Patent Publication Number: US-11660145-B2

Title: Method and apparatus for attaching a reference marker to a patient

Description:
BACKGROUND 
     Computer-assisted surgical procedures, which may include image guided surgery and robotic surgery, have attracted increased interest in recent years. These procedures include the integration of a “virtual” three-dimensional dataset of the patient&#39;s anatomy, typically obtained using pre-operative or intra-operative medical imaging (e.g., x-ray computed tomography (CT) or magnetic resonance (MR) imaging), to the actual position of the patient and/or other objects (e.g., surgical instruments, robotic manipulator(s) or end effector(s) in the surgical area. These procedures may be used to aid the surgeon in planning a surgical procedure and may also provide the surgeon with relevant feedback during the course of surgical procedure. There is a continuing need to improve the safety and ease-of-use of computer-assisted surgical systems. 
     SUMMARY 
     Various embodiments include systems and methods for attaching a reference marker to a patient in a computer-assisted image-guided surgery system. 
     Embodiments include an apparatus for attaching a reference marker to a patient includes an elongated member extending between a first end and a second end, a sharp tip located proximate to the first end of the elongated member that is configured to break through a cortical surface of a bone of the patient to enable the elongated member to be advanced into the bone, an anchoring device that is extendable from the elongated member in order to anchor the apparatus within the bone and inhibit relative movement of the apparatus and the bone, and a reference marker device comprising at least one optical marker configured to enable the apparatus to be tracked using a motion tracking system. 
     Further embodiments include a method of performing image guided surgery that includes inserting an apparatus comprising an elongated member having a sharp tip at an end of the elongated member into the body of a patient to cause the sharp tip to break through a cortical surface of a bone of the patient, anchoring the apparatus within the bone to inhibit relative movement of the apparatus and the bone, and tracking a reference marker device having at least one optical marker attached to the apparatus using a motion tracking system. 
     Further embodiments include a system for performing image guided surgery using an apparatus for attaching a reference marker to a patient. 
    
    
     
       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    is a perspective view of a system for performing robotically-assisted image-guided surgery. 
         FIGS.  2 A- 2 D  illustrate a first embodiment apparatus for attaching a reference marker to a patient. 
         FIGS.  3 A- 3 C  schematically illustrate an embodiment apparatus embedded within a bone of a patient. 
         FIG.  4    illustrates a second embodiment apparatus for attaching a reference marker to a patient. 
         FIG.  5    illustrates an embodiment system for performing image guided surgery that includes a plurality of reference markers fixed to a patient. 
         FIG.  6    illustrates another embodiment system for performing image guided surgery that includes a plurality of reference markers fixed to a patient 
         FIG.  7    illustrates yet another embodiment system for performing image guided surgery that includes a first reference marker fixed to a bone of a patient and a plurality of optical markers fixed over the skin surface of the patient. 
     
    
    
     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. 
       FIG.  1    illustrates a system  100  for performing computer-assisted image-guided surgery according to various embodiments. The system  100  in this embodiment includes an imaging device  103 , a motion tracking system  105  and a robotic arm  101  for performing a robotically-assisted surgical procedure. 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 robotic arm  101  may be fixed to a support structure at one end and may have an end effector  102  at the other end of the robotic arm  101 . 
     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. 
     An example imaging device  103  that may be used in various embodiments is the AIRO® intra-operative CT system manufactured by Mobius Imaging, LLC and distributed by Brainlab, AG. Other imaging devices may also be utilized. For example, the imaging device  103  may be a mobile CT device that is not attached to the patient support  60  and may be wheeled or otherwise moved over the patient and the support  60  to perform a scan. Examples of mobile CT devices include the BodyTom® CT scanner from Samsung Electronics Co., Ltd. and the O-Arm® surgical imaging system form Medtronic, plc. The imaging device  103  may also be a C-arm x-ray fluoroscopy device. In other embodiments, the imaging device  103  may be a fixed-bore imaging device, and the patient may be moved into the bore of the device, either on a surgical support  60  as shown in  FIG.  1   , or on a separate patient table that is configured to slide in and out of the bore. 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  206  (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, as described further below. The patient 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 . The patient reference marker device may be used to define the common, patient-based coordinate system during the procedure. 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  200 . 
     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. Each moiré pattern marker may also include a unique identifier or code that may enable different objects within the camera&#39;s field of view to be uniquely identified and tracked. 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  206  mounted to an arm  208  extending above the patient surgical area. The arm  208  may be mounted to or above the imaging device  103 . The arm  208  may enable the sensor device  111  to pivot with respect to the arm  208  and/or the imaging device  103  (e.g., via one or more ball joints  212 ). The arm  208  may enable a user to adjust the position and/or orientation of the sensor device  111  to provide the cameras  206  with a clear view into the surgical field while avoiding obstructions. The arm  208  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  120  as illustrated in  FIG.  1   . The display device  120  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  120  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  120  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  120 , and may be shown overlaying the image data. 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  120  may be a handheld display device, such as a tablet computer device. One or more handheld display devices  120  may be mounted to an arm  208  extending above the patient surgical area, as shown in  FIG.  1   . The arm  208  may also support the optical sensing device  111  for the motion tracking system  105 , as described above. The one or more display devices  120  may be suspended from the arm  208 , and the position of a display device  120  may be adjustable along the length of the arm  208 . In other embodiments, a handheld display device  120  may be mounted to the patient support  60  or column  50  or to any portion of the imaging system  103 , 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  120  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. 
     As shown in  FIG.  1   , the robotic arm  101  may be fixed to the imaging device  103 , such as on a support element  214  (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  208  to which the optical sensing device  111  is mounted may be mounted to the same or a similar support element  214  (e.g., curved rail) as the robotic arm  101 . The position of the robotic arm  101  and/or the arm  208  may be adjustable along the length of the support element  214 . 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. In further embodiments, the robotic arm  101  and/or the optical sensing device  111  may be mounted to a separate mobile shuttle, as described in U.S. Provisional Application No. 62/395,443, filed on Sep. 16, 2016, which is 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. In addition, various embodiments of a computer-assisted surgical method or system may include image-guided or navigation-supported surgery without the use of a robotic arm  101 . 
     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. The end effector  102  may include a hollow tube or cannula which may be used to guide an instrument  104  into the patient&#39;s body along the pre-defined trajectory and/or to the pre-defined target location. Alternately, the end effector  102  itself may be or may include an instrument that may be inserted into the patient&#39;s body and guided along the pre-defined trajectory and/or to the pre-defined target location. 
     Various embodiments include methods and systems for attaching a reference marker to a patient in a computer-assisted image-guided surgery system. As discussed above, a reference marker device (e.g., reference arc) 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 . During an image guided surgical procedure, the diagnostic imaging data of the relevant anatomy may be registered to a patient coordinate system based on the position and orientation of the reference marker device  115 , which may be continually tracked by the motion tracking system  105 . The registration may become inaccurate, however, if a marker device is not rigidly fixed to the patient and/or is accidentally bumped causing it to change its position relative to the patient during a surgical procedure. In some situations, the surgeon may not be aware that the marker device has moved relative to the patient and that the image registration is no longer accurate. 
     A first embodiment of an apparatus  200  for attaching a reference marker to a patient is shown in  FIGS.  2 A- 2 D . The apparatus  200  includes an outer cannula  201  with a hollow interior  203 . An elongated member  205  having a sharp tip end  207  may be insertable within the outer cannula  201 .  FIG.  2 A  illustrates the elongated member  205  outside of the outer cannula  201  and  FIG.  2 B  illustrates the elongated member  205  inserted within the outer cannula  201 . When the elongated member  205  is inserted in the outer cannula  201  as shown in  FIG.  2 B , the sharp tip end  207  may project beyond a first end  209  (e.g., tip end) of the cannula  201 . The sharp tip end  207  may be a cortical perforator that is configured to break through the cortical surface of a bone to enable the apparatus  200  to be advanced into the bone. The elongated member  205  may be attached to the outer cannula  201  when it is inserted, such as via a threaded connector at the second ends  211 ,  213  of the cannula  201  and the elongated member  205 . As shown in  FIGS.  2 A- 2 B , the elongated member  205  may optionally include a handle  215  (e.g., a t-handle) at the second end  213  of the elongated member  205 . The apparatus as shown in  FIG.  2 B  may be inserted into a patient such that the tip end of the apparatus passes through the patient&#39;s skin and punctures the outer cortical surface of a bone. After puncturing the cortical surface, the elongated member  205  may optionally be disengaged from the outer cannula  201  (e.g., via the threaded connector) and may be removed from the outer cannula  201 . The outer cannula  201  may be advanced further into the interior portion of the bone. The outer cannula  201  may include a flange  217  or other feature (e.g., a t-handle) to enable the user to grip and push the outer cannula  201  into the bone. 
     In some embodiments, the apparatus  200  may be a biopsy needle, including a bone marrow biopsy needle (e.g., a Jamshidi™ needle). The apparatus  200  may optionally be used to harvest tissue from the patient, and in particular may be used to harvest bone marrow from the patient. The obtained bone marrow cells may be used to promote bone growth in a spinal fusion or other surgical treatment of the patient. In one example, the outer cannula  201  may be pushed into the interior portion of the bone containing the marrow while optionally rotating the outer cannula  201  in order to collect the marrow within the hollow interior  203  of the cannula  201 . The sample (e.g., core) containing the marrow may then be removed from the outer cannula  201 , such as by aspiration or by inserting a separate instrument (e.g., a marrow acquisition cradle) into the cannula  201  to collect the sample. 
     In various embodiments, the apparatus  200  shown in  FIGS.  2 A- 2 B  may be inserted into the iliac crest of the patient. This is schematically illustrated in  FIGS.  3 A- 3 B , which are anterior and lateral views showing an outer cannula  201  of an embodiment apparatus  200  embedded within the iliac crest  301  of a patient. It will be understood that the apparatus  200  may be inserted into another bone or skeletal structure of a patient. In general, the apparatus  200  may be inserted into a structure that is relatively proximate (e.g., less than about 1 meter) from a site requiring surgical intervention. 
     The apparatus  200  also includes a marker device  219  that may be fixed to the outer cannula  201 . In one embodiment, as shown in  FIG.  2 C , the marker device  219  may include a plurality of optical markers  221  (e.g., reflective spheres) arrayed on a rigid-frame  223 . The optical markers  221  may be arranged in a unique pattern on the frame  223  to enable the array of markers  221  to be identified and tracked by a motion tracking system  105  as described above with reference to  FIG.  1   . The frame  223  may be attached to an elongated member  225  that may be sized and shaped to be inserted into the outer cannula  201 .  FIG.  2 D  illustrates the marker device  219  with the elongated member  225  inserted into the outer cannula  201 . The marker device  219  may be locked to the cannula  201  using a suitable attachment mechanism, such as a threaded connector, a latch mechanism, a locking collar, a snap-fit connector, an interference-fit, etc.  FIG.  3 C  illustrates the marker device  219  being inserted into an outer cannula  201  that is partially embedded within the bone  301  of a patient. 
     The apparatus may optionally also include an anchoring device  227  that prevents the apparatus  200  from moving relative to patient, including rotational movement of the apparatus  200  about the central axis of the outer cannula  201 . The anchoring device  227  may include a plurality of elements that may extend from the outer cannula  201  to fix the apparatus  200  within the surrounding bone structure. In embodiments, the anchoring device  227  may be selectively deployed after the outer cannula  201  is at a desired position within the bone of the patient. For example, the anchoring device  227  may be deployed by extending a plurality of anchoring elements radially outward with respect to the outer surface of the cannula  201  and/or longitudinally away from the tip end  209  of the cannula  201  in order to anchor the apparatus  200  within the bone and inhibit relative movement of the apparatus  200  and bone. 
     In one embodiment shown in  FIG.  2 D , the anchoring device  227  comprises a plurality of wires  229  that extend from the tip end  209  of the outer cannula  201 . The wires  229  may extend into the relatively softer (i.e., cancellous) bone tissue in the interior of the patient&#39;s bone to anchor the apparatus  200  within the bone. Although the embodiment shown in  FIG.  2 D  illustrates two wires  229  extending from the outer cannula  201 , it will be understood that an anchoring device  227  may include a different number of wires  229 . Further, as an alternative and/or in addition to wires  229  as shown in  FIG.  2 D , the anchoring device  227  may comprise one or more of a burr, a cleat, and expandable cage or other member that extends from the cannula  201  to help anchor the apparatus  200  within the patient&#39;s bone. 
     The anchoring device  227  is preferably comprised of a bio-compatible material, such as a nickel-titanium (nitinol) alloy. In some embodiments, the anchoring device  227  may be composed of a shape memory alloy, such as nitinol, and may be manufactured to a body temperature response so that the anchoring device  227  assumes a pre-determined austenitic shape within the patient&#39;s body. The pre-determined austenitic shape may be configured to aid in the anchoring of the apparatus  200  when the apparatus  200  is inserted into the patient&#39;s bone. 
     In some embodiments, the anchoring device  227 , such as wires  229 , may be located on or within the outer cannula  201  of the apparatus  200 . For example, the anchoring device  227  (e.g., wires  229 ) may be attached to the outer surface of the cannula  201  and may extend out from the cannula  201  when the cannula  201  is inserted into a bone. In further embodiments, the anchoring device  227  (e.g., wires  229 ) may be located within the cannula, such as within the central opening  203  of the cannula  201  or within one more housings formed within the sidewall of the cannula  201 . The anchoring device  227  may be deployed by pushing the anchoring device  227  (e.g. wires  229 ) out through the tip end  209  of the cannula  201  such that the anchoring device  227  may extend into the surrounding bone tissue. 
     In some embodiments, the anchoring device  227  may be deployed using a mechanical actuator  428 , such as a button, a rotatable handle or knob, or a mechanical slide, that is coupled to the anchoring device  227  and enables a user to cause the anchoring device  227  to extend into the surrounding bone tissue. The mechanical actuator  428  may be a spring-loaded actuator that causes the anchoring device  227  (e.g., wires  229 ) to project out from the tip end  209  of the cannula  201  and into the surrounding tissue. In some embodiments, the anchoring device  227  may deploy automatically when the marker device  219  is inserted within the outer cannula  201 . For example, the marker device  219  may include feature(s) that engage with and push down on the anchoring device  227  as the marker device  219  is inserted, causing the anchoring device  217  to extend out from the tip end  209  of the outer cannula  201 . In some embodiments, inserting the marker device  219  may engage a triggering device (e.g., a spring-loaded actuator) that causes the anchoring device  217  to extend out from the outer cannula  201 . The apparatus  200  may be configured such that the insertion of a different component into the outer cannula  201 , such as the elongated member  205  of  FIGS.  2 A- 2 B , does not cause the anchoring device  227  to deploy. 
     In some embodiments, the anchoring device  227  (e.g., wires  229 ) may be located on the marker device  219 , such as attached to the elongated member  225  shown in  FIGS.  2 C and  2 D . Fully inserting the elongated member  225  into the outer cannula  201  may cause the anchoring device  227  to project out through the tip end  209  of the cannula  201  and extend into the surrounding bone tissue. 
     In embodiments, the anchoring device  227  may be retracted from the surrounding tissue before the apparatus  200  is removed from the patient. For example, the anchoring device  227  (e.g., wires  229 ) may be pulled into the outer cannula  201  using a mechanical actuator, or automatically by removing the marker device  219  from the cannula  201 . 
       FIG.  4    illustrates another embodiment apparatus  400  for attaching a reference marker to a patient. Apparatus  400  is similar to apparatus  200  and includes an elongated member  401  (e.g., a rod) having a sharp tip  407  at a first end  409  of the apparatus  400 . A handle  415  (e.g., a t-handle) may be located at a second end  413  of the apparatus  400 . A marker device  419  is attached to the elongated member  401  near the second end  413  of the apparatus. The elongated member  401  may be a hollow cannula such as described above in reference to  FIGS.  2 A- 2 D , or may be a solid or partially solid rod that does not include a central opening extending through the elongated member  401 . The marker device  419 , handle  413  and/or sharp tip  407  may be non-removably attached to the elongated member  401 . 
     The apparatus  400  may be inserted into a patient such that the sharp tip  407  pierces the skin and punctures the cortical surface of an underlying bone (e.g., the iliac crest). In embodiments, the user may continue to advance the sharp tip  407  into the interior portion of the bone to attach the apparatus  400  to the patient. Alternately, after puncturing the cortical surface, the sharp tip  407  may be retracted into the elongated member  401  (e.g., retracted into a housing located near the first end  409  of the elongated member  401 , or pulled out through a central opening in the elongated member  401  similarly to the apparatus  200  described above). The elongated member  401  may be advanced into the interior of the bone with the sharp tip  407  removed or retracted. The apparatus  400  may optionally also include an anchoring device  427  (e.g., a plurality of wires  429 , shown in phantom in  FIG.  4   ) that may be deployed to anchor the apparatus  400  within the patient&#39;s bone. The anchoring device  427  may be selectively deployed using a trigger mechanism (e.g., a button  428 ). 
     Further embodiments include systems for performing image guided surgery that include multiple reference markers attached to a patient. Attaching multiple reference marker devices to the patient may provide redundancy such that if one marker device is not rigidly secured or becomes loose, any loss in the accuracy of the surgical navigation may be compensated for by one or more additional marker devices. The multiple marker devices may also be used to verify the accuracy of the patient registration, and in some cases, may enable a registration correction to be performed without needing to re-scan the patient using an imaging device. 
       FIG.  5    illustrates an embodiment system  500  for performing image guided surgery (IGS) that includes two reference markers  501 ,  503  attached to a patient  502 . The two reference markers  501 ,  503  may each comprise an apparatus  200 ,  400  such as described above in connection with  FIGS.  2 A- 4   , and may be embedded within different bone structures of the patient  502  (e.g., the left and right iliac crests). Although two reference markers  501 ,  503  are shown in  FIG.  5   , it will be understood that a greater number (e.g., 3, 4, 5, etc.) of reference markers may be utilized. 
     During image guided surgery, representations of tracked objects may be displayed in conjunction with diagnostic images obtained using an imaging device  103  (e.g., a CT scanner) in a patient-based coordinate system that may be based on the location(s) of reference marker  501  and/or reference marker  503 . In some embodiments, the patient-based coordinate system may be a blended or interpolated reference coordinate system that may be weighted by distance from the first and second reference marker devices  501 ,  503 . Various embodiments for performing image guided surgery using multiple reference markers attached to the patient are described in U.S. Provisional Application No. 62/385,552, filed Sep. 9, 2016, the entire contents of which are incorporated herein by reference. 
     The reference markers  501 ,  503  may be tracked by the motion tracking system  105  to detect a relative motion of reference markers  501 ,  503  during a surgical procedure. A detected relative motion between the markers may indicate that a reference marker  501 ,  503  is loose and/or has accidentally been bumped causing it to change its position relative to the patient during a surgical procedure. In such a case, the images displayed by the image guided surgery system may no longer accurately represent the current patient situation. In embodiments, the user may be notified (e.g., via an audible and/or visual alert) when there is a detected relative motion (e.g., greater than a pre-determined magnitude) between reference markers  501 ,  503 . In some embodiments, the IGS software may be configured to determine which of the reference markers  501 ,  503  most likely moved relative to the patient (for example, where a first reference marker abruptly changes its position/orientation with respect to the camera position while a second marker&#39;s position/orientation with respect to the camera(s) remains constant, it is significantly more likely that the first marker has moved). In response to determining that a first reference marker has moved relative to the patient, the IGS may automatically perform surgical navigation within a coordinate system based on the location of the second marker device. 
       FIG.  6    illustrates a further embodiment system  600  for performing image guided surgery (IGS) that includes two reference markers  601 ,  603  attached to a patient  602 . The two reference markers  601 ,  603  may be of different types and/or use different mechanisms for attaching to the patient. A first reference marker  601  may comprise an apparatus  200 ,  400  such as described above in connection with  FIGS.  2 A- 4   , and may be embedded within a first bone structure of the patient  602  (e.g., the iliac crest). One or more additional reference markers  603  may be attached to a different portion of the patient&#39;s skeletal structure, such as to a vertebra of the patient. The additional reference marker  603  may include a bone clamp that attaches to the spinous process, for example. The system  600  may otherwise be similar to system  500  as described above. 
       FIG.  7    illustrates yet another embodiment system  700  for performing image guided surgery (IGS). In this embodiments, a reference marker  701  may comprise an apparatus  200 ,  400  such as described above in connection with  FIGS.  2 A- 4   , and may be embedded within a first bone structure of the patient  702  (e.g., the iliac crest). A plurality of optical markers  703  (e.g., reflective spheres) may be attached over the skin surface of the patient  702 . The optical markers  703  may be attached using an adhesive. The plurality of optical markers  703  may at least partially surround the surgical area. 
     Reference marker  701  may be rigidly fixed to the patient&#39;s bone, while the additional optical markers  703  in this embodiment are attached to soft tissue of the patient and may have a limited degree of motion (e.g., both absolute motion relative to the anatomic region of interest and relative motion with respect to the other markers  703 ). Image registration and surgical navigation may utilize a patient-based coordinate system that may be based on the tracked position and orientation of the rigidly-attached reference marker  701 . The additional optical markers  703  may also be tracked and may be used to verify that reference marker  701  has not moved with respect to the patient  702 . For example, an average of the tracked positions of the additional optical markers  703  may be compared to the tracked position of reference marker  701 . Where the reference marker  703  is determined to have moved in a particular direction and/or by a particular magnitude with respect to the average position of markers  703 , this may indicate that the reference marker  701  has moved relative to the patient  702 . The user may be notified (e.g., via an audible and/or visual alert) when it is determined that the reference marker  701  has moved. 
     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 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.