for the purposes of promoting an understanding of the principles of the invention , reference will now be made to the embodiments illustrated in the drawings and described in the following written specification . it is understood that no limitation to the scope of the invention is thereby intended . it is further understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the invention as would normally occur to one skilled in the art to which this invention pertains . in accordance with one aspect of the present invention , a femoral positioner 50 is provided that can be used with a femoral resection guide , such as the guides 20 and 32 depicted in fig2 and 3 . the positioner 50 includes a surface alignment plate 52 that is configured to rest on the resected surface r of the tibia t , like the positioner 26 shown in fig2 . the alignment plate 52 defines a slot 54 that can engage a pin 51 disposed within the medullary canal of the tibia ( not shown ) to align the plate with the resected tibial plateau in a known manner . a connector plate 56 is arranged parallel with the surface alignment plate 52 and is configured to engage a mating feature in the resection guide . for instance , the connector plate 56 can engage the slot 23 in the main body of the resection guide 20 shown in fig2 , or the slot 34 or other mating feature in the guide 32 . a base 58 integrally spans between the plates 52 , 56 and establishes the distance between these two parallel plates . the base thus sets the distance between the surface of the tibia and a reference point by which the position of the resection guide is established . the base 58 can define a bore 60 therethrough to receive an alignment rod ( not shown ) that can be used to check ligament tension during the instrumentation procedure . the goal of the femoral positioner 50 is to properly orient the femur relative to the resected end r of the tibia . when the femur is properly positioned , the resection guide can be mounted on the exposed end of the femur and the necessary cuts made at the proper location on the bone . while the positioner 50 may be properly sized to achieve these results for some patients , the majority of the cases will require some augmentation for the surface alignment plate . in some cases , the necessary augmentation is simply to close the space between the alignment plate 52 and the posterior surface of the femur when the knee is flexed , as shown in fig2 . in other cases , the surface of either the femur or the tibia has surface defects that compromise the stable support of the femoral positioner 50 . in either case , an augment , such as the augment 70 shown in fig7 - 8 may be necessary . the augment 70 includes a mating surface 72 and an opposite surface 75 . the mating surface 72 contacts the surface alignment plate 52 of the positioner 50 , while the opposite surface 75 contacts the bone . in the most basic case , the opposite surface 75 is flat and parallel to the mating surface 72 . the thickness between these two surfaces can vary as necessary to fill the expected flexion / extension gap . nominally , several augments 70 can be provided , each having different thicknesses . where the augment 70 serves as a shim or spacer , the augment will normally be supported on the femoral - facing surface 62 a of the positioner 50 ( fig6 ). in other cases , the surface 75 of the augment 70 can include contours , such as the contours 76 shown in dashed lines . these contours are configured to match defects in the bone against which the augment bears . where the defects are in the tibia , the augment will be mounted to the underside or the tibia - facing surface 62 b of the positioner 50 ( fig6 ). the contours 76 fill the bone defects and ensure that the mating surface 72 will be supported in a proper parallel orientation . in order to facilitate mounting and removal of the augment 70 from the positioner 50 , means for removably coupling the components together are provided that incorporate a resilient member . in the preferred embodiment , the surface alignment plate 52 is provided with a pair of bores 64 on opposite sides of the notch 54 . the augment 70 includes a mating pair of pins 74 that are sized to be received within a corresponding one of the bores . as shown in the detail of fig6 , each of the bores defines an internal groove 66 configured to receive an elastomeric o - ring 68 . each pin 74 is sized to pass through the bore 64 into frictional contact with the o - ring 68 . the o - ring provides a tight elastomeric fit so that the pins are not easily dislodged from the bores during normal manipulation of the femoral positioner 50 . each pin can be provided with a groove ( not shown ) to receive the o - ring when the pin is properly positioned within the bore . in the preferred embodiment , the o - ring groove 66 is offset toward the tibial surface 62 b . the bore 64 has a diameter on either side of the groove 66 that provides a close running fit for the pin 74 . the o - ring defines an inner diameter that is less than the diameter of the bore . thus , the tip 74 a of the pin can be tapered to facilitate being pushed through the o - ring 66 . the base of the bore 64 at the tibial side can be provided with a chamfer 65 to further facilitate placement of the pin into the bore from the underside of the femoral positioner 50 . the augment 70 can also be used with a spacer block , such as the spacer block 80 shown in fig9 - 12 . the spacer block 80 includes a spacer body 82 connected to a handle 84 . the block defines a notch 83 therein that serves the same function as the notch 54 in the femoral positioner 50 discussed above . the handle 84 defines a number of angled bores 85 configured for receiving an alignment rod ( not shown ). the spacer block 80 can be used in a conventional manner to verify the flexion and extension gaps when the resection guide is mounted to the femur , or after the femoral implant has been mounted on the finished distal end of the femur . in order to accommodate a variety of joint anatomies , the body 82 of the spacer block defines a pair of bores 90 passing from the tibial surface 87 to the femoral surface 88 . the bores are sized to receive the pins 74 of an appropriate augment 70 . in accordance with the invention , the bores are provided with o - ring grooves and o - rings to firmly hold the pins within the bores . in one feature of the embodiment , the bores 90 are provided with two grooves 92 a , 92 b and two o - rings 94 a , 94 b . one o - ring 94 a is positioned near the femoral surface 88 and the other o - ring 94 b is positioned near the tibial surface 87 . it is contemplated that the pins 74 of the augment 70 have a predetermined height from the mating surface 72 that is calibrated to fit the bores 64 in the femoral positioner 50 . since the surface alignment plate 52 of the positioner is thinner than the body 82 of the spacer block 80 , the height of the pins 74 is less than the thickness of the spacer block . consequently , in order to orient an o - ring in a location where they can fully engage the pins , two o - rings 94 a , 94 b are provided , with a corresponding one offset to each surface of the spacer block . in an alternative feature , the bore 90 can define a larger bore portion 90 a and a smaller bore portion 90 b . the larger portion 90 a is adjacent the femoral surface 88 , while the smaller portion 90 b opens at the tibial surface 87 . while the invention has been illustrated and described in detail in the drawings and foregoing description , the same should be considered as illustrative and not restrictive in character . it is understood that only the preferred embodiments have been presented and that all changes , modifications and further applications that come within the spirit of the invention are desired to be protected . for instance , while the preferred embodiment calls for o - rings to provide the temporary fixation of the augment , the o - rings can be replaced with other resiliently gripping components . for instance , a slitted membrane can span the bores 64 or 90 , wherein the pin penetrates the membrane , which then resiliently grasps the surface of the pin . similarly , the o - rings can be replaced with a canted coil spring , similar to the canted spring coupling ring marketed by bal - seal engineering . in this case , the engagement pins can define a groove to engage the canted coil spring .
a system for establishing a prosthetic gap between first and second bones at a joint comprises an instrument for positioning within the gap between the first and second bones , and an augment for filling the gap when coupled to the instrument . in one embodiment , a resilient coupling member is provided to resiliently and removably couple the augment to the instrument . in another embodiment , the instrument and augment are configured so that the augment can be coupled to the instrument on either one of opposite surfaces of the augment facing the first and second bones .