Patent Document

BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to an orthopedic prosthesis utilized in knee joint replacements. More particularly, the present invention is directed to a tibial implant system comprising a tibial insert and interchangeable tibial trays, one made of a metal and one made of a polymer. 
         [0002]    Knee arthroplasty is a well-known surgical procedure to replace the weight-bearing surfaces of the knee joint to relieve pain and disability. It has enabled many individuals to function normally where it otherwise would be impossible. Artificial joints usually comprise metallic, ceramic, or plastic components implanted into existing bone. 
         [0003]    In a typical procedure, a tibial tray or baseplate is mounted on a prepared proximal tibia of a patient and a tibial bearing insert is mounted on the tibial tray. Methods and corresponding tibial trays exist that allow for implantation into the tibia with or without the use of bone cement. 
         [0004]    Earlier designs of tibial implant systems were typically composed primarily of metal trays or baseplates and bearings made of ultra-high-molecular-weight polyethylene (hereinafter “UHMWPE”). An earlier system, known as a monolithic UHMWPE device, comprised the bearing insert and bone-contacting portion as one piece implanted into the tibia through cement fixation. Although the system was resistant to stress shielding and had a long in vivo survival rate, the system fell out of favor with the medical industry because it was difficult to implant into the patient and because the bond between UHMWPE and bone with bone cement is not as strong as the bond with metal. 
         [0005]    An improvement which is still currently used is a metal-backed UHMWPE modular tibia, which comprises a metal tray and a separate UHMWPE tibial insert. Although the two-part system made the device much easier to implant, the system was expensive. 
         [0006]    Lastly, a system having a tibial tray composed of a polymer-porous metal composite has been recently developed for fixation without the need for bone cement, as can be seen in U.S. Pat. App. 2010/0100191, U.S. Pat. App. 2009/0084491, or U.S. Pat. App. 2011/0035018. Thus far, clinical results have shown that the cement-less bond is weaker than a bond using bone cement. In addition, devices comprising polymer-metal composites are expensive to manufacture. 
         [0007]    Thus, there is a need for a modular tibial tray and insert system which can produce an improved, sustained bond with bone cement, reduces stress shielding, has improved resistance to fracture and wear, provides ease of use for surgeons, and be inexpensive to manufacture. 
         [0008]    As used herein when referring to bones or other parts of the body, the term “proximal” means close to the heart and the term “distal” means more distant from the heart. The term “inferior” means toward the feet and the term “superior” means toward the head. The term “anterior” means toward the front part or the face and the term “posterior” means toward the back of the body. The term “medial” means toward the midline of the body and the term “lateral” means away from the midline of the body. The term “sagittal” refers to the plane dividing the body into left and right halves. The term “coronal” refers to the plane dividing the body into front and back halves. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    Accordingly, one aspect of the present invention is to provide a tibial implant system comprising a polymeric tibial insert which can securely engage with a first or second tibial tray, albeit through differing connections at their respective anterior portions. The preferred embodiment of the tibial insert includes a locking wire mounted in an anterior recess, a locking tab along an anterior surface, and a locking tab disposed along a posterior surface. The first tibial tray comprises polymer, which in a preferred embodiment is polyether ether ketone (PEEK), and includes a bead formed along an anterior wall and an undercut area along a posterior wall. The second tibial tray comprises metal and includes a plurality of barbs along an anterior wall and an undercut area along a posterior wall. The metal tray is similar to those already in use and may be made of titanium, titanium alloy, cobalt chrome, molybdenum alloy, or stainless steel. 
         [0010]    A further aspect of the present invention is to provide a method which improves the bond strength between a PEEK implant and bone cement, comprising the steps of loading a PEEK implant into a masking fixture, masking areas of the PEEK implant that will be subjected to high bending stresses, blasting a water soluble abrasive powder against the PEEK implant to achieve a certain average surface roughness, submerging the PEEK implant into water, and allowing the PEEK implant to air dry. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a front view of a UHMWPE tibial bearing insert for the tibial trays according to the present invention; 
           [0012]      FIG. 2  is a top view of the tibial insert of  FIG. 1 ; 
           [0013]      FIG. 3  is a cross-sectional side view of the tibial insert of  FIG. 1  along line  3 - 3 ; 
           [0014]      FIG. 4  is an enlarged view of detail A of the cross-sectional side view  FIG. 3 ; 
           [0015]      FIG. 5  is a top view of a polymeric tibial tray according to the present invention; 
           [0016]      FIG. 6  is a front view of a metallic tibial tray according to the present invention; 
           [0017]      FIG. 7  is a cross-sectional side view of the metallic tibial tray of  FIG. 6  along line  7 - 7 ; 
           [0018]      FIG. 8  is an enlarged view of detail B of the anterior wall of the metallic tibial tray of  FIG. 7 ; 
           [0019]      FIG. 9  is a top view of the metallic tibial tray of  FIG. 6 ; 
           [0020]      FIG. 10  is an enlarged schematic cross-sectional view of the engagement between the polyethylene tibial insert anterior surface and the polymeric tray anterior wall; and 
           [0021]      FIG. 11  is an enlarged cross-sectional view of the engagement between the polyethylene tibial insert anterior surface and the metallic tray anterior wall. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]      FIGS. 1 and 2  illustrate a representative UHMWPE tibial bearing insert according to the present invention, generally denoted as  10 . The tibial insert  10  comprises an anterior surface  11  including a locking wire  12  and locking tab  13  and a posterior surface  14  including an intracondylar recess  18  and locking recesses  15 , shown in  FIG. 3  (not in  FIG. 2 ). The insert also has a bearing surface  100 . In an alternative embodiment, an additional locking recess  15  may be placed within the intracondylar recess  18 . Locking recess  15  engages a protrusion  24   a,    34   a  formed on the posterior surfaces  23 ,  33  of the tibial trays  20 ,  30  as shown in  FIGS. 5 and 9 . 
         [0023]      FIGS. 3 and 4  illustrate a cross-sectional view of the tibial insert  10  along line  3 - 3  of  FIG. 1 . Referring to  FIG. 3 , the locking wire  12  is located above the locking tab  13  along the anterior surface  11 , whereas the locking recess is disposed along the lower portion of the posterior surface  14 . 
         [0024]      FIG. 4  is an enlarged view of detail A of  FIG. 3 , showing the anterior surface  11  of the tibial insert  10 , illustrating the placement of the locking wire  12  in a groove  12   a  formed above the locking tab  13 . As shown, the locking wire  12  is disposed above the locking tab  13 , with a space between the two parts. Furthermore, the locking wire  12  is disposed along a slot which allows the wire  12  to resiliently and posteriorly deflect into the groove  12   a  when a compressive force is applied during insertion of the insert  10  into the tray. Additionally,  FIG. 3  shows a view of the posterior surface  14  including locking recess  15  and a flange  17  extending outwardly of the tray contacting surface  19 . 
         [0025]      FIG. 5  illustrates a representative polymeric tibial tray  20  according to the present invention. The polymeric tray  20  comprises an anterior wall  21  including a bead  22  running along an inner posteriorly facing surface of anterior wall  21  and a posterior wall  23  including undercuts  24  located below protrusion  24   a  and receiving flange portion  17  of the insert  10 , and an intracondylar recess  25 . Alternately the bead  22  can be discontinuously spaced along the wall  21  such as at the medial corner  21   a  and lateral corner  21   c  of the anterior surface  21  and optimally along the central anterior surface  21   b.  The polymeric tray  20  further comprises a keel  206  (not shown) which is implanted into the tibia and a bone contacting surface  208  ( FIG. 10 ). An additional undercut  24  may also be placed within the intracondylar recess  25  for engaging a flange portion  17  located on the insert  10 . 
         [0026]      FIG. 6  illustrates a front view of a representative metallic tibial tray  30 . The metallic tray  30  comprises an anterior wall  31  including a plurality of barbs  32  ( FIGS. 7 ,  8 , and  9 ), disposed alongside and protruding posteriorly, and a posterior wall  33  including undercuts  34  and an intracondylar recess  35  ( FIG. 9 ). An additional undercut  34  may also be placed within the intracondylar recess  35  ( FIG. 9 ). The metallic tray  30  further comprises a keel  306  which is implanted into the tibia and a bone contacting surface  308 . 
         [0027]      FIG. 7  illustrates a cross-sectional side view of the metallic tray  30  along line  7 - 7  of  FIG. 6 . As shown, the barbs  32  protrude posteriorly and run continuously along a substantial portion of the anterior wall  31 . Furthermore, the intracondylar recess  35  is centrally disposed on a sagittal plane bisecting the posterior wall  33  with the undercuts  34  and protrusions  34   a  on the anteriorly facing surface of the posterior wall  33  ( FIG. 9 ). 
         [0028]      FIG. 8  is an enlarged view of detail B of  FIG. 7 , the anterior wall  31  of the metallic tray  30 , illustrating the placement of the barbs  32  along the anterior wall  31 . As shown, the barbs  32  include a chamfer  32   a  which extends posteriorly, a tip  32   b,  an underside  32   c  which faces distally, and a proximally facing surface  32   d  on which tibial insert  10  is mounted. The barbs  32  are placed along an upper portion of the anterior wall  31  so as to leave space below for insertion of the tibial bearing insert  10 . Furthermore, the locking wire is sufficiently thin to be capable of resiliently deflecting in the anterior-posterior direction when a force is applied against it by the barbs  32  during insertion of the insert  10 . 
         [0029]      FIG. 9  is a top view of the metallic tibial tray  30  according to the present invention. As shown, a plurality of barbs  32  are disposed alongside the anterior wall  31  and extend posteriorly for engaging the anterior surface of the tibial insert  10 . Furthermore, the posterior wall  33  includes undercuts  34 , protrusions  34   a,  and an intracondylar recess  35 . An additional undercut  34  and protrusion  34   a  may also be placed within the intracondylar recess  35 . The posterior wall undercuts  24 ,  34  and protrusions  24   a,    34   a  of the polymeric tray  20  and metallic tray  30  may be identical in design so that the same tibial insert  10  can be used in either tray. 
         [0030]      FIG. 10  is an enlarged schematic cross-sectional side view along line  10 - 10  of  FIG. 5  of the engagement between the tibial insert anterior surface  11  and the polymeric tray posterior surface of the anterior wall  21 . The tibial insert is first engaged with the polymeric tray  20  through insertion of the flanges  17  into the posterior undercuts  24 , wherein the protrusions  24   a  engage with the locking recesses (in  FIG. 5 ). Afterwards, the insert anterior surface  11  is snap-fit into engagement with the tray anterior wall  21 , wherein the locking tab  13  forces the anterior wall  21  to resiliently deflect anteriorly and snap posteriorly back into place with the bead  22  fitted between the locking wire  12  and locking tab  13 . Accordingly, the bead  22  should be sufficiently sized to protrude past the locking wire  12  and the locking tab  13  when the tibial insert  10  is snap-fit into the polymeric tray  20 . Thus, the tibial insert  10  is held in secure engagement with the polymeric tray  20  through connections at their posterior and anterior portions. 
         [0031]      FIG. 11  is an enlarged cross-sectional side view of the engagement between the tibial insert anterior surface  11  and the metallic tray anterior wall  31 . The tibial insert  10  is first engaged with the metallic tray  30  through insertion of the flanges  17  into the posterior undercuts  34 , wherein the protrusions  34   a  engage with the locking recesses  15  (in  FIG. 9 ). Afterwards, the insert anterior surface  11  is snap-fit into engagement with the tray anterior wall  31 , wherein the plurality of barbs  32  forces the locking wire  12  to resiliently deflect posteriorly into the groove  12   a  and then snap anteriorly back into place underneath the barbs  32 . Accordingly, for each barb  32 , the chamfers  32   a  must be sufficiently angled to extend posteriorly past the locking wire  12  such that the tips  32   b  may push the wire  12  into the groove  12   a  and, upon the wire  12  snapping back into place, the undersides  32   c  may subsequently prevent the locking wire  12  from moving in an upward direction. Thus, the tibial insert  10  is held in secure engagement with the metallic tray  30  through connections at their posterior and anterior portions. 
         [0032]    In a preferred embodiment of the present invention, the polymeric tray  20  is formed from polyether ether ketone (hereinafter “PEEK”), preferably with a crystallinity of less than 30 percent. Due to its relative inertness and nonporousness, PEEK biomaterials have been found to be an attractive platform upon which to develop implants such as the present invention. Benefits include lower stiffness which results in reduced stress shielding, sustained bonding strength to bone cement in body fluid, reduced backside wear between the polymeric tray  20  and the tibial insert  10 , ease of manufacture, and lower costs. 
         [0033]    Furthermore, in the preferred embodiment, the polymeric tray&#39;s  20  bone-contacting surface  208  is grit-blasted to a high surface roughness in order to increase its initial bond strength with bone cement. Preferably, the blast media used is sodium bicarbonate or a water-soluble grit of similar hardness. The polymeric tray  20  is grit-blasted at a pressure between 105 and 110 PSI to a high surface roughness in order to enhance its initial bond strength with bone cement. The grit-blasting method comprises the steps of loading the tray into a masking fixture, masking areas of the tray that will be subjected to high bending stresses, blasting an abrasive powder against the tray to achieve an average surface roughness of between 4 and 6 micrometers, submerging the tray in water having a temperature between 60 and 70 degrees Celsius for 2 minutes, and allowing the tray to air dry. 
         [0034]    In a preferred embodiment of the grit-blasting method, the grit-blasting machine is a suction-type or equivalent and has a 5/32 inch air jet and a nozzle having a 5/16 inch diameter orifice. The machine may be automated or manually operated. Furthermore, the preferred method includes focusing the nozzle perpendicular to the intended blasting area from a distance of 3 to 4 inches. If a perpendicular angle is impossible, the minimum allowable angle should be no less than 45 degrees. Blasting is continued until the desired surface roughness of 4 to 6 micrometers is reached. 
         [0035]    Preferably, the blast media used is sodium bicarbonate or another abrasive nonmetal, water-soluble, and noncorrosive powder having a particle size of distribution of less than 10 percent 210 micrometer particles, less than 25 percent 270 micrometer particles, less than 50 percent 350 micrometer particles, less than 75 percent 430 micrometer particles, and less than 90 percent 510 micrometer particles. In addition, the grit-blasting technique is performed below the glass transition temperature of the polymer being roughened by the technique. Below the glass transition temperature polymers are more brittle than they are above their glass transition temperature. As a result, it is easier to create a roughened surface on a polymer when it is below its glass transition temperature. Accordingly, there is a disadvantage to creating a roughened polymeric surface by grit-blasting the mold into which a polymer material could be injected. As a consequence of both material flow inherent to the molding (injection or compression) process and the molding process taking place above the glass transition temperature, the surface roughness achieved by molding roughness into a part is generally not equivalent to the surface roughness achieved by grit-blasting the part after molding. It has been discovered that grit-blasted polymeric surfaces of a given roughness have better adhesion to bone cement than molded polymeric surface of the same roughness. 
         [0036]    Thus, according to the invention, the tibial insert is shaped to lockingly engage with either the polymeric tibial tray  20  or the metallic tibial tray  30 , albeit through differing means of engagement. During implementation, either the polymeric tray  20  or metallic tray  30  may be chosen for implantation into the resected tibia and receive the same UHMWPE bearing insert  10 . 
         [0037]    The interchangeability of a polymeric tray  20  or metallic tray  30  provides the option of choosing between implantation with or without the use of bone cement. Implementing the polymeric tray  20 , particularly in the preferred embodiment in which the bone-contacting surface  208  is grit-blasted to a certain roughness, allows for a stronger bond with bone cement than metallic tibial trays. However, use of a metallic tray  30  would still be appropriate where bonding through cementless bone tissue ingrowth would be preferable to using bone cement. Because all cementless tibial trays are currently made of metal, a polymeric tray  20  which is compatible with an already-existing line of tibial inserts would reduce design and manufacturing costs and allow for the option of using bone cement without the need for a new tibial insert design. 
         [0038]    In addition, the interchangeability of the present invention allows for consideration of a patient&#39;s bone stock. Polymeric trays are more flexible than their metallic counterparts and are therefore less likely to shield from stress areas of the bone where bone resorption has occurred with metallic trays. Thus, a younger patient with relatively strong bone stock would benefit from the loading pattern provided by a polymeric tray whereas a metallic tray would be more appropriate for an older patient with weaker bone stock. Thus, the compatibility of both types of tibial trays with the same tibial insert provides more options and increases design and manufacturing efficiency. 
         [0039]    Lastly, the interchangeability between the polymeric and metallic trays would allow a provider to offer a more competitively priced implant system because a polymeric tray can be significantly less expensive to manufacture than a metallic tray. Thus, the company would be able to offer a less expensively priced polymeric tray in value markets while still being able to offer a metallic tray without having to design a different tibial insert for each tray. 
         [0040]    Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.

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