Abstract:
An artificial knee joint is described that includes a femoral component with a specially shaped bearing surface and a tibial component, whose surface interacts with the femoral surfaces. The interaction provides for the required motion and stability characteristics. The interaction between the femoral and tibial surfaces is such that as the knee is flexed to maximum, the femoral component moves posteriorly on the tibial surface, by an amount similar to that in the anatomic knee. The opposite motion, roll forward of the femur from a fully flexed to a more extended position, is accomplished by varying the outward radii of the lateral and medial femoral bearing surfaces, together with a ramp on the postero-lateral and postero-medial regions of the tibial surfaces.

Description:
FIELD OF THE INVENTION 
     The present invention relates to total knee prostheses and more specifically to a knee joint having femoral components and a tibial component that are specifically shaped for posterior cruciate retention and substituting. 
     BACKGROUND OF THE INVENTION 
     The large majority of the total knees implanted today are either of the posterior cruciate retaining (CR) or posterior stabilized (PS) types. These have both functioned well clinically, but a number of disadvantages remain. Due to variations in surgical techniques and between patients, when using the CR type of total knee, it has been difficult to obtain an optimal tightness of the posterior cruciate ligament (PCL) throughout flexion, resulting in variable rollback patterns. 
     For the PS, considerably extra bone needs to be resected from the center of the femur to accommodate the intercondylar housing of the femoral component, for the cam and post mechanism. Damage can sometimes occur to the plastic post in the long term, anteriorly due to impacts at full extension, and posteriorly due to high flexion loading. In addition, in many PS designs, there is a tendency for overconstraint in rotation due to the dishing of the tibial bearing surfaces, in combination with the construct of the cam-post. 
     A disadvantage common to both CR and PS types, although more pronounced with CR designs due to their shallower tibial bearing surfaces, is a paradoxical motion in which the femur slides forwards on the tibial surface during the first half of the flexion range, rather than backwards which is the required motion. Also, uneven or jerky motion occurs in many cases. Artificial knees have included configurations that accommodate medial pivotal rotation to resemble the anatomical motion in the anatomic knee. However, these configurations may not incorporate a mechanism for achieving lateral rollback in flexion in combination with a relatively immobile medial side, or do not provide sufficient laxity about the neutral path of motion. 
     A knee joint system is needed that has a femoral component and a tibial component that have specific contours and surfaces that guide the knee joint into an average anatomic neutral path of motion during flexion-extension, while having sufficient laxity about that neutral path to accommodate different individuals and activities; and that can be used as both a CR and PS types. 
     SUMMARY OF THE INVENTION 
     A knee joint for prosthesis for guiding the motion and preventing paradoxical motions is described that includes a tibial component having a lateral bearing surface and a medial bearing surface that are separated by a ramp. The medial bearing surface has a smaller sagittal plane radius of curvature than the lateral bearing surface. The medial bearing surface includes a raised pad in an anterior medial bearing surface 
     The knee joint includes a femoral component that includes a lateral condyle and medial condyle that are separated by a groove. The groove interfaces with the ramp of the tibial component to produce a posterior movement during flexion of the joint. The ramp with a plurality of smooth edges has a height that decreases gradually from an anterior portion to a posterior portion of the tibial component to guide the femoral component to displace posteriorly during flexion and displace anteriorly during extension. 
     The deeper medial tibial surface and the shallow lateral surface of the tibia results in most of the posterior displacement of the femur occurring laterally with only a small displacement medially. The femoral component includes a recess located antero-medially, which interfaces with the said tibia pad. The antero-medial recess-pad feature limits anterior femoral displacement in early flexion. This combined ramp system guides the motion of the knee into an anatomic motion pattern consisting of external femoral rotation of the femur about a medial axis on the tibia, as flexion proceeds. The femoral component also includes a patella flange in a proximal-anterior portion. 
     When the system is used as a CR device, the PCL provides some of the motion guidance. When used as a PS device, the features of the ramp guide the motion. Sufficient laxity about the neutral path allows for variations of motion. One of the benefits of the system is that anterior sliding of the femur on the tibia from approximately 0-60° flexion (paradoxical motion) is minimized due to the medial femoral recess as it interfaces with the antero-medial tibial pad feature. 
     The femoral component is advantageously used in PS type of knee prosthesis. In one more embodiment of the present invention, the femoral component is used for CR type of knee prosthesis with the similar tibial component. The two femoral components are identical except for a central recess to provide access to the PCL in the CR version of the femoral component. 
     Large contact surface areas between the femoral and tibial components, especially on the medial side, which is highly loaded than the lateral side, minimize wear and deformation of the polyethylene surfaces. Additionally, smooth contours around the central tibial ramp avoid stress concentrations which could result in damage to the plastic. Minimal bone resection is required from the central part of the femur for installation of the femoral component. 
     The same tibial component can be advantageously used with either the PS or CR femoral component. For application to a symmetric shape of tibial baseplate, which is commonly used, the tibial bearing surface can have 5° of external rotation built in, providing an anatomic position of the femur on the tibia at 0° flexion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a posterior-medial perspective view of a posterior substituting (PS) femoral component and tibial component in accordance with a preferred embodiment of the present invention; 
         FIG. 2  shows an anterio-medial perspective view of the femoral component of  FIG. 1 ; 
         FIG. 3  shows a posterior view of the femoral component of  FIG. 1 ; 
         FIG. 4  shows an anterior top perspective view of the tibial component of  FIG. 1 ; 
         FIG. 5  shows a top view of the tibial component of  FIG. 1 ; 
         FIG. 6  shows a sectional view taken along a plane- 6  of the tibial component that shows construction of a ramp and a tibial bearing surface; 
         FIG. 7  shows a sectional view taken along a plane- 7  of the tibial component that shows construction of a central ramp portion of the tibial component of  FIG. 1 ; 
         FIG. 8  shows a sectional view taken along a plane- 8  passing through a raised pad in an anterior medial bearing surface; 
         FIGS. 9   a - 9   c  show nature of contact between the femoral component and the tibial component at 0° flexion along planes-A, B and C that pass through a medial portion, a central portion and a lateral portion respectively of the tibial component of the  FIG. 1 ; 
         FIGS. 9   d - 9   f  show nature of contact between the femoral component and the tibial component at 75° flexion along planes-A, B and C that pass through a medial portion, a central portion and a lateral portion respectively of the tibial component of the  FIG. 1 ; 
         FIGS. 9   g - 9   i  show nature of contact between the femoral component and the tibial component at 120° flexion along planes-A, B and C that pass through a medial portion, a central portion and a lateral portion respectively of the tibial component of the  FIG. 1 ; 
         FIG. 10A  shows a sectional view along plane-P that passes through the pad and recess in the anterior medial bearing surface of the femoral and tibial components of  FIG. 1  at 0° flexion; 
         FIG. 10B  shows a sectional view along plane-P that passes through the pad and recess in the anterior medial bearing surface of the femoral and tibial components of  FIG. 1  at 45° flexion; and 
         FIG. 11  shows a cruciate-retaining (CR) femoral component in accordance with the preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , the knee joint  10  in a zero degree flexion in accordance with a preferred embodiment of the present invention is shown. The knee joint  10  of the present invention includes a femoral component  12  and a tibial component  14 . Femoral component  12  is preferably connected to a predefined distal end of a femur. The tibial component  14  is preferably connected to a predefined proximal end of a tibia. 
     The femoral component  12  is a one-piece arcuate construction that includes a pair of convex femoral condyles  16  and a patella flange  18 . Condyles  16  and patella flange  18  converge inwardly to meet in the distal portion  19  of the femoral component  12  to form the arcuate shape. The pair of condyles  16  forms a proximal-posterior portion  20  and the patella flange  18  forms an opposed proximal-anterior portion  22 . An interior side of distal portion  19  has a pair of pegs  23 . The interior side of the distal portion  19  of femoral component  12  includes approximately centrally placed low profile hump  24 . 
     The tibial component  14  is also a one piece construction having a proximal end portion  25  and a distal end portion  26 . The proximal end portion  25  includes a receiving surface  28  that is defined by a periphery  30  of the tibial component  14 . The distal end portion  26  includes three pegs  32 . In this one preferred embodiment, the tibial component  14  is one-piece, however it is understood, that the tibial component  14  can be a bearing component that is fixable into a tibial tray or a base plate. 
     Referring to  FIGS. 2-3 , the proximal posterior portion  20  of the femoral component  12  includes medial condyle  34  and a lateral condyle  36 . The medial condyle  34  and lateral condyle  36  are separated by an intercondylar groove  38  that runs centrally from the proximal anterior portion  22  to distal portion  19  and continues to a vicinity of proximal posterior portion  20  of the femoral component  12 . An anterio-medial side  42  of the medial condyle  34  includes a smooth recess  44 . 
     The patella flange  18  has rounded edges  50  at the proximal-anterior portion  22  of femoral component  12 . 
     The central groove  38  has a depth that varies from the proximal-anterior portion  22  to the proximal-posterior portion  20 . The depth of the groove  38  decreases towards proximal-posterior portion  20  of femoral component  12  such that the groove  38  preferably becomes shallow between a portion defined by medial condyle  34  and lateral condyles  36 . 
     Groove  38  is preferably approximately 8 mm-12 mm deep in the distal portion  19  of femoral component  12 . Beyond the distal portion  19 , in an upward direction of the femoral component, the central height of the groove  38  progressively reduces, such that the groove depth reaches 0-4 mm at proximal posterior portion  20 . It is, however, understood that the femoral component  12  is preferably contoured and dimensioned to be a close match to that of the anatomic femur. 
     Now referring to  FIGS. 4-7 , the receiving surface  28  of the tibial component  14  has a lateral receiving surface  54  and a medial receiving surface  56 , that are separated by a central ramp  58 . An anterior medial bearing surface  60  includes a pad  62  that extends up to the anterior edge of the tibial component  14 . The tibial component  14  also includes a cutout  66  that is positioned approximately centrally in a posterior portion  68  of the tibial component  14 . 
     The medial bearing surface  56  transits smoothly to ramp  58  and continues smoothly to lateral bearing surface  54 . The radii of curvature R 1  of the lateral condyle  56  and R 2  medial condyle  54  are only 1-4 mm larger than the equivalent radii of curvature of the femoral condyles  34  and  36  in the same section position. 
     The ramp  58  has a curved medial side  70  and a curved lateral side  72 . Surfaces defined by the sides  70  and  72  of the ramp  58  smoothly slope towards the adjacent bearing surfaces. The sides  70  and  72 , as shown in  FIG. 7  (which is a cross section along line  7 - 7  of  FIG. 5  converge smoothly with the respective bearing surfaces. Ramp  58  has a height that decreases gradually from an anterior portion  74  to a portion  75  and then to posterior portion  76 . 
     H 1  indicates a height of the anterior portion  74  of ramp  58 . H 2  indicates a height of approximately central portion  75  of the ramp  58 . H 3  indicates a height of a junction of the central portion  75  of the ramp  58 . H 4  indicates a height of the posterior portion  76  of the ramp  58 . H 5  indicates a height of posterior portion  68  of tibial component  14 . H 1  is greater than H 2  which is greater than H 3 . H 3  is greater in height than H 4 . H 5  is preferably higher than H 4 . 
     The posterior portion  76  of ramp  58  smoothly and gradually meets with the cutout  66  in the posterior portion  68  of tibial component  14 . The height and slope of the sides  70  and  72  of ramp  58  advantageously guide femoral component  12  to displace posteriorly during flexion and displace anteriorly during extension. The tibial receiving surface  28  has preferably approximately 5° of external rotation when used with a symmetric baseplate. The distal portion  26  of tibial component is preferably contoured in a predefined smooth shape. 
     In this preferred embodiment, the distal portion  26  includes three fixation pegs  32 , it is, however, understood that many other fixation means known in the art can be used. 
     Now referring to  FIG. 8 , (which is cross section of  FIG. 5  at  8 - 8 ) the medial bearing surface  56  defined by an arc ABD is shown. Arc AB with a center at point at P has a constant radius that is indicated by R 3 . The arc AB has been extended to BC with same radius. Arc AB meets with arc BD at point B. The arc BD has a radius which is greater than R 5  which matches the distal-anterior radius of the femoral component in extension. However, if this radius was used for tibial surface, in flexion, the femoral radius R 3  would readily slide up the slope BD. The pad  62  on the tibial surface is constructed with radius R 4 , between 0-3 mm greater in radius than R 3 . 
     The pad  62  with posterior portion  80  and anterior portion  82  locates within smooth femoral recess  44 . The pad  62  is thus in close proximity with the medial femoral condyle  34  in early part of flexion. This limits the anterior sliding of the medial side of the femoral component up the ramp along slope BC. This anterior sliding is also called as paradoxical motion, because in the anatomic knee, anterior sliding does not occur, especially on the medial slide. 
     The posterior portion  80  of the pad  62  is such that a profile formed by the posterior portion  80  is tangential to the arc BC having radius R 3 . The pad  62  smoothly joins with the medial bearing surface  56 . The pad  62  has a predefined height that is in accordance with the recess  44  on the femoral component  12 . Radius R 4  of the pad  62  in the posterior portion  80  is preferably 0-3 mm greater than R 3 , such that it is a close match to the recess  44  of the femoral component  12 . R 5  is the radius of the femoral component  12  in that region if the recess  44  was not present. It will be appreciated that if the pad and recess were not present, in flexion, radius R 3  of the femoral component could easily slide anteriorly up the slope of R 5 . In this preferred embodiment, femoral component  12  and tibial component  14  are preferably made of metal and a polymer material, respectively. The interface between femoral component  12  and receiving surface  28  is intended to provide the extended wear characteristics that are desirable in a knee replacement. 
     Now referring to in  FIGS. 9   a - 9   i , the behavior of condylar and tibial contact at various angles of flexion are described with  FIGS. 9   a, d  and  g  at cross section AA in  FIG. 4 ;  FIG. 9 , b, e and h at cross section BB in  FIG. 4 ; and  FIGS. 9   c, f , and  i  at cross sections CC in  FIG. 4 . At 0° flexion ( FIG. 9   a - c ). The contact on the lateral side is towards the anterior of the tibial receiving surface  26  that is indicated by C 1  such that an approximately complete conformity exists between the femoral component  12  and tibial component  14 , to provide a large area of contact C 1 . The section through the center at 0° flexion  FIG. 9   b  indicates that the femur will rock on the anterior platform  59  on the tibia if the femur is hyperextended. On the medial side, there is a large area of contact C 2  of the interacting femoral and tibial surfaces, a larger area of contact than on the lateral receiving surface. 
     As the flexion angle increases from zero to 75°, the lateral contact C 4  moves posteriorly while the central contact C 5  and medial contact C 6  also moves posteriorly. By 75° flexion, the central groove  38  of the femoral component  12  interacts with central ramp  58  of the tibial component  14 . The contact C 6  with the medial bearing surface moves posteriorly at 75° flexion by approximately 1 mm-2 mm. 
     From approximately 60° to 120° flexion and beyond, the contact on the lateral side moves further posteriorly, however the contact is still approximately 10 mm from a posterior edge of tibial component  14  to avoid damaging the edge. The contact between the femoral central groove  38  and the tibial ramp  58  is maintained in during the 60° to 120° flexion which results in the posterior displacement. 
     The lateral contact C 7  moves posteriorly to the posterior portion  68  of the tibial component  14  at 120° flexion. The central contact is along the sloping ramp  58  during flexion at 120° as indicated by C 8 . On the medial side at 120° flexion and beyond, the contact C 9  moves posteriorly 2 mm-4 mm to the posterior portion of the dished tibial surface  56 . Due to the dishing of the medial tibial surface  56 , at each angular position of flexion, the anterior and posterior stability of femoral component  12  on the medial tibial surface  56  is maintained. 
     In this one preferred embodiment, central groove  38  initially makes contact with ramp  58  at approximately 60° flexion as the posterior femoral displacement is preferably initiated at flexion angle of approximately 60°. As femoral component  12  flexes from 60° to maximum, the contact on the ramp  58  is towards the lower part of the ramp  58  to provide maximum stability. 
     The interaction between the ramp  58  and the femoral central groove  38  is such as to induce posterior displacement of the femoral component  12 . However, because of the stability on the medial side, the motion is effectively a rotation of the femoral component about points on the medial side, so-called medial pivot motion. This resembles natural anatomic motion. 
     At 120 degrees flexion and beyond, the contact location of femoral component  14  on the lateral tibial bearing surface  54  is advantageously at least 8 mm and as much as approximately 10-12 mm from the posterior of the tibial component  14 . The internal-external rotational laxity of the femur on the tibia is achievable without the femur contacting the extreme posterior edge of tibial component  1 ,  4  even at high angles of flexion. The rotational laxity is an important characteristic of anatomic knee motion. 
     Referring to  FIGS. 10A and 10B , the interaction of the recess  44  and pad  62  is described. The recess  44  and pad  62  fit together at 0° flexion. When femoral component  12  is flexed up to approximately 45°, for example, the medial condyle  34  articulates towards the steep pad  62 , preventing or limiting the anterior translation of femoral component  12  on tibial component  14 . The interaction of the pad  62  and recesses  44  prevents the femoral component  12  from skidding forward on the tibial component  14  during early flexion. 
     The edges of the recess  44  and pad  62  are advantageously rounded to avoid catching while the alignment of the knee is not exactly central as the knee is extended. The width of the recess  44  on the femoral component  12  is such that the recess  44  does not interfere with the region that the patella traverses on the patella flange  18 . However, it is understood, that the medial recess  44  has sufficient space as the patella bearing area is less extensive on the medial side, in contrast to the lateral side where it is more extensive. 
     Referring to  FIG. 11 , in one more embodiment of the present invention, the PS femoral component  12  is replaced by a CR (cruciate retaining) femoral component  90 . The CR femoral component  90  is approximately identical to the PS (posterior stabilized) femoral component  12  except for a central posterior portion  92 . A distal portion  94  of the CR femoral component  90  includes a contoured and smooth platform  96  that is approximately centrally positioned between a pair of pegs  98  that are mounted in an interior portion  99  of the distal portion  94 . 
     The CR femoral component  90  includes a central cutout  100  that is defined by smooth sides  102 ,  104  of the condyles  106 , and side  108  of the platform  96 . The sides  102 ,  104  and  108  are preferably smooth and rounded. In this preferred embodiment, the cutout  100  runs from the distal portion  94  towards the proximal posterior portion  110 . The purpose of the cut-out is to provide free access of the posterior cruciate ligament. The posterior cutout or recess  66  on the tibial component similarly provides such access. It is, however, understood that the posterior displacement of the CR femoral component on the tibial component is now provided by the posterior cruciate ligament (PCL) while the medial pivotal rotation is still provided by the deeper and more dished medial tibial bearing surface  56  and shallower lateral tibial bearing surface  54 . 
     In one another embodiment of the present invention, the tibial component  14  is mounted on a metal base plate. The base plate is fixed to the upper tibia such that the tibial component  14  rotatably flexes on the surface of the base plate, which is preferably called as a rotating platform design. 
     The femoral component  12  and the tibial component  14  of the knee joint are produced with known techniques in the art. It is, however, known in the art that smooth surfaces are preferably lofted from a multiplicity of frontal or sagittal plane sections. To generate this for the tibial surface, the outer surface of the geometrically defined femoral component  12  first positioned at zero degree flexion. The surface is then reproduced at a multiplicity of flexion angles. The position at each flexion angle is specified as being on the neutral path of motion resembling the motion of the anatomic knee. 
     The composite of the femoral positions at a full range of flexion up to, for example, in 135 degrees flexion is developed. The lower imprint of the composite of femoral surface is then made using a drape function to smooth the surface. The entire imprint is externally rotatable by approximately 5° relative to the tibia. This is done in order to reproduce the anatomic position of the femur on the tibia, when a symmetric tibial base plate is used. The required peripheral outline of the tibia is then superimposed on the imprint. 
     This peripheral outline is then treated as a ‘cookie cutter’ to cut a surface shape for the upper tibial surface. The surface is then extended downwards to create the tibial component  12 . The resulting tibial surface advantageously accommodates all of the motions required between the femoral component  12  and the tibial component  14  in the neutral path of motion. The tibial receiving surface  28  is preferably manufactured exactly from the surface model itself using numerical methods. Approximations could be made to the surface using defined radii and solids to permit more traditional machining processes to be used. 
     Now referring to  FIGS. 1 to 11 , one advantage of the ramp  58  of the tibial component  14  is that the knee is foolproof to possible dislocation. Even in high flexion when there is considerable laxity in the joint  10  and high shear or varus-valgus moments are applied. The central groove  38  of the femoral component  12  could slide up the ramp  58  in such extreme conditions, but on relief of the forces, the femoral component  12  would simply slide back down the ramp  58  again to a stable position. The posterior displacement of the PS femoral component  12  on the tibial component  14  is obtained by the interaction between central groove  38  and central ramp  58 . 
     The bearing receiving surface  28  advantageously receives and guides the position and motion of femoral component  12 . The tibial bearing surfaces  54  and  56  are smooth and do not have corners or edges that can interact with the femoral component  12  and cause damage to the structure. 
     The provision of a medial femoral recess  46  and tibial pad  62  limits the anterior sliding of the femur on the tibia to advantageously avoid the paradoxical movements. The recess-pad arrangement increases the anterior conformity of the medial bearing surfaces  56  from 0°-60° flexion that limits the anterior displacement of the femoral component  12  on the tibial component  14  to limit paradoxical motion. 
     Posteriorly located cutout  66  on the tibial component  14  matches the shape of the upper tibia and allows access of the PCL. The groove  38  is shallow in the medial condyle  34  and lateral condyles  36  of posterior portion such that the depth is not reduced to a groove depth of zero to provide medial-lateral stability in high flexion. 
     In the preferred embodiment, the tibial receiving surface  28  has 5° of external rotation built in, that provides an anatomical position of the femur on the tibia at 0° flexion, preferably when a symmetric tibial base plate is used. Rounded edges of the femoral component  50  and  52  advantageously provide smooth sliding of soft tissues including muscle, tendon and capsular tissue, over the edges of the femoral component  12 . 
     Large contact surface areas between the femoral components  12  or  90  and the tibial component  14  minimize wear and deformation of the polyethylene surfaces. Additionally, smooth contours of the central ramp  58  avoid stress concentrations and digging in at corners, which can result in damage to the plastic. The hump  24  is adapted to accommodate a distal portion of the groove  38 . 
     An important feature of the present invention is that the tibial component  14  may be used with PS component  12  and with CR component  90 . The knee joint  10  of the present invention requires only one tibial component  14  for either Cruciate Retaining femoral component  90  or Posterior Stabilized (PS) femoral component  12 . Cutout  66  in the tibial component  14  allows the retained posterior cruciate ligament (PCL) to go through both recess/notch  44  and femoral cutout  66  when used with CR tibial component  90 . 
     In the preceding specification, the invention has been described with reference to specific exemplary embodiments thereof. It will be evident, however, that various modifications, combinations and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the claims that follow. It is understood that the present invention can combine one or more novel features of the different embodiments. The specification and drawings are accordingly to be regarded in an illustrative manner rather than a restrictive sense.