Patent Publication Number: US-2023149053-A1

Title: Systems And Methods For Performing Lateral-Access Spine Surgery

Description:
RELATED APPLICATIONS 
     This application is a Continuation of U.S. patent application Ser. No. 16/605,743, filed on Oct. 16, 2019, which is the U.S. National Stage Entry of International Patent Application No. PCT/US2018/054395, filed on Oct. 4, 2018, which claims the benefit of priority to U.S. Provisional Patent Application No. 62/568,267, filed on Oct. 4, 2017, the disclosures of each of which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     Surgical procedures, such as minimally-invasive procedures, may require a surgeon to insert surgical tools inside the body of the patient to a particular depth to reach the target area inside the patient&#39;s body. For example, minimally invasive spinal surgical procedures have been used for stabilization of vertebral bones and spinal joints and for relieving of pressure applied to the spinal nerves. Such procedures may utilize relatively small incisions and insertion of tubular retractors and cannulas while minimizing damage to muscles and other surrounding anatomical features. Minimally invasive surgical approaches can be faster, safer and require less recovery time than conventional open surgeries. There is a continuing need for improvement to the safety and speed of surgical procedures, such as minimally-invasive surgical procedures. 
     SUMMARY 
     Various embodiments include systems and methods for performing spine surgery, including minimally invasive lateral access spine surgery. Embodiments include a retractor apparatus that may be used for robot-assisted minimally invasive lateral access spine surgery. 
     Embodiments include a retractor apparatus for a surgical robotic system that includes a frame defining a central open region, a connecting member that connects the frame to a robotic arm, a plurality of coupling mechanisms for attaching a set of retractor blades within the central open region of the frame such that blades define a working channel interior of the blades, and a plurality of actuators extending between the frame and each of the coupling mechanisms and configured to move the blades with respect to the frame to vary a dimension of the working channel. 
     Further embodiments include a surgical robotic system that includes a robotic arm and a retractor apparatus attached to the robotic arm, where the retractor apparatus includes a frame attached to the robotic arm and defining a central open region, a connecting member that connects the frame to a robotic arm, a plurality of coupling mechanisms for attaching a set of retractor blades within the central open region of the frame such that blades define a working channel interior of the blades, and a plurality of actuators extending between the frame and each of the coupling mechanisms and configured to move the blades with respect to the frame to vary a dimension of the working channel. 
     Further embodiments include a method for performing a robot-assisted surgical procedure that includes controlling a robotic arm having a frame of a retractor apparatus frame attached thereto to position the frame over a pre-set trajectory into the body of a patient, attaching a plurality of retractor blades to the frame such that the blades define a working channel into the body of the patient, and moving at least one retractor blade relative to the frame to vary a dimension of the working channel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the present invention will be apparent from the following detailed description of the invention, taken in conjunction with the accompanying drawings of which: 
         FIG.  1    illustrates a robotic-assisted surgical system according to an embodiment. 
         FIGS.  2 A- 2 E  illustrate an embodiment retractor apparatus for performing lateral-access spine surgery. 
         FIGS.  3 A- 3 E  schematically illustrate a robotic-assisted lateral access spine procedure performed on a patient. 
         FIGS.  4 A- 4 B  illustrate a further embodiment retractor apparatus. 
         FIG.  5    schematically illustrates a computing device which may be used for performing various embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The various embodiments will be described in detail with reference to the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. References made to particular examples and implementations are for illustrative purposes, and are not intended to limit the scope of the invention or the claims. 
     Various embodiments relate to apparatuses and methods for performing spine surgery, including minimally invasive lateral access spine surgery. Embodiments include a retractor apparatus that may be used for robot assisted minimally invasive lateral access spine surgery. 
     One common surgical procedure performed on the spine is an interbody fusion, which includes fusing two vertebrae together. To perform this procedure, the intervertebral space between the two vertebrae must be accessed to partially or completely remove the intervertebral disc and to insert an implant, such as a spacer or cage, that maintains the normal alignment of the spine while allowing the two vertebrae to fuse. Conventionally, the surgical space has been accessed from the posterior or anterior of the patient. However, this may require removing bony portions of the vertebral column to access the disc space. In addition, such approaches may risk damage to major vascular structures and other sensitive organs. More recently, a lateral approach has been utilized, in which the surgeon may access certain parts of the spine (e.g., the lumbar region of the spine) from the side of the patient. This may be less invasive for the patient, may result in less trauma, and can reduce operating time and recovery periods. 
     In various embodiments, a lateral-access spine procedure, such as lateral transpsoas interbody fusion, may be performed using a computer-assisted image guided surgery system. In embodiments, the system may be a surgical robotic system that may include at least one robotic arm that is configured to assist a surgeon in performing a surgical procedure.  FIG.  1    illustrates a system  100  for performing computer-assisted image-guided surgery that includes an imaging device  103 , a motion tracking system  105  and a robotic arm  101 . The robotic arm  101  may be fixed to a support structure at one end and may have an end effector  102  located at the other end of the robotic arm  101 . The robotic arm  101  may comprise a multi-joint arm that includes a plurality of linkages connected by joints having actuator(s) and optional encoder(s) to enable the linkages to rotate, bend and/or translate relative to one another in response to control signals from a robot control system. The motions of the robotic arm  101  may enable the end effector  102  to be moved to various positions and/or orientations, such as various positions and/or orientations with respect to a patient (not illustrated) that may be located on a patient support  60  (e.g., surgical table). In various embodiments described in further detail below, the end effector  102  of the robotic arm  101  may include a retractor apparatus that may be used to provide a working channel to a target site within the patient. 
     The imaging device  103  may be used to obtain diagnostic images of a patient (not shown in  FIG.  1   ), which may be a human or animal patient. In embodiments, the imaging device  103  may be an x-ray computed tomography (CT) imaging device. The patient may be positioned within a central bore  107  of the imaging device  103  and an x-ray source and detector may be rotated around the bore  107  to obtain x-ray image data (e.g., raw x-ray projection data) of the patient. The collected image data may be processed using a suitable processor (e.g., computer) to perform a three-dimensional reconstruction of the object. In other embodiments, the imaging device  103  may comprise one or more of an x-ray fluoroscopic imaging device, a magnetic resonance (MR) imaging device, a positron emission tomography (PET) imaging device, a single-photon emission computed tomography (SPECT), or an ultrasound imaging device. In embodiments, image data may be obtained pre-operatively (i.e., prior to performing a surgical procedure), intra-operatively (i.e., during a surgical procedure) or post-operatively (i.e., following a surgical procedure) by positioning the patient within the bore  107  of the imaging device  103 . In the system  100  of  FIG.  1   , this may be accomplished by moving the imaging device  103  over the patient to perform a scan while the patient may remain stationary. 
     Examples of x-ray CT imaging devices that may be used according to various embodiments are described in, for example, U.S. Pat. No. 8,118,488, U.S. Patent Application Publication No. 2014/0139215, U.S. Patent Application Publication No. 2014/0003572, U.S. Patent Application Publication No. 2014/0265182 and U.S. Patent Application Publication No. 2014/0275953, the entire contents of all of which are incorporated herein by reference. In the embodiment shown in  FIG.  1   , the patient support  60  (e.g., surgical table) upon which the patient may be located is secured to the imaging device  103 , such as via a column  50  which is mounted to a base  20  of the imaging device  103 . A portion of the imaging device  103  (e.g., an O-shaped imaging gantry  40 ) which includes at least one imaging component may translate along the length of the base  20  on rails  23  to perform an imaging scan of the patient, and may translate away from the patient to an out-of-the-way positon for performing a surgical procedure on the patient. It will be understood that other imaging devices may be utilized, including other mobile or fixed x-ray CT devices or a C-arm x-ray fluoroscopy device. 
     Further, although the imaging device  103  shown in  FIG.  1    is located close to the patient within the surgical theater, the imaging device  103  may be located remote from the surgical theater, such as in another room or building (e.g., in a hospital radiology department). 
     The motion tracking system  105  shown in  FIG.  1    includes a plurality of marker devices  119 ,  202  and an optical sensor device  111 . Various systems and technologies exist for tracking the position (including location and/or orientation) of objects as they move within a three-dimensional space. Such systems may include a plurality of active or passive markers fixed to the object(s) to be tracked and a sensing device that detects radiation emitted by or reflected from the markers. A 3D model of the space may be constructed in software based on the signals detected by the sensing device. 
     The motion tracking system  105  in the embodiment of  FIG.  1    includes a plurality of marker devices  119 ,  202  and a stereoscopic optical sensor device  111  that includes two or more cameras  207  (e.g., IR cameras). The optical sensor device  111  may include one or more radiation sources (e.g., diode ring(s)) that direct radiation (e.g., IR radiation) into the surgical field, where the radiation may be reflected by the marker devices  119 ,  202  and received by the cameras. The marker devices  119 ,  202  may each include three or more (e.g., four) reflecting spheres, which the motion tracking system  105  may use to construct a coordinate system for each of the marker devices  119 ,  202 . A computer  113  may be coupled to the sensor device  111  and may determine the transformations between each of the marker devices  119 ,  202  and the cameras using, for example, triangulation techniques. A 3D model of the surgical space in a common coordinate system may be generated and continually updated using motion tracking software implemented by the computer  113 . In embodiments, the computer  113  may also receive image data from the imaging device  103  and may register the image data to the common coordinate system as the motion tracking system  105  using image registration techniques as are known in the art. In embodiments, at least one reference marker device may be attached to the patient. The reference marker device may be rigidly attached to a landmark in the anatomical region of interest (e.g., clamped or otherwise attached to a bony portion of the patient&#39;s anatomy) to enable the anatomical region of interest to be continually tracked by the motion tracking system  105 . Additional marker devices  119  may be attached to surgical tools or instruments  104  to enable the tools/instruments  104  to be tracked within the common coordinate system. Another marker device  202  may be rigidly attached to the robotic arm  101 , such as on the end effector  102  of the robotic arm  101 , to enable the position of robotic arm  101  and end effector  102  to be tracked using the motion tracking system  105 . The computer  113  may also include software configured to perform a transform between the joint coordinates of the robotic arm  101  and the common coordinate system of the motion tracking system  105 , which may enable the position and orientation of the end effector  102  of the robotic arm  101  to be controlled with respect to the patient. 
     In addition to passive marker devices described above, the motion tracking system  105  may alternately utilize active marker devices that may include radiation emitters (e.g., LEDs) that may emit radiation that is detected by an optical sensor device  111 . Each active marker device or sets of active marker devices attached to a particular object may emit radiation in a pre-determined strobe pattern (e.g., with modulated pulse width, pulse rate, time slot and/or amplitude) and/or wavelength which may enable different objects to be uniquely identified and tracked by the motion tracking system  105 . One or more active marker devices may be fixed relative to the patient, such as secured to the patient&#39;s skin via an adhesive membrane or mask. Additional active marker devices may be fixed to surgical tools  104  and/or to the end effector  102  of the robotic arm  101  to allow these objects to be tracked relative to the patient. 
     In further embodiments, the marker devices may be passive maker devices that include moiré patterns that may enable their position and orientation to be tracked in three-dimensional space using a single camera using Moiré Phase Tracking (MPT) technology. Other tracking technologies, such as computer vision systems and/or magnetic-based tracking systems, may also be utilized. 
     As shown in  FIG.  1   , the optical sensor device  111  may include a plurality of cameras  207  mounted to an arm  209  extending above the patient surgical area. The arm  209  may be mounted to or above the imaging device  103 . The arm  209  may enable the sensor device  111  to pivot with respect to the arm  209  and/or the imaging device  103  (e.g., via one or more ball joints  213 ). The arm  209  may enable a user to adjust the position and/or orientation of the sensor device  111  to provide the cameras  207  with a clear view into the surgical field while avoiding obstructions. The arm  209  may enable the position and/or orientation of the sensor device  111  to be adjusted and then locked in place during an imaging scan or surgical procedure. 
     The system  100  may also include at least one display device  219  as illustrated in  FIG.  1   . The display device  219  may display image data of the patient&#39;s anatomy obtained by the imaging device  103 . In the case of CT image data, for example, the display device  219  may display a three-dimensional volume rendering of a portion of the patient&#39;s anatomy and/or may display two-dimensional slices (e.g., axial, sagittal and/or coronal slices) through the 3D CT reconstruction dataset. The display device  219  may facilitate planning for a surgical procedure, such as by enabling a surgeon to define one or more target positions in the patient&#39;s body and/or a path or trajectory into the patient&#39;s body for inserting surgical tool(s) to reach a target position while minimizing damage to other tissue or organs of the patient. The position and/or orientation of one or more objects tracked by the motion tracking system  105  may be shown on the display  219 , and may be shown overlaying the image data (e.g., using augmented reality technology). This may enable the surgeon to precisely navigate the tracked tools/implants within the patient&#39;s body in real-time. The use of tracked surgical instruments or tools in combination with pre-operative or intra-operative images of the patient&#39;s anatomy in order to guide a surgical procedure may be referred to as “image-guided surgery.” 
     In embodiments, the display device  219  may be a handheld computing device, such as a tablet device. One or more handheld display devices  219  may be mounted to the imaging device  103 , as shown in  FIG.  1   . In other embodiments, a handheld display device  219  may be mounted to the patient support  60  or column  50 , the arm  209  that supports the optical sensing device  111  for the motion tracking system  105 , or to any of the wall, ceiling or floor in the operating room, or to a separate cart. Alternately or in addition, the at least one display device  219  may be a monitor display that may be located on a mobile cart or mounted to another structure (e.g., a wall) within the surgical theater. In further embodiments, a display device  219  may be a head-mounted display that may be worn by a surgeon or other clinician. 
     As shown in  FIG.  1   , the robotic arm  101  may be fixed to the imaging device  103 , such as on a support element  215  (e.g., a curved rail) that may extend concentrically over the outer surface of the O-shaped gantry  40  of the imaging device  103 . In embodiments, an arm  209  to which the optical sensing device  111  is mounted may be mounted to the same or a similar support element  215  (e.g., curved rail) as the robotic arm  101 . The position of the robotic arm  101  and/or the arm  209  may be adjustable along the length of the support element  215 . In other embodiments, the robotic arm  101  may be secured to any other portion of the imaging device  103 , such as directly mounted to the gantry  40 . Alternatively, the robotic arm  101  may be mounted to the patient support  60  or column  50 , to any of the wall, ceiling or floor in the operating room, or to a separate cart.  FIG.  1    illustrates the robotic arm  101  mounted to a support element  215  (curved rail) that is directly attached to the imaging device  103 . Alternately, the robotic arm  101  may be mounted to a mobile shuttle  216  that may moved adjacent to the imaging device  103  such that a support member  218  (e.g., a curved rail) for mounting the robotic arm  101  extends at least partially over the gantry  40  of the imaging device  103 . Various exemplary systems for mounting a robotic arm  101  in a computer-assisted image guided surgery system are described in U.S. Provisional Patent Application No. 62/491,645, filed Apr. 28, 2017, the entire contents of which are incorporated by reference herein. Although a single robotic arm  101  is shown in  FIG.  1   , it will be understood that two or more robotic arms  101  may be utilized. Each robotic arm  101  may include an end effector  102  that may comprise or may be configured to hold an invasive surgical tool or implant. 
     The at least one robotic arm  101  may aid in the performance of a surgical procedure, such as a minimally-invasive spinal surgical procedure or various other types of orthopedic, neurological, cardiothoracic and general surgical procedures. In embodiments, the motion tracking system  105  may track the position of the robotic arm  101  (e.g., via marker device  202  on end effector  102  as shown in  FIG.  1   ) within the patient coordinate system. A control loop may continuously read the tracking data and the current parameters (e.g., joint parameters) of the robotic arm  101  and may send instructions to a robotic controller to cause the robotic arm  101  to move to a desired position and orientation within the patient coordinate system. 
     In embodiments, a surgeon may use an image-guided surgery system as a planning tool for a surgical procedure, such as by setting trajectories within the patient for inserting surgical tools, as well as by selecting one or more target locations for a surgical intervention within the patient&#39;s body. The trajectories and/or target locations set by the surgeon may be saved (e.g., in a memory of a computer device, such as computer device  113  shown in  FIG.  1   ) for later use during surgery. In embodiments, the surgeon may be able to select stored trajectories and/or target locations using an image guided surgery system, and the robotic arm  101  may be controlled to perform a particular movement based on the selected trajectory and/or target location. For example, the robotic arm  101  may be moved to position the end effector  102  of the robotic arm  101  into alignment with the pre-defined trajectory and/or over the pre-determined target location. 
     In addition to a robotic arm  101  as described above, an end effector  102  of the present embodiments may be attached to a moveable arm or boom, which may be motor-driven or manually moved. The arm may be moved to position the end effector  102  at a desired location with respect to the patient and the arm may be configured to hold its pose during a surgical intervention. 
       FIGS.  2 A- 2 E  schematically illustrate a retractor apparatus  200  for performing lateral-access spine surgery according to an embodiment.  FIG.  2 A  is an overhead view of the retractor apparatus  200  and  FIGS.  2 B- 2 E  are side views illustrating the retractor apparatus  200  with a plurality of retractor blades  227  mounted therein. The retractor apparatus  200  may be attached to the end of a robotic arm  101  (i.e., the retractor apparatus  200  may function as the end effector  102  of the robotic arm  101 ), such that the robotic arm  101  may move the retractor apparatus  200  to a desired position and/or orientation with respect to a patient. The retractor apparatus  200  includes a frame  221 , which may be made from a rigid structural material. The frame  221  may optionally be made of a radiolucent material. The frame  221  may surround a central open region  225 , as shown in  FIG.  2 A . The frame  221  in the embodiment of  FIGS.  2 A- 2 E  has a rectangular shape. It will be understood that the frame  221  may have a circular or other shape. A connecting member  223  connects the frame  221  to the end of a robotic arm  101  (not illustrated in  FIGS.  2 A- 2 E ). In embodiments, the robotic arm  101  may have an attachment mechanism that enables different end effectors  102 , such as the retractor apparatus  200 , to be attached to and removed from the end of the robotic arm  101 . The retractor apparatus  200  may be fastened to the robotic arm  101  using mechanical fasteners (e.g., bolts) and/or via a quick-connect/disconnect mechanism. Alternately, the retractor apparatus  200  may be permanently mounted to the robotic arm  101 . 
     The retractor apparatus  200  may be a sterile or sterilizable component that may not need to be draped during surgery. In some embodiments, the retractor apparatus  200  may be attached to a robotic arm  101  over a surgical drape that covers the arm  101 . All or a portion of the retractor apparatus  200  may be a single-use disposable component. Alternately, all or a portion of the retractor apparatus  200  multi-use component that may be re-sterilized (e.g., autoclavable). A marker device  202  (e.g., an array of reflective spheres) may be attached to the retractor apparatus  200  and/or to the robotic arm  101  to enable the retractor apparatus  200  to be tracked by a motion tracking system  105 , such as shown in  FIG.  1   . 
     The frame  221  may further include a coupling mechanism for mechanically coupling the frame  221  to a plurality of retractor blades  227  (see  FIGS.  2 B- 2 E ). The retractor blades  227  may extend downwards from the central open region  225  of the frame  221 . In this embodiment, the coupling mechanism comprises a plurality of guides  229  through which the retractor blades  227  may be inserted. In other embodiments, the coupling mechanism may be an attachment mechanism that engages with the side or top surface of the blades  227  to couple the blades  227  to the frame  221 . 
     As shown in the side view of  FIG.  2 B , the retractor blades  227  may slide through the guides  229  to couple the blades  227  to the frame  221 . The blades  227  may include clips  231  or another attachment mechanism to attach the blades  227  to the respective guides  229  when the blades  227  are fully inserted. 
     The retractor apparatus  200  may also include a plurality of actuators  233  for moving the retractor blades  227  radially inwards and outwards with respect to the frame  221 . Each actuator  233  may include, for example, a screw, a rack-and-pinion system, or a similar apparatus that extends from the frame  221  into the central open region  225 . The actuators  233  may be manually operated using a control knob, handle or other feature that enables a user to extend or retract the blades  227 . As shown in  FIG.  2 A , the frame  221  may include a plurality of sockets  235  into which a torque device (e.g., a key, an Allen wrench, screwdriver, etc.) may be inserted such that bi-directional rotation of the torque device causes the actuator  233  to extend and retract with respect to the frame  221 . In alternative embodiments, the actuators  233  may be motor-driven. For example, each actuator  233  may have an associated motor located on or within the frame  221 . The motor may drive the extension and retraction of the actuator  233  and blade  227  in response to control signals received from a system controller and/or a user input device. 
       FIG.  2 C  is a side view illustrating the retractor apparatus  200  in a first configuration in which the actuators  233  are fully extended from the frame  221  into the central open region  225 . The retractor blades  227  may be positioned adjacent to one another and may define a working channel  237  (see  FIG.  2 A ) radially inward from the plurality of blades  227 .  FIG.  2 D  illustrates the retractor apparatus  200  in a second configuration in which blades  227  are retracted by the actuators  233 . This is schematically illustrated in the overhead view of  FIG.  2 A , which shows the blades  237  retracted along the direction of arrows  239 . In the first configuration, the retractor blades  227  may define a working channel  237  having a generally circular cross-section and an initial width dimension (i.e., diameter) Di. The blades  237  may be retracted to a second configuration to increase the width dimension (i.e., diameter) of the working channel  237 , as shown in  FIG.  2 A . In embodiments, each blade  227  of the retractor apparatus  200  may be moved (i.e., extended or retracted) via its associated actuator  233  independently of any movement of the other blade(s)  227 . 
     In embodiments, the blades  227  may also pivot with respect to the frame  221  of the retractor apparatus  200 . This is illustrated by  FIG.  2 E , which shows a pair of blades  227  (depicted in phantom) that have been pivoted out with respect to the frame  221  in the direction of arrow  239 . Pivoting a retractor blade  227  as illustrated may enable the width dimension of the working channel  237  to be increased proximate to the area of surgical intervention (e.g., the spine) while minimizing the size of the opening through the skin surface and peripheral tissue. In one exemplary embodiment, the pivot motion of the blades  227  may be controlled using the same input feature (e.g., control knob, handle, torque device) that is used to extend and retract the blades  227 . For example, in the embodiment of  FIGS.  2 A- 2 E , turning a torque device in the socket  235  in one direction may cause the actuator  233  to retract the corresponding blade  227  towards the frame  221 . After the blade  227  is fully retracted, continuing to turn the torque device in the same direction may cause the blade  227  to pivot outwards as shown in  FIG.  2 E . Alternately, a set of separate manual controllers (e.g., control knobs, levers, handles, torque devices, etc.) may be utilized to control the pivoting motion of the blades  227 . In such a case, the pivoting motion of the blades  227  may be performed independently of the extension and retraction of the blades  227 . In addition, each blade  227  may be pivoted independently of any pivoting of the other blade(s)  227 . In further embodiments, the pivoting motion of the blades  227  be motor-driven, as described above. 
     Each retractor blade  227  may be made from a radiolucent material, such as carbon fiber. The retractor blades  227  may include electrically conductive material that forms one or more continuous electrical pathways through the blade  227 . The continuous electrical pathways may be used for performing intraoperative neurophysiological monitoring (IONM), as described further below. The retractor blade(s)  227  and/or the coupling mechanism (e.g., guide(s)  229 ) may optionally include a port  241  or other electrical connector to enable a probe device to electrically connect to the blade  227  (e.g., for neurophysiological monitoring). 
     In embodiments, the retractor blades  227  may include one or more channels  243  extending through the blade  227  (shown in phantom in  FIG.  2 B ). A channel  243  in the blade  227  may be utilized for illumination of the surgical area (e.g., by inserting an LED or other light source into the channel  243 ), for visualization of the surgical area (e.g., by inserting an endoscope into the channel  243 ) or for any other purpose (e.g., for removal of tissue/fluid via suction or other means). 
     A retractor apparatus  200  as described above may utilize retractor blades  227  having varying lengths. The length of the blades  227  used for a particular surgical procedure may be chosen based on the depth of the surgical site from the patient&#39;s skin surface. This depth may be determined using an image guided surgery system as described above. For example, a surgeon may use a tracked instrument to set a target trajectory and/or target location within the patient&#39;s anatomy. Based on the pre-set trajectory and/or location, the image guided surgery system may determine the appropriate size of the retractor blades  227  for insertion into the retractor apparatus  200  out of an available set of sizes for the retractor blades  227 . The image guided surgery system may provide an indication to the surgeon (e.g., via a display device  219  as shown in  FIG.  1   ) of an appropriate blade  227  size to use for the surgical procedure. In some embodiments, the image guided surgery system may control the robotic arm  101  to move the frame  221  of the retractor apparatus  200  to a pre-determined distance from the skin surface of the patient such that the tip ends of the selected retractor blades  227  are located at the proper anatomical depth within the patient. 
     The frame  221  of the retractor apparatus may include one or more rails  245  that may extend around the periphery of the central open region  225 . The one or more rails  245  may enable tools/instruments to be clipped or clamped on to the retractor apparatus  200 . For example, an illumination source or camera system (e.g., endoscope) may be attached to a desired position on a rail  245 , and may optionally extend at least partially into the working channel defined by the retractor blades  227 . Other tools that may be attached to a rail  245  include, for example, a suction device for removing fluids from the surgical site and/or a shim element that may be inserted into the disc space (e.g., to restore disc height and/or anchor the retractor apparatus  200  to the surgical site). 
     The retractor apparatus  200  of  FIGS.  2 A- 2 E  includes four retractor blades  227 . However, it will be understood that a retractor apparatus  200  according to various embodiments may have three blades, two blades or greater than four blades (e.g., five blades, six blades, etc.). 
     As discussed above, a retractor apparatus  200  may be configured to provide intraoperative neurophysiological monitoring (IONM). Use of IONM techniques may enable the surgeon to locate the proximity of tools to nerves and avoid damage or irritation to the nerves during surgery. A variety of IONM methods are known, including electromyography (EMG), including spontaneous EMG (S-EMG) and stimulus-triggered EMG (T-EMG), somatosensory evoked potentials (SSEPs) and motor evoked potentials (MEPs). 
     In one embodiment, IONM may be performed by electrically stimulating muscle tissue and neural structures surrounding the surgical area and measuring the evoked EMG response using sensor(s) located on or within the patient&#39;s body. A retractor apparatus  200  as described above may include at least one electrode  247  located on a retractor blade  227 , as schematically shown in  FIG.  2 D . The electrode  247  may be configured to electrically stimulate the surrounding tissue when the blade  227  is inserted into the patient. In embodiments, each of the blades  227  of the retractor apparatus  220  may include at least one electrode  247  for stimulating the surrounding tissue. The electrode  247  in  FIG.  2 D  is shown located at the tip end of the retractor blade  227 , although it will be understood that the electrode  247  may be located at another position on the blade  227 . In addition, a blade  227  may have multiple electrodes  247  located at different positions on the blade  227  for stimulating different portions of the surrounding tissue. 
     For performing neurophysiological monitoring, each electrode  247  may be electrically connected to a power source  246  (e.g., one or more batteries) and circuitry  249  for generating stimulation signals that may be transmitted to the electrode(s)  247  via a conductive lead  251 . The conductive lead  251  may be, for example, a wire located on or within the blade  227  or a conductive trace formed on a surface of the blade  227  via printing, spray coating, etc. In embodiments, the retractor apparatus  200  may include a conductive path  252  to conduct power from the power source  246  to the blade  237 . One or more sensors  253  (e.g., surface or needle electrodes) may be positioned at pre-determined locations on the patient&#39;s body corresponding to particular muscle(s) and/or neural features to measure the evoked EMG response. A processing device  255  (e.g., computer), operably coupled to the sensor(s)  253 , may include a nerve detection component  256  configured to process the sensor data according to defined algorithms to determine the proximity (including distance and/or direction) of a neural structure (e.g., a nerve) to a blade  227  or a portion thereof. The nerve detection component  256  may be implemented in electronic hardware, in computer software, or in combinations of both. 
     The nerve detection component  256  may be coupled to a user feedback device to provide audio and/or visual feedback to the surgeon. For example, the nerve detection component  256  may be coupled to a display device  219  (see  FIG.  1   ) configured provide feedback in the form of a visual indication on the display device  219  that a particular retractor blade  227  is proximate to a nerve and/or may be impinging on a nerve. In response to the nerve detection component  256  determining that a retractor  227  blade is too close to a nerve, the nerve detection component  256  may cause the display  219  to provide instructions to the surgeon to stop further movement of the blade  227  and/or to move the blade  227  away from the nerve. In some embodiments, a graphical depiction of one or more nerves detected using an IONM method may be overlaid with image data on the display screen of the image guided surgery system. In embodiments in which the movement of the blades  227  is motor-driven as described above, the nerve detection component  256  may be configured to send instructions to a motorized drive system of a blade to cause the motorized drive system to automatically stop movement (e.g., retraction or pivoting) of the blade  227 . The nerve detection component  256  may also cause the motorized drive system to move the retractor blade  227  away from a detected nerve. 
     In embodiments, the nerve detection component  256  may be configured to activate the electrodes  247  on the blades  227  of the retractor apparatus  200  to stimulate the surrounding tissue. The nerve detection component  256  may be operatively coupled to circuit  249  and to the electrodes  247  on the retractor blades  227  via a wired or wireless connection. The nerve detection component  256  may be configured to control the characteristics of the stimulation signals, such as the stimulation current, the duration of the signals, and/or the frequency of the signals. In embodiments, stimulation signals may be generated in response to a user input from a user input device. In some embodiments, a plurality of stimulation signals may be generated in a pre-determined sequence or cycle (e.g., each electrode  247  of a plurality of electrodes on the retractor blades  227  may be energized sequentially). 
     In the embodiment shown in  FIG.  2 D , the power source  246  and circuitry  249  for generating stimulation signals are located on the retractor apparatus  200 , and may be mounted to or within the frame  221  of the apparatus  200 . The circuitry  249  may include wireless transceiver circuitry configured to provide a wireless communication link  254  between the retractor apparatus  200  and an external entity, such as the processing device  255  shown in  FIG.  2 D . Signals, including data and/or command signals, may be transmitted wirelessly between the retractor apparatus  200  and the processing device  255  using a suitable wireless communication protocol or standard (e.g., an IEEE 802.15×(BLUETOOTH®) connection or IEEE 802.11 (WiFi) connection). In embodiments, command signals to energize particular electrodes  247  may be received wirelessly from a remote processing device  255 . Alternately or in addition, the retractor apparatus  200  may include a user interface component (e.g., one or more buttons) that may be used to trigger the stimulation signals. When the user actuates the user interface component, a signal may be sent to the processing device  255  to enable the device  255  to synchronize the recording of the evoked EMG response(s) with the triggering of the stimulation signal(s). 
     Although the embodiment of  FIG.  2 D  shows the power source  246  and circuitry  249  for generating stimulation signals located on the retractor apparatus  200 , it will be understood that one or both of these components may be omitted from the retractor apparatus  200 . For example, the retractor apparatus  200  and/or the individual retractor blades  227  may be connected to a separate neurophysiological monitoring device by a wire connection. In some embodiments, a neurophysiological monitoring device may include a handheld probe that may be selectively coupled to the retractor apparatus  200  or to individual retractor blades  227  to provide nerve stimulation signals. For example, a handheld probe may be inserted into the ports  241  as shown in  FIG.  1   . 
     In some embodiments, a connection  257  between the retractor apparatus  200  and the robotic arm  101  may be used for data/control signals and/or to provide power to the retractor apparatus  200 , as is schematically illustrated in  FIG.  2 D . The connection  257  between the robotic arm  101  and retractor apparatus  200  may need to pass through a sterile barrier (e.g., a surgical drape) covering the arm  101  and may utilize a non-contact transmission mechanism, such as inductive or capacitive coupling and/or optical or other electromagnetic transmission methods. 
       FIGS.  3 A- 3 E  illustrate a method of performing a surgical procedure using a retractor apparatus  200  such as described above. The surgical procedure may be a robot-assisted spinal procedure, such as a minimally-invasive lateral transpsoas interbody fusion. In  FIG.  3 A , a tracked instrument  304  may be used to define and set a target trajectory or target location within the body of the patient  300 . In embodiments, the instrument  304  may be a handheld instrument that may be gripped and easily manipulated by a user (e.g., a surgeon). The instrument  304  may be a handheld pointer or stylus device that may be manipulated by the surgeon to point to or touch various locations on the skin surface of the patient  300 . Alternately, the instrument  304  may be an invasive surgical instrument (e.g., dilator, cannula, needle, scalpel, etc.) that may be inserted into the body of the patient. The instrument  304  may further include at least one marker device  319  to enable the instrument  304  to be tracked using a motion tracking system  105 , as described above. In this embodiment, the at least one marker device  319  includes an array of reflective spheres that are rigidly fixed to the instrument  304 , although other types of active or passive markers may be utilized. The marker device  319  may be in a known, fixed geometric relationship with the instrument  304  such that by tracking the marker device  319  the motion tracking system  105  may determine the position and/or orientation of the instrument  304 . The motion tracking system  105  may also track the current position and orientation of the patient  300  via a reference marker device  315  which may be rigidly attached to the patient  300  (e.g., clamped or otherwise attached to a bony portion of the patient&#39;s anatomy). The motion tracking system  105  may thereby continuously track the position and/or orientation of the instrument  304  relative to the patient  300  (i.e., within a common, patient-centric coordinate system). 
     Patient images  318 , which may have previously-been obtained by an imaging device  103  (see  FIG.  1   ), may be registered to the common patient-centric coordinate system using an image registration technique, as described above. One or more patient images  318  may be shown on a display screen of a display device  219 , as shown in  FIG.  3 A . The patient images  318  on the display device  219  may be augmented by one or more graphical elements indicating the current position/orientation of the instrument  304  within the patient-centric coordinate system. For example, as shown in  FIG.  3 A , a dashed line  320  superimposed over the patient image  318  may indicate the trajectory defined by an imaginary ray projected forward from the tip of the instrument  304 . As the instrument  304  is moved relative to the patient  300 , the location of the graphical element(s)  320  on the display screen may be updated to reflect the current pose of the instrument  304  relative to the patient. 
     In embodiments, the user (e.g., surgeon) may manipulate the instrument  304  while viewing the augmented patient images on the display device  219  to identify a desired trajectory though the patient  300  to a surgical area. For example, for a lateral transpoas interbody fusion, the surgeon may utilize the instrument  304  to identify a path through the patient&#39;s anatomy to the surgical site (e.g., an intervertebral disc requiring a surgical intervention). The path may be selected to minimize disturbance to other anatomic features, such as neural structures (e.g., lumbar nerve plexus) located around or within the psoas muscle. The user may set a particular trajectory using a user-input command (e.g., a button push, a voice command, etc.). The selected trajectory within the common coordinate system may be saved in a memory (e.g., in computer  113 ). 
     After a trajectory is set, the surgeon may make an incision  331  in the patient&#39;s skin surface and insert an invasive surgical instrument through the incision  331  and into the patient&#39;s body. The invasive surgical instrument may be, for example, a K-wire, a needle, an awl or the like that may be advanced along the pre-determined trajectory to the surgical site of interest. In some embodiments, the invasive surgical instrument may be a tracked instrument that is pre-calibrated and registered within the image guided surgery system. This may enable the motion tracking system  105  to track the advancement of the instrument within the patient  300 . The display device  219  may graphically illustrate the position of the instrument as it is advanced along the pre-set trajectory. 
     In some embodiments, a robotic arm  101  such as shown in  FIG.  1    may be used to guide the insertion of an invasive surgical instrument along the pre-determined trajectory. For example, the robotic arm  101  may be controlled to move the end effector  102  of the arm  101  into alignment with the pre-defined trajectory and/or over the pre-determined target location. The end effector  102  may include a guide mechanism (e.g., such as a hollow tube) aligned with the pre-set trajectory and through which the surgical instrument may be inserted to guide the instrument along the trajectory. Alternately, the invasive surgical instrument may be inserted by the surgeon using a free-hand technique (i.e., without robotic assistance). The insertion may be performed with or without the use of image guidance/surgical navigation. 
     The surgeon may also perform intraoperative neurophysiological monitoring (IONM) such as by inserting a handheld neuro-monitoring probe device into the incision site of the patient to electrically stimulate the surrounding tissue and detecting the evoked EMG response to detect for the presence of nerve(s). Alternately or in addition, the invasive surgical instrument (e.g., K-wire, needle, etc.) that is inserted into the patient&#39;s body may be equipped with IONM functionality (e.g., it may include one or more electrodes configured to stimulate the surrounding tissue). This may enable the surgeon to repeatedly monitor for nerves as the instrument is advanced to the target site (e.g., an intervertebral disc). 
     In embodiments, after the surgeon has advanced an initial surgical instrument along the trajectory to the surgical site, one or more additional instruments may be inserted to dilate the tissue between the incision and the surgical site. For example, a series of dilating cannulas may be inserted over the initial surgical instrument (e.g., a K-wire).  FIG.  3 B  illustrates an outermost cannula  333  of a series of sequential dilating cannulas within the surgical opening  331 . It will be understood that each of the dilating instruments (e.g., cannulas  333 ) may optionally be tracked by the motion tracking system  105 . Also, each additional instrument inserted into the patient  300  may optionally include IONM functionality to detect nerve proximity during or after it&#39;s insertion into the patient  300 . 
     As shown in  FIG.  3 C , a robotic arm  101  having a retractor apparatus  200  attached thereto may be controlled to move the retractor apparatus  200  over the surgical site. In embodiments where a robotic arm  101  is used to guide the insertion of a K-wire or other instrument down to the surgical site, the end effector  102  used for guiding may be removed from the robotic arm  101  and a retractor apparatus  200  such as shown in  FIGS.  2 A- 2 E  may be attached to the end of the arm  101  (e.g., using a quick connect/disconnect attachment mechanism). The retractor apparatus  200  may be pre-calibrated and registered within the image guided surgery system and may include a marker device  322  to enable the position of the retractor apparatus  200  to be tracked and optionally shown on the display device  219 . The retractor apparatus  200  may be moved by the robotic arm  101  into a position such that a retractor axis, a, extending through the central open region  225  of the apparatus  200  may be aligned (i.e., collinear) with the pre-set trajectory into the patient  300 . The retractor axis a may extend down the center of the working channel  237  of the retractor apparatus  200  when the retractor blades  227  are attached. In embodiments, a controller for the robotic arm  101  may move the retractor apparatus  200  autonomously to align the retractor apparatus with the pre-set patient trajectory. Alternately, the robotic arm  101  may be manually moved using a hand guiding mode to align the retractor apparatus over the pre-set trajectory. 
     The retractor blades  227  may be attached to the frame  221  of the retractor apparatus  200  (see  FIGS.  2 A- 2 E ). In one embodiment, the diameter of the working channel  237  of the retractor apparatus in an initial configuration (i.e., Di in  FIG.  2 A ) may be approximately equal to the outer diameter of the tissue dilator  333 . The retractor blades  227  may be inserted through the respective guides  229  of the retractor assembly  200  and advanced along the outer surface of the dilator  333  into the patient  300 . In embodiments, the outer surface of the dilator  333  may include slots or similar features configured to guide the retractor blades  227  along the dilator  333  and into the surgical opening. After the retractor blades  227  are inserted into the patient  300 , the dilator  333  may be withdrawn from the patient  300  through the central open region  225  of the retractor apparatus  200  to expose the working channel  237  that may extend to the surgical site. 
     In an alternative embodiment, the retractor blades  227  may first be inserted into the patient  300  (e.g., over the outer surface of the dilator  333 ) and may then be attached to the frame  221  of the retractor assembly  200  via a coupling mechanism. The coupling mechanism may attach the distal ends of the actuators  233  to the retractor blades  227 . The coupling mechanism may be a latch (e.g., a mechanical or electromagnetic-based latch), a mechanical fastener, a clamp, a clip and/or mating features on the actuator  233  and the blade  227  that enable the blade  227  to be secured to the actuator  233 . In one example, the mating features may include a protrusion on the outer surface of the blade  227  that slides into a corresponding slot in the actuator  233  (e.g., to provide a dovetail or bayonet-type connection). Alternately, a protrusion on the actuator  233  may slide into a slot on the blade  227 . In embodiments, the retractor blades  227  may be secured to the frame  221  of the retractor apparatus  200  by rotating the blades in a first direction with respect to the frame  221 . The blades  227  may be detached from the frame  221  by rotating the blades  227  in the opposite direction. 
     In other alternative embodiments, the surgeon may create a pathway through the patient&#39;s anatomy to the surgical site with or without the use of an image guided surgery system. For example, the surgeon may optionally utilize image guidance/surgical navigation to pre-plan an initial path to the surgical site, and may then use a manual (i.e., non-navigated) approach for deep tissue dissection and/or cannulation. One or more invasive surgical instruments inserted into the patient (e.g., a K-wire, a needle, a cannula, etc.) may be tracked by the motion tracking system  105  (either directly by attaching a marker  319  to the invasive instrument, or indirectly by touching or aligning a tracked handheld probe  304  to the invasive instrument) to determine the actual trajectory of the instrument(s) (e.g., cannula  333 ) within the patient in the common coordinate system. The retractor apparatus  200  may then be moved by the robotic arm  101  to align the retractor axis, a, with the instrument trajectory, as described above. 
     In various embodiments, the retractor blades  227  may be used for performing IONM of the patient  300  as discussed above at any time before, during and/or after the blades  227  are attached to the frame  221  of the retractor apparatus  200 . 
     After the retractor blades  227  are attached to the frame  221 , the blades  227  may be retracted to increase the size of the working channel  237 , as shown in  FIG.  3 D . In embodiments, feedback data (e.g., encoder data) from the retractor apparatus  200  may be provided to the image guided surgery system to enable patient images shown on the display device  219  to be augmented by a graphical indication of real-time positions of the blades  227  and/or the size of the working channel  237  within the patient  300 . 
     During a surgical procedure, the robotic arm  101  may maintain the position of the retractor apparatus  200  relative to the patient  300 . In embodiments, the robotic arm  101  may be configured to compensate for any patient movement to maintain the working channel  237  aligned along the pre-set trajectory. The surgeon may perform a surgical procedure, such as in interbody fusion, through the working channel  237  defined by the retractor apparatus  200 . In particular, disc material or other pathologic tissue may be removed and an implant (e.g., a spacer or cage) may be inserted through the working channel  237  and placed in the intervertebral space. IONM may be utilized as desired to minimize damage or irritation to surrounding neural structures. 
     After the insertion of an implant, the retractor apparatus  200  may be removed from the patient  300  and the incision may be closed. The patient  300  may optionally be scanned using an imaging device  103  such as shown in  FIG.  1    to confirm the placement of the implant. The procedure may also include the insertion of stabilization elements (e.g., a rod and screw system) to stabilize the spine and allow the adjacent vertebra to properly fuse in the case of a fusion procedure. In some embodiments, the placement of screws (e.g., pedicle screws) may be performed using the robotic arm  101  and/or image guided surgery system without requiring the patient  300  to be repositioned or moved. In particular, patient images  318  on the display device  219  may be used by the surgeon to set one or more trajectories  323  for screw placement (e.g., via a posterior or anterior approach of the patient  300  lying on his/her side). The robotic arm  101  may be moved into position to align the end effector  102  over the pre-set trajectory. The retractor apparatus  200  may be removed and replaced on the robotic arm  101  by an end effector  102  that includes a guide mechanism (e.g., hollow tube  324 ) through which surgical instruments may be inserted along the pre-set trajectory. Various instruments, such as one or more cannulas, a drill, a screw and a screw driver, may be inserted through the end effector  102  and to place the screw in the patient. 
       FIGS.  4 A- 4 B  illustrate an alternative embodiment of a retractor apparatus  400 . The retractor apparatus  400  may be similar to retractor apparatus  200  shown in  FIGS.  2 A- 2 E . The retractor apparatus  400  includes a frame  421  having a central open region  425  as shown in the overhead view of  FIG.  4 A . The frame  421  may be coupled to a rigid support arm, such as robotic arm  101  shown in  FIG.  1   . In the embodiment of  FIGS.  4 A- 4 B , the frame  421  has a generally circular shape. The retractor apparatus  400  includes a plurality of actuators  433  extending into the central open region  425 . The actuators  433  may each be independently extended and retracted within the central open region  425  using control features (e.g., sockets  435 ). 
     The retractor apparatus  400  includes a coupling mechanism  420  for mechanically coupling the actuators  433  to a plurality of retractor blades  427 . In this embodiment, the coupling mechanism  430  comprises a projection  431  extending from the side of the retractor blade  427  that is received within a slot  432  in the actuator  433  to attach the retractor blade  427  to the actuator  433 . 
     The retractor apparatus  400  may also include a plurality of markers  434  (e.g., reflective spheres) attached to apparatus, such as on the rigid frame  421  of the apparatus  400 . A plurality of markers  434  (reflective spheres) are visible in the side view of the retractor apparatus  400  of  FIG.  4 B . The markers  434  may enable the retractor apparatus  400  to be tracked by a motion tracking system  105  as described above. 
       FIG.  5    is a system block diagram of a computing device  1300  useful for performing and implementing the various embodiments described above. While the computing device  1300  is illustrated as a laptop computer, a computing device providing the functional capabilities of the computer device  1300  may be implemented as a workstation computer, an embedded computer, a desktop computer, a server computer or a handheld computer (e.g., tablet, a smartphone, etc.). A typical computing device  1300  may include a processor  1301  coupled to an electronic display  1304 , a speaker  1306  and a memory  1302 , which may be a volatile memory as well as a nonvolatile memory (e.g., a disk drive). When implemented as a laptop computer or desktop computer, the computing device  1300  may also include a floppy disc drive, compact disc (CD) or DVD disc drive coupled to the processor  1301 . The computing device  1300  may include an antenna  1310 , a multimedia receiver  1312 , a transceiver  1318  and/or communications circuitry coupled to the processor  1301  for sending and receiving electromagnetic radiation, connecting to a wireless data link, and receiving data. Additionally, the computing device  1300  may include network access ports  1324  coupled to the processor  1301  for establishing data connections with a network (e.g., LAN coupled to a service provider network, etc.). A laptop computer or desktop computer  1300  typically also includes a keyboard  1314  and a mouse pad  1316  for receiving user inputs. 
     The foregoing method descriptions are provided merely as illustrative examples and are not intended to require or imply that the steps of the various embodiments must be performed in the order presented. As will be appreciated by one of skill in the art the order of steps in the foregoing embodiments may be performed in any order. Words such as “thereafter,” “then,” “next,” etc. are not necessarily intended to limit the order of the steps; these words may be used to guide the reader through the description of the methods. Further, any reference to claim elements in the singular, for example, using the articles “a,” “an” or “the” is not to be construed as limiting the element to the singular. 
     The various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention. 
     The hardware used to implement the various illustrative logics, logical blocks, modules, and circuits described in connection with the aspects disclosed herein may be implemented or performed with a general purpose processor, a graphics processing unit (GPU), a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but, in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. Alternatively, some steps or methods may be performed by circuitry that is specific to a given function. 
     In one or more exemplary aspects, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on as one or more instructions or code on a non-transitory computer-readable medium. The steps of a method or algorithm disclosed herein may be embodied in a processor-executable software module executed which may reside on a non-transitory computer-readable medium. Non-transitory computer-readable media includes computer storage media that facilitates transfer of a computer program from one place to another. A storage media may be any available media that may be accessed by a computer. By way of example, and not limitation, such non-transitory computer-readable storage media may comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that may be used to carry or store desired program code in the form of instructions or data structures and that may be accessed by a computer. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk, and blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of non-transitory computer-readable storage media. Additionally, the operations of a method or algorithm may reside as one or any combination or set of codes and/or instructions on a machine readable medium and/or computer-readable medium, which may be incorporated into a computer program product. 
     The preceding description of the disclosed aspects is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects without departing from the scope of the invention. Thus, the present invention is not intended to be limited to the aspects shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.