referring to fig1 , there is shown a perspective view of a femur 12 and a tibia 14 . femur 12 includes two planar resected distal condylar surfaces 16 and 18 respectively . tibia 14 includes a proximal planar cut surface 20 which will eventually receive a prosthetic tibial component ( not shown ). referring to fig2 there is shown an intramedullary rod 22 having a tip 24 for insertion into the femoral medullary canal and having a distal end 26 including three fins 28 preferably located at 120 ° increments about the outer circumference of distal end 26 . fins 28 are blade - like in form and will prevent rotation of rod 22 within the intramedullary canal . distal end 26 includes a mounting rod 30 , which , when rod 22 is placed in the canal , will extend distally beyond the planes of surfaces 16 and 18 . the intramedullary rod 22 extends along an axis 32 with projection 30 extending along an axis 34 . preferably there is an angle of between 3 and 6 ° between axes 32 and 34 to compensate for the angle between the anatomic axis of the femur and the mechanical axis of the femur . fig3 shows intramedullary rod 22 of fig2 mounted in femur 12 . as shown , fins 28 may be either internally formed on end 26 of rod 22 or may be removably mounted thereon . referring to fig4 , 4 a and 5 , there is shown distal plate 40 mounted on surfaces 16 and 18 in femur 12 . plate 40 includes a medial and a lateral slot 42 and a medial and lateral threaded mounting hole 44 , which will receive ligament tensioning body mounting screws shown in fig8 - 13 . distal plate assembly 40 includes a central bore 50 , which surrounds a bushing 52 for receiving end 30 of intramedullary rod 22 . bushing 52 includes a bore 74 for receiving end 30 of rod 22 . bushing 52 is mounted within an opening in a square or rectangular boss 54 of distal plate assembly 40 . boss 54 has a series of rack gear tooth elements 56 formed on a medially facing side 58 of boss 54 . as discussed below teeth 56 engage an acme screw for adjusting the anterior - posterior position of a tensioning element body . distal plate assembly 40 includes a posteriorally extending portion 60 which includes a curved gear element 62 adapted to engage a spur gear 64 mounted on an extension 66 of bushing 52 , best seen in fig5 , on which spur gear 64 is freely rotatable . fig4 a shows bushing 52 prior to assembly on projection 30 and in boss 54 . typically bushing 52 is mounted on end 30 of rod 22 prior to mounting the distal plate assembly 40 on the distal femur . at assembly , spur gear 64 is slightly rotated such that the teeth thereof will mesh with the curved teeth elements 62 integrally formed on the distal plate assembly 40 . spur gear 64 includes a drive socket 70 which , in the preferred embodiment , is hexagonal in shape to receive a normal allen wrench for rotating gear 64 . as shown in fig5 , slots 42 may have a kidney shape with a convex surface 72 facing outwardly from an axis through the center of the bore 74 in bushing 50 . referring to fig6 , there is shown the distal plate assembly 40 , including a pair of screws 80 about which distal plate assembly may pivot upon rotation of spur gear 64 . the kidney shape of slots 42 allows the distal plate 40 to rotate about partially inserted screws 80 . this movement sets the internal / external rotation of the plate 40 . screws 80 can then be fully tightened to lock the plate in place on the distal femur . referring to fig7 , there is shown a main body 100 of the gap balancing or tensioning instrument mounted on the distal plate 40 . the main body 100 is shown in fig7 - 13 . the main body includes a generally rectangular central opening 102 adapted to receive boss 54 of plate 40 . boss 54 and rectangular opening 102 are sized such that the main body may slide relative to the distal plate in the anterior - posterior direction . main body 100 includes a pair of slots 104 both medially and laterally of the center of opening 102 . slots 104 receive mounting screws threaded into bores 44 . a pair of slots 106 is also included to allow access to screws 80 located within slots 42 of distal plate 40 . also included on body 100 are medial and lateral trial condylar elements or feet 108 and 110 respectively . each condylar element 108 and 110 includes a slot 112 extending partially therethrough from the outer edges of body 100 . preferably , the outer curved surfaces of condylar portions 108 and 110 have a constant radius from the area of slots 112 to adjacent ends 114 of each condylar portion 108 , 110 . this radius matches the constant radius on the prosthetic femoral component to be implanted . main body 100 includes hole 116 to guide a drill for producing bores in the distal femur to locate the 4 in 1 cutting block for making the anterior and posterior chamfer and skin bone cuts . main body 100 also includes holes 116 a , preferably on both the lateral and medial sides of the main body to accommodate bone pins to allow the surgeon to secure the body ( 100 ) to the distal femur during femoral preparation . the main body additionally includes bores 118 for mounting body 100 to the distal femur . as best seen in fig8 prior to mounting the main body 100 onto the femur , screws 120 extend through slots 104 and into threaded bores 44 of distal plate assembly 40 . in the preferred embodiment , the slots 104 include a recessed surface 105 to thereby form a ledge around the circumference of each slot . screws 120 includes heads 122 , which have a head with a bottom surface able to slide on ledge 105 when main body 100 is moved in the anterior - posterior direction . heads 122 are sized such that they engage a peripheral wall 107 of the slot 104 between surface 105 and the outwardly facing surface 125 of main body 100 . this helps ensure that the main body does not rotate with respect to distal plate 40 during the anterior - posterior movement which is accomplished in a manner as described below . referring to fig9 , there is shown the assembly of fig8 including a drive tool 130 engaging socket 70 of spur gear 64 . as indicated above , rotation of tool 130 rotates the distal plate 40 about portion 30 of intramedullary shaft 22 . this movement sets the internal - external rotation of the instrument on the distal femur . also shown in fig9 are two shim plates 132 , 134 located on the proximal tibial surface 20 of tibia 14 . the two shim plates 132 and 134 provide a surface for the radius to articulate on in an easy manner . in the preferred embodiment , plate 132 is 2 mm and plate 134 is 4 mm such that 2 , 4 , and 6 mm adjustments can be possible . in use , it makes no difference whether the 2 or 4 mm plates are located on tibial surface 20 when a 6 mm correction is desired . referring to fig1 and 10 a , there is shown the assembly of fig9 , including a second drive tool 140 which drives an acme screw 143 mounted in body 100 . screw 143 is freely rotably mounted within a partial bore 109 a in body 100 and has a drive element at an anterior end thereof open to bore 109 for engaging drive tool 140 . screw 143 has a thread for engaging the gear teeth 56 on boss 54 . tool 140 extends into bore 109 in anterior facing surface 111 of main body 100 to engage screw 143 . also shown in fig7 - 11 are a pair of medial and lateral blind bores 142 which extend from an anterior facing bottom surface 144 internally of the main body in a posterior direction . a surface 146 extending between bottom surface 144 and anterior surface 111 faces outwardly and includes a series of femoral component size numbers 148 . medial and lateral bores 142 are adapted to receive the shaft of a femoral component sizer generally denoted as 177 shown in fig1 . fig1 is a perspective view of an assembled sizer 177 including a mechanical stylus 180 in addition to the body 100 and module 110 of fig1 . referring to fig1 , the stylus includes an arm 185 with a curved tip 185 a that is used for locating points on the anterior cortex of the femur 12 . the stylus also includes a handle 190 that is attached to the arm and a stylus body 195 that is attached to a shaft or rod 195 a . bore 142 accommodates shaft or rod 195 a such that the stylus 185 is free to rotate about the longitudinal axis of rod 195 a and is free to translate longitudinally such that the stylus may move in a direction along the hole 142 longitudinal axis . the handle 190 can be used to translate the arm within a bore in stylus body 195 through a hole 196 in a direction parallel to the stylus body 195 longitudinal axis . this is in the proximal - distal direction . handle 196 can also be used to rotate the arm about the arm &# 39 ; s longitudinal axis if necessary . a latch 200 is provided for locking the arm in a desired translation position and in the rotation position in which the handle is in a generally aligned with the longitudinal axis of rod 195 a . the latch is spring loaded with respect to the handle by a spring 205 . to release the latch , finger pressure is applied to the latch so as to compress the spring allowing arm 185 to be moved . notably , the orientation of lever 198 of handle 190 in the proximal direction is advantageous . orienting lever 198 as shown in fig1 facilitates manipulation of the stylus during knee arthroplasty by positioning the handle away from the incision area . a further indication of implant size is provided by a superior - inferior run - out scale (“ si scale ”) 250 . the si scale is associated with a multiple of notches 255 that are etched into the stylus body , each notch being associated with a corresponding size . when the curved tip of the stylus has been located at the desired run - out point , the latch is allowed to settle into the notch that most closely corresponds to the stylus position . the number associated with the notch into which the latch settles is the femur / implant size as measured by the si scale . for example , if the latch sits in the notch corresponding to the number “ 3 ” of si scale , then the si scale indicates a femur / implant size “ 3 .” the sizer of fig1 is used to determine correct implant size and cutting block pin position by referencing the posterior condyles of the distal femur . more specifically , the sizer is aligned with the posterior condylar axis through this two condylar skids 108 and 110 located on the base of the tensioned body 100 . the skids 108 , 110 are positioned to contact the posterior condyles while the body is centered , or approximately centered , on the femur with respect to the medial - lateral direction . fig1 shows the guide mounted on the distal femur just prior to resecting the posterior condyle prior to resecting the posterior condyle via slots 112 . bores 209 are provided to allow the use of pins 211 to hold the guide on femur 12 . once the body is properly positioned , pins can be passed through either one pair of pinholes 116 or 116 a , or through both pairs of pinholes 116 and 116 a , to secure the body to the femur . in a preferred embodiment , the body 100 , and stylus 180 are removably attached to each other to form a complete assembly , and then the complete assembly is attached to the femur via the pinholes . referring to fig1 - 16 there is shown an alternate embodiment of a main body 300 for the instrument of the present invention . main body 300 is similar to main body 100 with the main exception being the rotational mechanism shown in fig1 . referring to fig1 and 15 body 300 includes a rotating disk 302 mounted in a bore 304 therein . as shown in fig1 the proximally facing side of body 300 has a plate 305 mounted on disk 302 . plate 305 is initially mounted on the distal femur and includes an intramedullary rod portion 306 for insertion into the intramedullary canal of the femur . as with body 100 body 300 includes a pair of condylar portions 308 and 310 and slots 312 . as with body 100 body 300 includes a drive screw system for adjusting the body including a screw 320 mounted in body 300 for engaging a circular gear segment 322 fixed to disc 302 . this is best seen in fig1 . a screw element or rack element 324 having teeth 326 is mounted within disk 302 for engaging teeth 56 on plate 40 as shown in fig5 . thus rotation of screw 324 causes movement of body 300 in a proximal - distal direction while rotation of screw 320 causes internal and external rotation of body 300 about the axis of rod 306 . this design works the same way as that described below for body 100 , the main difference being the change in location of the internal / external rotation drive gear 322 on disk 302 . the use of the instrument assembly including body 100 set forth above will now be described . initially the surgeon drills a hole into the intramedullary canal of the distal femur . the surgeon then inserts intramedullary rod 22 such that portion 30 thereof extends distally of the prepared surfaces 16 and 18 of femur 12 . the surgeon then assembles distal plate 40 onto the distal femur over portion 30 of im rod 22 . the surgeon sets distal plate 40 at the approximate internal / external rotational position that is desired . the plate is then secured to the distal femur by means of bone screws 80 . main body 100 is then placed over distal plate 40 . posterior condylar feet 108 , 110 on main body 100 are placed under the posterior condyles of the femur . main body 100 is secured to distal plate 40 by means of screws 120 . screws 120 secure main body 100 to distal plate 40 but allow main body 100 to move in an anterior - posterior direction by means of adjustment screw 140 . the surgeon then performs a trial reduction with the instrument in place . trial reduction takes place by means of the offset single radius on the periphery of condylar skids 108 , 110 . the surgeon places a tibial shim plate ( 2 , 4 , or 6 mm ) 132 , 134 on the proximal tibia . this plate acts as a protective plate for the resected bone on the tibia and also as a firm surface for the condylar skids of the main body 100 to articulate on . the surgeon then puts the knee through a range of motion and assesses stability throughout the complete range . the surgeon has multiple thickness spacer blocks which allow the surgeon to assess the gap ( if any ) on each condyle . if the surgeon establishes that adjustment is required , then the following steps can be taken . if internal / external rotation adjustment is required then the surgeon opens ( not removes ) bone screws 80 from the distal plate and places drive tool 130 into spur gear 64 and rotates the distal plate 40 until the desired position has been obtained . the surgeon can then resecure the distal plate at this position if desired . however , the surgeon can adjust the ap position by means of actuating tool 140 to drive the acme screw . once the desired internal / external rotation of the main body 100 has been achieved , distal plate 40 is secured to the femur . the surgeon then begins the process of assessing the resection levels and the femoral component size . the resection level is assessed by means of a blade runner shown in fig1 placed into slots 112 of both condyles 108 , 110 , and by placing the stylus into one of the holes or bores 142 . after the internal - external rotation and anterior - posterior location of the implant is set , the stylus can be used to size the femur . that is , the stylus can be used to determine the appropriate size implant needed for the subject femur . to size the femur , a practitioner manipulates the stylus handle such that the curved tip 185 a of the stylus contacts the anterior cortex of the femur at the point where the anterior - superior point of the implant should contact the anterior cortex of the femur ( the “ desired run - out point ”). once the tip is contacting the desired run - out point , the practitioner reads the size from an anterior - posterior sizing scale (“ ap scale ”) 148 located on surface 146 . the reading is taken by comparing a ring 220 on the stylus 195 a to the ap scale . for example , if the ring is aligned toward the number “ 3 ” of the ap scale 148 , then the femur size is a “ 3 ” and the implant needed is a size “ 3 .” the sizes may be femoral component sizes 1 - 8 of the triathlon ® line of implants . finally , the surgeon drills two holes through bores 116 on the distal femur . the two bores 116 position a standard 4 - in - 1 cutting guide in the correct position so that the remaining cuts ( anterior and posterior chamfer and skim cuts ) can be made on the distal femur . although the invention herein has been described with reference to particular embodiments , it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention . it is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims .