Patent Application
20070162062
1. Field of the Invention
The present invention relates to removal of intervertebral discs and, more particularly, to apparatus and methods for removal of the nucleus pulposus of an intervertebral disc.
2. Description of the Related Art
The spine is made up of twenty-four bony vertebrae, each separated by a disc that both connects the vertebrae and provides cushioning between them. The lumbar portion of the spine has five vertebrae, the last of which connects to the sacrum. The disc is comprised of the annulus fibrosus, which is a tough, layered ligamentous ring of tissue that connects the vertebrae together, and the nucleus, a gelatinous material that absorbs water and nutrients through the endplates of the vertebrae. In a healthy disc, the nucleus pulposus is pressurized within the annulus much like the air is pressurized within an automobile tire.
Degenerative disc disease (DDD) is a condition that affects both the annulus fibrosus and nucleus pulposus of the disc, and is usually thought of as a cascade of events. In general, DDD is characterized by a weakening of the annulus and permanent changes in the nucleus, and may be caused by extreme stresses on the spine, poor tone of the surrounding muscles, poor nutrition, smoking, or other factors. In DDD, the nutrient flow to the nucleus is disrupted and the nucleus loses water content. As the nucleus dehydrates it loses pressure, resulting in a loss of disc height and a loss in the stability of that segment of the spine. In the lumbar spine, as the degenerative cascade continues, the annulus may bulge and press on a nerve root, causing sciatica (leg pain) among other problems. The loss of disc height can also result in leg pain by reducing the size of the opening for the nerve root through the bony structures of the spine. As the disc loses height, the layers of the annulus can begin to separate, irritating the nerves in the annulus and resulting in back pain.
Surgical treatment for early DDD, where the pain is primarily leg pain, is usually a discectomy where some of the nucleus material is removed to reduce the bulging of the disc and the pressure on the nerve root. For more severe cases of DDD, where the disc has completely collapsed and/or where a discectomy did not have long-term success, the traditional surgical treatment has been fusion of the vertebrae through the use of plates, rods, pedical screws, and interbody fusion devices. For years, surgeons and industry have been looking for ways to interrupt the degenerative cascade for patients with early stage disease, and for methods that retain motion at the affected disc in patients with more advanced disease. Just as the surgical treatment for degenerated knees and hips changed from fusion to motion preservation (arthrodesis to arthroplasty), innovative technologies are now creating a market for treatment of DDD without resorting to fusion. The field of spinal arthroplasty represents a significant emerging market in spinal surgery.
Surgical treatment for early stage disease that involves primarily leg pain as a result of a herniated disc is currently limited to a simple discectomy, where a small portion of the disc nucleus is removed to reduce pressure on the nerve root, the cause of the leg pain. While this procedure is usually immediately successful, it offers no means to prevent further degeneration, and a subsequent herniation requiring surgery will occur in about 15% of these patients.
A range of prosthetic techniques has been developed and continues to be developed for the treatment of DDD. These techniques typically use one of three types of prosthetic devices: total disc replacement (TDR) devices, which sacrifice much of the connective tissue of the disc and are intended for discs with severe degeneration; partial disc replacement (PDR) devices, which replace only the nucleus of the disc; and flexible springs and connectors attached to the posterior bony elements of the spine. The PDR will be marketed as the surgical treatment of choice for patients with slightly more advanced (mild-to-moderate) disc degeneration. This technology relies on the connective structures of the affected level, such as the annulus, facets, and longitudinal ligaments, to be relatively healthy. A fourth type of device, used for repairing the annulus after a herniation or implantation of a PDR, is also currently in development.
Current designs for nucleus replacement devices are typically not attached to the nucleus or vertebra, and are free to move within the nucleus cavity. Much like the healthy nucleus, these devices are subjected to the high forces and the twisting and bending motions that must be endured by the spinal structures, and some device movement is expected. Current PDR devices have a known complication of excessive device movement, however, and can move back out the annulus at the site of implantation. This device extrusion can occur in over 25% of cases for some designs. While the effect of the complication is not life threatening, the response is another surgery to reposition or replace the PDR, or to remove it altogether and likely replace it with a total disc replacement or a fusion procedure. There is mounting evidence that the nucleus material left in the disc cavity, even after an exhaustive removal procedure, can push against even a well-positioned PDR and be the cause of many of the device extrusions. When a posterior approach is used for removal, the remaining nucleus material left behind can push against a PDR. While more of this material could be removed if the disc is accessed via a lateral or an anterior approach, current information indicates that most spine surgeons prefer to use the posterior approach.
The annulus repair technologies that rely on mechanical means to close the annulus involve the need to contact and/or secure to the inside of the annulus tissue immediately adjacent to the site used to access the nucleus cavity. These designs will achieve the best deployment and surgical attachment to the annulus if the bulk of the relatively soft nucleus material near the access site has been adequately removed. Remaining nucleus material can have a negative impact on the performance of these devices if it is not removed. This material will be difficult to remove whether the access to the cavity is performed via a posterior, lateral, or an anterior surgical approach.
For annulus repair and PDR, among other procedures, implantation site preparation typically involves removal of the nucleus. A wide range of devices have been developed for this removal procedure. However, surgeons have historically utilized an array of pituitary rongeurs for the various procedures requiring removal of the nucleus pulposus or portions of the nucleus pulposus.
The rongeur is provided in a variety of configurations including “up-biting”; straight; and “down-biting”, and can be found in a variety of lengths, widths, and with razor or serrated jaws. However, even using the preferred posterior access to the disc with a rongeur, its useful range of motion within the intervertebral disc is limited. The bony structure of the posterior spinal elements, even though partially removed to provide access for PDR implantation, typically limits the angles through which the rongeur can be maneuvered. This limitation of movement serves to limit the amount of nucleus material that can be removed. More importantly, the limitation on movement may not allow adequate removal of material next to the annular access to provide good contact for an annular repair device and does not allow adequate removal of material contralateral to the annular access, preventing optimal placement for a PDR. Further, the use of a rongeur requires constant insertion and removal to clean the nucleus material from the tip of the device, resulting in dozens of insertion/removal steps to remove an adequate amount of material from the nucleus. This can increase the trauma to the surrounding annulus tissue and increase the risk of damaging the endplates.
An additional significant limitation of the rongeur instrument is the ability to easily remove the important annular tissue, especially when using rongeurs with a sharp cutting tip. Surgeons typically do not try to remove the entire nucleus in simple discectomy procedures, or intentionally remove annulus in preparation for fusion procedures. In this respect, a surgeon's “feel” for the tissue, or ability to distinguish softer nucleus tissue from tougher annulus tissue, may not be well developed and PDR site preparation may result in significant trauma to the annulus.
A range of more sophisticated devices for removing nucleus has been developed; however, the adoption of these devices has been very limited. Some of the more intricate devices utilize mechanized cutting mechanisms for removal of material from the nucleus pulposus. Frequently, these devices require suction and/or irrigation to remove material during the procedure.
One device uses a guillotine-style assembly that cuts nucleus material, aspirates the material into the instrument tip, and then evacuates the cut material is through the instrument. Movement of the guillotine assembly is automated and controlled by a mechanism in the handpiece of the instrument. The continuous removal of tissue without the need to repeatedly insert and remove the instrument minimizes trauma to the surrounding tissue. The guillotine type assembly is typically associated with a straight, stiff device that is intended for a minimally invasive, percutaneous approach. Because of their stiffness, although the devices may be somewhat effective for a lateral or anterior surgical approach for PDR implantation, they are generally not usable for nucleus removal utilizing a posterior approach.
Other devices have utilized an Archimedes type screw to pull nucleus material into the catheter and shear it when it reaches the tip of the catheter. Continued collection of nucleus material by the rotating Archimedes type screw pushes the sheared material through the catheter and into a collection chamber. While less complicated to use than the previously discussed guillotine type assembly, the devices utilizing the Archimedes type screw typically have the similar maneuverability disadvantages. Further, these devices can relatively easily be directed into and through the annulus of the intervertebral disc being treated.
Still other systems have used a high-pressure stream of water to remove nucleus material. In one device, the high-pressure stream of water produces a vacuum which pulls nucleus material into the stream. The high-pressure stream of water then cuts the nucleus material and pulls the material through a catheter to a collection bottle. Among other disadvantages, such systems are expensive. Further, although the tip of the instrument can be bent slightly, its lateral reach when used via the posterior approach is still very limited. Further, since the water stream is very narrow, successful nucleus removal can be technique dependent and time consuming.
Still other devices utilize radio frequency (RF) energy or plasma directed through electrodes for tissue resection and vessel cauterization in preparation for implanting a PDR. These devices typically include an RF generator that can be used with a variety of different types and shapes of electrodes. These devices are typically stiff and have little lateral reach when used making them relatively ineffective for use through the posterior approach. Further, the RF ablation technology can resect annulus or endplate cartilage as easily as nucleus material.
Still other devices utilize lasers to remove material from the nucleus pulposus. These lasers are typically transmitted through a laser fiber positioned within a multi-lumen catheter. These multi-lumen catheters have also included additional components such as imaging fibers, illumination fibers, and irrigation ports. Further, the tip of these catheters can be steerable. Although steerable, the bend radius of the catheters typically prevents them from being useful for removing nucleus near the annulus access. Accordingly, these devices have limited utility for removal of material in preparation for implantation of annulus repair devices. Further, the effective radius of the laser beam from these devices is typically only 0.5 mm, making removal of large amounts of nucleus very difficult and time consuming. Detrimentally, lasers can resect annulus or endplate cartilage as easily as nucleus material. Since the tip of the catheter is typically not protected, the laser beam has the ability to easily penetrate and damage the annulus and endplate tissue.
Other devices for nucleus removal are also available. However, these technologies possess their own limitations for the unique needs of annulus repair and PDR device site preparation. The limitations of these devices, along with those of the pituitary rongeur, are driving the need for a more advanced instrument for nucleus removal.
Apparatus and methods in accordance with the present invention may resolve many of the needs and shortcomings discussed above and will provide additional improvements and advantages as will be recognized by those skilled in the art upon review of the present disclosure.
In one aspect, the present invention may provide a reciprocating cutting apparatus for removing tissue from an intervertebral disc. The reciprocating cutting apparatus may include a guide tube, a drive shaft and a cutting cap. The guide tube may define a lumen extending through the guide tube from a proximal opening at a proximal end of the guide tube to a distal opening at a distal end of the guide tube. The lumen may include a bend at the distal end of the guide tube. Typically, the lumen of the guide tube extends linearly over a linear section extending between the bend and the distal opening. The guide tube may be slidably received within an outer guide tube. The cutting cap is typically movable between an extended position and a retracted position relative to the distal opening of the guide tube. The cutting cap has a trailing cutting edge and an atraumatic crown. The guide tube may include a cutting surface on a distal end of the guide tube to receive the trailing cutting edge of the cutting cap when the cutting cap is in a withdrawn position. The cutting surface may be defined on a cutting member secured to or within the distal end of the guide tube. The cutting member may be secured within the lumen of the guide tube at the distal opening of the lumen. The drive shaft has a proximal end and a distal end. The drive shaft may be received within the lumen of the guide tube. The drive shaft is operably connected to the cutting cap to confer a reciprocating motion to the cutting cap. The drive shaft may be operably connected to the cutting cap through a cam and cam follower system, through a direct mechanical connection, or through other indirect mechanisms for operably connecting the drive shaft to the cutting cap to confer a reciprocating motion to the cutting cap. The cutting cap may be secured directly to the drive shaft. When a cam and cam follower system is used to operably connect the drive shaft to the cutting cap, the cam system may include a cam, a cam follower and a cap shaft. The cam may be rotatably secured within the lumen of the guide tube. The cam defines a cam surface to slidably receive the cam follower. The cam may also include a shaft mount to secure the drive shaft to the cam. The cam follower may be biased against the cam surface to convert a rotation of the cam into a reciprocating motion. A spring may be used to bias the cam follower against the cam surface. The cap shaft may be secured at a first end of the cap shaft to the cam follower and at a second end of the cap shaft to the cutting cap. The reciprocating cutting apparatus may further include a motor connected to a distal end of the drive shaft to confer a reciprocating motion or rotational motion to the drive shaft. The motor may be slidably secured within a housing. A distal stop may be secured to the distal end of the guide tube. The distal stop may be secured to the guide tube by one or more stop supports. The stop supports may extend between the distal end of the guide tube and the distal stop to secure the distal stop relative to the distal opening of the guide tube. The cutting cap may include one or more cap guides secured to the cutting cap and slidably receiving at least one of the stop supports. The cap guides may be integral with the cutting cap.
All Figures are illustrated for ease of explanation of the basic teachings of the present invention only; the extensions of the Figures with respect to number, position, relationship and dimensions of the parts to form the preferred embodiment will be explained or will be within the skill of the art after the following description has been read and understood. Further, the exact dimensions and dimensional proportions to conform to specific force, weight, strength, and similar requirements will likewise be within the skill of the art after the following description has been read and understood.
Where used in various Figures of the drawings, the same numerals designate the same or similar parts. Furthermore, when the terms “top,” “bottom,” “right,” “left,” “forward,” “rear,” “first,” “second,” “inside,” “outside,” and similar terms are used, the terms should be understood to reference only the structure shown in the drawings as it would appear to a person viewing the drawings and utilized only to facilitate describing the illustrated embodiment.
The present invention provides a reciprocating cutting apparatus 10 and methods for removal of materials from an intervertebral disc positioned between adjacent vertebral bodies within the spine of a patient. Reciprocating cutting apparatus 10 in accordance with the present invention generally include a guide tube 12, and a cutting cap 14 as are illustrated generally throughout the figures for exemplary purposes. In some embodiments, reciprocating cutting apparatus 10 may also include a cutting member 16. The cutting cap 14 can reciprocate relative to the cutting member 16 to generate a cutting and/or abrading action to remove tissue from the nucleus pulposus of an intervertebral disc. In one aspect, the reciprocating cutting apparatus 10 may provide a cutting cap 14 and cutting member 16 which are extendable from the distal tip of an outer guide tube 20 for accessing tissues of an intervertebral disc. When extendable, the reciprocating cutting apparatus 10 may permit access to tissues remote from the distal end of the outer guide tube 20 positioned at a desired location within a patient. The reciprocating cutting apparatus 10 is typically configured to permit access to the intervertebral disc in a minimally invasive manner. In another aspect, the cutting cap 14 and cutting member 16 are retractable into the distal end of the outer guide tube 20 to better facilitate insertion and/or removal of the distal end of guide tube 20 from the intervertebral disc of a patient. In another aspect, the cutting cap 14 and cutting member 16 are configured to extend from and retract into the outer guide tube 20 while cutting cap 14 reciprocates relative to the cutting member 16 to remove or facilitate the removal of tissue from the intervertebral disc. The reciprocating cutting apparatus 10 may be generally configured to permit posterior access to the intervertebral disc wherein guide tube 12 may have sufficient flexibility to bend around various anatomical features and structures of the spine.
Guide tube 12 may be secured to the distal end of housing 100. Guide tube 12 defines a lumen 22. Generally, lumen 22 contains a drive shaft 18 to confer a reciprocating movement to cutting cap 14. Lumen 22 may further be operably connected to a vacuum apparatus, not shown, to provide suction through lumen 22 to a distal opening 32 at the distal end of guide tube 12. Suction may be used to assist in removal tissue fragments from the nucleus. In addition or alternatively, suction may assist in the removal of tissue cut by cutting cap 14 by urging the tissue toward the cutting surface 46 of the cutting cap 14. Guide tube 12 may be configured from a material which permits a surgeon to properly position the distal portion of the guide tube 12 within an intervertebral disc to remove the desired portions of the intervertebral disc. In one aspect, applications may required that the guide tube 12 have sufficient flexibility to bend and otherwise flex as the distal end of the guide tube 12 is inserted through a patient into the intervertebral disc. In other aspects, applications may require that the guide tube 12 have sufficient stiffness to permit a surgeon to advance the distal end into the intervertebral disc and to precisely maneuver the distal portion of the guide tube 12 within the intervertebral disc. In still other aspects, the guide tube 12 may have a variable stiffness along its length for applications requiring or benefiting from such variable stiffness. In still other aspects, the guide tube 12 may be configured to follow the curves within the lumen 34 of an outer guide tube 20.
Typically, the material used for the guide tube 12 is polymeric such as a high density polyethylene, PTFE, PEBAX, PEEK or other flexible polymeric material which will be recognized by those skilled in the art. However, the material may be a metal, composite materials or other material selected and configured for access to the intervertebral disc. Alternatively, the guide tube 12 may be configured from a stiff material such as a metal to allow precise positioning and movement of the cutting member 16.
As illustrated throughout the Figures, the guide tube 12 defines lumen 22 that may extend along the longitudinal axis 24 of the guide tube 12. Longitudinal axis 24 may be curvilinear over portions of its length. In one aspect, the lumen 22 may include a lubricious coating 26, shown in
As illustrated in FIGS. 1 to 2C, 5A to 5C, 7A to 8B, guide tube 12 may be received within an outer lumen 34 of an outer guide tube 20. When an the outer guide tube 20 is utilized, the guide tube 12 may be slidably received within the outer lumen 34 of the outer guide tube 20 to permit the extending and retracting of the cutting cap 14 and cutting member 16 relative to the outer distal opening 36 of the outer lumen 34. The guide tube 12 may be extended or retracted within the outer guide tube 20 to extend or retract the cutting cap 14 and cutting member 16 from the outer distal opening 36 of outer guide tube 20.
In these embodiments, the guide tube 12 is typically more flexible to accommodate following the outer lumen 34 of outer guide tube 20. The guide tube 12 may include a lubricious coating 26, shown in
A bend 28 may be located near the distal end of the outer guide tube 20. Bend 28 may be straightened to varying degrees by external forces to which outer guide tube 20 may be subjected but will typically resume the bent configuration when these forces are removed. The bend 28 directs the outer guide tube 20 and the associated guide tube 12 positioned within outer lumen 34 laterally at a desired angle 94 from the longitudinal axis 24. The angle 94 is typically between about sixty (60) degrees and one hundred twenty (120) degrees from the longitudinal axis 24. In one aspect, the angle 94, shown in
The drive shaft 18 may extend through at least a portion of lumen 22 and may extend through a least a portion of outer lumen 34. A distal end of the drive shaft 18 is connected to the cutting cap 14 to confer a reciprocating motion upon the cutting cap 14. The drive shaft 18 is typically operably connected to a motor 200 at a proximal end of the drive shaft 18. However, the drive shaft 18 may be otherwise connected to the motor 200 to confer a rotational or reciprocating motion upon the drive shaft 18 as will be recognized by those skilled in the art upon review of the present disclosure. The drive shaft 18 operably couples a motive component, such as for example a motor 200, conferring rotational or reciprocating movement to the cutting cap 14. A drive shaft 18 may, typically at a proximal end, be engaged with the motor 200, a transmission and/or clutch assembly connected to a motor 200, or to another rotational or reciprocal motivating component to confer a rotational or reciprocal force to a cutting cap 14. The drive shaft 18 are typically metals however a range of polymers and other materials may be used as will be recognized by those skilled in the art upon review of the present disclosure. Drive shaft 18 is frequently in the form of wires, cables, braided wires, coils, and tubes. In one aspect, the drive shaft 18 may define a driveshaft lumen such as may be the case when, for example, a coil is used as a drive shaft 18. A distal end of the drive shaft 18 typically engages the cutting cap 14. A drive shaft 18 in accordance with the present invention is typically of a diameter and configuration to be rotatably or reciprocatingly received within lumen 22 of guide tube 12. Typically, the drive shaft 18 will extend for a length greater than the length of the lumen 22. Such a length can permit the cutting cap 14 to be extended beyond the distal opening 32 of lumen 22 to engage a tissue within the intervertebral disc.
Cutting cap 14 is generally configured to cut, abrade or otherwise disrupt material to permit the concurrent or subsequent removal of tissue. Typically, cutting cap 14 is operably connected to the drive shaft 18 to reciprocate relative to the distal opening 32 of guide tube 20, an inner guide tube 20 and/or a cutting member 16. As illustrated in
Cutting member 16 may be received within the proximal end of lumen 22 of guide tube 12 an end view of which is illustrated in isolation in
The cutting member 16, as illustrated in
The cam 60 may be rotatably secured at a desired position within the lumen 22 of guide tube 12. As illustrated for exemplary purposes in
In some embodiments, the guide tube 12 may be extended from and retracted into the outer distal opening 36 in outer guide tube 20. A physician can extend the guide tube 12 relative to the outer distal opening 36 of the outer guide tube 20 to advance the reciprocating cutting cap 14 through the nucleus pulposus of an intervertebral disc.
As illustrated in
Generally, the cutting cap 14 reciprocates between the distal stop 74 and a distal opening 32. As illustrated, the cutting cap 14 reciprocates between a proximal surface 76 of distal stop 74 and a distal opening 32 of the guide tube 12. In one aspect, the proximal surface 76 of distal stop 74 may contact the crown 38 of cutting cap 14 when the cutting cap 14 is in a fully extended position or is approaching a fully extended position. In the embodiment illustrated in
Cutting member 16 is secured to the distal end of lumen 22 adjacent to the distal opening 32. As illustrated for exemplary purposes, the cutting member 16 is secured within the distal end of lumen 22. The cutting member 16 can include a guide 50 through which a cap shaft 44 or drive shaft 18 may be slidably or rotatably positioned. The guide 50 may maintain cutting cap 14 concentric with cutting member 16 as the cutting cap 14 rapidly reciprocates between an extended and a retracted position and as forces are conferred upon the cutting cap 14 and guide tube 12 as the reciprocating cutting apparatus 10 is advanced and as it cuts tissue. As illustrated, the posterior surface 40 of cutting cap 14 includes a beveled edge 43 and cutting member 16 includes a cutting edge 41. The cutting edge 41 of the cutting member 16 may contact the beveled edge 43 of cutting cap 14 when the cutting cap 14 is fully retracted. In one aspect, the juxtaposition or contact of beveled edge 43 of cutting cap 14 and the cutting edge 41 of cutting member 16 may cut tissue when the cutting cap 14 is in or is approaching a retracted position.
As illustrated in
The cutting cap 14 of the embodiment illustrated in
The distal crown 78 of distal stop 74 is again generally configured to be atraumatic upon incidental contact with the annulus fibrosus. Again for exemplary purposes, the distal crown 78 has been illustrated as rounded for exemplary purposes. Those skilled in the art will recognize additional configurations for distal crown 78 that may render any incidental contact of distal stop 74 with the annulus fibrosus substantially atraumatic upon review and understanding of the inventions of the present disclosure. The proximal surface 76 of distal stop 74 is configured to cooperate with the leading cutting edge 45 of cutting cap 14 in the cutting and/or abrading of tissue. In one aspect, the proximal surface 76 of distal stop 74 may contact the leading cutting edge 45 of cutting cap 14 when the cutting cap 14 is in a fully extended position or is approaching a fully extended position.
Cutting member 16 is secured to the distal end of lumen 22 adjacent to the distal opening 32. As illustrated for exemplary purposes, the cutting member 16 is secured within the distal end of lumen 22. The cutting member 16 can include a guide 50 through which a cap shaft 44 or drive shaft 18 may be slidably positioned. The guide 50 may maintain cutting cap 14 concentric with cutting member 16 as the cutting cap 14 rapidly reciprocates between an extended and a retracted position and as forces are conferred upon the cutting cap 14 and guide tube 12 as the reciprocating cutting apparatus 10 is advanced and as it cuts tissue. The cutting member 16, as illustrated, includes a beveled cutting surface 46. The cutting surface 46 is beveled inward which may permit the guiding of cut and/or abraded debris into the distal opening 32.
In operation, the cutting cap 14 reciprocates between the proximal surface 76 of distal stop 74 in an extended position and a distal opening 32 in a retracted position. As the cutting cap 14 is withdrawn, the trailing cutting edge 42 may cut and/or abrade tissue. When the trailing cutting edge 42 is fully retracted, the trailing cutting edge 42 of the cutting cap 14 and the cutting surface 46 of the cutting member 16 are brought into contact and/or close proximity to one another. In one aspect, this contact and/or proximity may cause tissue positioned between the trailing cutting edge 42 and cutting surface 46 to be cut from the bulk material of a patient's nucleus pulposus. Further, the cavity in the posterior surface 40 of the cutting cap 14 may tend to receive and maintain cut tissue and direct it to the distal opening 32 of guide tube 12. As the cutting cap 14 is extended, the leading cutting edge 45 may also tend to cut and/or abrade tissue. When the leading cutting edge 45 is fully extended, the leading cutting edge 45 of the cutting cap 14 and the proximal surface 76 of the distal stop 74 are brought into contact and/or close proximity to one another. In one aspect, this contact and/or proximity may cause tissue positioned between the leading cutting edge 45 and the proximal surface 76 of the distal stop 74 to be cut from the bulk material of a patient's nucleus pulposus.
The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. Upon review of the specification, one skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.
