Abstract:
A conical burr template is indexed to the intramedullary canal of a tibia, such that a burr may trace the periphery of the template to define a correspondingly shaped cavity in the bone which is properly sized, shaped, and positioned to receive a cone-shaped tibial augment component. Advantageously, use of the burr template promotes expedient surgery while maintaining optimal fit characteristics and proper spatial placement of the tibial cone augment.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims the benefit of priority under 35 U.S.C. §119(e) of Neal et al., U.S. Provisional Patent Application Ser. No. 61/522,872, entitled “PROSTHESIS RESECTION GUIDE”, filed on Aug. 12, 2011, which is herein incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present disclosure relates to orthopedic prostheses, more particularly, to guides for resecting bone in preparation to receive a prosthetic component. 
         [0004]    2. Description of the Related Art 
         [0005]    Orthopedic prostheses are commonly utilized to prepare and/or replace damaged bone and tissue in the human body. For example, a knee prosthesis may be used to restore natural knee function by repairing damaged or diseased articular surfaces of the femur and/or tibia. Knee prostheses may include a femoral component implanted on the distal end of a femur, which articulates with a tibial component implanted on the corresponding proximal end of tibia. The femoral and tibial components cooperate to restore the function of healthy natural knee. 
         [0006]    In some cases, the proximal tibia or distal femur may exhibit severe degeneration, trauma, or other pathologies, necessitating resection of more bone than can be compensated for by traditional femoral and tibial components. In such cases, augments may be used to effectively increase the size of an implanted component, thereby compensating for the additional volume of resected bone. 
         [0007]    In the proximal tibia, for example, poor quality bone stock may exist around the medullary canal in the diaphyseal or metaphyseal region of the bone. In such cases, an augment having a generally truncated cone-shaped outer profile corresponding to typically cone-shaped bone defect encountered around the medullary canal may be used. Such tibial cone augments may be used in order to mimic the exterior periphery of the natural bone and thereby limit resection of healthy bone stock. To this end, cone-shaped augments may define irregular conical shapes, such as shapes having differing tapers at the medial and lateral sides versus the anterior and posterior sides. Further, tibial cone augments may define a generally oval cross section, which accommodates the natural proximal tibial geometry having greater width in a medial-lateral direction compared to the anterior-posterior direction. 
         [0008]    Other indications for prosthetic implant augments include revision surgeries, in which formerly implanted prosthetic components are removed together with surrounding bone stock and replaced with new components. In such revision surgeries, the total volume of bone stock removed may be substantially greater than the bone stock removed during primary procedure, i.e., a procedure in which a first prosthesis is implanted to replace natural articular surfaces. 
         [0009]    Exemplary tibial cone augments are disclosed in U.S. patent application Ser. No. 11/560,276, filed Nov. 15, 2006 and entitled “PROSTHETIC IMPLANT SUPPORT STRUCTURE,” and in U.S. patent application Ser. No. 12/886,297, filed Sep. 20, 2010 and entitled “TIBIAL AUGMENTS FOR USE WITH KNEE JOINT PROSTHESES, METHOD OF IMPLANTING THE TIBIAL AUGMENT, AND ASSOCIATED TOOLS,” and in U.S. Provisional Patent Application Ser. No. 61/488,549, filed May 20, 2011 and entitled “STABILIZING PROSTHESIS SUPPORT STRUCTURE,” all of which are commonly assigned with the present application, the entire disclosures of which are hereby expressly incorporated by reference herein. 
         [0010]    In preparation for implantation of cone-shaped augment, a correspondingly cone-shaped cavity is formed in the bone. Instruments which aid in the expedient and accurate creation of this cavity have been the focus of substantial design efforts, particularly for the irregular cavities created for modern cone-shaped augments. 
       SUMMARY 
       [0011]    The present disclosure provides conical burr template which is indexed to the intramedullary canal of a tibia, such that a burr may trace the periphery of the template to define correspondingly shaped cavity in the bone which is properly sized, shaped, and positioned to receive a cone-shaped tibial augment component. Advantageously, use of the burr template promotes expedient surgery while maintaining optimal fit characteristics and proper spatial placement of the tibial cone augment. 
         [0012]    The burr template mounts directly to an intramedullary rod used in other aspects of a knee implantation procedure, such that the cone-shaped cavity created by use of the burr template references the intramedullary canal. Advantageously, because other instruments and the prosthetic components may also reference the intramedullary canal a resection cavity defined by the template facilitates proper placement and orientation of the final implanted prosthesis. Further, the present burr template provides an expedient and accurate guide for creating a bone resection with highly precise and complex geometrical configurations to provide an ideal match with the size of a given augment component. 
         [0013]    In one form thereof, the present disclosure provides a burr template comprising: a template track comprising a lateral track portion, a posterior track portion, and a medial track portion; a coupler joining the lateral track portion and the medial track portion, the coupler having a bore formed therethrough; and the bore spaced from the lateral track portion and the medial track portion, such that the template track defines an inner periphery corresponding to an outer periphery of tibial cone augment, the inner periphery adapted to be indexed to an intramedullary canal of a tibia. 
         [0014]    In one form thereof, the present disclosure provides a method for resecting cavity in a bone, comprising: inserting an intramedullary rod into an intramedullary canal of the bone; passing a template over the intramedullary rod and coupling the template to the intramedullary rod adjacent the bone, the template having a template track extending away from the coupler; and sweeping a cutting instrument around the template track to define an inner periphery of the resection void. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    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 descriptions of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
           [0016]      FIG. 1  is a perspective view of a conical burr template in accordance with the present disclosure, illustrated adjacent a proximal tibia; 
           [0017]      FIG. 2  is a perspective view of the template shown in  FIG. 1 , illustrating assembly of an alignment bushing to the template; 
           [0018]      FIG. 3A  is a top plan view of a centered alignment bushing in accordance with the present disclosure; 
           [0019]      FIG. 3B  is a top plan view of an offset alignment bushing in accordance with the present disclosure; 
           [0020]      FIG. 4  is a sagittal, elevation view of the template and bushing shown in  FIG. 2 , illustrating assembly thereof to a tibia with a positive anteroposterior slope; 
           [0021]      FIG. 5  is a perspective view of the template and bushing shown in  FIG. 2 , illustrating securement of the bushing to the template; 
           [0022]      FIG. 6  is a perspective view of the template and bushing shown in  FIG. 2 , together with a securement mechanism for securing the template to the tibia; 
           [0023]      FIG. 7  is a perspective view of the template, bushing, and securement mechanism shown in  FIG. 6 , illustrating attachment of the securement mechanism to the template; 
           [0024]      FIG. 8  is an exploded, perspective view of burr guard and burr in accordance with the present disclosure; 
           [0025]      FIG. 9  is a perspective view of the template, bushing, and securement mechanism shown in  FIG. 6 , together with the burr guard and burr shown in  FIG. 8 ; 
           [0026]      FIG. 10  is a perspective view of the tibia shown in  FIG. 1  after an initial resection, illustrating use of tibial cone augment to prepare for further resection 
           [0027]      FIG. 11A  is a perspective view of the tibia shown in  FIG. 10 , illustrating further resection thereof using a burr sleeve; 
           [0028]      FIG. 11B  is a side, elevation view of the tibia shown in  FIG. 11A  after completion of a cone shaped resection; 
           [0029]      FIG. 12  is a perspective view of the tibia shown in  FIG. 11A  after completion of a cone shaped resection, illustrating implantation of a tibial cone augment; 
           [0030]      FIG. 13  is a perspective view of an alternative conical burr template made in accordance with the present disclosure, together with the bushing, securement mechanism, burr guard and burr shown in  FIG. 9 ; and 
           [0031]      FIG. 14  is a perspective view of another alternative conical burr template made in accordance with the present disclosure, together with the bushing, securement mechanism, burr guard and burr shown in  FIG. 9 . 
       
    
    
       [0032]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate exemplary embodiments of the present invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
       DETAILED DESCRIPTION 
       [0033]    The present disclosure provides burr template which has an irregular conical shape corresponding to a similarly shaped tibial cone augment, such that the template may be used as a guide track for a burr to quickly and accurately form a cavity in the tibia sized to correspond to the tibial cone shaped augment. As described in detail below, the burr template references the intramedullary canal of the tibia, thereby ensuring that the cone-shaped void created by using the burr template has a desired geometrical and spatial relationship with the anatomic intramedullary canal (and, therefore, with the other anatomic shapes and features of the natural tibia). 
         [0034]    Although the exemplary embodiment described herein is adapted for use in conjunction with cone-shaped tibial augment components, it is contemplated that any template shape and size may be provided for use with any augment configuration, including cylindrical or other geometries, such as for other bones in human and animal anatomy. Further, although the exemplary template described herein is referred to as a “burr template” because a burr is an exemplary cutting tool used in conjunction with the template, it is contemplated that any suitable cutting tool may be used in conjunction with the present template as desired or required for a particular application. 
         [0035]    Turning now to  FIG. 1 , conical burr template  20  is shown positioned adjacent tibia T by surgeon&#39;s hand H. Template  20  includes an arcuate template track  22  having lateral track portion  24 , posterior track portion  26 , and medial track portion  28 . Lateral and medial track portions  24 ,  28  are joined at the anterior side of template  20  by coupler  30 , which is operable to couple template  20  to intramedullary rod  32  at desired a position and orientation (as described in detail below). 
         [0036]    Conical burr template  20  is sized to substantially encompass bone defect D in the metaphyseal and/or diaphyseal region of tibia T, as illustrated in  FIG. 1 . In order to accommodate a variety of bone sizes and varying geometries of bone defect D, a kit of conical burr templates similar to burr template  20  may be provided, in which each different template has a different overall size and/or geometrical configuration from each other template in the kit. Moreover, each template provided in the kit may be specifically sized and shaped to yield a resection void V ( FIGS. 11 and 12 ) which specifically fits an existing tibial cone augment size and shape. 
         [0037]    For example, burr template  20  defines anteroposterior taper α AP  ( FIG. 4 ) between posterior track portion  26  and an anterior face of coupler  30 , and medial-lateral taper α ML  ( FIG. 9 ) between lateral track portion  24  and medial track portion  28 . Taper angles α AP  and α ML  correspond with the analogous taper angles on a particular tibial cone augment, such as augments  34 ,  34 ′ ( FIGS. 10 and 12 ). Therefore, after burr template  20  is used to create resection void V as described in detail below, resection void V is sized and shaped to accept tibial cone augment  34 . Exemplary tibial augments, and geometrical details thereof, are disclosed in U.S. patent application Ser. Nos. 11/560,276, 12/886,297 and 61/488,549, incorporated by reference above. 
         [0038]    Before template  20  is placed adjacent tibia T as shown in  FIG. 1 , tibia T is prepared in accordance with conventional arthroplasty procedures. For example, tibia T may have its proximal surface resected to create a generally planar proximal tibial surface T S . In addition, the intramedullary canal of tibia T (not shown) may be reamed to accept intramedullary rod  32 , such that the longitudinal axis of intramedullary rod  32  is generally coaxial with the longitudinal axis of the intramedullary canal (and, therefore, of tibia T). With tibia T thus prepared, the size and extent of bone defect D may be measured or estimated in order to select an appropriate conical burr template from a provided kit (described above), or a plurality of templates may be overlaid on bone defect D for a visual estimation. 
         [0039]    Coupler  30  includes a coupler bore  36 , which is passed over intramedullary rod  32  to bring template  20  adjacent to resected surface T S  of tibia T. As illustrated in  FIG. 1 , coupler bore  36  is oversized, such that bore  36  provides an easy clearance fit over intramedullary rod  32 . Alignment bushing  38  is then passed over intramedullary rod  32  and into bore  36 , as shown in  FIG. 2 . The clearance between bore  36  and rod  32  provides a space for receipt of alignment bushing  38 , such that when alignment bushing  38  is received within bore  36 , the clearance is substantially consumed and template  20  is substantially immovable in an anteroposterior or medial-lateral direction with respect to tibia T. 
         [0040]    Coupler bore  36  further includes flat  40  formed therein, which corresponds to flat  42  of alignment bushing  38  ( FIG. 3A ). Flats  40 ,  42  cooperate to prevent rotation of template  20  with respect to the longitudinal axis of intramedullary rod  32  once template  20 , bushing  38 , and rod  32  are all affixed to one another (using set screw  44 , as shown in  FIG. 5  and described in detail below). 
         [0041]    Turning to  FIG. 3A , bushing  38  includes bore  46  sized to mount to intramedullary rod  32  ( FIG. 2 ). In the illustrated embodiment, alignment bushing  38  is provided as two halves, such that bore  46  is created by joining the two halves as shown. When bore  36  is occupied by rod  32 , gap  48  remains between the two halves of alignment bushing  38 . 
         [0042]    Turning to  FIGS. 4 and 5 , alignment bushing  38  is shown fully received within coupler bore  36 . When so received, flange  50  of bushing  38  abuts the upper or proximal face of coupler  30 . To affix alignment bushing  38  and template  20  to intramedullary rod  32 , set screw  44  is tightened as best shown in  FIG. 5 . More particularly, threaded engagement between set screw  44  and coupler  30  drives set screw  44  into flat  42 ′ ( FIG. 3A ), which in turn applies pressure onto flat  42 ′ to capture intramedullary rod  30  between the halves of alignment bushing  38 . As this pressure is applied, gaps  48  narrow slightly and flat  42  of bushing  38  is urged against flat  40  of bore  36 . The pressure generates friction between bore  46  and intramedullary rod  32 , and between coupler  30  and coupler bore  36 . This friction effectively fixes template  20 , bushing  38 , and intramedullary rod  32  to one another. Because intramedullary rod  32  is substantially immovably fixed to tibia T, template  20  is also immovably fixed to tibia T. 
         [0043]    As noted above, bushing  38  occupies the clearance space between intramedullary rod  32  and coupler bore  36 , such that bushing  38  cooperates with the geometry of template  20  to fully constrain the spatial location and orientation of template  20  with respect to intramedullary rod  32  (and, therefore, also with respect to tibia T). In some cases, centered bushing  38  also centers template  20  over tibia T. However, in other cases, a patient&#39;s natural intramedullary canal is not centered with respect to the geometry of the proximal tibia T, which leads to an off-center orientation of template T. In such cases, it may be appropriate to offset template  20  with respect to intramedullary rod  32 . 
         [0044]    To accommodate such offset, offset alignment bushing  38 A ( FIG. 3B ) may be provided. Offset bushing  38 A is similar to centered alignment bushing  38  described above, and reference numbers in  FIG. 3B  refer to analogous structures shown in  FIG. 3A  and described above with respect to bushing  38 . However, bore  46 A defines longitudinal axis A 2  which is offset with respect to longitudinal axis A 1  of bushing  38 A. In the illustrative embodiment of  FIG. 3B , the direction of this offset is generally parallel to flats  42 A,  42 A′, such that using offset bushing  38 A will laterally or medially offset template  20  (depending on which way bushing  38 A is installed in bore  36 ). It is contemplated that a similar offset may also be accomplished in an anterior or posterior direction by positioning bore  46 A closer to one of flats  42 A,  42 A′. 
         [0045]    Turning now to  FIG. 4 , burr template  20  can also be used when proximal tibial surface T S  is angled with respect to the longitudinal axis of intramedullary rod  32 . For example, in some surgical procedures it may be desirable to impart an anteroposterior slope to proximal tibial surface T S , such as to accommodate particular prosthesis design or to correct for natural deformity. In the illustrative embodiment of  FIG. 4 , a positive anterior slope having slope angle Θ has been provided, i.e., proximal tibial surface T S  elevates or “runs uphill” with respect to the longitudinal axis of intramedullary rod  32  as one traverses from the posterior portion of the tibia T is toward the anterior portion of tibia T. In addition to anteroposterior slope, a non-planar surface T S  may be provided, or medial-lateral slope may be used (such as to correct for varus or valgus deformity), or any combination thereof. Advantageously, template  20  is connected directly to intramedullary rod  32 , and is therefore indexed only to the longitudinal axis of intramedullary rod  32  (and, therefore, to the intramedullary canal of the tibia T) without regard to the geometry of proximal tibial surface T S . Thus, any slope or other geometry may be made on proximal tibial surface T S  without disturbing the intramedullary referencing of template  20 . 
         [0046]    In some instances, intramedullary rod  32  may not be pinned or otherwise fixed to tibia T. While intramedullary rod  32  will typically define a close tolerance fit with the reamed intramedullary canal of tibia T, intramedullary rod  32  may remain axially movable and rotatable about its longitudinal axis throughout knee replacement surgery. In order to prevent corresponding rotation and/or axial movement of template  20 , it may be desirable to provide a secondary fixation of template  20  to tibia T. 
         [0047]    Turning to  FIG. 6 , securement mechanism  52  may be provided to facilitate such fixation. Securement mechanism  52  includes anteroposteriorly extending arm  54  with set screw  56  extending therethrough. Set screw  56  is received in threaded bore  58  formed in an anterior portion of template  20 , and threaded engagement between set screw  56  and bore  58  fixes securement mechanism  52  to template  20  as shown in  FIG. 7 . In order to facilitate proper alignment of securement mechanism  52  with template  20 , shoulders  60  may be provided to engage a correspondingly shaped outer face  62  adjacent threaded bore  58 , as shown in  FIG. 6 . Fixation arm  64  extends distally from anteroposterior arm  54 , and includes a plurality of apertures  66  therethrough. Referring to  FIG. 7 , pin  68  may be passed through one or more apertures  66  and into tibia T to provide additional fixation of template  20 . 
         [0048]    With template  20  firmly affixed to tibia T, template track  22  may be used to define the perimeter of resection void V ( FIGS. 10 and 11 ). Referring to  FIG. 8 , the cutting tool used to define such perimeter includes burr  70 , which is received within burr guard  72 . Specifically, burr  70  includes cutting head  74  and drive shaft  76 , with drive shaft  76  received within bore  78  of burr guard  72 . When connected to rotary tool  80 , as shown in  FIG. 9 , burr  70  passes through burr guard  72  and into the bone stock of tibia T as described below. Burr guard  72  includes outer tube  82  with arm  84  extending therefrom. Arm  84  engages template track  22  as shown in  FIG. 9  to maintain cutting head  74  at particular angular orientation with respect to the longitudinal axis of intramedullary rod  32  as burr guard  72  is swept around the periphery of template track  22  from lateral track portion  24 , through posterior track portion  26  and to medial track portion  28  (or vice versa). 
         [0049]    As burr guard  72  is swept around the periphery of template track  22 , cutting head  74  can be plunged into tibia T by pushing on rotary tool  80  against the bias of spring  86  as shown in  FIG. 9  Inner tube  88  is received within outer tube  82 , and is axially movable within bore  78  to allow cutting head  74  to be selectively moved between a withdrawn position and a projected position. In the withdrawn position, cutting head  74  is received completely within bore  78  of outer tube  82 , such that the cutting head is incapable of effecting any resection. In the projected position, cutting head  74  projects outwardly from outer tube  82 , such that cutting head can resect material upon contact. Spring  86  urges cutting head into the fully withdrawn, stowed position in the absence of affirmative applied force overcoming such bias, such that outer tube  82  provides a protective sheath for cutting head  74 . Advantageously, this “normally withdrawn” configuration ensures that cutting head  74  is only exposed when the user engages burr guard  72  with template track  22  and then actively exerts an axial force on rotary tool  80 , as described below. 
         [0050]    A surgeon begins the resection process by engaging arm  84  of burr guard  72  with guide track  22 . Once arm  84  is so engaged, the surgeon applies a downward axial force to rotary tool  80 , which compresses spring  86  and causes cutting head  74  to emerge from within bore  78  of outer tube  82 . Cutting head  74  is then engaged with tibia T to begin the resection process. Cutting head  74  is then plunged to a specified depth, as shown in dashed lines in  FIG. 9 . Advantageously, cutting head  74  will automatically retract into outer tube  82  if the axial force is removed for any reason, such as upon completion of the resection operation or if arm  84  becomes inadvertently disengaged from guide track  22 . 
         [0051]    Resection continues by sweeping burr guard  72  around track  22 , while keeping arm  84  engaged with guide track  22 . At each new position, cutting head  74  is successively plunged into tibia T to the specified depth. Once cutting head has been plunged at each position around guide track  22 , a lateral, posterior, and medial periphery corresponding to tibial cone augments  34 ,  34 ′ ( FIGS. 10 and 12 ) is formed in tibia T. Pin  68  and intramedullary rod  32  may then be removed from tibia T, together with template  20  and securement mechanism  52 , leaving the medial, posterior and lateral portions of the resection periphery exposed. 
         [0052]    During the resection process, the depth of resection may be monitored by depth markings  90  on drive shaft  76 , which are visible through cut-out  92  formed in inner tube  88 . Cutting head  74  may be plunged to the full intended depth at each position on the first sweep around guide track  22 , such that the guide-track resection process is complete after single sweep. Alternatively, cutting head  74  may be plunged to a partial depth initially and to deeper depths in one or more subsequent sweeps around guide track  22  until the desired resection depth is achieved. 
         [0053]    Due to the presence of coupler  30 , the anterior portion of periphery P of resection void V ( FIGS. 11A and 11B ) is not defined through the use of template  20 . To define this portion of periphery P after removal of template  20 , tibial cone augment  34  may be placed upside down, as shown in  FIG. 10 , over the medial, posterior and lateral periphery created by the sweep-and-plunge resection process described above. Augment  34  is positioned on tibial surface T S  such that the lateral, posterior, and medial portions of tibial cone augment  34  are aligned with their corresponding lateral, posterior, and medial peripheral cuts made in tibia T. In an exemplary embodiment, tibial cone augment  34  used for this step is a provisional tibial cone augment, though it is contemplated that permanent tibial cone augment, such as augment  34 ′ ( FIG. 12 ) may also be used. 
         [0054]    With the proximal space of tibial cone augment  34  properly positioned upside down on proximal surface T S  of tibia T the anterior portion of periphery P ( FIG. 11A ) may be traced around tibial cone augment  34  with marking instrument  94 , such as manually by surgeon hand H as shown in  FIG. 10 . With the proper periphery thus marked, the remainder of periphery P may be created by freehand resection with any suitable tool, such as burr  72  described above. Coupler  32  may be maintained at a minimal size and extent within burr template  20 , which minimized the extent of the anterior freehand resection. 
         [0055]    Referring still to  FIGS. 11A and 11B , the remainder of the interior bone within the periphery P defined by the above described resection process may be removed to create resection void V. In an exemplary embodiment, this removal is conducted with the aid of burr sleeve  96 , which prevents the precisely formed periphery P and inner surface S of resection void V from being disturbed during the removal of the remaining interior bone. When this interior bone is fully removed to proper depth, resection void V is complete. 
         [0056]    Finally, as shown in  FIG. 12 , permanent tibial cone augment  34 ′ may be press-fit into resection void V. Ideally, such fit will be tight but will pose no risk for damage to tibia T by being too tight. Although a surgeon may make additional small resections at the margins of periphery P and/or resected surface S, such corrections are minimized or eliminated by the high degree of accuracy and precision afforded by template  20  and the associated components and methods described herein. 
         [0057]    Turning now to  FIG. 13 , an alternative embodiment is shown in which the depth of resection of periphery P is controlled by the upper face of a conical burr template. Rather than monitoring resection depth by depth markings  90  of burr  70  ( FIG. 8 ), resection depth is controlled by contact between burr guard  72  and proximal face  123  of template track  122  of burr template  120 , as described in detail below. Burr template  120  is similar to burr template  20  described above, and reference numbers in  FIG. 13  refer to analogous structures shown in  FIG. 1-9  and described above with respect to template  20 . However, template  120  includes a “rollercoaster”-like upper face  123  which defines variable height above resected surface T S  of tibia T. This variable height compensates for changes in the taper angles between various portions of template track  122 , thereby allowing the user to achieve a constant resection depth with respect to the longitudinal axis of intramedullary rod  32  without monitoring depth markings  90 . 
         [0058]    In the exemplary embodiment of  FIGS. 1-9 , medial lateral taper angle α ML  ( FIG. 9 ) is larger than anteroposterior taper angle α AP  ( FIG. 4 ). As cutting head  74  of burr  70  is swept around guide track  20  keeping arm  84  in contact therewith, cutting head  74  must be plunged relatively more deeply into tibia T at areas of high angulation (such as angle α ML  at lateral and medial track portions  24 ,  28 ) as compared with areas of lower angulation (such as angle α AP  at posterior track portions  26 ). The depth of plunge can be calculated for the different track portion angles to compensate for the additional axial, linear distance that must be traversed by cutting head  74  in the high-angulation areas, thereby producing a constant overall resection depth with respect to tibial surface T S . This calculated depth can then monitored by depth markings  90  as noted above. 
         [0059]    However, the embodiment of  FIG. 13  is configured to automatically adjust the plunge depth to accommodate varying track portion angles. Posterior track portion  126 , which defines a relatively smaller taper angle with respect to tibial surface T S , defines relatively elevated or “tall” portion of proximal face  123 , i.e., a portion set relatively far away from tibial surface T S . As template track transitions to lateral portion  124 , which defines relatively larger taper angle with respect to tibial surface T S  proximal face  123  slopes downwardly toward tibial surface T S  to define a relatively depressed or “short” portion of proximal face  123 , i.e., a portion set relatively close to tibial surface T S . Medial portion  126  (not shown) may be similarly configured with a dip or depression suitable for its particular angulation. 
         [0060]    In the embodiment of  FIG. 13 , cutting head  74  is limited to a fixed axial travel and is extended to the full extent of such axial travel throughout the resection process. For example, cutting head  74  may be advanced only up to a fixed amount before encountering a physical barrier to further advance, such a fully compressed spring  86 . At each position around template track  122  during the resection process, cutting head  74  may extend outwardly from outer tube  82  (i.e., “plunged”) and into tibia T by the same linear, axial amount. Assuming arm  84  remains in constant contact with template track  122 , this constant axial travel of cutting head  74  cooperates with the variable height of proximal face  123  to produce a constant resection depth around the entire periphery of template track  122 . 
         [0061]    Turning again to  FIG. 9 , it can be seen that burr guard  72  and burr  70  pass through template  20  along inner face  25  of template track  22 . As shown in  FIG. 14 , however, burr template  220  may be provided which facilitates the tracing of burr  70  and burr guard  72  around exterior face  225  of template track  222 . Burr template  220  is similar to burr template  20  described above, and reference numbers in  FIG. 14  refer to analogous structures shown in  FIG. 1-9  and described above with respect to template  20 . However, template track  222  defines a smaller circumference as compared to template track  22 , such that tracing template track  222  around exterior face  225  produces the same resection void V produced by tracing interior face  25  of template track  22 . 
         [0062]    Advantageously, this “exterior tracing” modality allows conical burr template  220  to be made smaller than burr template  20  for a given size and geometry of resection void V. Further, tracing exterior face  225  allows greater visual access to cutting head  74  during the resection procedure, because cutting head  74  is fully exposed to the surgeon rather than contained within the template track. 
         [0063]    Template track  222  may also be used in the same manner as template track  22 , i.e., with burr  70  and burr guard  72  traced around the inner face of track  222 , or vice-versa. For example, a surgeon may choose to sweep around the exterior of guide track  22  to convert resection void V from being sized for press-fit engagement with tibial cone augment  34 ′, as detailed above, to being sized for a clearance fit suitable for use with cemented fixation of augment  34 ′. 
         [0064]    It is contemplated that the wall thickness of the template track (e.g., the thickness along direction normal to the longitudinal axis of intramedullary rod  32  in template tracks  22 ,  222 ) may be varied to change the difference in size of the associated resection void. For example, relatively thick template track wall will result in relatively large difference in such sizes, such that one template may be suitable for two different implant sizes. In this example, the smaller implant would correspond to the resection void created by sweeping a cutting instrument around the inner face of the track, while the larger implant would correspond to the resection void created by sweeping the cutting instrument around the outer face of the track. On the other hand, relatively thin template track might be appropriate for defining press-fit and clearance fits, as described above 
         [0065]    Advantageously, the method and apparatus disclosed herein facilitate the creation of a resection cavity sized and shaped for an improved fit between the cavity and augment. 
         [0066]    While this invention has been described as having exemplary designs, the present disclosure 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 disclosure 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 disclosure pertains and which fall within the limits of the appended claims.