Abstract:
The invention is directed to an electrosurgical device having a shapeable elongate cutting electrode having a free distal end with an exposed length of at least 0.5 inch and secured by its proximal end to the distal end of a handle. The electrosurgical device is designed for use with a high frequency electrosurgical generator which has an output at a frequency of between about 1 MHz and about 10 MHz, preferably about 3 to about 8 MHz. Preferably, the output has an essentially sinusoidal waveform with little harmonic distortion. The methods provide for the enhanced cutting of a variety of tissue including muscular, connective, glandular and fatty tissue. The device is particularly suitable in performing a breast biopsy.

Description:
RELATED APPLICATIONS 
     This application is a CON of U.S. application Ser. No. 09/725,265, filed on Dec. 29, 2000, now U.S. Pat. No. 6,607,528, which is a CIP of U.S. application Ser. No. 09/337,666, filed Jun. 22, 1999, now U.S. Pat. No. 6,267,759, both of which are herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Surgical lesion removal has traditionally been performed using a variety of surgical tools and techniques, some of which are specially adapted for a particular procedure. For example, large lesion removal from, e.g., the human breast, is typically attempted through an open incision using an ordinary surgical knife or scalpel. While the use of scalpels is widely accepted, they are not designed to minimize the invasiveness of a surgical procedure. During a surgical procedure, it is usually necessary to form an incision which is much larger than the lesion which is targeted for removal, so that the surgeon can work around, under, and over the lesion to remove both the entire lesion and a margin of tissue surrounding the lesion. The removal of a margin of tissue around the lesion is typically indicated, to be more certain that all of the lesion has been removed by the surgical procedure. 
     While the practice of removing tissue adjacent to a tissue mass of interest, e.g., a malignant or benign lesion, is followed in many lumpectomy procedures, the tools provided for a surgeon to remove the tissue are not well suited for performing the procedure. Straight and sculpted blade scalpels do not assist the surgeon in making the smallest cut necessary, and often require the surgeon to essentially dig out the tissue mass. The damage to the remaining tissues can be significant, resulting in considerable postoperative pain, excessive bleeding, long recovery times, the potential for infection, the potential for depression of the tissues at the surgical site (poor cosmesis) due to the removal of excessive tissue, and surface tissue scarring which is much larger than necessary. Furthermore, use of these conventional tools and techniques may cause excessive damage to the removed tissue and thus create a tissue specimen having ragged and irregular margins or borders. This, in turn can lead to inaccurate pathology studies of the excised tissue. There are some practitioners who believe that a significant number of negative pathology reports (i.e. reports which indicate that the specimen margins are clear of malignant tissue) are false negatives that will most likely result in recurrence of cancer in the patient. It is felt that a surgical device that results in smooth uniform margins would result in far more accurate pathology reports, particularly with patients who have or who are thought to have breast cancer. Patient management based on these more accurate reports would in turn lead to lower recurrence in breast cancer patients. 
     Breast cancer is presently the most common cancer in women and is the second leading cause of cancer deaths in women. With approximately one in eight American women developing breast cancer sometime in her lifetime, it is apparent that improved methods of diagnosis, such as breast biopsy are needed. 
     Electrosurgical devices have previously been used for tissue cutting, and surgical procedures. However, such devices typically use small, often pointed active cutting surfaces and the types of devices available to the surgeon who uses electrosurgery are limited. Furthermore, breast tissue and various other tissues are heterogeneous tissues and contain a variety of tissue types such as connective tissue, glandular tissue, vascular tissue, and adipose (fatty) tissue. Connective glandular and vascular tissues have similar characteristics in the way they react to high frequency electrical energy and hence the electrosurgical device. However, adipose (fatty)) tissue presents a higher impedance to the flow of electrical current than do the other types of tissues, and presents more difficulty in cutting. Thus, during an electrosurgical procedure if fatty tissue is encountered, a surgeon must perform surgical cuts by “feathering”, making repetitive shallow cuts over the same area to attain a desired depth of cut. These repetitive shallow cuts expose the patient to an increased risk of receiving an unnecessary amount of electrical energy, greater injury to surrounding tissues, greater risk of infection, as well as potentially creating ragged or irregular margins in biopsied tissues. 
     From the discussion above, it should be apparent that there is need in the art for more effective surgical cutting devices which lead to less trauma to the surrounding and biopsied tissues and which can provide biopsy specimens having smooth regular margins. Furthermore there is a need in the art for additional types of tools used in electrosurgery which give the operator greater control over the types and configurations of cuts made in tissue during a surgical procedure. The present invention fulfills these needs. 
     SUMMARY OF THE INVENTION 
     The invention is directed generally toward an electrosurgical device for cutting tissue, and the method of use, which is particularly suitable for cutting heterogeneous tissue such as found in breast tissue. 
     The electrosurgical device embodying features of the invention has a handle and an elongated cutting electrode which is secured to the handle and which is configured to be electrically connected to a high frequency power source. The cutting electrode is an elongated conductive member with a free distal end and is preferably manually shapeable. The cutting electrode has an exposed cutting length of at least 0.5 inch (1.3 cm), and may extend up to 4 inches (10.2 cm). Preferably, the cutting electrode can have an exposed length ranging from about 0.8 inch to about 2.8 inches (2-7 cm). In other embodiments, the cutting electrode can have an exposed length in the range of about 1.2 inches to about 2.5 inches (3-6.4 cm). The elongated cutting electrode may be provided with an exterior insulating jacket which is slidable along the cutting electrode to allow the operator to adjust the length of the cutting electrode which is exposed. 
     The cutting electrode has a maximum transverse cross-sectional dimension of about 0.007 to about 0.03 inch (0.18-0.76 mm), preferably about 0.008 to about 0.02 inch (0.2-0.5 mm). Elongated cutting electrodes having transverse dimensions of this magnitude may cut large areas of tissue, particularly adipose tissue, with a very effective “clean sweeping motion” with very little pressure against the tissue, thereby creating less trauma to the surgical site and providing for smoother margins of excised tissues. For increased electrode flexibility, the distal section of the cuffing electrode may be distally tapered to smaller transverse dimensions. For example, the distal section may taper from a transverse dimension of about 0.01 to about 0.02 inch (0.25-0.51 mm) at the proximal section of the electrode to a smaller transverse dimension of about 0.004 to about 0.01 inch (0.1-25.4 mm) at a distal end of the tapered distal length. 
     A cutting electrode embodying features of the invention is formed of a conductive material and is preferably formed of a high strength metallic material such as tungsten, and alloys thereof and particularly tungsten alloys containing about 3 to about 25% (wt %) rhenium. The tungsten containing cutting electrodes are very suitable with high frequency electrical power. In alternative embodiments which operate at lower frequencies (e.g. less than about 2 megahertz) the electrode may be made of stainless steel and other metallic compositions. 
     The electrosurgical devices are preferably part of an electrosurgical system which includes a high frequency (e.g. RF) electrosurgery generator that is electrically coupled to the electrosurgical device. The high frequency generator is preferably configured to produce electrical power in a frequency range of about 1 to about 10 megahertz, particularly a frequency range of about 3 to about 8 megahertz with a current output of up to 4 amps. The voltage capacity is about 150 Vrms to about 800 Vrms to facilitate a wide variety of procedures, including coagulation at the lower voltages (e.g. about 150 to about 300 Vrms) and heterogeneous tissue cutting at the higher voltages (e.g. about 400 to about 800 Vrms). When cutting through heterogeneous tissue the voltage is controlled to a first range of about 550 to about 650 Vrms, typically about 600 Vrms during the initiating of the cut and then controlled at a lower level between about 450 and about 550 Vrms, typically about 480-500 Vrms. Amperage also may vary between initiation, e.g. about 1.75 amps, and normal running, e.g. about 1 amp. 
     The duty factor and the voltage generally should be higher at the initiation of the cut and less during the running period. For example, the duty factor may range from about 2 to about 10% up to 100% at a frequency of about 10 Hz up to the output frequency; however, generally the duty factor frequency is above 30 kHz with 50 kHz being typical. 
     The high frequency output of the electrical power generator has a periodic output and preferably has an essentially sinusoidal waveform and most preferably with a total harmonic distortion of less than about 5%. To complete the electrical circuit at least one additional electrode is needed to be in contact with the patient for a monopolar electrical configuration or on the electrosurgical cutting device for a bipolar mode. In one version of the invention the system has an electrode pad which is secured to the patient&#39;s exterior close to the electrosurgical site to complete the electrical circuit. 
     The power cable directing high frequency electrical power from the electrical generator should be shielded cable and be flexible enough so that it does not interfere with the physician&#39;s (or other operator&#39;s) handling of the electrosurgical device during the procedure. One cable construction which has been found to be very suitable has a central metallic conductor disposed within an outer jacket with a space between the central conductor and the inner surface of the jacket in order to reduce cable capacitance. The jacket has an outer polymer layer, an inner polymer layer and a shielding layer such as metallic braid, spiral wrap or foil disposed between the inner and outer layers. The inner polymer layer is essentially non-conductive The central conductor is not supported within the jacket, it is essentially free floating, so it will contact the inner surface of the jacket at multiple locations when the cable is bent during use. However, the capacitance of the cable remains relatively constant because the off-center conductor averages to be the same as an on center conductor. 
     The invention may also be directed toward a method of performing tissue excision wherein an electrosurgical device is provided having a shapeable elongated electrode with a proximal end that is electrically connected to a high frequency power source and a distal end have a length of exposed cutting surface. The device preferably has a handle configured to hold the electrode and preferably have a mechanism to extend a desired length of exposed electrode out the distal end of the handle for a particular use. The elongated cutting electrode may be preshaped to a desired configuration in its manufacturing process or it may be manually shaped by the physician or other operator just prior to or during the procedure. The cutting electrode is placed in contact with the tissue to be excised and the electrosurgical device is then energized by providing RF power to the cutting electrode from a high frequency power generator. The cutting electrode will readily and smoothly pass through a variety of tissue types including muscular, connective, glandular and fatty tissue. The electrosurgical device may also be energized by the high frequency power generator with wave forms suitable for coagulation of bleeding vessels and tissue. A finger actuated switch on the handle or a dual foot switch situated on the floor allow the user to choose the cutting or the coagulation modes. Other modes may also be provided for other procedures. 
     These and other advantages of the invention will become more apparent from the following detailed description thereof and accompanying exemplary drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 . is a perspective view of an electrosurgical cutting device which embodies features of the invention. 
         FIG. 2 . is an elevational view, partially in section, of the electrosurgical device illustrated in  FIG. 1 . 
         FIG. 3  is an elevational view of the electrosurgical electrode shown in the device of  FIG. 2 . 
         FIG. 4  is an enlarged plan view, partially in section, of the upper portion of the handle illustrated in  FIG. 2  taken along the lines  4 - 4 . 
         FIG. 5  is a transverse cross sectional view of the electrosurgical device illustrated in  FIG. 2  taken along lines  5 - 5 . 
         FIG. 6  is a transverse cross sectional view of the electrosurgical device of  FIG. 2  taken along lines  6 - 6 . 
         FIG. 7  is a transverse cross sectional view of the electrosurgical device illustrated in  FIG. 2  taken along the lines  7 - 7 . 
         FIG. 8 . is a longitudinal cross-sectional view taken of the area  8  shown in  FIG. 2 . 
         FIG. 9  is a schematic illustration of a female patient and an electrosurgical system embodying features of the invention for performing a breast biopsy. 
         FIG. 10  is an illustration of a preformed cutting electrode having a “J” shape. 
         FIG. 11  is an illustration of a preformed cutting electrode having an alternative arcuate shape. 
         FIG. 12  is an illustration of a preformed cutting electrode having another alternative arcuate shape. 
         FIG. 13  is a cut away perspective view of a flexible shielded cable embodying features of the invention. 
         FIG. 14  is a transverse cross sectional view of the shielded cable illustrated in  FIG. 13  taken along lines  14 - 14 . 
         FIG. 15  is an elevational view of an alternative bipolar electrosurgical device embodying features of the present invention. 
         FIG. 16 . is an enlarged elevational view, partially in section, of the distal end of the electrosurgical device illustrated in  FIG. 15 . 
         FIG. 17  is a transverse cross sectional view of the electrosurgical device shown in  FIG. 16  taken along line  17 - 17 . 
         FIG. 18  is an enlarged cutaway view of the distal extremity of an alternative bipolar electrosurgical device similar to the device shown in  FIGS. 15-17  but with parallel, side-by side electrodes. 
         FIG. 19  is a transverse cross sectional view of the electrosurgical device illustrated in  FIG. 18  taken along lines  19 - 19 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1-8  depict an electrosurgical cutting device  10  embodying various features of the invention which generally has a cutting electrode  11  with a free or exposed distal portion  12  and a proximal portion  13  which is secured within the distal end  14  of handle  15 . The exterior of handle  15  is provided with ridges  16  configured for gripping by the physician or other operator to allow control of the device during operation and a radially extending flange  17  to protect the hand of the operator during operation of the electrosurgical device. The handle  15  is provided with a button type switch  18  for switching an RF electrode power source (not shown) to an active or “on” position or to an inactive or “off” position. A switching function may also be provided for alternative modes such as for coagulation. As best shown in  FIGS. 2 and 4 , the handle  15  may be provided with a thumb slide  19  to allow axial translation of an electrode assembly within the handle with detents  20  being provided to lock the thumb slide at various positions along a length of the handle. This allows the length of exposed electrode  11  which extends out the distal end  14  of the handle  15  to be adjusted to a desired length. A flexible cable  22  is provided for electrically coupling the cutting electrode  11  of the electrosurgical device  10  to an energizing source (not shown). The cable  22  enters the proximal end  23  of the handle  15  through an appropriate opening provided in the proximal end of the handle. 
     Details of the interior of the handle  15  are illustrated in FIGS.  2  and  5 - 8 . The cable  22  extends through the interior of handle  15 . The outer layer  24  and the shielding layer  25  of the cable  22  are secured to the ring or proximal eyelet  26  connected to the thumb slide  19  through lever  27  as shown in  FIG. 7 . The inner layer  28  and the inner electrical conductor  29  extend through an inner lumen (not shown) in the proximal eyelet  26  into the distal portion of the handle  15 . As shown in more detail in  FIGS. 6 and 8 , the inner electrical conductor  29  extends beyond the distal end  30  of the inner layer  28  into the proximal end of metallic connector  31 . The proximal end  32  of the cutting electrode  11  extends into the distal end of the metallic connector  31  which brings the cutting electrode  11  into an end to end electrically conducting relationship with the inner electrical conductor  29  of cable  22  as shown or in an overlapping or other configuration if desired. The metallic connector  31  is crimped onto the ends of the conductor  29  and the electrode  11  and is preferably formed of conducting material such a brass to facilitate passage of the high frequency electrical current from the inner conductor  29  to the proximal end of the cutting electrode  11 . 
     The flexible cable  22  is shown in more detain in  FIGS. 9 and 10 . The inner conductor  29  of cable  22  is typically made of solid copper wire about 0.0126 to about 0.037 inch (0.32-0.94 mm) in diameter, preferably about 0.0159 to about 0.021 inch (0.4-0.53 mm). Typically 26 AWG copper wire is utilized. The inner conductor  29  may be plated with silver or gold to provide more surface conductivity to accommodate the skin effect of RF currents when such currents propagate in a conductor. Alternatively, a layer of flexible polymeric material such as a cross linked modified polyester, an amide-imide copolymer or a polyurethane may circumferentially coat a solid inner conductor  29 . A stranded inner conductor may be covered with a thin wall insulation material that is not affected by the sterilization method. The inner surface of the inner layer  28  defines an air gap or space between the inner layer and the inner conductor  29 . The inner conductor  29  is unsupported or minimally supported along the length of the cable  22  which allows the inner conductor  29  to move more freely or “float” within the cable  22  to provide greater flexibility to the cable and lower power losses through reduced cable capacitance. Typically, the air gap diameter is about 0.08 to about 0.14 inch (2-3.6 mm), preferably about 0.1 to about 0.11 inch (2.5-2.8 mm). The inner and outer polymeric layers are conventional silicone or polyethylene layers about 0.03 to about 0.045 inch (0.76-1.14 mm) in thickness. The shielding layer  25  is preferably a multistranded braid or spiral wrap of conductive material such as copper. The strands of the braid or wrap may be about 0.15 to about 0.230 inch (3.8-5.8 mm) in diameter. The construction of the cable  22  is controlled to provide a radius of curvature of about 2 to about 5 inches (5-12.7 cm), preferably about 3 to about 4 inches (7.6-10.2 mm) without kinking. In an alternate embodiment the cable shielding layer  25  may be formed of an electrically conductive foil. The shielding layer  25  is typically grounded to reduces exposure of the operator of the electrosurgical device, as well as the patient and others, to RF radiation. The cable  22  is generally less than about 12 feet, preferably less than about 10 feet in length. Cables having lengths greater than 12 feet will usually have too high an impedance for effective tissue cutting through a variety of tissues, particularly fatty tissue. 
     As shown in  FIG. 7  the outer layer  24  and shielding layer  25  are secured at the proximal end of proximal eyelet  26 . The outer layer  24  circumferentially surrounds the shielding layer  25 . The shielding braided layer  25  circumferentially surrounds the proximal eyelet and is typically joined to the proximal eyelet by soldering in which the solder is impregnated within the shielded layer. Alternatively, the shielding layer  25  may be joined to the proximal eyelet  26  by means of a crimp sleeve (not shown). The proximal eyelet  26  is generally made of brass but may alternatively be composed of other electrically conductive materials such as copper. The proximal eyelet  26  contains an aperture through which the inner layer  28  and inner conductor  29  pass through to the distal portions of the handle  15 . 
     The proximal eyelet  26  is typically cylindrical and may be about 0.1 to about 0.5 inch (2.5-13 mm) in length and is generally made of brass although it may alternatively be made of other electrically conductive materials. In some embodiments with fixed exposed electrode lengths, the proximal eyelet  26  may be captured within the handle member cavity to secure an electrode assembly with respect to the handle  15 . 
     Referring to  FIGS. 2 and 6 , an insulating ceramic hub or bushing  33  is disposed at the distal portion of the handle  15  and typically has a nosecone  34  which generally protrudes from the distal end  14  of the handle  15 . The proximal portion  13  of the electrosurgical tissue cutting blade  11  extends through a passageway of ceramic hub  33  which is configured to allow slidable longitudinal movement of the proximal electrode portion  13  through the inner lumen to adjust the exposed length of electrode  11  which extends out the distal end of the handle  15 . The ceramic hub  33  can be made of insulating material such as a mica glass material, e.g. Mycalex® which is available from Mykroy/Mycalex Ceramics of Clifton, N.J. Other insulating materials may also be utilized to form the hub  33 . The ceramic hub  33  prevents heating and melting of the handle and allows for tracking of the electrosurgical device during the performance of tissue excision. In alternate embodiments with fixed exposed electrode lengths, the hub  33  may be molded about the proximal portion  13  of the electrode  11  into a fixed position with the electrode. 
     As shown in  FIG. 3 , the cutting electrode  11  may have a proximal portion  13  of uniform transverse dimensions, a distal portion  12  having uniform transverse dimensions smaller than those of the proximal portion and an intermediate portion  35  which tapers from the transverse dimensions of the proximal portion to the smaller transverse dimensions of the distal portion  12 . The electrode  11  is made of tungsten and preferably an alloy of tungsten containing from 3 to about 25% (by wt) rhenium and typically about 5% rhenium In alternate embodiments the electrode may be made of high temperature stainless steel or other suitable alloy compositions. The tapered intermediate portion  35  and the distal portion  12  of the electrode  11  will form most, if not all of the exposed length of the electrode  11  which may range from about 0.5 inch up to about 4 inches (1.3-10 cm), and may range typically from about 1 inch to about 2.5 inches (2.5-5.4 cm). The transverse dimensions of the tungsten or tungsten alloy electrode is preferably formed by centerless grinding tungsten wire to achieve the desired dimensions which is a conventional technique. The lengths of the tapered and the distal portions of the electrode  11  will be selected to provide a desired stiffness to the exposed of the electrode for the particular procedure to be performed. Typically, the distal portion is about 0.5 inch (1.3 cm), the intermediate tapered portion about 1.25 inch (3.2 cm) and the proximal portion about 0.5 inch (1.3 cm). 
     The handle member  15  is typically made of a dielectric material such as ABS plastic and is generally tubular in shape with rounded edges but it may be formed of other suitable polymeric materials. The connector eyelet  31  is typically made of brass but can be made of any suitable conductive material. 
       FIG. 11 . illustrates the electrosurgical system for the performance of breast biopsy. The electrosurgical device  10  having the features and embodiments discussed above is energized by a high frequency RF generator  40  by means of a remote switch  41 . Typically, a transformer box  44  is connected to the patient by a waist strap  43 . The electrosurgical device  10  is connected to the transformer box  44  which is connected to the radio frequency generator  40 . An additional cable  45  connected to a transformer box  44  extends to an alternate or extra ground electrode or pad  46  secured to the patient with an electrically conductive gel at the interface to complete the electrical circuit from the active electrode and thus prevent shock or injury to the patient. In alternate embodiments the ground electrode may be placed on other regions such as the patient&#39;s back, or abdominal region. In another embodiment a cable in electrical connection with the on/off button may extend from the external aspect of the handle to the transformer box which is in electrical connection to the RF generator thus allowing for the operational switching of the RF generator with the press of a button by the operator of the device. Alternate embodiments may involve controlling the energizing source with the use of a remote switch such as a foot pedal or a voice activated controller. Switching elements (not shown) may also be provided for other operating modes such as for coagulation. The details of the RF generator are set forth in concurrently filed application of the present assignee entitled High Frequency Power Source which is incorporated herein in its entirety. 
       FIGS. 12 ,  13  and  14  illustrate examples of various shapes that can be provided for in the various embodiments of the invention having pre-shaped configurations of the electrode.  FIG. 12  shows an electrosurgical preshaped electrode having a “J” shape configuration.  FIGS. 13 and 14  show electrosurgical preshaped electrodes having alternative arcuate configurations In addition to the preshaped embodiments described above. Other useful shapes can be provided. 
       FIGS. 15-17  illustrate a bipolar electrosurgical device embodying features of the invention. In this embodiment the electrosurgical instrument  50  has an electrode assembly  51  with a manually shapeable elongated primary electrode  52  surrounded circumferentially by a return electrode conductor  53 . The proximal end of the electrode assembly  51  is secured to a handle  54  similar to handle  15  of the prior embodiment. The primary electrode  52  is electrically connected by a specialized cable/electrical conductor  55  (such as the cable  22  described above) which enters the proximal end of the handle  54  through an opening (not shown) in the proximal end of the handle and extends longitudinally to end in electrical connection with the primary electrode as indicated by the phantom lines. The return conductive electrode  53  is electrically connected by a separate cable  56  which enters the proximal portion of the handle member at the proximal end and travels longitudinally through the handle member cavity to end in electrical communication with the primary electrode as indicated by the phantom lines. The primary electrode is preferably formed of tungsten or alloys thereof as discussed above. The return conductive electrode  53  is generally a hollow cylindrical tube which circumferentially surrounds a portion of the proximal end of the primary electrode and is typically made of stainless steel but may alternatively be made of such material as tungsten or tungsten alloys as discussed above. An insulating material  57  is disposed between the primary and return electrodes  52  and  53  respectively The active electrode  52  generally extends beyond the distal end of the return electrode  53  and has an exposed length which is sufficient for the procedures to be conducted. 
       FIGS. 18 and 19  illustrate an alternative embodiment of a bipolar electrosurgical device  60  having a primary electrode  61  and a return electrode  62  with proximal and distal ends and a handle  63  similar to those previously described herein. Both the primary and return electrodes are preferably made of tungsten or alloys thereof but may be alternatively comprised of high temperature stainless steel or other: compositions. The primary and return electrodes typically have an exposed length in the range such as 0.25 to about 2.5 inches (0.63-6.35 mm). The primary electrode  61  and the return electrode  62  extend generally parallel with respect to each other and are joined or stabilized in their parallel positions by one or more insulating ceramic standoffs  64  which lie perpendicular between the longitudinal axis of the electrodes. The proximal ends of the primary and return electrodes  61  and  62  are attached to an insulating ceramic hub  65  which contains a lumen for receiving the proximal ends of the electrodes. The primary and the return electrodes  61  and  62  end in electrical communication with electrical conductors  66  and  67  respectively. The electrical conductors are configured for electrical connection to an RF generator (not shown). 
     The bipolar electrosurgical devices can be employed in a system such as that shown in  FIG. 11  and the method of using the bipolar devices for performing a tissue excision is essentially the same as with a monopolar device as previously described. 
     The foregoing description details certain embodiments of the invention. Various modifications to the invention may be made without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive to the scope of the invention. Moreover, those skilled in the art will recognize that feature found in one embodiment may be utilized in another embodiment. Terms such as elements, members, devices and words of similar import when used herein shall not be construed as invoking the provisions of 35 U.S.C. §112(6) unless the following claims expressly use the term “means” or “step” followed by a particular function.