Abstract:
A patellar implant is provided being of a two part construction having a strong base portion  32, 54, 72  and  92  (manufactured from a metal material or other medical grade strong material) with a molder outer material  34, 62, 80  and  100  (manufactured from a polymeric material or other softer, smoother material) at least on one side to provide a smooth friction surface to contact the femoral portion of a patient&#39;s knee.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a U.S. National Phase of International PCT Application No. PCT/US2011/047597 filed on Aug. 12, 2011, which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/373,585 filed on Aug. 13, 2010, the contents of each application hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to patellar implants that are designed to form a patellar portion (or knee cap) that replaces a part of a natural knee cap, and particularly to patellar implants that are designed to cooperate and articulate against a femoral component of a total or partial knee prosthesis. 
     BACKGROUND OF THE INVENTION 
     Joint replacement, and particularly knee replacement, has become increasingly widespread. Various knee prostheses and procedures have been developed to treat the debilitating effects of knee joint deterioration (e.g., such as that caused by arthritis, injury, or disease). A fairly common procedure used to repair a patient&#39;s knee is a total knee replacement, in which the tibia is resected and replaced with a tibial component, and the femur is resected and replaced with a femoral component. In some instances, the surgeon will also replace the articulating surface on the posterior aspect of the patella where it interfaces with the femoral component, which can help improve the results of the total knee replacements. A surgeon may also elect to perform a partial knee replacement where only some portions of the patient&#39;s knee will be replaced. It may also be desirable for the surgeon to replace the articulating surface of the patella in a partial knee replacement. 
     SUMMARY OF THE INVENTION 
     The various embodiments of the present invention described below and shown in the Figures provide patellar implants that are optimally shaped and designed to provide a two part implant having a strong base (metal or other significantly strong material) with a molded outer material (polymer or other softer, smoother material) at least on one side, to provide a smooth friction surface to contact the femoral portion of a patient&#39;s knee. 
     Further areas of applicability of the invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the particular embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a side perspective view of how a patellar implant may be secured to a portion of a natural patella. 
         FIG. 2  shows a top perspective view of a patellar implant according to one embodiment of the invention. 
         FIG. 3  shows a top perspective view of the base portion of the patellar implant. 
         FIG. 4  shows a bottom plan view of the base portion of the patellar implant. 
         FIG. 5  shows a top perspective view of the molded or snap fit polymer portion of the patellar implant. 
         FIG. 6  shows a bottom perspective view of the patellar implant with the base portion and polymer portion connected together to form the patellar implant. 
         FIG. 7  shows a side cross-sectional view of the patellar implant taken along line  7 - 7  shown in  FIG. 6 . 
         FIG. 8  shows a side cross-sectional view of an alternate embodiment of the patellar implant of the present invention. 
         FIG. 9  shows a side view of the patellar implant of the present invention in position under the skin and adjacent a patient&#39;s femoral bone portion. 
         FIG. 10  shows a side cross-sectional view of an alternate embodiment of the patellar implant of the present invention. 
         FIG. 11  shows an enlarged view of a portion of the patellar implant shown in  FIG. 10 . 
         FIG. 12  shows a bottom view of yet another alternate embodiment of the patellar implant of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     For the purpose of promoting an understanding of the principles of the present invention, reference will now be made to embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is being intended. Any alternatives and further modifications in the described embodiments, and any further applications of the principles of the invention as described herein are contemplated as would normally occur to one skilled in the art to which the invention relates. 
     Embodiments of the present invention provide patellar prostheses that are designed to form a patella portion or knee cap that replaces a part of a natural knee cap.  FIG. 1  shows the state of the art on how a patellar implant would typically be secured to a portion of a natural patella. The patient&#39;s natural knee bone is shown; the femoral bone portion  10  with a femoral knee implant  12  shown attached to the femor; a tibial bone portion  14  with a tibial tray implant  16  and spacer  18  is shown attached to the tibia; and a patellar bone portion  20  is shown having a patellar implant  22  being attached to the patellar bone portion  20 . It is common practice to not remove the whole patellar bone, or knee cap when doing a knee implant. It is always desirable to leave as much natural bone as possible. So typically, a patellar implant will only be affixed to the inside portion of the patellar bone portion to provide a smooth surface which will then interact with the femoral knee implant  12 . 
     In a first embodiment, shown in  FIGS. 2-7 , a patellar implant  30  is shown having a base portion  32  made of a metal (i.e., titanium, titanium alloy, zirconium, chrome cobalt, stainless steel or other medical grade material, such as a ceramic) having an appropriate rigidity to prevent bending when positioned within a patient&#39;s body and a polymeric portion  34  made of an appropriate polymeric material which is molded or snap fit onto the base portion of the patellar implant  30 . The polymeric portion  34  may be made of a polyethylene or a cross-linked polyethylene. References to  FIGS. 3 and 4 , the base portion  32  has a circular ring  36  with depending pegs  38  having a porous coating or surface treatment. The porous coating or surface treatment allows for the ingrowth of the patient&#39;s natural bone growth when the patellar implant is implanted into a portion of the patient&#39;s natural patella  20 . It is also possible that the entire base portion  32  be manufactured out of a porous material to allow for both the polymeric portion to be molded to such porous surface as well as to allow boney tissue ingrowth to those areas not covered by the polymeric portion. Reference to  FIGS. 5 and 6 , the polymeric portion  34  is shown either molded or snap fit onto the base portion  32  to form a patellar implant  30 . The polymeric portion  34  may be overmolded, compression molded, injection molded, or cold molded (or any other molding process may be used) over the base portion to complete the patellar implant  30 . In use, the pegs  38  of the patellar implant  30  may be press fit, glued with bone cement or otherwise adhered into the remaining natural portion of the patella  20 .  FIG. 7  shows a cross sectional view of the patellar implant  30  with the polymeric portion  34  molded or snap fit about the ring  36 . 
     In the embodiment shown in  FIGS. 2-7 , the base portion  32  is shown with three depending pegs  38 . However, it may also be desirable to have any number of pegs and the pegs can be of various shapes including various cruciform shapes. It may be desirable to have a single peg that has a plurality of spokes (not shown) extending from a portion of such pegs back to the annular ring portion  36 . The ring portion  32  may also have various cutouts or notches provided about its periphery. The ring portion  32  could also be a discontinuous ring depending on the overall thickness and diameter of the ring portion  32 . 
       FIG. 9  shows the patellar implant  30  of the present inventions shown connected to the remaining natural portion of a patient&#39;s patella and in position adjacent the patient&#39;s femoral knee implants  12 . As can be seen in  FIGS. 1 and 9 , the femoral bone portion  12  moves continuously with respect to the patient&#39;s patellar implant  30  in the course of normal leg movement. It is therefore desirable that the patellar implant  30  allow the femoral knee implant  12  and bone portion  10  to move freely and smoothly throughout the full range of motion by a patient during normal daily activities. 
     In an alternate embodiment,  FIG. 8  shows a patellar implant  50  having a base portion  52  with a central flat ring portion  54  with depending pegs  56  and an annular ring portion  58  positioned on the upper surface  60  of the flat ring portion  54 . The base portion  52  can be made of a metal or stiff portion having a porous coating or surface treatment. The base portion  52  can also be made of a porous material throughout as long as such material is strong enough to prevent bending of the patellar implant  50  when the patellar implant  50  is positioned within the patient&#39;s knee construct. The polymeric portion  62  of the patellar implant  50  is then molded or snap fit over the body of the base portion  52  covering the flat ring portion  54  and annular ring portion  58 . The annular ring  58  imparts a significant strength component to the overall implant  50  allowing the base component  52  to be as thin as possible to allow for a maximum thickness of the polymeric portion  62 . The rounded edges of the annular ring portion  58  and central flat ring  54  assist in minimizing stress concentrations to resist any metal/polymeric material de-bonding or separation. 
       FIG. 10  shows yet another embodiment of the present invention wherein the base portion  70  of the patellar implant  68  is manufactured out of a porous metal material having an upper surface  78 , a lower surface  79 , and a flat circular base portion  72  with depending pegs  74  therefrom. An annular ring  76  is machined or attached to the upper surface  78  of the flat circular base portion  72  such that no sharp edges are provided about the base portion  72 . The polymeric portion  80  is molded about the base portion  72  such that the annular ring  76  is encapsulated and the polymeric material  80  extends around the sides  82  of the flat circular base  72  to provide a dovetail lock about the sides of the patellar implant  68 .  FIG. 11  shows an enlarged portion of the base portion  72  and polymeric material  80  connection about the annular ring  76  and flat circular base portion  72 . The base portion  72  is shown with a thin impermeable membrane  84  about portions of the base portion  72  to prevent the flow of backside wear particles. Backside wear particles are created when a metal component rubs or wears against a polymeric material. The wear particles are irritants within the body which can cause infection or other medical issues within the body. The membrane  84  could also be provided slightly below the surface of the base portion  72  to allow the polymeric material to bond with the porous surface of the base member while still encapsulating any wear particles between the embedded membrane and the polymeric portion of the patellar implant. A depending peg  74  is also shown in  FIG. 11  illustrating how the thin membrane  84  could be applied to portions of the peg  74  to prevent or limit ingrowth of boney tissue along the sides of the pegs and yet allow for boney ingrowth at the bottom portions  86  of the pegs  74 . This would allow for the tissue ingrowth to assist in capturing the patellar implant  68  within the remaining natural patella ( 20  in  FIG. 1 ) while allowing for easier removal of the patellar implant in a revision situation. 
       FIG. 12  shows yet another embodiment of the present invention showing a patellar implant  90  having a flat circular base portion  92  having a pair of indentations provided about the periphery  96  of the base portion  92  provided at approximately 180 degrees apart. The polymeric material  94  may fill in this area  98  of the indentations and a polymeric rim  100  may be provided about the periphery of the circular base portion  92 . The indentations  94  would be positioned on the superior and inferior poles of the implant when positioned with respect to the patient&#39;s natural knee cap ( 20  in  FIG. 1 ) and the femoral components ( 12  in  FIG. 1 ). The indentations  94  of the patellar implant  90  allow for flexation of the patient&#39;s knee cap without creating undue stress risers of the flat metal circular base portion  92 . The bottom surface  102  of the base portion  92  would be provided with a boney tissue in-growth surface or be manufactured out of a metal material with boney in-growth properties. 
     It may be desirable to position the indentations  94  on medial to lateral poles of the implant when positioned with respect to a patient&#39;s natural knee cap. When the indentations are positioned in this fashion a greater volume of polymeric material would be existing at the sides of the patellar implant where the patellar implant would rub up against the femoral components of the artificial knee construct. 
     It may also be desirable to provide a patellar implant having a base portion and polymeric portion wherein the overall shape of the base portion can be modified intraoperatively such that the base portion could then be affixed to the polymeric portion of the implant. It may be necessary to cut away the pegs from the base portion such as during revision surgery. 
     Furthermore, the embodiments discussed above have all included a strong base portion with a softer polymeric portion for interacting with the other anatomy of the patient such as the femoral component. However, it may be desirable that the softer portion be made of a zirconium or other non-plastic material in case it is desirable to provide a hard-on-hard (or metal-on-metal) surface. 
     In some embodiments, the base portion may be manufactured using Selective Laser Sintered technologies (“SLS”) or other free-form fabrication technologies, such as one or more of the EOS Laser-Sintering systems available from EOS GmbH of Munich, Germany. For instance, in some embodiments, the entire base portion may be formed as a monolithic base portion (including any porous or other in-growth promoting surfaces or materials). In other embodiments, portions of the base portion may be formed using SLS technology and then additional in-growth materials or surfaces could be added or applied to the implant. In other embodiments, portions or the entire base portion can be formed using casting or other technologies or methods. In some embodiments, a non-porous base portion may be formed using SLS technologies and subsequently that base portion may be subjected to acid etching, grit blasting, plasma spraying (e.g. of titanium oxide or another metal to promote in-growth of tissue) or other treatments. 
     As various modifications could be made to the exemplary embodiments, as described above with reference to the corresponding illustrations, without departing from the scope of the invention, it is intended that all matter contained in the foregoing description and shown in the accompanying drawings shall be interpreted as illustrative rather than limiting. Thus, the breadth and scope of the invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the claims and their equivalents.