CA2026301A1

CA2026301A1 - Cast bone ingrowth surface

Info

Publication number: CA2026301A1
Application number: CA002026301A
Authority: CA
Inventors: Larry J. Gustavson; Melvin M. Schwartz
Original assignee: Pfizer Hospital Products Group Inc
Current assignee: MTG Divestitures LLC
Priority date: 1989-09-28
Filing date: 1990-09-26
Publication date: 1991-03-29
Also published as: JPH03123546A; US5108435A; KR930002565B1; IE903473A1; EP0420542A1; AU6329490A; IL95730A; JPH064082B2; IL95730A0; DE9011363U1; FI904758A0; KR910005829A; ZA907724B; AU619134B2

Abstract

PC(HO) 7655 ABSTRACT
A prosthetic part for use as an orthopaedic implant has a cast metal base member and a tissue ingrowth surface spaced outwardly therefrom. The tissue ingrowth surface is in the form of a cast metal lattice element which covers at least a part of the outer surface of the base member . The cast metal lattice element is cast simultaneously and integrally with the base member from the same metal. This metal may be any well known castable material for ortho-paedic implants such as Vitallium or titanium. The lattice element is in the form of a grid-like mesh which includes spacer members cast integrally with the wire mesh and the base member to space the lattice element a predetermined distance above the prosthesis surface. An investment casting technique wherein a meltable material is coated with a ceramic casting shell is utilized to produce the integrally cast orthopaedic implant and tissue ingrowth surface.

Description

hJ '.~ J ~ PC ~ J
CAST BONE INGROWTH SU~FACE

BACKGROUND OF THE INVENTION

Field of the Invention This invention relates to an integrally cast tissue ingrowth surface apparatus and a method for casting the same. Nore particularly, the invention relates to an integrally cast bone or tissue ingrowth surface in a cast metal orthopaedic prosthesis.
Description of the Prior Art Investment casting or the "lost wax process" has heen used for over 50 years in the production of medical and dental implants. The process derives its name from the investment of wax or other suitable mold material and ceramics used to produce an expendable mold for casting.
The investment casting process used to produce orthopaedic implants is common to the industry and is used to produce implants from Co-Cr-Mo ("Vitallium") alloys as well as titanium alloys and stainless steels. Being well suited to the manufacture of the complex shapes typical of many implant designs, investment casting is used exten-sively to produce components for bone implants or total joint prostheses such as knees and hips.
These prostheses typically consist of metallic and polymeric components where the metallic components rest against bone on one side of the joint and bear against the polymeric component on the other. The bearing surfaces of a total joint have evolved in design to closely mimic the movement of the natural joint, while the bone contacting sides have evolved to assure improved fixation of the implanted prosthesis with the surrounding bone.
Until recently, total joint prostheses were designed for implantation with bone cement. For example, a polymethyl-methacrylate (PMMA) grouting agent may be used to secure the prosthesis component against the surrounding bone. Implant surface~ contacting the cement were either 2~ Jv;j~ ~

cast smooth or with a two dimensional texture intended to improve fixation with the PMMA grout.
Recurrent loosening of these cemented implants, due to loss of support in underlying bone, lead to the development of prostheses with three dimensionally porous fixation surfaces which could be used without the PMMA bone cement.
These prostheses instead rely on fixation via the ingrowth of bone or other connective tissue directly into the prosthesis surfaces, thereby anchoring the prosthesis to lo the bone.
These three dimensionally textured surfaces are created by bonding a suita~le network of material, usually metal of the same composition as the implant, onto the implant's fixation surfaces to create a porous coating.
The nature of the porosity present in the coating is generally a direct function o~ the materials and methods used to produce the coating.
Porous surfaces have been created by plasma spraying (U.S. Patent 3,605,123) of fine metallic particles, or by sintering a loosely packed coating of metallic particles (U.S. Patent 4,550,448, British Patent 13168093, or by diffusion bonding kinked fiber metal pads (U.S. Patent 3,906,55 ), or overlapping mesh (U.S. Patent 4,636,219).
In another conceptj integrally formed ceramlc filled porous areas are formed on the prosthesis. U.S. Patent 4,722,870 discloses a method for investment casting a composite implant which produces a porous metal tructure filled with a ceramic (hydroxyapatite). However, this structure cannot be accurately controlled nor can it be spaced a predetermined distance above the outer surface of the implant.
Other U.S. patents describe mesh surfaces welded to the implant. Such a mesh is shown in U.S. Patent 3,905,777 to Lacroix, 4,089,071 to Kalnberz et al, 4,261,063 to Blanquaert and 4,636,219 to Pratt et al. None of these surfaces are integrally cast with the prosthesis.

'~ , ' , Each of the aforementioned methods for producing a porous ingrowth surface Pntails applying a porous network onto th2 surface of a metallic implant and bonding that network through the application of heat. Plasma spraying S employs super heated gases to melt the metal particles to be sprayed. Sintering develops interparticle bonds in a porous coating by exposing the coating and implant metal to temperatures approachiny their melting point, while diffusion bonding employs heat and pressure to promote atomic diffusion at the coating implant interface.
Each of these methods has its limitations. Plasma spraying cannot be adequately controlled to achieve a uniform interconnected pore structure in the coating. The temperatures required for sintering hava a deleterious effect on the implant materials strength, and diffusion bonding develops variations in pore structure and bond quality due to variations in pressure distribution during the coating procass. Each of the processes is limited in its achievable pore size by the loss in coating strength which occurs as coating porosity increases.
Particulate porous coatings are also inherently accompanied by a dramatic increase in surface area of metal exposed `to body fluids, increasing proportionally the corrosion products which are released after implantation.
Clinical reports exist of metal particles becoming loose from bonded coatings or fiber pads becoming detach~d on revision surgery. Furthermore, bonded coatings inher-ently develop stress concentrating surface notches at the coating-substrate interface which limit the locations a porous coating can be placed due to strength considera-tions. By their very nature, bonded coatings require the use of a secondary manufacturing process to affix the coating to the implant surfaces. These processes increase manufacturing costs through added labor, materials, tooling and fixturing.

- 4 - 646~0-562 SUMMARY OF THE INVENTION
An aspect of -the present invention provides a prosthetic part for use as an orthopaedic implant comprising:
a cast metal base member havlng an outer surface; and a tissue ingrowth surface in the form of a cast rnetal lattice element attached to the base member, over at least part of the outer surface by being cast integrally with the base member from the same metal.
Another aspect of t~e present invention provides a prosthetic part cast from a single metal for use as an orthopaedic implant, comprising: ;
a cast base member;
a cast tissue ingrowth surface spaced a predeterrnined distance from the base member, and ~ :
means for attaching the tissue ingrowth surface to the base member, the means for~attaching being integrally cast wlth the base member and the tissue ingrowth surface.
A still further aspect of the present invention pro-vides a method for forming tissue lngrowth surfaces on metal orthopaedic implants cast by an investment casting techni~ue where-in a meltable material is coated with a ceramic casting shell, the meltable material having a melting point lower than the ceramic, comprising the steps of:
forming a pattern corresponding to the orthopaedic implant from the meltable material;
forming a lattice from the meltable material, the - 4a - 64680-562 lattice corresponding to a predetermined tissue ingrowth surface;
~ oining the lattice to the pattern corresponding to the orthopaedic implant;
coating the joined combination of the pattern and the lattice with a ceramic slurry to form the casting shell;
removing the meltable material from the casting shell by heating; and filling the empty casting shell with mol'en metal and allowing it to cool to form a one-piece casting, in the form of the pattern corresponding to the orthopaedic implant with the lattice integral therewith.
The prosthetic part of the present invention has a cast metal base member having an outer surface designed to rest against a bone after implantation. A tissue Lng~h surface in the form of a cast metal lattice element composed of a grid-like element spaced a predetermined distance away from the base member outer surface may be integrally cast with the base member, from the .
same metal, over at least a part of the outer surface thereof.
The metal utilized may be "Vitallium", titanium alloy or`other suitable biocompatible metallic alloys.
Either the tissue ingrowth surface element or~the cast metal base member may include spacer elements to space the lattice element a predetermined distance above the outer surface of the base member. If the lattice element is in the form of a square or rectangular grid this permits tissue to grow over and under the cross members of the lattice element. The lattice element may be - 4b - 64680-562 in the form of a wire mesh integrally cast with the base member and spaced therefrom.

.

~ .3 An ~nvestment casting techniqu~, wherein a meltable material is coated with a ceramic casting shell may be utilized to cast the tissue ingrowth surface and the base member of the metal orthopaedic implant. As is well known, the meltable material, such as wax, has a melting point lower than the ceramic. A pattern i5 formed corresponding to the orthopaedic implant from the meltable material. The lattice element is then also formed from the meltable material in any well known manner such as utilizing a two-part die to mold the wax or other meltable material. Oncethe lattice element is formed, it is joined to the pattern corresponding to the orthopaedic implant. This may be done by using a solven~ which melts part of the materials to be joined, thereby allowing an integral, one-piece patte~n to be fo~ned from the meltable material after joining. The joined combination of the pattern and the lattice is then coated with a ceramic slurry to form a casting shell in a well known manner. The meltable material is then removed from the casting shell by heating. As is well known, the empty casting shell is filled with molten metal and allowed to cool, thereby forming a one piece casting in the form of the pattern corresponding to the orthopaedic implant with the lattice integrally cast therewith. While wAx is preferably used as the meltable material, other materials such as polystyrene may also be used. In order to achieve any proper spacing, spacers made of the meltable material are inserted between the lattice and the pattern corres-ponding to the orthopaedic implant during the joining operation. Alternately, the spacers may be formed as either part of the lattice element or the orthopaedic implant pattern. Usually this joining operation involves a solvent which causes the meltable material to partially liquify and therefore allows the various parts to be made integral with one another.

r~

It should be noted that the bone ingrowth surface can also be utilized in cemented application, since the cast surface will produce better adhesion between the cement and the prosthesis.
These features of the present invention will becume apparent from the following detailed description considered in connection with the accompanying drawings, which disclose several embodiments of the inven-tion. It is to be understood that the drawings are to used for the purposes of illustration only, and not as a defi-nition of the limits of the invention.

BRIEF DESCRIPTION QF THE DR~WINGS
In the drawings, wherein similar reference characters denote similar elements throughout the several views:
FIG. 1 is a plan view o~ the lattice eIement of the present invention in the form of a me}table material;
FIG. 2 is an elevation view of the lattice element of FIG. 1;
FIG. 3 is an elevation view of the lattice element of FIG. 1 joined to ~ pattern of an orthopaedic implant also made of meltable material;
FIG. 4 is an enlarged cross-sectional view of the joined lattice element and the orthopaed'c implant of FIG.
3 covered by a casting sh~ll;
FIG. 5 is a view of FIG. 4 after removal of the meltable material by heating;
FIG. 6 is a view of FIG. 5 after molten base metal has been introduced into the casting shell of FIG. 5;
FIG. 7 is a cross-se¢tionaI view of the integrally cast orthopaedic implant and lattice element of the present invention; and FIG. 8 is a partial cross-sectional view of a fe~oral component of a total hip prosthesi~, having the tissue ingrowth surface in the form of the lattice element in~egrally cast on the outer surface thereo~.

. .

FIG. 9 is a finished tibial plate with the tissue ingrowth surface of the invention cast thereon.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIGS. 1-3, there is shown the tissue ingrowth surface of the present invention, generally denoted as 10, in the form of a lattice element or grid-like mesh made out of a meltable material. Preferably, the meltable material is wax although polystyrene may also be used. Lattice element 10 may be molded with a two-piece die in any well known manner.
Lattice element 10 may be in the form of a uniform grid of crossing members 12 interse~ting at protrusions 14.
Preferably, each protrusions 14 includes a spacer member 16 extending downwardly therefrom. Spacers 16 serve to space connecting members 12 a predetermined distance from the outer surface of any suitable orthopaedic implant.
As stated above, wax or plastic lattice element 10 is produced by injection molding in a split die (not~shown) to form a grid-like three dimensional network. After cooling, lattice element 10 is removed ~rom the molding apparatus and is substantially as shown in FIGS. 1 and 2. Typically the cross-members 12 are .020" in diameter and the spherical protrusions 14 are .040" in diàmeter. The spacers 16 may be of any desirable length, typically .060".
Ref~rring to FIG. 3, it can be seen that lattice element 10 may be joined to the outer surface 18 of a pattern corresponding to the orthopaedic implant 20.
Orthopaedic implant 20, having a body also made from a meltable material such as wax or polystyrene. The joining operation may be accomplished by placing a glue or solvent on the outer surface 18 of the orthopaedic implank 20, which causes local melting or liquification of the meltable pattern. By placing the base 22 of spacer 16 of lattice element 10 on to the partially liquified areas of surface 18, a slight melting or liquification of base 22 of spacer ~ ~J ~

16 occurs. Upon evaporation of the solvent and the subsequent hardening of the meltable material, an integrally joined pat~ern, generally denoted as 24, is formed. Of course, the glue or solvent may be applied to base 22 of spacer 16, rather than to top surface 18 of orthopaedic implant pattern 20, with the same result.
While it is intended that each spacer 16 be integrally joined with the pattern 20, it can be seen that even if an inadequate bond is formed on a few spacers 16, sufficient attachment strength will still be developed between the final cast ingrowth su~rface and the implant body. It has been found that Testo~ s model cement acts as an adequate solvent when the lattice element is made of a Yates JW2 wax and the orthopaedic implant pattern is made from a ~ates'~
PX-12 wax. ~11 of these materials are readily available and are well known in the investment casting art.
After the one piece pattern 24 has been formed, the investment casting process proceeds with the pattern 24 being coated with a slurry of colloidal silica binder of the type including refractory powders of zirconia, alumina and silica. The first coat of the slurry used to form the shell in the investment casting process is critical. The first coat should have a viscosity o~ 12-14 seconds measured with a #4 Zahn cup. A preferred slurry for t~is first coat is a colloidal silica binder (such as Dupon ~ s 30% colloidal binder) base with refractory zirconia and silica flours. The viscosity can be varied by adding more or less binder. The dip pattern must be designed to make sure that one-piece pattern 24 is completely and evenly coated. Pattern 24 must be vibrated while draining, with air lightly blown over the lattice pattern to break up any air bubbles to prevent the slurry from bridging the grid openings. With care, it has been found that this technique can be used to produce grid openings o~ 0.020~ and above.
With the use of the injection molding process ~or the meltable material, various pattern shapes, such as square, `) f"/~

_9_ rectangular or triangular, may be used for the tissue ingrowth surface. With this process, the shapes and sizes can be accurately controlled. Thus, various pattern shapes can be fabricated to fit specific implant designs.
Furthermore, the potential variability of pore spacing would allow for the use of bone inductive coatings or fillers such as hydroxyapatite to facilitate tissue or bone ingrowth as well as more precise engineering and control of pore structures as required for improved osseo integration or vascularization.
The process for the formation of shell 30 is shown in FIGS. 4-7 with, after the initial coat is allowe~ to dry, additional slurry coats being applied in the well known manner forming the cPramic shell 30 as shown in FIG. 4.
Note that the usual stucco is not applied after the first coat, but is applied after application of the backup coats.
The combined pattern 24 made of meltable material is then removed from the shell by heating in a well known manner.
This results in a void being formed within a shell 30 as shown in FIG. 5. Molten metal, such as 'IVitallium" alloy or titanium is introduced into the void, as shown in FIG.

6, and allowed to cool. Of course, it is well known that in order to cast titanium, special foundry practices must be followed. If this is done, the cast ingrowth surface of the present invention can be produced.
After removal of shell 30, khe integral one-piece casting is shown in cross-section, in FIG 7. As can be seen, connecting elements lh~ forming lattice lO'j the protrusion 14', a spacer 16', and the orthopaedic implant pattern 20' having upper surface 18~ are now a one-piece casting of for example of Vitallium or titanium.
Referring to FIG. 8, it can be seen that the integral cast lattice element forming a tissue ingrowth surface can be easily produced on the outside of a femoral component of a hip prosthesis. This is accomplished by utilizing a formulation of wax or polystyrene which is suitably ~

flexible and can be wrapped around the outer surface 40 of the hip prosthesis 38. Such a wax is Yates JW-2 wax.
Referring to FIG. 9, there is shown a tibial component of a knee prosthesis in which the bottom surface of the tibial plate (the surface which comes in contact with the top surface of the tibia after implantation) includes the lattice element 10" thereby forming an ideal tissue ingrowth surface.
While several examples of the present invention have been described, it is obvious that many changes and modifications may be made thereunto, without departing the spirit and scope of the invention.

Claims

1. A prosthetic part for use as an orthopaedic implant comprising:
a cast metal base member having an outer surface;
and a tissue ingrowth surface in the form of a cast metal lattice element attached to said base member over at least part of said outer surface by being cast inte-grally with said base member from the same metal.

2. The prosthetic part as set forth in Claim 1 wherein said metal is Vitallium.

3. The prosthetic part as set forth in Claim 1 wherein said metal is titanium.

4. The prosthetic part as set forth in Claim 1 further including a means for spacing said metal base member and said cast metal lattice apart.

5. The prosthetic part as set forth in Claim 4 wherein said metal lattice is in the form of a grid-like mesh and said means for spacing are spacer members cast integrally with said mesh and said base member.

6. A method for forming tissue ingrowth surfaces on metal k implants cast by an investment casting technique wherein a meltable material is coated with a ceramic casting shell, the meltable material having a melting point lower than the ceramic, comprising the steps of:
forming a pattern corresponding to the orthopaedic implant from the meltable material;
forming a lattice from the meltable material, said lattice corresponding to a predetermined tissue ingrowth surface;
joining said lattice to said pattern corresponding to the orthopaedic implant;

coating said joined combination of said pattern and the said lattice with a ceramic slurry to form the casting shell;
removing the meltable material from said casting shell by heating; and filling said empty casting shell with molten metal and allowing it to cool to form a one-piece casting, in the form of said pattern corresponding to said orthopaedic implant with said lattice integral therewith.

7. The method for forming tissue ingrowth surfaces as set forth in Claim 6 wherein said meltable material is wax.

8. The method for forming tissue ingrowth surfaces as set forth in Claim 6 wherein said meltable material is polystyrene.

9. The method as set forth in Claim 6 further including the step of spacing said lattice a predetermined distance above said pattern by spacers made of said meltable material prior to said joining step.

10. The method as set forth in Claim 6 wherein said bonding of said lattice to said pattern corresponding to said surgical implant includes the step of coating one of said pattern or said lattice with a solvent which causes said meltable material to partially liquify

11. The method as set forth in Claim 6 wherein at least a first coat of said ceramic slurry is a colloidal silica slurry having a viscosity of 12-14 seconds as measured with a number 4 Zahn cup.

12. A prosthetic part cast from a single metal for use as an orthopaedic implant, comprising:
a cast base member;
a cast tissue ingrowth surface spaced a predeter-mined distance from said base member and means for attaching said tissue ingrowth surface to said base member, said means for attaching being integrally cast with said base member and said tissue ingrowth surface.

13. An orthopaedic implant comprising:
a cast metal base member having an outer surface, a plurality of generally cylindrical spacer elements integrally cast with the base member from the same metal as the base member and each spacer element extending the same predeter-mined distance outwardly from the outer surface thereof; and a metal attachment element integrally cast with the spacer elements, the attachment element having connecting elements for contact with a bone extending between the spacer elements to allow attachment to bone to occur between an underside of the con-necting elements and the outer surface of the base member, all of the connecting elements extending in the same plane.

14. The orthopaedic implant as set forth in claim 1 wherein the metal is a cobalt-chrome alloy.

15. The orthopaedic implant as set forth in claim l wherein the metal cobalt-chrome.

16. The orthopaedic implant as set forth in claim 1, 2 or 3, wherein the connecting elements define openings between the outer surface of the cast metal base member to allow for tissue in-growth.

17. The orthapaedic implant as set forth in claim 1, 2 or 3, wherein the connecting elements define openings between the outer surface of the cast metal base member to allow for the introduc-tion of bone cement.

18. The orthopaedic implant as set forth in claim 1, 2 or 3, wherein the metal attachment element is in the form of a grid-like mesh comprised of a multiplicity of cross-members.

19. The orthopaedic implant as set forth in claim 1, 2 or 3, wherein the connecting elements form a grid-like mesh having open-ings of at least 0.020 inches.