This invention relates to a method of producing a metal structure having an open cell structure at least partly covering its surface, useful as a component of bone implants. Upon implantation, bone material grows into the open-celled structure produced by the method of the invention thereby assuring permanent fixation of the implant in the bone. Another aspect of this invention pertains to a method of producing metal open-celled structures wherein the surface structure is very irregular. When implanted, the irregular surface structure stimulates bone growth.
Open-celled metal structures are produced using a lost positive pattern. In general, the voids of a suitable positive pattern are filled with a ceramic encapsulating medium and the material of the positive pattern is volatilized or otherwise removed by heating thereby forming a ceramic mold. The voids of the mold are then filled with metal and after hardening of the metal, the ceramic mold is removed.
In one prior art method of making an open-celled structure disclosed in German Offenlegungsschrift 3106917 (U.S. Pat. No. 4,781,721), a sponge of natural or synthetic material was used as a positive pattern. This method of producing open-celled metal structures has proved unsatisfactory in that the walls and the interlinking webs of a sponge are so thin that the metal structure produced using the sponge positive lacks sufficient strength.
Another method for producing open-celled metal structures has been disclosed in German Offenlegungsschrift 3224265 (U.S. Pat. No. 4,600,546). In this disclosure, wax is used as a strengthening means for the walls and cross-linked webs of a porous shaped plastic substrate. Wax in liquid form or as an emulsion in water is deposited on the plastic substrate and, after drying, is protected with a coating of plastic enamel. Within certain limits, the method disclosed produces thicker walls and interlinking webs in the plastic substrate than those of the prior art method discussed above.
However, this prior art method has a disadvantage in that the wax, which was applied by dipping or spraying onto the plastic substrate and as coated by the enamel, is inelastic compared to the elastic shaped plastic substrate. Therefore, under pressure, the wax can easily separate from the walls and webs of the pores of the plastic substrate. Another disadvantage is that the walls and webs of the plastic substrate take on a negative charge with respect to the wax or wax-in-water emulsion. Consequently, there is no uniform bond between the walls and webs of the plastic substrate and the wax, especially within the deep recesses of the plastic substrate. The rheological characteristics of the wax or wax-in-water emulsion do not allow the walls and webs deep within the plastic substrate to be strengthened to a satisfactory degree. As a result, the plastic substrate produced by this method has within it walls and webs which are in some cases too thin to withstand long-term stresses.
Accordingly, there exists a need for a method to produce open-celled metal structures wherein the walls, webs and pores of the plastic substrate are of a uniform and reliable strength throughout. Moreover, the means used to strengthen the walls and webs of the substrate must be capable of volatilization or destruction by heat, such as by incineration.