referring now to the drawings , and particularly to fig1 there is shown an embodiment of a mold assembly 10 which can be utilized to form a porous surface on the outer surface of an orthopaedic implant . mold assembly 10 generally includes a first mold half 12 and a second mold half 14 . second mold half 14 includes a flange 16 for positioning first mold half 12 relative thereto . further , second mold half 14 includes a contoured surface 18 which is either integral with or separably associated with second mold half 14 . as illustrated , second mold half 14 is configured to accommodate and otherwise support a metal acetabular cup 1 . it should be understood that while an acetabular cup is illustrated , this should not be considered a limitation on the invention . for example , the invention could apply equally to the fixation of a porous layer to the exterior surface of a femoral hip stem implant , or femoral knee joint component with the molds altered respectively . as another example , contoured surface 18 may be in the form of a prosthetic implant which is separately associated with second mold half 14 . first mold half 12 , with acetabular cup 1 supported thereon , and contoured surface 18 of second mold half 14 define a mold cavity 20 therebetween . mold cavity 20 is in communication with an inlet 22 which is sized and configured for receiving metallic particles 24 therein . metallic particles 24 may be in the form of metal beads . inlet 22 is also adapted to receive a binder therein , such as a water - soluble protein mixture 26 . in the embodiment shown in fig1 protein mixture 26 is in the form of gelatin 26 having an alloying material ( to be discussed hereinafter ) therein . gelatin 26 is preferably suspended in water at a concentration between 0 . 10 % to 50 % by weight . glycerin may also be added to the gelatin mixture as a plasticizer . in a preferred embodiment , the gelatin mixture consists essentially of 10 ml of water , 0 . 75 grams gelatin , and 0 . 02 to 0 . 05 grams glycerin . during use , metallic particles 24 and gelatin 26 may be mixed together and introduced into mold cavity 20 , such that the mixture substantially and entirely fills mold cavity 20 about the outer surface of acetabular cup 1 . mold cavity 20 is configured to form a shell about the cup having a desired shape with the mixture of metallic particles 24 and gelatin 26 . after the mixture fills the mold cavity , gelatin 26 is allowed to set - up or harden within mold cavity 20 . alternatively , the mixture , mold and implant may be frozen to allow the mold halves to be separated and the implant removed . in either instance , the implant with the gelatine and bead mixture adhered thereto is allowed to dry for a period of time . drying causes the gelatine to become very hard . after drying , implant 1 with shell 28 adhering thereto is placed within a furnace 30 shown in simplified form in fig2 . furnace 30 provides the dual functionality of both converting the gelatin mixture substantially to carbon , and bonding metallic particles 24 together via sintering . more particularly , after shell 28 is place within furnace 30 , the air within furnace 30 is evacuated using a pump or other suitable mechanism ( not shown ) down to a pressure of 1 × 10 - 5 torr pressure . furnace 30 is thereafter backfilled by pumping an inert gas such as high purity argon therein to a pressure zero ( 0 ) psig . subsequently , furnace 30 is again evacuated by pumping the high purity argon within furnace 30 down to a pressure of 1 × 10 - 5 torr . this evacuating and backfilling process is repeated two additional times and furnace 30 is thereafter backfilled with high purity argon to a partial pressure of at least 100 micrometers . reactive gases such as nitrogen , hydrogen and mixture of nitrogen and hydrogen gas can also be used in the process . the temperature within furnace 30 is then increased at a rate of 25 ° f . per minute to a predetermined temperature of 1 , 000 ° f . and held at this temperature for 15 minutes . furnace 30 is then heated again at a rate of 20 ° f . per minute to a sintering temperature of 2 , 350 ° f . and held at this temperature for a time period of between 15 minutes to 8 hours which is effective for carrying out a sintering process which provides an implant having a suitable bead bond strength for a particular application . it is anticipated that the preferred cycle time will be approximately 2 hours . in the embodiment shown in fig1 and 2 , metallic particles 24 and gelatin 26 are mixed prior to delivery within mold cavity 20 . however it is to be understood that metallic particles 24 can be introduced into mold cavity 20 , and gelatin 26 can thereafter be injected into mold cavity 20 . when shell 28 is disposed in furnace 30 and the temperature therein is increased at the predetermined rate as described above , at a temperature around 1300 degrees fahrenheit the gelatin mixture within shell 28 is converted essentially into residual carbon which then covers metallic particles 24 . the carbon defines an alloying material which is diffused into metallic particles 24 from which metallic particles 34 are made ( fig5 ). further , as the furnace temperature ramps up , all volatile constituents in the binder are removed leaving only the carbon . for example , fig5 illustrates a fragmentary , sectional view of a metal bead after the diffusion of carbon 32 therein from the binder as described above to form metal bead 34 . since the gelatine contacts only the periphery of the bead , carbon 32 may be disposed at a higher concentration about the periphery 36 of metal bead 34 , as shown ( fig5 ). the carbon illustrated deeper within the bead represents carbon already present in the bead . carbon 32 within alloyed metal bead 34 results in a lower melting point at the interface surface ( e . g ., periphery ) of alloyed metal bead 34 . that is , the eutectic composition of alloyed metal bead 34 having carbon 32 therein results in an interface surface 36 having a eutectic melting point which is less than the melting point of the co -- cr -- mo alloy from which alloyed metal bead 34 is initially constructed . the sintering process takes place at a temperature which is less than the eutectic melting point of alloyed bead 34 , which in turn is less than the melting point of the co -- cr -- mo alloy from which alloyed metal bead 34 is constructed . in the embodiment shown in fig5 the alloying material within alloyed metal bead 34 is in the form of carbon 32 , as described above . however , it is also possible that the alloying material can be in the form of silicon , ferrosilicon , f - 75 alloy , and / or iron . such alloying materials are typically in the form of a powder which is suspended within gelatin 26 ( fig1 ). referring now to fig3 an enlarged , fragmentary view of shell 28 shown in fig2 is illustrated prior to carbonization of gelatin 26 . metallic particles 24 are in the form of metal beads 24 which are held together using gelatin 26 . metal beads 24 may contact adjacent metal beads 24 along respective interface surfaces 36 ( fig3 and 5 ). referring now to fig4 an enlarged , fragmentary view of an alternate embodiment of a shell is shown . a metal layer is partially illustrated formed from metallic metal mesh as is well known in the industry . the metal mesh is formed from a plurality of metal fibers 38 which are in contact with each other at respective interface surfaces 40 . metal fibers 38 are held in place prior to sintering using gelatin 26 ( fig1 ) which has been allowed to set - up . in the embodiments described above , metallic particles 24 and binder 26 are introduced within mold cavity 20 of mold assembly 10 . however , it is also to be understood that a mixture of metallic particles 24 and binder 26 can be manually applied to an orthopaedic implant surface , such as a contoured surface 18 shown in fig1 . further , it is also possible to use a mixture of metallic particles 24 and binder 26 within a compression molding and / or injection molding machine to form a shell 28 &# 39 ;. as illustrated in fig6 a shell 28 &# 39 ; is formed using a mold 10 &# 39 ; having an upper mold portion 12 and a lower mold portion 14 . a mold cavity 20 &# 39 ; is formed between the upper and lower mold portions and may be filled with metal beads 24 and binder 26 . after molding , the shell 28 &# 39 ; is dried and then sintered in oven 30 ( fig7 ) to form a rigid porous shell . the rigid porous shell 28 &# 39 ; is may then be placed in an injection mold machine to form the porous backing for a molded acetabular cup . a further example of the usefulness of the binder method of this invention is illustrated in fig8 a wherein a fiber metal mesh pad 40 is connected to an implant 42 via a layer of small beads 44 . in use , the beads 44 are layered on the implant and the fiber mesh is supported by the layer of beads . the beads and mesh are coated or impregnated with the binder . the binder and implant are processed in a manner consistent with the above method . during sintering , the small beads are bonded to the fiber mesh and to the implant . alternatively , several spot welds 43 ( preferably formed by a laser welder ) may be used ( fig8 b ) to fix the metal pad 40 and beads 44 in place on the implant 42 prior to sintering . the advantage of such a construction would be two fold . first , the bead would present a greater surface area and thereby bond better with the implant as compared to fiber metal . second , the fiber metal is believed to form a better area for the ingrowth of bone as compared to the beads . while it is believed that the binder will , when dry , adequately hold a layer of fiber metal mesh to the implant during sintering , fig9 illustrates a potential variation of the method of the invention . in the embodiment of fig9 after the fiber metal is positioned on the implant , several spot welds 45 are made to fix the metal mesh 46 to the implant 48 prior to sintering . either before or after the spot welding , the metal mesh is impregnated with the binder material consistent with the above description . it has been found that if cobalt - chromium - molybdenum metallic beads 24 are used which have an initial concentration of 0 . 24 to 0 . 26 % carbon , the final carbon content within metal beads 34 ( fig3 and 5 ) using the process of the present invention is approximately 0 . 31 %. it is therefore possible to vary the final carbon content within metal beads 34 by varying the initial carbon concentration within metal beads 34 and the concentration of carbon within gelatin 26 . while this invention has been described as having a preferred design , the present invention can be further modified within the spirit and scope of this disclosure . this application is therefore intended to cover any variations , uses , or adaptations of the invention using its general principles . further , this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims .
the invention is directed to a method of forming an implant having a porous surface using an organic binder compound to enhance the bonding between the porous surface layer and implant . preferably , the binder is formed from a water - soluble protein that carbonizes during the sintering process to alloy with the metal of the porous surface layer . the porous surface layer may be in the form of beads or of fiber metal and can be preformed to fit with an implant or formed over the surface of the implant .