the unacceptable discharge value would result in a correction signal being sent by the controller  700 . The most important benefit of integrating the voltage characteristic curve of the second capacitor  410  is a “magnified” look at the acceptable range resulting in a more accurate account of events. 
     The integration technique can be combined with the statistical analysis approach, both described above, to obtain an extraordinarily accurate method of determining and adjusting the spark gap  315 . 
     Of course, it should be understood that a wide range of changes and modifications could be made to the exemplary embodiments described above. It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting and that it be understood that it is the following claims, including all equivalents, which are intended to define the scope of this invention.