referring to fig1 , 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 . 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 fig1 , 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 fig1 and 1a are outer scalloped surfaces 28 extending in a proximal - distal direction . referring to fig1 a , there is shown an elevation view of a modified version of the tibial component 10 shown in fig1 . 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 . referring to fig2 , there is shown a cross - sectional view along lines 2 - 2 of fig1 a . as shown , keel 16 and baseplate 12 have solid struts 26 , which connect these two parts , two of which are shown in fig2 . 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 fig1 , 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 . referring to fig3 , 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 fig1 - 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 fig5 ). 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 . referring to fig4 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 . fig5 is identical to fig4 with the exception that oscillating saw blade 144 is now in contact with porous area 118 of tibial component 100 . referring to fig6 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 fig4 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 . referring to fig9 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 fig9 and 10 include two or more spikes 330 , which have a porous portion 332 and a solid tip portion 334 . as shown in fig1 , 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 . referring to fig1 , there is shown a cross - section along lines 11 - 11 looking up at the bottom surface 314 of the tibial component 300 of fig9 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 . referring to fig1 , 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 fig1 , 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 fig1 in cross - section along lines 13 - 13 of fig1 . various design spikes are shown in fig1 - 16 with fig1 showing spikes similar to 330 a and fig1 showing spike 330 b with fig1 showing at an alternate spike design 330 c . all the spikes include solid cores on porous areas 332 a , b , c surrounding the cores . referring to fig1 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 fig1 , 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 fig1 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 . referring to fig1 - 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 fig2 , 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 ). 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 . 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 . 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 . 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 . 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 . 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 . 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 . as shown in fig5 a 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 . 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 fig7 a 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 . fig7 b 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 . 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 fig1 - 2 via additive ( layer - by - layer ) manufacturing . a solid ( non - porous ) tray preform ( e . g ., forging ) is partially or entirely machined to the final implant shape for example a tibial baseplate . a layer - by - layer ( additive manufacturing ) direct build onto the solid tray preform 12 ( e . g ., selective laser melting - slm ). 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 . 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 . 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 . 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 . 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 .
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 .