Bioactive Void Metal Composites for Orthopedic Implant Devices
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provide mechanically strong and non-toxic metals and alloys, biological integration of devices into natural tissues remains a problem (2-3). Thus many effective devices become loose over time and necessitate subsequent surgery to remove and replace the old device, a process fraught with high morbidity and mortality. Efforts to solve the problems associated with device anchoring have been highly fragmented among the biological, mechanical and surgical disciplines. The high loads required of many implants restricts the selection of many materials. In addition to being biocompatible, the material must possess adequate fracture and fatigue resistance. While metals or metal alloys meet many of the biomechanical requirements, they have poor or nonexistent interfacial bonding between the metallic surface and the surrounding bone. In order to alleviate this problem, porous metal coatings have been applied to many implant surfaces. This facilitates bone ingrowth into the porous layer thus improving fixation due to improved mechanical interlock. However, there remains concern about poor interfacial bonding between the porous metal coating and the surrounding bone. Many of the disadvantages of metallic implant devices can be diminished by the use of bioactive materials or coatings on the implant surface. Ducheyne (4) demonstrated that
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Mat. Res. Soc. Symp. Proc. Vol. 414 01996 Materials Research Society
hydroxyapatite (HAP) surface coatings increased the rate of bone formation within a porous metal sample. In addition, HAP surfaces do not form fibrous-tissues but form an extremely thin, epitaxial bonding layer with existing bone (5). Though there are many desirable features of bioactive surface coatings, an optimal technique for coating application has not been developed. Lacefield (6) found plasma coating formed a dense, adherent coating of HAP on a metal substrates, but cautioned that the coating of complex implant devices containing internal cavities was not feasible. More serious was the formation of an amorphous calcium phosphate in the film rather than HAP in more than 50% of the coating attempts (7). Materials and Methods Void Metal Composites (VMC)
The VMC process produces coatings that are open-celled with interconnecting porosity and the pore density can be controlled. "Wire VMC" (WVMC), has cylindrical pores of uniform diameter which may completely penetrate the structure of the material. The pores were formed with stacked pieces of brass mesh. A fine powder of the metal or alloy of the final structure, e.g. Ti-6A1-4V alloy, was packed by high frequency vibration into the interstices of the wire mesh. High speed compaction of the mixture, at elevated temperatures, produced a compact that had high density and good forming characteristics. The compact was then machined into its final shape. The brass mesh was removed from the densified material by dissolution in 6 N nitric acid. A final high temperature vacuum treatment (sintering) caused the metal structure around the spherical pores to fully densify, thereb
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