Abstract:
A trochlear implant for a prosthetic knee, a prosthetic knee system and a method for using the prosthetic knee system are disclosed. The trochlear implant mounts to the knee end of a femur and cooperates with a patellar implant mounted to the back side of a patella. The patellar implant is a component of a prosthesis including a femoral implant for replacing the knee end of the femur. The trochlear implant has an articulation surface shaped to slidably receive a portion of the patellar implant. The articulation surface is substantially similar in shape to a portion of a surface of the femoral implant such that the patellar implant is capable of being used with the femoral implant and the trochlear implant.

Description:
BRIEF DESCRIPTION OF THE INVENTION 
     The present invention relates in general to a modular knee replacement system for a prosthetic joint, and more particularly to a system including a trochlear implant. 
     BACKGROUND OF THE INVENTION 
     A knee joint connects three bones: the femur (thigh bone), the tibia (leg bone), and the patella (knee cap). Either through disease, injury, or pre-mature wear from mal-alignment, the knee joint can be damaged, and all or portions of the damaged joint surfaces may need to be replaced with a prosthesis. The most common prosthetic knee is referred to as a “total knee replacement” system because all knee joint surfaces are replaced. Typically the total knee replacement system includes a patellar prosthesis or implant, a femoral prosthesis or implant and a tibial, prosthesis or implant. 
     Different portions of the knee are referred to as compartments. For example, each condyle (rounded end of the femur) is a separate compartment. Other prostheses, called “unicompartmental” replacements, can be used when only the medial (toward the body&#39;s midline) compartment or the lateral (away from the body&#39;s midline) compartment of the femoral-tibial surface needs to be replaced. The unicompartmental replacement systems have femoral and tibial prostheses but do not include a patellar implant. Currently available unicompartmental implant systems are designed to be modular and work with their total knee counterparts from the same manufacturer with respect to using the same instruments and same bone contour. When these unicompartmental prosthetic systems need to be replaced with a total knee system, both the femoral and tibial implants must be removed. 
     Less frequently, the surface of the knee joint compartment between the patella If, and the groove on the front of the femur the (trochlea) requires replacement. Several prosthetic implants are available which replace this part of the knee joint and are called “total patello-femoral” prostheses or implants. Typically the total patello-femoral prostheses have a patellar implant that is installed on the patella and another implant which replaces the portion of the femur which contacts the patella(the trachlea). 
     However, there is no total patello-femoral prosthetic system which is modular with any total knee system. Therefore, if a total patello-femoral prosthesis has been implanted, and because of further deterioration in the rest of the knee (possibly years later), during a then second operation both the trochlear implant and the patellar prosthesis must be removed even though the patellar prosthesis is functioning well. The only reason for removing said patellar prosthesis is that its articular surface would not now match or articulate smoothly with the new femoral prosthesis being implanted. 
     A prosthesis system which does not require replacement of the patella implant when replacing the patello-femoral prosthesis with a total knee prosthesis is desirable. In particular, a trochlear implant which is applied to the surface of the trochlear groove of the femur and which cooperates with the components of the selected total knee implant is desirable. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is a primary object of the present invention to provide a prosthesis system which does not require replacement of the patella implant when replacing the trochlear implant with a full femoral implant. 
     It is another object of the present invention to provide a trochlear implant which cooperates with the components of the selected total knee implant system. 
     A more general object of the present invention is to provide a trochlear implant for a prosthetic joint that reduces the amount of bone removed for the implant. 
     It is yet another object of the present invention to provide a modular joint replacement system having interchangeable components. 
     In summary the present invention provides a prosthetic knee of the type which includes a femoral implant or a trachleas implant and a patellar implant. The femoral implant and the patellar implant have bearing surfaces that articulate with each other when the femoral implant and patellar implant move relatively. The trochlear implant has an articulation surface shaped to articulate with a portion of the bearing surface of the trochlear implant of the patellar implant. The articulation surface is substantially similar in shape to a portion of the bearing surface of the femoral implant such that the patellar implant is usable with either the femoral implant or the trochlear implant. 
     The present invention also provides a trochlear implant for use in a prosthetic knee. The trochlear implant mounts to the knee end of a femur and cooperates with a patellar implant mounted to the back side of a patella. The patellar implant is a component of a prosthesis system including a femoral implant for replacing the entire knee end of the femur. The trochlear implant has an articulation surface shaped to slidably receive a portion of the patellar implant. The articulation surface is substantially similar in shape to a portion of a surface of the femoral implant such that the patellar implant is capable of being used with either the femoral implant or the trochlear implant. 
     In addition, the present invention provides a method of knee replacement using a prosthetic knee system. A trochlear implant and a patellar implant are provided. The patellar implant cooperates with the trochlear implant and a femoral implant. The trochlear implant has an articulation surface shaped to slidably receive a portion of the patellar implant. The articulation surface is substantially similar in shape to a portion of a load bearing surface of a femoral implant. The patellar implant is installed in a patella in a knee. The trochlear implant is installed in the trochlear groove in the knee end of a femur bone. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be clearly understood from a reading of the following description in conjunction with the accompanying drawings. 
     FIG. 1 is a front view of an installed prior art total knee prosthesis. 
     FIG. 2 is a side view of the installed prior art total knee prosthesis of FIG.  1 . 
     FIG. 3 is an exploded front view of the components of the prosthetic knee implant system of the present invention. 
     FIG. 4 is a perspective view of one embodiment of the trochlear implant of FIG. 3 installed in a human knee. 
     FIG. 5 is a perspective view of an embodiment of the trochlear implant of the present invention suitable for installation in a left knee. 
     FIG. 6 is a side view of the trochlear implant of FIG.  5 . 
     FIG. 7 is a bottom view of the trochlear implant of FIG.  5 . 
     FIG. 8 is a front view of the trochlear implant of FIG. 5 installed in a human knee. 
     FIG. 9 is a side view of the trochlear implant of FIG. 5 installed in the human knee. 
     FIG. 10 is a diagram of a common generation curve used in a preferred embodiment of the trochlear implant of FIG.  3 . 
     FIG. 11 is a diagram showing the rotation of the common generation curve of FIG. 10 to generate the segments of surfaces of revolution that define the shape of the articulation surface of the trochlear implant and that define the shape of the femoral load bearing surface of a femoral implant, and that also define the shape of a load bearing surface of the patellar implant. 
     FIG. 12 is a bottom view of the trochlear implant of FIG. 5 showing the angles used to taper the peripheral edges. 
     FIG. 13 is an exploded view of a second prosthetic knee system using another embodiment of the trochlear implant of the present invention. 
     FIG. 14 is an exploded view of a third prosthetic knee system using yet another embodiment of the trochlear implant of the present invention. 
     FIG. 15 is an exploded view of a fourth prosthetic knee system using another alternative embodiment of the trochlear implant of the present invention. 
     FIG. 16 is a flowchart of a method of knee replacement using the components of the prosthetic knee system of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, a prosthetic knee implant  100  suitable for use with the present invention is installed in a human knee  102 . The knee  102  has a femur  104 , a patella  106  and a tibia  108 . The prosthetic knee  100  includes a femoral implant  110 , a patellar implant  112  and an optional tibial implant  114 . The tibial implant  114  has a tibial platform component  116  and a tibial bearing component  118 . 
     The patellar implant  112  is shaped to mate with the femoral implant  110 . The tibial implant  114 , and in particular the tibial bearing component  118 , is shaped to mate with the femoral implant  110 . The femoral implant  110  is shaped to slidably receive the patellar implant  112  and the tibial implant  114 . 
     FIG. 2 is a side view of the installed prosthetic knee of FIG.  1  and also shows a tendon  120  that attaches the muscles to the patella  106 . The femoral implant  110  and the patellar implant  112  have bearing surfaces,  122  and  124 , respectively. When the femoral implant and patella implant move relative to each other, the bearing surfaces  122  and  124  allow the femoral implant  110  and patella implant  112  to slidably engage. The tibial implant  114  also has a bearing surface  126  that slidably engages another portion of the femoral bearing surface  122  when the femur  104  and tibia  108  move relative to each other. 
     As shown in FIG. 3, the prosthetic knee system of the present invention includes a femoral implant  110 , a patellar implant  112  and a trochlear implant  130 . The trochlear implant  130  has an articulation surface  132  shaped to slidably receive a portion of the bearing surface  124  of the patellar implant  112 . The patellar implant  112  has an apex  134  that aligns with a longitudinal axis  136  forming a trochlear groove in both the femoral implant  110  and the trochlear implant  130 . 
     A noteworthy aspect of the invention is that the articulation surface  132  of the trochlear implant  130  is substantially similar in shape to a portion of the femoral bearing surface  122  of the femoral implant  110 . Therefore, the patellar implant  112  is usable with both the femoral implant  110  and the trochlear implant  130 . When a patient&#39;s remaining joint surface deteriorates to the point where the physician needs to replace the trochlear implant  130  with the femoral implant  110 , the patient&#39;s patella is not subjected to additional bone loss and trauma because the existing patellar implant  112  is usable with the new femoral implant  110 . 
     Fundamental differences among knee prostheses are found in the nature of the articulation or bearing surfaces. There are two basic types of articulation surfaces: those surfaces with theoretical line or point contact (referred to as incongruent contact), and those surfaces with area contact (referred to as congruent contact). Those surfaces with congruent contact more closely resemble the human body. 
     In a preferred embodiment, the articulation surface  138  of the trochlear implant  130  has a shape that substantially congruently engages the patellar implant  112 . In an alternate embodiment, the articulation surface  132  of the trochlear implant  130  is shaped to provide substantially point contact with the patellar implant  112 . In another alternate embodiment, the articulation surface  132  of the trochlear implant  130  is shaped to provide substantially line contact with the patellar implant  112 . In other embodiments, the articulation surface  132  of the trochlear implant  130  is shaped to provide a combination of congruent and line contact with the patellar implant  112 . 
     In one embodiment, the trochlear implant  130  is asymmetrical about the longitudinal axis  136  for installation in either a right knee or a left knee. In FIG. 3, the trochlear implant  130  is for installation in a right knee. In an alternate embodiment, the trochlear implant  130  is symmetrical about the longitudinal axis and can be installed in either the right knee or the left knee. 
     In a preferred embodiment, the bearing surface of the femoral implant  110 , the bearing surface of the patellar implant  112  and the articulation surface of the trochlear implant  130  are aligned to and are generated by a common generation curve  138 , FIG.  3 . Alternately, the articulation surface of the trochlear implant  130  is shaped to receive and engage a substantially spherical, dome-shaped patellar implant  112 . 
     The trochlear implant  130  may be made of cast cobalt-chrome-molybdenum and the articulation surface  132  is polished. Alternately, the trochlear implant  130  is made of cobalt-chrome, stainless steel or other suitable metal alloy. In another alternative embodiment, the trochlear implant  130  is made of a ceramic. In yet another embodiment, the trochlear implant  130  is made of titanium. In another alternate embodiment, a surface treatment is applied to harden and/or smooth the articulation surface of the trochlear implant  130 . In particular, a trochlear implant  130  made of titanium is treated to harden and smooth the articulation surface. 
     The trochlear implant  130  is installed in the trochlear groove between the condyles  142  of a knee-end of a femur  104  in a human knee  100 , FIG.  4 . In this embodiment, the trochlear implant  130  does not contact the tibia  108  or a tibial implant. In an alternate embodiment, the trochlear implant  130  can contact the tibia  108  or the tibial implant. 
     Referring to FIGS. 5,  6  and  7 , a peripheral edge  152  defines and forms the articulation surface  132  and a back surface  154 . In FIG. 7, the back surface  154  has a cement retaining rim  156  extending along a portion of the back surface  154  and portion of the peripheral edge  152 . Three fixation pins  158  project from the back surface  154 . In one embodiment, the back surface  154  is textured. Alternately, the back surface  154  has a porous coating. In an alternate embodiment, no cement retaining rim  156  is provided. In another alternate embodiment, no fixation pins are provided. In yet another alternate embodiment, any number, such as one, two, or more than three, fixation pins  158  project from the back surface  154 . Alternately the trochlear implant  130  is fixed to the femur using screws. 
     In FIGS. 8 and 9, the trochlear implant  130  of the present invention is shown in a human knee with the patellar implant  112 . The trochlear implant  130  contacts the patellar implant  112  for a range of knee motion. Primary and secondary load bearing regions,  162  and  164 , respectively, on the patellar implant  112  engage primary and secondary load bearing regions,  166  and  168 , respectively, of the articulation region  132  of the trochlear implant  130  to produce substantially anatomical patella-femoral articulation. As shown in FIG. 9, at full extension of the leg and knee, the primary load bearing region  162  of the patellar implant  112  lifts off the primary load bearing region  166  of the trochlear implant  130 , and the secondary load bearing region  164  of the patellar implant  112  slidably engages the secondary load bearing region  168  of the trochlear implant  130 . In contrast, at moderate and full flexion, the primary load bearing region  162  of the patellar implant  112  slidably engages the primary load bearing region  166  of the trochlear implant  130 . 
     In an alternate embodiment, at the extremes of knee motion, either when the knee is very straight or when the knee is extremely bent, the patellar implant  112  does not contact the trochlear implant  130 . Preferably, the patellar implant  112  slidably engages the trochlear implant  130  from an angle of about 20° when the knee is almost straight to an angle of about 110° when the knee is bent. 
     FIG. 10 shows the common generation curve  138  that is used to generate the shapes of the articulation and load bearing surfaces of the trochlear implant, femoral implant, patellar implant and tibial implant in a preferred embodiment of the present invention. The formation of the load bearing surfaces of the femur, patellar and tibial implants is described in detail in U.S. Pat. No. 4,470,158 to Pappas et al. which is incorporated herein by reference. 
     The primary and secondary load bearing regions of the articulation and load bearing surfaces are formed as surfaces of revolution and their shape is defined or generated by the common generation curve F  138 . The shape of the load bearing or articulation surfaces is defined by rotating the common generation curve F  138  through a predetermined angle about the generating axis  172  at the same major generating radii D 1  and D 2  where D 1  and D 2  are equal to each other and also equal to a predefined radius. The peak  174  of the common generation curve F  138  forms the apex  134  (FIG. 3) of the patellar implant and the longitudinal axis  136  (FIG. 3) of the femoral implant and the trochlear implant. 
     FIG. 11 shows the segments S 1  and S 2  of the load bearing regions of the load bearing surface and articulation surface of the femoral implant and trochlear implant of the present invention, respectively. Segment S 1  forms the secondary load bearing region  164 ,  168  (FIG.  9 ). Segment S 2  forms the primary load bearing regions  162 ,  166  (FIG. 9) of the patellar implant and trochlear implant, respectively. 
     In particular, to generate the articulation region of the trochlear implant. The common generating curve  138  is rotated at an angle of θ1, equal to 0°, at a radial distance from generating axis C 1  at ∞. In other words, the common generating curve  138  is substantially parallel to the line L 1  for a distance of S 1  or 0.314 inches. Tangent to line L 1 , the common generating curve  138  is rotated about generating axis C 2  for an angle of θ2 at a radial distance of R 2 . In one embodiment, θ2 and R 2  equal about 90° and 1.388 inches, respectively. The shape of the trochlear implant ends at line L 2 . 
     The load bearing regions of the patellar implant and femoral implant are generated in a similar manner. For the femoral implant, segment S 1  is formed substantially parallel to the line L 1  for a distance of 0.612 inches, and segment S 2  is formed for an angle θ2 of  107 . 75° with R 2  equal to 1.388 inches. Therefore, the trochlear implant load bearing regions  148 ,  146  (FIG. 9) formed with segments S 1  and S 2 , respectively, substantially match the load bearing regions S 1  and S 2  of the femoral implant. 
     Referring to FIG. 12, the trochlear implant  130  is tapered. To taper the trochlear implant  130 , a portion of the common generation curve  138  is used and peripheral side edges  176 ,  178  of the trochlear implant  130  are formed at predetermined angles θ3 and θ4 with respect to the longitudinal axis  136 . In one embodiment, the predetermined angles θ3 and θ4 are substantially equal to about 20° and 30°, respectively. In an alternate embodiment, θ3 and θ4 are the same. 
     FIG. 13 shows a second prosthetic knee system, such as the Johnson &amp; Johnson “PRIMARY CRUCIATE-SUBSTITUTING” (P.F.C.) modular total knee system, with a second embodiment of the trochlear implant  182  of the present invention. A patellar implant  184  is usable with either the femoral implant  186  or the trochlear implant  182 . 
     FIG. 14 shows a third prosthetic knee system, such as the Intermedics “NATURAL-KNEE,” with a third embodiment of the trochlear implant  188  of the present invention. A patellar implant  190  is usable with either a femoral implant  192  or the trochlear implant  188 . 
     FIG. 15 shows a fourth prosthetic knee system, such as the Zimmer “Insall/Burstein (I/B) II” modular knee system, with a fourth embodiment of the trochlear implant  194  of the present invention. A patellar implant  196  is usable with either a femoral implant  198  or the trochlear implant  194 . 
     In FIG. 16, a flowchart of an embodiment of a method of knee replacement using the components of the prosthetic knee system is shown. In step  202 , a trochlear implant of the present invention is provided, and in step  204 , a patellar implant suitable for use with the present invention is also provided. In step  206 , the patellar implant is installed in a patella in a knee. In step  208 , the trochlear implant is installed in the trochlear groove in a knee-end of a femur bone. 
     When a patient&#39;s remaining joint surface deteriorates to the point where the trochlear implant needs to be replaced, in step  210  a femoral implant suitable for use with the trochlear implant is provided. The shape of the articulation surface of the trochlear implant and the shape of the load bearing surface of the femoral implant are substantially similar. In step  212 , the trochlear implant is removed, and in step  214  the existing patellar implant is left in place in the patella. In an alternate embodiment, the patellar implant has a detachable load bearing surface, however, even in such patellar implants, the portion of the patellar implant that is attached to the patella bone remains in place. 
     In step  216 , the surgeon installs the femoral implant in the knee-end of the femur bone. If the tibia portion of the knee also needs to be replaced, a tibial implant is also provided in step  218 , and installed in step  220 . 
     Thus, there has been provided a prosthetic knee system that allows the same patellar implant to be used with both a trochlear implant and a femoral implant. Therefore, when a patient&#39;s remaining joint surface deteriorates to the point where the physician needs to replace the trochlear implant with the femoral implant, the patient&#39;s patella is not subjected to additional bone loss and trauma because the existing installed patella implant is usable with the new femoral implant. 
     While the invention has been described in detail and with reference to specific examples, it will be apparent to one skilled in the art that various trochlear implant shapes can be made without departing from the spirit and scope of the present invention.