Abstract:
The invention provides an apparatus and method for sizing a distal portion of a femur during knee arthroplasty. A sizer according to the invention includes a body that can be removably attached to the distal portion of the femur and that is operable to support a module that is removably attachable to the body. The body with attached module is operable for use in measuring the size of the distal portion of the femur. The module can be attached to the body either before or after the body has been attached to the distal portion of the femur and can be detached from the body either before or after the body is detached from the distal portion of the femur.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to methods and tools used in knee arthroplasty. More particularly, the invention relates to methods and tools used in knee surgery involving the installation of an artificial femoral component.  
       BACKGROUND OF THE INVENTION  
       [0002]     Total knee arthroplasty involves the replacement of portions of the patellar, femur and tibia with artificial components. In particular, a proximal portion of the tibia and a distal portion of the femur are cut away (resected) and replaced with artificial components.  
         [0003]     As used herein, when referring to bones or other body parts, the term “proximal” means closest to the heart and the term “distal” means more distant from the heart. When referring to tools and instruments, the term “proximal” means closest to the practitioner and the term “distal” means distant from the practitioner. However, when a tool or instrument is fixated to a bone or other body part the terms “proximal” and “distal” are applied to the tool or instrument as if the tool or instrument were itself a bone or body part.  
         [0004]     There are several types of knee pros theses known in the art. One type is sometimes referred to as a “resurfacing type.” In these pros theses, the articular surface of the distal femur and proximal tibia are “resurfaced” with respective metal and plastic condylar-type articular bearing components.  
         [0005]     The femoral component is typically a metallic alloy construction (e.g. cobalt-chrome alloy or 6A4V titanium alloy) and provides medial and lateral condylar bearing surfaces of multi-radii design of similar shape and geometry as the natural distal femur or femoral-side of the knee joint.  
         [0006]     One important aspect of these procedures is the correct resection of the distal femur and proximal tibia. These resections must provide planes which are correctly oriented in order to properly accept the prosthetic components. Among the factors that are considered when assessing resection of the distal femur and proximal tibia are the proximal-distal location of the resection planes, the varus-valgus angle of the planes, and the change in relative orientation of the planes in response to change in flexion-extension angle of the knee.  
         [0007]     Moreover, following distal resection the femur is shaped with the aid of a cutting block. To ensure correct shaping of the femur, the cutting block must be correctly positioned and sized. More specifically, the cutting block must be correctly positioned with respect to the anterior-posterior direction and must be correctly rotated about an axis perpendicular to the distal resection plane such that the block&#39;s rotation corresponds to the correct Internal/External (I/E) rotation of the femur relative to the tibia. The I/E rotation may be set in a number of ways. One way of setting I/E rotation is by referencing the angle formed between the cutting block&#39;s medial-lateral axis as projected onto the distal resection plane and the knee&#39;s posterior condylar axis as projected onto the distal resection plane. In a typical case, the angle formed between the cutting block&#39;s medial-lateral axis as projected onto the distal resection plane and the knee&#39;s posterior condylar axis as projected onto the distal resection plane is set to approximately 3 degrees and matches the angle formed between the epicondylar axis as projected onto the distal resection plane and the posterior condylar axis as projected onto the distal resection plane.  
         [0008]     In addition, the cutting block should be correctly positioned with respect to the medial-lateral direction. However, medal-lateral positioning of the block is not critical to the femur shaping procedure and, as such, does not require the same degree of precision as exercised during anterior-posterior positioning of the block and I/E rotation of the block.  
         [0009]     A typical cutting block includes two or more fixation pegs, or “pins” that are used for positioning the block on the distal resection plane and securing the block to the plane. In practice, the block to be used is known and thus the positions of the pins within the block are known. Therefore, one can set the block&#39;s position in space by setting the pins&#39; position in space. Accordingly, to position the block on the distal plane the appropriate pin positions are determined, pinholes are drilled at the determined positions, the pins in the block are lined up with the pinholes, and the pins are inserted into the pinholes to secure the block to the femur.  
         [0010]     In many cases, the appropriate cutting block and the correct pinhole positions are determined using an instrument referred to as an “Anterior-Posterior Sizer” (or “AP Sizer”). The Sizer is designed to determine the appropriate cutting block and correct pinhole positions based on the type and size of femoral component that will be implanted. For example, the implant could be from the line of implants associated with the Stryker® Triathlon® Knee System which includes femoral implants of sizes 1-8. In such context, the AP Sizer will determine the size of Triathlon® implant that is needed and will indicate where the pinholes should be located for a cutting block corresponding to the Triathlon® implant of the determined size.  
         [0011]     However, implants from different manufacturers or different lines may differ. Accordingly the cutting blocks and associated hole positions needed for each type of implant may differ, and therefore the AP Sizers required for each type of implant may differ. For this reason, a large number of different AP Sizers is typically required to support a large number of implant-types.  
         [0012]     Further, there are many different types of methodologies employed for determining the correct implant size and hole position. For example, implant size and hole position can be determined through use of a “mechanical stylus,” a “navigation stylus,” a “blade runner,” or “drill sizing.” The type of sizing used in a procedure is often left to the discretion of the practitioner, with most practitioners having a preference for one method over the others.  
         [0013]     For purposes of clarity of presentation, the various types of methodologies employed for determining the correct implant size and hole position will hereinafter be collectively referred to as “sizing methodologies.” 
         [0014]     Despite the many different sizing methodologies that may be employed, AP Sizers are typically designed for only one methodology. Thus, if a practitioner would like to change methodologies, the practitioner would have to change sizers. For example, a practitioner who prefers blade runner sizing may find that blade runner sizing is inaccurate on a particular patient due to patient specific conditions such as taught muscle tone or localized trauma to the desired referencing zone. In such event, the practitioner may wish to size through navigation rather than blade running. However, to make the change from blade runner sizing to navigation sizing, the practitioner would need to change from a sizer designed for blade runner sizing to a sizer designed for navigation sizing.  
         [0015]     In short, prior AP Sizers have been implant-specific and methodology-specific, and therefore large numbers of AP Sizers have been required to support the various implant types and sizing methodologies.  
       SUMMARY OF THE INVENTION  
       [0016]     In view of the desirability of an AP sizer that can efficiently support a number of different implant types and a number of different sizing methodologies, the present invention was conceived.  
         [0017]     The invention provides an apparatus and method for sizing a distal portion of a femur during knee arthroplasty. A sizer according to the invention includes a body that can be removably attached to the distal portion of the femur and that is operable to support a module that is removably attachable to the body. The body with attached module is operable for use in measuring the size of the distal portion of the femur. The module can be attached to the body either before or after the body has been attached to the distal portion of the femur and can be detached from the body either before or after the body is detached from the distal portion of the femur.  
         [0018]     In this manner, modules specific to various implant types and sizing methodologies can be provided and the sizer can be adapted to support the various types and methodologies by merely interchanging the modules. Further, modules can be changed while the sizer body is in situ, such that alternative types and/or methodologies can be employed during a procedure without the need to detach the sizer body from the femur. Moreover, the readily interchangeable modules facilitate the use of multiple sizing methodologies during a given procedure. The measurements provided according to multiple methodologies can then be used to cross-validate each other. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0019]     The following detailed description, given by way of example, but not intended to limit the invention solely to the specific embodiments described, may best be understood in conjunction with the accompanying drawings wherein like reference numerals denote like elements and parts, in which:  
         [0020]      FIGS. 1A-1F  provide an overview of how a sizer according to one embodiment of the invention can be assembled and attached to a distal portion of a femur.  
         [0021]      FIG. 2  shows an AP sizer body and three alternative sizer modules that can be removably attached to the body.  
         [0022]      FIG. 3  shows yet another alternative AP sizer module that can be removably attached to the body of  FIG. 2 .  
         [0023]      FIG. 4  is a perspective view of how module  110  of  FIG. 2  is removably attached to the body of  FIG. 2 .  
         [0024]      FIG. 5  is a perspective view of an assembled sizer including a mechanical stylus in addition to the body  100  and module  110  of  FIG. 4 .  
         [0025]      FIG. 6  is an alternative perspective view of the sizer of  FIG. 5 .  
         [0026]      FIG. 7  is a perspective view of an assembly including body  100  and module  105  of  FIG. 2 .  
         [0027]      FIG. 8  is a perspective view of an assembly including body  100  and module  115  of  FIG. 2 .  
         [0028]      FIG. 9  is a perspective view of a sizer including body  100  of  FIG. 2 , module  110  of  FIG. 2 . and a navigation stylus. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0029]      FIGS. 1A-1F  provide an overview of how a sizer according to one embodiment of the invention can be assembled and attached to a distal portion of a femur. The figures are introductory and a more detailed description of the preferred embodiments is provided in connection with  FIGS. 2-9 .  
         [0030]     Referring to  FIG. 1A , there is shown a sizer body  10  and sizer module  15 . The module is shown in position to be slidably mounted on the body. The module includes a through-hole  15   a  for mounting one or more types of styli to the module.  
         [0031]      FIG. 1B  shows a perspective view of a distal femur portion  20 . A generally anterior facing portion of the femur is indicated by reference numeral  25 , and a generally posterior facing portion of the femur is indicated by reference numeral  30 . The femur has been distally resected, and distal resection plane  35  is a product of such resection. The femur&#39;s epicondylar axis is indicated by dotted line  40 .  
         [0032]      FIG. 1C  shows how the body and module of  FIG. 1A  form an assembly  45  that can then be attached to the distal femur portion of  FIG. 1B . The assembly is attached to the distal portion by way of pinholes  50   a  and/or  50   b , through which pins (not shown) are passed to secure the assembly to the femur. Preferably, only one set of holes, either  50   a  or  50   b  is used to secure the assembly, the choice being made according to the size of the femur to which the assembly is attached.  
         [0033]      FIG. 1D  is an alternate view, from the medial side, of the assembly and distal portion of  FIG. 1C . As can be seen from  FIG. 1D , the assembly includes a base portion  55  that contacts a posterior portion of the distal femur and a proximal face  60  that contacts the distal resection plane.  
         [0034]      FIG. 1E  shows how a mechanical stylus  65  is used with the assembly of  FIGS. 1C and 1D . The mechanical stylus together with the body-module assembly makes up a complete AP sizer according to one embodiment of the invention. As can be seen from  FIG. 1E , the mechanical stylus is coupled to the assembly via a coupling rod  65   a  that is secured within through-hole  15   a . The rod is free to turn within the through-hole so that the stylus can be rotated about the rod&#39;s longitudinal axis, and is free to move in a longitudinal direction within through-hole  15   a  so that the stylus can moved in a direction parallel to the longitudinal axis of hole  15   a.    
         [0035]      FIG. 1F  is an alternate view, from the medial side, of the sizer and distal portion of  FIG. 1E . As can be seen in  FIG. 1F , the stylus includes an arm  70  that is used for locating points on the anterior cortex of the femur. The stylus also includes a handle  75  that is attached to the arm, and a stylus body  80  that is attached to rod  65   a  and includes a through-hole for accommodating the stylus arm. The handle is used to translate and rotate the arm within the stylus body&#39;s through-hole. A latch  85  is provided for locking the arm in a desired translation position and rotational orientation.  
         [0036]     Having provided an overview of one preferred embodiment of the invention, a more detailed description of preferred embodiments will be provided.  
         [0037]     Referring to  FIG. 2 , there is shown an AP sizer body  100  and three alternative sizer modules  105 ,  110  and  115 . Each of the sizer modules can be removably attached to the body. Module  105  is a module that can be used for implant sizing by either a mechanical stylus or navigation stylus inserted in through-hole  105   a , or by drill sizing via drill guide holes  105   b . Module  110  can only be used for sizing by a mechanical stylus or navigation stylus inserted in through-hole  110   a . Module  115  is a module that can be used for implant sizing by either a mechanical stylus or navigation stylus inserted in through-hole  115   a , or by blade runner sizing via blade slots  115   b.    
         [0038]     The body  100  of  FIG. 2  is made up a central part  120  and an outer part  125 . The central part includes a base  120   a  and a trunk  120   b . The outer part includes a top  125   a  and sides  125   b  and  125   c . For purposes of this description, the portions of the central and outer parts that face away from the distal portion of the femur when the body is attached to the femur will be referred to respectively as the “central part distal face” and “outer part distal face;” and the portions of central and outer parts that face toward the distal portion of the femur when the body is attached to the femur will be referred to respectively as the “central part proximal face” and “outer part proximal face.” 
         [0039]     In any event, the outer part of the body and the central part of the body can be moved relative to one another. More specifically, the outer part can be both translated relative to the central part and rotated relative to the central part. The outer part can be translated relative to the central part in a direction parallel to the central part&#39;s longitudinal axis (as depicted by line  130 ). The outer part can be rotated relative to the central part about an axis perpendicular to the central part distal face, such as an axis perpendicular to the central part distal face and passing through point  135 .  
         [0040]     Movement of the outer part of the body relative to the central part of the body is controlled by two independently operated mechanisms. Translational movement of the outer part relative to the central part is controlled by a rotating element  140 , and rotational movement of the outer part relative to the central part is controlled by a push-button  145 .  
         [0041]     To translate the outer part of the body relative to the central part, one inserts a suitably shaped instrument into a matching recess  140   a  in the rotation element, presses the element down toward the central part proximal face to unlock the element, and then rotates the instrument to rotate the element. A mechanical link causes the outer part to translate relative to the inner part when the element is rotated. Preferably, the translational movement is infinitely variable within a predetermined range. Further, the rotating element preferably includes a detent  140   b  that mates with a protrusion on the central part of the body when the translation position is in the middle of the predetermined range, such that a positive confirmation of the middle position is provided.  
         [0042]     To rotate the outer part of the body relative to the central part, one presses push-button  145  down toward the central part proximal face. The button is linked to a restraining element  150  having a multiple of teeth that mesh with teeth on the top of the outer part. When the button is pushed, the restraining element moves away from the top of the outer part (i.e. in a direction toward the base of the central part), and thereby the teeth of the restraining element are decoupled from the teeth on the top of the outer part. Once the restraining element and outer part are decoupled, the outer part is free to rotate about axis  135 . After the outer part has been rotated to the desired position, the button is allowed to return to its original position, causing the restraining element to once again mesh with the top of the outer part and thereby locking the outer part in the desired position. The meshed teeth arrangement of the restraining element and the top of the outer part preferably provide rotation in 1 degree increments. However, it should be noted that the teeth can be arranged such that the increments are other than 1 degree. Moreover, the teeth can be eliminated so as to provide an infinitely variable adjustment.  
         [0043]     Each of the modules depicted in  FIG. 2  are designed for use in left-knee arthroplasty. However, all of the embodiments detailed in this description are equally applicable in the context right-knee arthroplasty. In view of this description one skilled in the art of knee arthroplasty will readily appreciate how the embodiments detailed in this description apply in right-knee arthroplasty. For purposes of providing an example of how a right-knee embodiment is implemented,  FIG. 3  is provided.  FIG. 3  shows a module  155  that is the right-knee analogue of module  110 .  
         [0044]     In any case, each of the modules depicted in  FIGS. 2 and 3  may be removably attached to the outer part of the body. To illustrate how such attachment is achieved,  FIG. 4  is provided.  FIG. 4  is a perspective view of how module  110  of  FIG. 2  is removably attached to body  100  of  FIG. 2 . As can be seen from  FIG. 4 , the module is positioned over the top of outer part  125  and is moved downward onto the outer part, as indicated by arrow  160 . In a preferred embodiment, sides  125   b  and  125   c  of the outer part include grooves for guiding the module as it slides into its attached position. A latch  165  on the module mates with a catch  170   a  on the body to lock the module in place when the module reaches its attached position. A second catch  170   b  is provided for locking a module having a latch on the opposite side of the outer part, such as module  155  of  FIG. 3 .  
         [0045]      FIG. 5  is a perspective view of an assembled sizer  177  including a mechanical stylus  180  in addition to the body  100  and module  110  of  FIG. 4 .  FIG. 6  is an alternative perspective view of the sizer of  FIG. 5 .  
         [0046]     Referring now to  FIGS. 5 and 6 , the stylus includes an arm  185  with a curved tip  185   a  that is used for locating points on the anterior cortex of the femur. The stylus also includes a handle  190  that is attached to the arm and a stylus body  195  that is attached to a rod  195   a . Through-hole  110   a  (see  FIG. 2 ) accommodates the rod such that the stylus is free to rotate about the longitudinal axis of the rod and is free to translate longitudinally such that the stylus may move in a direction along the hole&#39;s longitudinal axis. The handle can be used to translate the arm within the stylus body&#39;s through-hole in a direction parallel to the stylus body&#39;s longitudinal axis. The handle is also used to rotate the arm about the arm&#39;s longitudinal axis. A latch  200  is provided for locking the arm in a desired translation position and in the rotation position in which the handle is in a generally aligned with the longitudinal axis of rod  195   a . The latch is spring loaded with respect to the handle by a spring  205 . To release the latch, finger pressure is applied to the latch so as to compress the spring.  
         [0047]     The sizer depicted in  FIG. 5  is similar to the sizer depicted in  FIGS. 1E and 1F . Accordingly, reference to  FIGS. 1E and 1F  is helpful in understanding how the sizer of  FIG. 5  is employed to size a femur.  
         [0048]     Notably, the orientation of handle  180  of the  FIG. 5  sizer is rotated 180 degrees relative to the orientation of handle  75  of the  FIG. 1E / 1 F sizer. Orienting the handle as shown in  FIG. 5  facilitates manipulation of the stylus during knee arthroplasty by positioning the handle away from the incision area.  
         [0049]     The sizer of  FIG. 5  is used to determine correct implant size and cutting block pin position by referencing the posterior condyles of the distal femur. More specifically, the sizer is aligned with the posterior condylar axis through two skids  210   a  and  210   b  located on the base of the sizer body. The skids are positioned to contact the posterior condyles while the body is centered, or approximately centered, on the femur with respect to the medial-lateral direction. An example of such positioning is seen in  FIGS. 1E and 1F .  
         [0050]     Once the body is properly positioned, pins can be passed through either pair of pinholes  220   a  or  220   b , or through both pairs of pinholes  220   a  and  220   b , to secure the body to the femur. In a preferred embodiment, the body  100 , module  110 , and stylus  180  are removably attached to each other to form a complete assembly, and then the complete assembly is attached to the femur via the pinholes.  
         [0051]     In another preferred embodiment, the body of the sizer is first attached to the distal resection plane and the appropriate module is mounted on the body after the body has been attached. Accordingly, it is not necessary that a module be attached to the body during attachment of the body to the femur.  
         [0052]     In still another preferred embodiment, the body is first attached to the femur, and the module and stylus are then attached to each other prior to attaching the module and stylus to the body.  
         [0053]     In yet another preferred embodiment, the stylus is not removably attached to the module. Rather, the stylus and module are fixed to each other such that they form a permanent sub-assembly. In such an embodiment, the preferred method of use is to attach the body to the femur apart from the module/stylus sub-assembly, and then removably attach the module/stylus sub-assembly to the body.  
         [0054]     In any event, once the complete sizer is correctly positioned on the femur, the internal-external rotation of the implant is set by setting the internal-external rotation of the sizer. In this regard, it is important to note that the sizer module includes two drill guide holes  225   a  and  225   b , which relate to a cutting block type which, in turn relates to a type of implant. Upon final setting of the sizer, holes are drilled in the femur at positions determined by the guide holes. Thus, the sizer setting determines the guide hole positions, which determines the cutting block position which, in turn, determines the implant position. Therefore, by setting the internal-external rotation of the sizer, the internal-external rotation of the implant is being set.  
         [0055]     To set the internal-external rotation of the sizer, the outer part of the sizer body is rotated relative to the central part of the sizer body. Since the central part of the sizer body is fixed relative to the posterior condylar axis, and both the central and outer parts of the sizer body are fixed relative to the distal resection plane, the rotation of the outer part of the body relative to the center part has the effect of changing the inclination between the posterior condylar axis as projected onto the distal resection plane and an imaginary line connecting the drill guide holes as projected onto the distal resection plane. The change in magnitude of such inclination is equal to the internal-external rotation.  
         [0056]     The outer part of the body is rotated by depressing push-button  145 , moving the outer part to the desired position, and then releasing the push button to lock the outer part in place. The degree of internal-external rotation is read from a scale  230  located at the top of the outer part. The scale is referenced to a mark  235  on the central part of the sizer body.  
         [0057]     After the internal-external rotation of the implant is set, the stylus can be used to size the femur. That is, the stylus can be used to determine the appropriate size implant needed for the subject femur. To size the femur, a practitioner manipulates the stylus handle such that the curved tip  185   a  of the stylus contacts the anterior cortex of the femur at the point where the anterior-superior point of the implant should contact the anterior cortex of the femur (the “desired run-out point”). Once the tip is contacting the desired run-out point, the practitioner reads the size from an anterior-posterior sizing scale (“AP scale”)  240  located on the distal face of the module. The reading is taken by comparing a lip  245  on the stylus body to the AP scale. For example, if the lip is pointed toward the number “3” of the AP scale, then the femur size is a “3” and the implant needed is a size “3.” 
         [0058]     A further indication of implant size is provided by a superior-inferior run-out scale (“SI scale”)  250 . The SI scale is associated with a multiple of notches  255  that are etched into the stylus body, each notch being associated with a corresponding size. When the curved tip of the stylus has been located at the desired run-out point, the latch is allowed to settle into the notch that most closely corresponds to the stylus position. The number associated with the notch into which the latch settles is the femur/implant size as measured by the SI scale. For example, if the latch sits in the notch corresponding to the number “3” of the SI scale, then the SI scale indicates a femur/implant size of “3.” 
         [0059]     If the size as measured by the SI scale does not match the size as measured by the AP scale, it is an indication that the anterior portion of the implant will not run-out of the femur at the desired run-out point. In other words, it is an indication that the anterior portion of the implant will not mate with the anterior portion of the resected femur at the ideal position. In common terminology, a mismatch in size measurements is an indication that the implant may “notch” or “overhang”.  
         [0060]     The sizer provides a mechanism for adjusting position of the implant to avoid “notching” and “overhanging.” More particularly, when the size as measured by the SI scale does not match the size as measured by the AP scale, the practitioner positions the stylus such that the latch settles into a notch  255  corresponding to size as measured by the AP scale. In this condition, with the stylus positioned such that the lip is at the measured AP size and the curved tip is over the desired run-out point, the practitioner can observe the degree to which the implant will “notch” or “overhang.” If the curved tip lies above the desired run-out point when the AP and SI scales read the same size, the distance between the tip and the desired run-out point indicates the “overhang” magnitude. If, the stylus can not be moved far enough down the AP scale due to premature contact of the femur and curved tip, the distance between the point on the scale where the stylus stops and the point corresponding to the measured AP size indicates the “notch” magnitude.  
         [0061]     It should be noted that it is the sizer geometry that makes run-out estimations possible. More particularly, the sizer is configured such that the plane within which the stylus arm  185  moves when the stylus is rotated about the longitudinal axis of rod  195   a  is parallel to the plane of the anterior resection. Thus, translation of the stylus arm along its longitudinal axis reflects translation of the plane of the anterior resection.  
         [0062]     The plane within which the stylus arm moves when the stylus is rotated about the longitudinal axis of rod  195   a  is determined by the inclination of through-hole  110   a  relative to the distal resection plane. When the stylus is positioned in the through-hole, the longitudinal axis of rod  195   a  is oriented at the same inclination as the through-hole. In this manner, the angle formed between the longitudinal axis of rod  195   a  and the distal resection plane determines the plane in which the stylus arm moves when rotated about the longitudinal axis of the rod. Accordingly, the inclination of through-hole  110   a  relative to the distal resection plane is set such that the plane within which the stylus arm moves is parallel to the plane of the anterior resection.  
         [0063]     In any event, when a “notch” or “overhang” is indicated, a correction can be made by shifting the implant. That is, for a given size implant, the implant can be shifted such that it properly mates with the desired run-out point. This is done using rotating element  140  to translate the outer part of the sizer body relative to the central part. For example, in an “overhanging” situation the rotating element is used to move the outer part in a generally posterior direction which, in turn, moves the module in the generally posterior direction, and thus moves the drill guide holes  225   a  and  225   b  in the generally posterior direction. Since the hole position corresponds to the cutting block and implant position, movement of the drill guide holes in a generally posterior direction results in corresponding movement of the implant in the generally posterior direction. In this manner, the run-out point for a given size implant can be adjusted to correct for an “overhanging” situation.  
         [0064]     In a similar manner, a shift of implant position can be made to correct a “notching” situation. To correct for a “notching” situation, the rotating element is used to move the outer part in a generally anterior direction which, in turn, moves the implant in the generally anterior direction.  
         [0065]     The sizer of  FIGS. 5 and 6  offers several advantages when used during minimally invasive surgery (MIS). One advantage is that the stylus  180 , through-hole  110   a , and AP scale  240  are positioned on the same side of the sizer so that the bulk at the other side of the sizer is minimized. Another advantage is that the stylus can rotate about the longitudinal axis of the stylus arm. Such rotation allows the curved tip  185   a  to be rotated for easy insertion under the soft tissue, and then rotated back into position to contact the desired run-out point. Thus, the stylus arm can be more easily maneuvered about the soft tissue, and therefore disturbance of the soft tissue is minimized.  
         [0066]     Turning now to  FIG. 7 , there is shown a perspective view of an assembly including body  100  and module  105  of  FIG. 2 . The assembly is an AP sizer  300  in accordance with an embodiment of the invention. The sizer  300  can be used to implement sizing through a mechanical stylus or through drill sizing.  
         [0067]     Sizer  300  is the same as sizer  177  of  FIGS. 5 and 6  with one exception, the module of  FIG. 7  includes the multiple drill guide holes  105   b . Each hole is associated with a femur/implant size, and to this end the drill guide holes are marked with sizes to clearly indicate the relationship between each hole and its respective size. For example, as seen in  FIG. 7 , there are eight drill guide holes and markings “1,” “3,” “5,” “7” and “8” are included on the module so that the association between holes and femur/implant sizes is clear. The holes corresponding to sizes “2,” “4” and “6” are clear from the markings for holes “1,” “3,” “5,” “7” and “8.” The sizes 1-8 may be the sizes associated, for example, with the Triathlon® line of implants.  
         [0068]     When sizer  300  is used for drill sizing, the procedures of attaching the sizer body to the femur, attaching the module to the body, and setting the I/E rotation are the same as for sizer  177 . However, once the sizer is attached to the femur and rotation has been set size is measured by drilling holes in the femur rather than through use of a stylus. More specifically, holes are drilled in the femur while using drill guide holes  105   b  to guide the drill bit. In this manner, the drill bit enters the femur through the distal resection plane. The bit may or may not exit the femur at the anterior cortex.  
         [0069]     The longitudinal axes of the drill guide holes  105   b  are inclined relative to a plane perpendicular to the distal resection plane such that the inclination of the holes matches the inclination of the anterior resection. Thus, when the drill bit passes into the bone, it follows a path that corresponds to the anterior resection. Therefore, the point where the drill bit exits the anterior cortex—or would exit the anterior cortex if it were long enough—is the point where the anterior resection will meet the anterior cortex of the femur. Ideally, the point of exit is the desired run-out point. To this end, the practitioner picks a drill guide hole that is likely to result in an exit point anterior to the desired run-out point and drills a hole using the selected guide. The practitioner observes the exit point and determines whether a better result can be achieved by selecting a different guide hole and drilling again, at all times being careful not to choose a hole that will result in an exit point posterior to the desired run-out point. If the practitioner believes a better result can be achieved by using a different guide hole, the practitioner drills again using the different guide hole to guide the drill bit. When the practitioner believes that further drilling can not achieve an exit point any closer to the desired run-out point than the last exit point, drill sizing is complete. The determined size of the femur/implant is read from the corresponding drill guide hole. The determined size is the size associated with the guide hole that was used to drill the best hole.  
         [0070]     As in the embodiment of  FIGS. 5 and 6 , in the embodiment of  FIG. 7 , the rotating element  140  can be used to shift the implant so as to correct for a “notching” or “overhanging” situation. Thus, if the exit point of the best drilled hole is posterior to the desired run-out point the rotating element can be used to move the outer part of the sizer body in a generally anterior direction which, in turn, moves the implant in the generally anterior direction. If the exit point of the best drilled hole is anterior to the desired run-out point the rotating element can be used to move the outer part of the sizer body in a generally posterior direction which, in turn, moves the implant in the generally posterior direction.  
         [0071]     When the sizer of  FIG. 7  is used for sizing with a mechanical stylus, sizing is performed in the same manner as discussed in connection with  FIGS. 5 and 6 , with the stylus being removably attached to module  105  via through-hole  105   a.    
         [0072]      FIG. 8  is a perspective view of an assembly including body  100  and module  115  of  FIG. 2 . The assembly is an AP sizer  400  in accordance with an embodiment of the invention. The sizer  400  can be used to implement sizing through a mechanical stylus or through blade runner sizing. Sizer  400  is the same as sizer  300  of  FIG. 7  with one exception, the module of  FIG. 8  includes multiple blade slots  115   b . Each slot is associated with a femur/implant size, and to this end the slots are marked with sizes to clearly indicate the relationship between each slot and its respective size. For example, as seen in  FIG. 8 , there are eight slots and markings “1” through “8” are included on the module so that the association between the slots and femur/implant sizes is clear. The sizes “1-8” may be the sizes associated, for example, with the Triathlon® line of implants.  
         [0073]     When sizer  400  is used for blade runner sizing, the procedures of attaching the sizer body to the femur, attaching the module to the body, and setting the I/E rotation are the same as for sizers  177  and  300 . However, once the sizer is attached to the femur and the rotation has been set, size is measured by inserting a blade into one of the blade slots rather than through use of a stylus. More specifically, the practitioner places the blade in a slot that he believes to be co-planar with the correct plane for the anterior resection.  
         [0074]     The planes of the slots are parallel to each other and are inclined relative to a plane perpendicular to the distal resection plane such that the inclination of the slots matches the inclination of the anterior resection. Thus, when the blade is sitting in a slot, it lies along a path that corresponds to the anterior resection. Therefore, by observing the plane of the blade one can visualize where the plane of the blade intersects the anterior cortex of the femur. Accordingly, the practitioner picks a slot that he believes will result in a resection plane that intersects the anterior cortex as the desired run-out point and inserts the sizing blade. The practitioner observes the intersection of the blade plane and the anterior cortex and adjusts the slot selection according to the result. When the practitioner believes that the blade is in the slot that corresponds to an intersection point closest to the desired run-out point and that no other slot can provide a better intersection point, blade runner sizing is complete. The determined size of the femur/implant is read from the slot&#39;s associated numeral. The size of the femur/implant is the size associated with the slot that provides the best intersection point.  
         [0075]     As in the embodiments of  FIGS. 5, 6  and  7 , in the embodiment of  FIG. 8  the rotating element  140  can be used to shift the implant so as to correct for a “notching” or “overhanging” situation. Thus, if the slot chosen in blade runner sizing indicates an intersection point anterior to the desired run-out point the rotating element can be used to move the outer part of the sizer body in a generally posterior direction which, in turn, moves the implant in the generally posterior direction. If the slot chosen during blade runner sizing indicates an intersection point posterior to the desired run-out point the rotating element can be used to move the outer part of the sizer body in a generally anterior direction which, in turn, moves the implant in the generally anterior direction.  
         [0076]     When the sizer of  FIG. 8  is used for sizing with a mechanical stylus, sizing is performed in the same manner as discussed in connection with  FIGS. 5, 6  and  7  with the stylus being removably attached to module  115  via through-hole  115   a.    
         [0077]      FIG. 9  is a perspective view of a sizer including body  100  of  FIG. 2 , module  110  of  FIG. 2 , and a navigation stylus  260 . The navigation stylus includes a head  260   a  to which a navigation tracker can be attached, a lip  260   b  which is used with scale  240  to obtain size readings, and a shaft  260   c  which is accommodated within though-hole  110   a . The longitudinal axis of the shaft is perpendicular to the plane defined by lip  260   b.    
         [0078]     The navigation tracker used with stylus  260  communicates with a navigation camera system. The communications between the tracker and camera system are used by a computer to generate a virtual three-dimensional Cartesian coordinate system (x-axis, y-axis, and z-axis) that is fixed in space relative to the tracker. Any movement of the tracker results in corresponding movement of the three-dimensional coordinate system. Further, the three dimensional coordinate system is established so that one of the planes defining the system (the x-y plane) is parallel to the plane defined by lip  260   b  and so that the coordinate axis perpendicular to such plane is parallel to the longitudinal axis of the stylus shaft. Thus, once the sizer is set to the correct I/E rotation, the x-y plane and lip plane are parallel to the plane of the anterior resection.  
         [0079]     When using sizer  500  to size a femur/implant, the procedures of attaching the sizer body to the femur and attaching the module to the body are the same as for sizers  177 ,  300  and  400 . The procedure of setting the I/E rotation may be the same as for sizers  177 ,  300  and  400 , though the I/E rotation may be alternatively set through navigation.  
         [0080]     In any event, the practitioner uses a hand-held tracker to map one or more femoral landmarks to the coordinate system associated with the navigation stylus. That is, a second tracker is used to determine the position of one or more femoral landmarks relative to the coordinate system associated with the navigation stylus.  
         [0081]     Once the sizer has been attached to the femur and the landmark(s) have been established, the stylus can be translated within through-hole  110   a  to size the femur. As the stylus is translated within through-hole  110   a , the computer compares the position of the x-y plane to the femoral landmark(s). When the stylus is positioned such that the plane of the anterior resection—as reflected by the x-y plane position—intersects the desired run-out point, the sizing is complete. The size reading is taken by observing the position of lip  260   b  relative to the AP scale. For example, if the lip is pointed toward the number “3” of the AP scale, then the femur/implant size is “3.” 
         [0082]     As in the embodiments of  FIGS. 5, 6 ,  7  and  8 , in the embodiment of  FIG. 9  the rotating element  140  can be used to shift the implant so as to correct for a “notching” or “overhanging” situation.  
         [0083]     As these and other variations and combinations of the features discussed above can be utilized without departing from the present invention as defined by the claims, the foregoing description of the preferred embodiments should be taken by way of illustration rather than by way of limitation of the invention as defined by the claims.