Patent Publication Number: US-6988009-B2

Title: Implant registration device for surgical navigation system

Description:
This application is a Continuation-in-Part of Ser. No. 10/357,754 filed Feb. 4, 2003, now U.S. Pat. No. 6,925,339. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a registration device and, more specifically, to a device for registering the position of an orthopedic implant in a computer assisted surgical navigation system. 
   2. Description of the Related Art 
   The controlled positioning of surgical instruments and other objects is of significant importance in many surgical procedures and various methods have been developed for properly positioning an object during a surgical procedure. Such methods include the use of both mechanical guides and computer assisted navigational systems. Computer assisted navigational techniques typically involve acquiring preoperative images of the relevant anatomical structures and generating a data base which represents a three dimensional model of the anatomical structures. The relevant tools and other objects used in the surgical procedure typically have a known and fixed geometry which is also defined preoperatively. During the surgical procedure, the position of the object being used is registered with the anatomical coordinate system and a graphical display showing the relative positions of the object and anatomical structure may be computed in real time and displayed for the surgeon to assist the surgeon in properly positioning and manipulating the object with respect to the relevant anatomical structure. 
   In such image guided procedures, a robotic arm may be used to position and control the object, or, the surgeon may manually position the object and use the display of the relative position of the object and anatomical structure to position the object. Examples of various computer assisted navigation systems, which are known in the art, are described in U.S. Pat. Nos. 5,682,886; 5,921,992; 6,096,050; 6,348,058 B1; 6,434,507 B1; 6,450,978 B1; 6,490,467 B1; and 6,491,699 B1, wherein the disclosures of each of these patents are hereby incorporated herein by reference. 
   SUMMARY OF THE INVENTION 
   The present invention provides a registration device which is engageable with a plurality of differently sized orthopedic implants. The registration device is engageable with each of the implants in a predefined relative position. A first reference structure is disposed on the registration device and a second reference structure is detachably secured to the implant. Each of the first and second reference structures have at least one reference element registerable in a computer assisted navigation system whereby the position and/or orientation of the implant relative to the second reference structure may be determined or calibrated. After calibrating the position of the implant relative to the second reference structure, the registration device is disengaged from the implant. The second reference structure may be mounted on a handling tool which is then used to place the implant in its implanted position using the computer assisted navigational system. 
   The invention comprises, in one form thereof, a system for registering an orthopedic implant in a computer assisted navigation system. The system includes a plurality of differently sized implants and a registration device engageable with each of the plurality of implants in a first predefined relative position. The system also includes a first reference structure and a second reference structure. Each of the reference structures has at least one reference element registerable in the computer assisted navigation system. The first reference structure is disposed on the registration device at a predetermined location and the second reference structure is detachably securable to each of the plurality of implants. The relative positions of the first and second reference structures differ for each of the plurality of implants when the registration device is engaged at the first predefined location and the second reference structure is secured to a selected one of the implants. 
   The first and second reference structures may include at least three non-linearly positioned reference elements. The reference elements of the first and second reference structures may define first and second patterns which are distinguishable. The second reference structure may also be mounted on a handling tool that has an attachment feature detachably securable to each of the plurality of implants. 
   The implants may be a plurality of hip stems adapted for insertion in a proximal femur. The hip stems may have a generally L-shaped configuration defining a stem portion and a neck portion. The stem portion has a distal end and a proximal end. Each of the hip stems also includes a mounting interface located proximate the proximal end which is securable to the handling tool. The registration device includes at least one graduated space for receiving a distal end of a first one and a second one of the plurality of hip stems. The first and second hip stems engage the registration device within the at least one graduated space whereby the engagement features of the first and second hip stems are respectively positioned at first and second non-equivalent distances from the first reference structure. Each of the hip stems may also include a projection disposed on the neck portion wherein each of the projections has a common configuration. The reference member may include an engagement feature for engaging the projections at a predefined second relative position. 
   The invention comprises, in another form thereof, a system for registering an orthopedic implant in a computer assisted navigation system wherein the implant is adapted for implantation on a bone. The system includes a plurality of differently sized orthopedic implants, each of the implants having an elongate stem defining a stem axis and a registration device engageable with the stem of each of the plurality of implants at a first predefined relative position along the stem axis of each of the plurality of implants. A first reference structure having at least one reference element registerable in the computer assisted navigation system is disposed on the registration device at a predetermined location. The registration device may include at least one graduated engagement feature for engaging the stems at the first predefined relative position. In one embodiment, the graduated engagement feature consists of a plurality of tapered slots, with each of the tapered slots configured for receiving at least a portion of at least one of the implants from one direction only. In an alternate embodiment, the tapered slots are each configured for receiving at least a portion of one of the said implants from either a first direction or from a second direction, where the second direction is directly opposite of the first direction. 
   The invention comprises, in yet another form thereof, a method of registering an orthopedic implant in a computer assisted navigation system. The method includes providing a implant having a stem defining a stem axis. The stem has a distal end and a proximal end wherein the distal end has a smaller cross sectional area than the proximal end. A registration device including a first reference structure having at least one reference element registerable in the computer assisted navigation system is also provided. The registration device is engageable with the stem at a predefined axial location. The method includes attaching a handling tool to the implant. The handling tool has a second reference structure mounted thereon. The second reference structure includes at least one reference element registerable in the computer assisted navigation system. The method also includes engaging the registration device with the implant at the predefined axial location with the implant secured to the handling tool and registering the positions of the first and second structures in the computer assisted navigation system and determining the position of implant stem relative to second reference structure. 
   The method may also include the step of disengaging the registration device from the implant stem following the step of determining the position of the implant stem relative to the second reference structure. The implant may also include a projection extending at an angle to the stem axis and the method further includes the steps of engaging the reference device with the projection at a predefined relative position, registering the relative positions of the first and second reference structures in the computer assisted navigation system, and determining the rotational position of the projection relative to the stem axis. 
   In alternative embodiments of the method, a plurality of implants may be provided with each of the implants having a differently sized stem defining a stem axis. The stems may be tapered. Each stem has a distal end and a proximal end wherein the distal end has a smaller cross sectional area than the proximal end. The handling tool is attachable to each of the implants at a predefined location and the method also includes the steps of selecting one of the implants for attachment to the handling tool and engagement with the registration device. The step of determining the position of the implant stem relative to the second structure also includes determining the size of the selected implant based upon the distance between the first and second reference structures. The registration device may define at least one graduated space and the step of engaging the registration device with the implant includes inserting the tapered stem into the graduated space. In an alternate embodiment, the at least one graduated space may consist of a space that is tapered inwardly with respect to both a first direction and a second direction, where the first direction is opposite of the second direction. 
   The invention comprises, in another form thereof, a system for registering a surgical tool in a computer assisted navigation system, where the system includes a plurality of differently sized surgical tools and a registration device engageable with each of the plurality of tools in a first predefined relative position. The system also preferably includes first and second reference structures, whereby each of the reference structures has at least one reference element registerable in the computer assisted navigation system, where the first reference structure is disposed on the registration device at a predetermined location and the second reference structure is detachably securable to each of the plurality of tools. Further wherein the relative positions of the first and second reference structures differ for each of the plurality of tools when the registration device is engaged at the first predefined location and the second reference structure is secured to a selected one of said tools. In one preferred embodiment, the plurality of surgical tools consist of a plurality of differently sized rasp handles. The present invention also relates to a method of registering a surgical instrument, such as a rasp handle, in a computer assisted navigation system. 
   The invention comprises, in yet another form thereof, an assembly for use in a computer assisted navigation system. The assembly includes an orthopedic implant, at least one wire loop removably mounted on said orthopedic implant and a communication means operably coupled between the wire loop and the computer assisted navigation system and communicating a signal from the wire loop to the navigation system indicative of the magnetic field sensed by the wire loop. 
   The communication means may be a communications cable operably coupled to the wire loop. The at least one wire loop may be at least two wire loops, each of the loops defining a loop axis, the loops disposed in relatively fixed locations wherein the loop axes are positioned in a mutually perpendicular orientation. The wire loop may be mounted on a surgical instrument attached to the implant. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
       FIG. 1  is a perspective view of a registration device in accordance with the present invention; 
       FIG. 2  is a top view of the registration device with a hip stem inserted into a registration slot; 
       FIG. 2A  is cross sectional view of  FIG. 2  taken through the slot having a hip stem inserted therein; 
       FIG. 3  is a top view of the registration device with the neck of a hip stem inserted into a registration opening; 
       FIG. 4  is a side view of the registration device engaged with an acetabular cup; 
       FIG. 5  is a top view of another embodiment of a reference device, shown with a rasp handle engaged therewith; 
       FIG. 6  is a perspective end view of the reference device of  FIG. 5 ; 
       FIG. 7  is a schematic representation of a computer assisted navigation system and the registration device engaged with a hip stem; and 
       FIG. 8  is an exploded schematic representation of an alternative embodiment of a reference element. 
   

   Corresponding reference characters indicate corresponding parts throughout the several views. Although the exemplifications set out herein illustrate embodiments of the invention, the embodiments disclosed below are not intended to be exhaustive or to be construed as limiting the scope of the invention to the precise forms disclosed. 
   DESCRIPTION OF THE PRESENT INVENTION 
   A registration device  20  in accordance with one embodiment of the present invention is shown in  FIG. 1 . Registration device  20  includes a grip or handle portion  22 , a body  24  and at least one slot. In the embodiment of  FIG. 1 , three differently sized tapered slots  26 ,  28 ,  30  are formed in body  24  (but of course the number of slots could be varied as necessary). Each of the slots is defined by two opposed side surfaces  26   a ,  26   b ;  28   a ,  28   b ;  30   a ,  30   b  and a bottom surface  26   c ,  28   c ,  30   c  and define a compound taper. Mounted on the substantially planar upper surface  32  of body  24  are preferably at least three non-linearly positioned reference elements, with four reference elements  34  being shown in this embodiment. In the illustrated embodiment, reference elements  34  are reflective spheres which are registerable in a computer assisted navigation system as discussed in greater detail below. As can be seen in  FIG. 1 , body  24  forms an integral reference structure having reference elements  34  mounted thereon in fixed locations. Reference elements  34  are mounted on posts  36  projecting from body  24 . It is also contemplated that the invention could also be used with at least one reference element of a suitable configuration. 
   On the distal edge  38  of body  24  opposite handle  22  are three depressions  40 ,  42  and  44 . Depression  40  is configured to closely fit the neck stem of a hip implant. Depression  42  has a conical shape and depression  44  has two concentric cylindrical portions of differing diameters. Of course, the number and configuration of the depressions can be varied, as required, in order to accommodate the desired number and type of components intended to be engaged therewith. The illustrated registration device is formed of a stainless steel material, however, other suitable materials such as aluminum or plastic materials may also be used. 
   As best seen in  FIG. 2 , registration device  20  may be engaged with a femoral component of a prosthetic hip joint, i.e., hip stem  46 . Examples of hip stems that may be used with the present invention are disclosed in U.S. Pat. Nos. 5,480,453 and 5,326,376 which are both hereby incorporated herein by reference. Hip stem  46  has a generally L-shaped configuration and includes an elongate stem portion  48  defining a stem axis  49  and a neck portion  50  defining a neck axis  51  ( FIG. 2A ). A projection  52  is located on the neck and a prosthetic ball (not shown) is mounted thereon for positioning in an acetabular cup. Typically, hip stems are manufactured in various sizes wherein the overall configuration of the hip stem remains substantially constant and proportional, but the dimensions are varied to provide a range of sizes to fit differently sized patients. Although the embodiments described herein relate to registration devices for registering prosthetic hip implants, the concepts of the present invention are also applicable to other types of prosthetic implants, as well as to surgical tools. 
     FIGS. 2 and 2A  illustrate a first implant  46  and a second implant  46   a  in dashed lines. First and second hip stems  46 ,  46   a  have a common design but are different sizes with second hip stem  46   a  being slightly smaller than hip stem  46 . In the illustrated embodiment, projections  52 ,  52   a  are identical in size and shape to provide a common mounting interface between the hip stems and femoral balls. Additionally, as shown in  FIG. 3 , the stems are tapered and distal end  54  has a smaller cross sectional area than proximal end  56  of stem  48 . The illustrated stems have a compound taper defining two taper angles. 
   Each of the hip stems  46  also includes a mounting interface which is located on the proximal edge of the hip stem near proximal end  56  of stem  48 . Hip stems typically include such mounting interfaces which are used to removably attach the hip stem to a handling tool, often referred to as a stem inserter. A variety of such interfaces are known. For example, as shown in  FIG. 2A , mounting interface  58  on stem  46  may be a threaded bore with handling tool or stem inserter  60  having a threaded shaft  62  which threadingly engages bore  58  to secure hip stem  46  to handling tool  60  in a manner known in the art. After attaching stem inserter  60  to hip stem  46 , stem inserter  60  is used to manipulate hip stem  46  instead of directly handling hip stem  46 . Stem inserter  60  is removed from hip stem  46  after positioning hip stem  46  in its final implanted position in a femur. 
   A rigid reference structure  64  having at least one reference element  34  mounted thereon in fixed relative positions is secured to stem inserter  60 . Reference structure  64  may be made of aluminum, another metal, a plastic, or of any suitable material. A dovetail joint  66  is used to removably mount reference structure  64  on stem inserter  60 . A threaded fastener  68  firmly secures reference structure  64  in a desired location on stem inserter  60 . In alternative embodiments, reference structure  64  may be permanently affixed to stem inserter  60  or be formed integrally therewith. 
   The underlying handling tool structure on which reference structure  64  is mounted at a predefined location may be a conventional handling tool. Although the illustrated embodiment utilizes a threaded shaft to secure tool  60  to implant  46 , other attachment features for securing the handling tool to the implant may be used. For example, the implant may have a smooth walled bore and the handling tool may have an expandable collet which may be releasably secured within the bore. Moreover, the present invention may be used with alternative implants, e.g., for a prosthetic knee joint, and handling tools adapted for use with such implants. 
   Examples of handling tools that may have reference structures mounted thereon and used with the present invention are described by Hoag et al. in U.S. patent application Ser. No. 10/194,874 entitled TOOL FOR GRIPPING AN ORTHOPEDIC IMPLANT filed on Jul. 12, 2002 and by Hoag et al. in U.S. patent application Ser. No. 10/194,744 entitled TOOL FOR RELEASABLY GRIPPING AN ORTHOPEDIC IMPLANT filed on Jul. 12, 2002 the disclosures of both of these applications are hereby incorporated herein by reference. 
   In addition to its stem handling function, by mounting reference structure  64  thereon, stem inserter  60  also serves to detachably secure reference structure  64  to stem  46  in a relative position which will be generally fixed until stem inserter  60  is disengaged from stem  46 . As discussed below, registration block  20  may be used to determine the relative position of hip stem  46  to reference structure  64  after attaching stem inserter  60  to hip stem  46 , thereby allowing a computer assisted navigation system to track hip stem  46  by sensing the location and orientation of reference structure  64 . 
   In other embodiments of the present invention, a reference structure having one or more reference elements may be directly and removably mounted to the implant instead of via a handling tool. Such a directly attached reference structure would provide for the tracking of the implant but not provide the handling function provided by tool  60 . 
   The position of hip stem  46  must be calibrated with the position of reference structure  64  for computer assisted navigation system  80  to accurately track the position and orientation of hip stem  46 . The use of registration device  20  to perform such a calibration will now be described. 
   Registration device  20  has at least one slot, with three tapered slots  26 ,  28 ,  30  being shown in this embodiment. The opposed side surfaces of the slots, e.g., surfaces  28   a ,  28   b , define a graduated space therebetween. The space defined by slots  26 ,  28 ,  30  are configured to uniquely engage registration device  20  with each of the differently sized hip stems  46  for which registration device  20  is intended for use. 
   In the illustrated embodiment, registration device  20  has been configured for use with a line of hip stems having approximately ten different nominal sizes. Each of the slots  26 ,  28 ,  30  are configured for use with three or four different nominal sizes, i.e., slot  26  receives the smallest sizes, slot  28  the middle sizes and slot  30  the largest sizes. If desired, markings can be added on or near slots  26 ,  28 ,  30  for indicating the appropriate size (or sizes) to be inserted into each slot. As best seen in  FIG. 3 , stems  48  are inserted into slots  26 ,  28 ,  30  in the directions indicated by arrows  27 ,  29 ,  31  respectively. 
   The dimensional tolerances inherent in the manufacture of stems  46  will result in a particular nominal size of a stem  46  being engaged with its associated slot within a narrow band. For example, the group of lines indicated by reference numeral  90  shown in  FIG. 3  represent the two extremes and midpoint of where implant  46   a  would engage slot  28  based upon the manufacturing tolerances of stem  46   a . These engagement locations translate into a range  91  which indicates the location of the engagement interface between stem  46   a  and handling tool  60 . Similarly, lines  92  indicate the two extremes and midpoint of where implant  46  would engage surfaces  28   a  and  28   b  and range  93  indicates the location of the engagement interface between stem  46  and handling tool  60 . This can also be seen with reference to  FIGS. 2 and 2A  which illustrate implant  46  engaged in the predefined relative position represented by lines  92  (lines  92  are only shown in  FIG. 3 ) and an outline of smaller implant  46   a  engaged in the predefined relative position represented by lines  90  (lines  90  are only shown in  FIG. 3 ). 
   Slots  26 ,  28 ,  30  are configured so that the ranges  91 ,  93  of the engagement interface between implant and handling tool of the differently sized implants do not overlap. Because the same tool  60  is used to engage each implant  46 , each different nominal size of implant  46  defines a range of positions of reference structure  64 , relative to registration device  20 , which is unique and does not overlap with the range of any other nominal size of implant  46 . This is exemplified in  FIG. 2 , which illustrates the reference elements  34  disposed on handling tool  60  in solid lines to represent their relative position when implant  46  is engaged with registration device  20  and in dashed outlines  34   a  to represent their relative position when smaller nominal sized implant  46   a  is engaged with registration device  20 . By configuring registration device  20  so that there is no overlap in the range of positions of reference structure  64  for the different nominal sizes of implants  46 , navigation system  80  can determine the nominal size of the implant after inserting its stem into the appropriate slot  26 ,  28 ,  30  on registration device  20 . The dimensions of the various nominal sizes of implants  46  are entered into the navigation system  80  prior to engaging registration device  20  with an implant  46 . 
   Although the illustrated embodiment utilizes tapered slots, alternative graduated engagement features could also be employed with the present invention. For example, in alternative embodiments, the orthopedic implant might include surfaces defining a space therebetween and the registration device might include a graduated projection which fits within the space to engage the implant at a predefined position relative to the implant. 
   As best seen in  FIGS. 2 and 2A , the axis of stem inserter  60  is positioned coaxially with stem axis  49  of an attached hip stem  46 . Thus, when stem  48  is engageably inserted into one of slots  26 ,  28 ,  30  and the relative positions of the reference structures  24 ,  64  respectively located on registration device  20  and stem inserter  60 , the processor of computer assisted navigation system  80  may calculate the nominal size of hip stem  46 , the orientation of stem axis  49 , the position of hip stem along the line defined by axis  49 , i.e., the axial position of hip stem  46 . It does not, however, calculate the rotational or angular orientation of neck  50  relative to axis  49  when stem  48  is inserted in one of the slots  26 ,  28 ,  30 . As shown in  FIG. 2A , the illustrated embodiment of registration device  20  is configured so that neck  50  of hip stem  46  projects in the same direction that reference elements  34  project from surface  32 . 
   To determine the angular orientation of neck  50 , stem  48  is removed from slot  28 , and projection  52  is engaged into registration device  20  by insertion into depression  40 . Depression  40  has slightly tapered sidewalls which match the taper on the common configuration of projections  52  located on implants  46 . By flushly engaging projection  52  of the stem  46  secured to handling tool  60  with depression  40  as shown in  FIG. 3 , the rotational position of projection  52  and neck  50  about axis  49  and relative to reference structure  64  can be determined by navigational system  80  from the relative positions of body  24  and reference structure  64 . 
   When used with an optical tracking system, registration device  20  and reference structure  64  require at least three non-linearly positioned reference points to define the location and orientation of the reference structure on which the reference points are located. The pattern defined by the reference elements disposed on registration device  20  and reference structure  64  may also differ whereby navigation system  80  may more readily distinguish and identify the object associated with each set of reference elements. 
   The registration device may be used to calibrate the position of other objects in a computer assisted navigation system in addition to hip stems  46 . For example,  FIG. 4  illustrates an acetabular cup  94  placed in engagement with the substantially planar surface  32  which has a known orientation to the reference structure defined by reference elements  34  mounted on body  24 . A handling tool  96  adapted for engaging cup  94  has a reference structure  64  mounted thereon and registration device  20  may be used in the calibration of the reference structure  64  mounted on handling tool  96  which is otherwise a conventional instrument for handling an acetabular cup during the implantation thereof as is known in the art. When tool  96  is secured to acetabular cup  94 , the distance of reference structure  64  from surface  32  will be dependent upon the nominal size of the acetabular cup  94 . Thus, the registration of cup  94  with surface  32  may be used to verify that the correct size of cup  94  has been mounted on instrument  96  prior to implanting cup  94 . 
   Similarly, depressions  42  and  44  have a known location and orientation relative to elements  34  mounted on body  24  and may be used to calibrate the coordinates of various surgical instruments or objects within a computer assisted navigation system. For example the tip of a digitizing probe, reamer, awl or other object could be engaged with a selected one of the depressions  42 ,  44 . 
   Turning now to  FIGS. 5 and 6 , a second embodiment of a registration device will be shown and described. Features of this embodiment that correspond to similar features of the embodiment shown in  FIGS. 1–4  will be given the same reference numbers, except with the addition of the prime (′) symbol. The second embodiment will be designated as registration device  20 ′, and it includes a handle  22 ′ and a body  24 ′. 
   As with the other embodiment, registration device  20 ′ includes reference elements  34 ′, at least one tapered slot, and at least one depression. However, in the embodiment of  FIGS. 5 and 6 , the slots are configured differently than those in the other embodiment. More specifically, this embodiment includes slots  102 ,  104  and  106  which are each tapered from both ends thereof. Thus, slots  102 ,  104  and  106  are each tapered inwardly from a first direction (indicated by arrow “A” in  FIG. 5 ), as well as being tapered inwardly from a second direction (indicated by arrow “B”) that is opposite the first direction. In particular, slot  102  includes a first tapered portion  102 A and a second tapered portion  102 B that are separated by ridge  102 C. Likewise, slots  104  and  106  each include, respectively, first tapered portions  104 A,  106 A; second tapered portions  104 B,  106 B; and ridges  104 C,  106 C. 
   Accordingly, in the embodiment of  FIGS. 5 and 6 , implants (such as hip stems) can be received into each slot from either the first direction or the second direction, which provides twice as many insertion points, compared to the embodiment of  FIGS. 1–4 , for the same number of slots. More specifically, in the first embodiment ( FIG. 3 ), the two outer slots receive hip stems from one direction (as indicted by arrows  27  and  31 ), and the third slot, which is located between the other two slots, receives hip stems from the opposite direction, as indicated by arrow  29  (for a total of three insertion points for all three slots). In contrast, in the embodiment of  FIGS. 5 and 6 , each of the three slots ( 102 ,  104  and  106 ) can receive a hip stem from either direction A or from direction B (for a total of six insertion points for all three slots). Of course, the number of slots may be varied from the three shown in either this embodiment or in the first embodiment. 
   In use, the hip stems  46  are engaged with registration device  20 ′ in the same manner as they are engaged with registration device  20  (which is shown in  FIG. 2 ), with the exception that the hip stems can be received into each slot from either direction A or from direction B, as described above. One advantage of the embodiment of  FIGS. 5 and 6  is that it allows for registration of a wider range of sizes of hip stems, as compared with the other embodiment, without requiring an increase in the size of the body of the registration device. 
   In order to facilitate matching the appropriate sizes of hip stem with the corresponding slot, markings may be provided on the registration device  20  in any of the embodiments described herein. For example, as shown in  FIG. 5 , marking  110  indicates that tapered portion  106 A is intended to receive hip stems with diameters of between 9 mm and 10 mm; marking  120  indicates that tapered portion  106 B is intended for hip stems with diameters of between 11 and 12 mm; marking  130  indicates that tapered portion  104 A is for diameters between 13 and 14 mm; marking  140  indicates that tapered portion  104 B for diameters between 15 and 16 mm; marking  150  indicates that tapered portion  102 A is for diameters between 17 and 18 mm; and marking  160  indicates that tapered portion  102 B is for diameters greater than 19 mm. The markings are preferably etched, but may be otherwise permanently provided, on the body of the registration device. 
   Another difference between registration device  20 ′ and registration device  20  relates to the arrangements provided on the distal edge  38 ′. In the first embodiment,  FIGS. 1 and 2  show depressions  40 ,  42  and  44 , which are also provided on the second embodiment, and are represented in  FIG. 6  by reference numbers  40 ′,  42 ′ and  44 ′. More specifically, depression  40 / 40 ′ is configured to engage with the neck of a hip stem implant (as shown in  FIG. 3 ). In a similar manner, depression  42 / 42 ′ (which has a conical shape) and depression  44 / 44 ′ (which has two concentric cylindrical portions of different diameters), are configured to engage with other objects, such as the tips of a digitizing probe, a reamer, an awl, a rasp, etc. The number and configuration(s) of the depression(s) or other arrangements can be varied to accept any desired number and type of object intended to be calibrated or verified into the system. 
   In addition to arrangements  40 ′,  42 ′ and  44 ′, the embodiment of  FIGS. 5 and 6  also includes arrangements for engaging a rasp handle and for engaging a neck of hip stem implant with a six degree taper. More specifically, arrangement  108  is provided to engage with a rasp handle, such as rasp handle  109 , whereby arrangement  108  includes a portion for engagement with the handle used for the rasp used for the right leg (designated “R”) and a portion for engagement with the handle used for the rasp for the left leg (designated “L”). In the embodiment shown, arrangement  108  includes two male projections  112  and two female depressions  114 , one set of each (i.e., one male projection and one female depression) of which are configured to engage with corresponding configurations on the distal end of the rasp handle  109 . Of course, arrangement  108  can be varied to accommodate other types of configurations that may be utilized on the ends of rasp handles for connecting the handle to the rasp. 
   In order to calibrate the rasp handle  109 , the distal end of the rasp handle is engaged with the appropriate portion of arrangement  108  (i.e., either the upper portion, marked “R”, for the right-side rasp handle or the lower portion, marked “L”, for the left-side rasp handle, as shown in  FIG. 6 ). Once engaged, in the manner shown in  FIG. 5 , the registration device  20 ′ can be used to calibrate (into the navigation system) the rasp handle  109 , to which is attached a reference structure  64 ′ that includes a plurality of referencing elements  34 ′ (where the reference structure  64 ′ and the referencing elements  34 ′ function in essentially the same manner as reference structure  64  and referencing elements  34  previously described). The reference structure  64 ′ is securely mounted upon rasp handle  109  using any known method, such as the methods described above for mounting reference structure  64  to stem inserter  60 . The rasp handle  109  is calibrated by itself to allow the navigation system  80  to understand the precise location of the reference structure  64  with respect to the handle  109 . After the handle is calibrated, a rasp (not shown) is attached to the handle  109 , and the rasp tip is inserted into an appropriate one of the arrangements on distal edge  38 ′, such as the large divot  42 ′, to allow the system to verify and record the position of the rasp. 
   Returning to the implantation of a hip stem  46 , the proper positioning of the hip stem in the femur is of great importance with respect to re-establishing the proper leg length of the patient. As is known in the art, the relevant dimensional data concerning an anatomical structure of interest, e.g., a femur, may be determined using data acquired from images of the anatomical structure to generate a data base representing a model of the anatomical structure. The model of the anatomical structure may be a three dimensional model which is developed by acquiring a series of two dimensional images of the anatomical structure. Alternatively, the model of the anatomical structure may be a set of two dimensional images having known spatial relationships or other data structure which can be used to convey information concerning the three dimensional form of the anatomical structure. The model of the anatomical structure may then be used to generate displays of the anatomical structure from various perspectives for preoperative planning purposes and intraoperative navigational purposes. A variety of technologies which may be employed to generate such a model of an anatomical structure are well known in the art and include computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), ultrasound scanning and fluoroscopic imaging technologies. 
   The model of the anatomical structure obtained by such imaging technologies can be used for the intraoperative guidance of an implant by facilitating the determination and display of the relative position and orientation of the implant with respect to the actual anatomical structure. For example, if the model of the anatomical structure is a set of two dimensional images having known spatial relationships, several such images may be simultaneously displayed during the surgical procedure. By also displaying the position of the implant in the images and displaying images taken from different perspectives, e.g., one image facilitating the display of implant movement along the x and y coordinate axes and another image facilitating the display of implant movement along the z axis, the individual images may together represent the movement of the implant in three dimensions relative to the anatomical structure. 
   For reference purposes, a coordinate system defined by the actual anatomical structure which is the subject of interest will be referred to herein as the anatomical coordinate system and a coordinate system defined by the model of the anatomical structure will be referred to as the image coordinate system. Data concerning the fixed size and shape of the implant which will be used in the image guided procedure is also determined preoperatively to obtain a three dimensional model of each of the different nominal sizes of the implant or the relevant portions thereof. 
   Rigid anatomical structures, such as skeletal elements, are well suited for such image guided surgical techniques and individual skeletal elements may be used to define separate coordinate systems. The different rigid structures, e.g., skeletal elements, may be subject to relative movement, for example, the femur and acetabulum of a patient may be relatively moved during the surgical procedure and separate three dimensional models and coordinate systems may be created for the different skeletal elements. For example, during a hip replacement procedure, a three dimensional model of the femur defining a first coordinate system may be utilized during the resection of the femur while a separate coordinate system defined by a three dimension model of the pelvis is utilized during the preparation of the acetabulum. 
   When using computer assisted navigation, also referred to as computer implemented image guidance, to conduct a surgical technique, the image coordinate system is registered with the anatomical coordinate system and the position of the implant or other tracked object is also registered within the image coordinate system. After the registration of both the actual anatomical structure and the implant, the relative position and orientation of the implant may be communicated to the surgeon by displaying together images of the anatomical structure and the implant based upon the three dimensional models of the anatomical structure and implant which were previously acquired. 
   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 or orthopedic implant within the image coordinate system to facilitate the display of the relative positions of the object and the actual anatomical structure are known in the art. Known methods of registering the anatomical structure with the image coordinate system include the use of implanted fiducial markers which are recognizable by one or more scanning technologies. Alternatively, implants may be located by physically positioning a digitizing probe or similar device in contact or at a known orientation with respect to the implant. Instead of using fiducial implants, it may also be possible to register the two coordinate systems by aligning anatomical landmark features. U.S. Pat. Nos. 6,236,875 B1 and 6,167,145 both describe methods of registering multiple rigid bodies and displaying the relative positions thereof and the disclosures of both of these patents are hereby incorporated herein by reference. 
   Tracking devices employing various technologies enabling the registration or localization of a surgical instrument or other object such as an orthopedic implant and the tracking of the object motion with respect to the anatomical coordinate system, which has also been registered with the image coordinate system, are also known. For example, optical tracking systems which detect light reflected from or emitted by reflective targets or localizing emitters secured in a known orientation to the object are known for determining the position of an object and registering the position of the object within an image coordinate system representing a three dimensional model of an anatomical structure. For example, such a tracking system may take the form of a sensor unit having one or more lenses each focusing on separate charge coupled device (CCD) sensitive to infrared light. The sensor unit detects infrared light emitted by three or more non-linearly positioned light emitting diodes (LEDs) secured relative to the object. A processor analyzes the images captured by the sensor unit and calculates the position and orientation of the object. By registering the position of the sensing unit within the image coordinate system, the position of the object relative to the anatomical structure, which has also been registered with the image coordinate system, may be determined and tracked as the object is moved relative to the anatomical structure. 
   Alternative localizing systems may employ localizing emitters which emit an electromagnetic signal in the radio frequency or which emit visible light. Other types of localizing systems that could be used with the present invention employ referencing elements or other distinguishing elements which are radio-opaque. It is also possible to employ digitizing physical probes which are brought into physical contact with the object at predefined locations on the object to register the position of the object. 
   In the disclosed embodiment, the localizing system includes a light source and reference elements  34 / 34 ′ reflect the light. The localizing system then detects the reflected light and computes the location of the individual reference elements  34 / 34 ′ in a known manner. Reference elements  34 / 34 ′ may be obtained from Northern Digital Inc. having a place of business at 103 Randall Dr., Waterloo, Ontario, Canada, N2V1C5. Northern Digital Inc. supplies image guidance systems under the brand names Optotrak® and Polaris® which may be used with the present invention. The present invention may also be used with other computer assisted navigation systems such as those described above or otherwise known in the art. For example, Medtronic, Inc. headquartered in Minneapolis, Minn. manufactures and sells various computer assisted surgical navigation systems under the trademark StealthStation® such as the FluoroNav™ Virtual Fluoroscopy System which could also be adapted for use with the present invention. 
     FIG. 7  schematically illustrates navigation system  80  which includes a position sensor  82  for detecting the position of reference elements  34  disposed on stem inserter  60  and registration device  20 , processing unit  84 , display screen  86  and input device  88 . A similar system is also used with reference elements  34 ′. 
   An alternative embodiment of the present invention could be employed with a computer assisted navigation system which utilizes magnetic fields instead of optical tracking to determine the position and orientation of the tracked object. A variety of referencing elements which are used with magnetic fields which could be adapted for use with the present invention are known in the art. For example, known systems using magnetic fields to determine the position and orientation of an object are described by U.S. Pat. Nos. 5,913,820; 6,381,485 B1; 6,402,762 B2; 6,474,341 B1; 6,493,573 B1; and 6,499,488 B1 the disclosures of these patents are all hereby incorporated herein by reference. 
     FIG. 8  schematically illustrates a reference element  70  which takes the form of a wire loop, in this case a copper wire coil  72  wound about a polymeric bobbin  74  and disposed in a polymeric housing  76  which may be used in a magnetic field navigation system. The axis of wire loop  72  is defined by the cylindrical shaft of bobbin  74  about which wire coil  72  is wound. Housing  76  includes a threaded shaft  78  projecting from one end which provides for the mounting of housing  76  and wire loop  72  located therein. Wire loop  72  is in communication with the processor of a computer assisted navigation system via cable  73 . Wireless communication between wire loop  72  and the processor using radio signals could alternatively be employed. Two or more such loops  72  may be advantageously fixed in mutually perpendicular orientations, e.g., each such loop may have an axis which is positioned parallel to one of the three axes of a Cartesian coordinate system. (In  FIG. 8  wire loop  72  is shown having an axis which extends parallel to the Z axis.) 
   By generating a magnetic field of known properties in the operative area and sensing the field with mutually perpendicular wire loops  72 , the position and orientation of the reference element defined by the loops  72  and the rigid object, such as a surgical instrument or orthopedic implant, attached thereto may be calculated. The determination of the position and orientation of such mutually perpendicularly oriented field sensors  72  is known in the art. It is also known to use a single wire loop  72  to form a field sensor and determine its position and orientation by generating magnetic fields from a plurality of locations. 
   In the illustrated embodiment, wire loop  72  is a cylindrical coil, however, other loop shapes may also be employed. A wire loop  72  may attached to a handling tool such as stem inserter  60  or a rasp handle  109  in a variety of methods. For example, a wire loop may be placed in a specially machined pocket and retained in place by a mechanical, adhesive, e.g., glue or epoxy, or other suitable means. It could also be mounted to an instrument or implant via a fixture that contains the loop such as housing  76  or a plastic screw that has a wire loop insert molded therein. Such a fixture would facilitate the mounting of the wire loop to existing instruments. The navigated instrument could also be manufactured with the coil integral to it or have a mounting for winding the wire loop thereon. Such instruments could be manufactured using various materials such as metal, nonferrous metal, plastic and composite materials. The choice of materials of such instruments and fixtures could facilitate the provision of single use disposable instruments or fixtures. 
   Other surgical tools which may be employed in a surgical procedure implanting a prosthetic hip joint and utilizing a computer assisted navigational system are described by McGinley et al. in a U.S. patent application entitled SURGICAL NAVIGATION INSTRUMENT USEFUL IN MARKING ANATOMICAL STRUCTURES having Ser. No. 10/357,959, filed on Feb. 4, 2003, and by McGinley et al. in a U.S. patent application entitled GUIDANCE SYSTEM FOR ROTARY SURGICAL INSTRUMENT having Ser. No. 10/357,592, also filed on Feb. 4, 2003, the disclosures of both of these applications are hereby incorporated herein by reference. 
   While this invention has been described as having exemplary designs, 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.