Abstract:
An implant providing for both short and long term stability and fixation is disclosed. The implant includes a plurality of projections extending from a bone contacting surface, and a porous material covering at least portions of the surface and projections. The orientation of the projections and the porous material provide for the stability and fixation. Methods of forming and utilizing the implant are also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of the filing date of U.S. Provisional Patent Application No. 61/794,339 filed Mar. 15, 2013, the disclosure of which is hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to an orthopedic medical device implant. In particular, the present invention is related to a unicondylar knee implant system&#39;s tibial component. 
     Orthopedic knee implant systems have been used for many years to treat patients with knee joints that have been damaged by trauma or disease, such as osteoarthritis, rheumatoid arthritis, and avascular neurosis. A knee arthroplasty resects, cuts, or resurfaces the damaged sections of the knee and replaces them with an endoprosthetic or implant. 
     Most knee implant systems are tricompartmental implants and the surgical procedure used with tricompartmental implants is commonly known as total knee arthroplasty. These implants are known as tricompartmental implants because they are used when the knee joint is prepared to receive an implant by resurfacing or resecting the three articulating compartments, i.e., the medial and lateral femorotibial and the patellofemoral surfaces. Regardless of the type of implant used, all arthroplasties require the bone to be specifically prepared to receive a corresponding implant by resecting, resurfacing, or deforming the bone to accept the implant. 
     Unicondylar or unicompartmental knee implants have become of great interest in the orthopedic industry due to their less invasive nature and the maintaining of the other healthy knee compartments. Unicondylar knees resurface or resect typically the medial or lateral femorotibial articulating surfaces thus allowing preservation of the other compartments which may not be suffering from damage due to trauma or disease. 
     Generally, the clinical outcomes for unicondylar knee implants have varied. Studies have reported long term survival rates for unicondylar implants to be less than that of comparable total knee implants. One particular cause for such discrepancies is due to the bone cement fixation technique associated with the tibial implant. Another cause is the limitations on longer term cement fixation integrity. And, another cause is the non-physiological tibial bone loading patterns of a required metal backed tibial component that is relatively stiff compared to the surrounding bone. 
     The development of orthopedic implant designs has been moving towards meeting the requirements of high demand patients. Patients today are requiring more from their implants and since patients are living longer, they are requiring implants to last longer. Accordingly, developments have been made in materials used to make orthopedic implants to improve implant survival rates, such as highly porous metals for biological bone fixation. 
     Orthopedic devices are mated with host bone by either cementing them in place using methyl methacrylate, generally termed bone cement, or by providing a rough or porous surface on the device for bone tissue to grow into, generally termed press-fit or cementless. 
     The use of bone cement in attaching a prosthesis within or onto a prepared bone provides an excellent immediate fixation but has various disadvantages that appear over time. Physical loads are repeatedly applied to the implant over its life. If bone cement is used to secure a unicompartmental knee prosthesis, the bone cement may fatigue and fracture under the repeated loading. In some instances, degradation of the bone cement integrity may cause the device to become loose, thereby necessitating replacement. Old bone cement must be removed from the host bone as part of the implant replacement procedure. This procedure can be complex, time consuming and potentially destructive to healthy bone structures surrounding the implant. Furthermore, conventional bone cement is cured after it has been dispensed into the patient&#39;s joint. Loose undetected cement fragments can remain in the joint space and, with patient mobility over time, increase the degradation rate of articulating implant surfaces. 
     Recognizing the disadvantages of cement fixation techniques, prior art devices have been developed that utilize mechanical attachment means to join an implant to bone for immediate stabilization. Various implant surface treatments intended to bond with bone biologically for long term stable attachment have proven successful. A simple technique of mechanically securing an implant, is to affix it within the bone with screws or other mechanical fasteners. However, due to the nature of the bone surrounding the surgical site, and other limiting factors such as artery location and the like, screws can only be applied in certain limited regions. The use of a screw for implant fixation should be considered only as an option by the surgeon depending upon implant placement and bone quality. 
     Primary fixation of an implant should come from a high friction interface with the prepared bone and in the long term with bone tissue ingrowth into a porous portion of the device. Specific instruments and surgical procedures are developed to match the implant and bone preparation. Often the bone cuts are undersized so that the implant or a portion of the implant such as a peg or keel is “press fit” into the bone. This assures an intimate contact between bone and implant. A high friction coating or porous portion of the implant assists with immediate bone fixation by mechanically locking the device in place. High friction will also resist any loading which may displace the device prior to bone ingrowth and more permanent biological fixation. 
     Prior art has established many methods for producing a high friction porous layer for implant designs. The use of metal beads, particles or wires which are metalurgically bonded to the implant surface is common. Plasma coating of metal surfaces with rough layers of metal particles is also utilized. More recently, porous metals of various chemical make up and structure have been developed which mimic the design of bone trabecular structure. These materials have been shown to have superior bone ingrowth results and should lead to improved implant fixation. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with a preferred embodiment, the present invention provides for a unicondylar tibial implant. The tibial implant includes a tibial keel positioned on a surface of the tibial implant to be submerged into prepared bone with a first projection extending along its lengthwise direction and a second projection extending along a direction perpendicular to the first projection. The first projection may be interrupted by a void to allow clearance for another implant or instrument. The second projection intersects the first projection. The tibial implant can be fabricated from a metal, a polymer, a biodegradable material, a porous metal material, or combinations thereof. The device as described could be produced through additive manufacturing techniques such as direct metal laser sintering. The foregoing description of the present invention is provided for the tibial implant when used on the medial condyle. However, the preferred embodiment can also be used on the lateral condyle, and when utilized in such a manner would have some features reversed in orientation. A description of the medial component features of the tibial implant is provided only for simplification. 
     The tibial keel is configured as an anterior-posterior projection with an intersecting keel segment that extends about a medial-lateral direction. The tibial keel is comprised of a solid material on a bone interfacing leading edge of the tibial keel i.e., a solid end portion, with the tibial keel having a porous material between the tibial tray and the solid end portion of the tibial keel. The tibial implant can optionally include a bone screw to secure the tibial implant to bone. 
     In accordance with another preferred embodiment, the present invention provides for a unicondylar tibial implant having a tibial keel configured as an anterior-posterior projection with at its most anterior aspect being an intersecting keel in the medial-lateral direction. The tibial keel is comprised of a solid material on a leading edge of the keel and porous material between the tibial tray and the solid end portion of the keel, and smaller protrusions on the medial facing portion of the tibial keel at the intersection of the tibial keel and tibial tray. The tibial implant is fabricated from a metal, a polymer and/or a biodegradable material. The tibial implant can optionally include a bone screw to secure the tibial implant to bone. 
     In accordance with yet another preferred embodiment, the present invention provides for a unicondylar tibial implant having a tibial keel configured as an anterior-posterior projection with at its most anterior aspect being an intersecting keel in the medial-lateral direction. The tibial keel is comprised of a solid material on the leading edge of the keel and porous material between the tibial tray and a solid end portion of the keel being implanted into an interference-fit created by an undersized preparation in the bone. The tibial implant is fabricated from a metal, a polymer and/or a biodegradable material. The tibial implant can optionally include a bone screw to secure the tibial implant to bone. 
     In accordance with another preferred embodiment, the present invention provides for a unicondylar tibial implant having a tibial keel configured as an anterior-posterior projection with at its most anterior aspect being an intersecting keel in the medial-lateral direction. The tibial keel is comprised of a solid material on a leading edge of the keel and porous material between the tibial tray and a solid end portion of the keel, and smaller protrusions on the medial facing portion of the keel at the intersection of the tibial keel and tibial tray where the protrusions preferentially force the tibial implant into the bone prepared about a resected mid-tibial eminence. The tibial implant is implanted into an interference fit created by an undersized preparation in the bone. The tibial implant is fabricated from a metal, a polymer and/or a biodegradable material. The tibial implant can optionally include a bone screw to secure the tibial implant to bone. 
     In accordance with yet another preferred embodiment, the present invention provides for a keel for a unicondylar tibial implant. The keel is connected to the tibial tray of the tibial implant and includes smaller protrusions on a medial facing portion of the keel at an intersection of the keel and the tibial tray where the protrusions push the tibial implant into the bone prepared about a resected tibial eminence. The keel is fabricated from a metal, a polymer and/or a biodegradable material. The tibial implant can optionally include a bone screw to secure the tibial implant to bone. 
     In accordance with another preferred embodiment, the present invention provides for a unicondylar tibial implant having a tibial tray with a porous keel and protrusions extending from the keel. The tibial tray accepts a polyethylene tibial bearing having an articulating surface for articulating with a femoral component. The tibial bearing can be a modular polyethylene tibial bearing. The tibial implant and tibial bearing can also be formed as a monoblock component. Alternatively, the tibial tray with a porous keel can be formed out of a singular biomaterial which is also used to form the tibial bearing. The tibial implant can optionally include a bone screw to secure the tibial implant to bone. 
     In accordance with yet another preferred embodiment, the present invention provides for a unicondylar tibial implant having at least one section of material that in its normal state forms at least one uninterrupted surface of the implant that is separable from the greater bulk of the tibial implant in a predictable shape defined by the presence of a shear section. The shear section of material when removed exposes a passageway for at least one additional implant, such as a bone screw. The removal of the shear section also exposes a passageway for surgical instrumentation, for the application of osteobiologic materials or for the application of bone cement. 
     In accordance with another preferred embodiment, the present invention provides for the ornamental design of a unicondylar tibial implant as shown and described in the figures below. 
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The foregoing summary, as well as the following detailed description of the preferred embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
    
    
     
       DETAILED DESCRIPTION OF THE DRAWINGS 
         FIGS. 1-8  illustrate a unicondylar tibial implant assembly in accordance with a preferred embodiment of the present invention; 
         FIGS. 9-18  illustrate a unicondylar tibial implant of the tibial implant assembly of  FIGS. 1-8 ; 
         FIGS. 19 and 20  illustrate the unicondylar tibial implant of  FIGS. 9-18  with a bone screw positioned within a through hole of the tibial implant; 
         FIGS. 21-29  are highly magnified photographic images of a porous portion of the unicondylar tibial implant of  FIGS. 9-18 ; 
         FIGS. 30-37  illustrate a unicondylar tibial implant in accordance with another aspect of the preferred embodiment of the present invention; 
         FIGS. 38-40  illustrate a unicondylar tibial implant in accordance with yet another aspect of the preferred embodiment of the present invention; and 
         FIGS. 41-43  illustrate a unicondylar tibial implant in accordance with another a further aspect of the preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the preferred embodiments of the present invention illustrated in the accompanying drawings. Wherever possible, the same or like reference numbers will be used throughout the drawings to refer to the same or like features. It should be noted that the drawings are in simplified form and are not drawn to precise scale. In reference to the disclosure herein, for purposes of convenience and clarity only, directional terms such as top, bottom, above, below and diagonal, are used with respect to the accompanying drawings. Such directional terms used in conjunction with the following description of the drawings should not be construed to limit the scope of the invention in any manner not explicitly set forth. Additionally, the term “a,” as used in the specification, means “at least one.” The terminology includes the words above specifically mentioned, derivatives thereof, and words of similar import. 
     Partial knee implants, also known as unicondylar or unicompartmental knee implants, replace either a medial or lateral compartment of a knee joint by resurfacing, either by itself or in conjunction with a resurfacing of the femoral condyle and an articulating surface of a proximal tibia with an engineered implant. The preparation of the bone to accept such implants may be facilitated by instrumentation such as bone files, burrs, saws, punches, and/or computer assisted instrumentation/navigation systems. Once the bone is prepared, the implant may be secured to the bone by bone cement which bonds to the implant and impregnates the bone resulting in fixation of the implant to the bone interface. 
     In order to remove bone cement from the surgical procedure of implanting partial knee implants, implants have been designed for fixation directly to the bone. Such fixation without bone cement is known as cementless fixation or press-fit fixation. The challenge of cementless fixation of tibial implant components is to have acceptable initial stability upon implantation to allow patient mobility immediately or a short time after surgery and promote adequate biologic fixation of the implant to the bone long term. The initial stability and long term fixation are requirements of the implant to reduce the incidence of implant loosening and reduce patient post-operative pain over time. 
     The present invention illustrated in  FIGS. 1-43  discloses preferred embodiments of a unicondylar tibial implant assembly  5  having a unicondylar tibial implant  10  and a unicondylar tibial implant bearing  12 . The unicondylar tibial implant  10  has been developed primarily for cementless application and includes a unique bone interfacing tibial keel  14  and a porous structured biomaterial interface i.e., a porous portion  16  ( FIGS. 21-29 ). The tibial implant  10  can be constructed from any combination of solid metal, porous metal, polymers or resorbable materials. 
     For purposes of convenience only, and not by way of limitation, the foregoing description of the preferred embodiments of the unicondylar tibial implant assembly  5  will be described and illustrated with respect to a unicondylar tibial implant assembly  5  for a medial tibial condyle. However, the foregoing description and features of the unicondylar tibial implant assembly  5  are equally applicable to a unicondylar tibial implant assembly for a lateral condyle, such similar features of the lateral unicondylar tibial implant assembly being substantially mirror images of such features of the medial unicondylar tibial implant assembly. 
     The tibial keel  14  is located on an undersurface of a tibial tray  18  of the tibial implant  10  which contacts a resected tibia bone (not shown). The tibial keel  14  is generally submerged into the bone to which the tibial implant  10  is to be implanted thereon. The tibial keel  14  can prepare its own cavity in the bone as it is inserted into the resected tibia or it can occupy cavities within the bone previously prepared by instrumentation or other implants. Any pre-cavities for receiving the tibial keel  14  when pre-prepared are generally smaller in size than the tibial keel  14  so as to generate compressive forces between the bone interface and the tibial keel  14  and increase frictional forces between the bone and the tibial keel  14 . That is, the tibial keel  14  is press-fitted into the bone. 
     Preferably, the tibial keel  14  is located on an underside of the tibial tray  18  of the tibial implant  10  and constructed out of a combination of a solid metal substrate and a porous portion  16  on the surfaces of the tibial keel  14 . 
     The tibial keel  14  is best shown in  FIGS. 2, 4-10 and 14-20  and includes a first projection  20  which is generally planar and has a height which corresponds to a depth within a prepared bone to which the tibial keel  14  will protrude into. The tibial keel  14  also includes a second projection  22  which is generally planar, has a height which corresponds to a depth within a prepared bone to which the tibial keel  14  will protrude into and is substantially perpendicular to the first projection  20 . The heights of first and second projections  20 ,  22  of the tibial keel  14  may be variable to accommodate access limitations while maximizing the fixation of the tibial implant  10  into bone. Preferably, the tibial keel  14  is positioned on an underside or inferior surface  24  of the tibial tray  18  with the first projection  20  running along the anterior-posterior direction. The second projection  22  intersects the first projection  20  towards the anterior edge of the first projection  20 . Both of the first and second projections  20 ,  22  of the tibial keel are substantially normal to the underside of the tibial tray  18 . Further, the first protrusion  20  can be configured to have a height that varies along its length. 
     Each of the first and second protrusions  20 ,  22  of the tibial implant  10  can be configured to have one or more extensions i.e., a plurality of extensions  26 .  FIGS. 2 and 5  illustrate the extensions  26  extending from the second protrusion  22 . The extensions  26  that emanate from the protrusions are oriented out of plane with the protrusion. That is, the extensions  26  extend outwardly from the lateral surfaces of the protrusions. The extensions  26  are designed to create and fill cavities within the bone so as to create and maximize compressive frictional forces between the tibial keel  14  and the surrounding bone. The extensions  26  are preferably located so that resultant forces during insertion of the tibial implant  10  into a resected tibia bias the position of the tibial implant  10  in a predetermined or desired direction. The extensions  26  are configured as substantially wedge shaped extensions that extend along substantially the entire height of the keel. Further, the extensions  26  preferably tapered in the distal direction. The plurality of extensions  26  on the second protrusion  22  are spaced apart from each other and substantially circumscribes the second protrusion  22 . Preferably, the second protrusion includes five extensions  26 , but can include more or less than five. 
     The extensions  26  are preferably located around the periphery of both the first and second protrusions  20 ,  22  with a higher number of extensions  26  or higher density of extensions emanating from the second protrusion  22  located about the anterior region of the tibial implant  10  where higher frictional forces are able to make a greater contribution to address anterior lift-off stability issues of the tibial implant  10  when implanted within the bone. The number of extensions  26  is greater on the sides of the protrusion  22  that face away from a central region of the tibial implant  10  so that bone reaction forces will push/direct the tibial implant  10  into the central region of the tibia. 
     The tibial implant  10  can optionally be configured with a through hole  28  ( FIGS. 2, 5 and 21 ) through which another device, instrument or material e.g., a bone screw  30  ( FIGS. 19 and 20 ) can be inserted therethrough. The through hole  28  may pass through one or more of the protrusions  20 ,  22  thereby interrupting their general shape. However, material is removed from protrusions  20 ,  22  around or adjacent the through hole  28  to provide for clearance of the device, instrument or material to be inserted therethrough. 
     A solid edge  32  ( FIGS. 2, 7 and 21 ) at the distal end of the tibial keel  14  prevents bone from growing into the tibial keel  14  from the bottom up. The majority of the surface area of the tibial implant  10  for fixing i.e., via bone ingrowth, the tibial implant  10  to the bone occurs at the perimeter of the tibial keel  14 , i.e., the lateral side surfaces of the tibial keel  14 . The bone which engages and contacts the bottom of the tibial keel  14  represents a small fraction of the overall surface area of the tibial implant  10 . 
     That is, the tibial implant  10  is configured to prevent any bone ingrowth or fixation about a distal surface of the tibial keel  14  via the solid edge  32 . Preventing bone ingrowth about the distal surface of the tibial keel  14  allows for easier removal of the implant, if necessary, since bone ingrowth on such distal surfaces of the tibial keel  14  represents areas that are most problematic to achieving separation of the implant from bone during revision procedures. In other words, as an implant is pulled out of bone, bony ingrowth into the bottom portion of the tibial keel might not separate from the greater volume of the bone exactly at the implant interface but rather somewhere deeper within the volume of bone beneath the implant. If this occurs during implant removal, the additional bone that would otherwise be inadvertently removed would complicate the revision procedure and drive the use of more significant revision components. 
     The general shape of the tibial keel  14  is designed to maximize surface area to volume ratio for the tibial keel  14  to enhance bone ingrowth thereto while minimizing the amount of bone removal during bone preparation. The amount of surface area available for bone ingrowth is important for both short and long term fixation of the implant to the bone. Short term fixation is achieved by “press-fitting” a larger body into a smaller preparation. Once in place, the residual stresses from the compressed bone around the tibial keel  14  increase the frictional forces against the tibial keel  14  and increase the stability of the tibial implant  10  into the prepared bone. Increasing the surface area over which the press-fit interference is effective helps to increase the total frictional forces available to contribute to stability of the implant and to distribute frictional forces over a greater effective area of the tibial implant  10 . 
     Long term fixation of the tibial implant  10  is enhanced by the areas of the tibial implant  10  having the porous structure and surface, hereafter referred to as ‘porous metal’  26 . As the bone remodels and grows into the porous metal  26 , the frictional retention forces will be replaced and/or supplemented with bone ingrowth. The degree of this fixation via bone ingrowth is, in part, a function of the amount and distribution of the porous metal surface area available for ingrowth. The large distributed tibial keel surface area thereby provides a structure for increased stability via a larger area of bone ingrowth. 
     The tibial keel  14  also includes a plurality of fins  34  which extend beyond the nominal volume of the tibial keel  14 . The fins  34  enter bone that has not been prepared to receive the fins  34 . Instead, the fins  34  prepare their own receiving volume within the bone as they are inserted into the bone, i.e., the fins  34  create their own preparation into the bone. In other words, the fins  34  are inserted into bone without the need to prepare the bone to receive such fins  34 . The fins  34  are sized to maximize their surface area, minimize their volume and are shaped to ease entry into the bone. As shown in  FIGS. 2 and 7 , the fins  34  are preferably configured as shown and are substantially wedge shaped or shaped as a dual inclined plane structure. Further, the fins  34  are tapered as they extend from a proximal end of the tibial keel  14  distally. The fins  34  are also preferably configured to extend an overall length about half way the overall height of the tibial keel  14 . 
     Preferably, the through hole  28  is shaped and sized for the passage of the bone screw  30  ( FIGS. 19 and 20 ) through a superior aspect of the tibial implant  10  into the bone beneath the underside or inferior surface of the tibial tray  18 . The bone screw  30  can be angulated to achieve a desired direction by the user. Further, with material from adjacent protrusion  20  removed, the protrusion  20  does not interfere with the passage of the bone screw  30  through the through hole  28 . Such bone screws  30  are readily known in the art and a detailed description of their structure and operation is not necessary for a complete understanding of the present invention. 
     The tibial implant  10  may employ the use of a knockout plug  36  formed within the through hole  28  and out of a material that is metallurgically continuous with the greater bulk of the tibial implant  10 . The knockout plug  36  is configured to be removed from the remainder of the tibial implant  10  via a boundary shear section  38  around the plug  36 . The plug  36  may be machined into the tibial tray  18  or built in final form through an additive manufacturing process such as by direct metal laser sintering. 
     Preferably, the through hole  28 , designed for the passage of the bone screw  30  therethrough, is obstructed by the knockout plug  36  so that the superior surface  40  of the tibial tray  18  facing the bearing component  12 , which can be assembled thereto, is fully continuous without any path through which debris or material could pass through the tibial tray  18  to the bone engaging underside of the tibial implant  10 . 
     In sum, the tibial tray  18  has a through hole  28  into which a screw  30  can be placed to further stabilize the tibial implant  10  to the prepared bone upon implantation. This is especially advantageous for initial implant stability and when placing the tibial implant into bone of questionable density where the user/surgeon is not confident the bone itself is stable enough to support adequate short term stability. 
     The through hole  28  can be covered during the manufacturing process of the tibial implant  10  with the knockout plug or shear plug  36 . The knockout plug  36  has a weak cross section which will yield to an appropriate level of force. When the knockout plug  36  is in place, there exists an uninterrupted tibial tray surface between the poly (i.e., bearing component  12 ) and the bone interface. In the event of backside wear of the bearing component  12 , wear particles are less likely to migrate out of the tibial tray  18  than if an already present through hole were in place. The knockout plug  36  can optionally include a threaded stud  42  ( FIG. 12 ), which mates to instrumentation to facilitate removal of the knockout plug  36 . 
     The porous metal  16  is formed from a porous structured biomaterial, and includes a plurality of struts  44  ( FIGS. 21-29 ) having varying lengths and cross sections. At least one strut of the porous metal  16  has an end connected to one or more other struts at node points  46  ( FIG. 29 ) thereby forming the porous geometry of the porous metal  16 . The porous metal  16  also includes boundary struts  48  ( FIGS. 26, 27 and 28 ) that are configured to extend beyond a nominal boundary of the porous metal  16 . That is, the porous metal  16  has boundary struts  48  that extend away from the surface of the porous metal  16  in a finger-like or hair follicle-like fashion. The extending boundary struts  48  impart a roughness to the surface, the degree of which is dependent upon the number and length of boundary struts  48  present. The average or main direction of the boundary struts  48  also impart a surface roughness that varies dependent upon which direction the device is driven for implantation. 
     Preferably, the tibial keel  14  is formed from a metal substrate and a layer of porous metal  16  adjacent the substrate. The porous metal  16  on the tibial keel  14  includes extending boundary struts  48  with unconnected ends pointing or extending towards the bottom or inferior surface of the tibial tray  18 . Under similar loading conditions, sliding over the angled struts toward the bottom surface of the tibial tray  18  will experience less frictional forces than bone sliding away from the bottom face of the tibial tray  18 . Preferably, the boundary struts  48  are angled about +/−10 degrees from normal to a surface of the substrate to which the porous metal  16  is applied to. 
     Another element of the present invention is that the boundary struts  48  are oriented in a predetermined direction such that they push or are directed towards the bone interface surface. While the surface of the porous metal  16  may exhibit characteristics of a rougher surface, the boundary struts  48  of the porous metal  16  implanted into a bone interface embed themselves into the bone and provide a mechanical interlock to the surrounding bone. This is especially advantageous during initial implantation for initial fixation purposes. In the aggregate, the plurality of boundary struts  48  significantly improves the overall stability of the tibial implant  10  upon initial implantation. 
     Preferably, the bottom surface of the tibial tray  18  has extending boundary struts  48 ′ ( FIGS. 26 and 27 ) in a direction substantially normal to the bottom surface of the tibial tray  18 . As the tibial implant  10  is definitively seated against the bone interface surface, the boundary struts  48 ′ pierce the surface of the prepared bone to increase stability of the tibial implant  10  to the bone. 
     The tibial implant  10  has the porous metal  16  on all surfaces that make contact with bone. The surface of the porous metal  16  is tailored for each specific region of the tibial implant  10  to have specific surface roughness and thereby specific amounts of friction when engaged with bone. That is, the tibial implant  10  is configured to have a porous metal  16  with boundary struts  48  at predetermined angles dependent upon the location of the porous metal  16  on the tibial implant  10 . 
     In sum, the surfaces of the porous metal  16  have extending boundary struts  48  which serve to modify the surface roughness of the tibial implant  10 . The size and average direction of the extending boundary struts  48  impart different frictional coefficients depending upon the direction the boundary struts  48  extend. The boundary struts  48  can also be directed in a direction largely normal to the surface from which they extend from. This can have an additive anchoring effect which enhances stability of the tibial implant  10  to the bone. 
     Referring to  FIGS. 30-37 , in accordance with another preferred embodiment, the present invention provides for a tibial implant  10 ′. The tibial implant  10 ′ is similarly configured as tibial implant  10 , excepted are follows. The tibial implant  10  includes a first protrusion  20 ′ segmented by a void and a second protrusion  22 ′. The second protrusion  22 ′ is similarly configured as the second protrusion  22  discussed above, but is spaced from the first protrusion  20 ′. The second protrusion  22 ′ has a height equivalent to the height of the first protrusion  20 ′ adjacent the second protrusion  22 ′. As best shown in  FIGS. 34-36 , the height of the first protrusion  20 ′ slopes towards the posterior end of the tibial implant  10 ′ such that the height of the first protrusion decreases as it extends from the anterior end towards the posterior end. 
     Referring to  FIGS. 38-40 , the tibial implant  10 ′ can alternatively include a third protrusion  23 ′. The third protrusion  23 ′, like the second protrusion  22 ′, is slightly spaced apart from the first protrusion  20 ′. Preferably, the third protrusion  23 ′ is positioned more towards the rear or posterior to the first protrusion and has a height similar to the height of the posterior end of the first protrusion  20 ′ to which it is adjacent to. The height of the third protrusion  23 ′ is not equal to the height of the second protrusion  22 ′ or the height of the first protrusion adjacent the anterior end of the first protrusion  20 ′. The third protrusion  23 ′ is also configured not to intersect the first protrusion  20 ′. 
     Referring to  FIGS. 41-43 , the third protrusion  23 ′ can also alternatively be positioned toward or about a middle section of the first protrusion  20 ′ and spaced apart from the first protrusion  20 ′. When positioned about the middle section of the first protrusion  20 ′, the third protrusion  23 ′ has a height substantially the same as the area of the first protrusion  20 ′ that it is adjacent to. 
     It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. For example, additional components can be added to the tibial implant assembly. It is to be understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as described above. 
       FIGS. 36 and 37  depict yet another embodiment tibial implant  410 . Unlike the above-discussed implants, implant  410  includes a keel  414  that is of a unitary design. A hole or aperture  428  is situated offset with respect to the keel  414 . This allows for the keel  414  to have a unitary construction (i.e., it is not broken up by the hole  428  as in the above designs). Like the foregoing embodiments, keel  414  includes two projections  420 ,  422 , with the projection  420  including fins  434  and the projection  422  including extensions  426 . Of course, as in the above embodiments, either projection could include either or both of the extensions  426  or fins  434 , and such structures can be of any shape and or size with respect to the projections. Moreover, keel  414  includes a rounded cut out  421 , which allows for a screw  430  to angulate with respect to implant  410 . In other words, the cut out  421  provides clearance for the screw  430  to move with respect to the plate in directions towards the keel  414 . 
     It is also to be understood that the disclosure set forth herein includes all possible combinations of the particular features described. For example, where a particular feature is disclosed in the context of a particular aspect, arrangement, configuration, or embodiment, or a particular claim, that feature can also be used, to the extent possible, in combination with and/or in the context of other particular aspects, arrangements, configurations, and embodiments of the invention, and in the invention generally. 
     Furthermore, 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.