Abstract:
An instrument ( 10 ) and method are provided for total knee arthroplasty (TKA). The instrument separates a patient&#39;s tibia and femur, in both extension and flexion, to measure a gap and an angle therebetween. The instrument includes various modular accessories ( 16, 54, 70, 80, 90, 100 ) that provide flexibility of usage throughout the TKA procedure and that accommodate different surgical philosophies.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from U.S. Provisional Patent Application Ser. No. 61/509,355, filed Jul. 19, 2011, the disclosure of which is hereby expressly incorporated by reference herein in its entirety. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    The present disclosure relates to knee arthroplasty. More particularly, the present disclosure relates to an instrument for use during a knee arthroplasty procedure, and to a method for using the same. 
       BACKGROUND AND SUMMARY OF THE DISCLOSURE 
       [0003]    In a total knee arthroplasty (TKA) procedure, a patient&#39;s distal femur is resected and replaced with a prosthetic femoral implant, and the patient&#39;s proximal tibia is resected and replaced with a prosthetic tibial implant. The prosthetic femoral implant articulates with the prosthetic tibial implant to restore joint motion. 
         [0004]    Many factors influence joint motion after the TKA procedure. The size and shape of each prosthetic implant will impact joint motion. Additionally, the location and orientation of each prosthetic implant, which is determined by the location and orientation of the corresponding bone resections, will impact joint motion. The tension or laxity of the surrounding soft tissue will also impact joint motion. For example, if the surrounding collateral ligaments are too tense, joint motion may be limited, but if the surrounding collateral ligaments are too lax, improper femoral rotation or femoral lift-off may occur. Also, the soft tissue balance around the joint will impact joint motion. 
         [0005]    Different surgical philosophies have traditionally influenced TKA instruments and procedures. For example, a first, “measured resection” philosophy emphasizes bone resections while preserving the natural joint axis and soft tissue. A second, “soft tissue balancing” philosophy emphasizes soft tissue modifications while preserving bone. 
         [0006]    The present invention provides an exemplary TKA instrument and procedure. The instrument separates the patient&#39;s tibia and femur, in both extension and flexion, to place the knee joint in tension and to measure a gap and an angle therebetween. The instrument includes various modular accessories. The accessories provide flexibility of usage throughout the TKA procedure. For example, the instrument may be used before resecting or otherwise manipulating the patient&#39;s knee joint to evaluate the natural knee joint and plan the TKA procedure, as well as after resecting or otherwise manipulating the patient&#39;s knee joint to evaluate and/or further plan the TKA procedure. The accessories also allow each individual user to select accessories that accommodate his or her own surgical philosophy and the needs of the particular patient. The accessories also allow the user to incorporate multiple surgical philosophies into a single surgical procedure, such as by comparing the potential outcome of one accessory with the potential outcome of another accessory. 
         [0007]    According to an embodiment of the present invention, a knee arthroplasty instrument is provided for use in a patient&#39;s knee joint. The knee joint includes a tibia and a femur. The instrument may include a tensioning tool, a first sizer, and a second sizer different from the first sizer. The tensioning tool includes a tibial component configured for placement against the tibia and a femoral component configured for placement against the femur, the femoral component being movably coupled to the tibial component to place the patient&#39;s knee joint in tension by separating the tibia and the femur. The first sizer is removably coupled to the tensioning tool, the first sizer including at least one first reference indicator that references the femur to locate a cut guide relative to the femur. The second sizer is different from the first sizer and is removably coupled to the tensioning tool, the second sizer including at least one second reference indicator that references the femur to locate the cut guide relative to the femur. 
         [0008]    According to another embodiment of the present invention, a knee arthroplasty instrument is provided for use in a patient&#39;s knee joint. The knee joint includes a tibia and a femur. The instrument may include a tensioning tool, a cut guide, and a sizer. The tensioning tool includes a tibial component configured for placement against the tibia and a femoral component configured for placement against the femur, the femoral component being movably coupled to the tibial component to place the patient&#39;s knee joint in tension by separating the tibia and the femur. The cut guide is removably coupled to the tensioning tool. The sizer is removably coupled to the tensioning tool to locate the cut guide relative to the femur. 
         [0009]    According to yet another embodiment of the present invention, a knee arthroplasty method is provided for a patient&#39;s knee joint. The knee joint includes a tibia and a femur. The method may include: using a tensioning tool to place the patient&#39;s knee joint in tension by separating the tibia and the femur; selecting one of a first sizer and a second sizer, the first sizer differing from the second sizer; coupling the selected sizer to the tensioning tool; and using the selected sizer to locate a cut guide relative to the femur. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The above-mentioned and other features and advantages of this disclosure, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
           [0011]      FIG. 1  is a perspective view of an exemplary tensioning instrument of the present disclosure, the instrument including a base, a lower tibial component, an upper femoral component, and an intermediate arm; 
           [0012]      FIG. 2  is a top plan view of the instrument of  FIG. 1 ; 
           [0013]      FIG. 3A  is a front elevational view of the instrument of  FIG. 1 ; 
           [0014]      FIG. 3B  is a front elevational view similar to  FIG. 3A  with the base, the lower tibial component, and the upper femoral component of the instrument removed to show a shaft of the intermediate arm; 
           [0015]      FIG. 4A  is a rear elevational view of the instrument of  FIG. 1  showing the upper femoral component coupled to the instrument; 
           [0016]      FIG. 4B  is a rear elevational view similar to  FIG. 4A  showing the upper femoral component rotated for removal from the instrument; 
           [0017]      FIG. 5A  is a perspective view of the instrument of  FIG. 1  with a first, arcuate femoral plate; 
           [0018]      FIG. 5B  is a perspective view of the instrument of  FIG. 1  with a second, flat femoral plate; 
           [0019]      FIG. 6  is a perspective view of the instrument of  FIG. 1  with a first, measured resection type sizer; 
           [0020]      FIG. 7  is a perspective view of the instrument of  FIG. 1  with a second, soft tissue balancing type sizer; 
           [0021]      FIG. 8  is a perspective view of the instrument of  FIG. 1  with a third, pure gap type sizer; 
           [0022]      FIG. 9  is a perspective view of the instrument of  FIG. 1  with a femoral cut guide; 
           [0023]      FIG. 10A  is an anterior elevational view of a knee joint in extension; 
           [0024]      FIG. 10B  is an anterior elevational view of the knee joint in flexion; 
           [0025]      FIG. 11A  is a perspective view of the instrument positioned within the knee joint in extension, the instrument including the second, flat femoral plate of  FIG. 5B ; 
           [0026]      FIG. 11B  is a perspective view of the instrument positioned within the knee joint in flexion, the instrument including the second, flat femoral plate of  FIG. 5B ; 
           [0027]      FIG. 12  is a perspective view of the instrument positioned within the knee joint in flexion, the instrument including the first, measured resection type sizer of  FIG. 6 ; 
           [0028]      FIG. 13  is a perspective view of the instrument positioned within the knee joint in flexion, the instrument including the second, soft tissue balancing type sizer of  FIG. 7 ; 
           [0029]      FIG. 14  is a perspective view of the instrument positioned within the knee joint in flexion, the instrument including the third, pure gap type sizer of  FIG. 8 ; and 
           [0030]      FIG. 15  is a perspective view of the instrument positioned within the knee joint in flexion, the instrument including the femoral cut guide of  FIG. 9 . 
       
    
    
       [0031]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate exemplary embodiments of the invention and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
       DETAILED DESCRIPTION 
       [0032]    With reference to  FIGS. 1 and 2 , a tensioning instrument  10  is provided for separating a patient&#39;s tibia and femur and measuring a joint gap and a joint angle therebetween. Instrument  10  includes a base  12 , a lower tibial paddle or component  14 , an upper femoral paddle or component  16 , and an intermediate arm  18  that couples tibial component  14  to femoral component  16 . Tibial component  14  and femoral component  16  are illustratively offset from base  12 , as shown in  FIG. 2 , to accommodate the patient&#39;s patella. Additional information regarding instrument  10  may be found in U.S. Pat. No. 7,156,853 to Muratsu, the disclosure of which is expressly incorporated herein by reference in its entirety. 
         [0033]    Femoral component  16  is configured to translate vertically along arrows V, V′ relative to tibial component  14  via arm  18 , as shown in  FIG. 3A . Instrument  10  may be opened by moving femoral component  16  apart from tibial component  14  along arrow V, and instrument  10  may be closed by moving femoral component  16  toward tibial component  14  along arrow V′. As shown in  FIG. 3A , arm  18  includes shaft  20  that translates vertically through base  12 . Femoral component  16  is coupled to arm  18  for movement therewith relative to base  12 . Shaft  20  may be keyed to base  12  to permit vertical translation of shaft  20  through base  12  while preventing rotation of shaft  20  in base  12 . 
         [0034]    A driving means is provided for selectively translating femoral component  16  relative to tibial component  14 . The illustrative driving means of  FIG. 3B  includes a pinion gear  22  in base  12  that cooperates with a linear rack  24  on shaft  20 . In use, a hex driver or another suitable tool is used to turn gear  22 , and gear  22  meshes with rack  24  to drive rack  24  vertically along arrows V, V′. When opening instrument  10  along arrow V, the tool may be rotated until the patient&#39;s knee joint reaches a predetermined tension, which may occur when the user detects soft tissue resistance from the patient&#39;s knee joint to the further opening of instrument  10 . In certain embodiments, instrument  10  may be opened to apply a load to the patient&#39;s knee joint of about 40 lbs., about 60 lbs., about 80 lbs., or more, although the load may vary depending on the surgeon&#39;s preference, the state of the patient&#39;s surrounding soft tissue, and other factors. It is also within the scope of the present disclosure that the tool may be torque-limited to open instrument  10  until a predetermined rotational torque of the tool is reached, wherein the predetermined rotational torque of the tool could be selected to correspond with the predetermined tension of the patient&#39;s knee joint. 
         [0035]    A locking means is also provided to hold femoral component  16  in place relative to tibial component  14 . The illustrative locking means of  FIG. 3B  includes a spring-biased lever  40  having an actuator end  42  and a pawl end  44 . The illustrative locking means also includes a linear ratchet  46  on shaft  20  that interacts with pawl end  44  of lever  40 . As shown in  FIG. 3B , ratchet  46  and rack  24  are located on opposite sides of shaft  20 . The locking means may allow instrument  10  to be freely opened, but the locking means may prevent instrument  10  from being closed until lever  40  is operated by the user. In the illustrated embodiment of  FIG. 3B , pawl  44  permits vertically upward movement of ratchet  46  along arrow V when opening instrument  10  but resists vertically downward movement of ratchet  46  along arrow V′ when closing instrument  10 . When actuator end  42  of lever  40  is pressed inwardly by the user, pawl end  44  of lever  40  disengages ratchet  46 , thereby permitting vertically downward movement of ratchet  46  along arrow V′ to close instrument  10 . Another suitable ratchet mechanism is described in the above-incorporated U.S. Pat. No. 7,156,853 to Muratsu. The locking means may also include a detent mechanism or another suitable locking mechanism, for example. 
         [0036]    A distance measuring means is provided to measure a distance or gap G between tibial component  14  and femoral component  16  along arrows V, V′. The illustrative distance measuring means includes a distance scale  26  on arm  18  having corresponding values and a pointer  28  on base  12 . As shaft  20  of arm  18  translates relative to base  12  along arrows V, V′, distance scale  26  moves relative to pointer  28  on base  12 . The gap G may be determined by reading the value from distance scale  26  that is aligned with the top end of pointer  28 . 
         [0037]    In addition to femoral component  16  translating vertically relative to tibial component  14 , femoral component  16  is also configured to rotate relative to tibial component  14 . More specifically, in addition to femoral component  16  translating vertically relative to tibial component  14  via arm  18  along arrows V, V′, femoral component  16  is also configured to rotate relative to arm  18  and tibial component  14  about axis A. As shown in  FIG. 2 , post  30  extends into femoral component  16  from arm  18  along the rotation axis A. Femoral component  16  is configured to rotate around post  30  and rotation axis A. According to an exemplary embodiment of the present disclosure, and as shown in  FIG. 1 , the rotation axis A of femoral component  16  is perpendicular to the translation axes V, V′ of femoral component  16 . 
         [0038]    An angle measuring means is provided to measure an angle α between tibial component  14  and femoral component  16  about the rotation axis A. The illustrative angle measuring means of  FIG. 1  includes a scale plate  32  on arm  18  having corresponding values, where scale plate  32  defines an arcuate slot  34 . The illustrative angle measuring means also includes a pointer  36  on femoral component  16 . As femoral component  16  rotates relative to arm  18  about axis A, pointer  36  moves along or through the arcuate slot  34  of scale plate  32 . The angle α may be determined by reading the value from scale plate  32  that is adjacent to pointer  36 . When femoral component  16  is oriented parallel to tibial component  14 , pointer  36  may be centered in slot  34  corresponding to an angle α of 0 degrees. When femoral component  16  deviates from this parallel orientation, on the other hand, pointer  36  may move along slot  34  to a positive angle α greater than 0 degrees or a negative angle α less than 0 degrees. As discussed further below, angle α may indicate a varus/valgus angle of the patient&#39;s knee joint and/or internal/external rotation of the patient&#39;s knee joint. 
         [0039]    Instrument  10  includes a set of modular accessories, each of which is described further below. Instrument  10  and the accessories may be provided together in a kit. In this manner, a surgeon or another user may select a desired accessory from the kit and attach that first accessory to instrument  10 . As the surgical procedure progresses, the user may select a second accessory from the kit and attach the second accessory to instrument  10 . In certain embodiments, the first accessory may be left in place when the second accessory is attached to instrument  10 . In other embodiments, the first accessory may be removed from instrument  10  to accommodate the second accessory. A variety of different coupling mechanisms (e.g., dovetail joints) and locking mechanisms (e.g., ball detents) may be used to selectively receive and retain the desired modular accessory on instrument  10 , as exemplified below. 
         [0040]    The above-described femoral component  16  may be considered a first modular accessory of instrument  10  that is removably coupled to instrument  10 . As discussed above, femoral component  16  is configured to rotate around post  30  of arm  18 . A tab or key  50  may be provided on post  30  to retain femoral component  16  on post  30 , as shown in  FIG. 4A . A keyway  52  may be provided in femoral component  16  to allow for selective removal and replacement of femoral component  16  when keyway  52  is rotated into alignment with key  50 , as shown in  FIG. 4B . According to an exemplary embodiment of the present disclosure, key  50  and keyway  52  are positioned such that key  50  remains offset from keyway  52  during normal rotation of femoral component  16  (e.g., 0 to 24 degrees) to resist unwanted removal of femoral component  16  during use. When removal of femoral component  16  is desired, femoral component  16  may be manually rotated beyond its normal range of motion (e.g., 25 degrees or more) to align keyway  52  with key  50 . 
         [0041]    Referring next to  FIG. 5A , a second modular accessory of instrument  10  is provided as femoral plate  54 . Femoral plate  54  includes an articular surface  56 . In the illustrated embodiment of  FIG. 5A , articular surface  56  of femoral plate  54  includes concave regions  58  to facilitate contact and/or articulation with convex condyles of the patient&#39;s femur. Another femoral plate  54 ′ is shown in  FIG. 5B . Femoral plate  54 ′ of  FIG. 5B  is generally similar to femoral plate  54  of  FIG. 5A , with like reference numerals indicating like elements, except that femoral plate  54 ′ has a generally flat articular surface  56 ′ to facilitate contact and/or articulation with flat, resected surfaces of the patient&#39;s femur. Femoral plates  54 ,  54 ′ may be provided in different shapes and sizes for use as spacers. 
         [0042]    Femoral plate  54  may be removably coupled to femoral component  16  of instrument  10 , such that femoral plate  54  may translate vertically and rotate relative to tibial component  14  along with femoral component  16 . In  FIG. 5A , femoral plate  54  is attached to femoral component  16  via tongues  60  and corresponding grooves  62 , where tongues  60  on femoral plate  54  are sized to slide through corresponding grooves  62  in femoral component  16 . Other coupling mechanisms may also be used between femoral plate  54  and femoral component  16 . 
         [0043]    When femoral plate  54  slides into place on femoral component  16 , femoral plate  54  may be selectively retained or locked in place. In  FIG. 5A , femoral plate  54  may be locked onto femoral component  16  by aligning each indentation or recess  64  in femoral plate  54  with a corresponding lock  66  on femoral component  16 . Each lock  66  may be in the form of a ball detent, a spring pin, or another suitable locking mechanism, for example. When removal of femoral plate  54  is desired, locks  66  may be released and freed from recesses  64  of femoral plate  54 . According to an exemplary embodiment of the present disclosure, locks  66  act in a direction perpendicular to tongues  60  and grooves  62  of the coupling mechanism. 
         [0044]    As discussed further below with reference to  FIGS. 6-8 , various sizers are provided as accessories of instrument  10 . Exemplary sizers of the present disclosure are configured to size the patient&#39;s femur, to identify an appropriately sized femoral cut guide and an appropriately sized prosthetic femoral implant, and to locate the selected femoral cut guide and the selected femoral implant relative to the patient&#39;s femur, all with a single device. It is also within the scope of the present disclosure that the sizers may include one device to size the patient&#39;s femur and another distinct device to locate the femoral cut guide, for example. 
         [0045]    Referring next to  FIG. 6 , a third modular accessory of instrument  10  is provided as a measured resection type (MR-type) sizer  70 . The illustrative MR-type sizer  70  includes a posterior-referencing component  71  with posterior feet  72  and an adjustable anterior-referencing component  73  with an anterior probe or stylus  74 . The anterior-referencing component  73  also includes a plurality of distal-referencing indicators, illustratively holes  75 . Holes  75  may be arranged in sets with each set corresponding to a desired angle of internal/external rotation, such as 0 degrees, 3 degrees, and 5 degrees of internal/external rotation. In another embodiment, bone-engaging pins may be provided in MR-type sizer  70 , the pins being slidable (e.g., between holes  75 ) to select the desired angle of internal/external rotation. 
         [0046]    The anterior-referencing component  73  is configured to translate vertically relative to the posterior-referencing component  71 . As a result, anterior probe  74  and the distal-referencing holes  75  of the anterior-referencing component  73  translate vertically relative to posterior feet  72  of the posterior-referencing component  71 . An anterior/posterior (A/P) sizing scale  76  may be provided between the adjustable anterior-referencing component  73  and the posterior-referencing component  71 . More specifically, the A/P sizing scale  76  may be provided between anterior probe  74  of the adjustable anterior-referencing component  73  and posterior feet  72  of the posterior-referencing component  71 . In certain embodiments, this measurement is a component size (e.g., size 12) corresponding to the separation between anterior probe  74  and posterior feet  72 . In other embodiments, this measurement is the vertical distance (e.g., 60 millimeters) between anterior probe  74  and posterior feet  72 . The measurement communicated by the A/P sizing scale  76  would increase as anterior probe  74  moves apart from posterior feet  72 , which would also inform the user that, along with anterior probe  74 , distal-referencing holes  75  have moved apart from posterior feet  72 . In addition to moving vertically, anterior probe  74  is also configured to move horizontally relative to the posterior-referencing component  71  and the anterior-referencing component  73  to reference the patient&#39;s femur, as discussed further below. 
         [0047]    The MR-type sizer  70  may be removably coupled to femoral component  16  of instrument  10 , such that the MR-type sizer  70  may translate vertically and rotate relative to tibial component  14  along with femoral component  16 . As a result, the distal-referencing holes  75  in the MR-type sizer  70  may also translate vertically and rotate relative to tibial component  14 . According to an exemplary embodiment of the present disclosure, the MR-type sizer  70  is attached to and selectively locked onto femoral component  16  in the same manner as the above-described femoral plate  54  ( FIG. 5A ). For example, the MR-type sizer  70  may include tongues  77  similar to tongues  60  of femoral plate  54  for receipt in grooves  62  of femoral component  16 . Also, the MR-type sizer  70  may include recesses (not shown) similar to recesses  64  of femoral plate  54  for receipt of locks  66  of femoral component  16 . Other coupling mechanisms and locking mechanisms may also be used between the MR-type sizer  70  and femoral component  16 . 
         [0048]    Referring next to  FIG. 7 , a fourth modular accessory of instrument  10  is provided as a soft tissue balancing type (STB-type) sizer  80 . The STB-type sizer  80  includes a base component  81  and an adjustable anterior-referencing component  83  with an anterior probe or stylus  84 . The anterior-referencing component  83  also includes a plurality of distal-referencing indicators, illustratively holes  85 . 
         [0049]    The anterior-referencing component  83  is configured to translate vertically relative to the base component  81 . As a result, anterior probe  84  and the distal-referencing holes  85  of the anterior-referencing component  83  also translate vertically relative to the base component  81 . An anterior/posterior (A/P) sizing scale  86  may be provided between the adjustable anterior-referencing component  83  and the base component  81 . More specifically, the A/P sizing scale  86  may be provided between anterior probe  84  of the adjustable anterior-referencing component  83  and base component  81  to provide an A/P measurement between the anterior cortex and the posterior condyles. In certain embodiments, this measurement is a component size (e.g., size 12) corresponding to the separation between anterior probe  84  of the anterior-referencing component  83  and base component  81 . In other embodiments, this measurement is the vertical distance (e.g., 60 millimeters) between anterior probe  84  of the anterior-referencing component  83  and base component  81 . The measurement communicated by the A/P sizing scale  86  would increase as anterior probe  84  moves apart from base component  81 , which would also inform the user that, along with anterior probe  84 , distal-referencing holes  85  have moved apart from base component  81 . In addition to moving vertically, anterior probe  84  is also configured to move horizontally relative to the base component  81  and the anterior-referencing component  83  to reference the patient&#39;s femur, as discussed further below. 
         [0050]    The illustrative STB-type sizer  80  of  FIG. 7  is an anterior-referencing sizer, with distal-referencing holes  85  translating vertically relative to base component  81  along with anterior probe  84 . It is also within the scope of the present disclosure to have a posterior-referencing sizer, with distal-referencing holes  85  being vertically fixed to base component  81  and anterior probe  84  translating vertically relative to distal-referencing holes  85 . 
         [0051]    The STB-type sizer  80  may be removably coupled to arm  18  of instrument  10 , such that the STB-type sizer  80  may translate vertically relative to tibial component  14  while remaining rotatably fixed relative to tibial component  14 . Therefore, unlike the above-described MR-type sizer  70  ( FIG. 6 ) that both translates vertically and rotates relative to tibial component  14 , the STB-type sizer  80  may translate vertically relative to tibial component  14  without rotating relative to tibial component  14 . In the illustrated embodiment of  FIG. 7 , the STB-type sizer  80  is attached to arm  18  via a tongue  87  and a corresponding dovetail groove  88 , where tongue  87  on arm  18  is sized to slide into the corresponding groove  88  in the STB-type sizer  80 . Other coupling mechanisms may also be used between the STB-type sizer  80  and arm  18 . 
         [0052]    When the STB-type sizer  80  slides onto place on arm  18 , the STB-type sizer  80  may be selectively retained or locked in place. In the illustrated embodiment of  FIG. 7 , the STB-type sizer  80  may be locked onto arm  18  by aligning each recess (not shown) in the STB-type sizer  80  with a corresponding lock  89  in arm  18 . Each lock  89  may be in the form of a ball detent, a spring pin, or another suitable locking mechanism, for example. When removal of the STB-type sizer  80  is desired, the locks  89  may be released and freed from the recesses of the STB-type sizer  80 . According to an exemplary embodiment of the present disclosure, locks  89  act in a direction perpendicular to tongue  87  and groove  88  of the coupling mechanism. 
         [0053]    Referring next to  FIG. 8 , a fifth modular accessory of instrument  10  is provided as a pure gap type (PG-type) sizer  90 . The PG-type sizer  90  includes a plurality of distal-referencing indicators, illustratively holes  95 . Corresponding holes  95  are arranged in horizontal rows  96 . In certain embodiments, each row  96  corresponds to a component size, with the row  96  closest to tibial component  14  corresponding to a relatively small component size (e.g., size 10), and the row  96  farthest from tibial component  14  corresponding to a relatively large component size (e.g., size 20). In other embodiments, each row  96  corresponds to a vertical distance from tibial component  14 , with the row  96  closest to tibial component  14  corresponding to a relatively small distance, and the row  96  farthest from tibial component  14  corresponding to a relatively large distance. In this manner, rows  96  of holes  95  may serve as an A/P sizing scale of the PG-type sizer  90 . 
         [0054]    The PG-type sizer  90  may be removably coupled to tibial component  14  of instrument  10 , such that the PG-type sizer  90  remains both vertically and rotatably fixed relative to tibial component  14 . Therefore, unlike the above-described MR-type sizer  70  ( FIG. 6 ), which is both rotatably and slidably coupled to tibial component  14 , and the above-described STB-type sizer  80  ( FIG. 7 ), which is slidably coupled to tibial component  14 , the PG-type sizer  90  is fixedly coupled to tibial component  14 . In the illustrated embodiment of  FIG. 8 , the PG-type sizer  90  is attached to tibial component  14  by sliding legs (not shown) on the PG-type sizer  90  into corresponding openings  98  in tibial component  14  ( FIG. 2 ). Other coupling mechanisms may also be used between the PG-type sizer  90  and tibial component  14 . 
         [0055]    When the PG-type sizer  90  slides onto place on tibial component  14 , the PG-type sizer  90  may be selectively retained or locked in place. For example, the PG-type sizer  90  may be locked onto openings  98  of tibial component  14  by aligning recesses (not shown) in the legs of the PG-type sizer  90  with corresponding locks  99  on tibial component  14  ( FIG. 2 ). Each lock  99  may be in the form of a ball detent, a spring pin, or another suitable locking mechanism, for example. When removal of the PG-type sizer  90  is desired, the locks  99  may be released and freed from the legs of the PG-type sizer  90 . 
         [0056]    Referring next to  FIG. 9 , a sixth modular accessory of instrument  10  is provided as a femoral cut guide  100 . Cut guide  100  includes body  102  that defines a plurality of cut slots, illustratively a posterior cut slot  104   a , a posterior chamfer cut slot  104   b , an anterior chamfer cut slot  104   c , and an anterior cut slot  104   d . The posterior chamfer cut slot  104   b  and the anterior chamfer cut slot  104   c  may extend in opposite directions from the same opening. Because body  102  of cut guide  100  includes four (4) cut slots in  FIG. 9 , cut guide  100  may be referred to as a “4-in-1 cut guide.” Body  102  of cut guide  100  also defines a plurality of fixation holes  106 . In one exemplification, cut guide  100  further includes a connector piece  108 . Connector piece  108  is illustratively removably coupled to body  102  via pegs  110 , which allows body  102  to be separated from connector piece  108  during cutting. It is also within the scope of the present disclosure that connector piece  108  may be integrally formed with body  102 . 
         [0057]    Cut guide  100  may be removably coupled to and selectively locked onto arm  18  of instrument  10  in a manner similar to the above-described femoral component  16 . Femoral component  16  and cut guide  100  may be interchangeably connected to instrument  10 , requiring removal of one accessory (e.g., femoral component  16 ) to accommodate the other accessory (e.g., cut guide  100 ). In the illustrated embodiment of  FIG. 9 , for example, connector piece  108  of cut guide  100  is sized and shaped to rotate around post  30  of arm  18  in the same manner as femoral component  16 . Therefore, like femoral component  16 , cut guide  100  may translate vertically relative to tibial component  14  via shaft  20  of arm  18  and may also rotate relative to tibial component  14  about post  30  of arm  18 . Connector piece  108  of cut guide  100  may also include a keyway  112  that is similar to keyway  52  of femoral component  16  ( FIG. 4B ) to allow for selective removal and replacement of cut guide  100  when keyway  112  is rotated into alignment with key  50  on post  30 . 
         [0058]    When cut guide  100  is coupled to instrument  10 , the user may refer to the same distance measuring means and angle measuring means that were described above with respect to femoral component  16 . For example, to measure the gap G between tibial component  14  and cut guide  100 , the user may refer to distance scale  26  on shaft  20  and pointer  28  on base  12 . Also, to measure the angle α between tibial component  14  and cut guide  100  about post  30 , the user may refer to scale plate  32  on arm  18  and pointer  114  on connector piece  108 , with pointer  114  on connector piece  108  being similar to pointer  36  on femoral component  16  ( FIG. 2 ). 
         [0059]    A method of using instrument  10  and its accessories will now be described with reference to  FIGS. 10A-15 . The ordering of the following steps may vary depending on the surgeon&#39;s preference, the patient&#39;s bone quality, the state of the patient&#39;s surrounding soft tissue, the types of prosthetic implants being used, and other factors, for example. 
         [0060]    First, the user may perform pre-operative planning. The planning step may involve taking X-rays or other images of the patient&#39;s knee joint  200  and selecting prosthetic implants to accommodate the patient&#39;s needs, for example. 
         [0061]    Next, as shown in  FIGS. 10A and 10B , the user exposes tibia  202  and femur  204  of the patient&#39;s knee joint  200 . The exposing step may involve incising the patient&#39;s skin, incising the patient&#39;s joint capsule, and removing osteophytes, for example. 
         [0062]    With the patient&#39;s knee joint  200  now exposed, the user uses instrument  10  to separate tibia  202  and femur  204  of the patient&#39;s knee joint  200  to a predetermined tension, and to plan and identify the desired bone resections of tibia  202  and femur  204 . With the patient&#39;s knee joint  200  tensioned in extension ( FIG. 10A ), the user is able to plan and identify a proximal tibial resection  206  and a distal femoral resection  208  that will produce a desired gap G and angle α therebetween. The extension angle α may be referred to as a varus/valgus angle. With the patient&#39;s knee joint  200  tensioned in flexion ( FIG. 10B ), the user is able to plan and identify the proximal tibial resection  206  and a posterior femoral resection  210  that will produce a desired gap G and angle α therebetween. The flexion angle α may be referred to as an internal/external rotation angle. Gap G and angle α between tibia  202  and femur  204  may be selected based on the patient&#39;s age, the patient&#39;s bone quality, the state of the patient&#39;s surrounding soft tissue, the types of prosthetic implants being used, and other factors, for example. 
         [0063]    Tibia  202  and femur  204  may be resected using suitable cut guides. For example, the Minimally Invasive Surgery (MIS) Tibial Cut Guide Assembly, which is available from Zimmer, Inc. of Warsaw, Ind., may be used to form the proximal tibial resection  206  in tibia  202 . Suitable cut guides may also be used to form the distal femoral resection  208  and the posterior femoral resection  210  in femur  204 . 
         [0064]    In addition to evaluating bone resections, the user may also evaluate soft tissue resections, releases, or other soft tissue operations that would impact gap G and angle α between tibia  202  and femur  204 . For example, if the surgeon desires a balanced angle α of 0 degrees between tibia  202  and femur  204 , the surgeon may release or otherwise relax ligaments on one side of the patient&#39;s knee joint  200  (e.g., the medial side) relative to the other side of the patient&#39;s knee joint  200  (e.g., the lateral side). As another example, if the surgeon desires a larger gap G between tibia  202  and femur  204  without resecting more bone from tibia  202  or femur  204 , the surgeon may release or otherwise relax ligaments around the patient&#39;s knee joint  200 . 
         [0065]    According to an exemplary embodiment of the present disclosure, knee joint  200  is prepared such that gap G and angle α between tibia  202  and femur  204  are the same or substantially the same in extension ( FIG. 10A ) as in flexion ( FIG. 10B ). In this embodiment, a three-dimensional space may be maintained between tibia  202  and femur  204  in extension and flexion. For example, a surgeon implanting a prosthetic femoral implant having equally thick distal and femoral condyles may prepare an extension gap G that is the same as the flexion gap G, while a surgeon implanting a prosthetic femoral implant having distal and femoral condyles of different thicknesses may prepare an extension gap G that is not the exactly the same as the flexion gap G to account for the different thicknesses. When angle α is 0 degrees, such that the proximal tibial resection  206  is parallel to the distal femoral resection  208  in extension ( FIG. 10A ) and the posterior femoral resection  210  in flexion ( FIG. 10B ), the three-dimensional space between tibia  202  and femur  204  will be rectangular in shape in extension and flexion. It is also within the scope of the present disclosure that the user may tolerate differences between the extension angle α ( FIG. 10A ) and the flexion angle α ( FIG. 10B ), such as differences of 1 degree, 2 degrees, or 3 degrees. 
         [0066]    Instrument  10  may be used to separate tibia  202  and femur  204  of the patient&#39;s knee joint  200  to a predetermined tension, and to measure gap G and angle α therebetween, in both extension and flexion. Before resecting or otherwise manipulating knee joint  200 , instrument  10  may be used to measure the natural gap G and angle α between tibia  202  and femur  204  in tension. Also, instrument  10  may be used to plan or identify the proximal tibial resection  206 , the distal femoral resection  208 , the posterior femoral resection  210 , and/or any soft tissue resections that will produce a desired gap G and angle α between tibia  202  and femur  204  in tension. After resecting or otherwise manipulating knee joint  200 , instrument  10  may be used to verify the desired gap G and angle α between tibia  202  and femur  204  in tension. Therefore, instrument  10  and its accessories may be used before and/or after resecting or otherwise manipulating knee joint  200 . 
         [0067]    The use of instrument  10  to measure gap G and angle α between tibia  202  and femur  204  is described further with reference to  FIGS. 11A and 11B , for example. In  FIG. 11A , instrument  10  is being used with the patient&#39;s knee joint  200  in extension. The proximal tibial resection  206  has already been formed in tibia  202 , and the distal femoral resection  208  has already been formed in femur  204 , so instrument  10  is being used to verify the resected gap G and the resected angle α between tibia  202  and femur  204 . Tibial component  14  of instrument  10  is placed against the proximal tibial resection  206 . Femoral plate  54 ′ ( FIG. 5B ) is coupled to femoral component  16  of instrument  10  and placed against the distal femoral resection  208 . With tibial component  14  and femoral component  16  of instrument  10  opened to a predetermined tension to separate tibia  202  and femur  204 , the user may measure the extension gap G between the proximal tibial resection  206  and the distal femoral resection  208  by referencing distance scale  26  on shaft  20  and pointer  28  on base  12 . Also, the user may measure the extension angle α between the proximal tibial resection  206  and the distal femoral resection  208  by referencing scale plate  32  on arm  18  and pointer  36  on femoral component  16 . 
         [0068]    In  FIG. 11B , instrument  10  is being used with the patient&#39;s knee joint  200  in flexion. The proximal tibial resection  206  has already been formed in tibia  202 , and the posterior femoral resection  210  has already been formed in femur  204 , so instrument  10  is being used to verify the resected gap G and the resected angle α between tibia  202  and femur  204 . Tibial component  14  of instrument  10  is placed against the proximal tibial resection  206 . Femoral plate  54 ′ ( FIG. 5B ) is coupled to femoral component  16  of instrument  10  and placed against the posterior femoral resection  210 . With tibial component  14  and femoral component  16  of instrument  10  opened to the predetermined tension to separate tibia  202  and femur  204 , the user may verify that the flexion gap G of  FIG. 11B  is the same as or substantially the same as the extension gap G of  FIG. 11A . Also, the user may verify that the flexion angle α of  FIG. 11B  is the same as or substantially the same as the extension angle α of  FIG. 11A . Although  FIGS. 11A and 11B  show the distal femoral resection  208  and the posterior femoral resection  210  in femur  204 , other resections (e.g., chamfer cuts and the anterior cut) may also exist in femur  204  when instrument  10  is in use. 
         [0069]    If necessary, the patient&#39;s knee joint  200  may be manipulated to adjust the measured gap G and/or the measured angle α between tibia  202  and femur  204 . For example, if the user determines that the flexion gap G of  FIG. 11B  is too small compared to the extension gap G of  FIG. 11A , the user may cut a deeper posterior femoral resection  210  to increase the flexion gap G of  FIG. 11B . The user may also make any necessary ligament adjustments to balance the soft tissue around knee joint  200 . For example, the user may release the patient&#39;s posterior cruciate ligament (PCL), which has been shown to increase the flexion gap G relative to the extension gap G. 
         [0070]      FIGS. 11A and 11B  depict post-resection use of instrument  10 , with instrument  10  being positioned against resected bone surfaces of tibia  202  and femur  204 . As discussed above, instrument  10  may also be used pre-resection, with instrument  10  being positioned against natural, un-resected bone surfaces of tibia  202  and femur  204 . In this pre-resection condition, instrument  10  would communicate the pre-resection gap G and the pre-resection angle α between the natural, un-resected bone surfaces in tension. The user could predict the post-resection values by combining the pre-resection values with the planned resections. For example, the user could estimate the post-resection gap G by adding the planned resection depths to the corresponding pre-resection gap G. 
         [0071]    The surgeon may also use instrument  10  to size the patient&#39;s femur  204 , to select an appropriately sized femoral cut guide (e.g., cut guide  100  of  FIG. 9 ), and to determine the location and orientation of the femoral cut guide relative to the patient&#39;s femur  204 . The location and orientation of the femoral cut guide will determine the location and orientation of subsequent femoral resections and, ultimately, the location and orientation of an appropriately sized prosthetic femoral implant corresponding to the femoral cut guide. These steps may be performed by attaching a desired sizer to instrument  10 , as discussed further below. The sizer type may vary depending on the surgeon&#39;s preference, the patient&#39;s bone quality, the state of the patient&#39;s surrounding soft tissue, the type of prosthetic femoral implant being used, and other factors, for example. 
         [0072]    The MR-type sizer  70  is shown attached to instrument  10  in  FIG. 12  (see also  FIG. 6 ). With the patient&#39;s knee joint  200  in flexion, tibial component  14  of instrument  10  is positioned against the patient&#39;s tibia  202 , posterior feet  72  of the MR-type sizer  70  are placed against the patient&#39;s uncut posterior femur  204 , and anterior probe  74  of the MR-type sizer  70  is placed against the patient&#39;s anterior femoral cortex  212 . In this arrangement, the user may reference the A/P sizing scale  76  to size the patient&#39;s femur  204  and select an appropriately sized cut guide. The user may also use a set of distal-referencing holes  75  in the MR-type sizer  70  as guides to mark or drill distal fixation holes (not shown) into the patient&#39;s femur  204  for eventual receipt of the cut guide, with the selected set of distal-referencing holes  75  controlling internal/external rotation of the corresponding femoral cut guide and the corresponding prosthetic femoral implant. Because the MR-type sizer  70  is coupled to femoral component  16  of instrument  10  for rotation therewith, rotation of the patient&#39;s femur  204  on femoral component  16  will also cause rotation of the distal-referencing holes  75  in the MR-type sizer  70 . As a result, the distal-referencing holes  75  may track or follow the bone of the patient&#39;s femur  204  as it rotates relative to the patient&#39;s tibia  202 . In this manner, the bone of the patient&#39;s femur  204  (i.e., the “measured resections” of the patient&#39;s femur  204 ) will impact internal/external rotation of distal-referencing holes  75 , as well as the placement of the corresponding femoral cut guide and the corresponding prosthetic femoral implant. 
         [0073]    The STB-type sizer  80  is shown attached to instrument  10  in  FIG. 13  (see also  FIG. 7 ). With the patient&#39;s knee joint  200  in flexion, tibial component  14  of instrument  10  is positioned against the patient&#39;s tibia  202 , femoral component  16  of instrument  10  is placed against the patient&#39;s uncut posterior femur  204 , and anterior probe  84  of the STB-type sizer  80  is placed against the patient&#39;s anterior femoral cortex  212 . Although  FIG. 13  shows femoral component  16  of instrument  10  in contact with the patient&#39;s posterior femur  204 , one of femoral plate  54  and femoral plate  54 ′ will be attached to femoral component  16  of instrument  10  to directly contact the patient&#39;s posterior femur  204 . In these arrangements, the user may reference the A/P sizing scale  86  to size the patient&#39;s femur  204  and select an appropriately sized cut guide. The user may also use the distal-referencing holes  85  in the STB-type sizer  80  as guides to mark or drill distal fixation holes (not shown) into the patient&#39;s femur  204  for eventual receipt of the cut guide. Because the STB-type sizer  80  is not rotatable relative to tibial component  14  of instrument  10 , rotation of the patient&#39;s femur  204  on femoral component  16  will not rotate the STB-type sizer  80 . As a result, the distal-referencing holes  85  may remain rotatably fixed in space, even as soft tissue of the knee joint  200  causes the patient&#39;s femur  204  to rotate relative to the patient&#39;s tibia  202  as the joint is placed in tension. In this manner, “soft tissue balancing” around the patient&#39;s femur  204  will impact internal/external rotation of distal-referencing holes  85  relative to the patient&#39;s femur  204 , as well as the placement of the corresponding femoral cut guide and the corresponding prosthetic femoral implant. 
         [0074]    The PG-type sizer  90  is shown attached to instrument  10  in  FIG. 14  (see also  FIG. 8 ). With the patient&#39;s knee joint  200  in flexion, tibial component  14  of instrument  10  is positioned against the patient&#39;s tibia  202 , and femoral component  16  of instrument  10  is placed against the patient&#39;s uncut posterior femur  204 . Although  FIG. 14  shows femoral component  16  of instrument  10  in contact with the patient&#39;s posterior femur  204 , one of femoral plate  54  and femoral plate  54 ′ will be attached to femoral component  16  of instrument  10  to directly contact the patient&#39;s posterior femur  204 . The patient&#39;s knee joint  200  may also be moved to extension with the PG-type sizer  90 . 
         [0075]    Based on the size of the patient&#39;s femur  204  and the gap G produced when the patient&#39;s knee joint  200  is tensioned in flexion and/or extension, the user may identify an appropriate row  96  of distal-referencing holes  95  in the PG-type sizer  90  and select the corresponding cut guide. For example, after forming the distal femoral resection  208  in femur  204 , the user may measure the extension gap G produced when the patient&#39;s knee joint  200  is tensioned in extension. Then, the user may tension the patient&#39;s knee joint  200  in flexion to the same tension as in extension and select the row  96  of distal-referencing holes  95  in the PG-type sizer  90  that correspond to the previously-measured extension gap G. The user may then use the selected distal-referencing holes  95  in the PG-type sizer  90  as guides to mark or drill distal fixation holes (not shown) into the patient&#39;s femur  204  for eventual receipt of the cut guide, thereby arriving at a posterior femoral resection  210  ( FIG. 10B ) that produces the same flexion gap G as the previously-measured extension gap G. Because the PG-type sizer  90  is coupled to tibial component  14  of instrument  10 , rotation or translation of the patient&#39;s femur  204  on femoral component  16  will not rotate or translate the PG-type sizer  90 . As a result, the distal-referencing holes  95  may remain fixed in space, even as soft tissue of the knee joint  200  causes the patient&#39;s femur  204  to rotate or translate relative to the patient&#39;s tibia  202  as the joint is placed in tension. In this manner, soft tissue around the patient&#39;s femur  204  will impact internal/external rotation of distal-referencing holes  95  relative to the patient&#39;s femur  204 , as well as the placement of the corresponding femoral cut guide and the corresponding prosthetic femoral implant. 
         [0076]    In certain embodiments, the user may interchangeably couple more than one sizer  70 ,  80 ,  90  to instrument  10  during a single surgical procedure and compare the potential outcomes of each sizer  70 ,  80 ,  90  in that surgical procedure. For example, the user may compare the planned location and orientation of distal-referencing holes  75 ,  85 ,  95 , for each sizer  70 ,  80 ,  90 , and then continue with a desired sizer  70 ,  80 ,  90 . 
         [0077]    After using instrument  10  to mark or drill distal fixation holes (not shown) into the patient&#39;s femur  204 , the user uses instrument  10  in  FIG. 15  to confirm the location and orientation of the selected cut guide  100 . Cut guide  100  is illustratively coupled to instrument  10  via connector piece  108 , as discussed further above with respect to  FIG. 9 . During this confirming step, the user may visually or otherwise align fixation holes  106  in cut guide  100  with the pre-marked or pre-drilled distal fixation holes on the patient&#39;s distal femur  204 . If the markings are in the form of symbols (e.g., circles, crosses) drawn onto the patient&#39;s distal femur  204 , for example, fixation holes  106  in cut guide  100  may be visually aligned with the symbols to overlap the symbols. If the markings are in the form of pins or other reference structures extending from the patient&#39;s distal femur  204 , fixation holes  106  in cut guide  100  may be placed onto the pins. In such arrangements, the user may verify the location and orientation of cut guide  100  relative to the patient&#39;s femur  204 . More specifically, the user may verify the location and orientation of cut slots  104   a ,  104   b ,  104   c ,  104   d , relative to the patient&#39;s femur  204 . When cut guide  100  is attached to instrument  10 , the user may also use instrument  10  to verify gap G and angle α between tibia  202  and femur  204 . 
         [0078]    Advantageously, before cutting the patient&#39;s bone, instrument  10  enables the user to adjust the location and/or orientation of cut guide  100 . For example, if the patient&#39;s bone stock along one or more cut slots  104   a ,  104   b ,  104   c ,  104   d , is inadequate, the user may repeat some or all of the above-described method steps to re-evaluate and adjust the location and/or orientation of cut guide  100 . 
         [0079]    After using instrument  10  to confirm the location and orientation of cut guide  100  in  FIG. 15 , the user may separate connector piece  108  from body  102  and affix body  102  to the patient&#39;s femur  204 . Body  102  may be affixed to the patient&#39;s femur  204  by driving supports (e.g., pins, nails, screws, or other anchors) through fixation holes  106  of body  102  and into the patient&#39;s femur  204 . Based on the previous alignment step, the anchors would extend into the pre-marked or pre-drilled distal fixation holes (not shown) in the patient&#39;s femur  204 . 
         [0080]    With body  102  of cut guide  100  affixed to the patient&#39;s femur  204 , the user may cut bone by drawing an oscillating saw or another suitable tool (not shown) through some or all of the cut slots  104   a ,  104   b ,  104   c ,  104   d , in cut guide  100 . If posterior femoral resection  210  was previously formed in femur  204 , it may be unnecessary to use posterior cut slot  104   a  of cut guide  100 . However, the surgeon will generally form distal femoral resection  208  in femur  204 , and then form the remaining resections in femur  204  (including posterior femoral resection  210 ) using cut slots  104   a ,  104   b ,  104   c ,  104   d , of cut guide  100  based on the outcome of the selected sizer  70 ,  80 ,  90 . 
         [0081]    Next, the user may temporarily secure trial implants (not shown) to the patient&#39;s resected tibia  202  and femur  204  and use instrument  10  to confirm the location and orientation of the trial implants. For example, with tibial component  14  of instrument  10  positioned against a trial tibial implant and femoral component  16  of instrument  10  positioned against a trial femoral implant, the user may use instrument  10  to verify gap G and angle α therebetween. 
         [0082]    Finally, the user may affix final prosthetic implants (not shown) to the patient&#39;s resected tibia  202  and femur  204 . The final prosthetic implants may be secured in place with anchors and/or bone cement, for example. Again, the user may use instrument  10  to confirm the location and orientation of the final prosthetic implants. 
         [0083]    While this invention has been described as having exemplary designs, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.