Abstract:
A method for removing a stem portion of an orthopedic implant from a bone comprises exposing an implanted orthopedic implant having a body portion, a stem portion interconnected to the body and a porous metal section forming an interconnection between the body and the stem portion. A cutting tool is mounted on a holder connected to an exposed surface of the orthopedic implant. The porous section is aligned with the cutting tool mounted on the holder. The entire porous section is cut by moving the cutting tool therethrough in a direction transverse to the stem portion axis. The implant body portion is then removed and then the stem portion is removed from the bone. The cutting tool may be a saw or chisel which may be mounted on a guide fixed to the body portion.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Current keel implant devices, such as tibial components, are solid with high stiffness are commonly associated with diaphyseal load transfer or stress shielding. This invention provides a porous section for allowing bone ingrowth and a reduced stiffness keel device for more anatomical load transfer. In addition, the porous section allows easier removal of the implant during revision procedure. 
         [0002]    The present invention also relates to a device having two different porous surfaces attached directly or indirectly to one another and a method for forming the same. The two porous surfaces may be separated by a solid or fully dense (non-porous) layer. 
         [0003]    The present application is particularly directed toward a method of forming porous or partially porous metallic structures having different porosities for bone ingrowth and soft tissue ingrowth or attachment. Porous structures may also be formed for polymer attachment. 
         [0004]    One method of producing the different porous structures uses rapid prototyping to produce low density three-dimensional structures. This is useful in applications where porous and partially porous metallic structures, and more particularly metal porous structures with interconnective porosity are advantageous for use. In addition, composite structures of metal and porous ceramics or porous polymer can be used. 
         [0005]    Many structures, especially in the medical arts, require two different surfaces, each adapted for their own purposes. Along this line, a structure may have a first surface which needs to be porous for tissue ingrowth and a second surface which could have porosity adapted to be a bearing surface. Those structures can be produced by Selective Laser Melting (SLM). See for example U.S. Patent Publication No. 2007/0142914, the disclosure of which is incorporated herein by reference. Further, the first surface or portion may include different layers having different gradients of porosity. For example, the first surface may include an outer region having a porosity of approximately 80%. Moving more inwardly normal, with regard to the first surface, the porosity may alter such that the porosity is increased or in a preferred embodiment, the porosity decreases even until the porosity is almost zero. Of course, the present invention contemplates a situation where the porosity changes from position to position depending on the requirements of the device. 
         [0006]    Cementless bone implant technologies provide a variety of porous surfaces that allows for bone ingrowth into the implant. This ingrowth allows for a better transfer of mechanical loads to the surrounding bone tissues and decreases bone resorption due to stress shielding. As with all primary implants sometimes a revision surgery is necessary to remove the implant. With non-cemented implants osseointegration blurs the bore/implant interface. This allows for the potential to lose large quantities of bone stock, when the implant is removed under conventional means. Other disadvantages of the current removal techniques include increased metal debris at and around the surgical site, increased heat and increased time in the operating room (OR). The proposed system addresses these issues and provides a method to remove porous material from cementless implants. 
         [0007]    The porous material and manufacturing methods of the present invention can be used for other applications. One of the current concerns with modern total knee arthroplasty is the issue of femoral bone resorption due to stress shielding. This is most commonly found in the distal/anterior region behind the patellar groove of the implant. As femoral components are most commonly manufactured from Cobalt Chromium or titanium alloy, they have a significantly higher modulus of elasticity than the bone. Additionally, most femoral components are substantially rigid due to the amount of solid material that encompasses the space between the articular surface and the surfaces that mate with the resected femur. Bone remodels in the presence of load and in the natural knee, the articular surfaces are loaded by the patella and tibia. The replacement of that bearing with a stiff metal component shields the bone from much of the load in the anterior and posterior regions, leading to a lack of remodeling and ultimately, resorption. By making the femoral component essentially “hollow” and filling the space with foam, it becomes a more flexible component, which is better able to transfer the loads from the articular surfaces to the resected bone. This allows for remodeling of the bone in all regions and prevents stress shielding. 
         [0008]    Current literature suggests several surgical methods to remove cementless implants during revision surgeries. These methods include the use of an oscillating cutting system such as oscillating saws and instruments which apply blunt striking force. Both methods have yielded positive effects, but present obvious disadvantages when used with solid implants. 
         [0009]    Oscillating cutting systems are used to quickly cut up to the bone/implant interface and remove enough bone to allow the implant to be pulled from the bone. However, if the implant is not completely free, this results in large amounts of bone stock loss when the implant is extracted. With the oscillating system, when the blade reaches the porous coating of the otherwise solid implant, the high frequency motion can cause porous material to be dispersed throughout the surgical site. This may lead to subsequent revisions due to host response from debris particles or degradation of articulating surfaces due to body wear caused by the debris. Furthermore, friction between the blade and the implant can cause heat generation, in which the implant will act as a heat sink. The distribution of heat throughout the implant can transfer to the bone and cause bone tissue necrosis. 
         [0010]    Blunt striking with an osteotome is also an effective means for separating the anchoring mechanisms of an implant from the planar force bearing surfaces. This allows access to the smaller anchoring point, which requires smaller, more precise tools to extract, or the surgeon can choose to leave them in. However, without a counter force, the bone provides the only resistance to implant movement. As such, the force from blunt striking may cause inadvertent damage to the bone surrounding the implant. 
         [0011]    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. 
       BRIEF SUMMARY OF THE INVENTION 
       [0012]    A cementless bone fixation keel, extending from the metaphyseal seating surface of an orthopedic implant, such as a tibial implant, comprises a porous portion adjacent to the bone metaphyseal porous seating surface, and a solid portion extending to the diaphysis. 
         [0013]    The extent of porosity of the keel portion can be adjusted to achieve a desired implant stiffness. The pore size of the porous portion is preferred in the range of 10 micron to 1000 micron, in which 100-400 micron is preferred for bone ingrowth application. The porosity is preferred between 40% to 80%. The porous portion of the keel can have either a surface porosity or a through-thickness porosity. The later will allow tissue to grow through the proximal porous portion of a tibial keeled implant. 
         [0014]    The strength of the porous portion could be reinforced by inner high density elements such as a gradually decreasing porosity towards the center in the direction perpendicular to the surface of the keel or by means of near solid or totally solid reinforcing members or struts directionally oriented to best support applied loads. 
         [0015]    The diaphyseal keel portion outer surface is closed off to prevent bone ingrowth but the stiffness of the construct can be reduced by inclusion of a lower density core structure. The inner lower density structure has a porosity of 10% to 80%. The solid outer surface could also contain struts that are directionally oriented to best reduce bending stiffness. 
         [0016]    The total stiffness of the keel is preferred to be 20-80% less than that of a solid keel with the same material. 
         [0017]    The bone fixation keel of the present invention is made from a bio-compatible material such as Ti, Ti alloy, or CoCr alloy. 
         [0018]    The porous portion of the keel encourages the bony ingrowth for metaphyseal load transfer and the solid portion of the keel provides the device alignment and stability. A fully porous keel section also provides the surgeon full visibility of the implant/bone interface on x-ray inspection. 
         [0019]    The porous keel section allows for an easier revision of the component since a tool can be provided to cut through the porous area. 
         [0020]    One cementless implant removal tool described herein is a linear blunt force applicator that holds the implant and drives a cutting tool, such as a chisel, through the cementless material, i.e., bone and the porous metal. The cementless implant removal tool of the present invention allows for precise extraction of implants that lie on a single planar surface. One embodiment specifically applies to revision of novel implants described above which have a porous band of material integrated into a tibial baseplate and keel of a tibial implant. The removal device is comprised of an adapter that fits between the tibial baseplate holder and an impaction handle. Attached is a linear rod that supports a driving mechanism such as described by Goul (U.S. Pat. No. 4,874,155). The driving mechanism supports an adapter that holds a blade such as in a chisel. 
         [0021]    One embodiment of the removal device is assembled by attaching the baseplate holder to the top of the baseplate. This may be accomplished by using the same system on the baseplate as is used to connect the bearing to the baseplate. The adapter is attached to the holder and the impactor handle attached to the adapter. The blade is attached to the adapter supported by the driving mechanism. Preferably the design allows the blade to be up to about 3 inches below the bottom of the baseplate. The device is operated by holding the tibial impactor handle and engaging the driving mechanism until it reaches the cancellous bone and implant interface. Once at the interface the cutting mechanism is driven through the porous baseplate keel material either by a power tool or manually. The adapter can preferably be rotated incrementally to −60 and 60 degree orientations and the device driven again through the remaining porous material. The baseplate will then be free from its anchoring points and able to be removed easily. The remaining keel may have a central threaded portion to allow for its removal. 
         [0022]    Other embodiments include gauging and adjustment mechanisms to allow for offsets in any and all directions. Also other driving methods may be used, including ratcheting and motorized drivers. Also cutting may be achieved by different blades including flat, pointed, serrated and notched. 
         [0023]    The present invention may provide a blunt cutting force as performed with the osteotome, but includes a counter force to prevent the implant from moving and potentially damaging the surrounding bone. The gauging and adjustment mechanisms provide a more precise execution of implant removal as well reproducibility of procedure. Though metal debris may be generated, the cutter may be designed with a lack of moving parts which help keep the debris localized. Also a stationary blade, such as a chisel, decrease friction and reduces heat generation. 
         [0024]    Alternately an embodiment utilizing an oscillating saw can be used. The holder which attaches to the baseplate and to a saw can be identical to that described above with respect to a blunt force instrument. The holder preferably locks on the same undercut surfaces formed on the proximally facing side of the tibial baseplate used to lock the polyethylene bearing to the baseplate. 
         [0025]    Another aspect of the invention is a semi-hollow, foam filled femoral component that has more flexibility than current solid femoral components. The semi-hollow femoral component replaces the articular surface of the distal femur. The device is comprised of a thin solid articular surface, which provides a bearing surface to articulate with a tibial bearing and a patellar component or the natural patella. Solid re-enforcing struts protrude from the non-bearing side of the thin solid articular surface and are shaped to transfer load from the resected femur to the articular surface. These struts may be added using SLM technology. The volume remaining between the reinforcing struts, the resected femur and the thin solid articular surface is filled with an engineered foam structure such as a urethane foam. The thin articular surface is of constant or variable thickness, sufficient to provide the strength needed to handle the stresses induced at the articular interface. The solid re-enforcing struts provide mechanical strength and some stiffness to the component in order to handle the loads transferred between the resected femur and the articular surface. The profile of the surface formed by the ends of these struts can match the geometry of existing resections of contemporary knee systems (typically five; planar cuts formed in the distal end of the femur) or can be customized to a unique geometry to match alternate methods of resecting the distal femur. The foam filler provides some additional mechanical strength to the component as well as provides a surface in direct opposition to the bone for either biological fixation or cement interdigitation. 
         [0026]    In addition, in another aspect of the invention, a patella with porous surfaces may be manufactured using the material and methods of the present invention. The patella has three distinct layers produced entirely by the Selective Laser Melting (SLM) process to optimize the performance of the physical and mechanical properties of the component. The component is designed as a hybrid structure with a porous structure designed for bone ingrowth, an engineered structure designed for polyethylene interlock, and a fully dense structure that serves as barrier between the two porous structures. In addition, there is a fully dense core within each of the three patella pegs that provide additional mechanical strength to the structure. The porous layer designed to contact tissue and allow tissue ingrowth is approximately 1 mm thick. A 600 μm octahedral unit cell structure with 30 percent randomization is applied to replicate cancellous bone and provide optimal pore size and porosity for bone ingrowth. This structure has openings of 100-1000 μm in diameter with most recent measurements averaging 319 μm. The structure is 60-75% porous with most recent measurements averaging 68%. The porous layer designed for polymer interlock is designed to be under 1 mm thick. A 1000 μm octahedral unit cell structure is optimized for polymer molding operations. The polymeric material infiltrates the metallic grid and is stopped by the fully dense metal layer, preventing penetration into the tissue ingrowth layer. Likewise tissue cannot grow into the metallic grid. The fully dense intermediate layer is intended to separate the two porous layers preventing the polymer from penetrating the ingrowth layer during polymer molding operations. In addition, it provides structural support for the patella component to maintain dimensional stability. This layer is designed to be 381 μm thick. The peg portion of the patella component also features a fully dense region for structural and mechanical support. See WO 2011/056422 (PCT/US 2010/053314), the disclosure of which is incorporated herein by reference. 
         [0027]    One aspect of the invention is a tibial implant comprising a solid metal baseplate having a bone contacting surface. A porous metal spacer section is fused to the bone contacting surface of the baseplate. A fully dense stem portion having an attachment area is also fused to the porous metal spacer section of the baseplate. The fully dense stem portion is spaced at least 2 mm from the baseplate bone contacting surface by the porous metal spacer section. The tibial implant porous metal spacer area and the fully dense stem portion may be in the form of a v-shaped keel. The porous metal spacer area and the fully dense stem portion may comprise a plurality of solid reinforcing struts extending through the 2 mm porous area. The porous metal spacer section has a pore size of 100 to 400 μm and preferably has pores formed by open polyhedron structures. The polyhedron is an preferably octahedron. The fully dense stem portion may surround a porous core so that only the outer layer of stem is fully dense. 
         [0028]    Another aspect of the invention is a method for manufacturing the tibial baseplate implant, which includes providing a solid metal baseplate substrate and constructing a porous metal section on the metal baseplate substrate by layer by layer additive manufacturing using a laser (SLM). The substantially non-porous (fully dense) metal stem portion is constructed on the porous area by layer by layer additive manufacturing using a laser in a similar manner. The implant may be constructed in a manner where the porous area and the stem portion have a v-shape. A plurality of non-porous, i.e., solid struts may extend through the v-shaped porous area which struts can also be formed by SLM. The metal used is selected from the group consisting of Titanium, Titanium alloy and cobalt chrome molybdenum alloy. 
         [0029]    Another aspect of the invention is a method for removing a stem portion with a porous area as described above of an already implanted orthopedic implant from a bone comprising exposing an implanted orthopedic implant having a body portion and a stem portion interconnected to the body. The stem portion extends along an axis into a bone canal and has a porous metal section forming an interconnection between the bone and the stem portion. A cutting tool is mounted on a holder connected to an exposed surface of the orthopedic implant. The porous section is aligned with the cutting tool mounted on the holder. The surgeon cuts through the entire porous section of the implant by moving the cutting tool therethrough in a direction transverse to the stem portion axis. The surgeon then removes the implant body portion and stem portion from the bone. The method may further comprise slidably mounting the cutting tool on a guide rail extending from the holder by mounting a first end of the guide rail on the holder connected to the body portion of the implant. The guide rail extends from the holder along an axis transverse to the axis of the stem portion. The cutting tool is moved along the rail into contact with the porous metal section to cut the stem portion from the body portion. 
         [0030]    The method for removing the stem portion may further comprise slidably mounting a carriage on the rail, the carriage is coupled to the cutting tool and moving the carriage with a drive system towards the stem axis to perform the cutting operation. The drive system for moving the carriage may comprise a hand actuated clamp or may be electrically driven. 
         [0031]    Another aspect of the invention is an instrument for separating a stem portion of an already implanted orthopedic implant from a body portion thereof. The orthopedic implant having a porous interface area between the stem and body portions. The instrument includes a rail having a first end and a second end. A coupling element at the first end of the rail is provided for coupling the rail to the body portion of the orthopedic implant. The rail extends in a direction transverse to a longitudinal axis of the stem. A carriage is mounted on the rail for movement along the rail from the second end to the first end. A cutting tool is mounted on the carriage and spaced from the rail along the stem axis and positioned to engage the porous interface section between the body portion and the stem portion of the orthopedic implant. A hand actuated drive system engaging the carriage is capable of moving the carriage towards the first end of the rail. The implant may be a tibial component and the stem portion is a v-shaped keel having a proximal portion connected to a distal facing surface of the tibial component by the porous interface section. The cutting tool may have a stop element thereon spaced toward the second end of the rail from a cutting edge of the cutting tool. The cutting tool may be a saw or a chisel. The v-shaped keel typically has a cylindrical central stem portions with medially and laterally extending wing portions having a relatively narrow thickness in the anterior-posterior direction of 2-6 mm. 
         [0032]    Another aspect of the invention is a prosthetic femoral component for implantation on a prepared distal femur. The femoral component has a bearing element having an outer condylar surface for contesting a tibial bearing surface. The bearing element has an inner surface matching a shape of the outer condylar surface. A plurality of reinforced struts are formed on the inner surface of the bearing element and extend a distance therefrom. The struts have ends aligned in planes matching an anterior surface, a posterior surface, and a distal surface of a prepared distal femur. The struts form a plurality of voids therebetween, the voids stretching between the inner surface of the bearing and the ends of the struts. A polymeric foam is provided for filling the voids. 
         [0033]    The bearing element of the femoral component is formed of sheet metal or other metal having a thickness less than 0.125 inches. The thickness of the metal between the outer condylar surface and the inner surface of the bearing element may vary or may be uniform. The struts are made by additive layers by selective laser melting (SLM) titanium powder. 
         [0034]    The polymer foam is preferably made from urethane. 
         [0035]    Another aspect of the invention is a patellar component and a method of making the prosthetic patellar component. The method includes forming a bone contacting first metal layer by selective laser melting. The first layer has openings of 100-1000 μm and 60-75% porous. A fully dense intermediate metal second layer is formed on the first layer. A polymer contacting third metal layer in the form of a porous grid is formed on the solid metal layer. 
         [0036]    The method of making the patellar implant includes forming the fully dense layer with a plurality of solid pegs extending outwardly from a side the fully dense layer on which the bone contacting layer is formed. The pegs are received in bores formed in the remaining patella bone. Preferably the plurality of pegs are covered by the first metal layer. Ultrahigh molecular weight polyethylene is molded into the third porous grid metal layer to form the bearing surface. The polyethylene receiving grid openings are about 1000 μm. The third porous layer may have a convex outer surface such that a convex polyethylene bearing is formed. 
         [0037]    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. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]      FIG. 1  is a perspective view of a tibial baseplate, including a porous area intermediate the bone contacting surface of the baseplate and a solid metal v-shaped keel; 
           [0039]      FIG. 1A  is an elevation view of the tibial component of  FIG. 1 , including three solid struts extending through the porous area, the struts connecting the bone contacting surface and keel of the tibial baseplate; 
           [0040]      FIG. 2  is a cross-sectional view of the tibial component of  FIG. 1  along lines  2 - 2  including the central solid strut shown in  FIG. 1A ; 
           [0041]      FIG. 3  is an exploded view of an instrument for separating the keel portion of the tibial component from the bone contacting surface of the baseplate by cutting through the porous area connecting the same with an oscillating; 
           [0042]      FIG. 4  shows the instrument of  FIG. 3  assembled to the proximal surface of the tibial component, including an oscillating saw guide slot aligned with the porous area of the implant; 
           [0043]      FIG. 5  is an elevation view of the assembled instrument of  FIG. 4 ; 
           [0044]      FIG. 6  shows an alternate embodiment of the instrument of  FIG. 4  for separating the keel from the tibial baseplate using a chisel-like instrument; 
           [0045]      FIG. 7  is a perspective view of the instrument of  FIG. 6  coupled to the proximal surface of a tibial component with the chisel-like instrument aligned with the porous area of the tibial component; 
           [0046]      FIG. 8  is an elevation view similar to  FIG. 7 ; 
           [0047]      FIG. 9  shows an alternate tibial component, including the porous area between the tibial baseplate, the keel, and a plurality of spikes; 
           [0048]      FIG. 10  shows the tibial component of  FIG. 9  with the porous area removed to reveal solid reinforcing struts extending between the baseplate and the keel and through the spikes adapted to engage the proximal tibia; 
           [0049]      FIG. 11  shows a partial cross-sectional bottom view through lines  11 - 11  of  FIG. 10 ; 
           [0050]      FIG. 12  is a perspective view of a tibial component similar to that shown in  FIG. 9  with alternate design for the spikes; 
           [0051]      FIG. 13  is a cross-sectional through lines  13 - 13  of  FIG. 12 ; 
           [0052]      FIGS. 14-16  show alternate designs for the spikes used on the tibial component as shown in  FIGS. 9-13 ; 
           [0053]      FIG. 17  is a cross-sectional view of a semi-hollow femoral component manufactured by the process described herein; 
           [0054]      FIG. 18  is a plan view of a femoral component as shown in  FIG. 17 ; 
           [0055]      FIG. 19  shows a patella implant manufactured using the techniques described herein having a porous layer on either side of a fully dense layer and ultrahigh molecular weight polyethylene molded into one layer; 
           [0056]      FIG. 20  is a cross-sectional view of the patella implant shown in  FIG. 19  along lines  20 - 20 ; 
           [0057]      FIG. 21  shows the three layer metal backing of the patella component shown in  FIGS. 19 and 20 ; and 
           [0058]      FIG. 22  is a bottom view of the patella component shown in  FIGS. 19-21 . 
       
    
    
     DETAILED DESCRIPTION 
       [0059]    Referring to  FIG. 1 , there is shown a perspective view of a tibial component generally denoted as  10 , including a baseplate  12  with a bone contacting surface  14  and a proximally facing surface  15 . A keel  16  extends distally from the bone contacting surface  14  of baseplate  12 . Typically, keel  16  has a v-shape, including wings  18  and  20  on a medial and a lateral side of the tibial component  10 . While a tibial component is shown other implants such as patellar implants and femoral implants could incorporate pegs or stems having porous cross-sections. 
         [0060]    Plate  12  may be either solid or porous to allow bone ingrowth upon implantation of the tibial component bone connecting surface  14 . As shown in  FIG. 1 , a porous area  22  extends distally from bone contacting surface  14 . A porous area  22  is integrally formed on both bone contacting surface  14  and a proximal surface  24  of keel  16  below area  22 . While baseplate  12  and keel  16  are preferably solid, they could be porous or, to improve implant stiffness, could have a solid outer surface surrounding a porous core to improve flexibility. Also shown in  FIGS. 1 and 1A  are outer scalloped surfaces  28  extending in a proximal-distal direction. 
         [0061]    Referring to  FIG. 1A , there is shown an elevation view of a modified version of the tibial component  10  shown in  FIG. 1 . In this version, baseplate  12  and keel  16  are identical to that described above, however, porous area  22  is reinforced by three or more solid struts  26  extending from surface  24  to surface  14 . Struts  26  add portional strength to the construct while, as will be described below, without hindering the ability to separate baseplate  12  from keel  16  during revision surgery. Struts  26  are shown located centrally and adjacent ends of wings  18  and  20  of keel  16 . These struts are located to resist loads applied to implant  10  during use, which for a tibial implant is mainly in a proximal-distal direction. 
         [0062]    Referring to  FIG. 2 , there is shown a cross-sectional view along lines  2 - 2  of  FIG. 1A . 
         [0063]    As shown, keel  16  and baseplate  12  have solid struts  26 , which connect these two parts, two of which are shown in  FIG. 2 . Porous area or layer  22  is shown extending between top surface  24  of the solid portion of keel  16  to bottom bone contacting surface  14  of baseplate  12 . It should be noted that tibial components typically include features formed on their proximally locking surfaces  15  for mounting a polyethylene bearing element, and these features have been omitted from  FIGS. 1 ,  1 A, and  2  for simplification. It should also be noted that while a v-shaped keel has been described, a circular or part circular stem can be coupled via the porous area  22  to baseplate  12 . Such a stem would include a central solid strut or a plurality of struts extending from a bone contacting surface through the porous area  22  to provide extra strength for the tibial component  10 . It can also be seen that the outer surface of keel  16  includes scalloped areas  28 , which aid in resisting implant rotation but which could easily be eliminated rendering the keel outer surface planar. 
         [0064]    Referring to  FIG. 3 , there is shown a first embodiment of an implant removal tool generally devoted as  20  for removing an already implanted tibial component  100  from the proximal tibia during revision surgery. As is typical, the proximal portion of implant  100  includes an outer peripheral wall  102  surrounding a proximally facing surface  104 . As shown, a central intracondylar portion  106  separates condylar medial and lateral areas  108  and  110 . Also shown are typical features  111  adapted to lock a polyethylene bearing component (not shown) to the tibial component  100 . The baseplate portion  112  includes a bone contacting surface  114  and keel  116 , which are identical to that shown in  FIGS. 1-2 . Also shown is an adapter or holder  120 , which can be selectively mounted on proximally facing surface  104  of tibial component  100 , preferably using the same locking elements  111  used to couple the polyethylene bearing component to the tibial component. Adapter or holder  120  includes a locking element  122 , which can, in a first position, lock the adapter  120  to the tibial component  100  and in a second position allow for its removal. Also shown is a rail element  130 , which has a tongue  131  which mounts on in an opening  127  in adapter or holder  120  and slidably receives a saw-guide mounting element  134 , which can move in an anterior/posterior direction so that a slotted saw-blade guide  140  can be moved toward and away from the porous area between baseplate  112  and keel  116  (better seen in  FIG. 5 ). If desired, element  130  can be rotatably mounted with respect to baseplate  100  so it may swing in a medial-lateral direction ±60 degrees around axis  124  of holder  120 . A saw blade  144  is also shown, which blade can be mounted on any typical oscillating saw power unit. Typically slotted guide  140  can be slid up and down on a post  136  to vary the proximal-distal height of the slots. 
         [0065]    Referring to  FIGS. 4 and 5 , there is shown tibial component  100 , adapter  120 , rail  130 , and saw guide  140  assembled to slidable mounting element  134  for guiding saw blade  144  into the porous area  118  of baseplate  100 . Adapter or holder  120  is shown fixably mounted to central boss  106  of tibial baseplate  100  with locking element  122  in the lock position. Guide rail  130  has a leading end  132  mounted on actuator  120  such that slidable mounting element  134  can toward or away move cutting guide  140  toward and away from tibial component  100 . Saw guide  140  includes a plurality of slots  142  for guiding saw blade  144  at different proximal-distal levels. Saw guide  140  may be moved in a proximal-distal direction along post  136  to properly align saw blade  144  with the porous area  118  of tibial component  100 . The preferred embodiment a pair of spring elements  146  are used to selectively lock and release cutting guide  140  in the desired proximal-distal location along post  136 . As indicated above, saw blade  144  can be driven by any electric or pneumatic oscillating saw drive system and can be moved along rail  130  and, if desired, rotated about axis  124  to be able to cut the entire porous areas  22 ,  118  connecting the keel  16 ,  116  to baseplate  12 ,  112 .  FIG. 5  is identical to  FIG. 4  with the exception that oscillating saw blade  144  is now in contact with porous area  118  of tibial component  100 . 
         [0066]    Referring to  FIGS. 6 and 7 , there is shown an alternate instrument for separating keel  116  from baseplate  112 . In this system, an adapter  220  is mounted on tibial component  100  in a manner similar to that described above with regard to  FIGS. 4 and 5  with a rail  230  extending therefrom. A guide element  240  is slidably mounted on rail  230  via a slotted opening  242  for movement toward and away from tibial component  100 . Guide element  240  connects to an adapter  245 , which adapter  245  in turn is slidably mounted on element  240  for movement in a proximal-distal direction. Adapter  245  has a mounting element  246  with a bore  246 , which can releasably couple a chisel-like blade element  250  thereon via a coupling element  254 . Sliding element  240  may be impacted such as by a hammer once tip  252  of blade  250  engages with porous area  118  of tibial component  100 . In order to facilitate the separation of baseplate  112  from keel  116  through porous area  118 , rail  230  may be rotated on adapter  220  about axis  260  so that chisel-like blade  250  can impact area  118  at any position between the medial and lateral ends of v-shaped keel  116 . Normally, at least ±60° rotation about the sagittal plane is allowed by the structure. Thus, during use in revision surgery, the chisel-like blade can be advanced toward baseplate  100  as well as rotated with respect thereto about axis  260  thus allowing the surgeon to cut through porous area  118 . 
         [0067]    Referring to  FIGS. 9 and 10 , there is shown an alternate tibial component  300  with a baseplate  312  having an optional distal porous baseplate area  314 , a solid keel  316 , and a porous area  322  toward keel  316  and area  312 . Keel  316  is essentially the same as that described with regard to tibial component  10  and  100  as above. However, the tibial component  300  of  FIGS. 9 and 10  include two or more spikes  330 , which have a porous portion  332  and a solid tip portion  334 . As shown in  FIG. 10 , the porous portions  332  of the spikes  330  can have a solid core  336  to reinforce the connection of the spikes  330  to baseplate  312 . If baseplate  312  has a partially porous area  314 , the solid extension portions would extend therethrough to solid portions of the tibial baseplate  312 . As with tibial component  10 , tibial component  300  can have a central solid strut  326  and solid areas  328  further connecting keel  316  to baseplate  312 . 
         [0068]    Referring to  FIG. 11 , there is shown a cross-section along lines  11 - 11  looking up at the bottom surface  314  of the tibial component  300  of  FIGS. 9 and 10  showing solid portions  326 ,  328  which connect keel  316  and struts to tibial baseplate  312 . Spikes  330  are also shown with porous portions  332  and solid portions  336 . 
         [0069]    Referring to  FIG. 12 , there is shown an alternate embodiment  300   a  of tibial component  300 . This embodiment is in all respects the same as  300  with the exception of the design of the spikes  330   a , which have a cruciform shape. As can be seen, there are identical porous areas  332   a  on the v-shaped keel  316   a  and on areas  332   a  or spikes  330   a . As seen in  FIG. 13 , spikes  330   a  have solid cores  336   a  surrounded by porous areas  332   a . Also shown is solid central strut  326   a  surrounded by porous area  322   a . The porous area  314   a  and  322   a  are also identical as those shown with regard to tibial component  300 . These elements are shown in  FIG. 13  in cross-section along lines  13 - 13  of  FIG. 12 . Various design spikes are shown in  FIGS. 14-16  with  FIG. 15  showing spikes similar to  330   a  and  FIG. 16  showing spike  330   b  with  FIG. 16  showing at an alternate spike design  330   c . All the spikes include solid cores on porous areas  332   a, b, c  surrounding the cores. 
         [0070]    Referring to  FIGS. 17 and 18 , there is shown a semi-hollow femoral component generally denoted as  400  manufactured by the process described hereinbelow in which a thin solid articular element  401  is formed with an outer condylar bearing surface  414  followed by a plurality of solid plate-like reinforcing struts  402 , forming a grid pattern and having outwardly facing surfaces  403 ,  404 ,  406 ,  408 , and  410  matching the surface of a prepared femur (not shown). The femur is prepared in a well-known fashion to have distal, anterior, posterior, anterior chamfer, and posterior chamfer planar surfaces on which the prosthetic femoral component  400  is mounted. As shown in  FIG. 17 , the anterior inner surface  402  matches with the prepared planar femur anterior surface, surface  404  matches with the planar anterior chamfer surface, surface  406  matches with the planar distal surface of the femur, surface  408  matches with the planar posterior chamfer surface of the femur and surface  410  matches the prepared planar posterior surface of the femur. As indicated above, surfaces  402 ,  404 ,  406 ,  408 , and  410  are formed by the end surface of spaced struts  403 , which extend inwardly from an inner surface  413  of element  401  a distance sufficient to reinforce the thin articular element  401 . As shown in  FIGS. 17 and 18 , a pair of pegs  412  extend proximally of distal surface  406  for insertion into blind bores formed in the distal surface of the prepared femur. The articulating surface  414  of element  401  of femoral component  400  is identical or similar to known solid femoral components. Surface  414  has a posterior condylar areas  418  and anterior condylar areas  416 , including a recessed area  420  on which the patella (not shown) articulates. In the preferred embodiments, the areas between plate-like struts  403  are filled with a foam material  430 , such as urethane foam, to further stiffen the femoral component. Plate-like outer bearing surface  401  may be hydroformed from sheet metal such as Titanium to have medial and lateral condyles on an intracondylar notch. The metal may be less than 0.125 inches thick. 
         [0071]    Referring to  FIGS. 19-22 , there is shown a patella generally denoted as  500 , having a three-layer metal backing which is fabricated by the methods set forth below. Patella  500  includes a polyethylene bearing surface  502  mounted onto a first layer comprising a porous layer  504  of the patella  500 , which layer  504  is formed on top of a solid or fully dense intermediate metal element  506 , which in turn has a porous bone contacting layer  508  made of a second porous element formed thereon. Elements  504 ,  506 , and  508  are formed by the selective laser melting (SLM) techniques set forth below in a single additive manufacturing operation. After the metal backing is formed, the polyethylene bearing element  502  is molded on porous layer  504 . Solid or fully dense portion  506  gives the patellar implant structural integrity. Referring to  FIG. 22 , there is shown a bottom view in which three pegs  512  are shown which are adapted to engage bores in the prepared bone of the natural patella. Pegs  512  have solid cores  516  forming an integral part of fully dense or solid area  506 . Cores  516  are surrounded by a porous metal area  508 , which is adapted to allow bone tissue to grow therein. Porous area  504  is adapted to allow the infusion of the polyethylene bearing surface  502  as described in co-pending U.S. PCT Application PCT/US 2010/053314 (WO 2011/056422), the disclosure of which is incorporated herein by reference. The porous surface  504  may be convex to help form a convex UHMWPE bearing surface of the patella which typically engages the intracondylar area of the femoral implant. 
         [0072]    The tibial component  10 ,  100 , and  300  of the present invention as well as femoral component  400  and patella  500  are made at least in part by forming solid, porous and partially porous metallic structures. The method makes use of SLM laser technology by employing a variety of scanning strategies. Typical metal and metal alloys employed include stainless steel, cobalt chromium alloys, titanium and its alloys, tantalum and niobium, all of which have been used in medical device applications. This method can be used for medical device applications such as in the tibia where bone and soft tissue interlock with the tibial component is required, or where a controlled structure is required to more closely match the mechanical properties of the device with surrounding tissue. 
         [0073]    The method produces a three-dimensional structure using a laser remelting process, for example, for building structures utilizing unit cells onto a premade solid tibial baseplate  12 . When applied to a tibial component, the three-dimensional structure is used to provide porous spacer area  22  between the baseplate and a stem or keel  16 . Keel  16  is preferably solid and is formed by the same laser remelting or fusion process as the porous spacer area  22 . Thus a tibial component can be made by building porous spacer  22 /solid keel  16  on premade baseplate  12  during a continuous operation to form a bone ingrowth structure and a solid keel or stem portion or a keel or stem with a solid surface and a porous core. The preferred materials used for both the baseplate and stem are titanium, cobalt chrome and tantalum but both stainless steel and niobium can also be used as well as any other suitable material. The tibial component  10  may be built from any of these materials, with the preferred material being titanium (Ti). The described method can be exploited on a commercial basis for the production of, for example, bone interlock surfaces on a device although it has many other uses. 
         [0074]    According to the preferred method of forming a three-dimensional structure includes building the shape by laser melting powdered Ti and its alloys, stainless steel, cobalt chrome alloys, Ta or Nb using a continuous or pulsed laser beam. Individual layers of metal are scanned using a laser. Each layer or portion of a layer is scanned to create a portion of a plurality of predetermined unit cells, as will be described below. Successive layers are deposited onto previous layers and also may be scanned. The scanning and depositing of successive layers continues the building process of the predetermined unit cells. As disclosed herein, by continuing the building process refers not only to a continuation of a unit cell from a previous layer but also a beginning of a new unit cell as well as the completion of a unit cell. 
         [0075]    The method can be performed so that the structure is either porous or solid and, if porous, the pores can be interconnecting to provide an interconnected porosity. 
         [0076]    The method includes using a tibial baseplate of cobalt chrome alloy, titanium or alloy, stainless steel, niobium and tantalum, on which to build the porous layer of any one of the aforementioned metals and alloys by laser melting using a continuous or pulsed laser beam. Thus, a mixture of desired mixed materials can be employed. 
         [0077]    The method includes a laser melting process which precludes the requirement for subsequent heat treatment of the structure, thereby preserving the initial mechanical properties of the baseplate metal. The equipment used for the manufacture of such a device could be one of many currently available including the MCP Realiszer, the EOS M270, Trumpf Trumaform 250, the Arcam EBM S12 and the like. The laser may also be a custom produced laboratory device. 
         [0078]    The pore density, pore size and pore size distribution of the porous spacer area can be controlled (varied) from one location on the structure to another. It is important to note that successive powder layers can differ in porosity by varying factors used for laser scanning powder layers. Additionally, the porosity of successive layers of powder can be varied by either creating a specific type of unit cell or manipulating various dimensions of a given unit cell. 
         [0079]    To produce a porous spacer area structure, the nature of the material formed as a result of laser melting of powdered beads is principally dependent on the thermal profile involved (heating rate, soaking time, cooling rate); the condition of the raw material (size and size distribution of powder particles); and atmospheric conditions (reducing, inert or oxidizing chamber gas). 
         [0080]    There have been a number of studies to determine the optimum pore structure for maximization of bone ingrowth on prostheses. The general findings suggest that optimum porosity is between approximately 20% and 40%, and aim to mid value with a mean volume percent of voids of about 70%. The preferred pore structure is interconnected, with a minimum pore size between about 80 μm and 100 μm and a maximum pore size between 80 μm and 800 μm. The structured thickness for ingrowth is 1.4-1.6 mm, but can be larger or smaller depending on the application. 
         [0081]    In the present method the porous spacer structure is built in the form of a plurality of unit cells. Many designs of unit cells are possible to give the shape, type, degree, and size of porosity required. Such unit cell designs can be dodecahedral, octahedral, diamond, as well as many other various shapes. Additionally, besides regular geometric shapes as discussed above the unit cells of the present invention may be configured to have irregular shapes where various sides and dimensions have little if any repeating sequences. The unit cells can be configured to constructs that closely mimic the structure of trabecular bone for instance. Unit cells can be space filling, all the space within a three-dimensional object is filled with cells, or interconnected where there may be some space left between cells but the cells are connected together by their edges. 
         [0082]    The cells can be distributed within the construct a number of ways. Firstly, they may be made into a block within a computer automated design system where the dimensions correspond to the extent of the solid geometry. This block can then be intersected with the geometry representing the component to produce a porous cellular representation of the geometry. Secondly, the cells may be deformed so as to drape over an object thus allowing the cells to follow the surface of the geometry. Thirdly, the cells can be populated through the geometry following the contours of any selected surface. 
         [0083]    The unit cell can be open or complete at the surface of the construct to produce a desired effect. For instance, open cells with truncated lattice struts produce a surface with a porosity and impart the surface with some degree of barb. 
         [0084]    Modifying the lattice strut dimensions can control the mechanical strength of the unit cell. This modification can be in a number of key areas. The lattice strut can be adjusted by careful selection of build parameters or specifically by changing the design of the cross-section of each strut. The density of the lattice can similarly be adjusted by modification of the density of the unit cells as can the extent and shape of porosity or a combination thereof. Clearly the overall design of the unit cell will also have a significant effect of the structural performance of the lattice. For instance, dodecahedral unit cells have a different mechanical performance when compared to a tetrahedral (diamond) structure. 
         [0085]    The two key parameters used to define the relations regarding height, surface area, space height, volume of tetrahedron, and the dihedral angle of a tetrahedron are the strand length of the tetrahedron and, i.e., the diameter or height and width, cross section area of the strand, i.e., strut. These two parameters control the pore size and porosity of the structure. The parameter editor and relation editor within a typical CAD system can be used to control these parameters. Hence, by changing the parameters one can change the fundamental properties of the porous structure. The diamond or octahedral structure may have a circular cross-section strands or square cross-section strands. 
         [0086]    A diamond or octahedral lattice structure can be made with and without laser beam compensation. Laser beam compensation essentially allows the diameter of the beam to be taken into account. Without it the constructed geometry is one beam diameter too wide as the beam traces out the contour of the particular section being grown. When laser beam compensation is utilized, the contour is offset half a beam diameter all around the constructed geometry which is represented in the CAD file. Although various parameters may be used, the parameters employed to create the porous ingrowth spacer include a laser power of 90-100 watts with an exposure time of 1,000 psec from a point distance of about μm. 
         [0087]    As shown in FIGS. 5A and 5B of U.S. Publication 2006/0147332, the preferred unit cell for the porous structures of the present invention may be constructed in the shape of a truncated octahedron. A truncated octahedron has eight regular hexagonal faces, six regular square faces, twenty-four vertices, and thirty-six edges. A square and two hexagons meet at each vertex. When the octahedron is truncated, it creates a square face replacing the vertex, and changes the triangular face to a hexagonal face. This solid contains six square faces and eight hexagonal faces. The square faces replace the vertices and thus this leads to the formation of the hexagonal faces. It should be noted here that these truncations are not regular polydra, but rather square-based prisms. All edges of an archamedian solid have the same length, since the features are regular polygons and the edges of a regular polygon have the same length. The neighbors of a polygon must have the same edge length, therefore also the neighbors and so on. As with previous unit cells, various dimensions such as the octahedron height, octahedron volume, octahedron surface area, octahedron dihydral angle, and truncated octahedron volume, truncated octahedron height, truncated octahedron area, truncated octahedron volume, truncated octahedron dihydral angle can be determined by simple trigonometry and are known by those skilled in the art. 
         [0088]    In a method of use, a CAD model of the truncated octahedron is constructed using the sweep feature and calculations of between 10 and 1000 um and dimensions are incorporated using basic trigonometry. To tessellate the unit cell, the unit cell is first reoriented to enable easy tessellation and to reduce the number of horizontal struts in the model. Further, the model can be modified to remove all of the horizontal struts as shown in FIG. 7A of U.S. 2006/0147332. The modified structure is reproduced in order to save file size in the Steriolithography (“STL”) format of the program. Next, in order to create the unit cells, the method of using a laser melting process is performed. In one preferred embodiment, the parameter chosen includes a laser power of 2260 watts for a bone ingrowth structure, 2512 watts for the solid structure an exposure of 410 and 200 psec respectively with a focus distance of 1593 μm and, for the porous construct a point of distance of 65 μm. FIG. 7b of US 2006/0147332 illustrates a lattice structure formed using a plurality of individual truncated octahedron. The removal of various struts can create a barb effect on the exterior surface of the lattice structure. 
       Example 
       [0089]    The following is a method for making a partially porous implant and porous area by near-net shape building of a modular tibial tray component  10  of  FIGS. 1-2  via additive (layer-by-layer) manufacturing. 
         [0090]    A solid (non-porous) tray preform (e.g., forging) is partially or entirely machined to the final implant shape for example a tibial baseplate. 
         [0091]    A layer-by-layer (additive manufacturing) direct build onto the solid tray preform  12  (e.g., Selective Laser Melting-SLM). 
         [0092]    The construct is a hybrid SLM-built structure that is a combination of (a) porous SLM with a preferred porosity between 4-80% and a pore size in the range of 10 μm to 1000 μm, in which 100-400 μm is preferred for bone ingrowth and a “fully dense” SLM-built material that is less than 20% porous. This porous structure  22  is between 1 and 5 mm thick followed by a solid keel section  16 . Both structures  16  and  22  may have a v-shape. 
         [0093]    The SLM-built structure consists of a fully porous area  22  and a bone fixation keel  16 , extending from the metaphyseal seating surface  14  of an orthopedic implant (such as tibial component  10 ) comprised of a porous portion adjacent to the metaphyseal porous seating surface and a fully dense portion extending to the diaphysis. Additionally, the fully dense keel portion  16  may be another hybrid structure where the center may be porous to reduce implant stiffness. 
         [0094]    The SLM-built structure and the substrate the SLM-built structure is built onto are both integral parts of the implant. There is not removal of the SLM-built structure from the substrate (no EDM). That is, the fixturing for the additive manufacture process (SML) is part of the implant. 
         [0095]    The SLM process uses only localized heating and, therefore, there is little to no affect from the heat on the underlying substrate mechanical properties. Further details of this manufacturing process are described in commonly owned U.S. patents and applications 2006/0147332, 2007/0142914, 2008/0004709, 2010/0010638, 2010/0291286, 2011/0014081, and U.S. Pat. No. 7,537,664, all the disclosures of which are incorporated herein by reference. 
         [0096]    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.