Abstract:
A bone resection tool for resecting an end of a bone along a surface having a curvature comprises a guide, a cutting tool and a track follower. The guide is configured to be removably attached in a fixed position to the end of the bone. The guide is configured to include a track exhibiting a curvature generally corresponding to the curvature of the surface to be resected in the bone. The cutting tool includes a cutting face. The track follower is configured to couple to the cutting tool and cooperate with the track to facilitate reciprocation of the cutting tool relative to the guide to induce the cutting face to resect the bone along the surface having the curvature. A method for cutting a bone along a curved surface conforming to the curvature of a curved surface of the underside of a prosthetic component comprises the steps of incising the tissue surrounding the surface of the bone, positioning a guide alongside the surface of a bone to be cut, affixing the guide to the bone, interconnecting a cutter having a cutting face with the guide, maintaining the cutting face generally parallel to the tangent of the curved surface and traversing the cutting face along the bone while guiding the cutter along the track. The tissue incision incises the tissue surrounding the surface of the bone to be cut in a minimally invasive fashion. The positioned guide includes a track configured to assimilate the curvature of the curved surface of the underside of the prosthetic component.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to joint arthroplasty and more particularly to tools and techniques for shaping a bone to receive a component of a joint replacement system.  
       BACKGROUND AND SUMMARY  
       [0002]     It is well known to provide prosthetic joint components for replacing damaged and deteriorating joints. Typical joint replacements require resection of distal or proximal end of one or more of the bones forming the joint to be replaced to permit the prosthetic device to be firmly attached to the bone without altering the length of the limb in which the joint is being replaced. Joint component manufactures have recognized the benefits to be derived from joint replacement procedures that require less invasive surgeries with smaller incisions and more kinematically correct implants. Additionally, as younger patients are receiving joint replacements, there is a desire to provide joint replacements that allow the patient to recover from the replacement surgery more quickly, provide better function and are more durable.  
         [0003]     One key to a kinematically correct durable implant is the interface between the bone and the implant. Due to the likelihood of eventual failure of a prosthesis there is always the possibility of the need for a replacement prosthetic procedures that will require further bone resections. Thus, it is preferable to resect a sufficient area of the bone to permit proper seating of the implant on the bone and to facilitate in-growth and/or on-growth of the bone to the implant, while reducing the amount of bone resected to allow further bone resections to accommodate revision or replacement of the implant. One method utilized to address the competing concerns of providing adequate bone resection to properly seat an implant and reducing the amount of bone resected to facilitate revision or replacement of the implant is to provide a surface replacement implant. Surface replacement implants require shaping the bone into curved or multiple non-coplanar surfaces rather than flat surfaces. It would be preferable to provide instruments for resecting the articulating surfaces of a bone to receive an implant that are minimally sized and that accurately guide a cutting tool to create curved surfaces for receipt of a surface replacement implant.  
         [0004]     One type of surface implant in common use is the uni-compartmental knee system. The tibia (leg bone) and femur (thigh bone) meet at the knee and divide into medial (inner) and lateral (outer) tibio-femoral compartments. The patella (kneecap) in the front of the knee articulates with the trochlea (kneecap socket) of the femur to form the patello-femoral compartment. The tibio-femoral compartments are used for walking. The patello-femoral compartment is used for kneeling, squatting and stair-climbing. In unicompartmental knee arthroplasty, one of the knee&#39;s tibio-femoral compartments, usually the medial one, is resurfaced. This should be contrasted to total knee arthroplasty (TKA), which resurfaces both of the tibio-femoral compartments and usually the patello-femoral compartment. With the increased prevalence of unicompartmental knee arthroplasty (UKA), a prosthetic system which provides a conservative approach in terms of both instrumentation and implant design is important.  
         [0005]     Age and activity level factor into all reconstructive procedures. Typically, the state of arthritis determines the treatment. With the advancement of minimally invasive techniques that support uni-compartmental knee reconstruction, a growing number of patients are offered this alternative for relief from the disabling pain of arthritis and for the potential benefits of a rapid recovery. Some patients have very significant arthritis changes in only one part of their knee, frequently on their inner or medial side. If there are no other major arthritis changes on the other side of the knee or under the knee cap, partial knee replacement (officially “uni-compartmental knee replacement”) is indicated. Uni-compartmental knee replacement is a less invasive partial knee replacement procedure.  
         [0006]     Benefits of uni-compartmental knee arthroplasty over total knee arthroplasty include 1) faster recovery, 2) less pain, 3) greater range of motion, 4) greater feeling of normalcy, 5) better alternatives when the prosthesis wears out, 6) no blood transfusions and 7) no need for blood thinners. Many of these benefits arise because with a uni-compartmental arthroplasty the surgery is less extensive, the incision is smaller, and there is less tissue trauma than in a TKA. A primary TKA will usually last 10-15 years. When it fails it must be replaced with a Revision TKA (RTKA). RTKAs have a high complication rate and don&#39;t last as long as primary TKAs. The uni-compartmental arthroplasty buys time. When it wears out it is replaced by a TKA. Patients may never need an RTKA or if they do, they generally will have gotten many more years use out of their TKA. By utilizing a uni-compartmental arthroplasty instead of a TKA, patients of any age can benefit. Older patients benefit from the reduced severity of the procedure and easier recovery. Younger patients benefit because when the prosthesis eventually fails (all prostheses fail faster in younger patients), they will be able to have it replaced with the better primary TKA as opposed to the less desirable RTKA.  
         [0007]     Some prior art uni-compartmental knee systems have provided either limited instrumentation, making reproducible alignment difficult, or bulky instrumentation which requires more intrusive surgery. A few prior art uni-compartmental knee systems are designed with bone conserving femoral and tibial components that provide reproducible results utilizing a minimal incision. The resurfacing femoral implant conserves more quality bone stock compared to contemporary full resection femoral implants.  
         [0008]     One such uni-compartmental knee system is the PRESERVATION™ Uni-compartmental Knee, available from DePuy Orthopaedics. The PRESERVATION™ Uni-compartmental Knee offers the surgeon fixed and mobile bearing tibial options specific to patient requirements. The system supports a less invasive procedure with instruments that provide for joint line restoration, load sharing balance, and component alignment. The PRESERVATION™ uni-compartmental knee system utilizes a technique that minimizes bone resections for later total knee arthroplasty options. The system is adapted to be utilized with Computer Aided Surgical (“CAS”) technology.  
         [0009]     The technical challenges of less invasive uni-compartmental knee surgery are becoming more apparent, even for the specialist arthroplasty surgeon. CAS technology has a clear role to play in less invasive surgery. CAS technology provides enhanced surgical vision which optimizes visualization of the critical anatomical landmarks, irrespective of the length of the incision. CAS technology offers the surgeon a level of vision and control that is difficult to achieve with non-CAS enabled, less invasive, procedures. With the key anatomy fully visualized, finger-tip instrument adjustment allows the surgeon to transfer on-screen planning to the table with greater precision. Virtual planning and kinematic assessment software provided with CAS technology allows implant positioning for each patient prior to any bone cuts being made.  
         [0010]     The disclosed invention provides guides and a cutting tools configured to follow the guides to resect a bone to receive a surface replacement implant. The guide includes features to maintain the correct alignment of the cutting tool while providing sufficient freedom to create cuts having the desired depth for receiving a surface replacement prosthesis. The guides and cutting tools are configured to be effectively used in a minimal incision procedure while still offering the surgeon the alignment guides needed for consistent results. The disclosed guides are configured for utilization with CAS technology. Thus, reproducible outcomes are possible within a minimal incision.  
         [0011]     According to one aspect of the disclosure, a bone resection tool for resecting an end of a bone along a surface having a curvature comprises a guide, a cutting tool and a track follower. The guide is configured to be removably attached in a fixed position to the end of the bone. The guide is configured to include a track exhibiting a curvature generally corresponding to the curvature of the surface to be resected in the bone. The cutting tool includes a cutting face. The track follower is configured to couple to the cutting tool and cooperate with the track to facilitate reciprocation of the cutting tool relative to the guide to induce the cutting face to resect the bone along the surface having the curvature.  
         [0012]     According to another aspect of the disclosure, a method for cutting a bone along a curved surface conforming to the curvature of a curved surface of the underside of a prosthetic component comprises the steps of incising the tissue surrounding the surface of the bone, positioning a guide alongside the surface of a bone to be cut, affixing the guide to the bone, interconnecting a cutter having a cutting face with the guide, maintaining the cutting face generally parallel to the tangent of the curved surface and traversing the cutting face along the bone while guiding the cutter along the track. The tissue incision incises the tissue surrounding the surface of the bone to be cut in a minimally invasive fashion. The positioned guide includes a track configured to assimilate the curvature of the curved surface of the underside of the prosthetic component.  
         [0013]     The accompanying drawings, which are incorporated in and constitute part of the specification, illustrate the preferred embodiments of the invention, and together with the description, serve to explain the principles of the invention. It is to be understood, of course, that both the drawings and the description are explanatory only and are not restrictive of the invention.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The illustrative devices will be described hereinafter with reference to the attached drawings which are given as non-limiting examples only, in which:  
         [0015]      FIG. 1  is an exploded view of a first embodiment of a guide, a CAS orientation device and a first embodiment of a guided cutting tool including a cutter, a retaining collar and a depth stop of the bone shaping tool disclosed herein;  
         [0016]      FIG. 2  is a top plan view of the guide of the bone shaping tool of  FIG. 1 ;  
         [0017]      FIG. 3  is side elevation view of the guide of  FIG. 2 ;  
         [0018]      FIG. 4  is an end view of the guide of  FIG. 3 ;  
         [0019]      FIG. 5  is a side view of the first embodiment of the guide of  FIG. 1  and a second embodiment of a guided cutting tool including a posterior cutter, an anterior cutter and a driver of the bone shaping tool disclosed herein;  
         [0020]      FIG. 6  is a top view of the guide and guided cutting tool of  FIG. 5 ;  
         [0021]      FIG. 7  is a sectional view taken along line  7 - 7  of the guide and guided cutting tool of  FIG. 6 ;  
         [0022]      FIG. 8  is a perspective view of a the bone shaping tool of  FIG. 1  pinned to the medial condyle on the distal end of a femur of a patient;  
         [0023]      FIG. 9  is a side elevation view of the bone shaping tool of  FIG. 8  showing the first embodiment of the guided cutting tool received in the guide prior to beginning resection of the medial condyle of the femur;  
         [0024]      FIG. 10  is a side elevation view similar to  FIG. 9  showing the guided cutting tool inserted into the medial condyle of the femur to the desired depth of the resection after being plunged into the femur from the position shown in  FIG. 9 ;  
         [0025]      FIG. 11  is a side elevation view similar to  FIG. 10  showing the guided cutting tool inserted into the medial condyle of the femur to the desired depth of the resection after being translated anteriorly guided by the guide;  
         [0026]      FIG. 12  is a side elevation view of a second embodiment of a guide and a third embodiment of a guided cutting tool similar wherein the medial and lateral side rails of the guide to guide two sets of four guide pins extending from the retainer collar;  
         [0027]      FIG. 13  is an exploded view of a third embodiment of a guide configured to include a guide channel, and a fourth embodiment of a guided cutting tool including a cutter, a retaining collar configured to receive guide pins configured to be captured in the guide channel of the guide of the bone shaping tool disclosed herein;  
         [0028]      FIG. 14  is a perspective view of the third embodiment of a guide and the fourth embodiment of a guided cutting tool of  FIG. 13  affixed to the bone to be resected;  
         [0029]      FIG. 15  is a perspective view of a fourth embodiment of a guide configured to be affixed to the bone to be resected by sliding into a slot formed in the bone and a fifth embodiment of a guided cutting tool of the bone shaping tool disclosed herein;  
         [0030]      FIG. 16  is a perspective view of the fourth embodiment of the guide and fifth embodiment of the guided cutting tool of the bone shaping tool of  FIG. 15 ; and  
         [0031]      FIG. 17  is a perspective view of the fourth embodiment of the guide and fifth embodiment of the guided cutting tool of the bone shaping tool of  FIG. 15  affixed to the bone to be resected. 
     
    
       [0032]     Corresponding reference characters indicate corresponding parts throughout the several views. Like reference characters tend to indicate like parts throughout the several views.  
       DETAILED DESCRIPTION  
       [0033]     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the invention is thereby intended. It is further understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the invention as would normally occur to one skilled in the art to which this invention pertains.  
         [0034]     The disclosed bone shaping instruments facilitate preparation of a bone to receive a surface replacement implant such as a modified femoral component of a PRESENTATION™ uni-compartmental knee, available from DePuy Orthopedics, a Johnson &amp; Johnson company modified to have an underside comprising a surface exhibiting a curvature. The instruments are described as being configured to prepare the distal femoral condyles or trochlea for knee arthroplasty. The instruments provide the options or preparing curved saggital and coronal cuts.  
         [0035]     In one embodiment of the bone shaping instrument, an end and side cutting tool is translated within and guided by a tracked guide over a bone. The depth of the cut is controlled by a depth stop on the cutting tool engaging the guide and by the length of the legs of the guide which may be extendable or may be effectively extended using shims. The depth stop also controls the radius of curvature of the resection surface created by the instrument. Thus, if a deeper resection is required but the radius of curvature of the resected surface is not to be reduced, the depth of the resection should be adjusted by decreasing the length of the feet of the guide. However, if a deeper resection requiring a smaller radius of curvature is desired, the depth of the resection should be adjusted by moving the depth stop relative to the shaft of the cutting tool away from the cutting face of the cutting tool.  
         [0036]     The depth stop is configured to be adjusted longitudinally with respect to the cutting tool to permit a surgeon to select the depth and the radius of curvature of the resection to be made. The depth stop and guide are configured to maintain the proper orientation of the cutting tool while it is being translated within the guide. In the illustrated embodiment the depth stop and guide are configured to maintain the cutting tool perpendicular to a guide track formed in the guide.  
         [0037]     In a second embodiment of a cutting tool for use with the first embodiment of the guide, two rasps are configured to be driven by a hand drill while being guided by the guide. The rasps reciprocate on the surface of the bone creating the desired resection shape. Each of the cutting tools illustrated are configured to be driven by a flexible drive shaft to facilitate use of the bone shaping instrument in a surgical procedure utilizing minimally invasive incisions.  
         [0038]     While only one size of each embodiment of the guide and each of the guided cutting tools are described in the disclosure, those skilled in the art will recognize that knee prosthesis systems typically include a plurality of differently sized femoral components and that for such a system a plurality of appropriately configured and sized guides and guided cutting tools could be provided. Thus, the disclosed guides and guided cutting tools will be described with reference to a femoral component of a uni-compartmental knee prosthesis system associated with the guide and guided cutting tool.  
         [0039]     As shown, for example, generally in  FIGS. 1-11  and more particularly in  FIGS. 1-4 , the first embodiment of the guide  10  includes a medial rail  12 , a lateral rail  14 , a posterior cross member  16 , an anterior cross member  18 , a posterior leg  19 , an anterior leg  21 , and a plurality of ears  24 ,  26 ,  28 ,  30 ,  32  and  34 . The posterior cross member  16  extends between and couples the posterior end  36  of the medial rail  12  and the posterior end  38  of the lateral rail  14 . The anterior cross member  18  extends between and couples the anterior end  40  of the medial rail  12  and the anterior end  42  of the lateral rail  14 . The medial rail  12 , lateral rail  14 , posterior cross member  16  and anterior cross member  18  cooperate to define a tool-receiving slot  44  through which portions of the cutting tools  100 ,  200  and/or drive shafts  112  of the power source  114  extend to drive the cutting tools  100 ,  200 .  
         [0040]     In the illustrated embodiment, the medial rail  12  and the lateral rail  14  extend longitudinally parallel to one another to define the side walls of the tool-receiving slot  44 . The medial rail  12  and the lateral rail  14  are spaced apart from one another by a displacement which also defines the width  46  of the tool-receiving slot  44 . In one illustrated embodiment, the width  46  of the tool-receiving slot  44  is approximately thirteen millimeters to facilitate use in a minimally invasive procedure.  
         [0041]     The posterior cross member  16  and anterior cross member  18  extend laterally parallel to one another to define the end walls of the tool-receiving slot  44 . The posterior cross member  16  and anterior cross member  18  are spaced apart from one another by a displacement which also defines the length  48  of the tool-receiving slot  44 .  
         [0042]     Illustratively the medial rail  12  includes a concave bottom wall  50 , a convex top wall  52 , an inner wall  54  and an outer wall  56 . The inner wall  54  and outer wall  56  are each flat and are generally parallel to one another. The inner wall  54  and outer wall  56  extend between and couple the concave bottom wall  50  and convex top wall  52 . The concave bottom wall  50  and concave top wall  52  each exhibit a curvature that corresponds to the curvature of the underside of a femoral component of the uni-compartmental knee prosthesis associated with the particular guide  10 .  
         [0043]     The medial rail  12  is formed to include a curved slot  58  defined by a concave top wall  60 , a convex bottom wall  62 , a posterior wall  64  and an anterior wall  66 , each of which extend between the inner wall  54  and the outer wall  56 . The concave top wall  60  and convex bottom wall  62  each exhibit a curvature that corresponds to the curvature of the underside of a femoral component of the uni-compartmental knee prosthesis associated with the particular guide  10 .  
         [0044]     The concave top wall  60  and convex bottom wall  62  are spaced apart from one another by a displacement that serves as the width  68  of the curved slot  58 . The width of the curved slot  58  is sized to capture guide pins  120 ,  214  therein for anterior-posterior longitudinal movement within the curved slot  58 . The anterior wall  66  and posterior wall  64  of the curved slot  58  are spaced apart from one another by a displacement that serves as the length  70  of the curved slot  58 . The anterior wall  66  and posterior wall  64  act as stops that limit the anterior-posterior longitudinal movement of guide pins  120 ,  214  when captured within the curved slot  58 .  
         [0045]     The medial rail  12  is formed to include a posterior medial ear  24 , a central medial ear  26  and an anterior medial ear  28  extending outwardly from the outer wall  56 . The posterior medial ear  24  is positioned adjacent the posterior end  36  of the medial rail  12 . The anterior medial ear  28  is positioned adjacent the anterior end  40  of the medial rail  12 . The central medial ear  26  is positioned near the middle of the medial rail  12  between the posterior medial ear  24  and the anterior medial ear  28 . Each of the posterior medial ear  24 , central medial ear  26  and anterior medial ear  28  are formed to include a fastener-receiving hole  29  configured to receive a fastener  31  for securing the guide  10  to the femur, as shown, for example, in  FIGS. 8-11 . The fastener-receiving holes  29  are formed concentrically about an axis that extends radially toward the focus of the center of curvature of the underside of a femoral component of the uni-compartmental knee prosthesis associated with the particular guide  10 . The orientation of the fastener-receiving holes  29  facilitates insertion of the fasteners  31  through a minimally invasive incision to secure the guide  10  to the femur.  
         [0046]     Illustratively, the medial rail  12  and the lateral rail  14  are formed symmetrically about a plane extending perpendicular to the rails  12 ,  14  through the center of the tool-receiving slot  44 . Thus, the lateral rail  14  includes a concave bottom wall  72 , a convex top wall  74 , an inner wall  76  and an outer wall  78 . The inner wall  76  and outer wall  78  are generally parallel to one another and extend between and couple the concave bottom wall  72  and convex top wall  74 . The concave bottom wall  72  and concave top wall  74  each exhibit a curvature that corresponds to the curvature of the underside of a femoral component of the uni-compartmental knee prosthesis associated with the particular guide  10 .  
         [0047]     The lateral rail  14  is formed to include a curved slot  80  defined by a concave top wall  82 , a convex bottom wall  84 , a posterior wall  86  and an anterior wall  88  each of which extend between the inner wall  76  and outer wall  78 . The concave top wall  82  and convex bottom wall  84  each exhibit a curvature that corresponds to the curvature of the underside of a femoral component of the uni-compartmental knee prosthesis associated with the particular guide  10 . The concave top wall  82  and convex bottom wall  84  are spaced apart from one another by a displacement that serves as the width  90  of the curved slot  80 . The width  90  of the curved slot  80  is sized to capture a guide pin  120 ,  214  therein for anterior-posterior longitudinal movement within the curved slot  80 . The anterior wall  88  and posterior wall  86  of the curved slot  80  are spaced apart from one another by a displacement that serves as the length  92  of the curved slot  80 . The anterior wall  88  and posterior wall  86  act as stops that limit the anterior-posterior longitudinal movement of guide pins  120 ,  214  when captured within the curved slot  80 .  
         [0048]     The lateral rail  14  is formed to include a posterior lateral ear  30 , a central lateral ear  32  and an anterior lateral ear  34  extending outwardly from the outer wall  78 . The posterior lateral ear  30  is positioned adjacent the posterior end  38  of the lateral rail  14 . The anterior lateral ear  34  is positioned adjacent the anterior end  42  of the lateral rail  14 . The central lateral ear  32  is positioned near the middle of the lateral rail  14  between the posterior lateral ear  30  and the anterior lateral ear  32 . Each of the posterior lateral ear  30 , central lateral ear  32  and anterior lateral ear  34  are formed to include a fastener-receiving hole  29  configured to receive a fastener  31  for securing the guide  10  to the femur.  
         [0049]     The posterior leg  19  extends radially inwardly from the bottom surface of the posterior cross member  16  of the guide  10  terminating in a posterior foot  20 . The anterior leg  21  extends radially inwardly from the bottom surface of the anterior cross member  18  of the guide  10  terminating in an anterior foot  22 . The posterior foot  20  and the anterior foot  22  are configured to engage a condyle of the femur, or a shim  212 , as described hereafter with reference to the second guided cutter  200 , when the guide  10  is positioned for resection of the condyle. The posterior leg  19  and anterior leg  21  cooperate to displace the concave bottom wall  50  of the medial rail  12  and the concave bottom wall  72  of the lateral rail  14  from the condyle of the femur when the posterior and anterior feet  20 ,  22 , respectively, engage the condyle or a shim  212 . Since the effective length of the posterior leg  19  and the anterior leg  21  control the depth of the resection, they may be configured to be extendable and retractable if shim usage is not desired. Alternatively, a plurality of guides  10  may be provided each having posterior legs  19  and anterior legs  21  of varying lengths.  
         [0050]     As shown, for example, in  FIGS. 1, 4  and  8 , the anterior leg  21  is formed to include an aperture  98  sized and configured to receive a shaft  94  of a CAS orientation device  96  in a fixed orientation. When the shaft  94  of the CAS orientation device  96  is received in the aperture  98 , the orientation device  96  provides details of the orientation of the guide  10  in three dimensions. Thus, when the CAS orientation device  96  is received in the aperture  98 , CAS techniques may be utilized to ensure the proper positioning of the guide  10 . Those skilled in the art will recognize that other couplings in different locations can be provided for attaching a CAS orientation device  96  to the guide  10  within the scope of the disclosure.  
         [0051]     The first embodiment of a guided cutting tool  100  includes a depth stop  102 , a cutter  104  and a retaining collar  110 . The cutter  104  includes a cutting face  106  and a shaft  108 . Illustratively, the cutter  104  includes a cutting face  106  that provides for end and side cutting. Cutter  104  may be a reamer, such as a concave spherical, convex spherical or planar reamer, an end mill or a burr. The cutting face  106  is mounted to the shaft  108  to be rotated about the longitudinal axis  140  of the shaft  108  when the shaft  108  is turned.  
         [0052]     Illustratively, the shaft  108  is configured to be coupled to a drive shaft  112  of a power source  114 . The power source  114  may be a hand drill with a rigid, or preferably flexible, drive shaft  112 . The hand drill may be manually, electrically, pneumatically or hydraulically powered. It is within the scope of the disclosure for the power source  114  to be some other device configured to rotate the shaft  108  of the cutter  104 .  
         [0053]     The retaining collar  110  includes a body  116  formed to include a shaft-receiving aperture  118  and a plurality of guide pins  120 . The body  116  of the retaining collar  110  includes parallel spaced apart side walls  122 ,  124  extending between and coupling a top wall  126  to a bottom wall  128  and an anterior wall  130  to a posterior wall  132 . The top wall  126  is planar and is configured to act as a stop surface against which the depth stop  102  engages to limit the depth of the cut of the cutter  104 . The shaft-receiving aperture  118  extends through the top wall  126  and the bottom wall  128 . Illustratively, the shaft-receiving aperture  118  is defined by a cylindrical side wall  134  formed concentrically about a longitudinal axis  136  normal to the top wall  126 . The cylindrical side wall  134  has an inside diameter  138 . Illustratively, the cylindrical side wall  134  is configured to receive the shaft  108  of the cutter  104  therein and is sized to permit the shaft  108  of the cutter  104  to rotate freely therein about its longitudinal axis  140 . The cylindrical side wall  134  cooperates with the shaft  108  of the cutter  104 , and with the retaining collar  110  and depth stop  102 , to maintain the alignment of the cutter  104 . Thus, the inside diameter  138  of the shaft-receiving aperture  118  is slightly larger than the outside diameter  142  of the shaft  108  of the cutter  104  to limit misalignment of the cutter  104 . In the illustrated embodiment, the shaft  108  of the cutter  104  is aligned to extend radially to facilitate utilization of the bone shaping instrument through a minimally invasive incision.  
         [0054]     In the illustrated embodiment, the plurality of guide pins  120  include an anterior medial guide pin  144  and a posterior medial guide pin  146  extending perpendicularly from the medial side wall  122  of the retaining collar  110  and an anterior lateral guide pin  148  and a posterior lateral guide pin  150  extending perpendicularly from the lateral side wall  124  of the retaining collar  110 . The guide pins  120  define a plane parallel to the top wall  126  of the retaining collar  110 . Illustratively, each of the plurality of guide pins  120  have a diameter  152  slightly less than the widths  68 ,  90  of the curved slots  58 ,  80  permitting the guide pins  120 , when received in the curved slots  58 ,  80 , to reciprocate anteriorly-posteriorly within the curved slots  58 ,  80 . The guide pins  120  may be spring loaded to permit the guide pins  120  to be reciprocated along their longitudinal axis into and out of the body  116  of the retaining collar  110 .  
         [0055]     The medial side wall  122  and the lateral side wall  124  of the body  116  of the retaining collar  110  are spaced apart by a displacement  154 . The displacement  154  between the medial side wall  122  and the lateral side wall is slightly less than, but approximately equal to, the width  46  of the tool-receiving slot  44  of the guide  10 . In use, portions of the retaining collar  110  are received in the tool-receiving slot  44  of the guide  10  for reciprocal movement anteriorly and posteriorly within the tool-receiving slot  44 . Portions of the medial side wall  122  are disposed adjacent the inner wall  54  of the medial rail  12  and portions of the lateral side wall  124  are disposed adjacent the inner wall  76  of the lateral rail  14 . The medial side wall  122  of the retaining collar  110  and the inner wall  54  of the medial rail  12  and the lateral side wall  124  of the retaining collar  110  and the inner wall  76  of the lateral rail  12  cooperate to restrict rotation of the retaining collar  110  about the longitudinal axis  136  of the shaft-receiving aperture  118  when the retaining collar is received within the tool-receiving slot  44 . Since rotation of the retaining collar  110  within the tool-receiving slot  44  is limited, the guide pins  120  are retained within the curved slots  58 ,  80 .  
         [0056]     Illustratively, guide pins  120  act as track followers configured to induce the retaining collar  110  to follow a path having a curvature conforming to the curvature of the convex bottom walls  62 ,  84  of the curved slots  58 ,  80 . Those skilled in the art will recognize that other track following devices may be utilized within the scope of the disclosure including, but not limited to walls, bosses, ears, and flanges extending from the body  116  of the retaining collar  110  and configured to be received in and guided by the curved slots  58 ,  80  during anterior-posterior reciprocation within the curved slots  58 ,  80 . Also, it is within the scope of the disclosure for the medial rail and lateral rail to be formed with a curved bottom wall and a curved top wall that act as tracks and the retaining collar to include track followers such as guide pins, walls, bosses, ears, and flanges extending from the body  116  and configured to follow the curved bottom wall and a curved top wall, as shown, for example, in  FIG. 12 .  
         [0057]     In use, the guide pins  120 , when captured within the curved slots  58 ,  80 , ride on the convex bottom walls  62 ,  84  of the curved slots  58 ,  80  which define a curved guide surface or track. The concave top walls  60 ,  82  of the curved slots  58 ,  80  act to restrict the proximal and distal movement of the guide pins  120  relative to the guide  10 . The guide pins  120  ride within the curved slots  58 ,  80  and act to restrict the cutting face  106  of the cutter  104  to move along a curvature approximating the curvature of an inner surface of a femoral component of the uni-compartmental knee prosthesis associated with the particular guide  10 .  
         [0058]     The illustrated depth stop  102  includes a cylindrical body  160  formed to include a planar circular top wall  162 , a cylindrical outer wall  164  and a planar circular bottom wall  166  formed concentrically about an axis  168 . A shaft-receiving aperture  170  extending between and through the top wall  162  and bottom wall  164  is defined by a cylindrical side wall  172  formed concentrically about the axis  168 . Illustratively, the cylindrical side wall  172  is configured to receive the shaft  108  of the cutter  104  therein and is sized to permit the shaft  108  of the cutter  104  to reciprocate longitudinally along its longitudinal axis  140 . Thus, the inside diameter  174  of the shaft-receiving aperture  170  is slightly larger than the outside diameter  142  of the shaft  108  of the cutter  104  to facilitate adjusting the position of the depth stop  102  along the shaft  108  of the cutter  104 . A set screw-receiving hole  176  extends radially between and through the outer wall  164  and cylindrical side wall  172  of the body  160  of the depth stop  102 . The set screw-receiving hole  176  is appropriately sized and threaded to receive a set screw  178  therein for tightening to secure the depth stop  102  in a fixed position relative to the shaft  108  of the cutter  104 . Those skilled in the art will recognize that other configurations of the depth stop  102  may be utilized to permit the depth stop  102  to be temporarily or permanently affixed to the shaft  108  of the cutter  104   
         [0059]     The planar circular bottom wall  166  of the depth stop  102  cooperates with the planar top wall  126  of the retaining collar  110  to limit the proximal movement of the cutter  104  relative to the femur thereby limiting the depth of the resection performed by the cutter  104 . The planar circular bottom wall  166  of the depth stop  102  also engages the planar top wall  126  of the retaining collar  110  to facilitate proper alignment of the longitudinal axis of the shaft  108  of the cutter  104  with the axis  136  of the retaining collar  110  thereby maintaining correct alignment of the cutting face  106  when at the full resection depth. The cylindrical side wall  134  of the retaining collar  110  also cooperates with the shaft  108  of the cutter  104 , and with the retaining collar  110  and depth stop  102 , to maintain the alignment of the cutter  104  at other depths.  
         [0060]     As shown, for example, in  FIGS. 5-7 , the second embodiment of a guided cutting tool  200  includes a posterior cutter  202 , an anterior cutter  204 , a posterior return spring  206 , an anterior return spring  208 , a driver  210 , a plurality of pairs of shims  212  and a plurality of guide pins  214 . The plurality of pairs of shims  212  are provided for placing under the anterior foot  22  and posterior foot  20  of the guide  10  for adjusting the effective length of the posterior leg  19  and the anterior leg  21  and, thus, the depth of the resection performed by the guided cutting tool  200 . The plurality of pairs of shims  212  will be provided in pairs having incrementally differing thickness to facilitate incrementally increasing the depth of the resection by replacing the shims  212  after the posterior and anterior cutters  202 ,  204  cease removing bone from the femur.  
         [0061]     The posterior cutter  202  includes a body  216  having a top wall  218 , a cutting surface  220 , a medial side wall  222 , a lateral side wall  224 , a posterior end wall  226  and an anterior end wall  228 . The medial side wall  222  and lateral side wall  224  extend between and couple the top wall  218  to the cutting surface  220  and the anterior end wall  228  to the posterior end wall  226 .  
         [0062]     In the illustrated embodiment, the top wall  218  and cutting surface  220  exhibit a posterior-anterior curvature that corresponds to the anterior-posterior curvature of the inner surface of a femoral component of the uni-compartmental knee prosthesis corresponding to the posterior cutter  202 . Those skilled in the art will recognize that the curvature of the top wall  218  is not critical to the operation of the posterior cutter  202 , but rather provides some aesthetic and possibly safety features to the cutter  202 . Thus, it is within the scope of the disclosure for the top wall  218  to be flat or exhibit a different curvature than the cutting surface  220 .  
         [0063]     The cutting surface  220  is formed to include rows of teeth  230  extending medially-laterally across the cutting surface  220 . The cutting surface  220 , and its associated rows of teeth  230 , may be medially-laterally flat or exhibit a concave or convex medial-lateral curvature to correspond to the medial-lateral curvature of a femoral component of the uni-compartmental knee prosthesis corresponding to the posterior cutter  202 .  
         [0064]     In the illustrated embodiment, the plurality of guide pins  214  include an anterior medial guide pin  232  and a posterior medial guide pin  234  extending perpendicularly from the medial side wall  222  of the posterior cutter  202  and an anterior lateral guide pin  236  and a posterior lateral guide pin  238  extending perpendicularly from the lateral side wall  224  of the posterior cutter  202 . The plurality of guide pins  214  defines a plane parallel to a tangent of the cutting surface  220  of the posterior cutter  202 . Illustratively, each of the plurality of guide pins  214  have a diameter  240  slightly less than the widths  68 ,  90  of the curved slots  58 ,  80  permitting the guide pins  214 , when received in the curved slots  58 ,  80 , to reciprocate anteriorly-posteriorly within the curved slots  58 ,  80 . The guide pins  214  may be spring loaded to permit the guide pins  214  to be reciprocated along their longitudinal axis into and out of the body  216  of the posterior cutter  202 .  
         [0065]     The medial side wall  222  and the lateral side wall  224  of the body  216  of the posterior cutter  202  are parallel and spaced apart by a displacement  242 . The displacement  242  between the medial side wall  222  and the lateral side wall  224  is slightly less than, but approximately equal to, the width  46  of the tool-receiving slot  44  of the guide  10 . In use, portions of the posterior cutter  202  are received in the tool-receiving slot  44  of the guide  10  for reciprocal movement anteriorly and posteriorly within the tool-receiving slot  44 . Portions of the medial side wall  222  are disposed adjacent the inner wall  54  of the medial rail  12  and portions of the lateral side wall  224  are disposed adjacent the inner wall  76  of the lateral rail  14 . The medial side wall  222  of the posterior cutter  202  and the inner wall  54  of the medial rail  12  and the lateral side wall  224  of the posterior cutter  202  and the inner wall  76  of the lateral rail  12  cooperate to restrict rotation of the posterior cutter  202  with respect to the guide  10  when the posterior cutter  202  is received within the tool-receiving slot  44 .  
         [0066]     The posterior end wall  226  and the anterior end wall  228  of the body  216  of the posterior cutter  202  are spaced apart by a displacement  244 . The displacement  244  between the posterior end wall  226  and the anterior end wall  228  is less than the half the length  48  of the tool-receiving slot  44  of the guide  10 . In the illustrated embodiment, the posterior cutter  202  is configured to be driven by the driver  210  to reciprocate anteriorly-posteriorly within the tool-receiving slot  44  of the guide  10 . The illustrated driver  210  is configured to induce the posterior cutter  202  to reciprocate anteriorly-posteriorly within the tool-receiving slot by a distance of approximately two millimeters. Thus, in the illustrated embodiment, the displacement  244  between the posterior end wall  226  and the anterior end wall  228  is approximately two millimeters less than the half the length  48  of the tool-receiving slot  44  of the guide  10 .  
         [0067]     As shown, for example, in  FIG. 7 , the posterior end wall  226  of the posterior cutter  202  is formed to include a bore  246  sized to receive portions of the posterior return spring  206 . The bore  246  has a diameter  248  greater than the diameter  250  of the posterior return spring  206 . The bore  246  has a depth  252  sufficient to prevent the posterior return spring  206  from dislodging from the bore  246  when received therein and compressed between the bottom wall  254  of the bore  246  and the posterior cross member  16  of the guide  10 . The posterior return spring  206  is configured to return the posterior cutter  202  anteriorly to a central position within the guide  10  following posterior movement of the posterior cutter  202  relative to the guide  10 . While not shown, it is within the scope of the disclosure for the posterior cross member  16  of the guide  10  to be configured to include a similar bore for receiving the opposite end of the posterior return spring  206 .  
         [0068]     As shown for example, in  FIG. 7  and in phantom lines in  FIG. 6 , the anterior end wall  228  of the posterior cutter  202  is formed to include cavity  258  extending posteriorly into the body  216  of the posterior cutter  202 . The cavity  258  includes a riding surface  260  for engaging the cam surface  262  of the driver  210 . In the illustrated embodiment, the cavity  258  is a semi-elliptical cavity having its major axis  264  coplanar with the anterior end wall  228  and its semi-minor axis  266  perpendicular to the anterior end wall  228 . The major axis  264  is slightly greater than the major axis  268  of the elliptical cam  270  of the driver  210  and the semi-minor axis  266  is approximately equal to half the minor axis  272  of the elliptical cam  270 .  
         [0069]     The cavity  258  has a height  274  slightly greater than the thickness  276  of the elliptical cam  270  of the driver  210  to capture the elliptical cam  270  within the cavity  258  for rotation therein. The riding surface  260  of the cavity  258  and the cam surface  262  of the driver  210  are configured so that rotation of the driver  210  about the longitudinal axis  278  of its shaft  280  induces reciprocal movement of the posterior cutter  202  within the guide  10  anteriorly and posteriorly by a distance equal to the difference between the major axis  268  and the minor axis  272  of the elliptical cam  270 .  
         [0070]     A semi-cylindrical groove  282  is formed in the anterior end wall  228  of the posterior cutter  202  extending between the cavity  258  and the top wall  218  of the posterior cutter  202 . A semi-circular opening  284  is formed in the top wall  218  of the cavity  258  at the junction of the cavity  258  and the groove  282 . A semi-circular opening is also formed in the top wall  218  of the posterior cutter  202  at the junction of the top wall  218  and the groove  282 . The semi-cylindrical groove  282  is formed concentrically about an axis  286  coplanar with the anterior end wall  228 , perpendicular to a tangent of the top wall  218  and parallel to the medial side wall  222  and lateral side wall  224 . When the posterior cutter  202  and the anterior cutter  204  are both in their central positions within the guide  10 , the semi-cylindrical groove  282  cooperates with a similar semi-cylindrical groove  382  in the anterior cutter  204  to receive the shaft  280  of the driver  210  therein for rotational movement about its longitudinal axis  278 . Thus, the semicircular groove  282  has a radius  288  slightly greater than half the diameter  290  of the shaft  280  of the driver  210 .  
         [0071]     In the illustrated embodiment, the anterior cutter  204  is symmetrical to the posterior cutter  202 . To reduce cutter components in the second embodiment of the cutting tool  200 , a plurality of single cutters could be manufactured for orientation in opposite directions within the guide  10  to act as the anterior cutter  204  and posterior cutter  202 .  
         [0072]     The anterior cutter  204  includes a body  316  having a top wall  318 , a cutting surface  320 , a medial side wall  322 , a lateral side wall  324 , a posterior end wall  326  and an anterior end wall  328 . The medial side wall  322  and lateral side wall  324  extend between and couple the top wall  318  to the cutting surface  320  and the anterior end wall  328  to the posterior end wall  326 .  
         [0073]     In the illustrated embodiment, the top wall  318  and cutting surface  320  exhibit a posterior-anterior curvature that corresponds to the anterior-posterior curvature of the inner surface of a femoral component of the uni-compartmental knee prosthesis corresponding to the anterior cutter  204 . Those skilled in the art will recognize that the curvature of the top wall  318  is not critical to the operation of the anterior cutter  204 , but rather provides some aesthetic and possibly safety features to the cutter  204 . Thus, it is within the scope of the disclosure for the top wall  318  to be flat or exhibit a different curvature than the cutting surface  320 .  
         [0074]     The cutting surface  320  is formed to include rows of teeth  330  extending medially-laterally across the cutting surface  320 . The cutting surface  320 , and its associated rows of teeth  330 , may be medially-laterally flat or exhibit a concave or convex medial-lateral curvature to correspond to the medial-lateral curvature of a femoral component of the uni-compartmental knee prosthesis corresponding to the anterior cutter  204 .  
         [0075]     In the illustrated embodiment, the plurality of guide pins  214  include an anterior medial guide pin  332  and a posterior medial guide pin  334  extending perpendicularly from the medial side wall  322  of the anterior cutter  204  and an anterior lateral guide pin  336  and a posterior lateral guide pin  338  extending perpendicularly from the lateral side wall  324  of the anterior cutter  204 . The plurality of guide pins  214  defines a plane parallel to a tangent of the cutting surface  320  of the anterior cutter  204 . Illustratively, each of the plurality of guide pins  214  have a diameter  240  slightly less than the widths  68 ,  90  of the curved slots  58 ,  80  permitting the guide pins  214 , when received in the curved slots  58 ,  80 , to reciprocate anteriorly-posteriorly within the curved slots  58 ,  80 . The guide pins  214  may be spring loaded to permit the guide pins  214  to be reciprocated along their longitudinal axis into and out of the body  316  of the anterior cutter  204 .  
         [0076]     The medial side wall  322  and the lateral side wall  324  of the body  316  of the anterior cutter  204  are parallel and spaced apart by a displacement  342 . The displacement  342  between the medial side wall  322  and the lateral side wall  324  is slightly less than, but approximately equal to, the width  46  of the tool-receiving slot  44  of the guide  10 . In use, portions of the anterior cutter  204  are received in the tool-receiving slot  44  of the guide  10  for reciprocal movement anteriorly and posteriorly within the tool-receiving slot  44 . Portions of the medial side wall  322  are disposed adjacent the inner wall  54  of the medial rail  12  and portions of the lateral side wall  324  are disposed adjacent the inner wall  76  of the lateral rail  14 . The medial side wall  322  of the anterior cutter  204  and the inner wall  54  of the medial rail  12  and the lateral side wall  324  of the anterior cutter  204  and the inner wall  76  of the lateral rail  12  cooperate to restrict rotation of the anterior cutter  204  with respect to the guide  10  when the anterior cutter  204  is received within the tool-receiving slot  44 .  
         [0077]     The posterior end wall  326  and the anterior end wall  328  of the body  316  of the anterior cutter  204  are spaced apart by a displacement  344  The displacement  344  between the posterior end wall  326  and the anterior end wall  328  is less than the half the length  48  of the tool-receiving slot  44  of the guide  10 . In the illustrated embodiment, the anterior cutter  204  is configured to be driven by the driver  210  to reciprocate anteriorly-posteriorly within the tool-receiving slot  44  of the guide  10 . The illustrated driver  210  is configured to induce the anterior cutter  204  to reciprocate anteriorly-posteriorly within the tool-receiving slot  44  by a distance of approximately two millimeters. Thus, in the illustrated embodiment, the displacement  344  between the posterior end wall  326  and the anterior end wall  328  is approximately two millimeters less than the half the length  48  of the tool-receiving slot  44  of the guide  10 .  
         [0078]     As shown, for example, in  FIG. 7 , the anterior end wall  328  of the anterior cutter  204  is formed to include a bore  346  sized to receive portions of the anterior return spring  208 . The bore  346  has a diameter  348  greater than the diameter  350  of the anterior return spring  208 . The bore  346  has a depth  352  sufficient to prevent the anterior return spring  208  from dislodging from the bore  346  when received therein and compressed between the bottom wall  354  of the bore  346  and the anterior cross member  18  of the guide  10 . The anterior return spring  208  is configured to return the anterior cutter  204  posteriorly to a central position within the guide  10  following anterior movement of the anterior cutter  204  relative to the guide  10 . While not shown, it is within the scope of the disclosure for the anterior cross member  18  of the guide  10  to be configured to include a similar bore for receiving the opposite end of the anterior return spring  208 .  
         [0079]     As shown, for example, in  FIG. 7  and in phantom lines in  FIG. 6 , the posterior end wall  326  of the anterior cutter  204  is formed to include cavity  358  extending anteriorly into the body  316  of the anterior cutter  204 . The cavity  358  includes a riding surface  360  for engaging the cam surface  262  of the driver  210 . In the illustrated embodiment, the cavity  358  is a semi-elliptical cavity having its major axis  364  coplanar with the posterior end wall  326  and its semi-minor axis  366  perpendicular to the posterior end wall  326 . The major axis  364  is slightly greater than the major axis  268  of the elliptical cam  270  of the driver  210  and the semi-minor axis  366  is approximately equal to half the minor axis  272  of the elliptical cam  270 .  
         [0080]     The cavity  358  has a height  374  slightly greater than the thickness  276  of the elliptical cam  270  of the driver  210  to capture the elliptical cam  270  within the cavity  358  for rotation therein. The riding surface  360  of the cavity  358  and the cam surface  262  of the driver  210  are configured so that rotation of the driver  210  about the longitudinal axis  278  of its shaft  280  induces reciprocal movement of the anterior cutter  204  within the guide  10  anteriorly and posteriorly by a distance equal to the difference between the major axis  268  and the minor axis  272  of the elliptical cam  270 .  
         [0081]     A semi-cylindrical groove  382  is formed in the posterior end wall  326  of the anterior cutter  204  extending between the cavity  358  and the top wall  318  of the anterior cutter  204 . A semi-circular opening  384  is formed in the top wall  318  of the cavity  358  at the junction of the cavity  358  and the groove  382 . A semi-circular opening is also formed in the top wall  318  of the anterior cutter  204  at the junction of the top wall  318  and the groove  382 . The semi-cylindrical groove  382  is formed concentrically about an axis  386  coplanar with the posterior end wall  326 , perpendicular to a tangent of the top wall  318  and parallel to the medial side wall  322  and lateral side wall  324 . When the posterior cutter  202  and the anterior cutter  204  are both in their central positions within the guide  10 , the semi-cylindrical groove  382  cooperates with a similar semi-cylindrical groove  282  in the posterior cutter  202  to receive the shaft  280  of the driver  210  therein for rotational movement about its longitudinal axis  278 . Thus, the semicircular groove  382  has a radius  388  slightly greater than half the diameter  290  of the shaft  280  of the driver  210 .  
         [0082]     The driver  210  includes an elliptical cam  270  and a shaft  280 . Illustratively, the shaft  280  is formed concentrically about a longitudinal axis  278  and is configured to be coupled to the drive shaft  112  of the power source  114 . The elliptical cam  270  includes a cam surface  262  extending between and coupling a top wall  294  and a bottom wall  296 . The top wall  294  is displaced from the bottom wall  296  by the thickness  276  of the elliptical cam  270 . The cam surface  262  has a major axis  268  and a minor axis  272 . The shaft  280  of the driver  210  is coupled to the top wall  294  of the elliptical cam  270  with the longitudinal axis  278  of the shaft  280  extending perpendicularly from the intersection of the major axis  268  and minor axis  272  of the elliptical cam  270 . The major axis  268  has a length that exceeds the length of the minor axis  272 . In the illustrated embodiment, the major axis  268  is approximately four millimeters longer than the minor axis  272 . Thus, when the driver  210  is rotated about the longitudinal axis  278  of its shaft  280 , the cam surface  262  of the elliptical cam  270  engages the riding surfaces  260 ,  360  of the cavities  258 ,  358  to drive both the posterior cutter  202  and the anterior cutter  204  to each move approximately two millimeters anteriorly-posteriorly within the guide  10 .  
         [0083]     While not illustrated, the second embodiment of the cutting tool  200  may also include an alignment collar configured to be attached to the guide  10  to maintain the alignment of the shaft  280  of the driver  210  during rotation. Such alignment collar may include a body formed to include a shaft-receiving aperture and a plurality of pin holes. The pin holes may be formed in the body so that fasteners  31  extending through the fastener holes would also extend through the fastener-receiving holes  29  formed in the central medial ear  26  and central lateral ear  32  of the guide  10 . The bottom wall of the alignment collar may be configured to contiguously engage the top walls of the lateral rail  14  and medial rail  12  to provide stability to the collar. The shaft-receiving aperture may be defined by a cylindrical side wall formed concentrically about a longitudinal axis. The cylindrical side wall of the alignment collar would be configured to receive the shaft  280  of the driver  210  therein and sized to permit the shaft  280  of the driver  210  to rotate freely therein about its longitudinal axis  278 . The cylindrical side wall of the alignment collar would cooperate with the guide  10  to maintain the alignment and central position of the shaft  280  of the driver  210 . Thus, the inside diameter of the shaft-receiving aperture of the alignment collar would be slightly larger than the outside diameter of the shaft  280  of the driver  210  to limit misalignment of the driver  210 .  
         [0084]     Illustratively, guide pins  214  act as track followers configured to induce the posterior cutter  202  and anterior cutter  204  to follow a path having a curvature conforming to the curvature of the convex bottom walls  62 ,  84  of the curved slots  58 ,  80 . Those skilled in the art will recognize that other track following devices may be utilized within the scope of the disclosure including, but not limited to walls, bosses, ears, and flanges extending from the bodies  216 ,  316  of the posterior cutter  202  and anterior cutter  204 , respectively, and configured to be received in and guided by the curved slots  58 ,  80  during anterior-posterior reciprocation within the curved slots  58 ,  80 . Also, it is within the scope of the disclosure for the medial rail and lateral rail to be formed with a curved bottom wall and a curved top wall that act as tracks and the posterior cutter  202  and anterior cutter  204  to include track followers such as guide pins, walls, bosses, ears, and flanges extending from the body  216 ,  316  and configured to follow the curved bottom wall and a curved top wall that act as tracks.  
         [0085]     In use, the guide pins  214 , when captured within the curved slots  58 ,  80  of the guide  10 , ride on the convex bottom walls  62 ,  84  of the curved slots  58 ,  80  which define a curved guide surface or track. The concave top walls  60 ,  82  of the curved slots  58 ,  80  act to restrict the proximal and distal movement of the guide pins  214  relative to the guide  10 . The guide pins  214  ride within the curved slots  58 ,  80  and act to restrict the cutting surfaces  220 ,  320  of the posterior cutter  202  and anterior cutter  204 , respectively, to move along a curvature approximating the curvature of an inner surface of a femoral component of the uni-compartmental knee prosthesis associated with the particular guide  10  and cutters  202 ,  204 .  
         [0086]     An alternative embodiment of the bone shaping instrument comprising a second embodiment of a guide  1210  and a third embodiment of a cutting tool  1300  is shown, for example, in  FIG. 12 . The second embodiment of guide  1210  is very similar to the first embodiment of guide  10 , so that identical reference numerals are used to identify identical components and similar reference numerals are used to identify similar components. Like guide  10 , guide  1210  is formed symmetrically about a plane extending anteriorly to posteriorly through the tool-receiving slot. Thus, since  FIG. 12  is a lateral side view of the guide  1210 , only the lateral components will be described and identified by reference numerals, it being understood that the medial components are similarly formed. Guide  1210  includes a medial rail, a lateral rail  1214 , a posterior cross member  16 , an anterior cross member  18 , a posterior leg  19 , an anterior leg  21 , and a plurality of ears  1230 ,  1232  and  1234 . The posterior cross member  16  extends between and couples the posterior end of the medial rail and the posterior end of the lateral rail  1214 . The anterior cross member  18  extends between and couples the anterior end of the medial rail and the anterior end of the lateral rail  1214 . The medial rail, lateral rail  1214 , posterior cross member  16  and anterior cross member  18  cooperate to define a tool-receiving slot through which portions of the cutting tool  1300  and/or drive shafts of the power source extend to drive the cutting tool  1300 .  
         [0087]     In the illustrated embodiment, the medial rail and the lateral rail  1214  extend longitudinally parallel to one another to define the side walls of the tool-receiving slot. The medial rail and the lateral rail  1214  are spaced apart from one another by a displacement which also defines the width of the tool-receiving slot. In one embodiment, the width of the tool-receiving slot is approximately thirteen millimeters to facilitate use in a minimally invasive procedure.  
         [0088]     The posterior cross member  16  and anterior cross member  18  extend laterally parallel to one another to define the end walls of the tool-receiving slot. The posterior cross member  16  and anterior cross member  18  are spaced apart from one another by a displacement which also defines the length of the tool-receiving slot.  
         [0089]     The lateral rail  1214  includes a concave bottom wall  1272 , a convex top wall  1274 , an inner wall and an outer wall  1278 . The inner wall and outer wall  1278  are generally parallel to one another and extend between and couple the concave bottom wall  1272  and convex top wall  1274 . The concave bottom wall  1272  and convex top wall  1274  each exhibit a curvature that corresponds to the curvature of the underside of a femoral component of the uni-compartmental knee prosthesis associated with the particular guide  1210 .  
         [0090]     The lateral rail  1214  is formed to include a posterior lateral ear  1230 , a central lateral ear  1232  and an anterior lateral ear  1234  extending outwardly from the outer wall  1278 . The posterior lateral ear  1230  is positioned adjacent the posterior end  1238  of the lateral rail  1214 . The anterior lateral ear  1234  is positioned adjacent the anterior end  1242  of the lateral rail  1214 . The central lateral ear  1232  is positioned near the middle of the lateral rail  1214  between the posterior lateral ear  1230  and the anterior lateral ear  1232 . Each of the posterior lateral ear  1230 , central lateral ear  1232  and anterior lateral ear  1234  are formed to include a fastener-receiving hole configured to receive a fastener  31  for securing the guide  1210  to the femur. The fastener-receiving holes are formed concentrically about an axis that extends radially toward the focus of the center of curvature of the underside of a femoral component of the uni-compartmental knee prosthesis associated with the particular guide  1210 . The orientation of the fastener-receiving holes facilitates insertion of the fasteners  31  through a minimally invasive incision to secure the guide  1210  to the femur.  
         [0091]     The third embodiment of the cutting tool  1300  is very similar to the first embodiment of the cutting tool  100 , so that identical reference numerals are used to identify identical components and similar reference numerals are used to identify similar components. The third embodiment of a guided cutting tool  1300  includes a depth stop  102 , a cutter  104  and a retaining collar  1310 . Since only the retaining collar  1310  of cutting tool  1300  differs from the components of cutting tool  100 , only the retaining collar  1310  will be described in detail, it being understood that the description of the depth stop  102  and cutter  104  of guided cutting tool  100  is applicable to the depth stop  102  and cutter  104  of guided cutting tool  1300 .  
         [0092]     The retaining collar  1310  includes a body  1316  formed to include a shaft-receiving aperture and a plurality of guide pins  1320 . The body  1316  of the retaining collar  1310  includes parallel spaced apart side walls extending between and coupling a top wall  1326  to a bottom wall  1328  and an anterior wall  1330  to a posterior wall  1332 . The top wall  1326  is planar and is configured to act as a stop surface against which the depth stop  102  engages to limit the depth of the cut of the cutter  104 . The shaft-receiving aperture extends through the top wall  1326  and the bottom wall  1328 . Illustratively, the shaft-receiving aperture is defined by a cylindrical side wall formed concentrically about a longitudinal axis  1336  normal to the top wall  1326 . The cylindrical side wall has an inside diameter. Illustratively, the cylindrical side wall is configured to receive the shaft  108  of the cutter  104  therein and is sized to permit the shaft  108  of the cutter  104  to rotate freely therein about its longitudinal axis  140 . The cylindrical side wall cooperates with the shaft  108  of the cutter  104 , and with the retaining collar  1310  and depth stop  102 , to maintain the alignment of the cutter  104 . Thus, the inside diameter of the shaft-receiving aperture is slightly larger than the outside diameter of the shaft  108  of the cutter  104  to limit misalignment of the cutter  104 . In the illustrated embodiment, the shaft  108  of the cutter  104  is aligned to extend radially to facilitate utilization of the bone shaping instrument through a minimally invasive incision.  
         [0093]     In the illustrated embodiment, the plurality of guide pins  1320  include an upper anterior medial guide pin, a lower anterior medial guide pin, an upper posterior medial guide pin and a lower posterior medial guide pin extending perpendicularly from the medial side wall of the retaining collar  1310  and an upper anterior lateral guide pin  1348 , a lower anterior lateral guide pin  1349 , an upper posterior lateral guide pin  1350  and a lower posterior lateral guide pin  1351  extending perpendicularly from the lateral side wall  1324  of the retaining collar  1310 . The guide pins  1320  define a plane parallel to the top wall  1326  of the retaining collar  1310 . Illustratively, the upper anterior medial guide pin and upper posterior medial guide pin ride on the convex top wall of the medial rail and the lower anterior medial guide pin and lower posterior medial guide pin ride against the concave lower wall of the medial rail when the retaining collar  1310  is reciprocated posteriorly-anteriorly within the tool-receiving slot. The upper anterior lateral guide pin  1348  and upper posterior lateral guide pin  1350  ride on the convex top wall  1274  of the lateral rail  1214  and the lower anterior lateral guide pin  1349  and lower posterior lateral guide pin  1351  ride against the concave bottom wall  1272  of the lateral rail  1214  when the retaining collar  1310  is reciprocated posteriorly-anteriorly within the tool-receiving slot. The guide pins  1320  may be spring loaded to permit the guide pins  1320  to be reciprocated along their longitudinal axis into and out of the body  1316  of the retaining collar  1310 .  
         [0094]     The medial side wall and the lateral side wall  1324  of the body  1316  of the retaining collar  1310  are spaced apart by a displacement that is slightly less than, but approximately equal to, the width of the tool-receiving slot of the guide  1210 . In use, portions of the retaining collar  1310  are received in the tool-receiving slot of the guide  1210  for reciprocal movement anteriorly and posteriorly within the tool-receiving slot. Portions of the medial side wall are disposed adjacent the inner wall of the medial rail and portions of the lateral side wall  1324  are disposed adjacent the inner wall of the lateral rail  1214 .  
         [0095]     Illustratively, guide pins  1320  act as track followers configured to induce the retaining collar  1310  to follow a path having a curvature conforming to the curvature of the concave bottom wall and convex top wall of the medial rail and the concave bottom wall  1272  and convex top wall  1274  of the lateral rail  1214 . Those skilled in the art will recognize that other track following devices may be utilized within the scope of the disclosure including, but not limited to walls, bosses, ears, and flanges extending from the body  1316  of the retaining collar  1310  and configured to engage and be guided by the concave bottom wall and convex top wall of the medial rail and the concave bottom wall  1272  and convex top wall  1274  of the lateral rail  1214 .  
         [0096]     Yet another alternative embodiment of the bone shaping instrument comprising a third embodiment of a guide  1410  and a fourth embodiment of a cutting tool  1500  is shown, for example, in  FIGS. 13 and 14 . Guide  1410  includes a guide plate  1412 , an anterior pin-receiving cylinder  1428  and a posterior pin-receiving cylinder  1424 .  
         [0097]     Illustratively the guide plate  1412  includes a concave bottom wall  1450 , a convex top wall  1452 , an inner wall  1454  and an outer wall  1456 . The inner wall  1454  and outer wall  1456  are each flat and are generally parallel to one another. The inner wall  1454  and outer wall  1456  extend between and couple the concave bottom wall  1450  and convex top wall  1452 . The concave bottom wall  1450  and concave top wall  1452  each exhibit a curvature that corresponds to the curvature of the underside of a femoral component of the uni-compartmental knee prosthesis associated with the particular guide  1410 .  
         [0098]     The guide plate  1412  is formed to include a curved slot  1458  defined by a concave top wall  1460 , a convex bottom wall  1462 , a posterior wall  1464  and an anterior wall  1466 , each of which extend between the inner wall  1454  and the outer wall  1456 . The concave top wall  1460  and convex bottom wall  1462  each exhibit a curvature that corresponds to the curvature of the underside of a femoral component of the uni-compartmental knee prosthesis associated with the particular guide  1410 .  
         [0099]     The concave top wall  1460  and convex bottom wall  1462  are spaced apart from one another by a displacement that serves as the width of the curved slot  1458 . The width of the curved slot  1458  is sized to capture guide pins  1520  therein for anterior-posterior longitudinal movement within the curved slot  1458 . The anterior wall  1466  and posterior wall  1464  of the curved slot  1458  are spaced apart from one another by a displacement that serves as the length of the curved slot  1458 . The anterior wall  1466  and posterior wall  1464  act as stops that limit the anterior-posterior longitudinal movement of guide pins  1520  when captured within the curved slot  1458 .  
         [0100]     The guide plate  1412  is formed to include a posterior pin-receiving cylinder  1424  and an anterior pin-receiving cylinder  1428 . The posterior pin-receiving cylinder  1424  is coupled to the posterior end  1436  of the guide plate  1412 . The anterior pin-receiving cylinder  1428  is coupled to the anterior end  1440  of the guide plate  1412 . Each of the anterior pin-receiving cylinder  1428  and the posterior pin-receiving cylinder  1424  are formed to include a fastener-receiving hole  1429  configured to receive a fastener  31  for securing the guide  1410  to the femur. The fastener-receiving holes  1429  are formed concentrically about an axis that extends radially toward the focus of the center of curvature of the underside of a femoral component of the uni-compartmental knee prosthesis associated with the particular guide  1410 . The orientation of the fastener-receiving holes  1429  facilitates insertion of the fasteners  31  through a minimally invasive incision to secure the guide  1410  to the femur.  
         [0101]     The fourth embodiment of the cutting tool  1500  is very similar to the first embodiment of the cutting tool  100 , so that identical reference numerals are used to identify identical components and similar reference numerals are used to identify similar components. The fourth embodiment of a guided cutting tool  1500  includes a cutter  104  and a retaining collar  1510 . While not illustrated, those skilled in the art will recognize that the guided cutting tool  1500  may also include a depth stop  102 . Since only the retaining collar  1510  of cutting tool  1500  differs from the components of cutting tool  100 , only the retaining collar  1510  will be described in detail, it being understood that the description of the depth stop  102  and cutter  104  of guided cutting tool  100  is applicable to the depth stop  102  and cutter  104  of guided cutting tool  1500 .  
         [0102]     The retaining collar  1510  includes a body  1516  formed to include a shaft-receiving aperture and a plurality of threaded guide pin holes  1517  each configured to receive one of a plurality of guide pins  1520 . The body  1516  of the retaining collar  1510  includes parallel spaced apart side walls extending between and coupling a top wall  1526  to a bottom wall  1528  and an anterior wall  1530  to a posterior wall  1532 . The top wall  1526  is planar and is configured to act as a stop surface against which the depth stop  102  engages to limit the depth of the cut of the cutter  104 . The shaft-receiving aperture extends through the top wall  1526  and the bottom wall  1528 . Illustratively, the shaft-receiving aperture is defined by a cylindrical side wall formed concentrically about a longitudinal axis normal to the top wall  1526 . The cylindrical side wall has an inside diameter. Illustratively, the cylindrical side wall is configured to receive the shaft  108  of the cutter  104  therein and is sized to permit the shaft  108  of the cutter  104  to rotate freely therein about its longitudinal axis  140 . The cylindrical side wall cooperates with the shaft  108  of the cutter  104 , and with the retaining collar  1510  and depth stop  102 , to maintain the alignment of the cutter  104 . Thus, the inside diameter of the shaft-receiving aperture is slightly larger than the outside diameter of the shaft  108  of the cutter  104  to limit misalignment of the cutter  104 . In the illustrated embodiment, the shaft  108  of the cutter  104  is aligned to extend radially to facilitate utilization of the bone shaping instrument through a minimally invasive incision.  
         [0103]     In the illustrated embodiment, the plurality of threaded guide pins  1520  includes an anterior guide pin  1548  and a posterior guide pin  1550 . Each guide pin  1520  includes a head  1549  and a threaded shaft  1551  configured to be received in a guide pin-receiving hole  1517  in the body  1516 . When so received, the guide pins  1520  extend perpendicularly from the outer side wall  1524  of the retaining collar  1510 . The guide pins  1520  define a plane parallel to the top wall  1526  of the retaining collar  1510 . When the guide pins  1520  are extended through the guide slot  1458  to couple the body  1516  of the retaining collar  1510  to the guide  1410 , the inner walls of the heads  1549  of the guide pins  1520  are closely adjacent to, or in engagement with, the outer wall  1456  of the guide  1410 . When so coupled to the guide  1410 , the outer wall  1524  of the body  1516  is adjacent to, or in engagement with, the inner wall  1454  of the guide  1410 . Illustratively, the anterior guide pin  1548  and the posterior guide pin  1550  ride on the convex bottom wall  1462  of the guide slot  1458  and the ride against the concave top wall  1460  of the guide slot  1458  when the retaining collar  1510  is reciprocated posteriorly-anteriorly. Thus, inner walls of the heads  1549  of the guide pins  1520  and the outer wall  1456  of the guide  1410 , the outer wall  1524  of the body  1516  of the retaining collar  1510  and the inner wall  1454  of the guide  1410 , the shafts  1551  of the guide pins  1520  and the convex bottom wall  1462  and concave top wall  1460  of the guide slot  1458  cooperate to maintain the proper alignment of the cutting tool  104  during resection of the bone.  
         [0104]     Illustratively, guide pins  1520  act as track followers configured to induce the retaining collar  1510  to follow a path having a curvature conforming to the curvature of the concave top wall  1460  and convex bottom wall  1462  of the guide slot  1458 . Those skilled in the art will recognize that other track following devices may be utilized within the scope of the disclosure including, but not limited to walls, bosses, ears, and flanges extending from the body  1516  of the retaining collar  1510  and configured to engage and be guided by the concave top wall  1460  and convex bottom wall  1462  of the guide slot  1458 .  
         [0105]     Yet another alternative embodiment of the bone shaping instrument comprising a third embodiment of a guide  1610  and a fourth embodiment of a cutting tool  1700  is shown, for example, in  FIGS. 15-17 .  
         [0106]     Guide  1610  includes a guide plate  1612  and is very similar to guide  1410  except for the lack of pin-receiving cylinders  1424  and  1428 . Guide  1610  is intended to be attached to the bone to be resected by inserting guide  1610  into a resected slot  1611  formed in the bone at a desired location.  
         [0107]     Illustratively the guide plate  1612  includes a concave bottom wall  1650 , a convex top wall  1652 , an inner wall  1654  and an outer wall  1656 . The inner wall  1654  and outer wall  1656  are each flat and are generally parallel to one another. The inner wall  1654  and outer wall  1656  extend between and couple the concave bottom wall  1650  and convex top wall  1652 . The concave bottom wall  1650  and concave top wall  1652  each exhibit a curvature that corresponds to the curvature of the underside of a femoral component of the uni-compartmental knee prosthesis associated with the particular guide  1610 .  
         [0108]     The guide plate  1612  is formed to include a curved slot  1658  defined by a concave top wall  1660 , a convex bottom wall  1662 , a posterior wall  1664  and an anterior wall  1666 , each of which extend between the inner wall  1654  and the outer wall  1656 . The concave top wall  1660  and convex bottom wall  1662  each exhibit a curvature that corresponds to the curvature of the underside of a femoral component of the uni-compartmental knee prosthesis associated with the particular guide  1610 .  
         [0109]     The concave top wall  1660  and convex bottom wall  1662  are spaced apart from one another by a displacement that serves as the width of the curved slot  1658 . The width of the curved slot  1658  is sized to capture central shafts  1751  of guide bushings  1720  therein for anterior-posterior longitudinal movement within the curved slot  1658 . The anterior wall  1666  and posterior wall  1664  of the curved slot  1658  are spaced apart from one another by a displacement that serves as the length of the curved slot  1658 . The anterior wall  1666  and posterior wall  1664  act as stops that limit the anterior-posterior longitudinal movement of guide bushings  1720  when captured within the curved slot  1658 .  
         [0110]     The fifth embodiment of the cutting tool  1700  is very similar to the first embodiment of the cutting tool  100 , so that identical reference numerals are used to identify identical components and similar reference numerals are used to identify similar components. The fifth embodiment of a guided cutting tool  1700  includes a cutter  1704  and a retaining collar  1710 . Cutter  1704  is very similar to cutter  104  except that cutter  1704  is modular including a drive shaft  1708  formed to include a large diameter section  1707  adjacent the proximal end  1709  and a small diameter section  1711  adjacent the distal end  1713  and a cutting face  1706  configured for removable attachment to the distal end  1713  of the shaft  1708 . The distal wall  1715  of the large diameter section  1707  of the shaft  1708  acts as a depth stop. The cutting face  1706  is removably attached to the shaft  1708  to facilitate mounting the retaining collar  1710  on the small diameter section  1711  of the shaft  1708  and to facilitate changing cutting faces  1706 .  
         [0111]     The retaining collar  1710  includes a body  1716  formed to include a shaft-receiving aperture and a male portion  1780  of a coupling  1782 , a body plate  1784  formed to include a female portion  1786  of the coupling  1782 , a plurality of modular guide bushings  1720 , an outer plate  1788  and a plurality of fasteners  1790 .  
         [0112]     The small diameter section of the shaft  1708  extends through the shaft receiving aperture in the body  1716  to mount the retaining collar  1710  to the cutter  1704 . The body  1716  includes a cylindrical side wall  1724  extending between and coupling a top wall  1726  and a bottom wall  1728 . The top wall  1726  is planar and is configured to act as a stop surface against which the distal wall  1715  of the large diameter section  1707  of the shaft  1708  engages to limit the depth of the cut of the cutter  1704 . The shaft-receiving aperture extends through the top wall  1726  and the bottom wall  1728 . Illustratively, the shaft-receiving aperture is defined by a cylindrical side wall formed concentrically about a longitudinal axis normal to the top wall  1726 . The cylindrical side wall has an inside diameter. Illustratively, the cylindrical side wall is configured to receive the small diameter section  1711  of the shaft  1708  of the cutter  1704  therein and is sized to permit the small diameter section  1711  of the shaft  1708  of the cutter  1704  to rotate freely therein about its longitudinal axis  1740 . The cylindrical side wall cooperates with the small diameter section  1711  of the shaft  1708  of the cutter  1704  to maintain the alignment of the cutter  1704 . Thus, the inside diameter of the shaft-receiving aperture is slightly larger than the outside diameter of the small diameter section  1711  of the shaft  1708  of the cutter  1704  to limit misalignment of the cutter  1704 .  
         [0113]     In the illustrated embodiment, the plurality of modular guide bushings  1720  includes a modular anterior guide bushing  1748  and a modular posterior guide bushing  1750 . Each modular guide bushing  1720  includes a first portion  1747  having a head  1749  and a shaft  1751  and a second portion  1753  comprising a cap  1755 . The shaft  1751  is configured to be received in a guide slot  1658  of the guide  1610 .  
         [0114]     The male portion  1780  of a coupling  1782  on the body  1716  is coupled to the female portion  1786  of the coupling  1782  on the body plate  1784  to couple the body  1716  to the body plate  1784 . In the illustrated embodiment, the male portion  1780  of a coupling  1782  is a T-shaped protrusion extending radially from the cylindrical wall  1724  of the body  1716  and the female portion  1786  of the coupling  1782  is a slot formed horizontally in the body plate  1784 . The T-shaped protrusion is inserted through the slot and the body plate is rotated ninety degrees to couple the body  1716  to the body plate  1784 . When the retaining collar  1710  is fully assembled, the caps  1755  engage the T-shaped protrusion and prevent the body  1716  from rotating relative to the body plate  1784 . It is within the scope of the disclosure for other common forms of couplings to be utilized to couple the body  1716  to the body plate  1784 .  
         [0115]     After attaching the body  1716  to the body plate  1784 , the shafts  1751  of the modular guide bushings  1720  are inserted into the guide slot  1658  of the guide plate  1610  and the retaining collar  1710  is assembled. The caps  1755  of the modular guide bushings  1720  are attached to the shafts  1751  of the modular guide bushings  1720  and fasteners  1790  are inserted through the outer plate  1788 , the modular bushings  1720  and the body plate  1784  to complete the assembly of the retaining collar  1710 . In the illustrated embodiment, fastener holes are provided in the outer plate  1788 , the modular bushings  1720  and the body plate  1784  to facilitate assembly of the retaining collar  1710 .  
         [0116]     When so assembled, the guide bushings  1720  extend perpendicularly from the longitudinal axis of the body  1716  of the retaining collar  1710 . The guide bushings  1720  define a plane parallel to the top wall  1726  of the retaining collar  1710 . When the shafts  1751  of the guide bushings  1720  are extended through the guide slot  1658  to couple the retaining collar  1710  to the guide  1610 , the inner walls of the heads  1749  of the guide bushings  1720  are closely adjacent to, or in engagement with, the outer wall  1656  of the guide  1610 . When so coupled to the guide  1610 , the outer walls of the caps  1755  of the guide bushings  1720  are adjacent to, or in engagement with, the inner wall  1654  of the guide  1610 . Illustratively, the shafts  1751  of the anterior guide bushing  1748  and the posterior guide bushing  1750  ride on the convex bottom wall  1662  of the guide slot  1658  and ride against the concave top wall  1660  of the guide slot  1658  when the retaining collar  1710  is reciprocated posteriorly-anteriorly. Thus, inner walls of the heads  1749  of the guide bushings  1720  and the outer wall  1656  of the guide  1610 , the outer walls of the caps  1755  of the guide bushings  1720  of the retaining collar  1710  and the inner wall  1654  of the guide  1610 , the shafts  1751  of the guide bushings  1720  and the convex bottom wall  1662  and concave top wall  1660  of the guide slot  1658  cooperate to maintain the proper alignment of the cutting tool  1704  during resection of the bone.  
         [0117]     Illustratively, guide bushings  1720  act as track followers configured to induce the retaining collar  1710  to follow a path having a curvature conforming to the curvature of the concave top wall  1660  and convex bottom wall  1662  of the guide slot  1658 . Those skilled in the art will recognize that other track following devices may be utilized within the scope of the disclosure including, but not limited to walls, bosses, ears, and flanges extending from the body  1716  of the retaining collar  1710  and configured to engage and be guided by the concave top wall  1660  and convex bottom wall  1662  of the guide slot  1658 .  
         [0118]     One embodiment of a uni-compartmental knee replacement procedure appropriate for utilization of the disclosed bone shaping tools begins with the surgeon performing an antero-medial or antero-lateral skin incision. The incision should begin 1 cm proximal to the superior border of the patella. It should extend 6 to 10 cm distally along the edge of the patella and patella tendon, and end 2 cm distal to the joint line. A longer incision is may be advised when first starting to use the procedure or if the patient is obese.  
         [0119]     The surgeon then enters the joint capsule with a parapatellar incision. Once the joint is exposed, may make a final assessment of the extent of arthritic damage and the suitability of the joint for this procedure. The surgeon reflects the deep menisco-tibial layer of the medial or lateral capsule to provide good access to any tibial osteophytes and allow accurate wound closure. The surgeon also excises any excess deep synovium to provide clear sight of the joint. If required, all or part of the fat pad may also be excised to improve vision and allow inspection of the opposite compartment.  
         [0120]     Retractors may be introduced to maintain access to the joint at all stages of the procedure. No ligament releases should be necessary. In order to achieve medial/lateral (M/L) alignment and joint stability, all osteophytes should be removed from the entire medial or lateral edges of the femur and tibia. A retractor is used on the patella rim to draw the patella into a central position.  
         [0121]     Next the surgeon proceeds to the tibial resection of the UKR procedure. The knee is placed in 90 degrees of flexion to perform the tibial resection. A tibial alignment guide, such as the Preservation Tibial Cutting Block, Cat. No. 2498-60-002, 004 available from DePuy may be utilized in performing the tibial resection. The tibial cutting block of such an alignment guide should be raised to just below the level of the joint line.  
         [0122]     When a medial UKR procedure is being performed, the tibial cutting block is now positioned 2 mm medial to the center of the tibial tubercle. M/L adjustment is made proximally to achieve varus/valgus alignment that is perpendicular to the mechanical axis of the tibia. The A/P slope of the tibia is then set using the anterior crest of the tibia as a reference. The posterior tibial slope is adjusted to approximately 3 degrees to 5 degrees or to the figure determined from presurgical templating with a lateral X-ray.  
         [0123]     The inferior/superior and A/P slope is then fixed. The surgeon then makes an L-cut using a reciprocating saw, using anatomic landmarks as reference points. For a medial Uni, make the L-cut should be made just lateral to the lateral border of the medial femoral condyle. The L-Cut is aligned in the sagittal plane using the midpoint of the lateral border of the insertion of the ACL as a landmark.  
         [0124]     The surgeon then makes a transverse cut in the tibia in the proper orientation using a saw blade attached to an oscillating saw. The resected tibial bone chip is then removed.  
         [0125]     The resection of the medial condyle of the femur may be accomplished with or without an initial distal femoral resection. If the initial distal femoral resection is utilized, the alignment guide from the PRESERVATION™ system may be used in accordance with procedures recommended for its use. With the leg held in full extension, the alignment guide and extramedullary alignment rod are assembled into the slot of the distal femoral cutting block to check overall alignment. Varus/valgus alignment can be assessed in the frontal plane. Neutral position of the distal femoral cut can be verified, from the lateral view. An oscillating saw is utilized to perform distal femoral cut. The bone removed from the distal femur is minimal. Once the resected distal bone is removed, any remaining soft tissue is cleared from the meniscal rim and posterior tibia.  
         [0126]     Utilization of an initial distal femoral cut permits the femoral size established during preoperative templating to be checked using a femoral rotation and sizing guide in a known manner. The anterior tip profile and M/L width of the guide is the same as the femoral prosthesis. The aim is to ensure a smooth transition of the patella from the trochlea onto the anterior tip of the femoral prosthesis and avoid implant overhang. A femoral component is selected that covers the distal femoral cartilage erosion without overhanging the patello femoral joint. If between sizes, the smaller of the two is generally chosen.  
         [0127]     To facilitate rotation of the anterior aspect of the femur, make a longitudinal mark on the condyle in full extension, indicating the central MIL articulating point of the femur over the tibial plateau. Aligning the anterior tip of the guide with this mark will help verify proper femoral alignment. It is also important to avoid femoral component overhang on the anterior aspect of the femur. Once the appropriate femoral component is selected, the guide  10  and guided cutting tool  100 ,  200  corresponding with the selected component is assembled prior to being attached to the femur.  
         [0128]     The guide is positioned and aligned on the femoral component using the following steps: (1) With the knee placed in flexion, the guide is positioned with the anterior foot seated proximally of the load bearing portion of the anterior condyle. (2) The guide  10  is positioned in the correct medial-lateral and rotational position (along the distal weight bearing surface) to obtain maximum femoral coverage. The track of the guide  10  replicates the outer medial-lateral geometry of the femoral implant. (3) Ensure the guide is rotated correctly for femoral alignment. The femoral implant provides a 7° anatomic angle, between the distal and posterior surfaces, which should closely replicate the geometry of the anatomic femur. (4) Determine the proper varus/valgus positioning of the guide  10  by ensuring that the tangent line at the center of the guide  10  is perpendicular to the long axis of the femur.  
         [0129]     Alternatively, if CAS technology is available, it may be utilized to accurately position the guide by based upon the registered geometry of the bone. Those skilled in the art will be familiar with the techniques registering when a CAS system utilized.  
         [0130]     With the guide correctly positioned, the guide  10  is pinned in place with 0.125″ (3.2 mm) headless fixation pins through both the holes  29  in the anterior lateral ear and posterior medial ear, as shown, for example, in  FIGS. 8-11 . An additional pin is placed in the hole  29  provided in central medial ear of the guide  10  to ensure it remains stable during resection of the posterior condyle. For hard or sclerotic bone, it may be necessary to pre-drill the holes with the 0.125″ (3.2 mm) quick disconnect drill bits. Additional stabilization pins may be placed in the holes  29  left vacant by the guide. It may be necessary to utilize a high speed burr to remove the hard layer of sclerotic bone prior to using the cutter.  
         [0131]     The power source should be started prior to plunging the end-mill into the bone to avoid chipping and gouging. After initial bone engagement, the end-mill is plunged into the bone three to four times down the length of the guide. The end-mill is then used in a sweeping motion to clear our remaining bone (end-mill blades are both side-cutting and end-cutting). The depth stop  102  coming into contact with the retaining collar will automatically stop the progress of the end-mill when it reaches the appropriate depth.  
         [0132]     The appropriate level of preparation is complete when the depth stop comes into contact with the retaining collar as it is moved along its entire path of travel. It is critical that this is completed along the entire path of travel for the implant to fit properly.  
         [0133]     Final preparatory steps can be performed to add any necessary features such as peg or keel shapes. The same guide rails would be used to guide the necessary preparatory instruments such as a drill or burr.  
         [0134]     Although specific embodiments of the invention have been described herein, other embodiments may be perceived by those skilled in the art without departing from the scope of the invention as defined by the following claims. For example, although the invention has been described in terms of the implantation of the femoral portion of a knee prosthesis, it can be used with prostheses for other joints such as the shoulder, hip, or elbow.