Abstract:
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.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to devices for use in orthopaedic surgery, and especially for proper alignment of surgical instruments used in preparing a bone for an implant. The invention has particular application in preparing the distal end of the femur to receive a femoral prosthesis. 
     Damage or disease can deteriorate the bones, articular cartilage and ligaments of human joints, such as the knee, which can ultimately affect the ability of the natural joint to function properly. To address these conditions, prosthetic joints have been developed that are mounted to prepared ends of the bones of the joint, namely the tibia and femur in the case of a knee prosthesis. Among the many knee prostheses, a mobile bearing knee simulates the condylar and bearing surfaces of the knee to emulate the natural movement of the knee during flexion and extension. The tibial component is configured to permit rotation about the axis of the tibia to accurately replicate the effects of differential rollback in the transverse plane. 
     Implantable mobile bearing knee prostheses, such as the prosthesis  10  shown in  FIG. 1 , for diseased and/or damaged knees typically include three components, namely a tibial component  12 , a femoral component  16  and a meniscal component (not shown). The tibial component  12  includes a platform  13  with a stem  14  configured for engagement in the prepared proximal end of the tibia. Generally, in a total knee joint replacement the platform  13  replaces the entire superior surface of the tibial plateau and substitutes for the tibial condylar surfaces. The femoral component can also include laterally-spaced condylar portions joined by an inter-condylar bridge and a patellar surface. 
     The femoral component  16  defines interior mounting surfaces  17  that often require involved cuts into the distal end of the femur. Since the components of the mobile bearing knee prosthesis  10  are generally configured to restore or emulate as much of the natural motion of the knee joint as possible, the femoral component often has a complicated geometry, which requires significant modification to the femur to accept and support the implant. The selection of the particular prosthesis components is usually dictated by the condition of the patient&#39;s knee. For instance, the condition of the distal end of the femur and proximal end of the tibia, as well as the patency of the surrounding ligaments and soft tissue can affect the form of the joint prosthesis. 
     In addition to the overall implant geometry, implant positioning with respect to the natural bone is critical. For instance, a proper implant will maintain the proper tension in the retained ligaments supporting the joint. In total knee reconstruction surgery, the menisci, bone ends and other stabilizing tissues are removed and replaced with implants. The thicknesses of the implants are ideally equal to the thickness of the removed material. Exceptions occur in reconstruction of severe deformity, where ligament length and tension after tissue releases during the reconstruction vary significantly form the preoperative state and from the normal knee. 
     Intraoperatively, the gap between the facing ends of the bones of the joint, which are related to the final implant position, can be manipulated. In the knee, a critical measure is the gap when the knee is in flexion or extension. The bone gaps in an ideal surgical reconstruction will have be the same in flexion and extension, the only exception being with implant systems having uneven implant thicknesses between anterior and posterior, or between medial and lateral compartments on either the tibial or femoral implants. The bone gaps for implants with unequal thicknesses must be accommodated for by the measuring tool or in the measurements when accessing potential implant fit. An ideal implant will maintain the same tension in flexion and extension, and the resulting joint tension and the stability of the implant will be substantially identical to the joint tension and stability of the patient&#39;s natural knee. 
     In preparing a knee joint, for instance, to receive a prosthesis, the orthopaedic surgeon typically uses templates to determine the proper size of the implant components. The surgeon may also measure the joint gap and choose a spacer that can be used in the procedure to maintain that gap. Since the femoral component of the knee prosthesis requires complex cuts in the femur, a femoral resection guide is used, such as the resection guide  20  shown in  FIG. 2 . The main body  22  of the guide  20  is aligned at the distal end of the femur F and held in place by one or more guide pins  24 . The resection guide  20  may include other structure and components for maintaining the guide in a proper orientation as the femur is resected. 
     In order to ensure that the resulting femoral implant achieves the proper flexion and extension gaps, a femoral positioner  26  is often used. The femoral positioner shown in  FIG. 2  includes a surface alignment plate  28  that rests on the previously resected surface R of the tibia. The alignment plate  28  is integral with a connector plate  30  that fits within a slot  23  in the main body  22  of the resection guide  20 . The femoral positioner  26  is thus used to help position the resection guide so that the femur is properly resected. 
     Another known femoral resection guide  32  is depicted in  FIG. 3 . This guide includes a body  33  defining a slot  34  for receiving a saw. A stylus  36  is used to align the depth of the saw cut. Handles  40  can be provided to help stabilize the resection guide during a cut. Guide pins  38  extend into the femur F to align and support the resection guide. 
     It is important that the resection guide be properly oriented when the distal end of the femur is prepared, otherwise the femoral implant will be produce undue strain or laxity in the knee joint. It is critical to maintain equal flexion and extension gaps to restore the proper anatomic tension as much as possible, regardless of the nature of the knee prosthesis. For instance, most mobile bearing knees are modular, meaning that several bearing elements can be provided depending upon the patient&#39;s anatomy. Obviously, thicker bearing elements correspond to greater flexion/extension gaps. 
     Similar modularity is important in the guide instruments used to ensure proper manipulation of the bones of the joint. There is a need, therefor, for an augment that can be readily used in the orthopaedic procedure to allow the guide instruments to properly emulate the natural anatomy of the instrumented joint. 
     Moreover, there is a need for an augment that can account for variations in the quality of the underlying bone. This need is particularly acute for revision surgeries in which the bone may have defects that make finding a stable platform difficult. 
     SUMMARY OF THE INVENTION 
     In one embodiment of the invention a system is provided for establishing a prosthetic gap between first and second bones at a joint. The system comprises an instrument for positioning within the gap between the first and second bones, the instrument having a first surface facing the first bone and a second surface facing the second bone. The system further comprises an augment for filling the gap when coupled to the instrument. 
     The augment and instrument include a mating connection mechanism that permits ready mounting and removal of the augment to the instrument. In certain embodiments, the instrument defines at least one bore between the first and second surfaces and the augment includes at least one pin sized to be received within the at least one bore with the augment in contact with either the first surface or the second surface. Other mating connection mechanisms can include other male-female constructs, such as dovetail or snap-fit mechanisms, or a canted coil spring mechanism. 
     In one specific embodiment, the instrument is a femoral positioner that includes a surface alignment plate configured to engage the tibia and a connector plate configured to engage a femoral resection guide. The surface alignment plate defines the at least one bore and is contacted by a mating surface of the augment form which the pin projects. In another specific embodiment, the instrument is a spacer block having a spacer body and a handle projecting therefrom. The spacer block defines the at least one bore. 
     In one aspect of the invention, the bore includes a resilient member disposed therein. The resilient member is configured to resiliently engage the pin when the pin extends through the bore. In one embodiment, the bore defines an internal groove, and the resilient member is an O-ring mounted within the groove. In an alternative embodiment, the bore defines a pair of internal grooves, one each adjacent each of the first and second surfaces, and the resilient member includes an O-ring mounted within each of the pair of grooves. 
     The augment includes a mating surface for contacting the instrument when the pin is within the bore, and an opposite surface. In certain embodiments, the opposite surface is substantially parallel to the first or second surface of the instrument. In other embodiments, the opposite surface defines a contour substantially similar to the contour of the first or second bones. 
     In another embodiment of the invention, 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, an augment for filling the gap when coupled to the instrument, and means for removably coupling the augment to the instrument including a resilient member disposed between the augment and the instrument. In one embodiment, the means for removably coupling includes a bore defined in the instrument and a pin disposed on the augment sized for engagement within the bore, with the resilient member disposed within the bore. The resilient member can be one or more O-rings disposed within the bore. 
     The illustrated embodiment is used for a knee prosthesis. However, it is contemplated that the present invention can be used in other human joints that may benefit from the features of the present invention. 
     It is one object of the invention to provide an augment that can serve as a spacer or a shim as part of a system for establishing a prosthetic gap for a human joint. Another object is to provide an augment that can be readily and securely mounted and disengaged from an instrument used in the system for establishing a prosthetic gap. 
     A further object resides in features of the invention that allow the augment to be mounted on different surfaces of the instrument to contact different bones of the joint. These and other objects and benefits of the invention can be discerned from the following written description, taken together with the accompanying figures 
    
    
     
       DESCRIPTION OF THE FIGURES 
         FIG. 1  is a perspective view of one type of knee prosthesis. 
         FIG. 2  is a side representation of a femoral resection guide as it is being positioned on the femur. 
         FIG. 3  is a perspective view of another known femoral resection guide. 
         FIG. 4  is a perspective view of a femoral positioner according to one embodiment of the invention. 
         FIG. 5  is a top elevational view of the femoral positioner shown in  FIG. 4 . 
         FIG. 6  is an enlarged cross-sectional view of a portion A in  FIG. 5 . 
         FIG. 7  is a top elevational view of an augment in accordance with one embodiment of the invention. 
         FIG. 8  is a side elevational view of the augment shown in  FIG. 7 . 
         FIG. 9  is a perspective view of a spacer block in accordance with a further embodiment of the invention. 
         FIG. 10  is a top elevational view of the spacer block shown in  FIG. 9 . 
         FIG. 11  is a side elevational view of the spacer block illustrated in  FIG. 9   
         FIG. 12  is a cross-sectional view of the spacer block depicted in  FIG. 9  taken along line B-B. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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  FIGS. 2 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  FIG. 2 . 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  FIG. 2 , 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  FIG. 2 . 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  FIGS. 7-8  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  ( FIG. 6 ). 
     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  ( FIG. 6 ). 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  FIG. 6 , 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  FIGS. 9-12 . 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.