Abstract:
An apparatus for delivering energy to a surgical site from an energy source is provided. The apparatus includes; a handle, an actuator, a shaft, and a deflection wire. The actuator is movably coupled to the handle. The actuator moves from a first position to a second position. The shaft includes a proximal end, a deflectable portion and an energy delivery portion. The proximal end of the shaft is coupled to the handle. The energy delivery portion is coupled to the energy source to deliver energy to the surgical site. The deflection wire includes a proximal portion and a distal portion. The distal portion is affixed to the shaft. The proximal portion is affixed to the actuator. A movement of the actuator from the first position to the second position moves the deflection wire to produce a deflection of the deflectable portion. In an alternate embodiment of the invention a sensor and feedback control module are provided.

Description:
This application is a continuation-in-part of: PCT/US97/19189, filed Oct. 22, 1997, entitled Method and Apparatus For Treating Intervertebral Discs, which is a continuation-in-part of U.S. application Ser. No. 60/029,735, entitled “Method and Apparatus for Treating Intervertebral Discs,” filed Oct. 23, 1996, now abandoned; U.S. application Ser. No. 60/029,600, entitled “Method and Apparatus for Treating Intervertebral Discs,” filed Oct. 23, 1996, now abandoned; U.S. application Ser. No. 60/029,734, entitled “Method and Apparatus for Treating Intervertebral Discs,” filed Oct. 23, 1996, U.S. application Ser. No. 60/029,602, entitled “Method and Apparatus for Treating Intervertebral Discs,” filed Oct. 23, 1996, now abandoned; U.S. application Ser. No. 60/046,002, entitled “Method and Apparatus for Treating Intervertebral Discs with Radio Frequency Heating,” filed May 8, 1997, now abandoned; U.S. application Ser. No. 60/046,001, entitled “Method and Apparatus for Treating Intervertebral Discs with Resistive Heating,” filed May 8, 1997, now abandoned; U.S. application Ser. No. 60/047,820, entitled “Method and Apparatus for Treating Intervertebral Discs with Electromagnetic Energy,” filed May 28, 1997, now pending; U.S. application Ser. No. 60/047,681, entitled “Method and Apparatus for Treating Intervertebral Discs with Thermal Energy,” filed May 28, 1997, now pending; U.S. application Ser. No. 60/045,941, entitled “Method and Apparatus for Delivering or Removing Material from the Interior of an Intervertebral Disc,” filed May 8, 1997, now abandoned; U.S. application Ser. No. 60/047,818, entitled “Method and Apparatus for Treating Annular Fissures in Intervertebral Discs,” filed May 28, 1997, now pending; U.S. application Ser. No. 60/047,848, entitled “Method and Apparatus for Treating Intervertebral Disc Degeneration,” filed May 28, 1997, now pending; U.S. application Ser. No. 08/881,525, entitled “Method and Apparatus for Treating Intervertebral Discs with Thermal Energy,” filed Jun. 24, 1997, now U.S. Pat. No. 6,122,549; U.S. application Ser. No. 08/881,692, entitled “Method and Apparatus for Treating Intervertebral Discs with Electromagnetic Energy,” filed Jun. 24, 1997, now U.S. Pat. No. 6,073,051; U.S. application Ser. No. 08/881,527, entitled “Method and Apparatus for Delivering or Removing Material from the Interior of an Intervertebral Disc,” filed Jun. 24, 1997, now U.S. Pat. No. 5,980,504; U.S. application Ser. No. 08/881,693, entitled “Method and Apparatus for Treating Annular Fissures in Intervertebral Discs,” filed Jun. 24, 1997, now U.S. Pat. No. 6,007,570; and U.S. application Ser. No. 08/881,694, entitled “Method and Apparatus for Treating Intervertebral Disc Degeneration,” filed Jun. 24, 1997, now U.S. Pat. No. 6,095,149. This application is a continuation of application Ser. No. 08/547,510, filed Oct. 24, 1995, now U.S. Pat. No. 5,785,705 enitled RF Method For Controlled Depth Ablation of Soft Tissue, application Ser. No. 08/637,095, filed Apr. 24, 1996, entitled Method And Apparatus For Controlled Contraction of Soft Tissue, application Ser. No. 08/696,051, filed Aug. 13, 1996, entitled In VIVO Formed Thermally Contracted Collagen Tissue, application Ser. No. 08/714,987, filed Sep. 17, 1996, entitled Method And Apparatus For Controlled Contraction of Soft Tissue, application Ser. No. 08/888,359, filed Jul. 3, 1997, entitled Orthopedic Apparatus For Controlled Contraction of Collagen Tissue, are all incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to an apparatus for delivery of energy to a surgical site and more particularly to an apparatus which can provide a band of energy to the surgical site. 
     DESCRIPTION OF THE RELATED ART 
     Instability of peripheral joints has long been recognized as a significant cause of disability and functional limitation in patients who are active in their daily activities, work or sports. Diarthrodial joints of the musculoskeletal system have varying degrees of intrinsic stability based on joint geometry and ligament and soft tissue investment. Diarthrodial joints are comprised of the articulation of the ends of bones and their covering of hyaline cartilage surrounded by a soft tissue joint capsule that maintains the constant contact of the cartilage surfaces. This joint capsule also maintains, within the joint, the synovial fluid that provides nutrition and lubrication of the joint surfaces. Ligaments are soft tissue condensations in or around the joint capsule that reinforce and hold the joint together while also controlling and restricting various movements of the joints. Ligaments, joint capsules, and connective tissue are largely comprised of collagen. 
     When a joint becomes unstable, its soft tissue or bony structures allow for excessive motion of the joint surfaces relative to each other, and in direction not normally permitted by the ligaments or capsule. When one surface of a joint slides out of position relative to the other surface, but some contact remains, subluxation occurs. When one surface of the joint completely disengages and losses contact with the opposing surface, a dislocation occurs. Typically, the more motion a joint normally demonstrates, the more inherently loose the soft tissue investment is surrounding the joint. This makes some joints more prone to instability than others. The shoulder, glenohumeral joint, for example, has the greatest range of motion of all peripheral joints. It has long been recognized as having the highest subluxation and dislocation rate because of its inherent laxity relative to more constrained “ball and socket” joints such as the hip. 
     Instability of the shoulder can occur congenitally, developmentally, or traumatically and often becomes recurrent, necessitating surgical repair. In fact, subluxations and dislocations are a common occurrence and cause for a large number of orthopedic procedures each year. Symptoms include pain, instability, weakness and limitation of function. If the instability is severe and recurrent, functional incapacity and arthritis may result. Surgical attempts are directed toward tightening the soft tissue restraints that have become pathologically loose. These procedures are typically performed through open surgical approaches that often require hospitalization and prolonged rehabilitation programs. 
     More recently, endoscope (arthroscopic) techniques for achieving these same goals have been explored with variable success. Endoscopic techniques have the advantage of being performed through smaller incisions, and therefor are usually less painful. Such techniques are performed on an outpatient basis, associated with less blood loss and lower risk of infection and have a more cosmetically acceptable scar. Recovery is often faster postoperatively than using open techniques. However, it is often more technically demanding to advance and tighten capsule or ligamentous tissue arthroscopically because of the difficult access to pathologically loose tissue, and because it is very hard to determine how much tightening or advancement of the lax tissue is clinically necessary. In addition, fixation of advanced or tightened soft tissue is more difficult arthroscopically than through open surgical methods. 
     Collagen connective tissue is ubiquitous in the human body and demonstrates several unique characteristics not found in other tissues. It provides the cohesiveness of the musculoskeletal system, the structural integrity of the viscera as well as the elasticity of integument. There are basically five types of collagen molecules, with Type I being most common in bone, tendon, skin and other connective tissues, and Type III is common in muscular and elastic tissues. 
     Intermolecular cross links provide collagen connective tissue with unique physical properties of high tensile strength and substantial elasticity. A previously recognized property of collagen is hydrothermal shrinkage of collagen fibers when elevated in temperature. This unique molecular response to temperature elevation is the result of rupture of the collagen stabilizing cross links and immediate contraction of the collagen fibers to about one-third of their original lineal distention. Additionally, the caliber of the individual fibers increases greatly, over four fold, without changing the structural integrity of the connective tissue. 
     There has been discussion in the existing literature regarding alteration of collagen connective tissue in different parts of the body. One known technique for effective use of this knowledge of the properties of collagen is through the use of infrared laser energy to effect tissue heating. The importance in controlling the localization, timing and intensity of laser energy delivery is recognized as paramount in providing the desired soft tissue shrinkage effects without creating excessive damage to the surrounding non-target tissues. 
     Shrinkage of collagen tissue is important in many applications. One application is the shoulder capsule. The capsule of the shoulder consists of a synovial lining and three well defined layers of collagen. The fibers of the inner and outer layers extend in a coronal access from the glenoid to the humerus. The middle layer of the collagen extends in a sagittal direction, crossing the fibers of the other two layers. The relative thickness and degree of intermingling of collagen fibers of the three layers vary with different portions of the capsule. The ligamentous components of the capsule are represented by abrupt thickenings of the inner layer with a significant increase in well organized coarse collagen bundles in the coronal plane. 
     The capsule functions as a hammock-like sling to support the humeral head. In pathologic states of recurrent traumatic or developmental instability this capsule or pouch becomes attenuated, and the capsule capacity increases secondary to capsule redundance. In cases of congenital or developmental multi-directional laxity, an altered ratio of Type I to Type III collagen fibers may be noted. In these shoulder capsules, a higher ratio of more elastic type III collagen has been described. 
     There exists a need for an apparatus to effect controlled ablation of soft tissue along a painted band or line created by the introduction of energy. It would be desirable to provide an ablation apparatus which can provide controlled ablation depth of soft tissue to shrink the tissue to a desired state along a selectable surface, including but not limited to a narrow line. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a surgical apparatus which creates controlled delivery of energy to a desired tissue site. 
     Another object of the present invention is to provide a surgical apparatus which can paint a band or line of ablation along a selected tissue site. 
     A further object of the present invention is to provide a surgical apparatus which provides selectable painting ablation of a tissue site. 
     Yet another object of the present invention is to provide a surgical apparatus which provides continuous and controllable, adjustable ablation of soft tissue. 
     A further object of the present invention is to provide a surgical apparatus which provides for the maximum amount of collagen contraction without dissociation of the collagen structure. 
     Yet another object of the present invention is to provide a surgical apparatus to deliver a controlled amount of energy to tissue site in order to contract and restrict the soft tissue elasticity and improve joint stability. 
     These and other objects of the invention can be obtained with an apparatus for delivering energy to a surgical site from an energy source. The apparatus includes; a handle, an actuator, a shaft, and a deflection wire. The actuator is movably coupled to the handle. The actuator moves from a first position to a second position. The shaft includes a proximal end, a deflectable portion and an energy delivery portion. The proximal end of the shaft is coupled to the handle. The energy delivery portion is coupled to the energy source to deliver energy to the surgical site. The deflection wire includes a proximal portion and a distal portion. The distal portion is affixed to the shaft. The proximal portion is affixed to the actuator. A movement of the actuator from the first position to the second position moves the deflection wire to produce a deflection of the deflectable portion. 
     In an alternate embodiment of the invention an apparatus for delivering energy to a surgical site from an energy source is provided. The apparatus includes; a handle, an actuator, a shaft, a deflection wire, a sensor and a feedback control module. The actuator is movably coupled to the handle. The actuator moves from a first position to a second position. The shaft includes a proximal end, a deflectable portion and an energy delivery portion. The proximal end of the shaft is coupled to the handle. The energy delivery portion is coupled to the energy source to deliver energy to the surgical site. The deflection wire includes a proximal portion and a distal portion. The distal portion is affixed to the shaft. The proximal portion is affixed to the actuator. A movement of the actuator from the first position to the second position moves the deflection wire to produce a deflection of the deflectable portion. The sensor is positioned adjacent the energy delivery portion of the shaft to provide a signal indicating an amount of energy delivered from the energy source to surgical site. The feedback control module is coupled to the sensor and to the energy source. The feedback control module includes a logic responsive to the signal for adjusting a level of energy delivered from the energy source to the energy delivery portion of the shaft. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-B is an isometric view of a surgical apparatus. 
     FIG. 2 illustrates a shaft for use with a surgical apparatus according to the present invention. 
     FIGS. 3A-C show various cross-sectional views of the shaft shown in FIG.  2 . 
     FIG. 4A-B are cross sectional views of the handle portion of a surgical apparatus. 
     FIGS. 5A-C illustrate a spinal disc pierced by a cannula through which a shaft of a surgical apparatus is introduced. 
     FIG. 6 is a block diagram of an energy delivery and control unit. 
     FIG. 7 is a block diagram of an embodiment illustrating an analog amplifier, analog multiplexer and microprocessor used with the energy delivery and control unit of FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1A illustrates an energy delivery system according to the present invention. The system includes a surgical apparatus  110 , an energy delivery and control unit  112 , an energy supply cord  114  and a sensor signal cord  116 . The surgical apparatus  110  includes a handle  118  and a shaft  120  with a tip  122 . The handle  118  includes a stationary handle  124 , an actuator  126  and a utility port  128 . 
     The utility port  128  is coupled with the energy supply cord  114  and the sensor signal cord  116 . The energy supply cord  114  and the sensor signal cord  116  are in turn coupled with the energy delivery and control unit  112 . The actuator  126  is pivotally coupled with the stationary handle  124  such that the actuator  126  can be moved relative to the stationary handle  124 . 
     In operation, the shaft tip  122  can be deflected by pulling the actuator  126  toward the stationary handle  124  as illustrated in FIG.  1 B. Energy is supplied from the energy delivery and control unit  112  to the surgical apparatus  110  via the energy supply cord  114 . The energy delivery and control unit  112  receives a signal from a sensor (not shown) via the sensor signal cord  116 . In an embodiment of the invention, the sensor is positioned at the tip  122  adjacent an energy delivery portion. The energy delivery and control unit  112  adjusts the amount of energy delivered from the energy delivery and control unit  112  in response to the signal received from the sensor. 
     FIG. 2 is a detailed diagram of the shaft  120  illustrated in FIG.  1 A. The shaft  120  is divided into a proximal section  200 , a transition section  202  and an energy delivery section  204 . Suitable lengths for the energy delivery section  204  include, but are not limited to, two inches long starting from the tip. 
     FIGS. 3A-C provide cross-sectional views of the proximal section  200 , transition section  202  and energy delivery section  204  illustrated in FIG.  2 . FIG. 3A illustrates the proximal section  200  of the shaft  120 . This view is taken along lines A—A illustrated in FIG.  2 . The proximal section  200  includes an outer polyamide sheath  302 , a stainless steel tube  304 , a stainless steel core  306 , a deflection wire  308 , and sensor and energy delivery wires  310 . The proximal section has no preferred bending axis since its moment of inertia is determined primarily by the rigid stainless steel tube  304  and the core member  306 . Thus, the proximal section of the shaft  120  resists both bending and buckling. 
     FIG. 3B illustrates the transition section  202  of the shaft  120 . This view is taken along lines B—B illustrated in FIG.  2 . The stainless steel tube  304  found in FIG. 3A has been replaced with a relatively flexible polyamide tube  330 . The polyamide tube  330  is housed within the outer polyamide sheath  302 . Both of these tubes are flexible. The stainless steel core has transitioned to a cross member  334 . The cross member  334  has a preferred bending axis  340  which is orthogonal to the longitudinal axis  342  of the stainless steel cross-member  334 . This cross-member  334  provides the needed resistance to deflection along axis  342  while allowing deflection along axis  340 . 
     FIG. 3C illustrates the energy delivery section  204  of the shaft  120 . This view is taken along lines C—C illustrated in FIG.  2 . Outer polyamide sheath  302  wraps around an energy delivery device  370 . In the interior of the shaft  120  stainless steel cross-member  334  provides the needed resistance to deflection along axis  342 . In FIGS. 3B and 3C, the sensor and energy delivery wires  310  include a coating  332  for insulating the sensor and energy delivery wires  310  from the energy delivered to the energy delivery device  370 . 
     In FIGS. 3A-C, the sensor and energy delivery wires  310  extend from the shaft  120  to the utility port  128 . The wires are coupled at the utility port to the energy delivery device  370  to allow energy to be delivered to the energy delivery portion  370  of the shaft, and to allow feedback from a sensor in the energy delivery portion to be used to regulate the level of energy delivered by the energy delivery and control unit  112 . 
     Suitable energy delivery devices include, but are not limited to, resistive heaters, monopolar RF electrode and bipolar RF electrodes. Suitable locations for the sensor include, but are not limited to, the inside of stainless steel tube  304 , inside of the flexible polyamide tube  330  and/or at the tip  122 . Suitable sensors include, but are not limited to, impedance sensors and temperature sensors such as thermocouples. 
     In operation bending along axis  340  is brought about by drawing the deflection wire  308  proximally. The deflection wire  308  is fastened to the distal portion of cross-member  334  but is otherwise free to move longitudinally along the inside of shaft  120 . Drawing deflection wire  308  proximally causes compressional forces on cross-member  334 . Cross-member  334  bends along axis  340  in response to the compression. The bend of cross-member  334  causes deflection of the tip  122  as illustrated in FIG.  1 B. 
     FIG. 4A is a cross section of the handle  118 . The handle  118  includes a shaft seat  402 , a pivot structure  404 , a spring seat  406 , a spring  408 , a forward stop  410  and a rearward stop  412 A. The actuator  126  includes an arcuate groove  414 , a grip and a biasing support  416 . 
     The actuator  126  is pivotally coupled with the pivot structure  404  by positioning an aperture in the actuator  126  over the pivot structure  404 . The actuator  126  can be pivoted about the pivot structure  404  from a first position to a second position. 
     The spring  408  is seated within the spring seat  406  so as to apply pressure to the biasing support  416  of the actuator  126 . The pressure of the spring  408  on the biasing support biases the actuator toward a forward position where the actuator contacts the forward stop  410 . 
     The shaft  120  is seated in the shaft seat  402 . The deflection wire  308  exits the shaft  120 . The distal end of the deflection wire  308  includes a resilient arcuate portion positioned within the arcuate groove  414 , to affix the wire to the actuator. 
     In operation, the actuator  126  is pulled toward the stationary handle  124  as illustrated in FIG.  4 B. The movement of the actuator  126  causes the arcuate groove  414  to be pulled rearward. Since the deflection wire  308  is fixed within the arcuate groove  414 , the deflection wire  308  is also pulled rearward. The proximal motion of the deflection wire causes the tip  122  to become deflected as illustrated in FIG.  1 B. An increased degree of proximal motion of the deflection wire causes an increased deflection of the tip. The motion of the actuator can be limited by the rear stop  412 A in order to prevent damage resulting from over deflection. The rearward stop  412 A can include an adjustable pin  412 B which can be used to set the degree of deflection. 
     The surgical apparatus  110  is suitable for use in numerous tissues sites within a body. Suitable tissue sites include, but are not limited to, a nucleus pulposus of a spinal disk. FIGS. 5A-C illustrate use of the surgical apparatus  110  to deliver energy to the nucleus pulposus  504  of a spinal disc  506 . In FIG. 5A a cannula/introducer  502  is introduced through a patient&#39;s back along a path which intercepts a selected disc  506 . The insertion of the cannula  502  into the appropriate location may be accomplished with x-ray imaging which allows visualization of the cannula tip  508  with respect to the selected spinal disc  506 . The cannula tip  508  pierces the spinal disc  506  and extends a desired distance into the nucleus pulposus  504 . A visual indicator  510  on hub  512  of the cannula  502  shows the orientation of the opening in the cannula tip  508 . 
     In FIG. 5B the shaft  120  is inserted through the hub  512  of the cannula  502 . The surgical apparatus  110  can include depth markings on the shaft  120  to indicate the point at which the tip  122  and energy delivery section  204  of the shaft  120  has reached the nucleus pulposus  504 . 
     In FIG. 5C the continued insertion of the shaft  120  causes the tip  122  to traverse the interior walls of the annulus fibrosus. The tip  122  is then deflected to an angle which approximates the curvature of the inside of the disc  506 . This deflection allows an increased portion of the shaft  120  to be contacted with the inside of the disc  506 . In this orientation energy can be delivered to a specific tissue site, i.e. a fissure or rupture. The deflection of the tip  122  the motion of the actuator  126  from the forward to the reverse positions shown in FIGS. 4A-B can also be used to aid in steering the tip  122  to a particular tissue site within the body. 
     FIGS. 6-7 illustrate one embodiment of the energy delivery and control unit  112 . The illustrated energy delivery and control unit  112  is used with a thermocouple as the sensor, a resistive heating element as the energy delivery device and an electrical energy source as the energy source. The embodiment illustrated in FIGS. 6 and 7 are readily adaptable to other energy sources, sensors and energy delivery devices. Suitable energy sources include, but are not limited to, RF energy sources and thermal energy sources. 
     The energy delivery control unit  112  includes an energy source  602 , a controller  604  with a microprocessor, a current sensor  606 , a voltage sensor  608 , a power calculator  610 , a user display interface  612 , a power circuit  614  and a temperature calculator  616 . 
     When the surgical apparatus  110  is positioned at a tissue site, a signal indicating the temperature of the tissue site is provided by a thermocouple sensor. The signal from the thermocouple sensor is provided to the temperature calculator. The signal from the temperature calculator is provided to the controller  604 . 
     The resistive heating element within the surgical apparatus  110  is in energy communication with the energy source  602 . The current delivered through the resistive heating element is passed through the current sensor  606 . The voltage of the delivered current can be measured by the voltage sensor  608 . The output of the current sensor  606  and voltage sensor  668  are provided to a power calculation device  610 . The output from the power calculation device  610  is provided to the controller  604 . 
     In operation, the controller  604  monitors the output from the temperature calculator  616  and the power calculation device  610 . The controller  604  can display these values at the user display and interface  612 . When controller  604  detects that the temperature falls outside a desired range, the controller  604  provides an output to the power circuit  614 . The output to the power circuit  614  is used to adjust the level of energy delivered from the energy source  602 . For instance, when the temperature at the temperature site is below the desired temperature range, the output from the controller causes the power circuit  614  to increase the amount of energy delivered from the energy source  602 . The degree of energy level change is a function of the amount of power energy being delivered from the energy source  602  as indicated by the power calculation device  610 . 
     Controller  604  can be a digital or analog controller or a computer with software. When controller  604  is a computer, it can include a CPU coupled through a system bus. Included in this system can be a keyboard, a disc drive or other nonvolatile memory system, a display, and other peripherals, as are known in the art. Also coupled to the bus can be a program memory and a data memory. 
     User interface and display  612  includes operator controls and a display. Controller  604  can be coupled to imaging systems well known in the art. 
     The output of current sensor  606  and voltage sensor  608  can be used by controller  604  to maintain a selected power level at resistive heating elements. A predetermined profile of power, temperature or energy to be delivered can be incorporated in controller  604 . 
     Circuitry, software, and feedback to controller  604  result in process control and in the maintenance of the selected power that is independent of changes in voltage or current. Control can include (i) the selected power and (ii) the duty cycle (wattage and on-off times). These process variables are controlled and varied while maintaining the desired delivery of power independent of changes in voltage or current, based on temperatures monitored at the thermocouple. 
     In the embodiment illustrated in FIG. 7, current sensor  606  and voltage sensor  608  are connected to the input of an analog amplifier  702 . Analog amplifier  702  can be a conventional differential amplifier circuit for use with a sensor. The output of analog amplifier  702  is sequentially connected by an analog multiplexer  704  to the input of A/D converter  706 . The output of analog amplifier  702  is a voltage which represents the respective sensed parameters. Digitized amplifier output voltages are supplied by A/D converter  706  to microprocessor  700 . Microprocessor  700  may be a type 68HCII available from Motorola. However, it will be appreciated that any suitable microprocessor or general purpose digital or analog computer can be used for the parameters of temperature, voltage or current. 
     Microprocessor  700  sequentially receives and stores digital representations of temperature. Each digital value received by microprocessor  700  corresponds to different parameters. 
     Calculated power and temperature values can be indicated on user interface and display  612 . Alternatively, or in addition to the numerical indication of power, calculated power values can be compared by microprocessor  700  with power limits. When the values exceed predetermined power or temperature values, a warning can be given on user interface and display  612 , and additionally, the delivery of electromagnetic energy can be reduced, modified or interrupted. A control signal from microprocessor  700  can modify the power level supplied by energy source  602 . 
     The foregoing description of preferred embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications, as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.