# Patent

## Title
Multiple-cam, posterior-stabilized knee prosthesis

## Abstract

A total knee replacement system includes a tibial component having a tibial post with a posterior portion, and a femoral component with medial and lateral condylar protrusions forming separated bearing surfaces configured to articulate with the tibial component. In one disclosed embodiment the posterior portion of the tibial post has a concave surface in the sagittal plane and a convex surface in the transverse plane, and the femoral component includes an intercondylar femoral cam mechanism configured to articulate with the posterior surface of the tibial post. The femoral cam mechanism may include an intercondylar bridging structure with distinct bars or interconnected structural elements providing convex and concave distal surfaces in the sagittal plane, and a concave surface in the transverse plane to facilitate rotation of the femoral component relative to the tibial component in the transverse plane.

## Background
REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. patent application Ser. No. 12/337,280, filed Dec. 17, 2008, now U.S. Pat. No. 8,721,730, which is a continuation of U.S. patent application Ser. No. 10/430,548, filed May 6, 2003, now U.S. Pat. No. 8,273,132, 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 all of which are incorporated herein in their entirety by reference.


FIELD OF THE INVENTION
This invention relates generally to orthopedic surgery and, in particular, to a posterior stabilized knee prosthesis.
BACKGROUND OF THE INVENTION
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.
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.
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.
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 '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.
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.
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.
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 '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.
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.
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 provide more normal range of motion for cruciate substituting knee replacement.
SUMMARY OF THE INVENTION
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.
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.
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.
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 allow 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
FIG. 1A is a drawing which illustrates a prior-art cam-and-post mechanism in extension;
FIG. 1B is a drawing which illustrates the prior-art cam-and-post mechanism of FIG. 1B in flexion;
FIG. 2A illustrates a preferred embodiment of the invention in extension;
FIG. 2B shows the system of FIG. 2A at 90 degrees flexion;
FIG. 2C illustrates the system of FIG. 2A in flexion at 120 degrees or more;
FIG. 2D illustrates the alternative use of interconnected cams with physically separate contact points;
FIG. 2E illustrates the distal femoral component of FIG. 2D and the post configuration of FIG. 6A at full extension;
FIG. 2F shows the distal femoral component of FIG. 2D and the post configuration of FIG. 6A at 90 degrees of flexion;
FIG. 2G illustrates the distal femoral component of FIG. 2D and the post configuration of FIG. 6A in flexion beyond 90 degrees;
FIG. 2H shows a degree of hyperflexion wherein the extended cam in FIG. 2D first makes contact with the posterior surface of the post;
FIG. 2I illustrates the distal femoral component of FIG. 2D and the post configuration of FIG. 6E (with rounded corners) at full extension;
FIG. 2J illustrates the distal femoral component of FIG. 2D and the modified post configuration of FIG. 6E at about 45 degrees of flexion;
FIG. 2K illustrates the distal femoral component of FIG. 2D and the modified post configuration of FIG. 6E before 90 degrees of flexion;
FIG. 2L illustrates the distal femoral component of FIG. 2D and the modified post configuration of FIG. 6E after 90 degrees of flexion;
FIG. 3 shows an anterior view of a prior-art cruciate-substituting knee-replacement component;
FIG. 4 shows a knee prosthesis according to the invention having multiple cams as seen in a distal-to-proximal view;
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
FIGS. 6A through 6E illustrate various alternative post configurations, all of which are applicable to the invention.



DETAILED DESCRIPTION OF THE INVENTION
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.
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.
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 allow 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.
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.
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 posterior aspect of the post or for that matter, use more than three if desired.
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.

## Claims

1. A total knee replacement system, comprising:
a tibial component having a tibial post with a posterior surface;
a femoral component having medial and lateral condylar protrusions which form separated bearing surfaces configured to articulate with the tibial component, the femoral component further including an intercondylar femoral cam mechanism configured to articulate with the posterior surface of the tibial post;
wherein a majority of the posterior surface of the tibial post is concave in a sagittal plane, defined as a vertical plane extending from front to back;
wherein the cam mechanism of the femoral component has a superior convex portion, a concave central portion, and an inferior convex posterior portion;
wherein the inferior convex posterior portion contacts the posterior surface of the tibial post at or before 90 degrees of flexion;
wherein at least a portion of the posterior surface of the tibial post is convex in a transverse (horizontal) plane; and
wherein at least a portion of the cam mechanism of the femoral component is concave in the transverse (horizontal) plane.

2. The total knee replacement system of claim 1, wherein a portion of the femoral cam mechanism forms an “s” shaped surface portion in the sagittal plane; and
wherein the “s” shaped surface portion is formed from the superior aspect of the most superior surface of the femoral cam at its superior convex portion and terminates at the most inferior aspect of the concave surface.