Abstract:
An apparatus for reaming a patella includes a clamp for securing the patella and a reamer. The reamer includes a depth scale to indicate a depth to which the patella has been reamed. The clamp includes a pivoting collet to enable the clamp to stably secure different patellas having a variety of shapes and sizes. The stability of the collet is ensured by a plurality of prongs on the side contacting the patella, which are arranged to be equally spaced around the peak of the patella. Using this reaming apparatus, the depth to which the patella is reamed can be accurately determined. The reamed patella can then be fitted with a prosthesis to coordinate with a femoral prosthesis fitted on a resected femur and a tibial prosthesis fitted on a resected proximal end of a tibia to form a reconstructed knee joint which is substantially at anatomic level and has a proper degree of laxity in both flexion and extension.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application to U.S. patent application Ser. No. 09/177,334, filed Oct. 22, 1998, entitled METHOD AND APPARATUS FOR LOCATING BONE CUTS AT THE DISTAL CONDYLAR FEMUR REGION TO RECEIVE A FEMORAL PROSTHESIS AND TO COORDINATE TIBIAL AND PATELLAR RESECTION AND REPLACEMENT WITH FEMORAL RESECTION AND REPLACEMENT, now U.S. Pat. No. 6,077,270 which is a continuation-in-part application to U.S. patent application Ser. No. 09/049,781, filed Mar. 27, 1998 entitled METHOD AND APPARATUS FOR LOCATING BONE CUTS AT THE DISTAL FEMORAL CONDYLES TO RECEIVE A FEMORAL PROTHESIS AND TO COORDINATE TIBIAL AND PATELLAR RESECTION AND REPLACEMENT WITH FEMORAL RESECTION AND REPLACEMENT, now U.S. Pat. No. 6,024,746, which is a continuation-in-part application to U.S. patent application Ser. No. 08/956,015, filed Oct. 22, 1997 entitled METHOD AND APPARATUS FOR LOCATING BONE CUTS AT THE DISTAL CONDYLAR FEMUR REGION TO RECEIVE A FEMORAL PROTHESIS AND PROPERLY ARTICULATED WITH PATELLAR AND TIBIAL PROTHESIS, now U.S. Pat. No. 6,059,788, which is a continuation-in-part application to U.S. patent application Ser. No. 08/455,985, filed May 31, 1995, entitled METHOD AND APPARATUS FOR LOCATING BONE CUTS AT THE DISTAL CONDYLAR FEMUR REGION TO RECEIVE A FEMORAL PROSTHESIS, now U.S. Pat. No. 5,776,137, issued Jul. 7, 1998, the disclosures of which are incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to methods and apparatus for locating bone cuts on the medial and lateral femoral condyles to form seating surfaces for a femoral knee prosthesis, and to coordinate tibial and patellar resection and replacement with femoral resection and replacement. 
     The invention further relates to a tool for locating said cuts. 
     BACKGROUND OF THE INVENTION 
     Over the years, the concepts of designs for the total knee arthroplasty have evolved to the point where with few exceptions, most are quite comparable in the design of femoral, tibial and patellar prostheses. 
     Major discrepancies and problems encountered are caused by physician error and failure to understand the principles of more complex alignment or ligament problems to be corrected at surgery. With the more complex alignment or “routine” degenerative knee, the major differences are the ease and consistency of instrumentation for alignment and proper bone cuts allowing proper ligament balance. This allows satisfactory motion and stability post operatively. 
     The distal femoral cuts must be placed to provide the knee prosthesis with a proper flexion and extension gap, proper varus-valgus alignment, proper patellofemoral relationship and proper rotation. It is customary to use an intramedullary rod placed in a retrograde fashion between the medial and lateral femoral condyles just anterior to the intercondylar notch to establish a single point of reference for subsequent bone cuts. A major problem is in the instrumentation to indicate the location of the femoral cuts which relies upon the “experience” or “eyeballing” of the surgeon. Over the years, two basic instrument system designs have become popular. 
     In one design (anterior referencing), the total knee alignment system takes its point of reference from a centrally placed rod and careful attention is given to the patellofemoral joint by using an anteriorly placed feeler gage. The distal femoral cut is consistent with the thickness of the prosthesis. 
     This instrument system operates on the principle of anatomic anterior and distal femoral cuts to allow proper ligament balancing and stability in extension as well as consistent patellofemoral placement on the anterior surface. The femur is not notched, and the anterior surface of the femoral prosthesis not elevated above the anterior surface of the femur. Notching the femur may cause a decrease in strength of the distal femur. Elevation of the anterior surface of the prosthesis will cause extremely high patellofemoral pressures in a joint that seems to be prone to a high rate of post-operative failure. 
     By establishing the anterior femoral cut as the benchmark or datum starting point, however, the anterior referencing instruments result in the installation of a knee prosthesis which sacrifices consistent stability in flexion due to the formation of a posterior femoral condylar cut that may leave the posterior space either too wide or too narrow. This can cause instability in flexion, or restrict flexion and cause increased wear. 
     The second type of instrument design (posterior referencing) is based on the concept that the flexion and extension stability are more important and the patellofemoral joint is of secondary importance. This system also uses an intramedullary rod for referencing. Although I consider all three joints as “important”, when a compromise must be made, the posterior referencing systems compromise the patellofemoral joint while the anterior reference systems sacrifice stability in flexion (the posterior tibial femoral joint). Both systems allegedly equally address the distal tibial-femoral space. Neither consistently addresses the distal rotation of the femoral component. 
     Neither system tries to preserve the joint line at or near an “anatomic” level. By elevating the jointline, the patella is distalized. The femur is also shortened. Since the arthritic knee often has a loss of cartilage, there may be a patella infera of 2-3 mm initially. Elevating the distal femoral resection beyond this will: 
     1) Further alter the patellofemoral relationship. 
     2) Change the isometric and rotational balance of the MCL and the LCL. 
     3) Shorten the femur in flexion and may cause increased roll back, anterior lift off, and increased posteromedial wear. 
     4) Elevate the level of tibial resection necessitating a major amount of posterior femoral resection to achieve a satisfactory flexion space. 
     When performing a unicompartmental knee replacement, it is imperative to maintain the jointline. As a consequence, it is desirable to maintain a full range of motion. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide methods and apparatus for locating bone cuts on the medial and lateral femoral condyles to form seating surfaces for a femoral knee prosthesis, and to coordinate tibial and patellar resection and replacement with femoral resection and replacement which reliably and anatomically provide: 
     1. Consistent distal tibio-femoral stability. 
     2. Consistent distal femoral rotation. 
     3. Consistent placement of the anterior cut flush with the anterior surface of the femoral cortex, i.e., without notching or elevation. 
     4. Consistent placement of the posterior femoral cut such that the distal and posterior cuts are equal (when indicated) allowing for satisfactory extension and flexion stability and motion. 
     The method and apparatus of the invention contemplate placement of the anatomic joint line which, in extreme cases, varies up to the difference between the anterior-posterior A-P internal measurements of the size prostheses. Based on my knowledge of total knee replacement, personal experience with numerous routine total knee replacements, numerous more complicated cases consisting of knees with flexion deformities and revision surgery, a maximum of a few mm proximal or distal displacement of the joint line is considerably less harmful than: 
     1. A lax flexion gap; 
     2. Sloping the proximal tibial cut to accommodate for an inconsistent posterior femoral condylar cut; 
     3. Significantly notching the femur anteriorly; 
     4. Raising the anterior flanges of the prosthesis and thus the patellofemoral joint; 
     5. Not allowing full extension; 
     6. Raising the joint line; 
     7. Tightness in flexion; 
     8. Malrotation; and 
     9. Patient pain. 
     With an understanding of the measurements involved in total knee replacement, a new instrument system and methodology has been developed to allow flexion 120-130 degrees; to perform less soft tissue releasing; and decrease surgical time. Starting with a “normal” knee, the goal should be to maintain the anatomic landmarks as close to normal as possible. Then, if deformities are present, the procedure can be modified to accommodate the situation. 
     In accordance with the invention, a method is provided for forming planar cuts on the medial and lateral condyles of the femur to form seating surfaces to receive a femoral knee prosthesis, comprising: 
     determining a prospective planar cut at the posterior of the condyles of the femur at which the distance between the anterior surface of the femoral cortex and the prospective planar cuts is substantially equal to the interior dimension of a knee prosthesis to be fitted on said femur at the anterior surface and the cut planar surface, 
     determining the thickness of the posterior lateral or medial condyle which will be resected by said prospective planar cut, 
     cutting the distal ends of the condyles along a plane at which the maximum thickness of resection of the more prominent condyle at said distal end is substantially equal to the thickness determined to be resected at the posterior medial or lateral condyle by said prospective planar cut, and 
     cutting the condyles along a plane substantially flush with the anterior surface of the femoral cortex, and along said prospective planar cut. 
     The method further contemplates loosely placing a longitudinal intramedullary rod in the femur such that an end of the rod projects from the femur, mounting a tool on the projecting end of the rod, establishing, by said tool, an angular position of said prospective planar cut along a plane rotated at an angle of between 0 and 15° with respect to a tangential plane at the posterior of the lateral and medial condyles about an axis located in said tangential plane. 
     In further accordance with the method, the tool is rotated with said rod through said angle and a datum or benchmark is established by the rotated rod or by pins installed in the condyles on the basis of the rotated position of the tool. A cutting guide can be mounted on said tool, to enable the distal end of the condyles to be cut along said plane. Thereafter, the tool is removed and a second A-P cutting guide is mounted on the selected benchmark, i.e., the rod or the pins and the posterior and anterior cuts are made. The axis about which the plane of the prospective cut is rotated is located in said tangential plane at the posterior surfaces of the medial and lateral condyles and can be located at either of the condyles or at any location therebetween. It is a feature of the invention that the tool may remain on the rod both for the measurements and for the cutting of the distal end of the femur. 
     The invention also contemplates that the cutting guide supports a means which enables the cutting guide to be secured to the condyles during the cutting of the distal ends of the condyles. 
     The invention further contemplates an apparatus for forming planar resections on the medial and lateral condyles of a femur to form seating surfaces to receive a femoral prosthesis and to properly articulate with a tibial and patellar prosthesis comprising: 
     a caliper feeler and measurement plate to measure for the size of the femoral prosthesis to be received, said caliper feeler and measurement plate adapted to determine a first distance between an anterior surface of the femoral cortex and a plane tangent to a posterior surface of the medial and lateral condyles of a femur, the caliper feeler referencing the anterior surface of the femoral cortex and the measurement plate referencing the plane tangent to the posterior surface of the medial and lateral condyles; 
     a graduated scale to compare the first distance to at least two standard femoral prosthesis sizes and to determine the smaller of the at least two standard femoral prosthesis sizes; 
     a graduated scale to measure a second distance between the first distance and the size of the smaller standard femoral prosthesis size, so that a thickness or thicknesses can be measured to be resected at the posterior surface of the medial and lateral condyles of the femur by adding the average thickness of the posterior condyles of the smaller standard femoral prosthesis and the second distance; 
     a tool to resect the medial and lateral condyles along a plane at the anterior surfaces thereof flush with the anterior surface of the femoral cortex; and 
     a tool to resect distal ends of the medial and lateral condyles at a resected thickness equal to the average thickness of the distal condyles of the smaller standard femoral prosthesis plus the second distance. 
     The apparatus further contemplates a tool to resect the measured thickness at the posterior surface of the medial and lateral condyles of the femur. 
     The invention also contemplates a method for forming planar resections on the medial and lateral condyles of a femur to form seating surfaces to receive a femoral prosthesis and to properly articulate with a tibial and patellar prosthesis comprises: 
     measuring for the size of the femoral prosthesis to be received by determining a first distance between an anterior surface of the femoral cortex and a plane tangent to a posterior surface of the medial and lateral condyles of a femur; 
     using a graduated scale to compare the first distance to at least two standard femoral prosthesis sizes; 
     measuring a second distance between the first distance and the size of the smaller standard femoral prosthesis size; and 
     measuring a thickness or thicknesses to be resected at the posterior surface of the medial and lateral condyles of the femur, the thickness being equal to the average thickness of the posterior condyles of the smaller standard femoral prosthesis plus the second distance. 
     The method further contemplates the steps of resecting the medial and lateral condyles along a plane at the anterior surfaces thereof substantially flush with the anterior surface of the femoral cortex; and 
     measuring a thickness or thicknesses to be resected at the distal ends of the medial and lateral condyles, the thickness being equal to the average thickness of the distal surface of the smaller standard femoral prosthesis plus the second distance, and resecting the distal ends of the medial and lateral condyles at the measured thickness. 
     Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic, lateral view of the femur and tibia at a knee joint showing prospective cuts to be made on the femur for installation of a femoral prosthesis. 
     FIG. 2 is a diagrammatic illustration of the knee joint of FIG. 1 seen anteriorly of the joint. 
     FIG. 3 is an end view from the distal end of the femur of the knee joint. 
     FIG. 4 is a sectional view of a femoral knee prosthesis adapted for placement on the femur after the planar cuts have been made on the femur. 
     FIG. 4 a  is a side view identifying each of the cuts made to the femur. 
     FIG. 5 is a side view similar to FIG. 1 in which the tibia has been turned 90° to expose the distal end of the femur, an intramedullary rod has been inserted into the femur and a tool placed on the rod, the tool being partly broken away and shown in section. 
     FIG. 6 is an end view of the tool taken in the direction of arrow  6 — 6  in FIG.  5 . 
     FIG. 7 is a sectional view taken on line  7 — 7  in FIG.  6 . 
     FIG. 8 is a broken, perspective view of a lower caliper feeler of the tool. 
     FIG. 9 is similar to FIG.  6  and illustrates a first stage in which the rod is angularly rotated by a specific amount. 
     FIG. 10 is similar to FIG. 9 in a subsequent stage. 
     FIG. 11 is an exploded view showing a cutting guide to be installed on the tool. 
     FIG. 12 is a top, plan view showing the cutting guide installed on the tool. 
     FIG. 13 illustrates the distal end of the femur after the distal end has been cut and an AP cutting guide has been placed on the rod. 
     FIG. 14 is an end view similar to FIG. 6 of a second embodiment of the tool. 
     FIG. 15 shows the tool of FIG. 14 in a rotated state. 
     FIG. 16 is an end view similar to FIG. 6 of a third embodiment of the tool. 
     FIG. 17 shows the tool of FIG. 16 in a rotated state. 
     FIG. 18 is an end view similar to FIG. 6 of a fourth embodiment of the tool. 
     FIG. 19 shows the tool of FIG. 18 in a rotated state. 
     FIG. 20 is similar to FIG. 13 but shows a modification adapted to the embodiment of FIGS. 16 and 17. 
     FIG. 21 is an end view similar to FIG. 6 of a fifth embodiment of the tool including an A-P measuring guide. 
     FIG. 22 is a side view similar to FIG. 5 of the tool of FIG. 21 mounted on the distal femur. 
     FIG. 23 is a top view of the tool of FIG. 21 mounted on the distal femur. 
     FIG. 24 is an exploded view of a distal cutting block to be installed on the tool of FIG.  21 . 
     FIG. 25 is a side view of the tool of FIG. 21 mounted on the femur installed with the distal cutting block. 
     FIG. 26 is a top view of the tool of FIG. 25 mounted on the femur. 
     FIG. 27 is an enlarged fragmentary view of the sliding scale of the distal cutting block. 
     FIG. 28 is a side view of the femur with the distal cutting block mounted thereon. 
     FIG. 29 is a top view of the A-P cutting block mounted on the distal femur. 
     FIG. 30 is a view similar to FIG. 13 illustrating the distal end of the femur after the distal end has been cut and the A-P cutting block has been mounted thereon. 
     FIG. 31 is a top view of the A-P cutting block mounted on the distal end of the femur after the distal end has been cut and the A-P cutting block has been mounted thereon. 
     FIG. 32 is a side view of the distal end of a femur after it has been cut and a preferred prosthesis is ready to be mounted thereon. 
     FIG. 33 is a front view of the preferred prosthesis to be used with the tool of FIG.  21 . 
     FIG. 34 is an alternative embodiment of the tool of FIG. 22, including posterior clips. 
     FIG. 34 a  is a rear perspective view of a posterior clip of FIG.  34 . 
     FIG. 34 b  is a front perspective view of a posterior clip of FIG.  34 . 
     FIG. 35 is a top view of the tool of FIG.  34 . 
     FIG. 36 is a front view of the tool of FIG.  34 . 
     FIG. 37 is rear view of the tool of FIG.  34 . 
     FIG. 38 is a top view of the tool of FIG. 34 mounted on the femur installed with a distal femoral cutting block and a distal femoral resection caliper. 
     FIG. 39 is a top view of the femur with the distal femoral cutting block mounted thereon. 
     FIGS. 40 and 41 are perspective views of distal femoral resection calipers for use in right and left femurs. 
     FIG. 42 is a front view of a tibial resection guide of the present invention mounted on a tibia. 
     FIG. 43 is a side view of the tibial resection guide of FIG.  42 . 
     FIG. 44 is a top view of the tibial resection guide of FIG.  42 . 
     FIG. 45 is a side view of a tibial external resection guide of the present invention mounted on a tibia. 
     FIG. 46 is a top view of a spacer of the present invention. 
     FIG. 46 a  is an end view of the top extension portion of the spacer of FIG.  46 . 
     FIG. 46 b  is an end view of the bottom flexion top extension portion of the spacer of FIG.  46 . 
     FIG. 47 is a side view of the spacer of FIG.  46 . 
     FIG. 48 is a front view of the knee space including a spacer in flexion. 
     FIG. 49 is a front view of the knee space including a spacer in extension. 
     FIG. 50 is a side view of a tibial reresection guide in accordance with the present invention mounted on a tibia. 
     FIG. 51 is a front view of the tibial reresection guide of FIG.  50 . 
     FIG. 52 is a top view of a tibial reresection guide of FIG.  50 . 
     FIG. 53 is a partial side view of a patellar clamp including a hinge feature in accordance with the present invention. 
     FIG. 53 a  is a partial top view of the hinge feature of the patellar clamp of FIG.  53 . 
     FIG. 54 is an end view of the patellar clamp of FIG. 53 showing a scale to measure the patella thickness. 
     FIG. 55 is a side view of a patellar clamp in accordance with the present invention without the hinge feature shown reaming a patella. 
     FIG. 56 is a top view of the patellar clamp of FIG. 55 reaming a patella. 
     FIG. 57 is a side partially broken away view of a patella having a patella insert fitted therein. 
     FIG. 58 is a side view of an improved nail in accordance with the present invention. 
     FIG. 59 is a side view of the tool of FIG. 34 mounted on a femur installed with a distal femoral cutting block and a distal femoral resection caliper of the present invention, showing the nail of FIG. 58 being removed from the femur in accordance with an improved slap hammer of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     When performing a unicompartmental knee replacement, it is imperative to maintain the jointline at or near anatomic level. As a consequence, this maintains a full range of motion. The instrument system of the present invention has been developed which combines the advantages of anterior and posterior referencing systems to maximize motion in a reproducible fashion and can easily be incorporated into an operative protocol. With the instrument system of the present invention, orthopedic surgeons can reconstruct a knee and retain “anatomic” landmarks. This makes it possible to deal with many of the deformities confronting the orthopedic surgeon in an arthritic knee. 
     There are three ways that joint surgeons can insert a total knee replacement: 
     1) Resect the distal femur to accommodate the thickness of the femoral prosthesis. Resect the proximal tibia to accommodate the thickness of the tibial prosthesis. This recreates any lost motion and requires major soft tissue releasing. The flexion and extension resection spaces are not coordinated. 
     2) Rebuild the “normal” knee by compensating for loss articular cartilage and bone in the measurements for bony resection; then soft tissue releases can be performed to accommodate the proper dimensions. This places even greater demands on contracted soft tissues. Although this may be most anatomically correct, it requires such major tissue releases as to make it impractical. 
     3) Accept bony and articular cartilage loss. Resect the amount of bone in flexion and extension to accommodate full extension and as much flexion as deemed necessary. This method relies on a coordinated resection of the flexion-extension spacing. It relies on accurate measurements to allow for the resection of bone and minor soft tissue release. Within certain parameters, this method is preferable and can only be possible with better instrumentation, such as the instrumentation of the present invention. 
     Referring now to FIG. 1, the drawing diagrammatically illustrates the femur  1  and tibia  2  of a knee joint  3 . The invention is concerned with the placement of planar resections or cuts at the distal condylar region  4  of the femur  1  to receive a femoral knee prosthesis  5  (FIG.  4 ). Typically, a total knee replacement also requires placing a planar cut at the proximal tibia of the tibia  2  to receive a tibial prosthesis, not shown. The tibial prosthesis typically consists of a tibial baseplate, not shown, that is fitted on the proximal tibia after the tibial cut is made, and an articular insert, not shown, secured to the baseplate to articulate with the femoral prosthesis  5 . 
     The cut made on the tibia  2  and installation of the tibial knee prosthesis should be as close to the anatomic level as possible and should be substantially parallel to the floor in the mediolateral plane. This maintains the joint line at or close to anatomic level. Moreover, the angle of the proximal tibial resection should correspond to the angle of the distal femoral resections  12 . For example, the proximal tibia is in mild varus and is resected such that the resection in the mediolateral plane is parallel to the floor and oriented posteriorly about 3°. Accordingly, the cuts made on the femoral prosthesis, discussed below, must also take into account this 3° mediolateral orientation in order to align the femoral prosthesis with the tibial prosthesis as will be explained later. 
     Assuming normal anatomy, it is also important that the resected space medially in extension between the tibia  2  and the femur  1  of the knee equals the combined thickness of the medial tibial prosthesis and the distal medial femoral prosthesis; that the resected space laterally in extension between the tibia  2  and the femur  1  of the knee equals the combined thickness of the lateral tibial prosthesis and the distal lateral femoral prosthesis; that the resected space medially in flexion between the tibia  2  and the femur  1  of the knee equals the combined thickness of the medial tibial prosthesis and the posterior medial femoral prosthesis; that the resected space laterally in flexion between the tibia  2  and the femur  1  of the knee equals the combined thickness of the lateral tibial prosthesis and the posterior lateral femoral prosthesis; and that the resected space between the tibia  2  and the femur  1  of the knee in flexion must be equal to or greater than the resected space between the tibia  2  and the femur  1  of the knee laterally in extension, assuming normal ligament balance. 
     The condylar region  4  of the femur  1  is formed with a medial condyle  6  and a lateral condyle  7  separated by an intercondylar notch  8 . The femur  1  includes a shaft  9  forming the femoral cortex, the condylar region  4  being at the distal end of the shaft  9 . 
     In order to install the femoral knee prosthesis  5  on the distal condylar region  4  of the femur  1 , three planar cuts are made in the condylar region  4  to form seating surfaces for the prosthesis  5 . These cuts consist of an anterior cut  10 , a posterior cut  11  and a distal end cut  12 . The placement of these cuts  10 ,  11 ,  12  is crucial to the installation of the prosthesis  5  and its effect on the overall function of the prosthetic knee joint. 
     The invention is based on complying with the following conditions. 
     1. Forming the planar cut  10  at the anterior surface of the femoral condylar region flush with the anterior surface  13  of the femoral cortex so as to form a continuous surface therewith free of formation of either a notch or elevation at the juncture of cut  10  and surface  13 . 
     2. Forming the planar cut  11  at the posterior surface of the femoral condylar region at a distance D from planar cut  10  equal to the interior dimension S between the anterior and posterior mounting surfaces  14 ,  15  of the prosthesis  5 . The dimension S is the so-called A-P distance of the prosthesis and this distance varies for different size prostheses. For example, prosthesis are categorized as small, small(+), medium, large, large(+) and extra large and the A-P distance increases in proportion to the size increase. 
     With reference to FIG. 3, therein is seen a plane T tangential to the medial and lateral condyles at the posterior surface  16  of the condylar region. The planar cut  11  is made at an angle A, with respect to plane T to angularly align the femoral prosthesis with the tibial prosthesis. Normally, the angle would be 3° to match the angle of the tibial prosthesis, however, due to anatomical conditions of the patient such as wear of the medial or lateral condyles posteriorly the angle A can vary substantially, generally between 0 and 15°. The planar cut  11  will result in resection of bone of a thickness t 1  at the medial condyle  6  and a thickness t 3  at the lateral condyle. The thickness t 3  is usually less than t 1  and controls the location of planar cut  11  so that a minimum thickness of bone is resected at the posterior surfaces of the condyles. In this regard, the thickness t 3  is established as the difference between distance D′ between the anterior surface  13  of the femoral cortex and a plane P tangent to the posterior surface of the lateral condyle  7  and parallel to planar cut  11  and distance D between the anterior surface of the femoral cortex  13  and planar cut  11 . 
     The thickness t 3  and the location of the prospective planar cut  11  therefore can be established based on measurement of the distance D and the A/P dimension of the selected size of the prosthesis. The size of the prosthesis is determined on the basis of the measurement of the distance D′ and in general, the prosthesis size will be selected so that the thickness t 3  falls within a relatively narrow range, generally at least 6 mm and between 6 and 11 mm. The resected thickness of bone t 1  and t 3  at the medial and lateral condyles are generally unequal. 
     The distal end cut  12  is made so that the maximum thickness t 2  of bone resected at the distal end is substantially equal to t 3 , i.e., the maximum thickness t 2  of bone resected at the more prominent condyle at the distal end (the medial condyle  6  in FIG. 2) is equal to the minimum thickness t 3  of bone resected at the posterior surface. 
     Referring now to FIG. 5, in order to establish the precise positions of the three planar cuts  10 ,  11 ,  12  to be made on the femur  1 , a referencing or datum system is utilized which in the description herein is in the form of an intramedullary rod  20  installed in a bore  21  formed in the femur  1 . The use of the intramedullary rod  20  as a benchmark or datum is known in the art and is illustrated herein by way of example. Other referencing or datum systems can be employed as well, for example, utilizing two pins placed in the condyles as set distance below the anterior femoral cut to position an AP cutting guide thereon. This will be described later. 
     The bore  21 , which is approximately 8 mm in diameter, is formed longitudinally in the shaft  9  and in the condylar region  4  of the femur  1  at a location which is slightly anterior and medial of the intercondylar notch  8 . The rod  20  has a cylindrical portion  22  which snugly fits in the bore  21  but is able to be rotated in the bore  21 . The rod  20  may include radial flutes  23  extending outwardly a distance slightly greater than the diameter of the bore  21 . The flutes  23  are initially outside the bore  21  and are intended to be driven into the bore  21  to fixedly secure the rod  20  in the bore  21 . For this purpose, the flutes  23  are tapered to facilitate driving them into the bore  21  and grip the bore tightly in the distal femur  1  when driven therein. The outer ends of the flutes  23  can be saw-tooth or jagged as shown in FIG. 7 to provide a resilient gripping action. 
     The rod  20  includes an adjunct end or stub  24  which is non-circular in cross-section. The stub  24  may extend at an angle with respect to the longitudinal axis of the rest of the shaft so as to be substantially perpendicular to the joint and the prospective distal end cut  12  and parallel to the weight bearing mechanical axis of the leg. Shafts having stubs with different angles varying about 5-7° may be provided and selection is made on the sex, anatomical condition, and other conditions of the patient. This is conventional in prior usage. 
     The angular position of the non-circular stub  24  in bore  21  when the flutes  23  are driven into the bore  21  is a measure of the angle A at which the posterior and anterior cuts  11 ,  10  are made and, consequently, of the angular position of the knee prosthesis  5  on the femur  1  relative to the weight bearing mechanical axis of the leg. 
     The anatomical conditions governing the angular position of the rod  20  in the bore  21  is based on anatomy to maintain a straight line between the hip joint or the center of the femoral head in neutral rotation, the center of the knee joint and the midmedial third of the tibial plafond. 
     If the rod  20  initially assumes an angular position parallel to plane T, the rod is rotated by angle A to reach its datum position from which the cuts  10 ,  11 ,  12  will eventually be made. Nominally, the rotation is at an angle 3° to match the angle of the tibia prosthesis. However, due to wear of the condyles, and anatomical conditions of the patient the rotation of the rod must be varied from 3° to match the tibia prosthesis. The surgeon is readily able to estimate this angle based on the anatomy and on X-rays of the patient. Heretofore, however, the surgeon had to estimate the angle at which to set the rod  20  when the rod is driven into the bore  21 . An imprecise estimate of the rotational orientation of the stub  24  can lead to angulation and placement errors of the prosthesis. Stated succinctly, the estimate of the surgeon of the angulation of cut  11  based on patient anatomy is accurate, but the “eyeballing” of the rotational position of the stub is often inaccurate. 
     The invention provides a tool or instrument  30  which is fitted on the stub  24  of rod  20  and accurately establishes rotation of the rod  20  when it is driven into the bore  21  and which measures the distance D′ which in turn will determine the location of the planar cuts  10 ,  11 ,  12 . 
     The tool  30  includes a sleeve  31  having a circular-like bore  32  of the same shape as the stub  24  in order to be fitted on the stub  24  for common rotation therewith. The bore  32  should include longitudinal slots or striations, e.g., star-shaped. The sleeve  31  has grooves  33  aligned with flutes  23  to permit passage of the flutes  23  through the sleeve  31  when the rod  20  is driven into the bore  21  in the shaft  9  of the femur  1 . The sleeve  31  is rotatably supported in a slider  34  which is slidably supported by a lower half  35  of a caliper means whose upper half  36  is slidably engaged with lower half  35 . The upper and lower halves  36 ,  35  are formed as open U-shaped members forming adjacent legs  37 ,  38  which are slidably engaged by tongue and groove engagement means  39 . The slider  34  is slidably engaged in the legs  37  of the lower half  35  of the caliper means by a tongue and groove engagement means  40 . 
     A cross leg  41  at the closed end of the lower half  35  of the caliper means engages a bar  42  for slidable movement in a direction substantially perpendicular to the direction of slidable movement of slider  34 . The bar  42  is formed with opposed flats  43  on which the cross leg  41  can slide without undergoing rotation. The bar  42  is provided with forwardly facing pins  44  at end regions thereof. 
     A posterior caliper  45  is mounted on the pins  44 . The posterior caliper  45  includes a caliper plate  46  with spaced caliper feelers  47  (FIG. 8) for respectively contacting the posterior surfaces of the medial and lateral condyles. A pair of upright legs  48  are provided on plate  46  and the legs  48  are provided with respective slots  49  to receive respective pins  44  of bar  42 . The slots  49  are part-circular in extent and have a common center such that either pin  44  can ride its respective slot  49  and change the angle of bar  42  relative to the caliper plate  46 . The ends of the pins  44  are threaded and nuts  50  are engaged on the threaded ends to lock the position of the pins  44  in the slots  49 . 
     At the top of upper half  36  of the caliper means is an integral upstanding projection  60  which is integral with a guide bar  61 . The guide bar  61  extends substantially perpendicular to the plane of the caliper halves  36 ,  37 . The guide bar  61  has a bore  62  at one end thereof in which is slidably fitted an end of a rod  63  of an anterior caliper feeler  64  for extension and retraction adjustment movement of the anterior caliper feeler  64 . A nut  65  secures the position of the rod  63 . At the end of the rod  63  of the anterior caliper feeler  64  is a sector plate  66  which is pivotally supported at  67  by the rod  63 . The sector plate  66  has a part-circular surface  68  adapted to contact the anterior surface  13  of the femoral cortex. The surface  68  has its center at the pivotable support point  67 . 
     In operation, the femur  1  is rotated 90° from the position shown in FIG. 2 to the position in FIG. 3 or  5  so that the distal end of the femur  1  is exposed. The bore  21  is formed in the femur  1  and the rod  20  is inserted into the bore  21 . The tool  30  is then installed in the rod  20  by fitting the bore  32  in sleeve  31  on the stub  24  of the rod  20  projecting from the distal end of the femur  1 . The posterior caliper feelers  47  are respectively brought into contact with the posterior surfaces of the respective medial and lateral condyles. This effectively establishes the position of plane T as described in FIG.  3 . 
     A radially projecting tab  70  on the sleeve  31  is manually engaged to rotate the sleeve  31  through angle A representing the angle determined by the surgeon as explained previously. A scale  71  is provided to indicate the angle through which the sleeve  31 , and thereby the rod  20 , has been turned. The scale  71  comprises an index marker  72  on the sleeve and an angle scale  73  on the slider  34 . The scale  73  is marked for left and right femurs. For left femurs (described and illustrated in the drawing) the sleeve and rod are rotated to the right (clockwise) whereas when the tool is mounted on a rod in the right femur, the sleeve and rod are rotated to the left (counter clockwise). When the scale  71  indicates the desired angle of rotation, the sleeve  31  is rotatably locked in the slider  34  by suitable means (not shown) and the rod  20  is driven into the bore  21  of the femur  1  to be angularly secured thereon in the desired rotational position relative to the plane T tangential to the posterior surfaces of the medial and lateral condyles. This is the position shown in FIG.  9 . 
     In order to set the caliper means in position to measure the distance D′, the nuts  50  on pins  44  are loosened and the upper and lower caliper halves  36  and  37  are rotated as a unit around pin  44  at the lateral femoral condyle until the index marker  72  returns to its zero setting on the scale  73  as shown in FIG.  10 . The nuts  50  are then tightened and the caliper halves are now in a position to measure distances perpendicular to the plane P tangent to the posterior surface of the lateral condyle. The capability of slidable movement of the slider  34  on the lower caliper half  35  and of the caliper half  35  relative to bar  42  and posterior caliper feeler  45  permits the rotation of the caliper halves about pin  44  at the lateral condyle while the sleeve  31  and the slider  34  are engaged with the stub  24  of rod  20 . 
     The anterior feeler  64  is then positioned so that sector plate  66  contacts the anterior surface  13  of the femoral cortex. A distance scale  80  is provided between the upper and lower caliper halves  36 ,  35  and comprises a marker  81  on leg  37  and a scale  82  on leg  38 . The scale  82  indicates the prosthesis size and hence is a measure of the distance D. The calibration is such that when the marker  81  is in correspondence with a mark on scale  82  for a particular prosthesis, when this prosthesis is utilized, the difference between D and D′ (the thickness t 3  resected at the posterior condyle) will be substantially equal to the thickness of the prosthesis to be inserted. If the scale falls between prosthesis markings on scale  82 , generally the smaller prosthesis is selected and the resected thickness of the lateral condyle will be slightly increased accordingly. The scale markings can also be calibrated with reference to the resected thickness t 1  at the medial condyle to reflect the normally greater thickness resected thereat. 
     With the tool still mounted on the rod  20 , FIG. 11, the anterior feeler  64  is removed and a guide  90  is slidably fitted on guide bar  61 . At the top of the guide bar  61  another scale  91  is provided. The scale  91  is marked in millimeters and represents the distance from a plane perpendicular to the rod and tangent to the high point of the distal end surface of the more prominent of the medial or lateral condyles. In other words, when the tool  30  remains on the rod  20  and is brought into abutment with the condyles, this is the zero position of the scale  91 . The guide  90  has four upstanding pegs  92  which fit into four holes  93  of a distal end cutting guide  94 . 
     The cutting guide  94  is provided with slots  95  extending in a plane substantially perpendicular to the axis of stub  24 . The slots  95  extend from the medial and lateral side surfaces of the cutting guide  94  towards the center thereof. The slots  95  are adapted to guide a narrow cutting blade (not shown) for respectively cutting the medial and lateral condyles  6 ,  7  along planar cut  12 . The slots  95  are separated by a solid, intermediate section  96 . 
     The position of the slots  95  relative to the scale indicate the thickness t 2  to be resected by the planar cut  12  at the distal end of the femur  1 . The invention contemplates that the thickness t 2  may be equal to the thickness t 3  determined by the measurement of distance D′. Therefore, the guide  90  is moved until the slots  95  are aligned with the distance on scale  91  equal to the determined thickness t 3 . The guide  90  is then locked on guide bar  61  by suitable means (not shown). 
     Depending feet  97  are slidably mounted on cutting guide  94  in respective pairs on opposite sides of each slot  95 . After the cutting guide  94  has been moved to its cutting position as indicated on scale  91 , the depending feet  97  are slidably moved to abut against respective portions of the condyles. The feet  97  are provided with nail holes  98  and nails (not shown) are driven into the holes  98  to secure the cutting guide  94  to the femur  1 . A conventional cutting blade is then inserted in guide slots  95  to cut the distal ends of the condyles  6 ,  7  along the planar cut  12 . The feet  97  nailed to the condyles prevent skewing or sliding of the cutting guide during the cutting operation. 
     The tool  30  is then removed from the rod  20  and a conventional AP cutting guide  100  (FIG. 13) is fitted on the end of the rod  20  and abutted against the planar surface  12  now cut at the distal end of the femur  1 . The cutting guide  100  is provided with guide slots  101  and  102  which can be precisely placed for guiding a cutting blade to produce the anterior and posterior cuts  10 ,  11  respectively. The cut  10  will be flush with anterior surface  13  of the femoral cortex and the cut  11  will be at distance D therefrom. The AP cutting guide  100  also includes angular slots  103 ,  104  to form chamfer cuts  105 ,  106  on the femur  1  which match corresponding angular surfaces  107 ,  108  on the knee prosthesis  5 . 
     FIGS. 14 and 15 illustrate a second embodiment of a tool  30 A which is a simpler version of the first embodiment of FIGS. 5-10 and wherein the same reference characters are used to designate like elements. 
     Essentially, the embodiment of the tool  30 A of FIGS. 14 and 15 differs from that of FIGS. 5-10 in eliminating the rotatable sleeve  31  and directly engaging the stub  24  of rod  20  in bore  32  now provided directly in the slider  34 . The slider  34  thus serves as the engaging means for the stub  24 . The legs  48  on the caliper plate  46  are provided with spaced holes  149  instead of the continuous slot  49  of the embodiment of FIGS. 5-10 and angular markings  173  are provided adjacent to the holes  149  to indicate the magnitude of angle A between the caliper plate  46  and bar  42 , serving as a measurement plate, when the pin  44  is in the respective hole  149 . In the illustrated embodiment in FIGS. 14 and 15, the holes  149  are placed to provide angulations of 0, 3, 5, 7 and 9° left and right between bar  42  and caliper plate  46 . 
     In operation, the stub  24  is engaged in the bore  32  in slider  34  and pins  44  are placed in the 0° holes in respective legs  48 . The caliper feelers  47  are placed into tangential contact with the posterior surfaces of the medial and lateral condyles  6 ,  7  respectively. The pin  44  in the leg  48  corresponding to the medial condyle is then removed from the 0° hole and placed in the hole  149  corresponding to the desired angulation of the rod  20 . This is shown in FIG. 15 where pin  44  is set in the hole  149  to angulate the bar  42 , 7° relative to the caliper plate  46  and thereby relative to the plane T tangent to the medial and lateral condyles. By virtue of the slidable support of slider  34  in legs  37  and the slidable support of cross leg  41  on bar  42 , the tool  30 A is capable of remaining in position on stub  24  and rotating around pin  44  at the posterior surface of the lateral condyle  7 . 
     The measurement by the caliper means to determine the size of the prosthesis and the resected thickness t 3  at the lateral condyle is carried out in the same way as in the first embodiment and the planar cuts are then made on the condyles as previously described. 
     As was described for the first embodiment of tool  30 , it is also possible to effect measurement with the tool  30 A to determine thickness t 1  at the medial condyles and to utilize this thickness to establish the thickness t 2  distal cut  12 . 
     Both the first and second embodiments have been described with regard to the intramedullary rod  20  with radial flutes  23  to embed the rod securely in the bore  21  in the femur  1  to establish the datum or benchmark position for attaching the cutting guide  94  to effect the distal end cut  12  and thereafter the AP cutting guide  100  to effect the anterior and posterior planar cuts  10 ,  11 . However, other suitable means can be employed to secure the angular position of the rod instead of the flutes  23 . Moreover, since the rod  20  is ultimately removed from the femur  1  after the planar cuts  10 ,  11 ,  12  have been made, the absence of the flutes  23  makes removal simpler. 
     FIGS. 16 and 17 illustrate a third modified embodiment of the tool  30 B which secures the angular datum position by use of a rod without flutes  23 . The same reference characters as in the first two embodiments designate like elements. 
     In the third embodiment, the rod  20  is smooth and devoid of flutes  23 . The rod  20  is rotated to its adjusted angular position, as in the first and second embodiments, and in order to secure an angularly adjusted datum position, lateral plates  110 ,  111  are secured to the legs  38  of the upper caliper half  36 . Each plate  110 ,  111  contains two vertical rows  112  of overlapped holes  113 . The rows  112  are designated for right and left femurs and the holes  113  are respectively graduated in size order from the scale  82 . When the caliper means of the tool  30 B has been rotated to the desired degree of angulation, pins  115  or similar fasteners are placed in the appropriate holes  113  in the lateral plates  110 ,  111  and secured in the distal ends of the medial and lateral condyles so that the pins  115  project from the distal ends of the condyles. The pins  115  establish an angular datum position representing the rotation of the tool. The steps of measurement of prosthesis size, and of effecting the planar cut with the guide  94  are carried out as in the previously described embodiments. However, after the distal end cut  12  is made, the tool  30 B is removed leaving the pins  115  in place in the condyles, the rod  20  is removed from the femur  1 , and a guide  100 ′, FIG. 20, is mounted on the pins  115  which serve to accurately position the guide  100 ′ so that the slots  101 - 104  will be precisely located for exact placement of the cuts  10 ,  11 ,  105  and  106 . The guide  100 ′ has holes  116  to receive the pins  115  which are precisely located with regard to the slots  101 - 104  to insure accurate location of the cuts when the guide  100 ′ is mounted on the pins  115 . After the cuts have been made, the pins  115  are removed from the condyles. As evident from the above, the embodiment contemplates the use of the pins  115  as the means to provide the datum position for the cutting guide  100 ′ in lieu of the rod  20 . The use of the plates  110 ,  111  and of the pins  115  is applicable to the other embodiments as well. 
     FIGS. 18 and 19 illustrate a fourth modified embodiment of the tool  30 C which is a simplified version of the second embodiment of FIG.  14  and uses the same reference-characters to designate like elements. 
     The tool  30 C utilizes slider  34  which engages the rod end  24  and is slidably engaged in the legs  37  of the lower caliper half  35 . The legs  37  of the lower caliper half  35  are slidably engaged with the legs  38  of the upper caliper half  36 . 
     At its lower end, the lower caliper half  35  includes a cross bar  141  from which a leg  142  depends. The leg  142  supports a pivot  143  which slidably rides in a slot  144  in a bracket  145  integral with posterior caliper plate  146 . The posterior caliper plate  146  is similar to caliper plate  46  of the second embodiment and includes posterior caliper feelers for contacting the medial and lateral condyles  6  and  7 . The slot  144  extends substantially parallel to the caliper plate  146  in the plane of tangential contact of the posterior feelers with the posterior surfaces of the medial and lateral condyles. An angle scale  147  is provided between the leg  142  and the bracket  145 . 
     In the initial position of the tool, the slider  34  is fitted on the end  24  of the rod and the posterior feelers are brought into tangential contact with the medial and lateral condyles. The caliper means  35 ,  36  are rotated, while the rod  24  is held fixed, until the angle scale  147  reads zero. The pivot  143  is disposed in the slot  144  substantially in the plane T tangent to the posterior surfaces of the medial and lateral condyles. The tool  30 C is then rotated to cause the end  24  to rotate through an angle A. corresponding to the determined angle of rotation. The angle A is read on the angle scale  147 . The pin  143  undergoes slidable movement in slot  144  while the slider  34  undergoes slidable movement in lower caliper half  35  to accommodate the rotation of the tool. The pin  144  remains in the tangential plane T. The scale  80  is a measure of the distance from the anterior feeler in contact with the anterior femoral cortex and the pin  143  along a perpendicular line from the anterior femoral cortex to a plane P passing through the pin  144  and inclined relative to posterior caliper plate  146  by the angle of rotation A of the tool. Any difference between the distance from pivot point  67  to the surface  68  of the sector plate  66  and the corresponding distance measured along the perpendicular to the incline plane P is negligible and even for an angle A of 12° the difference will be less than one-third mm. 
     As an alternative to the slot  144 , the bracket  145  can be provided with a series of holes representing different angles of the caliper means  35 ,  36  relative to the plate  146 , corresponding to different angles A, as in FIG.  14 . The holes are provided along the axis of slot  144  in order to be in tangential plane T of the posterior feelers on the posterior surfaces of the condyles. When the pin  143  is secured in a respective hole the caliper means is secured at the angle designated by the associated hole. In the use of this alternative, with the tool not yet fitted on the end  24 , the angle of the caliper means is set by inserting the pin  143  into the selected hole and the posterior feelers on plate  146  are brought into tangential contact with the condyles  6 ,  7 . The tool is then fitted on the end  24  which now assumes the angle of the caliper means relative to the plate  146 . The rod  20  is then driven into the femur  1  as before, or alternatively, as in the embodiment of FIGS. 16 and 17, pins  115  are installed in the condyles through holes in plates  110 ,  111  installed on the upper caliper half of the tool. The subsequent operations are the same as previously described. 
     FIGS. 21-33 illustrate a fifth modified embodiment of a tool  30 D which is a simplified version of the first four embodiments of FIGS. 1-20 and wherein like reference characters are used to designate like elements. Preferably, the tool  30 D is used in connection with a GENESIS II Total Knee System supplied by Smith &amp; Nephew Richards, Inc. of Memphis, Tenn. It should be realized, however, that the tool  30 D can be adapted to be used with knee systems of other manufacturers. 
     Referring now to FIG. 33, there is shown a GENESIS II femoral prosthesis  198 . The thickness of the distal femoral condyles of the prosthesis is about 9.5 mm (about 9-9.5 mm), i.e., the thickness of the distal medial femoral condyle  204  and the distal lateral femoral condyle  206  are about the same. 
     The prosthesis  198  has a  30  external rotation or varus angulation built therein. This is accomplished by altering the thickness of the femoral condyles posteriorly. For example, the thickness of the posterior lateral femoral condyle  200  of the prosthesis is about 3 mm thicker (about 2.5-3 mm) than the posterior medial femoral condyle  202 , assuming that the most prominent portions of the medial and lateral condyles are two inches apart. The difference of thickness of the posterior condyles of the prosthesis will vary directly with the distance between the condyles. The GENESIS II tibial prosthesis assembly, not shown, has a tibial baseplate (metal) thickness of about 2 mm and a minimal tibial prosthetic (plastic) thickness of about 7.5 mm. 
     When using the tool  30 D and the GENESIS II prosthesis  198 , the joint line will be realigned parallel to the floor. This changes a normally 3° varus angle to 0°. A 3° angle amounts to approximately 1.5 mm per linear inch. Assuming the tibio femoral weight-bearing area is 2 inches apart on average, then 3 mm more laterally than medially must be resected from the tibia to achieve the resection parallel to the floor. To achieve a rectangular extension space and a trapeziodal flexion space, it follows that 3 mm more from the distal medial femoral condyle than the distal lateral femoral condyle must also be resected. The posterior condyles, however, are neutrally resected. 
     Referring now to FIGS. 21-33, once the bore  21  is formed longitudinally in the shaft  9  and in the condylar region  4  of the femur  1 , the tool  30 D is fitted over the rod  20  until it contacts the distal femur, i.e., the distal end of the femur  1 . Before the tool  30 D is fitted over the rod  20 , the tool  30 D is first fitted with a collet  206  which is similar to the stub  24 . Like stub  24 , collets having different angles varying about 5-7° may be provided and selection is made based on the anatomical condition and other conditions of the patient. The collet  206  is similar to the valgus angle bushing available from Smith &amp; Nephew Richards, Inc. 
     The tool  30 D is somewhat similar in structure to, but an improved version of, the valgus alignment guide and/or valgus alignment assembly available from Smith &amp; Nephew Richards, Inc. The tool  30 D includes a distal femoral sizer made up of a lower half  208  and an upper half  210  slidable in the lower half  208 . When the distal femoral sizer is fitted with collet  206 , collet  206  fixes the angle of the distal femoral sizer and may be referred to as a valgus alignment guide. 
     The lower half  208  includes a pair of posterior caliper feelers  47  for respectively contacting the posterior surfaces of the medial and lateral condyles. The caliper feelers  47  can be elongated to accommodate smaller and larger femurs, corresponding to prosthesis sizes 1-5 and 4-8 respectively. 
     The tool  30 D includes a graduated scale  212 . The graduated scale  212  includes markings  213  on the upper half  210  and a marker  214  on the lower half. The markings  213  on the graduated scale  212  indicate prosthetic sizes and hence is a measure of the distance D or S. For example, the markings  213  in FIG. 21 indicate prosthetic sizes 3-8. The upper half  210  can be adapted to indicate other prosthetic sizes as well. 
     The lower half  208  includes scale  215  to indicate differences in size between respective prosthetic sizes, i.e., the number of millimeters over or under the prosthetic size. In a preferred embodiment, the scale  215  is calibrated in one millimeter increments. The calibration is such that when the marker  214  directly corresponds with a mark  213  on the scale  212  for a particular prosthesis, e.g., size 4, when this size 4 prosthesis is utilized, the difference between D and D′ (the thickness t 3  resected at the posterior condyles and the distal femoral condyles) will be substantially equal to the thickness S (FIGS. 4 and 34) of the size 4 prosthesis to be inserted. 
     If the marker  214  falls between prosthesis markings  213  on scale  215 , generally the smaller prosthesis size is selected and the resected thicknesses of the posterior condyles and the distal femur will be slightly increased accordingly. For example if the marker  214  falls one increment, i.e. one millimeter, beyond prosthesis size 4, then the resected thickness at the posterior condyles will be the average thickness of the posterior condyles of the size 4 prothesis (e.g., 19.5 mm) plus 1 mm. similarly, the resected thickness of the distal femoral condyles may be electively increased 1 mm, i.e., 9.5 mm plus 1 mm. 
     It should be realized that if a surgeon were to choose the larger prosthesis, then the marker  214  and the scale  215  would indicate how much less thickness from the prosthetic size would be resected at the posterior and distal femoral condyles. In this case, appropriate compensation must be made on the distal femoral resection and possibly the proximal tibial resection, depending on the deformity of the knee, to achieve satisfactory motion and ligament balance. 
     The tool  30 D includes an anterior-posterior (A-P) measuring guide or anterior caliper feeler  64  which along with the posterior caliper feelers  47  measure distance D′. The A-P measuring guide  64  includes a tab  216  which allows the A-P measuring guide to be releasably attached to the tool  30 D. 
     The A-P measuring guide  64  is somewhat similar in structure to, but an improved version of, a femoral sizing guide available from Smith &amp; Nephew Richards, Inc. The A-P measuring guide  64  includes a rod  63  and a sector plate  66  adapted to contact the anterior surface  13  of the femoral cortex. Unlike the tool  30  of the first preferred embodiment of FIGS. 1-3, the sector plate  66  need not be pivotally attached to the rod  63 . 
     The tool  30 D is also adapted to be used in connection with a distal femoral resection caliper  88 , a distal femoral cutting block  89  (FIGS. 24-28) and an A-P cutting block  100  (FIGS.  29 - 31 ). The distal femoral cutting block  89  is used to resect the distal ends of the femur  1 . The A-P cutting block  100  is used to resect the posterior medial and lateral condyles, to make the final anterior resection and to make the posterior and anterior chamfer resections as described above. The distal femoral cutting block  89  and the A-P cutting block  100  are somewhat similar in structure to, but an improved version of, the distal femoral resection stylus and cutting block, and the femoral A-P cutting block, respectively, available from Smith &amp; Nephew Richards, Inc. Like the A-P measuring guide  64 , the distal femoral cutting block  89  (and resection caliper  88  when joined, as explained below) are releasably attached to tool  30 D by tab  218 . 
     Referring now to FIGS. 21-33, in use, once the tool  30 D with the properly angled collet  206  is fitted over the rod  20 , which is inserted in the bone  21  of the femur  1 , the tool  30 D is made to contact the distal femur  1 . As best seen in FIGS. 23 and 26, the side of tool  30 D that contacts the distal femur  1  should include a 3 mm lateral offset  220  to contact the distal surface of the lateral femoral condyle. This ensures that the distal resection is substantially parallel to the proximal tibial resection in the medial lateral plane, and that the resultant distance between the tibial and femoral resections will be substantially equal to the thickness of the combined tibial and femoral prosthesis in flexion and extension. 
     In an alternative embodiment, if collet  206  is angled 8-10° instead of 5-7°, the 3 mm lateral offset  220  is not necessary. The 8-10° angulation is preferable because it reflects the true angulation of the distal femur. 
     Next, external rotation must be oriented from the posterior condyles (or any other consistent anatomic landmark). This requires adjustable posterior feelers or “feet”  47  to contact the posterior condyles at 3° of external rotation, or to be able to compensate for deformities and achieve posterior proper rotation. 
     To achieve the 3° external rotation, the tool  30 D is then rotated so that the posterior caliper feelers  47  contact both corresponding posterior surfaces of the medial and lateral condyles (FIGS. 22 and 23) assuming equal or no bone substance loss. This sets the rotation or angle of the preliminary anterior resection  10  and the posterior resection  11  which is made by the A-P cutting block  100  (FIGS.  29 - 31 ), and equal amounts of substance will be resected from the medial and lateral posterior condyles. The rotation or angle of the posterior resection  11  is also set because an anterior portion  248  of the A-P cutting block  100  rests on the preliminary anterior resection  10  and thus orients the posterior resection  11  from the rotation or angle as the anterior resection  10 . Referring now to FIG. 23, nails  222  should then be inserted in nail holes  224  to secure the lower half  208  to the distal femur. 
     If the posterior surfaces have unequal bone loss, the corresponding caliper feeler  47  should be made to contact the posterior surface with the least amount of bone loss. The tool  30 D should then be rotated on the rod  20  so that the other caliper feeler  47  corresponding to the posterior condyle with the greater amount of bone loss is a distance away from that posterior condyle about equal to the amount of bone loss. This sets the rotation, or angle, of the preliminary anterior resection  10  and the posterior resection  11  which is made by the A-P cutting block  100  (FIGS.  29 - 31 ). Unequal amounts of substance may now be resected from the medial and lateral posterior condyles. 
     Referring now to FIGS. 34-38, in an alternative embodiment, clips  300  sized to make up for bone loss to the posterior condyles can be added to the posterior feelers  47 . With clips  300 , tool  30 D does not need to be rotated as explained above to achieve the 3° external rotation. Preferably, clips  300  are made in 2, 4, 6, 10, and 12 mm sizes, although clip  300  can be made of any other appropriate size. The user of the tool  30 D can estimate the amount of bone loss to the nearest clip size. In FIGS. 34-38, clip  300  is attached to the posterior feeler  47  corresponding to the medial posterior condyle because the medial posterior condyle suffered bone loss. 
     If both the medial and lateral condyles suffer bone loss, then the user of tool  30 D uses the appropriately sized clip  300  based upon relative bone loss between the medial and lateral condyles. For example, if the medial posterior condyle suffers 6 mm bone loss and the lateral posterior-condyle suffers 2 mm bone loss, then a 4 mm clip  300  will be attached to posterior feeler  47  corresponding to the medial posterior condyle. 
     The clips  300  can be attached to the posterior feelers  47  by any of the known methods. Preferably, posterior feelers  47  include a slot  302  and edges  304  which receive a tab  306  and grooves  308 , respectively, formed in clip  300 . Clips  300  may even include a spring activated post  310  or the like to secure clip  300  to posterior feelers  47  once slot  302  and edges  304  of the posterior feelers  47  receive tab  306  and grooves  308  of clip  300 . Posterior feelers  47  may also include posts  310  to even more securely attach clips  300  to posterior feeler  47 . 
     In the first preferred embodiment of the invention, the scale  71  should be set to rotate the sleeve  31  and thereby the rod  20  through angle A at 1° for every millimeter of bone loss. For example if the surgeon determines that there is 2 mm bone loss at one of the posterior condyles, the index marker should be set to correspond to a 2° angle on the angle scale  73 . The sleeve  31  is then rotatably locked in the slider  34  and the rod  20  is driven in the bone  21  of the femur  1  to be angularly secured thereon in the desired rotational position relative to the plane T targeted to the portion surface of the medial and lateral condyles. See FIG.  9 . 
     In order to set the caliper means in position to measure the distance D′, the nuts  50  on pins  44  are loosened and the upper and lower caliper halves  36  and  37  are rotated as a unit around pin  44  at the lateral femoral condyle until the index marker  72  returns to its zero setting on the scale  73  as shown in FIG.  10 . The nuts  50  are then tightened and the caliper halves are now in a position to measure distances perpendicular to the plane P tangent to the posterior surface of the lateral condyle. 
     In the fourth preferred embodiment of the invention, the tool  30 C is rotated to cause the end  24  to rotate through angle A corresponding to the 2° angle of rotation. The angle A is read in the angle scale  147 . 
     It should be realized that as explained above, if a prosthesis other than a GENESIS II prosthesis is used, i.e., a symmetrical prosthesis, the posterior condyles will be resected asymmetrically to reflect the 3° external rotation that was otherwise built into the GENESIS II prosthesis. This angulation may be greater than or less than 3° to compensate for any bone loss posteriorly. 
     Referring back now to tool  30 D, in order to set the caliper means in position to measure the distance D′, the upper half  210  of the tool  30 D fitted with the anterior caliper feeler  64  is then lowered until the sector plate  66  of the anterior caliper feeler  64  contacts the lateral portion of the anterior cortex, i.e., the sector plate  66  should contact the lateral side of the anterior cortex (FIGS.  22  and  23 ). The marker  214  then indicates a prostheses size S or distance D. If the marker  214  falls between two prosthetic sizes, normally the smaller prosthetic size is chosen. The upper half  210  is then fixed to the distal femur by inserting a nail  226  in the nail hole  228  that corresponds to the smaller chosen prosthetic size. 
     A measurement is now made to determine the appropriate size A-P cutting block  100  to later be used to resect the posterior medial and lateral condyles. The approximate size cutting block  100  corresponds to the chosen prosthetic size. If the marker  214  fell between two prosthetic sizes and the smaller size is chosen, i.e., anterior referencing, a measurement must be made to determine how many millimeters extra would be resected posteriorly (i.e., the thickness of the prosthesis posteriorly plus the number of mm over resection). This measurement is then taken from the scale  215  and is equal to the number of millimeters the marker  214  is away from the smaller prosthetic size. This measurement is then added to the average thickness of the posterior condyles of the prosthesis to determine the posterior resections. Each type of prosthesis has its own average thickness. For example, if the marker  214  indicates 1 mm greater than prosthetic  198  size 3, 1 mm extra will be resected posteriorly. The total posterior resection would then be the average thickness of the posterior condyles of the prosthesis  198  (e.g., 8.5 mm) plus 1 mm for a total thickness of 9.5 mm. 
     With the tool  30 D still mounted on the rod  20 , the anterior caliper feeler  64  is removed from the upper half  210  by depressing the tab  216 . A saw blade, not shown, is then inserted into guides or slot  230  to make a preliminary cut. of the anterior condyles to meet the surface of the anterior cortex in proper rotational alignment in the mediolateral plane. 
     The distal femoral cutting block  89  secured to distal femoral resection caliper  88  is then attached to the upper half  210  of tool  30 D. Distal femoral resection caliper  88  is releasably attached to distal femoral cutting block  89  through a channel  234  fixed therein. Distal femoral resection caliper  88  is secured to distal femoral cutting block  89  by a cam mechanism  236  and to the upper half  210  by tab  218 . 
     The distal femoral resection caliper  88  includes a sliding scale  238  that is calibrated at one millimeter increments from the average size or thickness of the distal femoral condyles of the prosthesis. The average size or thickness of the distal femoral condyles of a prosthesis ranges from about 6 mm to 12 mm depending on the particular prosthesis chosen. A typical thickness of the distal femoral condyles of prosthesis  198  is about 9.5 mm. 
     The distal femoral cutting block  89  should be inserted on the upper half  210  until it abuts the resected surface of the anterior cortex (FIGS.  25 , 26 ). The cutting block  89  should then be set at “size” plus (or minus) the previously taken measurement of how many extra (or fewer) millimeters would be resected posteriorly, i.e., 1 mm, at the surgeon&#39;s discretion. As explained above, “size” equals the average of the expected resection of the medial and lateral distal femoral condyles. This equals 9.5 mm assuming normal anatomy. This will resect 11 mm from the distal medial femoral condyle and 8 mm from the distal lateral femoral condyle assuming a two inch distance between the two, i.e., the two most prominent portions of the distal femoral condyles. 
     The distal femoral cutting block  89  is then locked into place on sliding scale  238  by the cam mechanism  236 . The distal femoral cutting block  89  is then secured to the anterior cortex by nails  240  through nail holes  242 . 
     The rod  20  is then removed from the tool  30 D. The cam mechanism  236  is disengaged and the distal femoral resection-caliper  88  and the tool  30 D should be removed from the distal femoral cutting block  89 . Only the distal femoral cutting block  89  should remain on the femur  1  (FIG.  28 ). 
     The distal femur should then be resected along the mediolateral plane  244  of the distal end  246  of the distal femoral cutting block  89 . The preliminary anterior and final distal cuts  10  and  12 , respectively, have now been made as illustrated in FIGS. 1 and 4 a.    
     The plane  244  or cut  12  should be substantially parallel in the mediolateral plane to the proximal tibial resection, i.e., parallel to the floor, assuming normal ligament balance. If the ligaments are not normally balanced, then the ligaments should be released by any of the known methods until the planes are parallel in the mediolateral direction. 
     Referring now to FIGS. 38-41 in an alternative embodiment, if collet  206  is angled 8-10° and the 3 mm lateral offset  220  is not used as explained above, distal femoral resection caliper  88  must compensate for the 3° increase in the angle of collet  206 . Preferably, distal femoral resection caliper  88  compensates for the increase in the angle of collet  206  by itself being angled 3° as shown in FIGS. 38,  40 ,  41 . For the left femur as shown in FIG. 38, distal femoral resection caliper  88  is angled 3° laterally as shown in FIG. 40 (i.e., follows the direction of the intramedullary rod). For the right femur, not shown, distal femur resection caliper  88  is angled 3° laterally as shown in FIG. 41 (and would also follow the direction of the intramedullary rod). The angulation of the distal femoral resection caliper  88  insures that the angle of the distal femoral resection corresponds to the medio-lateral plane of the tibial resection in a normal knee. Thus if one of the distal condyles suffers bone loss, the distal resection will remain at the proper level. In FIG. 38, there is shown a collet  206  angled 9°. Using distal femoral resection caliper  88  angled at 3° insures that the distal femoral resection is made 3° less, or 6°. 
     Referring now to FIGS. 29-32, the distal femoral cutting block  89  should be removed from the distal femur, and the appropriately sized A-P cutting block  100  should be inserted thereon. The A-P cutting block  100  is used to make the final anterior resection  10  and to resect the posterior surfaces of the medial and lateral condyles  11 . If the flexion space is of concern, the preliminary anterior resection, distal femoral resection, and posterior condylar resection should be performed. Preferably, the flexion and extension spacing or “balance” with the appropriately-sized spacers are tested before continuing. See FIGS. 48-49. 
     The A-P cutting block  100  is placed onto the distal femur secured by angled nails through the sides of the cutting block  100 , not shown. The A-P cutting block  100  includes an anterior portion  248  that sits flush with the anterior cortex of the femur  1 . If desired, the A-P cutting block  100  can also be secured to the distal femur by nails (not shown) in nail holes  250 . The A-P cutting block  100  should now sit flush with the cut anterior surface  10  and the distal surface  12 . 
     The A-P cutting block  100  includes slots  102  and  101  which are precisely placed for guiding a resector or cutting blade to produce the final posterior and anterior cuts  11 ,  10  respectively. Because of the asymmetric buildup of metal on the posterior condyles of the GENESIS II femoral prosthesis  198 , e.g., about 2.5-3 mm thicker on the posterior lateral condyle, the posterior femoral resection  11  must be altered to accommodate this difference. The resultant posterior femoral condylar joint line should be parallel to the resultant tibial joint line, i.e., parallel to the floor. 
     The posterior femoral resection  11  should be approximately 3° of varus (e.g., if using the Genisis II knee) in the mediolateral plane referenced from the horizontal assuming no wear or equal wear posteriorly. The A-P cutting block  100  assures this due to its alignment with the preliminary anterior femoral condyle resection  10 . The A-P cutting block  100  is so aligned because the anterior portion  248  rests on the preliminary anterior cut  10  which has already been resected at the desired rotation or angle. The posterior condylar resection will be equal posterior medially and posterior laterally assuming no wear or equal wear of the posterior condyles. Moreover, the posterior cut  11  will be made in a constant relationship, e.g., diverge 3°, from the proximal tibial resection. In other words there will be an opening wedge laterally. 
     As explained above, if there is unequal wear, then tool  30 D would have been rotated appropriately to account for the asymmetric loss of substance. The posterior resections will not be equal posterior medially and laterally under this circumstance. 
     The A-P cutting block  100  also includes angular slots  103  and  104  to form chamfer cuts  105 ,  106  which also match corresponding angular surfaces  107 ,  108  on the prosthesis  198 . Preferably, the femur  1  should be resected in the following order: the posterior resection  11 , the posterior chamfer  106 , the final anterior resection  10  and the anterior chamfer  105  (see FIG. 4 a ). The A-P cutting block  100  is then removed and the prothesis  198  is installed on the distal femur by any of the known methods. 
     As shown above, since the prosthetic femoral condylar dimensions were 9.5 mm distally and posteriorly, a “standard” resection would be set to resect 11 mm from the distal medial and posteromedial condyles, and 8 mm from the distal lateral and posterolateral condyles. This would then produce a 9.5 mm resection at the midline. When combined with the tibial resection, explained below, this would give a 19 mm rectangular space to receive the prosthetic components. 
     Preferably, the instrument system of the present invention is for femoral resection and replacement, tibial resection and replacement and patellar resection and replacement, i.e., a total knee replacement system. Preferably, in the total knee replacement system of the present invention, the tibia is resected before the femur and the tibia then reresected, if necessary, to properly articulate the tibial and femoral prosthesis. It should be realized, however, by those skilled in the art that the tibia and femur can be resected and replaced in any order. 
     Referring now to FIGS. 42-44, there is shown a tibial resection guide  400  of the present invention mounted on tibia  2 . Tibial resection guide  400  is similar to, but an improved version of, the tibial alignment assembly marketed by Smith &amp; Nephew Richards, Inc., in Memphis, Tenn. under the PROFIX® total knee system and adapted to GENESIS II prosthetic dimensions and specifications. 
     Tibial resection guide  400  includes a bore  402  for receiving a reamer rod  404  therethrough. Bore  402  is drilled in tibial resection guide  400  at a 3° posterior tilt, i.e. tilted down from the anterior to posterior, or down from the horizontal, 3°. Tibial resection guide  400  is mounted on tibia  2  by any of the known methods. 
     Initially, the knee should be exposed in the standard fashion everting the patella. The anterior cruciate ligament and the PCL should be released, and the medial osteophytes removed, if necessary. Preferably, using a standard femoral drill, e.g., ⅜″, the proximal tibial medullary canal  406  is opened at, or just posterior to, the tibial attachment of the anterior cruciate ligament. The reamer rod  404  is then placed in the drilled tibial shaft just anterior to and between the tibial spines by any of the known methods. 
     Tibial resection guide  400  is mounted on rod  404  in the manner shown in FIGS. 42-44. Tibial resection guide  400  includes a pair of styluses  408  rotatable in tibial resection guide  400 . Preferably, the styluses are spaced about 2 inches apart. Styluses  408  are independently rotatable so they can contact the highest, or the most intact side, of the tibial plateau. Styluses  408  include angled or curved foot extensions  410 . Preferably foot extensions  410  are angled so they extend approximately 3-4 mm further posterially than reamer rod  404 . 
     Once a stylus  408  is made to contact the most intact or highest point on the tibial plateau, that particular stylus  408  remains stationary while tibial resection guide  400  is moved up or down rod  404  until it coincides with an “M” marking  412  if the most intact side is the medial side, or with “L” marking  414  if the most intact side is the lateral side. It is not necessary to use both styluses. Tibial resection guide  400  is then secured in that position by for example tightening a sliding screw. The markings  412 ,  414  set the proper tibial resection. Preferably, for a tibial prosthesis 9.5 mm thick, the proper resection corresponding to the “L” marking  414  is 11 mm and 8 mm for the “M” marking  412 , thus representing a 3 mm difference between the markings, assuming the styluses are two inches apart. One millimeter or more markings can be utilized therebetween. For prostheses with differing thicknesses, the resection lengths for the markings  412 ,  414  are adjusted taking into account that the tibial plateau varies approximately 1.5 mm per inch. Thus, the thickness of the prosthesis, e.g., 9.5 mm, should represent an average thickness between the medial lateral resection, namely 8 mm and 11 mm, respectively. 
     Tibial resection guide  400  is now in the proper position to remove the desired amount of bone described above, i.e., resect at 8 mm medially or 11 mm laterally. At this time it is preferable to protect the posterior cruciate ligament (PCL) if still intact. 
     To resect the bone, a resector is inserted through slot  416  by any of the known methods. Because of the 3° proximal medio-lateral varus tilt of the tibia, the bone is resected on a 3° valgus medio-lateral tilt relative to the tibial plateau. Regardless of the tilt, using either an intramedullary or extramedullary rod, the resultant resection of the tibial plateau in the medio-lateral plane must be parallel to the floor. 
     Referring now to FIG. 43, because bore  402  is drilled in tibial resection guide  400  at a 3° posterior tilt, i.e. tilted down from the anterior side to the posterior side 3°, tibia  2  is resected 3° down from the horizontal, i.e., tilted higher anteriorly than posteriorly. 
     Preferably, slot  416  of tibial resection guide  400  extends through most of its width, and does not include a stop in the center like tibial resection apparatus of the prior art. Without such a center stop, better access is afforded to the tibia. 
     In a preferred embodiment, tibial resection guide  400  includes stops  418  at the ends thereof. Stops  418  protect the patellar tendon and the medial and lateral ligaments. 
     Referring now to FIG. 45 there is shown an alternative embodiment to the present invention. FIG. 45 shows a tibial external alignment guide  420 . Preferably, rod  404  should be angled 3° to account for the 3° posterior tilt bored into tibial resection guide  400  as explained above. Tibial external alignment guide  420  is otherwise constructed, and resects tibia  2 , in the same way as tibial resection guide  400 . 
     Now that tibia  2  and preferably femur  1  have been resected, tibial secondary finishing apparatus is employed to achieve a properly articulated knee in flexion and extension. For example, tibial resection guide  422 , FIG. 50 described below, can be used to downsize, i.e., reresect, tibia  2 , versus downsizing femur  1 , i.e., resect more bone off of, to gain increased flexion if the knee is tight following the femoral and tibial resections. 
     Referring now to FIGS. 46-47, there is shown a spacer apparatus which is used to check the space between the tibial and femoral surfaces in flexion and extension after the appropriate femoral and tibial resections. The spacer apparatus is typically necessary only in cases where the spacing is put in question, e.g., where the knee has a complex deformity or a severe loss of motion. The goal is to cut off the least amount of bone and have proper ligament balance medially and laterally in flexion and extension. The ligament balance should be solid in extension and slightly loose in flexion. The spacer apparatus is advantageous because it determines how much additional bone to resect before the bone is actually resected. 
     Referring now to FIG. 46, spacer apparatus includes paddle  424  having an extension spacer  426  and a flexion spacer  428  located at opposite ends thereof. As shown in FIG. 46A, flexion spacer  428  tapers from 10.5 to 7 mm in thickness. As shown in FIG. 46B, extension spacer  426  is uniformly 7 mm thick. 
     Paddle  424  can be used in connection with snap-on spacers, not shown, which are available from any of the know suppliers and can be secured to paddle  424  by any of the known methods. The snap-on spacers are used to increase the thickness of extension and flexion spacers  426 ,  428 . Preferably, the snap-on spacers should come in sizes of 2-16 mm or 2-27 mm, and be available in 2 mm increments, e.g., 2, 4, 6 mm etc. 
     Heretofore, paddles of the prior art had extension spacers 19 mm thick and flexion spacers that tapered from 19-20 mm in thickness. These paddles are disadvantageous because (1) they can not be used in spaces less than 19 mm thick; and (2) they do not accommodate ligamentous laxity (trapezoidal space). 
     Paddle  424  and snap-on spacers are used in accordance with known methods to ensure a properly articulated knee in flexion and extension and proper ligament balance. 
     Referring now to FIG. 50, there is shown a tibial reresection guide  430  that is used to take additional bone off of tibia  2  if, for example, this is found to be necessary as a result of using the spacer apparatus described above. Tibial reresection guide  430  is similar to, but an improved version of, the tibial secondary prep guide marketed by Smith &amp; Nephew Richards, Inc., in Memphis, Tenn. under the PROFIX® total knee system. 
     Like tibial resection guide  400 , tibial reresection guide  430  includes a bore  432  drilled in the reresection guide at a 3° posterior tilt, i.e. tilted down from the anterior to posterior, or down from the horizontal, 3°. Tibia  2  is reresected an appropriate amount using any of the known methods. 
     Referring now to FIGS. 53-57, there is shown an apparatus in accordance with the present invention for patellar replacement. The apparatus of the present invention allows for medialization of the patellar prosthesis and offers the following advantages: 
     containment with circumferential bone 
     load sharing with the “intact” patella 
     maintenance of the patellar ridge 
     “replaced” surface conforming to trochlear notch 
     minimal sacrifice of the patellar bone. 
     Referring now to FIG. 53, a patellar clamp  500  is shown therein. The knee must first be fully extended and the patella completely exposed on the tendinous surface. The patella is then inverted and the patellar clamp  500  placed on the lateral aspect of the patella. Patellar clamp  500  is similar to, but an improved version of, the patellar clamp marketed by Smith &amp; Nephew Richards, Inc., in Memphis, Tenn. under the PROFIX® total knee system. 
     Patellar clamp  500  is first used to measure the thickness of patella  502  so the surgeon knows how much of patella  502  will be left while accommodating the patellar insert  504 , FIG.  57 . It is undesirable to leave too little bone. It is desirable to have patellar insert  504  well contained and stable in the patellar bone supporting it circumferentially. 
     To measure the thickness of patella  502 , the patella is placed between an appropriately sized collet  506  and a base plate  511 . Collet  506  has four prongs  508  equally spaced around and over the peak  509  of the patella  502 . Prongs  508  are separated by arcuate sections  512 . Collet  506  is then centered over the patellar ridge and patellar clamp  500  is tightened by a thumb screw  513  or by any of the known methods. 
     Preferably, collet  506  is pivotably hinged to the arm  507  of patellar clamp  500 , FIGS. 53,  53   a.  This hinged arrangement allows all of the prongs  508  to contact the patellar peak  509  if it is disposed at an angle. 
     The thickness of patella  502  for purposes of the present invention is a distance d between where prongs  508  intersect the patella, and base plate  511 . As a result, patella  502  is reamed or drilled from where prongs  508  intersect patella  502 . 
     Patellar clamp  500  includes a scale  514 , FIG. 54, located at an end thereof which measures distance d. The scale determines if patella  502  is thick enough for reaming. In FIG. 54, for example, distance d is determined to be 20 mm. 
     Next, patellar reamer  516  corresponding in size to collet  506 , is placed in patellar collet  506  to drill or ream away the required amount of patella  502  from where prongs  508  intersect patella  502  to accommodate patellar insert  504  by any of the known methods. In a preferred embodiment, patellar reamer  516  includes a depth scale  518 , FIG. 55, located therein to indicate how much patellar reamer  516  reams or drills into patella  502 . Preferably, markings  518  correspond to the thickness of patellar insert  504 , e.g., 8 mm or 12 mm. 
     By way of example, in FIGS. 55-56, (hinged embodiment not shown) scale  514  measured a patellar thickness of 20 mm. As a result, if a surgeon decides to use an 8 mm patellar insert  504 , and accordingly ream patella  502  8 mm&#39;s, he then knows there will be 12 mm of bone left anteriorly, which is a sufficient amount left for reaming and to accommodate patellar insert  504 . As there is a significant amount of intratendonous patella distally, at least a 4-5 mm margin superiorly (if possible) should be left. 
     The surgeon would then ream patella  502  8 mm from where prongs  508  intersect patella  502  or until depth scale  518  indicates 8 mm. Preferably, it is recommended to rongeur the excess substance superiorly and inferiorly to level off the patellar ridge. The surgeon would then insert patellar insert  504  by any of the known methods, FIG.  57 . 
     Referring now to FIGS. 58-59, there is shown an improved nail and slap hammer apparatus of the present invention. FIG. 58 shows an improved nail  600  of the type to secure apparatus, e.g., distal femoral cutting block  89 , FIG. 59, to femur  1 , or tibia  2 . 
     Nail  600  includes two flat sides  602  adjacent two round sides  604 . This allows nail  600  to be drilled rather than hammered into the bone. Preferably, nail  600  is lengthened ½-¾″ from prior art nails, and includes a stop  606  to prevent nail  600  from being drilled too far into the apparatus. This leaves sufficient room between the head  608  of the nail and slap hammer  609  to remove nail  600  as shown in FIG.  59 . Nail  600  further includes a hexagonal head  608  so the nail can be used in connection with a quick-release chuck. 
     Referring now to FIG. 59, slap hammer  609  includes a flat shaft  610  which creates a path of travel for a head  611  to travel up and down. Preferably, shaft  610  should be flat, and a channel in head  611 , not shown, similarly shaped, to prevent head  611  from spinning on shaft  610 . Preferably, slap hammer  609  should include a claw like end  612  to more easily secure nail  600  and remove it from the bone. 
     As mentioned above, an objective of the apparatus of the present invention is to maintain the prosthetic joint line as near anatomic as possible. For example, assuming the above prosthetic dimensions for the GENESIS II femoral prosthesis  198 , 8 mm must be resected from the medial tibial condyle or 11 mm from the lateral tibial condyle. This will give a 9.5 mm resection at the midpoint at 0°. With the tibial insert of 9.5 mm replaced, the joint level will be elevated 1.5 mm medially and lowered 1.5 mm laterally, but the patellofemoral joint level will be near anatomic. It follows then that 11 mm from the medial femoral condyle and 8 mm from the lateral femoral condyle (both distally and posteriorly) must be resected to achieve the desired 19 mm bony resection to accommodate the 19 mm dimensions of the prosthetic implants. 
     The surgeon must decide whether to resect the thickness of the combined tibial-femoral prosthesis (i.e., 19 mm) from either the medial or lateral side of the femur  1 . The most intact side, least affected or the convex side should be chosen. 
     When calculating the amount to be resected for the convex side, the surgeon should include an estimate of the “millimeters” of convex ligament laxity. Otherwise, a too thick tibial prosthesis may be necessary. 
     Normally the distal femoral resection guide i.e., the lower half  208  of tool  30 D (FIG. 21) will contact the medial femoral condyle and be approximately 3 mm off the lateral femoral condyle. In this situation, 11 mm would be resected from the medial femoral condyle. If the knee is in varus and the distal femoral resection guide contacts the lateral femoral condyle, only 8 mm of bone from the lateral femoral condyle must be excised. Therefore, the medial side must be under-resected by 3 mm. Any measurements between these extremes can be easily calculated. 
     Resecting more than 8 mm medially or 11 mm laterally from the tibial plateau (proximal tibia) may detach or significantly weaken the posterior cruciate ligament (“PCL”) insertion more than is compatible with useful function. A tibial plateau that compensates for a lost PCL may be necessary. Also, lowering the tibial resection may place the tibial prosthesis on to a less supportive cancellous surface. 
     It is important to achieve proper ligament balancing and not equal flexion-extension spacing. The “normal” knee is stable in full extension and has some laxity in flexion. A surgeon should allow an extra 2-3 mm of laxity in flexion to accommodate the normal laxity. If the knee has full unhindered motion, resection in flexion and extension are equal. If flexion is limited, however, an extra few mm in flexion must be resected. (See “Loss of Flexion” situation described below). 
     A 1 mm resection is approximately equal to 5° increased motion in both flexion and extension. Although this varies slightly from smaller patients to larger patients, the results are relatively consistent. This also implies that if the femur  1  is under-resected distally by 1 mm or over-resected posteriorly by 1 mm, 5° extra flexion should be achieved. 
     When checking range of motion before closing, at least 10° of laxity is required in flexion and extension. When arthroscoping a knee, initially the joint is quite snug. After “wrestling” with the knee for 5-10 minutes, the joint seems to “loosen up”. This can be attributed to “stretching” the ligamentous structures around the knee. There is no normally organized elastin or reticulin in the soft tissues surrounding the,knee, but these structures are capable of approximately lengthening by approximately 10% before failure (i.e., loss of elastic deformation and recovery). Generally, this is approximately 2 mm, which translates to approximately 10° of motion lost after closure. 
     Any varus or valgus malalignment must be compensated by removal of osteophytes and by appropriate medial or lateral ligament and soft tissue release, (i.e., concave balancing). Flexion and extension deformities are managed with a combination of bony resection, soft tissue release, and possibly posterior cruciate release. 
     The placement of the tibial plateau is also important. Aside from establishing proper rotation, posterior placement of the tibial plateau may be useful to: 
     1) compensate for shortening of the femur by allowing the tibia to move posteriorly under the femur; 
     2) decrease posterior impingement and rollback; 
     3) decrease posteromedial tibial wear; 
     4) decrease posterior soft tissue impingement; 
     5) allow for maintenance of the posterior cruciate ligament if the femur is shortened only a few millimeters; and 
     6) decrease patellofemoral pressure by anteriorly placing the tibial tubercle relative to the femur. 
     If the femur  1  is shortened more than approximately 4 mm from “anatomic,” a surgeon should consider releasing the posterior cruciate ligament to allow the tibial prosthesis to fall back under the femoral prosthesis in flexion. In addition, releasing the posterior cruciate ligament allows increased laxity in flexion. Generally, up to about 3-7 mm of extra space can be achieved in flexion and 0-2 mm in extension. 
     If the posterior cruciate ligament is left intact, flexion often causes posterior impingement (i.e. the “kinetic conflict”). The PCL must be released if: 
     1) There is significant deformity (varus, valgus, flexion, extension). 
     2) It has become significantly contracted. 
     3) There has been a patellectomy. 
     4) The jointline is elevated and/or the femur shortened greater than or equal to 4 mm. 
     5) The patient has an inflammatory arthritis. 
     6) Flexion under anesthesia is less than 115°. 
     If the knee is unstable in flexion, consider under-resection of the proximal tibia. A thicker plastic insert can also be inserted to make up for the laxity. 
     If the collateral ligaments are compromised, a more constrained knee may be indicated. Care must be taken not to create a patella infera. If this is of concern, then one must pre-operatively determine the proper combination of under-resection of the femur and/or under-resection of the tibia. 
     If the knee is stable in flexion or lacks full flexion, the distal femur should be under-resected 1 mm for every 5° of desired motion. If the knee is still too tight, more can be resected after trialing. If the flexion space is too loose after resection, then a slightly thicker plastic insert can be inserted with a corresponding loss of flexion. 
     Preferably, the instrumentation system of the present invention should correspond to the protocol described below. 
     The following standard protocol can be followed if a simple procedure is desired. 
     Standard Protocol 
     Resect proximal tibia. Measure from most intact or convex side (least affected) 
     Evaluate and “replace” for asymmetric loss of bone from the posterior condyles 
     Measure AP size 
     Resect distal femur 
     Downsize femur and resect condyles and chamfers 
     Maintain PCL 
     Resect posterior osteophytes 
     If the knee lacks 0°-15° extension and 115°—full flexion, then: 
     1. Resect proximal tibia at standard 
     2. Measure AP and downsize to next smaller size 
     3. Resect distal femur≦3 mm extra as needed 
     4. Leave PCL and resect posterior osteophytes 
     5. Downsize to next smaller femur 
     6. If tight in flexion, release PCL and ream for PCL substitution 
     A more advanced protocol for more specific knee conditions is described below. 
     Advanced Protocol 
     “Solving” the Flexion Space 
     In order to get “full” (i.e., 130°) flexion, the surgeon must balance the size of the prosthesis, bony resection, soft tissue releasing, proper rotation and proper relationship to the extension space. 
     If the parameter of PCL release is assumed to be 4 mm, wherein 1 mm bone resection results in 5° gain of motion, then an adequate flexion space with an “anatomic” 2-3 mm laxity is easily produced. 
     The following situations maintain the jointline between 3 mm distally and or 4 mm proximally starting with flexion from 90°-130° and extension from 55°-0°. The above measurements and parameters will allow for a resultant range of motion of 0°-130° in most patients. 
     Initial Observations Re: Measurement Variations 
     As the radius of curvature of the prosthetic condyles increase, the amount of motion gained from: 
     a. 1 mm bony resection 
     b. PCL release 
     c. Posterior capsular release decreases as the radius of curvature of the prosthetic condyles increase, i.e., increased size of the distal femur. 
     PCL Resection 
     This procedure generally produces about 3-7 mm of flexion space laxity, which seems to be in inverse relationship to the AP size of the distal femur (i.e., size 8=3 mm laxity, size 4-5=4 mm laxity, size 2-3=5-6 mm. In the present example, the “gained” space is determined to be 4 mm. 
     1 mm Resection=5° Gained Motion 
     It has been found that 1 mm resection allows 3°-6° of increased motion which also appears to be in inverse relationship to the AP size of. the distal femur. This is similar to the motion gained with PCL release. 
     Therefore, for simplicity, 4 mm of gained space for PCL resection and 5°/1 mm bone resection has been allowed in the present example. If the motion range is still tight at the end of the procedure, it would be a simple matter to resect an extra 1-2 mm from proximal tibia. 
     Varus or Valgus Deformity 
     Before measuring flexion or extension loss, release the contracted medial or lateral ligments. 
     0°-15° Lack of Extension and 95°-105° Flexion 
     1. Resect proximal tibia at standard 
     2. Measure, AP, femoral size 
     3. Under-resect distal femur 3 mm 
     4. Downsize to next smaller femur—Resect only posterior condyles now. 
     5. Release the PCL 
     6. Release posterior capsule as needed 
     7. Re-resect tibia≦3 mm as needed if tight in flexion 
     8. Check flexion and extension spaces. Make sure the flexion space is ample and that extension is stable at 0°-10° hyperextension. 
     9. Resect distal femur as needed 
     10. Resect posterior and anterior condyle 
     11. Ream for PCL substitution and resect chamfer 
     15°-25° Lack of Extension and Full Flexion 
     1. Resect tibia as needed 
     2. Measure AP femoral size 
     3. Resect distal femur per standard 
     4. Upsize to next larger femur 
     5. Release PCL 
     6. Release posterior capsule and osteophytes 
     7. Measure extension space and re-resect distal femur as necessary 
     8. Resect anterior and posterior condyles 
     9. Ream for PCL substitution and resect chamfers 
     15°-25° Lack of Extension and 115°-120° Flexion 
     1. Resect tibia as indicated 
     2. Measure AP femoral size 
     3. Resect distal femur per standard 
     4. Downsize to next smaller femur 
     5. Release PCL 
     6. Resect anterior and posterior condyles 
     7. Release posterior capsule and osteophytes 
     8. Measure extension space with blocks 
     9. Re-resect distal femur as necessary 
     10. Ream for PCL substitution and resect chamfers 
     15°-25° Lack of Extension and 95°-115° Flexion 
     1. Resect tibia as indicated 
     2. Measure AP femoral size 
     3. Under-resect distal femur 3 mm 
     4. Downsize to next smaller femur 
     5. Release PCL 
     6. Resect posterior condyles 
     7. Release posterior capsule and osteophytes 
     8. Measure extension space with blocks 
     9. Re-resect proximal tibia as needed 
     10. Re-resect proximal distal femur as needed 
     11. Ream for PCL substitution and resect chamfers 
     25°-40° Lack of Extension&gt;120° Flexion 
     1. Resect tibia as indicated 
     2. Measure distal femoral AP size 
     3. Resect distal femur as standard 
     4. Upsize to next larger femur 
     5. Resect posterior condyles 
     6. Release posterior capsule PCL and osteophytes 
     7. Measure extension space with blocks 
     8. If full extension not achieved, over-resect distal femur≦3 mm and/or proximal tibia as needed 
     9. Complete resection of anterior condyles 
     10. Ream for PCL substitution and resect chamfers 
     25°-40° Lack of Extension and 105°-115° Flexion 
     1. Standard resection of tibia 
     2. Standard resection of distal femur 
     3. Measure AP size of femur 
     4. Downsize to next smaller femur 
     5. Resect posterior condyles 
     6. Release posterior capsule PCL 
     7. Measure extension with spacer block 
     8. Re-resect distal femur≦3 mm if full extension not achieved and/or proximal tibia as needed 
     9. Re-resect posterior and anterior condyles, if needed 
     10. Ream for PCL substitution and resect chamfers 
     25°-40° Lack of Extension and 90°-105° Flexion 
     1. Resect tibia&lt;3 mm as needed (choose amount as needed to accommodate flexion or extension) 
     2. Measure AP size of femur 
     3. Resect distal femur as standard 
     4. Downsize to next smaller femur and resect posterior condyles 
     5. Release PCL 
     6. Release posterior capsule 
     7. Check flexion-extension space with blocks 
     8. Re-resect distal femur&lt;3 mm if full extension not achieved 
     9. Complete resection of anterior and posterior condyles 
     10. Ream for PCL substitution and resect chamfers 
     40°-55° Lack of Extension and Full Flexion 
     1. Resect tibia as needed 
     2. Measure AP size of femur 
     3. Resect distal femur as standard 
     4. Upsize to next larger femur 
     5. Resect posterior condyles 
     6. Release posterior capsule PCL and osteophytes 
     7. Measure extension with spacer block 
     8. Resect≦4 mm if from distal femur if needed 
     9. 
     a. If full extension, resect anterior condyles and proceed 
     b. If still not extending, and soft tissue has been released, re-resect proximal tibia 3≦mm. Then complete resection of anterior and posterior condyles 
     10. Ream for PCL substitution and resect chamfers 
     40°-55° Lack of Extension and 115°-120° Flexion 
     1. Resect tibia as indicated 
     2. Measure distal femoral AP size 
     3. Over-resect distal femur 4 mm 
     4. Downsize to next smaller femur 
     5. Resect posterior condyles 
     6. Release posterior capsule PCL and osteophytes 
     7. Measure extension with spacer block 
     8. If still not extending and soft tissue has been released, re-resect Proximal tibia≦3 mm 
     9. Ream for PCL substitution and resect chamfers and anterior condyles 
     40°-55° Lack of Extension and 105°-115° Flexion 
     1. Resect proximal tibia 
     2. Measure AP size of femur 
     3. Resect distal femur at standard 
     4. Downsize to next smaller femur 
     5. Resect posterior condyles 
     6. Resect PCL and release posterior capsule 
     7. Measure extension and flexion space with spacer block 
     8. 
     a. Re-resect proximal tibia≦3 mm if tight in both flexion and extension. 
     Then re-resect distal femur≦4 mm if still not extended. 
     b. If flexion ok and lack full extension, re-resect distal femur≦4 mm 
     9. Complete resection of anterior and posterior condyles 
     10. Ream for PCL substitution and complete chamfers 
     40°-55° of Extension and 90°-105° Flexion 
     1. Resect proximal tibia plus≦3 mm extra resection as needed 
     2. Measure AP size of femur 
     3. Resect distal femur at standard minus overresection of tibia. 
     4. Downsize to next smaller femur 
     5. Resect posterior condyles 
     6. Resect PCL 
     7. Release posterior capsule and resect osteophytes 
     8. Measure flexion-extension space with spacer block 
     9. Re-resect distal femur as necessary 
     10. Complete resection of anterior and posterior condyles 
     11. Ream for PCL substitution and complete chamfers 
     Hyperextension Deformity 
     1. 
      If the knee is unstable in flexion, consider under-resecting the proximal tibia. A thicker plastic insert can also be inserted to make up for the laxity. 
      If the collateral ligaments are compromised, a more constrained knee may be indicated. 
      Care must be taken not to create a patella infera. If this is of concern, then one must pre-operatively determine the proper combination of under-resection of the femur and/or under-resection of the tibia. 
     2. If the knee is stable in flexion or lacks full flexion, under-resect the distal femur 1 mm per 5°. If too tight, more can be resected after trialing. If too loose, then a slightly thicker plastic insert can be inserted with a corresponding loss of flexion. 
     3. If there is and initial hyperextension deformity, leave the knee in neutral at the end and not in 10° hyperextension. 
     Varus or Valgus Laxity 
     Pre-operative assessment of medial or lateral laxity is important. If on standing films one can ascertain excess laxity on the convex side of the knee, then appropriate compensation must be made. 
     1. If there is full flexion, then under-resect the distal femur 2-3 mm 
     2. If there is a lack of full flexion, under-resect the distal femur 2-3 mm and consider releasing the PCL 
     If there is significant instability, consider a more constrained knee. 
     It should be realized that to achieve better results with total knee replacement, orthopedic surgeons must: 
     1) have accurate measurements; 
     2) have coordinated measurements; 
     3) maintain an “anatomic” joint line; 
     4) have access to dimensions of components; and 
     5) have the ability to compensate for deformities with a minimum of soft tissue release and bony resection. 
     Although the invention has been described with reference to specific embodiments thereof, it will become apparent to those skilled in the art that numerous modifications and variations can be made within the scope and spirit of the invention as defined in the attached claims.