Abstract:
A universal tracking apparatus for a surgical instrument, the tracking apparatus including a support arm and tracking array. The support arm allows position adjustment of the tracking array between a number of predefined orientations relative to the instrument which are recognizeable and registerable in the navigation system. The support arm may be coupled with multiple types of surgical instruments and used to accurately define the position of the instrument in an anatomical model generated by the navigation system. The adjustable coupling between the tracking array and the support arm allows the surgeon to determine which orientation is best suited for the surgical application and also allows the surgeon to adjust the position of the tracking array during a surgical procedure without the need to re-register the tracking apparatus. The tracking array may also include at least one repositionable reference element to allow a single tracking array to be configured for use with a plurality of different instruments, wherein differing geometries defined by the tracking array and corresponding to particular instruments are recognizable and registerable by the navigation system.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is related to and claims the benefit under Title 35, U.S.C. §119(e) of U.S. Provisional Patent Application Ser. No. 60/550,460, entitled UNIVERSAL SUPPORT ARM AND TRACKING ARRAY, filed on Mar. 5, 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to computer-assisted navigation for use in surgical procedures and, more specifically, to a tracking apparatus including a support arm which may be detachably coupled to a surgical instrument, the support arm including a tracking array for use in accurately locating the position of the surgical instrument during a surgical procedure.  
         [0004]     2. Description of the Related Art  
         [0005]     Various methods and guide instruments have been developed to facilitate the proper positioning of a surgical instrument during a surgical procedure. Such instruments and methods include the use of mechanical surgical guides which can be positioned in relation to one or more anatomical structures to function as mechanical guides for cutting, reaming, and drilling instruments, for example. For example, these types of mechanical guides may be used to locate and align a surgical instrument with respect to a bone when preparing the bone for receiving an implant, such as a component of an artificial joint.  
         [0006]     Computer-assisted navigation systems are well known, and can also be used to facilitate proper instrument placement during a surgical procedure. Computer-assisted navigation techniques often involve acquiring preoperative images of the relevant anatomical structures of a patient, and generating an anatomical coordinate system database that represents a three-dimensional model of the anatomical structures. The relevant surgical instruments typically have a known and fixed geometry that is also defined in the database preoperatively.  
         [0007]     During the surgical procedure, the position of the instrument being used, as well as the location of one or more relevant anatomical landmarks, are registered with the anatomical coordinate system. Generally, registration is the process of relating pre-procedural or intra-procedural scans of the relevant anatomy and/or data corresponding to an instrument, to the surgical or medical position of the corresponding anatomy and/or instrument. A graphical display showing the relative position of the instrument with respect to the relevant anatomical structures may then be computed in real time and displayed for the surgeon to assist in properly positioning and manipulating the surgical instrument with respect to the relevant anatomical structures. In such image-guided procedures, a robotic arm may be used to position and control the instrument, or alternatively, the surgeon may manually position the instrument using the display of the relative position of the instrument with respect to the anatomical structures to facilitate proper positioning of the instrument.  
         [0008]     Surgical instruments have typically been adapted for use with computer-assisted navigation systems by clamping a fixed tracking array that is registrable in the navigation system onto the surgical instrument at an arbitrary location and orientation relative to the instrument. Because the location and orientation is arbitrary, the geometric relationship between the tracking array and the surgical instrument must then be calibrated in order to register the combination of the tracking array and the surgical instrument in the navigation system and to properly graphically display the relative position and orientation of the instrument with respect to the anatomical structures. Thus, each time the tracking array is coupled to a surgical instrument, the combination must again be carefully calibrated and registered to ensure that the graphical display viewed by the surgeon on the computer-assisted navigation system accurately reflects the true position of the instrument relative to the anatomical structures.  
         [0009]     The variability associated with arbitrarily clamping a tracking array to a surgical instrument, as well as the possibility of a clamped tracking array slipping relative to the surgical instrument, causes uncertainty in the geometry of the combination, thus requiring careful and possibly repeated instrument calibrations. Surgical instruments including a tracking array permanently fixed to the instrument eliminate this uncertainty. However, a surgical instrument so equipped may not be suitable for procedures which require the array to be in a different position relative to the instrument, or may not be suitable for use without the computer-assisted navigation system because of the permanently-affixed tracking array.  
         [0010]     After use of a surgical instrument having a tracking array clamped thereto has begun, it may become necessary to reposition the tracking array relative to the surgical instrument. For example, the position of the tracking array may present a barrier to accessing the anatomical structures visually or with other instruments, or the position of the tracking array may need to be adjusted to place the tracking array properly in the “line of sight” of, or otherwise within the applicable field of detection of, the computer-assisted navigation system. Repositioning the tracking array may require re-clamping the tracking array to the instrument, followed by recalibration and re-registration of the assembly with the computer-assisted navigation system.  
         [0011]     Also, when multiple surgical instruments are independently positioned relative to the anatomical structures, different tracking arrays each having a different pattern of reference elements may be clamped to the instruments, thus enabling the computer-assisted navigation system to distinguish between the different tracking arrays and in turn the different instruments. Disadvantageously, such a system requires a supply of many different tracking arrays for each surgical procedure.  
         [0012]     What is needed is a tracking apparatus for use in a computer-assisted navigation system which is an improvement over the foregoing.  
       SUMMARY OF THE INVENTION  
       [0013]     The present invention provides a universal tracking apparatus for a surgical instrument, the tracking apparatus including a support arm and tracking array. The support arm allows position adjustment of the tracking array between a number of predefined orientations relative to the instrument which are recognizeable and registerable in the navigation system. The support arm may be coupled with multiple types of surgical instruments and used to accurately define the position of the instrument in an anatomical model generated by the navigation system. The adjustable coupling between the tracking array and the support arm allows the surgeon to determine which orientation is best suited for the surgical application and also allows the surgeon to adjust the position of the tracking array during a surgical procedure without the need to re-register the tracking apparatus. The tracking array may also include at least one repositionable reference element to allow a single tracking array to be configured for use with a plurality of different instruments, wherein differing geometries defined by the tracking array and corresponding to particular instruments are recognizable and registerable by the navigation system.  
         [0014]     In one exemplary embodiment, the tracking apparatus of the present invention includes a support arm having first, second, and third members. The first member includes a mounting interface for releasably coupling the first member to a surgical instrument in a predefined geometric relationship with respect to the support arm. The second member is moveable between a plurality of predefined positions relative to the first member to permit adjustment of the support arm. For example, the second member may be rotationally coupled with the first member about a first axis, wherein the second member is rotatable relative to the first member about the first axis between a plurality of predefined positions. Also, the third member is moveable between a plurality of predefined positions relative to the second member to permit further adjustment of the support arm. For example, the third member may be rotationally coupled with the second member about a second axis which is substantially perpendicular to the first axis for rotational adjustment about the second axis between a plurality of predefined positions.  
         [0015]     The third member includes a mounting interface for coupling a tracking array which may be registered in the computer-assisted navigation system. By repositioning the second and third members to selected predefined positions, the position of the tracking array relative to the instrument may be adjusted. Also, at least one of the first, second, and third members may include a reference indicator registerable in the navigation system, such that after the tracking apparatus is adjusted, the orientation of the tracking apparatus may be automatically recognized by the navigation system, eliminating the need for the surgeon to manually re-register the tracking apparatus in the navigation system.  
         [0016]     In one exemplary embodiment, the first and/or third members may be coupled to the second member so that they may be rotated to a limited number of predefined positions relative to the second member, thereby simplifying the task of indicating to the computer-assisted navigation system the relative geometry between the instrument and the tracking array. For example, a set of matching protuberances and recesses may be located on adjacent members and the members biased by a spring to engage the protuberances in the recesses. To reposition the first or third member relative to the second member, the members may be pulled slightly apart against the force of a spring to disengage the protuberances from the recesses and then rotated to another predefined position in which the protuberances may again engage matching recesses. The selected predefined position may then be input manually to the computer-assisted navigation system, or a reference element may be coupled to the first member in order for the computer-assisted navigation system to determine the geometry between the tracking array and first member, thereby defining the geometry between the tracking array and instrument.  
         [0017]     In another exemplary embodiment, the tracking apparatus includes a tracking array usable for tracking an instrument in a computer-assisted navigation system. The tracking array includes a body member, at least three reference elements coupled with the body member in a nonlinear pattern, and a mounting interface coupled with the body member for coupling the body member with an instrument. At least one of the reference elements is adjustable in its position relative to the body member. For example, the reference elements may be coplanar, with one reference element movable within the plane relative to the remaining reference elements, thus forming a different reference element pattern. The pattern may be distinct from other patterns and/or other tracking arrays used with the computer-assisted navigation system in order to uniquely identify the tracking array, and the instrument to which it is coupled, from the other tracking arrays and instruments.  
         [0018]     The reference elements of the exemplary tracking array may be passive elements such as reflective spheres, for example, the positions of which are detectable by a position sensor of the navigation system. Alternatively, the reference elements may be active elements which emit a signal detectable by a position sensor of the navigation system.  
         [0019]     The adjustable or repositionable reference element may be repositioned by providing, for example, multiple mounting interfaces to which the reference element may be coupled, or by providing a translating, pivoting, or otherwise movable mounting interface which is coupled to the body of the tracking array and upon which the reference element is mounted. The mechanism for moving the mounting interface relative to the body may include detents or other mechanical devices to provide predetermined positions. Optionally, a template may be used for accurately repositioning the movable reference element relative to the other reference elements. The template may include recesses corresponding to the fixed reference elements and the multiple positions of the movable reference element.  
         [0020]     Advantageously, the support arm or tracking array may be repositioned to provide an optimal view of the tracking array by the position sensor, or to provide unobstructed access for the surgeon to the anatomical structures of the patient without having to uncouple the support arm from the instrument or having to re-register the tracking apparatus.  
         [0021]     Further, once an instrument is positioned and secured relative to the anatomical structures of the patient, the support arm and tracking array may be detached from a surgical instrument, providing increased access to the instrument and anatomical structure. Additionally, the flexibility of adjusting the pivotable couplings of the support arm and the movable reference element of the tracking array reduces the number of different components that need be available to perform a surgical procedure.  
         [0022]     In one form thereof, the present invention provides a tracking apparatus for use with a surgical instrument in a computer-assisted surgical navigation system, including a support arm, including a first member having a surgical instrument mounting interface, and a second member adjustably coupled to the first member; and a tracking array adjustably coupled to the second member, the tracking array including at least one reference element which is registerable in the navigation system.  
         [0023]     In another form thereof, the present invention provides a tracking apparatus for use with a surgical instrument in a computer-assisted surgical navigation system, including a support arm including a surgical instrument mounting interface; and a tracking array adjustably coupled to the support arm, the tracking array including a plurality of reference elements which are registerable in the navigation system, at least one of the reference elements adjustably coupled to the tracking array whereby the relative position of the at least one reference element with respect to others of the reference elements may be varied.  
         [0024]     In a further form thereof, the present invention provides a tracking array for use with a surgical instrument in a computer-assisted surgical navigation system, including a body member; and a plurality of reference elements coupled to the body member, the reference elements registerable in the navigation system, at least one reference element adjustably coupled to the body member, whereby the relative position of the at least one reference element with respect to others of the reference elements may be varied.  
         [0025]     In a further form thereof, the present invention provides a method of using a tracking apparatus in a computer-assisted surgical navigation system, including the steps of providing a surgical instrument; providing a tracking apparatus including a support arm having first and second members adjustably coupled to one another, and a tracking array adjustably coupled to the second member, the tracking array registerable in the navigation system; coupling a surgical instrument to the first member of the support arm; registering the tracking apparatus in the navigation system; adjusting at least one of the position of the second member with respect to the first member, and the position of the tracking array with respect to the second member; and re-registering the tracking apparatus with the navigation system.  
         [0026]     In the foregoing method, the re-registering step may occur automatically without manual intervention by an operator. Also, the adjusting step may include at least one of rotationally adjusting the position of the second member with respect to the first member about a first axis; and rotationally adjusting the position of the tracking array with respect to the second member about a second axis different from the first axis. Further, the tracking array may include a plurality of reference elements registerable in the navigation system, the method further including the additional step, prior to the registering step, of adjusting the position of at least one reference element of the tracking array with respect to at least one other reference element of the tracking array. Still further, the coupling step may include threading a threaded member associated with one of the support arm and the instrument into a threaded bore of the other of the support arm and the instrument; and inserting a locating member associated with one of the support arm and the instrument into a locating recess in the other of the support arm and the instrument.  
         [0027]     In a still further form thereof, the present invention provides a method of using a tracking apparatus in a computer-assisted surgical navigation system, including the steps of providing a surgical instrument; providing a tracking apparatus including a plurality of reference elements registerable in the navigation system; coupling the surgical instrument to the tracking apparatus; adjusting the position of at least one of the reference elements with respect to others of the reference elements; and registering the tracking apparatus with the navigation system.  
         [0028]     In the foregoing method, the adjusting step may further include adjusting the position of at least one of the reference elements with respect to others of the reference elements to define a predetermined reference element geometry corresponding to the surgical instrument. Also, the adjusting step may further include positioning the tracking apparatus with respect to a template; and securing the at least one reference element in a position defined by the template. Further, the coupling step may include threading a threaded member associated with one of the support arm and the instrument into a threaded bore of the other of the support arm and the instrument; and inserting a locating member associated with one of the support arm and the instrument into a locating recess in the other of the support arm and the instrument. Still further, the method may include the additional steps of decoupling the surgical instrument from the tracking apparatus; coupling a different surgical instrument to the tracking apparatus; adjusting the position of at least one of the reference elements with respect to others of the reference elements; and re-registering the tracking apparatus with the navigation system.  
         [0029]     In a still further form thereof, the present invention provides a tracking apparatus for use with a surgical instrument in a computer-assisted surgical navigation system, including a support arm including a surgical instrument mounting interface; a tracking array adjustably coupled to the support arm, the tracking array including at least one reference element which is registerable in the navigation system; and the support arm and the tracking array further including cooperating reference indicators registerable in the navigation system for determining a relative orientation between the support arm and the tracking array. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]     The above mentioned and other features and objects of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:  
         [0031]      FIG. 1  is a perspective assembly view of a tracking apparatus, including a support arm coupling a surgical instrument with a tracking array in accordance with the present invention;  
         [0032]      FIG. 2  is an exploded perspective view of the components of the tracking apparatus of  FIG. 1 ;  
         [0033]      FIG. 3  is a perspective view of an operating room arrangement having a computer-assisted navigation system utilizing the tracking apparatus of  FIG. 1 ;  
         [0034]      FIG. 4  is a side view of a registration probe for use with the support arm of the tracking apparatus of  FIG. 1 ;  
         [0035]      FIG. 5A  is a top perspective view of a second exemplary tracking array in accordance with the present invention;  
         [0036]      FIG. 5B  is an end view of the tracking array of  FIG. 5A ;  
         [0037]      FIG. 6  is a top perspective view of a third exemplary tracking array in accordance with the present invention;  
         [0038]      FIG. 7A  is a plan view of a first exemplary template for use with a tracking array of the present invention;  
         [0039]      FIG. 7B  is a top view of a tracking array of the present invention coupled with the first exemplary template of  FIG. 7A ;  
         [0040]      FIG. 8  is a plan view of second exemplary template which may be used with a tracking array of the present invention;  
         [0041]      FIG. 9  is a schematic representation of exemplary method steps for using the exemplary tracking apparatus of  FIGS. 1 and 2 ; and  
         [0042]      FIG. 10  is a schematic representation of exemplary method steps for using the second and third exemplary tracking assemblies of  FIGS. 5 and 6 . 
     
    
       [0043]     Corresponding reference characters indicate corresponding parts throughout the several views. Although the drawings represent embodiments of the present invention, the drawings are not necessarily to scale and certain features may be exaggerated to better illustrate and explain the present invention. The exemplifications set out herein illustrate embodiments of the invention, in several forms, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.  
       DETAILED DESCRIPTION  
       [0044]     Referring initially to  FIG. 1 , an exemplary tracking apparatus  20  according to the present invention includes support arm  22  for coupling tracking array  24  to surgical instrument  26 . Support arm  22  displaces tracking array  24  from instrument  26 , for example for placing array  24  within the field of view or detection of position sensing unit  28  of computer-assisted navigation system  30 , shown in  FIG. 3 . Additionally, support arm  22  allows positional adjustment of tracking array  24  relative to instrument  26 , thereby providing adjustment of the geometry between instrument  26  and array  24 . Instrument  26  may be any instrument used with navigation system  30 , for example a cut guide for orthopedic implant surgery, or a saw, reamer, drill, or other surgical instrument.  
         [0045]     Referring to  FIG. 3 , operating room arrangement  32  includes computer-assisted navigation system  30 , position sensing unit  28  for detecting the position of tracking array  24  of tracking apparatus  20 , and patient  34 , who is positioned on operating table  36 . Navigation system  30  assists a surgeon in positioning instrument  26  relative to anatomical structures of patient  34 , for example anatomical structures of knee  38  when performing knee arthroplasty procedures.  
         [0046]     Navigation system  30  is well known in the art, and generally includes a monitor for displaying an image of one or more body elements, such as portions of the knee  38  of patient  34 . The image is generated from an image data set stored within a computer, and the image data set is typically obtained from preoperative computed tomography (CT) or magnetic resonance image (MRI) scan. The image data set includes reference points for at least one body element which have a fixed spatial relation relative to the body element. These reference points may be sensors attached to the body, or sensors on an auxiliary frame which is fixed with respect to the body. The position sensing unit  28  may be a sensor array or digitizer for identifying, during the surgical procedure, the position of the reference points, and the computer modifies the image data set during the procedure according to the positions of each of the reference points. These and other types of surgical navigation systems are well known in the art and will not be further described herein.  
         [0047]     Referring to  FIGS. 1 and 2 , support arm  22  includes first member or portion  40 , second member or portion  42 , and third member or portion  44 . First member  40  and third member  44  are pivotably coupled to second member  42 . In one alternative embodiment, first member  40  and third member  44  are coupled to second member  42  in a manner allowing translation of first member  40  relative to second member  42  as well as translation of third member  44  relative to second member  42 . Support arm  22  may include fewer or more than three members or portions that form a variable geometry in one or more degrees of freedom. In support arm  22 , first member  40  may be pivotably coupled to second member  42  such that second member  42  is pivotably adjustable relative to first member  40 . Specifically, second member  42  rotates about first axis  46 , which, in one exemplary embodiment, is defined by the longitudinal axis of second member  42 . Similarly, third member  44  may be pivotably coupled with second member  42  so that third member  44  is rotatable about second axis  48 , which is defined by the longitudinal axis of third member  44 . In the exemplary embodiment, second axis  48  is substantially perpendicular to first axis  46 .  
         [0048]     Referring to  FIG. 2 , first member  40  includes cylindrical portion  47  and frame  49  having opening  51 . Distal to cylindrical portion  47 , frame  49  includes a surgical instrument mounting interface  50  for coupling surgical instrument  26 , for example a cut guide, to support arm  22 . First mounting interface  50  provides a universal mount for multiple types of instruments. For example, exemplary first mounting interface  50  includes two noncoaxial bosses  52  and  54  which are engageable in mating bores or recesses  58  and  60  of instrument  26 , and a threaded screw  56  is inserted through boss  52  of interface  50  and is threaded into recess  58  of instrument  26  to secure instrument  26  to first member  40 . Advantageously, first mounting interface  50  and mating recesses  58  and  60  are formed so that support arm  22  may be repeatably precisely coupled to instrument  26  and/or other instruments in a predefined geometry with respect to support arm  22  and tracking array  24 .  
         [0049]     A second mounting interface  62  is located on third member  44  for coupling tracking array  24  to support arm  22 . Second mounting interface  62  may be a universal mount for repeatably precisely coupling tracking array  24  to support arm  22  in a predefined geometry. In the exemplary embodiment, second mounting interface  62  is a dovetail projection which is receivable by dovetail receptacle  64  of body  66  of tracking array  24  so that tracking array  24  may be repeatably precisely coupled to support arm  22  in a predefined geometry with respect to support arm  22  and instrument  26 .  
         [0050]     Tracking array  24  includes body  66  having arms or projections  68  projecting therefrom. The end of each projection  68  distal from body  66  includes a mounting interface, for example, posts  70  ( FIG. 2 ) for coupling reference elements  72  to tracking array  24 . Reference elements  72  are detectable by position sensing unit  28  ( FIG. 3 ) of computer-assisted navigation system  30 . Many types of reference elements usable in computer-assisted surgical navigation systems are known. For example, the reference elements may be “active” reference elements which emit a signal to position sensing unit  28  for detection, or may be “passive” reference elements which reflect signals emitted from position sensing unit  28  for detection. In particular, active reference elements include light emitting diodes (“LED&#39;s”), or may generate acoustic, magnetic, electromagnetic, or radiologic signals. In the exemplary embodiment, reference elements  72  are passive, in the form of spherical reflectors arranged in a nonlinear planar pattern. The exemplary embodiment includes four reference elements  72 ; however, greater or fewer reference elements may be utilized.  
         [0051]     In order to minimize or eliminate the need for repeated calibration after adjustment of tracking apparatus  20 , first member  40  and third member  44  are movably coupled such that they may be repositioned relative to second member  42  in predetermined positions defining the relative location and relative orientation between the components. Specifically, in the exemplary embodiment, the pivotable joints about axes  46  and  48  provided between first member  40  and second member  42 , and between third member  44  and second member  42 , are biased together by springs  74  and  76 , respectively, so that projections  78  of second member  42  engage with recesses  80  of first member  40 , and projections  82  of third member  44  engage with recesses  84  of second member  42  to place tracking apparatus  20  in one of a number of predetermined positions.  
         [0052]     In one exemplary embodiment, second member  42  is substantially cylindrical, and includes four equilaterally spaced V-shaped projections  78  projecting longitudinally from the circumference of lower portion  86  of second member  42 . First member  40  includes substantially cylindrical portion  47  having recesses  80  defined adjacent the circumference of top surface  88  of first member  40 . Recesses  80  correspond to the spacing and shape of projections  78 . Because projections  78  and recesses  80  include four equilaterally spaced pairs, the exemplary embodiment includes four possible predetermined rotational positions in which first member  40  may be engaged with, and located with respect to, second member  42 .  
         [0053]     The rotational coupling of first member  40  and second member  42  is supported by cylindrical sleeves  90  and  92  which engage one inside the other from oppositely faced ends of bore  98  of first member  40  and bore  100  of second member  42 . Bores  98  and  100  are located coaxially with first axis  46 , which extends longitudinally and centrally through first member  40  and second member  42 .  
         [0054]     Heads  96  and  94  of sleeves  90  and  92  restrict relative translation of members  40  and  42 . Bores  98  and  100  may be countersunk to accommodate heads  96  and  94 . Spring  74  is located within the hollow central lengths of cylinders  90  and  94  and is welded or otherwise secured at its opposite ends to heads  94  and  96 , thus providing a biasing force to compress first member  40  against second member  42 , maintaining the engagement of projections  78  within recesses  80 .  
         [0055]     In order to rotationally reposition second member  42  relative to first member  40 , second member  42  may be manually translated away from first member  40  along first axis  46  sufficient to disengage projections  78  from recesses  80 . Upon disengagement of projections  78  from recesses  80 , second member  42  may be rotated about axis  46  relative to first member  40  until the desired predetermined position between members  42  and  40  is achieved. Upon release of second member  42 , the bias of spring  74  re-engages projections  78  within recesses  80 .  
         [0056]     In exemplary support arm  22 , third member  44 , which may be substantially rectangular, is coupled to second member  42  in a fashion similar to that of members  40  and  42 . However, along with the coupling between first and second members  40  and  42 , other forms of coupling and mechanisms for adjustment may also be utilized. Third member  44  includes V-shaped projections  82  on an end thereof opposite second mounting interface  62  to which tracking array  24  is attached. Projections  82  are engagable within complementary-shaped recesses  84  defined within substantially flat wall  101  of second member  42 . Specifically, four recesses  84  provide four predefined positions of engagement for second and third members  42  and  44  in 90° increments about second axis  48 .  
         [0057]     Projections  82  of third member  44  may be biased into engagement with recesses  84  of second member  42  by spring  76 . Spring  76  is located within cylinder  102 . Cylinder  102  includes threads  104  and  106 , located at opposite ends thereof. Thread  104  is fastened in threaded receptacle  108  located in second member  42  and centered on axis  48 . Thread  106  of cylinder  102  extends through bore  110  in third member  44 , and into opening  112  which extends from face  114  through third member  44 . Retaining nut  116  is fastened on threads  106  of cylinder  102  to retain third member  44  to second member  42 . However, cylinder  102  is of sufficient length to allow third member  44  to slide along axis  48  against the bias of spring  76  and away from second member  42 , disengaging projections  82  from recesses  84  so that third member  44  may be rotated about axis  48  relative to second member  42 . Spring  76  may be anchored to cylinder  102  adjacent threads  104  and, at an opposite end of spring  76 , to anchor  118  located in opening  112  of third member  44 . Spring  76  pulls anchor  118  toward second member  42 , thereby biasing projections  82  into engagement within recesses  84 , while allowing for disengagement and rotation of members  44  and  42  when desired.  
         [0058]     Indicators  312  and  316  located on member  42 , and labels  314  and  318  located on first member  40  and third member  44 , respectively, may be used to indicate, as described below, the predetermined positions of members  40 ,  42  and  44  relative to one another. Similarly, reference element  320  mounted on post  319  of first member  40 , and recess or indentation  328  on first member  40 , which is sized to receive engagement feature  326  of probe  322  (illustrated in  FIG. 4  and discussed in detail below) may, in conjunction with navigation system  30 , also serve the same function. Advantageously, after an initial registering or calibration of tracking apparatus  20  with navigation system  30 , the foregoing features allow the position of second member  42  with respect to first member  40 , as well as the position of third member  44  and tracking array  24  with respect to second member  42 , to be adjusted without the need to re-register or re-calibrate tracking apparatus  20  with navigation system  30 .  
         [0059]     Although support arm  22  includes rotatable couplings, alternative mechanisms for moving each member of support arm  22  relative to the other members of support arm  22  may be utilized. For example, a pivoting or translating member, a worm gear, or other known couplings and mechanisms may be utilized. Additionally, although projection and recess pairs  78 ,  80  and  82 ,  84  provide a limited number of predetermined positions between members  40  and  42  and between members  44  and  42 , other mechanisms may be utilized that provide fewer or additional predefined positions between the members.  
         [0060]     Referring to  FIGS. 5A and 5B , a second, alternative tracking array  150  includes body  152 , mounting interface  154 , and reference elements  156   a - 156   d.  Tracking array  150  may be coupled to support arm  22  in the manner described above with reference to tracking array  24  to track an instrument, such as instrument  26  shown in  FIGS. 1-3 , in computer-assisted navigation system  30  ( FIG. 3 ) relative to anatomical structures of patient  34 .  
         [0061]     Advantageously, reference elements  156   a - 156   d  may be arranged in various nonlinear patterns to enable navigation system  30  to distinguish between multiple tracking arrays  150  and in turn, to distinguish between various different instruments to which arrays  150  may be coupled. Reference elements  156   a - 156   d  may be active or passive reference elements, as described above, the positions of which are detectable by position sensing unit  28  ( FIG. 3 ). At least one of reference elements  156   a - 156   d  is adjustably movable relative to the remaining reference elements so that various distinguishable patterns of elements  156   a - 156   d  may be utilized.  
         [0062]     Second exemplary tracking array  150  includes reference elements  156   b  and  156   c  which are movable along projections  158   b  and  158   c  of body  152 , respectively. Specifically, posts  160   a - 160   d  and  162   a - 162   d  may be used to selectively position reference elements  156   b  and/or  156   c  relative to reference elements  156   a  and  156   d,  which are mounted on projections  158   a  and  158   d,  respectively. For example, reference element  156   b  is shown mounted on post  160   a;  however, reference element  156   b  may also be mounted on any one of posts  160   b,    160   c  and  160   d.  Likewise, reference element  156   c  is shown mounted on post  162   a;  however, reference element  156   c  may also be mounted on any one of posts  162   b,    162   c  and  162   d.  Reference elements  156   b  and  156   c  may be retained on posts  160   a - 160   d  and  162   a - 162   d  by conventional hardware, for example, threads, or by a mechanical detent or a press fit, for example.  
         [0063]     The number of movable reference elements  156   a - 156   d  and mounting posts  160   a - 160   d  and  162   a - 162   d  may be selected as desired and located on one or more of projections  158   a - 158   d.  Tracking array  150  includes four possible mounting positions for each of reference elements  156   b  and  156   c,  thereby providing sixteen unique patterns of reference elements  156   a - 156   d  that may be distinguished by computer-assisted navigation system  30 . The pattern of reference elements  156   a - 156   d  identify and aid tracking of each configuration of array  150  and instrument to which the array is coupled, independently of any other configurations of array  150  and other associated instruments. Computer-assisted navigation system  30  may be programmed to automatically recognize the instrument associated with a particular reference element pattern, or may be manually instructed to do so by the surgeon or other operator prior to or during a surgical procedure.  
         [0064]     Referring to  FIG. 6 , another alternative tracking array  170  includes body  172 , mounting interface  174  ( FIG. 7B ) and reference elements  176   a - 176   d.  At least one of reference elements  176   a - 176   d  of tracking array  170  is movable relative to the remaining reference elements. For example, third exemplary tracking array  170  may include one or both of slidably coupled adjustment projection  178   b  and pivotably coupled adjustment projection  178   c,  which include movable reference elements  176   b  and  176   c,  respectively.  
         [0065]     Adjustment projection  178   b  may slidably engage sleeve  180  defined in body  172 . Projection  178   b  is thereby capable of extending outwardly relative to body  172 , for example to predefined positions  182   a - 182   d  shown in  FIG. 6 . By repositioning reference element  176   b  relative to the remaining reference elements, computer-assisted navigation system  30  ( FIG. 3 ) is enabled to distinguish between various configurations of tracking array  170  and the instrument to which each tracking array is coupled. Similarly, adjustment projection  178   c  may be pivotably coupled to body  172  such that reference element  176   c  may be moved, for example between the predefined positions  184   a - 184   e  shown in  FIG. 6 .  
         [0066]     Referring to  FIGS. 7A and 8 , templates  190  and  192  may be utilized to accurately reposition and calibrate the position of movable reference elements  176   b  or  176   c.  For example, template  190  ( FIG. 7A ) is designed for a tracking array having a slidably coupled adjustment projection  178   b,  and template  192  ( FIG. 8 ) is designed for a tracking array having a pivotably coupled adjustment projection  178   c.  Other templates accommodating both a slidably coupled projection and a pivotally coupled projection are also contemplated.  
         [0067]     For example, referring to  FIG. 7B , tracking array  172 , including fixed reference element  176   c  and movable reference element  176   b,  may be placed in template  190 . Specifically, upper face  186  ( FIG. 6 ) of array  170  is faced toward template  190  and reference elements  176   a - 176   d  are positioned in their respective calibration bores  194   a - 194   d.  Template  190  includes four bores  194   b  for positioning reference element  176   b.  Bores  194   b  correspond to the four exemplary predefined positions  182   a - 182   d  of reference element  176   b.    
         [0068]     Referring to  FIG. 7B , sleeve  180  of array  170  may include device  196  for securing adjustment projection  178   b  relative to array body  172 . For example, device  196  may be a set screw, a ball and spring, or other positioning device or fastener for securing adjustment projection  178   b  relative to array  172 . Also, device  196  may engage one of a plurality of recesses  198  defined along the length of support projection  178   b.  Advantageously, recesses  198  enable device  196  to accurately position support projection  178   b  and therefore reference element  176   b  in a predefined position relative to array body  172  so that calibration of array  170  after adjustment may not be required.  
         [0069]     Referring to  FIG. 8 , template  192  may be similarly utilized to locate reference element  176   c  and adjustable projection  178   c  of tracking array  170  having pivotably adjustable reference element  176   c.  Specifically, reference elements  176   a - 176   d  may be located in the respective matching ones of bores  200   a - 200   d.  Five bores  200   c  are provided to correspond to predefined positions  184   a - 184   e  of reference element  176   c.  Adjustable projection  178   c  may be fixed in a selected position with respect to tracking array  170  in a similar manner as those described above with respect to adjustment projection  178   b.    
         [0070]     Referring to  FIG. 9 , an exemplary method  300 , which begins at step  302 , includes steps for using computer-assisted navigation system  30  to position an instrument, for example instrument  26  ( FIGS. 1 and 3 ), with respect to anatomical structures of patient  34 . In step  304 , a support arm is provided, for example, support arm  22  of  FIG. 1 , having at least one rotatable adjustment coupling, a mounting interface for instrument  26 , and a tracking array  24 ,  150  or  170  ( FIGS. 1, 5A  and  6 ).  
         [0071]     In step  306 , tracking apparatus  20  is assembled by coupling instrument  26  to support arm  22  using mounting interface  50  shown in  FIG. 2 , for example, and by coupling tracking array  24 ,  150  or  170  to support arm  22  using mounting interface  62  shown in  FIG. 2 , for example. The tracking array may be coupled to support arm  22  either before or after instrument is coupled to tracking arm  22 .  
         [0072]     In step  308 , the surgeon adjusts the rotatable coupling(s) of support arm  22  as desired. For example, first element  40  and third element  44  may be rotatably repositioned relative to second element  42  to provide a desired geometry between support arm  24 ,  150  or  170  and instrument  26  to allow easier access to particular anatomical structures during the surgical procedure, or to more accurately position the tracking array within the field of detection of position sensing unit  28 .  
         [0073]     In step  310 , indication of the relative position of the rotatable couplings and the type of instrument  26  utilized are indicated to computer-assisted navigation system  30  ( FIG. 3 ). Indicating step  310  may occur by preprogramming navigation system  30  to recognize that a particular reference element pattern of tracking array  24 ,  150  or  170  indicates a particular type of instrument is in use, for example instrument  26 . Alternatively, the surgeon or other operator may manually enter the type of instrument associated with tracking array  24 ,  150  or  170  into the computer of navigation system  30  using a keyboard, graphic pointer, touch screen, or similar input device. Similarly, the positions of third member  44  and first member  40  relative to second member  42  of support arm  22  may be indicated manually by the operator. For example, first indicator  312  ( FIG. 1 ) of second member  42  may be located adjacent one of labels  314  of first member  40 , and second indicator  316  may be located adjacent one of labels  316  of third member  44 . Thus, the appropriate labels  314  and  316  which relate to the predefined geometry of instrument  26  relative to tracking array  24 ,  150  or  170  may be manually entered in navigation system  30  by the operator.  
         [0074]     Alternatively, navigation system  30  may be preprogrammed to automatically recognize the predefined positions of support arm  22 . First member  40  may include a mounting post  319 , or another mounting interface, for coupling reference element  320  to first member  40 . Reference element  320  is detectable by position sensing unit  28  so that navigation system  30  may determine the geometric relationship between tracking array  24 ,  150  or  170  and first member  40  and thereby automatically recognize the predefined position of support arm  22 .  
         [0075]     Another alternative method of indicating the position of the rotatable couplings of support arm  20  is to utilize reference probe  322 , shown in  FIG. 4 . Reference probe  322  includes reference element  324  and engagement feature  326 . Engagement feature  326  may be positioned in recess  329  ( FIG. 1 ) of first member  40  to allow navigation system  30  to determine the position of first member  40  relevant to tracking array  24 ,  150  or  170 , and thus the relevant geometry of instrument  26  to tracking array  24 ,  150  or  170 .  
         [0076]     In step  312 , tracking apparatus  20 , including support arm  22 , tracking array  24 ,  150  or  170 , and instrument  26 , is registered in navigation system  30 . Computer implemented image guidance systems which provide for the registration of an actual anatomical structure with a three dimensional model representing that structure, together with the registration or localization of another object such as a surgical instrument within the image coordinate system to facilitate the display of the relative positions of the object and the actual anatomical structure are well known in the art, and thus will not be described in detail herein. Registration enables navigation system  30  to track and to assist in the positioning of instrument  26  relevant to anatomical structures of patient  34 .  
         [0077]     In step  320 , the position of instrument  26  with respect to the anatomical structures of patient  34  may be adjusted as required. Additionally, support arm  22  may be adjusted as required to reposition tracking array  24 ,  150  or  170  relative to instrument  26 . For example, the surgeon may require tracking array  24 ,  150  or  170  to be moved to allow for unobstructed access to an anatomical structure of patient  34 . Alternatively, tracking array,  24 ,  150  or  170  may require repositioning relative to instrument  26  in order to provide an improved orientation for detection by position sensing unit  28 .  
         [0078]     Further, after instrument  26  is positioned with respect to the anatomical structures of patient  34 , instrument  26  may be secured with other instrumentation (not shown) to a patient anatomical structure, and tracking array  24 ,  150  and  170  and support arm  22  may then be uncoupled from instrument  26 . Method  300  is complete in step  322 .  
         [0079]     Method  400 , illustrated in  FIG. 10 , includes steps for identifying an instrument, for example instrument  26  shown in  FIGS. 1 and 3 , in computer-assisted navigation system  30 . Method  400  begins in step  402 .  
         [0080]     In step  404 , a tracking device, for example tracking array  150  or  170  ( FIG. 5A and 6 ) having reference elements  156   a - 156   d  and/or  176   a - 176   d,  is provided. In step  406 , the tracking array is coupled with instrument  26 . Advantageously, tracking array  150  or  170  includes reference element  156   b  or  176   b  which is movable relative to the remaining reference elements, thereby providing a distinct reference element pattern or configuration which is distinguishable from other configurations of tracking arrays  150  or  170  by navigation system  30 .  
         [0081]     In step  406 , the operator couples instrument  26  with tracking array  150  or  170 . Tracking array  150  or  170  may be coupled directly to instrument  26 , or may include a support arm, such as support arm  22  described above. In step  408 , reference element  156   b  or  176   b  is repositioned with respect to tracking array body  152  or  172 , as discussed above.  
         [0082]     In step  410 , navigation system  30  receives an indication of the type of instrument  26  coupled to tracking array  150  or  170 . This indication may be in the form of an operator manually identifying the instrument type to navigation system  30 , or by navigation system  30  being preprogrammed to associate a particular instrument type with a particular reference element pattern, as determined by the position of movable reference element  156   b  and  176   b  relevant to the remaining reference elements.  
         [0083]     In step  412 , the assembly consisting of at least tracking array  150  or  170  and instrument  26  may be registered with a navigation system  30 . Registration enables navigation system  30  to track and to determine and guide the position of instrument  26  via the tracking of tracking array of  150  or  170  by reference device  28 .  
         [0084]     In step  414  the surgeon adjusts the position of instrument  26  as desired relative to the anatomical structures of patient  34 . Method  400  is complete in step  416 .  
         [0085]     While this invention has been described as having exemplary embodiments, the present invention may be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains.