Patent Publication Number: US-2006015018-A1

Title: CAS modular body reference and limb position measurement system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This is a continuation of International Application No. PCT/CA2004/000159 filed Feb. 4, 2004, designating the United States, which itself claims priority on U.S. provisional application 60/404,758 and 60/444,691 which were respectively filed Apr. 30, 2003 and Feb. 4, 2003, the specifications of all of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD  
      The present invention relates generally to a trackable reference for use in conjunction with a Computer Assisted Surgery (CAS) system.  
     BACKGROUND OF THE INVENTION  
      CAS systems capable of real time location and tracking of a plurality of CAS identifiable markers in a surgical field are desirable. A variety of systems are now employed, some of which require that the necessary bone elements of the patient be identified and registered to pre-operatively taken anatomical scans or intra-operatively taken images of the same bone elements. In order for the relevant bone elements to be located and tracked by the CAS system, trackable reference members must be fastened thereto. These bone reference members will vary depending on the type and specific requirements of the particular CAS system used. However, CAS surgical procedures have more recently tended towards systems and surgical methods which do not require such anatomical scans or images in order to identify the bone elements of the patient. As such procedures do not require CT scans to generate the pre-operative anatomical models, these procedures are often termed CT-free or CT-less operations.  
      No matter the CAS operation system employed, in order for the relevant bone elements to be located and tracked by the CAS system, trackable reference members are nevertheless typically used to identify the position and orientation in space of the bone element. These bone reference members vary depending on the type and specific requirements of the particular CAS system used.  
      For example, for an optical CAS system, the trackable bone reference members comprise at least three optically detectable markers whose exact positions can be determined by each of the at least two cameras of the optical CAS system. This therefore permits the position in space of each detectable marker to be determined by the CAS system, and therefore permits the position and orientation of the bone reference member, and consequently also the position and orientation of the bone element to which it is affixed, to be determinable by the CAS system.  
      For optically based CAS systems, the ability to maintain an unobstructed line of sight view between the system cameras and the detectable marker elements of the trackable member is of prime importance. This can, however, become difficult in some surgical installations, where numerous medical staff and a large quantity of medical equipment are required within the surgical field. The cameras of the CAS system must be able to simultaneously visually locate both the bone reference trackable member and any additional trackable members disposed on tracked tools employed. While tracked surgical instruments can more easily be displaced such that their trackable members are in an optimal position relative to the cameras, it is often more difficult and impractical to adjust the trackable bone reference member, being fastened to a bone element of the patient.  
      No matter what type of positioning reference block is used, all such reference members used in conjunction with a computer assisted surgery must comprise a trackable member. It is well known to permanently fix such trackable members to the reference block by such methods as welding, press-fitting, and pinning. However, as it can be desirable in particular circumstances to be able to separate the trackable member portion from the base reference block, it is known to fasten the trackable member to the reference block with releasable engagement mechanisms. These generally permit the trackable member to be completely removed from the reference member fastened to the bone element. This can be useful if temporary removal of the trackable member provides better access for the surgeon to a particular location, for example as may be helpful in hip surgeries.  
      However, once the trackable member is removed from the reference block fixed to the bone element, the position and orientation of the bone element is no longer known. As such, when the trackable member is re-attached to the reference member in an alternate position, the bone element must be re-registered in order for the CAS model or image to correspond to the position and orientation of the actual bone element, such that the reference member can then be again used to accurately track the bone element to which it is fixed.  
      Therefore, while the ability to remove a trackable member from a bone reference and re-engage it therewith intra-operatively is desirable, the re-registration that is subsequently required is time consuming and impractical. Additionally, known bone reference members provide limited adjustability of the trackable member. Maintaining an optimal, unobstructed visual contact between the bone reference trackable member and the cameras of the CAS system is consequently often difficult.  
      Hip surgeries in general, and total hip replacements in particular, are common. When total hip replacements are performed, there can be a discrepancy between the leg length on the treated hip side relative to the length of the non-treated leg. Additionally, replacement of the natural hip with a prosthetic replacement can also result in a change in the position of the leg of the treated hip along the medio-lateral axis of the pelvic coordinate system. However, any post-operative change in the longitudinal and medio-lateral positioning of the limb relative to the pre-operative values of the natural hip cannot easily be determined unless a trackable bone reference member is fastened to the limb in question.  
     SUMMARY OF THE INVENTION  
      It is accordingly an object of the present invention to provide an improved CAS bone reference assembly having a trackable member adapted for communication with an image guided surgical system.  
      It is another object of the present invention to provide a CAS bone reference assembly having a trackable member that is selectively removable from a base reference member to which it is engaged, and re-engageable intra-operatively.  
      It is another object of the present invention to provide a CAS bone reference assembly comprising a selectively disengageable articulated support for a trackable member.  
      It is also an object of the present invention to provide a CT-free CAS system capable of determining limb position change relative to a pre-operative position of the limb.  
      It is another object of the present invention to provide a method for determining length discrepancy and medio-lateral offset of an un-tracked limb using a CT-free CAS system.  
      The present invention is generally directed to a bone reference having a selectively removable articulated support for a position identifying element trackable by a CAS system, and a method for determining the limb length discrepancy and limb medio-lateral offset in a computed tomography (CT) free total hip replacement surgery using the bone reference and the CAS system.  
      Therefore, in accordance with the present invention, there is provided a surgical bone reference assembly, adapted for communication with a computer assisted surgical system, comprising: a bone anchor member, engageable to a bone element of a patient such that substantially no relative movement therebetween is possible; a trackable member comprising a detectable element adapted to be located and tracked in three dimensional space by the computer assisted surgical system, thereby defining position and movement of said trackable member; an adjustable support member having said trackable member disposed at a first end thereof, a second end of said support member being removably fastenable to said bone anchor member by an attachment member, said support member permitting variable positioning of said trackable member relative to said bone anchor member and being lockable to fix said trackable member in a desired position relative to said bone anchor member; and said support member, via said attachment member, being intra-operatively detachable from said bone anchor member and subsequently re-fastenable thereto such that said trackable member is in said desired position and orientation relative to the bone element.  
      There is also provided, in accordance with the present invention, a method for monitoring position and movement of a bone element using a computer assisted surgical system comprising: fastening a bone anchor member to the bone element; attaching an adjustable support member to said bone anchor member, said adjustable support member having a trackable member fixed thereto, said trackable member including a detectable element being locatable and trackable in three dimensional space by said computer-assisted surgical system; adjusting said trackable member into a desired position and orientation relative to sensing elements of said computer-assisted surgical system; locking said adjustable support member in place such that said trackable member is fixed in said desired position and orientation relative to sensing elements of said computer-assisted surgical system; performing a registration of the bone element; detaching said adjustable support member from said bone anchor member; and re-fastening said adjustable support member to said bone anchor member, said trackable member being in said desired position and orientation without requiring readjustment and said bone element being locatable and trackable using said computer assisted surgical system without requiring re-registration of said bone element.  
      There is further provided, in accordance with the present invention, a method of using a computed tomography (CT) free computer assisted surgery (CAS) system for determining a change in position of an un-tracked target limb undergoing orthopaedic surgery, the method comprising: engaging a bone reference member, trackable by said CAS system, to a bone element distinct from said target limb, and using said bone reference member to define a base coordinate system; locating a position identifying landmark on said target limb; performing a first digitization of said landmark; performing a second digitization of said landmark following joint reduction; and determining at least one of a post-joint reduction limb length discrepancy value and a target limb medio-lateral offset value.  
      There is also provided, in accordance with the present invention, a computed tomography (CT) free computer assisted surgery (CAS) system for determining a change in position of an un-tracked target limb undergoing orthopaedic surgery, comprising: a bone reference member trackable by said CAS system and engaged with a bone element distinct from said target limb; means for locating said bone reference member and determining a base coordinate system relative thereto; a digitizer, trackable by said CAS system, for performing a first and a second digitization of a landmark on said target limb; means for determining pre-joint dislocation coordinates in said base coordinate system from said first digitization and post-joint reduction coordinates in said base coordinate system from said second digitization, and for determining longitudinal axis components and medio-lateral axis components of said pre-joint dislocation coordinates and said post-joint reduction coordinates; and means for determining at least one of a post-joint reduction limb length discrepancy value and a target limb medio-lateral offset value, said post-joint reduction limb length discrepancy value being computed using said longitudinal axis components and said target limb medio-lateral offset value being computed using said medio-lateral axis components.  
      There is finally provided, in accordance with the present invention, a method of using a computed tomography (CT) free computer assisted surgery (CAS) system for determining a change in position of a target limb undergoing orthopaedic surgery, the method comprising: engaging a first bone reference member, trackable by said CAS system, to a bone element distinct from said target limb, and using said bone reference member to define a base coordinate system; engaging a second bone reference member, trackable by said CAS system, to said target limb; performing a first digitization of said second bone reference member to identify a pre-joint dislocation position thereof relative to said base coordinate system; performing a second digitization of said second bone reference member, following joint reduction, to determine a post-joint reduction position thereof relative to said base coordinate system; and determining at least one of a post-joint reduction limb length discrepancy value and a target limb medio-lateral offset value. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which:  
       FIG. 1  is a front perspective view of a surgical bone reference assembly according to the present invention;  
       FIG. 2  is a front perspective view of a disengageable trackable member portion of the surgical bone reference assembly of  FIG. 1 ;  
       FIG. 3  is a front perspective view of a bone reference base portion of the surgical bone reference assembly of  FIG. 1 ; and  
       FIG. 4  is a flow chart of a method of limb length discrepancy and medialization value determination according to a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Referring to  FIG. 1 , a surgical bone reference assembly  10  generally comprises a bone anchor member  12 , an articulated tracker support  14 , having a trackable member  16  engaged at one end thereof, and being removably engageable and disengageable with the bone anchor member  12  by an attachment member  18 . The trackable member  16  is adapted to be communicable with a computer assisted surgery (CAS) system capable of detecting and tracking the device in three-dimensional space within a surgical field. As best seen in  FIG. 3 , the bone anchor member  12  comprises preferably a cylindrical body  22  having at least one pin hole  24  extending axially therethrough for receiving at least one bone mounting pin  20  ( FIG. 1 ). Preferably, however, three bone mounting pins  20  are used to fasten the bone anchor block  12  to a bone element of a patient such that no movement of the bone anchor member  12  relative to the bone element is possible. Three pin holes  24  are accordingly provided in the cylindrical body  22  of the bone anchor member  12 , within which the bone mounting pins are received. Locking screws  26  extend transversely through the cylindrical body  22 , such that their hidden tips can frictionally engage the bone mounting pins  20  disposed within the pin holes  24 . The locking screws  26  permit the cylindrical body  22  to be axially adjusted on the bone mounting pins  20  and engaged thereto such that the bone anchor member  12  is fixed in place on bone mounting pins  20 . The bone mounting pins are fastened into a bone element of a patient, preferably such that they protrude sufficiently therefrom to permit exposed distal ends of the pins extend beyond the soft tissue surrounding the bone element. Consequently, once the bone mounting pins are fixed in place, the bone anchor member  12  can be engaged thereto super-cutaneously (i.e.: above the skin), thereby being fixed relative to the patient without being directly fastened thereto. This reduces the invasiveness of the installation of the bone anchor member  12 .  
      The pin holes  24  are preferably parallel to one another, however they can also be slightly inclined relative to one another. This requires each of the bone mounting pins  20  to be anchored into the bone element at a corresponding angle. This alternate arrangement can be used to provide better stability of the anchor member  12  when engaged to the bone mounting pins  20 .  
      Although three bone mounting pins  20  are preferably used to fasten the bone anchor member  12  to a bone element of a patient, it is also possible to engage the bone anchor member  12  to at least one bone mounting pin or rod. Such a bone mounting pin or rod having a non-circular cross-sectional area received into a correspondingly shaped aperture or bore in the bone anchor member  12 , would similarly prevent the possibility of the reference assembly  10  from rotating relative to the bone element  11 , and the anchor member  12  could similarly be axially fastened thereto. Although not as secure as the use of three bone mounting pins  20 , the use of a single, non-circular bone mounting pin would substantially eliminate relative movement between the anchor member  12  and the bone element, while requiring only a single insertion point for mounting the bone reference assembly  10  to the bone element. The use of two or three pins with such non-circular cross-sectional area is also possible. Further, a single pin can alternately be used which permits engagement of a toothed body directly with the bone element, the toothed body being fastened to, or integrally formed with, the bone anchor member  12 .  
      The bone anchor member further comprises a central mounting element  28 , which is integrally formed with the cylindrical body  22  and distally extends therefrom. The outer circumferential surface  30  of the mounting element  28  preferably has external threads  36  thereon. A central bore  32  extends through both the mounting element  28  and the cylindrical body  22 , and is sized to receive a proximal end of a base link member  38  ( FIG. 2 ) therein. A transverse alignment groove  34  diametrically extends across the distal surface of the mounting element  28  on either side of the central bore  32 . Each side of the alignment groove  34  receives transversely projecting alignment pins  39  of the base link member  38 , as will be described in further detail below. The alignment groove  34  is preferably V-shaped, such that a corresponding pin, having a circular cross-sectional area, when disposed therein will always tend to be centrally located.  
      The trackable member  16  generally comprises a detectable tracker head element  17 , including detectable element mounting posts  15  for receiving detectable markers thereon, which is connected to the bone anchor member  12  by an articulated support member  14  that will be described in further detail below. To each mounting post  15  is removably fixed a detectable marker element, such as an optically detectable sphere element  19 . The detectable spheres  19  are preferably coated with a retro-reflective layer in order to be detected by, for example, an infrared sensor using axial illumination. Cameras of an optical CAS system can therefore detect the position of each optically detectable sphere  19  illuminated by infrared light. Each detectable marker element can equally be any other type of position indicator such as a light emitting diode or detectable electromagnetic indicator, provided each can be detected by the type of sensor used by the particular CAS system employed. Although the present surgical bone reference assembly  10  is preferably adapted for use with an optically based CAS system, one skilled in the art will appreciate that in addition to the optical and electromagnetic systems mentioned above, other types of CAS systems can also be used, such as, for example, those which use ultrasound or laser as a means for position identification. In such cases, it is to be understood that the detectable sphere elements  19  will be such that they are able to indicate to, or be detected by, sensors of the particular CAS position identification system used.  
      The articulated support  14  adjustably links the trackable member  16  to the anchor member  12 . The articulated support  14  permits selective adjustability of the position in space of the trackable member  16  relative to the bone anchor member  12 , and therefore to the bone element to which the bone anchor member  12  is fixed. The articulated support member  14  preferably comprises at least two independently articulated joint assemblies, such as first and second joint assemblies  44  and  46  in  FIGS. 1 and 2 . However, a single joint is equally possible. For example, a single rotating joint can be used between the bone anchor member  12  and an angled, rigid support arm having a trackable member on the end thereof. Although providing less adjustability and range of motion, such an arrangement would be simpler and less expensive. No matter the number, each joint preferably provides an independent single degree of freedom. However, a selectively lockable, ball-and-socket type joint could also be used, and would provide itself three rotational degrees of freedom. While joints providing rotational movement are preferred, other types of joints, for example those providing a translational degree of freedom, are equally possible, but preferably used in combination with at least one rotational joint.  
      Referring to the preferred embodiment as depicted in  FIGS. 1 and 2 , the articulated support member  14  comprises a first link member  40  and a second link member  42 , interconnected by the first joint assembly  44  therebetween. The second link member  42  comprises a rigid rod element, fixed at one end to the tracker head element  17  of the trackable member  16 , and having a preferably integrally formed annular second link end  54  at an opposing end. The annular second link end  54  includes a serrated, or toothed ring  56 , disposed substantially perpendicularly to the surface of the tracker head element  17 . The toothed rings are all preferably integrally formed with their annular link ends, however the toothed rings can also be separately formed and press fit, or otherwise securely engaged, with the link ends. The serrations or teeth of the toothed ring  56  inter-engage with corresponding teeth of a toothed ring  50 , preferably integrally formed on an annular first link end  48  of the first link member  40 . When the two toothed rings  50  and  56  are pressed into engagement together, the teeth interlock to prevent rotational movement relative to one another.  
      The annular first link end  48  comprises a central aperture defined therethrough, about which the toothed ring  50  disposed. The central aperture in the distal first link end  48  is concentric with a first joint axis of rotation  62 , substantially perpendicular to a longitudinal axis of the first link member  40 . A first axle pin  58  is permanently fixed at one end to the second link end  54 , and extends through the central aperture in the annular first link end  48 . The first joint axle pin  58  has an externally threaded central portion, not seen in the figures but disposed generally partially beneath each of a first joint locking nut  52  and the annular first link end  48 . The central aperture through the first link end  48  has a diameter sufficiently large enough such that the axle pin  58  is free to rotate within the aperture. The axle pin  58  also comprises a disc flange  60  at the free end of the pin  58  opposed to the end fixed to the second link end  54 . The disc flange  60  prevents the first joint locking nut  52  from being completely separable from the first joint assembly  44 . When the locking nut  52 , having internal threads corresponding to those on the axle pin  58 , is tightened, it forces the annular first link end  48  towards the second link end  54 , such that the corresponding toothed rings  50  and  56  engage one another. This thereby engages the first and second link members  40  and  42  in a specific angular relation to one another. The first joint assembly  44  therefore permits selective rotational adjustment of the second link member  42 , to which the trackable member  16  is fastened, about the first axis of rotation  62 .  
      The articulated support  14  further comprises a second joint assembly  46 , providing selective rotational adjustment between the first link member  40  and the base link member  38  about a second joint axis of rotation  78 , collinear with a longitudinal axis of both the base link member  38  and the bone anchor member  12 . The second joint assembly  46  operates much as the first joint assembly  44 , permitting selective rotation of the first link member  40  relative to the base link member  38  when a second joint locking nut  72  is disengaged, and fixed engagement between the base link member  38  and the first link member  40  when the second joint locking nut  72  is tightened. The second joint assembly  46  includes a proximal first link end  68 , disposed at an opposite end of the first link member  40  from the distal first link end  48 . The proximal first link end  68  comprises a toothed ring  70 , having proximally projecting teeth for engagement with the distally projecting teeth of a corresponding toothed ring  83 , centrally disposed on a distal base link end  82 . A second joint axle pin  74  is fixed to the base link member  38  with the toothed ring  83 , and distally extends therefrom. Much as the first joint axle pin  58 , the second joint axle pin  74  has a threaded central body portion, such that the second joint locking nut  72  can be engaged thereto, thereby forcing the first link member  40  into fixed engagement with the base link member  38  when the locking nut  72  is tightened. The corresponding teeth of the mating toothed rings  70  and  83  on both the proximal end  68  of the first link member  40  and the distal end  82  of the base link member  38 , are consequently engaged such that these two components are rotationally fixed relative to one another.  
      All surfaces of the present bone reference assembly  10  can be easily cleaned. Particularly, all surfaces of the joints can be sufficiently exposed such that thorough pressure cleaning is possible. The ability to sterilize all surfaces of the bone reference assembly  10  by pressure cleaning and autoclaving is important to ensure that all contaminating biological matter can be safely removed. Such potentially dangerous contaminating biological matter can include unwanted bacteria and proteins, which can cause infections or diseases. The free end flanges  60  and  76  of the joint axle pins  58  and  74  are spaced sufficiently away from the joints that the joint locking nuts  52  and  72  can be completely unscrewed and the two halves of the joints separated such that all surfaces, including the outer threads of the joint axle pins, can be substantially exposed to permit pressure cleaning thereof. Further, the articulated support member  14  is removable from the bone anchor member  12 , as will be described below, which permits intra-operative sterilization of the articulated support member  14  and the trackable member  16  when required.  
      The articulated support member  14  is removably engageable to the bone anchor member  12  with an attachment member  18 . The attachment member  18  comprises a main body  31  having a central bore  35  axially extending therethrough such that the main body  31  can be freely rotated on the base link member  38 . The main body  31  includes a radially extending finger grip portion  33 , integrally formed or permanently fixed to the main body  31 , such that the main body  31  can be manually rotated. The bulbous proximal end  37  of the main body  31  has a greater outer diameter than the central portion of the main body  31 , and comprises internal threads in the central bore  35  therewithin. The attachment member  18  thereby provides a tightening nut for screwed engagement with the bone anchor member  12 . The internal threads of the nut portion of the main body  31  are co-operable and engageable with the external threads  36  on the circumferential outer surface  30  of the mounting element  28  of the bone anchor member  12 , such that the proximal end  37  of the attachment member  18  can removably fasten the base link member  38  to the bone anchor member  12 .  
      To fasten the articulated support member  14  to the bone anchor member  12 , the proximal end of the base link member  38  is inserted into the central bore  32  of the bone anchor member  12 , and transversely projecting alignment pins  39  which extend from the base link member  38  are aligned with, and inserted into, the alignment grooves  34  in the mounting element  28  of the bone anchor member  12 . The alignment pins  39  thereby prevent unwanted rotation of the base link member  38 , and consequently the entire articulated support member  14 , relative to the fixed bone anchor member  12 . The attachment member  18  can then be screwed into engagement with the mounting element  28  on the bone anchor member  12 . Removal of the articulated support member  14  and trackable member  16  from the bone anchor member  12 , is accordingly quickly and easily possible, by unscrewing the attachment member  18  from the mounting element  28 , and axially sliding the base link member  38  out of the central bore  32  of the bone anchor member  12 . The entire articulated support member  14 , having the trackable member  16  disposed at an end thereof, can thereby be disengaged from the bone anchor member  12  which is fixed to the bone of the patient. This can be done intra-operatively, if for example, the patient has to be displaced or repositioned, and the articulated support member  14  would impede such required movement.  
      Similarly, the articulated support member  14  and trackable member  16  can thus be intra-operatively removed, sterilized and easily re-installed, without having to remove the bone anchor member  12 , in substantially the exact same position and orientation relative to the bone element. As the bone anchor member  12  and the bone mounting pins  30  fixed to the bone element of the patient do not have to be removed, significant time savings can thus be made. The alignment pins  39  of the base link member  38  permit the articulated support member  14  to be re-positioned in the same orientation relative to the bone anchor member  12  when the trackable member  16  is to be re-engaged to the bone element. However, when the attachment member  18  is re-engaged with the bone anchor member  12 , care must be taken to ensure that the articulated support member  14  is disposed in the same orientation as it was during the initial registration or digitization of the coordinate system, before the articulated support member  14  and trackable member  16  were disengaged from the bone anchor member  12 , and is not replaced 180 degrees out of position.  
      In a preferred application, the surgical bone reference assembly  10  is used in a total hip replacement surgery, and is fixed to the ilium of the patient. Particularly, the surgical bone reference assembly  10  is preferably used in computer assisted hip surgery procedures, such as the CT-less THR surgery as described below, which do not use pre-operatively taken scans, such as computed tomography (CT) scans, to create a computerized bone model. Generally, the pelvic region of the patient is intra-operatively digitized to create a pelvic coordinate system. Although the actual hip replacement surgery is performed with the patient in a lateral decubitus position, the bone reference assembly  10  must be fixed to the ilium while the patient is in a supine decubitus position, to allow digitization of the pelvic coordinate system. Therefore, the articulated support member  14  and the trackable member  16  can be detached from the bone anchor member  12 , which is fixed to the ilium by the bone mounting pins  20 , once the digitization of the pelvic coordinate system is complete. As described above, this is done by unscrewing the attachment member  18 . With the articulated support member  14  disengaged, the patient can then be displaced into the lateral decubitus position, without concern for the trackable member  16  and the associated articulated support structure. Additionally, the articulated support member  14  and the trackable member  16  can be sterilized if required once removed. Once the patient has been placed in the desired position for the surgical operation, the articulated support member  14  can subsequently be re-attached to the bone anchor member  12  in the same position it was in when the digitization was performed, and can be used to accurately locate and track the bone element without requiring a further registration or calibration of the trackable member  16  relative to the bone element.  
      As mentioned above, the CAS bone reference assembly  10  of the present invention is preferably intended to be used in conjunction with an optical tracking CAS system which employs a network of cameras to locate the trackable member  16 , or more specifically to locate identification markers  19  of a detectable element  17  thereof, so that their position and movement can be tracked during the surgery. Therefore, when the bone reference assembly  10  is fixed to the desired patient bone element, such as the pelvic bone, the anatomical position and orientation of the bone element can be determined and tracked in space by the CAS system.  
      Although the present invention is preferably used with a CT-free CAS system, it is nevertheless to be understood that the step of performing a registration of the bone element, as used herein, comprises all means of relating the actual bone element to a corresponding model or image of the same bone element. Those skilled in the art will appreciate that there are a plurality of ways of creating such a model or image of the bone element, and of relating or matching the actual bone element to the model or image thereof.  
      When a CT-free surgical procedure is being used, once the bone reference assembly  10  is securely engaged to the bone element, thereby fixing the bone element relative to the location of the trackable member  16  of the bone reference assembly  10 , a plurality of points on the relevant surfaces of the bone element can then be digitized to create a computer model of the surface. This is preferably done by acquiring the plurality of points, either pre-determined and sequentially identified by the CAS to the surgeon or randomly selected by the surgeon, on the surface of the bone element using a calibrated CAS probe.  
      Such landmark digitization techniques permit intra-operatively acquired surface points, preferably acquired on specific predetermined landmarks of the bone element surface, to be used to create the computerized anatomical reference model of the bone element. This eliminates the need for a CT scan, taken pre-operatively for example, to be used to generate the computer reference model of the bone element.  
      All methods of generating a computerized model or displaying image of the bone element, and of relating or matching the position and orientation of the actual bone element thereto, will be understood herein to be, included in the process of performing a registration of the bone element.  
      Generally, as will be described in greater detail below, once a landmark on the bone surface of the target limb is digitized, the CAS system can identify the position in the base coordinate system defined by the bone reference assembly  10  fixed to the bone element that is distinct from the target limb, such as the pelvic bone. The second digitization performed after the limb reduction similarly positions the target limb in the base coordinate system at this later time. These coordinates can then be used, as defined above, in order to determine the limb length discrepancy and the limb medio-lateral offset.  
      While another aspect of the present invention will be particularly described below with regard to a total hip replacement (THR) using a CT-free CAS system, it is to be understood that the present invention can be similarly used with other orthopaedic surgical operations and applications which would be evident to one skilled in the art. Particularly, the present invention is preferably used in conjunction with a CT-free CAS system for THR, for instance as defined in U.S. Application Ser. No. 60/415,809 filed Oct. 4, 2002, the full contents of which are incorporated herein by reference.  
      Further, while the bone reference member described above is preferably used in connection with the subsequent aspect of the present invention, an optically trackable CAS bone reference member as defined in U.S. application Ser. No. 10/263,711 filed Oct. 4, 2002 or U.S. application Ser. No. 10/263,708 filed Oct. 4, 2002 may alternately be employed. The above-noted references are assigned to the same assignee as the present invention, and are incorporated by reference herewith.  
      In a CT-free surgical procedure, a plurality of points on a surface of the bone element can then be digitized to create a digitized computer surface model of a portion of the bone element. The surface digitization is preferably done by acquiring the plurality of points, either pre-determined and sequentially identified by the CAS to the surgeon or randomly selected by the surgeon, on the surface of the bone element using a digitizer such as a calibrated CAS probe. Such landmark digitization techniques permit intra-operatively acquired surface points, preferably acquired on specific predetermined bony landmarks of the bone element surface, to be used to create a computerized anatomical reference model of the bone element.  
      Replacement of a natural hip with a prosthetic hip replacement can result in a change in the position of the leg of the treated hip in the medio-lateral axis of the pelvic coordinate system. A discrepancy between the final leg length of the limb on the treated hip side relative to the length of the non-treated limb can also result. Using CT-free landmark digitization techniques as defined above, the present CAS system comprises means for determining these variations in the limb positions based on pre-hip joint dislocation and post-hip joint reduction values, without requiring that the limb in question to be tracked using a bone reference member fastened thereto. The system and method preferably used to determine these limb position measurements follows.  
      An antero-posterior (AP) X-ray, taken pre-operatively or at least prior to the joint dislocation and subsequent joint reduction, is preferably used to determine a pre-operative, or pre-joint dislocation, value of the natural limb length discrepancy, which is the difference in length between the two limbs. In severe cases, such a length discrepancy between legs can be quite pronounced. Even in less extreme cases where bone degeneration has less severely affected the total leg length of the worn hip, and the natural leg length discrepancy is accordingly slighter, accurate measurement of the pre-operative limb length discrepancy is preferably determined. Although this is preferably done using an AP X-ray of the patient, other means of determining such a pre-operative difference between natural leg lengths can also be used.  
      While in a preferred embodiment the present invention is particularly described with regard to a THR, the intra-operative limb displacement required by the surgery includes a hip joint dislocation and subsequent joint reduction, once the natural hip has been replaced by the necessary prosthetic implants. However, as the present invention can be used in connection with other limb surgical procedures, the term reduction as used herein is defined to include the replacement or realignment of a body part in normal position or an initial position. Similarly, surgical limb displacement, as used herein, is intended to include hip joint dislocation and other limb movement as required for any particular orthopaedic surgical procedure. The term pre-operative, as used herein, is defined as being prior to such a joint dislocation or other surgical limb displacement. While this can be prior to the actual entire surgical procedure, in the traditional sense of the word pre-operative, it nonetheless similarly includes actions taken during the surgical procedure, but prior to the surgical limb displacement as defined.  
      Initially, a first digitization of a position defining landmark on the target limb to be treated is performed, prior to the joint dislocation and subsequent reduction of the hip joint being replaced, or before any other similar limb displacement required by the particular orthopaedic surgery being performed. The term pre-joint dislocation position as used herein is defined to include the position of the target limb prior to the dislocation of the natural joint. The term post-joint reduction position as used herein is defined to comprise the position of the target limb following the reduction of the artificial joint of the target limb. The position defining landmark is preferably a bony landmark chosen such that it is easily recognizable and identifiable, and is preferably located on the femur of the target leg, in the case of a total hip replacement surgery. The points chosen for the landmark digitization can be marked with an electro-surgical cutter or other bone identification means, which ensures that precise indication of the location of the digitized bony landmark is provided, such that a second post-joint reduction digitization can be performed by choosing points on the same landmark. The second digitization of the exact same landmark is performed after the installation and reduction of the artificial hip joint. It is important to ensure that the target limb, namely the treated leg, is placed in the same position with regard to the pelvis bone for the digitization of both points or surfaces. Although the position defining landmark on the target limb is preferably an actual bony landmark thereon, the term position defining landmark as defined herein is intended to include man-made landmarks, such as for example, bone reference members which are trackable by the CAS system.  
      The digitization of the bone surface, by a digitizer such as a calibrated probe or pointer that is identifiable by the CAS system, permits the means for determining coordinates of the CAS system to identify the position and orientation of at least the digitized surface in relation to a bone reference member fixed to another point on the body, such as the pelvic bone for example. Therefore, although the digitized limb surface is not operatively tracked, its position relative to a bone reference member on an independent bone element can thereby be determined both before and after the hip replacement. The digitized points can be projected onto the longitudinal and medio-lateral axes of the pelvic coordinate system of the patient, as defined by the first tracked bone reference member which defines the base coordinate reference system.  
      Although the digitization of the position defining landmarks on the un-tracked target limb preferably comprises acquiring points thereon using a CAS pointer, the process of digitization of the position defining landmarks, as defined herein, is intended to include the position identification of a trackable bone reference member by the CAS system. Accordingly, although the preferred embodiment of the present invention permits the position of an un-tracked limb to be determined, it is similarly possible to determine limb length discrepancy and limb medio-lateral offset, as will be described in further detail below, of a target limb which has a bone reference member fixed thereto, and is thus tracked by the CAS system. In this case, the first and second digitizations of the position defining landmark, which is a second trackable CAS bone reference member, comprise using the CAS system to identify the positions thereof in the base coordinate system, which is defined by the first bone reference member fixed to the pelvis of the patient. The relative positions of one trackable bone reference member to the other, determined both before joint dislocation and after the joint reduction by the first and second digitizations respectively, are therefore similarly used to determine the limb length discrepancy and limb medio-lateral offset. It therefore follows, in this alternate embodiment, that a first bone reference member that is trackable by the CAS system is fixed to a bone element that is distinct from the target limb, such as the pelvic bone, and a second trackable bone reference member is fixed to the target limb, such as the femur of the target leg. However, in this alternate embodiment in which the target limb is tracked, the additional steps of performing a digitization of the target limb coordinate system, and performing a digitization of the center of rotation of the treated joint, either on the acetabulum or the femoral head for example, are preferably performed.  
      The difference between the projected coordinates of the first pre-joint dislocation digitized points and the second post-joint reduction digitized points is accordingly computed by the CAS system&#39;s means for determining one of a limb length discrepancy value and a limb medialization value, in order to determine the limb length discrepancy and the operated limb medialization. Limb medio-lateral offset, or medialization, as used herein is broadly defined as the difference between the pre-joint dislocation and post-joint reduction positions of a limb along the medio-lateral axis. Limb length discrepancy as used herein is broadly defined as the difference between the length of the target limb and the length of the untreated limb.  
      These two measurements are computed slightly differently. The limb medio-lateral offset is calculated by determining the difference between the medio-lateral axis coordinates of the projected pre and post-operative digitized points on the treated leg. Namely: 
          limb medialization =−Z1+Z2, 
 
 where Z1 and Z2 are respectively the pre and post operative coordinates of the digitized points projected on the medio-lateral axis of the pelvic coordinate system. Therefore, the limb medio-lateral offset as calculated by the CAS system determines the difference in position, in the medio-lateral axis, of the treated hip after the total hip replacement has been performed, in comparison with its position pre-operatively. This generally provides information regarding the effect of the total hip replacement on the medio-lateral positioning of the treated leg. 
       

      While the primary concern to surgeons is generally the post-THR limb medio-lateral offset for the treated leg only, it is nevertheless possible to determine the treated limb medialization relative to the non-treated leg, rather than relative to the pre-operative treated limb medio-lateral position. However, as this may require digitization of a precisely corresponding points or bone surface landmark on both the target and non-treated leg, which may require unnecessary invasiveness on the non-treated leg, post-THR limb medialization of the treated leg relative to the non-treated leg is usually not determined. However, as per the limb length discrepancy determination, a pre-operatively taken X-ray can similarly be used to determine the position of the non-treated leg so that post-THR limb medialization values of the treated leg relative to the non-treated leg can be measured without undue invasiveness.  
      Limb length discrepancy between the treated and non-treated legs is determined as follows by the CAS system. The pre and post-operative values of the treated leg digitized landmarks projected onto the longitudinal axis, are related to the pre-operative leg length discrepancy between the treated and non-treated leg, which is measured from the pre-operative antero-posterior X-ray. Namely: 
          limb length discrepancy=ΔpreopLL−Y1+Y2, 
 
 where ΔpreopLL is the pre-operatively measured leg length discrepancy and Y1 and Y2 are respectively the coordinates of the pre and post-operative digitized points on the longitudinal axis of the pelvic coordinate system. The post-operative leg length discrepancy between the treated leg and the non-treated leg can thereby be determined by the CAS system. 
       

      The embodiments of the invention described above are intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.