Abstract:
A distal femoral knee-replacement component provides additional points of cam action by either distinct bars or interconnected structural elements such as cam extensions to prevent early translation of the knee or dislocation of the femoral component over the tibial post which can occur in cruciate-substituting designs.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 10/430,548, filed May 6, 2003, which is a divisional of U.S. patent application Ser. No. 09/724,100, filed Nov. 28, 2000, now U.S. Pat. No. 6,558,426, the content of both of which are incorporated herein in their entirety by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates generally to orthopedic surgery and, in particular, to a posterior stabilized knee prosthesis. 
       BACKGROUND OF THE INVENTION 
       [0003]    In total knee-replacement (TKR) surgery, there are four broad classes of implants used for resurfacing of the distal femur. In one configuration, the posterior cruciate ligament is retained. In another design the ligament is sacrificed, relying on the articular geometry to provide stability. The third type of device is constrained, in the sense that an actual linkage is used between the femoral and tibial components. According to a fourth arrangement, the posterior cruciate is replaced with a cam on the femoral component and a post on the tibial component. 
         [0004]    Many patents have been issued in relation to these design configurations, including the cam-and-post design configuration. Some of the earlier patents in this area include U.S. Pat. No. 4,213,209 to Insall et al; U.S. Pat. No. 4,298,992 to Burstein et al. 
         [0005]    Other patents include U.S. Pat. No. 4,888,021 to Forte et al., which teaches a cam-and-post mechanism as well as a linking mechanism. Essentially, each component includes a varying surface and a cam member, so that both the tibial and the femoral component have separate and distinct cams that cooperate with a single tibial post. 
         [0006]    U.S. Pat. No. 5,824,100 to Kester et al. discloses a cam/post type of arrangement with a unique type of cam and box enclosure. A portion of the box enclosure is intended to prevent hyperextension and posterior translation. As noted in particular in FIGS. 3 and 4 of the &#39;100 patent, a large space exists between the cam  110  and the post  100  which permits a translation to occur prior to engagement of the cam left of post. 
         [0007]    U.S. Pat. No. 5,997,577 to Herrington et al. provides a cam on the femur with a geometry meant to contact the post through a large range of motion. This design attempts to provide the function of multiple cams by providing an area that acts as a separate bearing surface. As such, the cam effectively moves through a range of motion while contacting the post. Depending on the articular geometry which differs than the geometry of the cam post mechanism, this could lead to a variety of problems as well as significantly constrained motion, either between the cam and the post or between the two articulating surfaces. 
         [0008]    U.S. Pat. No. 5,658,342 to Draganich et al. describes a cam member with including a bearing surface at complimenting an articulating surface. As in other previous designs, this represents a complex cam geometry meant to capture the post in certain degrees of the range of motion. 
         [0009]    U.S. Pat. No. 5,147,405 to Van Zyle et al. Discloses a femoral component with two distinct cam structures, one located at point  44 , the other located at  46  in the drawings. The cam member  44  is meant to contact the anterior surface of the post  24  to prevent hyperextension, while cam surface  46  is a posterior located cam meant to have contact throughout the range of flexion. As noted in FIG. 6A of the &#39;405 patent, there is a space between the cam and the post when the knee is in extension, necessitating anterior translation of the femur on the tibia prior to contacting the posterior cam. 
         [0010]    Many other patents directed to knee-replacement surgery include cam-and-post mechanisms. But in all cases, either the full range of joint motion is precluded, or translation is allowed to occur which could lead to premature wear.  FIG. 1  is a drawing which illustrates a typical prior-art cam-and-post mechanism. Item  102  is a tibial insert or tibial component having a post  103  protruding into a box-like recess of the femoral component  100 .  FIG. 1A  shows the system in extension, whereas  FIG. 1B  shows the system in flexion. In  FIG. 1A , a femoral component  100  includes a cam  101  which has not yet engaged with a post  103 . 
         [0011]    In  FIG. 1B , following a considerable amount of flexion, the cam  101  finally engages with the post  103 . Until engagement occurs, however, the component  100  may be permitted to slide relative to the tibial insert. The need remains, therefore, for an improved distal femoral prosthesis having multiple distinct cams contacting a post on its posterior surface to a provide more normal range of motion for cruciate substituting knee replacement. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention resides in a distal femoral knee-replacement component configured for use in a cruciate-substituting situation involving a tibial component having a bearing surface and a superior post with a posterior aspect. As with existing configurations, the component is comprised of a body having a pair of medial and lateral condylar protrusions and an intercondylar region therebetween dimensioned to receive the tibial post. In contrast to prior-art devices, however, the inventive component provides additional points of cam action to facilitate a more normal range of knee motion. 
         [0013]    In the preferred embodiment, the invention facilitates a more normal rollback while inhibiting initial translation which could lead to increased wear and sub-optimal patella femoral mechanics. To accomplish this goal, the inventive component includes a distinct point of cam action to prevent early translation at the initiation of flexion, and a distinct point of cam action to prevent a dislocation of the femoral component over the tibial post which is known to occur in cruciate-substituting designs. According to the invention, these points of cam action may be used separately or in combination. 
         [0014]    In the preferred embodiment, the component includes three distinct points of cam action. The first is preferably located substantially where existing cams are found, namely, at a point spaced apart a slight distance posteriorly relative to the post in full extension. According to the invention, however, a second point of cam action is located immediately adjacent the posterior aspect of the superior post to minimize and, ideally, prevent anterior translation at the initiation of flexion. The third point of cam action is preferably located more posteriorly to allow enhanced flexion without a dislocation of the knee. 
         [0015]    In terms of structure, the points of cam action may be implemented using any member or combination of elements operative to provide distinct stages of cooperation with the posterior aspect of the superior post. For example, transverse bars may be used which bridge, or partially bridge, the intercondylar space. The members or elements need not be straight across, but may instead be curved, with the post being curved to allows for a rotation, if so desired. The cam structures according to the invention may also be connected to one another forming points of contact as opposed to complete transverse elements such as distinct bars. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0016]      FIG. 1A  is a drawing which illustrates a prior-art cam-and-post mechanism in extension; 
           [0017]      FIG. 1B  is a drawing which illustrates the prior-art cam-and-post mechanism of  FIG. 1B  in flexion; 
           [0018]      FIG. 2A  illustrates a preferred embodiment of the invention in extension; 
           [0019]      FIG. 2B  shows the system of  FIG. 2A  at 90 degrees flexion; 
           [0020]      FIG. 2C  illustrates the system of  FIG. 2A  in flexion at 120 degrees or more; 
           [0021]      FIG. 2D  illustrates the alternative use of interconnected cams with physically separate contact points; 
           [0022]      FIG. 3  shows an anterior view of a prior-art cruciate-substituting knee-replacement component; 
           [0023]      FIG. 4  shows a knee prosthesis according to the invention having multiple cams as seen in a distal-to-proximal view; 
           [0024]      FIG. 5  is a drawing which shows how cam-acting members according to the invention need not be straight across, but may be curved in conjunction with a curved post to facilitate rotation; and 
           [0025]      FIGS. 6A through 6E  illustrate various alternative post configurations, all of which are applicable to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]      FIGS. 2A through 2C  illustrate one embodiment of the invention.  FIG. 2A  shows the configuration in extension,  FIG. 2B  shows the system at 90 degrees flexion, and  FIG. 2C  illustrates flexion of 120 degrees or more. In addition to a conventionally placed cam at  101 , two additional points of cam action are preferably provided. In particular, a feature at  201  acts to prevent translation from extension into the initiation of the flexion. Feature  201  preferably disengages as conventional cam  101  is engaged. As the knee follows through a range of motion to 90° of flexion, and beyond, cam  101  disengages and feature  202  engages, if necessary, to prevent dislocation of the component. 
         [0027]    In  FIG. 2B , the cam which is usually present at  101  is engaging the tibial post, cam  201  has disengaged, and cam  202  has not yet engaged but is available for engagement on further flexion. In  FIG. 2C , cam  202  is now engaged the post in the presence of additional flexion. Cam  101  can now disengage, cam  201  had disengaged earlier.  FIG. 2D  illustrates the alternative use of interconnected cams with physically separate contact points. 
         [0028]    In  FIGS. 2A through 4 , the features depicted to provide the various stages of cam/pivoting function are depicted as bars which cross the intercondylar recess or box portion of a cruciate substituting design knee. However, although the terms “cam” or “bar” are used to reference the stages of cam action, it should be understood that the responsible structures may be implemented using any member or combination of elements operative to provide distinct stages of cooperation with the posterior aspect of the superior post. Thus, the members or elements need not be complete or straight across, but may instead be curved, with the post being curved to allows for a rotation, if so desired. The cam structures according to the invention may also be connected to one another forming points of contact as opposed to complete transverse elements. The structure may be provided as part of an open- or closed-type of a box structure, both being familiar to those of skill in the art. 
         [0029]    Whereas  FIGS. 1 and 2  represent lateral or side views of a knee through various ranges of motion,  FIG. 3  shows an anterior view of a prior-art cruciate substituting knee component at  300  having an open-type box  302  including a single transverse member  301  for illustrative purposes.  FIG. 4  shows a knee prosthesis  400  according to the invention, viewed again from the distal-to-proximal perspective, having three distinct points of cam action. In particular, cam  401  is conventionally located, an anterior cam is disposed at  402  in support of a greater range of enhanced flexion, and a more posterior cam at  403  is used primarily to prevent dislocation of the cams over the post, as discussed above. 
         [0030]      FIG. 5  is a drawing which shows various cams from a top view looking down. Note that bars of need not be straight across, but may be curved with the post being curved so that it allows for a rotation to occur if desired. The cam structures according to the invention may be individual distinct bars or may be connected to one another forming points of contact as opposed to distinct structures themselves. It should also be noted that the cam structures may be located at different locations from the posterior to the anterior aspect of the knee design, as well as from the distal or proximal, depending upon implant size, patient physiology, desired range of motion, and other requirements. It should further be noted that as opposed to using three separate cams, one could use two cams intended to contact the posted or aspect of the post or for that matter, use more than three if desired. 
         [0031]    It will also be apparent to one of skill in the art that the posterior aspect of the post may be modified to affect the timing and/or operation of the cam engagement.  FIGS. 6A through 6E  illustrate various alternative post configurations, all of which are applicable to the invention. As opposed to a substantially straight configuration, as depicted in  FIG. 6A , the post may be posteriorly oriented along the posterior aspect, as shown in  FIG. 6B . Alternatively, the post may be anteriorly oriented along the posterior aspect, as shown in  FIG. 6C . As further alternatives, the post may be stepped, as shown in  FIG. 6D , or curved, as shown in  FIG. 6E . Also, as opposed to the sharp corners shown, they may be rounded off, and the bars or recesses may be adjusted from the positions shown in  FIG. 2 through 5  to achieve a desired operation.