Abstract:
An orthopedic joint replacement has first and second joint components that can be placed in load-bearing articulation with one another. The first joint component has first and second convex spherical condylar segments defining first and second radii. The second joint component has a spherical first concave condylar segment with a radius equal to the radius of the first convex spherical condylar segment. The second joint component also has a non-spherical second concave condylar segment. The first convex spherical condylar segment of the first joint component is in congruent contact with the first spherical concave condylar segment of the second joint component. The second spherical convex condylar segment of the first joint component is in line contact with the non-spherical concave condylar segment of the second joint component.

Description:
[0001]    This application claims priority on U.S. Provisional Patent Appl. No. 61/098,824 filed on Sep. 22, 2008, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to prosthetic joints, such as prosthetic knee joints. 
         [0004]    2. Description of the Related Art 
         [0005]    A typical prosthetic knee includes a tibial component for mounting to the resected proximal end of the tibia, a femoral component for mounting to the resected distal end of the femur, a bearing between the tibial and femoral components and a patellar component mounted to the posterior face of the patella. The tibial and femoral components typically are made of metal and the bearing typically is made of plastic, such as UHMWPe. The proximal or superior surface of the bearing is formed to define medial and lateral concave regions. The distal or inferior surface of the femoral component is formed to define medial and lateral convex condyles that articulate in bearing engagement with the concave regions of the bearing. Some prosthetic knees include a mobile bearing that is permitted to undergo controlled rotational and translational movement relative to the tibial component. Other prosthetic knees include a bearing that is fixed relative to the tibial component. 
         [0006]    Knee motion is highly complex and includes flexion-extension, axial rotation, anterior-posterior translation, and adduction-abduction. Incongruency between the femoral component and the bearing enables these complex motions to be carried out with enhanced mobility for the patient who has a prosthetic knee joint Accordingly, many prosthetic knee joints provide highly incongruent contact between the femoral component and the bearing. Incongruent contact causes a specified load to be applied to a small area, and hence causes the contact stress (load per unit area) to be higher than in a knee joint with more congruent contact. The metallic and plastic materials currently used in joint replacement permit normal knee motion with contact stresses that can accommodate normal physiological loads over an extended period of time in mobile bearing prosthetic knees. For example, U.S. Pat. Nos. 4,309,778 and 4,340,978 disclose mobile bearing prosthetic knee joints with tibiofemoral articulation surfaces that have demonstrated an ability to last for an extended time. 
         [0007]    Incongruent contact is particularly important in fixed bearing designs in view of the complex combinations of flexion-extension, axial rotation, anterior-posterior translation, and adduction-abduction associated with knee motion. However, fixed bearing prosthetic knee joints can produce contact stresses greatly in excess of acceptable limits associated with the strength of UHMWPe normally used for the tibial articulation surface. The dilemma for designers of fixed bearing knees is to effect a compromise between the conflicting requirements for joint motion mobility (which is accomplished by increasing contact surface incongruity and thus contact stress) and low contact stress (which requires high congruity and thus low joint mobility) to prevent rapid failure of the plastic used in current prosthetic joint articulations. Unfortunately a satisfactory compromise has yet to be found where fixed bearing knee components can be considered safe for extended use under normal physiological loads. A similar situation is true for other load bearing condylar joints such as the tibiotalar ankle joint. 
         [0008]    The United States Food and Drug Administration (USFDA) requires extensive and rigorous clinical testing before approval of most mobile bearing joint replacements, and hence inhibits the use of such devices. The USFDA does not require similar testing for fixed bearing devices. Thus, most knee devices and all ankle devices that are generally available in the United States are the lower performing fixed bearing devices. 
       SUMMARY OF THE INVENTION 
       [0009]    Improved fixed bearing articulating surfaces are possible by limiting the degree of incongruity in such devices. This may be accomplished by using a congruent, spherical surface on the medial condyle of the knee or ankle and mildly incongruent line contact on the more lightly loaded lateral condyle rather than the typical point contact on both sides used for fixed bearing designs. This design recognizes the fact that the medial condyles of both the femur and the patella of the knee joint and the medial condyle of the ankle joint are subject to greater loads than the lateral condyles thereof. The congruent contact at the more highly loaded medial condyle results in lower stress (i.e. force per unit area) due to the higher surface contact area achieved with congruency. On the other hand, the line contact at the less highly loaded lateral condyle results in acceptably low stress despite the smaller surface area due to the lower load on the lateral condyle. However, the line contact at the lateral condyles can achieve greater joint mobility without using a mobile bearing joint design. 
         [0010]    Such a surface can be designed to accept normal walking loads within the allowable stress limits of the materials used in such joint replacement while still providing needed joint mobility. Expected stresses on the lateral condyle will, however, be substantially greater than that of a comparable mobile bearing with congruity on both sides. The combined congruent-incongruent articulating surface is thus an acceptable, although less desirable, design compromise to accommodate the regulatory requirements of the USFDA and the many surgeons who have become accustomed to fixed bearings. 
         [0011]    Many patients who receive knee and ankle implants are quite elderly and inactive and thus produce loads that are substantially less than normal. This lower loading level (producing lower contact stresses for a given articulation geometry), coupled with the reduced time and frequency of use (which reduce the accumulated damage for given contact stresses) can allow articulating surfaces with a greater degree of incongruity and thus allow the use of fixed bearing components. Since fixed bearings do not require a supporting prosthetic platform, they can be fixtured directly to bone, saving the cost of the platform. The US medical care system is under considerable pressure to lower costs, and hence many hospitals would prefer to use a low cost device. A low cost, fixed bearing, device can be used as tibial or patellar components of a total knee in an elderly, inactive, patient. Therefore, the added cost of multi-part tibial or patellar replacements are not justified economically if a lower cost set of components are adequate. 
         [0012]    An articulation surface with partially incongruent contact surfaces can produce substantially lower contact stresses than existing incongruent, fixed bearing devices. Lowering contact stresses in incongruent fixed bearing devices reduces wear and fatigue damage of the prosthetic articulating surfaces, thereby increasing their service life and increasing the population group to which such components can safely be used. The articulating surfaces of the subject invention can have similarities to the articulating surfaces shown in U.S. Pat. No. 5,871,539 and U.S. Pat. No. 6,074,425, the disclosures of which are incorporated herein by reference. However, the articulating surfaces of the subject invention are formed by means that are different from the means used to generate the articulating surfaces in these earlier patents. Additionally, the articulating surfaces of the subject invention are configured to achieve line contact in only one of the condyles of the subject invention as compared to both condyles of the earlier patents. Thus, this invention improves the fixed bearing articulating surfaces. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  side elevational view of a knee that has a total knee replacement prosthesis in accordance with one embodiment of the invention. 
           [0014]      FIG. 2  is a front elevational view of the knee and prosthesis of  FIG. 1 . 
           [0015]      FIG. 3  is of the assembled components of the prosthesis of  FIGS. 1 and 2  independent of the knee. 
           [0016]      FIG. 4  is a front elevational view of the prosthesis of  FIG. 3   
           [0017]      FIG. 5  is a top plan view of the tibial articular surface of the knee prosthesis of  FIGS. 3 and 4 . 
           [0018]      FIG. 6  is a front elevational view of a blank for forming the tibial component of the prosthesis and a cutter for forming the articular surface on the blank. 
           [0019]      FIG. 7  is an exploded side elevational view of the blank for forming the tibial component of the prosthesis and the cutter of  FIG. 6 . 
           [0020]      FIG. 8  is a front elevation view of the tibial component and the cutter near the completion of a cutting operation. 
           [0021]      FIG. 9  is a side elevational view of the tibial component and the cutter in the relative positions shown in  FIG. 8 . 
           [0022]      FIG. 10  is a cross sectional view of the tibial component of the knee prosthesis formed by the cutter as taken along an anterior-posterior line through the lateral condylar surface. 
           [0023]      FIG. 11  is a side elevational view of the tibial and femoral components assembled and articulated relative to one another. 
           [0024]      FIG. 12  is a front elevational view of the femoral component and the patellar component of the prosthesis. 
           [0025]      FIG. 13  is a top plan view, partly in section, shown the assembled knee prosthesis at full extension. 
           [0026]      FIG. 14  is a front elevational view, partly in section, of an ankle prosthesis in accordance with the invention. 
           [0027]      FIG. 15  is a side elevational view of the ankle prosthesis of  FIG. 14 . 
           [0028]      FIG. 16  is a cross sectional view of the tibial component of the ankle prosthesis of  FIGS. 14 and 15 . 
           [0029]      FIG. 17  is a bottom plan view of the ankle prosthesis of  FIGS. 14 and 15 . 
           [0030]      FIG. 18  is a front elevational view of the bearing of the ankle prosthesis of  FIGS. 14 and 15 . 
           [0031]      FIG. 19  is a side elevational view of the bearing shown in  FIG. 18 . 
           [0032]      FIG. 20  is a cross-sectional view taken along line A-A of  FIG. 17 . 
           [0033]      FIG. 21  is a cross-sectional view taken along line B-B of  FIG. 17 . 
           [0034]      FIG. 22  is a bottom plan view of the bearing. 
           [0035]      FIG. 23  is a cross-sectional view taken along line C-C of  FIG. 22 . 
           [0036]      FIG. 24  is a front elevational view of the talar component of the ankle prosthesis of  FIGS. 14 and 15 . 
           [0037]      FIG. 25  is a side elevation component of the talar component of  FIG. 24 . 
           [0038]      FIG. 26  is a side elevational view of a knee prosthesis in accordance with a third embodiment of the invention. 
           [0039]      FIG. 27  is a front elevational view of the knee prosthesis of  FIG. 26 . 
           [0040]      FIG. 28  is a front elevational view similar to  FIG. 27 , but showing the bearing and the tibial component in section along a medial-lateral line. 
           [0041]      FIG. 29  is a top plan view of the bearing and the tibial component. 
           [0042]      FIG. 30  is a top plan view of the tibial component. 
           [0043]      FIG. 31  is a bottom plan view of the bearing. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0044]      FIGS. 1 and 2  show a cruciate sacrificing total knee replacement prosthesis  100 . The knee prosthesis has a metallic (Co—Cr or Titanium alloy) femoral component  10  which is fixtured to the distal femur  11 , a plastic (UHMWPe) tibial component  20  fixtured to the proximal tibia  21 , and a plastic patellar component  30  fixtured to the posterior patella  31 . Alternately, both components may be metallic, ceramic coated metal or ceramic. 
         [0045]    The geometry of the femoral articulating surface  12  of the femoral component  10 , as shown in  FIGS. 3 and 4 , is a compound surface of revolution generated by revolving a generating curve  13  consisting of radii  14 , radius  15 , and connecting tangents  16 . This geometry is described in additional detail in the above-referenced patents, including U.S. Pat. No. 4,309,778 and U.S. Pat. No. 5,507,820. 
         [0046]    The tibial component  20  has a tibial articulating surface  22  that is generated using the same generating curve  13 , except for different connecting tangents. However, only the medial articulation  28  of the tibial articulating surface  22  is a surface of revolution, and the lateral surface  29  is not a surface of revolution. Rather, the lateral tibial articulating surface  29  is generated by simultaneously rotating a surface of revolution about different axes. This generation method is unique and useful. The surface of revolution for the medial articulation  28  of the tibial articulating surface  22  preferably is configured relative to the compound surface of revolution of the femoral articulating surface  12  to achieve congruency to at least about 40-50 degrees of flexion. Line contact may exist between the femoral component  10  and the tibial component  20  at greater flexion. 
         [0047]    The tibial articulating surface  22  may be formed on a tibial component blank  23 , as shown in  FIGS. 6 and 7 , by a cutter  24  made in the form of a surface of revolution formed by the generating curve  13  shown in  FIG. 4 . The cutter  24  initially is rotated about axis X-X, fixed in the cutter, as shown in  FIG. 6 . The axis X-X is parallel to the face  25  of tibial component blank  23  from a position where axis X-X of the cutter  24  intersects the Z axis. In this initial position, the cutting surface  26  of the cutter  24  is above the top surface  27  of tibial component blank  23 , as shown in  FIGS. 6 and 7 . The cutter  24  then is moved along the Z axis into the blank  23  until the cutter  24  has cut the blank  23  to the desired depth D as shown in  FIG. 8 . From this position the cutter  24  simultaneously is rotated about the Y and Z axes, as shown in  FIG. 9  to create a lateral condylar surface  29  with principal radii R and G at the line of lateral contact where R is larger than radius G, as shown in  FIGS. 10 and 11 . This manufacturing method results in an articulating surface  22  that is congruent to the femoral surface  12  on the medial condyle and in line contact on the lateral condyle under compressive loading of the joint  100  during axial rotation of the tibia  21  relative to the femur  11 . The desired size of the radius R compared to the radius G is dependent on the degree of axial rotation needed in normal joint motion. An increase in radius R decreases valgus-varus tibial rotation about the Y axis during axial (Z axis) rotation and increases the amount of axial rotation before line contact is lost on the lateral articulating surfaces. Unfortunately increasing radius R also increases the degree of incongruity. 
         [0048]    This resulting surface will be referred to here as a “medial-pivot” surface since motion on the medial articulation of the tibia  21  relative to the femur will take place about the origin of the X, Y and Z axes, fixed to the tibia with the X, Y, Z coordinate system origin at the center of the spherical medial articulating surfaces. 
         [0049]    Loads that press the patellar component  30  to the femoral component articulating surface  12  are low at full extension. However, at about 35-45 degrees flexion, the substantial load caused by the quadriceps pulls the patellar component  30  medially into the sulcus. Thus, the medial patellar articulation surface  32  carries most of the load, Often the lateral patellar articulating surface  33  lifts off the femoral component articulating surface  12 , as shown in  FIG. 12 . Where this occurs a medial-pivot surface will produce congruent contact on the medial articulation  32  and since the contacting surfaces are spherical it allows rotation about three independent axes under congruent contact. 
         [0050]    Where the medial component of the patellofemoral compressive load is sufficient so as not to produce lift off of lateral patellar articulation surface  33 , as shown in  FIG. 11 , congruent articulation at the medial patellar articulation surface  32  will still occur but articulation at the lateral patellar articulation surface  33  will be incongruent. The normal axial rotation of the patella  31  is less than associated with the tibiofemoral articulation. Thus, somewhat smaller radius R may be used to reduce the degree of incongruity, thereby reducing the lateral surface contact stress. 
         [0051]      FIGS. 14-25  illustrate an ankle prosthesis  300  in accordance with the invention. The ankle prosthesis  300  has a tibial component  310 , a bearing  320  and a talar component  330 . The bearing  320  has a plate  321  that fits snugly into cavity  311  of the talar component  310  to prevent movement of the bearing relative to the talar component under compressive load. This arrangement causes the bearing  320  to be considered a “fixed” bearing. The bearing  320  also has a bearing articulating surface  322  of bearing that articulates with a talar articulating surface  331  of the talar component  330 . The talar articulating surface  331  of the talar component  330  is a surface of revolution generated by rotating a generating curve similar in shape to  13 , except reduced in scale. The bearing articulating surface  322  of the bearing  320  is generated in exactly the same fashion as the knee tibial articulating surface  22 . However, axial rotation of the ankle is small compared to the knee. Therefore, a radius R′ may be much closer in size, proportionately, to the radius G′ than the radius R is to the radius G. Thus, the increase in contact stress due to the introduction of incongruity is substantially less in the ankle than in the knee. Such reduction is needed because contact stresses in the ankle, even for congruent contact, are substantially greater than in the knee due to the fact that, although loads in the knee and ankle are similar, the ankle is much smaller than the knee. 
         [0052]    A replacement knee in accordance with a third embodiment is identified by the numeral  400  in  FIGS. 26-31 . The replacement knee  400  has a femoral component  410  and tibial articulating surface  422  that are the same as in the replacement knee  100  of the first embodiment. However, the replacement knee  400  differs from the replacement knee  100  in that the tibial component  420  of the replacement knee  400  comprises two parts, namely, a bearing  430 , made of a plastic such as UHMWPe and a metallic  440  tray, made of Co—Cr or Titanium alloy. 
         [0053]    Referring to  FIG. 30 , the tray  440  has a platform  441  with two vertical walls  442 . A button  443  projects up from the platform  441 , as shown in  FIG. 28 . The button  443  is formed with a ridge  444  and an undercut  445 . Fixation surfaces  446  are defined on a lower or inferior part of the tray  440 , as shown in  FIG. 26 . Referring to  FIG. 31  the bearing  430  has a flat inferior surface  431  and side surfaces  432  extend up from the inferior surface  431 . A hole  433  extends into the inferior surface  431  and is formed with a ridge  435 , as shown in  FIG. 28 . The bearing  430  is assembled onto the tray  440  by placing the hole  433  on the button  443  and pushing the bearing toward the tray  440 , while aligning the tray sidewalls  442  with the bearing side surface  432  until the ridge  435  of the hole  433  expands over the ridge  444  of the button  443  and the bearing  430  snaps into place, as shown in  FIG. 28 . The dimensions of the side surfaces  432  of the bearing  430  and the sidewalls  442  of the tray  440  are selected to produce a close slip, to light press fit so as to minimize any motion between the bearing  430  and the tray  440 . 
         [0054]    The medial-pivot surface need not be formed by use of a cutter such cutter  34 , which is used primarily for purposes of illustration. A medial-pivot surface can be machined by a variety of cutters including form cutters, point cutters, and ball mills using two and three dimensional computer driven machines. 
         [0055]    A medial-pivot surface is unique within and without the field of orthopedic surgical appliances. In human replacement joints its primary application is in condylar joints such as the knee, ankle great toe, pip joint of the finger, and the thumb and in the elbow.