Abstract:
An instrument for setting ligament tension between a distal femur and a proximal tibia has a rod mounted on a femur. The rod has an extension extending along a first axis. A first drive element is mounted on the distal femur and is rotatable about a second axis. A body is rotatably mounted on the rod extension for rotation about the first axis. The body has a second drive element engaging the first drive element and a third drive element extending about a third axis offset from the first and second axes. A ligament tensioning element is mounted on the body and has a foot for engaging a posterior condyle of a femur and has a fourth drive element. A fifth drive element engages both the third and fourth drive elements for moving the ligament balancer with respect to the body along the third axis.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to the implant of prosthetic joints and pertains, more specifically, to the preparation of the distal femur for the implant of a femoral knee prosthesis, utilizing a femoral guide, such as a femoral sizing guide for the subsequent location of a femoral cutting guide, to assist in establishing the surfaces necessary for locating and securing the prosthesis in place on the femur. More particularly, the invention relates to an instrument that allows the assessment of the balance of the knee ligaments by using a trial femoral condylar portion having a constant radius and an internal/external rotation adjustment mechanism as well as an anterior-posterior adjustment mechanism. 
         [0002]    The implant of a prosthetic knee joint requires that the distal femur be prepared to receive the femoral component of the knee prosthesis by cutting the bone of the femur to establish accurately located surfaces against which the femoral knee prosthesis will rest upon implantation of the femoral component. Various guides are available to the surgeon for assisting in guiding a saw blade during use of the saw blade to make the femoral cuts which establish the desired surfaces. These guides usually are located and secured on the distal femur, often upon a transverse surface established initially at the distal femur, to provide guide surfaces for guiding the saw blade during the execution of an axially directed anterior femoral cut, an axially directed posterior femoral cut, an anterior chamfer and a posterior chamfer, all specifically related to the size of the femoral knee prosthesis to be implanted and to the position and orientation of the femoral knee prosthesis at the site of the implant. The appropriate location of a femoral cutting guide, then, generally requires the use of a femoral sizing guide to determine the size of the femoral knee prosthesis which will be implanted at an implant site in a particular recipient, and to locate the corresponding femoral cutting guide appropriately on the transverse distal femoral surface for proper placement of the femoral knee prosthesis upon implant at the implant site. 
         [0003]    Femoral knee prostheses are made available in a range of standard sizes. A femoral sizing guide is used to assist in the selection of a standard size femoral knee prosthesis which will best fit the requirements of a particular implant site. Once selected, the femoral knee prosthesis must be located and oriented so as to attain appropriate rotational alignment and create a symmetric flexion gap. 
         [0004]    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 or constant radius design of similar shape and geometry as the natural distal femur. 
         [0005]    One important aspect of the known component implantation 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. 
         [0006]    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. 
         [0007]    In addition, the cutting block should be correctly positioned with respect to the medial-lateral direction. However, medial-lateral positioning of the block is not as 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. 
         [0008]    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. 
         [0009]    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. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    The instrument of the present invention allows the assessment of the balance of the ligaments by means of a single radius on the external geometry of the tensioning element. The design of the instrument allows internal/external rotation of the device to be adjusted by means of an Allen key in combination with a gear mechanism. The device allows the anterior-posterior (AP) adjustment of the instrument by means of a hexagonal key in combination with an Acme Screw. 
         [0011]    Once the desired position has been achieved, the instrument can be secured to the femur. A plate is positioned on the proximal tibia, and a trial reduction is performed. This is done by means of the single radius on the periphery of the instrument. The surgeon assesses stability throughout the complete range of motion. If required, the surgeon has the ability to open screws, make further adjustments (I/E rotation and/or AP Adjustment), secure the instrument in place again, and assess the stability again. Once the desired position of the instrument has been achieved, the instrument can be finally secured in position and the pin or peg holes drilled. The device also allows for the AP sizing of the femur for the correct implant size. 
         [0012]    In the surgical method for using the instrument of the present invention, the surgeon inserts an intramedullary (IM) rod into the hole drilled in the femoral canal. The IM rod has a portion protruding distally from the resected distal femur. The surgeon then assembles distal plate on to the distal femur over the IM rod. The surgeon sets the distal plate at the approximate internal/external rotation position. The plate is then secured to the distal femur by means of two bone screws. The main body of the instrument is then placed over distal plate. The medial and lateral posterior feet on the main body are placed under the posterior condyles. The main body is secured to the distal plate by means of two screws. These two screws secure the main body to the distal plate, but still allow the main body to move in an AP direction by means of an adjustment screw. The surgeon performs a trial reduction with the instrument in place. 
         [0013]    Trial reduction takes place by means of the offset single radius on the outer periphery of the posterior feet of the instrument main body device. The surgeon places tibial shim plate (2 mm or 4 mm thick) on the proximal tibia. This plate acts as a protective plate for the resected bone on the tibia, and also as a firm surface for rotation of the outer profile on the main body. The surgeon puts the knee through a range of motion and assesses stability throughout the complete range. The surgeon has multiple thickness spacer blocks which allow the surgeon to assess the gap (if any) on each condyle. 
         [0014]    If the surgeon establishes that adjustment is required, then the following steps can be taken. If internal/external rotation is required, then the surgeon opens the two bone screws on the distal plate and places a hex key into centre screw mechanism and rotates the distal plate until the desired position has been obtained. The surgeon can secure the distal plate at this stage if desired. However, he can if he wishes adjust the AP position by means of the actuating screw. This is adjusted by means of a hex key placed onto the screw. Once the desired position has been achieved, then distal plate is secured to the femur. 
         [0015]    The surgeon begins the process of assessing the resection levels and femoral component size. The resection level is assessed by means of a blade runner placed into the slots on the posterior side of the instrument. Two AP sizing stylus receiving holes exist on the medial and lateral sides of the main body. The femoral stylus is inserted into one of the holes. The stylus works by placing a tip of the stylus on the anterior cortex of the femur and an indicator line on the rod, indicates the size of femur on the main body by means of a scale. 
         [0016]    Finally the surgeon drills two holes on the distal femur which ultimately position a standard 4-in-1 cutting guides in the correct position, which cutting guide directs an oscillating saw to make the standard anterior and posterior skim cuts and anterior and posterior chamfer cuts on the distal femur. 
         [0017]    The above described instrument and method of use is accomplished by an instrument system for setting ligament tension between a distal femur and a proximal tibia during total knee arthroplasty. The instrument has an IM rod for mounting in an intramedullary canal of a distal femur, the rod having a portion extending outwardly of the distal femur along a first axis. A first drive elements is mounted on the rod portion extending from the distal femur and is rotatable about a second axis parallel to and offset from the first axis. A body is rotatably mounted on the extension portion of the rod for rotation about the first axis. The body has a second drive element engaging the first drive element whereby rotation of the first drive element about the second axis rotates the body with respect to the first axis. The body further comprising a third drive element extending about a third axis offset from the first and second axis. A ligament tensioning element is mounted on the body for movement with respect thereto. The ligament tensioning element has a foot for engaging a posterior condyle of a femur and having a fourth drive element extending generally parallel to and spaced from the third drive element; and a fifth drive element engaging both the third and fourth drive elements for moving the ligament tensioning element with respect to the body along the third axis. 
         [0018]    The first drive element is a gear having teeth meshing with teeth on the second drive element. The third and fourth drive elements are a plurality of teeth and the fifth drive element is a screw operatively engaging the teeth. The ligament tensioning element has a first foot for engaging a posterior surface of a medial condyle and a second foot for engaging a lateral condyle of a femur. The first and second feet have outer surfaces for contacting a tibial surface, the outer surfaces having a portion with a constant radius in a saggital plane. The body includes first and second elongated slots elongated along an axis parallel to the third axis. 
         [0019]    The instrument system as set forth in claim  6 , wherein the system further comprises a pair of screws extending through the first and second slots of the body for mounting the body to a planar surface on the distal femur. The body also includes a first threaded bore on a medial side of the body and a second threaded bore on a lateral side of the body. The ligament tensioning element has first and second elongate openings respectively aligned with the first and second threaded bores in the body, and the system further comprises first and second screws extending respectively through the first and second slots and threaded into the first and second bores for mounting the ligament tensioning element to the body. 
         [0020]    The elongated slots have elongated portions extending along axes parallel to the third axis. 
         [0021]    The instrument further comprises a femoral sizing module mounted on the ligament balancer, the femoral sizing module moveable in an anterior-posterior direction with respect to the balancer and stylus slidably mounted on the module for movement in a proximal distal direction and the stylus having a tip for contacting an anterior surface of the distal femur. 
         [0022]    Another aspect of the invention includes an instrument system for setting ligament tension between a distal femur and a proximal tibia during total knee arthroplasty. The instrument comprises a plate which is rotatably mounted on a resected planar distal surface of a femur for rotation about a first axis. A ligament tensioning element is mounted on the plate for rotation therewith about the first axis and for movement with respect to the plate in a plane parallel to the resected planar surface of the femur. The plate has a foot or feet for engaging a posterior surface of the distal femur. A device is provided for rotating the plate and ligament tension about the first axis. A system is provided for moving the tensioning element with respect to the plate in the plane in a generally anterior-posterior direction of the distal femur. Elements for locking the ligament tensioning element in a desired location with respect to the distal femur. 
         [0023]    The instrument further comprises an intramedullary rod having a first portion aligned with a longitudinal axis of the femur and a second portion angled with respect to the first portion extending along the first axis. 
         [0024]    The angle between the first and second rod portions, i.e., the longitudinal axis of the femur and the first axis is between 3° and 6°. The second portion comprises anti-rotation elements extending radically outwardly from the first axis. The system for rotating the plate and ligament about the first axis comprises a gear body having a bore engaging the second portion of the rod. The bore has a central axis coaxial with the first axis and a gear element rotatable with respect to the gear body about a third axis parallel to the first axis. The gear element having teeth operatively engaging a portion of the plate having teeth. 
         [0025]    The plate includes an opening for receiving the gear body, this opening sized to permit the plate to rotate ±5° about a saggital plane bisecting the resected distal femur. 
         [0026]    As used herein when referring to bones or other parts of the body, the term “proximal” means close to the heart and the term “distal” means more distant from the heart. The term “inferior” means toward the feet and the term “superior” means toward the head. The term “anterior” means toward the front part or the face and the term “posterior” means toward the back of the body. The term “medial” means toward the midline of the body and the term “lateral” means away from the midline of the body. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  is a perspective view of a femur and a tibia after having their respective distal and proximal surfaces resected to form planar cuts; 
           [0028]      FIG. 2  is a perspective view of an intramedullary rod element for insertion into the intramedullary canal of the resected femur of  FIG. 1 ; 
           [0029]      FIG. 3  shows the intramedullary rod element of  FIG. 2  inserted in the distal femur of  FIG. 1 ; 
           [0030]      FIG. 4  shows a distal drive plate assembly mounted on a portion of the intramedullary rod element of  FIG. 2 , which extends beyond the distal surface shown in  FIG. 1 ; 
           [0031]      FIG. 4   a  is a detail of a modular bushing prior to being assembled to the plate of  FIG. 4 ; 
           [0032]      FIG. 5  is an enlarged perspective view of the drive plate shown in  FIG. 4 ; 
           [0033]      FIG. 6  is a perspective view of the drive plate shown in  FIG. 4 , including two guide screws mounted in slots of the drive element; 
           [0034]      FIG. 7  is a perspective view of a ligament tensioning element mounted on the distal drive plate; 
           [0035]      FIG. 8  shows the ligament tensioning element of  FIG. 7 , including two mounting screws engaging the distal plate but allowing anterior-posterior movement of the ligament tensioning element with the knee joint in the flex position; 
           [0036]      FIG. 9  shows the assembly of  FIG. 8 , including a drive tool for bearing the internal external rotation of the distal body with respect to the femur and including a pair of tibial shim plates engaging the outer surfaces of the medial and lateral condylar portions of the ligament tensioning element; 
           [0037]      FIG. 10  is identical to  FIG. 9  with the exception that the tool for changing the internal external rotation of the instrument is removed and replaced by a second tool for adjusting the anterior-posterior location of the ligament tensioning instrument on the distal femur; 
           [0038]      FIG. 10   a  shows a rear view of the ligament tensioning element of  FIG. 10  including an ACME screw mounted therein; 
           [0039]      FIG. 11  is the ligament tensioning element of  FIG. 10  in the position ready for ligament assessment; 
           [0040]      FIG. 12  is a perspective view of a ligament tensioning element including a mechanical stylus mounted on the ligament tensioning element and the stylus having a tip engaging the anterior femur; 
           [0041]      FIG. 13  shows the ligament tensioning element in its final position with a pair of cutting slots respectively aligned with the medial and lateral posterior condyles; 
           [0042]      FIG. 14  is a view of the distally facing surface of an alternate main body of a gap balancing or tensioning instrument; 
           [0043]      FIG. 15  is a view of the main body of the alternate gap balancing or tensioning instrument as shown in  FIG. 1  showing the proximally facing side; and 
           [0044]      FIG. 16  is a partial view of the mechanism which moves the alternate body of the gap balancing and tensioning instrument in the anterior-posterior direction and rotates the body in internal/external rotation. 
       
    
    
     DETAILED DESCRIPTION 
       [0045]    Referring to  FIG. 1 , there is shown a perspective view of a femur  12  and a tibia  14 . Femur  12  includes two planar resected distal condylar surfaces  16  and  18  respectively. Tibia  14  includes a proximal planar cut surface  20  which will eventually receive a prosthetic tibial component (not shown). 
         [0046]    Referring to  FIG. 2  there is shown an intramedullary rod  22  having a tip  24  for insertion into the femoral medullary canal and having a distal end  26  including three fins  28  preferably located at 120° increments about the outer circumference of distal end  26 . Fins  28  are blade-like in form and will prevent rotation of rod  22  within the intramedullary canal. Distal end  26  includes a mounting rod  30 , which, when rod  22  is placed in the canal, will extend distally beyond the planes of surfaces  16  and  18 . The intramedullary rod  22  extends along an axis  32  with projection  30  extending along an axis  34 . Preferably there is an angle of between 3 and 6° between axes  32  and  34  to compensate for the angle between the anatomic axis of the femur and the mechanical axis of the femur.  FIG. 3  shows intramedullary rod  22  of  FIG. 2  mounted in femur  12 . As shown, fins  28  may be either internally formed on end  26  of rod  22  or may be removably mounted thereon. 
         [0047]    Referring to  FIGS. 4 ,  4   a  and  5 , there is shown distal plate  40  mounted on surfaces  16  and  18  in femur  12 . Plate  40  includes a medial and a lateral slot  42  and a medial and lateral threaded mounting hole  44 , which will receive ligament tensioning body mounting screws shown in  FIGS. 8-13 . Distal plate assembly  40  includes a central bore  50 , which surrounds a bushing  52  for receiving end  30  of intramedullary rod  22 . Bushing  52  includes a bore  74  for receiving end  30  of rod  22 . 
         [0048]    Bushing  52  is mounted within an opening in a square or rectangular boss  54  of distal plate assembly  40 . Boss  54  has a series of rack gear tooth elements  56  formed on a medially facing side  58  of boss  54 . As discussed below teeth  56  engage an ACME screw for adjusting the anterior-posterior position of a tensioning element body. Distal plate assembly  40  includes a posteriorally extending portion  60  which includes a curved gear element  62  adapted to engage a spur gear  64  mounted on an extension  66  of bushing  52 , best seen in  FIG. 5 , on which spur gear  64  is freely rotatable.  FIG. 4   a  shows bushing  52  prior to assembly on projection  30  and in boss  54 . Typically bushing  52  is mounted on end  30  of rod  22  prior to mounting the distal plate assembly  40  on the distal femur. At assembly, spur gear  64  is slightly rotated such that the teeth thereof will mesh with the curved teeth elements  62  integrally formed on the distal plate assembly  40 . Spur gear  64  includes a drive socket  70  which, in the preferred embodiment, is hexagonal in shape to receive a normal Allen wrench for rotating gear  64 . 
         [0049]    As shown in  FIG. 5 , slots  42  may have a kidney shape with a convex surface  72  facing outwardly from an axis through the center of the bore  74  in bushing  50 . 
         [0050]    Referring to  FIG. 6 , there is shown the distal plate assembly  40 , including a pair of screws  80  about which distal plate assembly may pivot upon rotation of spur gear  64 . The kidney shape of slots  42  allows the distal plate  40  to rotate about partially inserted screws  80 . This movement sets the internal/external rotation of the plate  40 . Screws  80  can then be fully tightened to lock the plate in place on the distal femur. 
         [0051]    Referring to  FIG. 7 , there is shown a main body  100  of the gap balancing or tensioning instrument mounted on the distal plate  40 . The main body  100  is shown in  FIGS. 7-13 . The main body includes a generally rectangular central opening  102  adapted to receive boss  54  of plate  40 . Boss  54  and rectangular opening  102  are sized such that the main body may slide relative to the distal plate in the anterior-posterior direction. Main body  100  includes a pair of slots  104  both medially and laterally of the center of opening  102 . Slots  104  receive mounting screws threaded into bores  44 . A pair of slots  106  is also included to allow access to screws  80  located within slots  42  of distal plate  40 . Also included on body  100  are medial and lateral trial condylar elements or feet  108  and  110  respectively. Each condylar element  108  and  110  includes a slot  112  extending partially therethrough from the outer edges of body  100 . Preferably, the outer curved surfaces of condylar portions  108  and  110  have a constant radius from the area of slots  112  to adjacent ends  114  of each condylar portion  108 ,  110 . This radius matches the constant radius on the prosthetic femoral component to be implanted. Main body  100  includes hole  116  to guide a drill for producing bores in the distal femur to locate the 4 in 1 cutting block for making the anterior and posterior chamfer and skin bone cuts. Main body  100  also includes holes  116   a , preferably on both the lateral and medial sides of the main body to accommodate bone pins to allow the surgeon to secure the body ( 100 ) to the distal femur during femoral preparation. The main body additionally includes bores  118  for mounting body  100  to the distal femur. 
         [0052]    As best seen in  FIG. 8  prior to mounting the main body  100  onto the femur, screws  120  extend through slots  104  and into threaded bores  44  of distal plate assembly  40 . In the preferred embodiment, the slots  104  include a recessed surface  105  to thereby form a ledge around the circumference of each slot. Screws  120  includes heads  122 , which have a head with a bottom surface able to slide on ledge  105  when main body  100  is moved in the anterior-posterior direction. Heads  122  are sized such that they engage a peripheral wall  107  of the slot  104  between surface  105  and the outwardly facing surface  125  of main body  100 . This helps ensure that the main body does not rotate with respect to distal plate  40  during the anterior-posterior movement which is accomplished in a manner as described below. 
         [0053]    Referring to  FIG. 9 , there is shown the assembly of  FIG. 8  including a drive tool  130  engaging socket  70  of spur gear  64 . As indicated above, rotation of tool  130  rotates the distal plate  40  about portion  30  of intramedullary shaft  22 . This movement sets the internal-external rotation of the instrument on the distal femur. Also shown in  FIG. 9  are two shim plates  132 ,  134  located on the proximal tibial surface  20  of tibia  14 . The two shim plates  132  and  134  provide a surface for the radius to articulate on in an easy manner. In the preferred embodiment, plate  132  is 2 mm and plate  134  is 4 mm such that 2, 4, and 6 mm adjustments can be possible. In use, it makes no difference whether the 2 or 4 mm plates are located on tibial surface  20  when a 6 mm correction is desired. 
         [0054]    Referring to  FIGS. 10 and 10   a , there is shown the assembly of  FIG. 9 , including a second drive tool  140  which drives an ACME screw  143  mounted in body  100 . Screw  143  is freely rotably mounted within a partial bore  109   a  in body  100  and has a drive element at an anterior end thereof open to bore  109  for engaging drive tool  140 . Screw  143  has a thread for engaging the gear teeth  56  on boss  54 . Tool  140  extends into bore  109  in anterior facing surface  111  of main body  100  to engage screw  143 . Also shown in  FIGS. 7-11  are a pair of medial and lateral blind bores  142  which extend from an anterior facing bottom surface  144  internally of the main body in a posterior direction. A surface  146  extending between bottom surface  144  and anterior surface  111  faces outwardly and includes a series of femoral component size numbers  148 . Medial and lateral bores  142  are adapted to receive the shaft of a femoral component sizer generally denoted as  177  shown in  FIG. 12 . 
         [0055]      FIG. 12  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. 11 . 
         [0056]    Referring to  FIG. 12 , 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  12 . The stylus also includes a handle  190  that is attached to the arm and a stylus body  195  that is attached to a shaft or rod  195   a . Bore  142  accommodates shaft or rod  195   a  such that the stylus  185  is free to rotate about the longitudinal axis of rod  195   a  and is free to translate longitudinally such that the stylus may move in a direction along the hole  142  longitudinal axis. The handle  190  can be used to translate the arm within a bore in stylus body  195  through a hole  196  in a direction parallel to the stylus body  195  longitudinal axis. This is in the proximal-distal direction. Handle  196  can also be used to rotate the arm about the arm&#39;s longitudinal axis if necessary. 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 allowing arm  185  to be moved. 
         [0057]    Notably, the orientation of lever  198  of handle  190  in the proximal direction is advantageous. Orienting lever  198  as shown in  FIG. 12  facilitates manipulation of the stylus during knee arthroplasty by positioning the handle away from the incision area. 
         [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 SI scale, then the SI scale indicates a femur/implant size “3.” 
         [0059]    The sizer of  FIG. 12  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 this two condylar skids  108  and  110  located on the base of the tensioned body  100 . The skids  108 , 110  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. 
         [0060]      FIG. 13  shows the guide mounted on the distal femur just prior to resecting the posterior condyle prior to resecting the posterior condyle via slots  112 . Bores  209  are provided to allow the use of pins  211  to hold the guide on femur  12 . 
         [0061]    Once the body is properly positioned, pins can be passed through either one pair of pinholes  116  or  116   a , or through both pairs of pinholes  116  and  116   a , to secure the body to the femur. In a preferred embodiment, the body  100 , 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. 
         [0062]    Referring to  FIGS. 14-16  there is shown an alternate embodiment of a main body  300  for the instrument of the present invention. Main body  300  is similar to main body  100  with the main exception being the rotational mechanism shown in  FIG. 16 . Referring to  FIGS. 14 and 15  body  300  includes a rotating disk  302  mounted in a bore  304  therein. As shown in  FIG. 15  the proximally facing side of body  300  has a plate  305  mounted on disk  302 . Plate  305  is initially mounted on the distal femur and includes an intramedullary rod portion  306  for insertion into the intramedullary canal of the femur. As with body  100  body  300  includes a pair of condylar portions  308  and  310  and slots  312 . As with body  100  body  300  includes a drive screw system for adjusting the body including a screw  320  mounted in body  300  for engaging a circular gear segment  322  fixed to disc  302 . This is best seen in  FIG. 16 . A screw element or rack element  324  having teeth  326  is mounted within disk  302  for engaging teeth  56  on plate  40  as shown in  FIG. 5 . Thus rotation of screw  324  causes movement of body  300  in a proximal-distal direction while rotation of screw  320  causes internal and external rotation of body  300  about the axis of rod  306 . This design works the same way as that described below for body  100 , the main difference being the change in location of the internal/external rotation drive gear  322  on disk  302 . 
         [0063]    The use of the instrument assembly including body  100  set forth above will now be described. Initially the surgeon drills a hole into the intramedullary canal of the distal femur. The surgeon then inserts intramedullary rod  22  such that portion  30  thereof extends distally of the prepared surfaces  16  and  18  of femur  12 . The surgeon then assembles distal plate  40  onto the distal femur over portion  30  of IM rod  22 . The surgeon sets distal plate  40  at the approximate internal/external rotational position that is desired. The plate is then secured to the distal femur by means of bone screws  80 . Main body  100  is then placed over distal plate  40 . Posterior condylar feet  108 ,  110  on main body  100  are placed under the posterior condyles of the femur. Main body  100  is secured to distal plate  40  by means of screws  120 . Screws  120  secure main body  100  to distal plate  40  but allow main body  100  to move in an anterior-posterior direction by means of adjustment screw  140 . The surgeon then performs a trial reduction with the instrument in place. 
         [0064]    Trial reduction takes place by means of the offset single radius on the periphery of condylar skids  108 ,  110 . The surgeon places a tibial shim plate (2, 4, or 6 mm)  132 ,  134  on the proximal tibia. This plate acts as a protective plate for the resected bone on the tibia and also as a firm surface for the condylar skids of the main body  100  to articulate on. The surgeon then puts the knee through a range of motion and assesses stability throughout the complete range. The surgeon has multiple thickness spacer blocks which allow the surgeon to assess the gap (if any) on each condyle. 
         [0065]    If the surgeon establishes that adjustment is required, then the following steps can be taken. If internal/external rotation adjustment is required then the surgeon opens (not removes) bone screws  80  from the distal plate and places drive tool  130  into spur gear  64  and rotates the distal plate  40  until the desired position has been obtained. The surgeon can then resecure the distal plate at this position if desired. However, the surgeon can adjust the AP position by means of actuating tool  140  to drive the acme screw. Once the desired internal/external rotation of the main body  100  has been achieved, distal plate  40  is secured to the femur. The surgeon then begins the process of assessing the resection levels and the femoral component size. The resection level is assessed by means of a blade runner shown in  FIG. 11  placed into slots  112  of both condyles  108 ,  110 , and by placing the stylus into one of the holes or bores  142 . 
         [0066]    After the internal-external rotation and anterior-posterior location 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”)  148  located on surface  146 . The reading is taken by comparing a ring  220  on the stylus  195   a  to the AP scale. For example, if the ring is aligned toward the number “3” of the AP scale  148 , then the femur size is a “3” and the implant needed is a size “3.” The sizes may be femoral component sizes 1-8 of the Triathlon® line of implants. 
         [0067]    Finally, the surgeon drills two holes through bores  116  on the distal femur. The two bores  116  position a standard 4-in-1 cutting guide in the correct position so that the remaining cuts (anterior and posterior chamfer and skim cuts) can be made on the distal femur. 
         [0068]    Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.