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`(19) United States
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`(12) Patent Application Publication (10) Pub. N0.: US 2002/0106611 A1
`(43) Pub. Date: Aug. 8, 2002
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`Bhaduri et al.
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`US 20020106611A1
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`Related US. Application Data
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`(54) METAL PART HAVING A DENSE CORE AND
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`POROUS PERIPHERY, BIOCOMPATIBLE
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`PROSTHESIS AND MICROWAVE
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`SINTERING
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`(76)
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`Inventors: Sutapa Bhaduri, Moscow, ID (US);
`Sarit B. Bhaduri, Moscow, ID (US);
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`Muralithran G. Kutty, Moscow, ID
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`(US)
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`Correspondence Address:
`Ormiston & McKinney, PLLC
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`802 W. Bannock, Suite 400
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`PO. Box 298
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`Boise, ID 83701-0298 (US)
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`(21) Appl. No.:
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`10/052,291
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`(22)
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`Filed:
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`Jan. 18, 2002
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`(60) Provisional application No. 60/262,730, filed on Jan.
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`19, 2001.
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`Publication Classification
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`Int. Cl.7 ....................................................... A61C 8/00
`(51)
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`433/2011; 623/2355; 623/2357
`(52) U.S.Cl.
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`(57)
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`ABSTRACT
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`Monolithic metallic parts having a dense core surrounded by
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`a porous periphery. Metallic parts having a dense core
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`surrounded by a porous periphery characterized by a mul-
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`titude of interconnected pores. Dental implants and other
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`prosthesis using such metal parts as a substrate coated with
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`a bioactive material. Microwave sintering a compacted
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`metal powder to produce such parts and prosthesis
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`Sheet 1 0f 8
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`m”J?©wv-é.Ewwwwégg‘
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`,/
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`\\\\.
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`FIG. 3
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`FIG. 2
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`4b
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`5
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`“FIG.“5“
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`‘N‘-~‘§“‘N\~:fi~‘§‘§-~§§§-~§\\
` V‘“§“~§‘\‘§§§)‘/-‘~‘§\§‘~‘~\~/§‘§”/fl//
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`7///
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`FIG. 6
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`Porosity
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`Distance from Surface (um)
`FIG. 10
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`(%)
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`Intensity
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`29
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`FIG. 11
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`FIG. 14
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`US 2002/0106611 A1
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`Aug. 8, 2002
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`METAL PART HAVING A DENSE CORE AND
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`POROUS PERIPHERY, BIOCOMPATIBLE
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`PROSTHESIS AND MICROWAVE SINTERING
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`CROSS REFERENCE TO RELATED
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`APPLICATION
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`[0001] This application claims subject matter disclosed in
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`provisional patent application serial No. 60/262,730 filed
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`Jan. 19, 2001 titled Microwave Sintering, Bioactive Coating
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`By Electrodeposition And Osseointegration, which is incor-
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`porated herein by reference in its entirety.
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`STATEMENT OF RIGHTS TO INVENTIONS
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`MADE UNDER FEDERALLY FUNDED
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`RESEARCH AND DEVELOPMENT
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`[0002] Part of the work performed during the development
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`of this invention was funded by the National Science Foun-
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`dation under contract no. DMII-0085100. The United States
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`government may have certain rights in the invention.
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`FIELD OF THE INVENTION
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`[0003] The present invention is directed to solid metallic
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`parts having a dense core and a porous periphery and
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`microwave sintering techniques used to form such parts
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`BACKGROUND
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`[0004] Osseointegration is the formation of a direct struc-
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`tural and functional bond between living bone and the
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`surface of an implant without any intervening soft tissue.
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`Osseointegrated dental implants have been used since the
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`early 1980s in the restoration of toothless people all over the
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`world. Several factors influence the success of osseointe-
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`gration, including the strength, elasticity, surface composi-
`tion, biocompatibility and design of the implant and the
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`surgical techniques used for implantation.
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`[0005] Osseointegrated dental implants are usually made
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`out of titanium. FIG. 1 illustrates a typical titanium dental
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`implant 10. FIG. 2 is a partial cut-away view showing
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`implant 10 after implantation. As shown in FIG. 2, implant
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`10 is embedded in bone 12 with surrounding gum 14. New
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`bone growth 16 occurs in the region immediately around
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`implant 10. The crown or artificial tooth is cemented to
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`abutment 18 which screws in to implant 10.
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`[0006] Presently, titanium dental implants are machined
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`out of titanium and titanium alloys. Some implants are
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`plasma spray coated with titanium or hydroxyapatite.
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`Hydroxyapatite is a material found in bone and naturally
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`promotes a biochemical bond with osseous tissues. The
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`plasma spray process melts fine particles, typically 30 nm to
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`50 nm, at high temperatures, about 3000° C., and provides
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`a porous surface on the implant. Bone tends to appose to this
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`porous structure. However, due to the presence of undesir-
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`able phases, plasma spraying of hydroxyapatite is only
`partially effective in the osseointegration process. The
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`porous coating by itself does not have enough strength.
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`Strength is provided by a monolithic structure underneath
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`the coating. Further, it is important to have the right surface
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`composition for proper osseointegration. While conven-
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`tional implants are machined and turned at room tempera-
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`ture, plasma spraying exposes the surfaces to much higher
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`temperatures. The
`surface obtained by machining is
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`smoother than the surface obtained by plasma spray coating.
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`[0007] Conventional methods for fabricating titanium and
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`other metallic implants include investment casting, comput-
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`erized manufacturing, forging, and powder metallurgy. In
`most cases, the final product is formed by machining the raw
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`product. The risk of surface damage during machining is a
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`disadvantage because surface damage can appreciably lower
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`the fatigue life of the implant. Titanium is one of the most
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`difficult materials to machine. Titanium is chemically reac-
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`tive and has a tendency to weld to the tool during machining,
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`which leads to premature tool failure. Its high reactivity
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`limits the use of state-of-the-art ceramic tools. The low
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`thermal conductivity of titanium increases the temperature at
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`the interface of tool and work piece, reducing the effective
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`life of the tool. The low elastic modulus of titanium permits
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`deflections of the work piece and, therefore, requires proper
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`backup. As a result of these difficulties, titanium should be
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`machined using low cutting speeds, maintaining high feed
`rates, spraying generous quantities of cutting fluid and
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`maintaining sharp tools and rigid setups. Even if these
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`precautions are taken, machining titanium ingots still can
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`not produce interconnected surface pores.
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`[0008]
`In spite of the machining difficulties, titanium is
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`still the most popular material for dental implants. Electro-
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`chemical machining, chemical milling, and laser beam-
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`machining have been tried recently in an attempt to avoid the
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`difficulties encountered during conventional machining of
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`titanium. Electrochemically machining or chemically mill-
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`ing an intricate contour shape like a dental
`implant
`is
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`difficult. Laser beam machining is an effective technique for
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`making intricate contour shapes, but
`it requires special
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`equipment and may not be cost effective for the commercial
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`production of titanium dental implants.
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`[0009] Biocompatibility is another important consider-
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`ation in the fabrication of metal implants. The optimum
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`biocompatible implant is non-toxic, inert, stabile, and non-
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`carcinogenic. Fabrication conditions can affect biocompat-
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`ibility. A metal can be non-toxic but unstable, corroding in
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`body fluids. Corrosion results in the loss of implant mate-
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`rials, which eventually weakens the implant. Corrosion
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`debris that escapes from the corroded surface can penetrate
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`the body. A porous surface helps osseointegration but may
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`increase the risk of corrosion. Also, pores may trap machin-
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`ing debris and cutting fluids. Bioactive coatings on implants
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`are desirable. As an intermediate between resorbable and
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`bioinert materials, bioactive
`coatings promote bonds
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`between tissues and the implants. Also, a bioactive coating
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`can prevent corrosion of porous metal.
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`[0010] Hydroxyapatite can provide the desired bioactivity
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`on the implant surface. Hydroxyapatite is a ceramic and
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`hence a brittle material. The fracture toughness of synthetic
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`hydroxyapatite is 0.8-1.2 MPa. \/m with an average of 1
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`MPa.\/Wm. Human bone has a fracture toughness of 2-12
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`MPa.\/m. Presently, hydroxyapatite ceramics cannot be used
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`as monolithic implants, such as those used for teeth and
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`bones. If the calcium to phosphorous (Ca/P) ratio is lower
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`than 1.67,
`(X and [3
`tricalcium phosphates (TCPs) form.
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`However, with a low Ca/P ratio, the strength increases. The
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`tricalcium phosphate
`hydroxyapatite
`presence
`of
`in
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`increases biodegradability and susceptibility to slow crack
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`growth. The Weibull modulus of dense hydroxyapatite is
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`reported to be between 5 and 18. Slow crack growth
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`coefficients (n) vary greatly from 12-49 under wet condi-
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`tions to 26-80 under dry conditions. Grain boundaries with
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`Ca/P ratio lower than that of hydroxyapatite are especially
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`susceptible to slow crack growth. Vickers hardness of dense
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`hydroxyapatite is 3-6 GPa and Young’s modulus is 44-88
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`GPa. This data indicates that
`in its monolithic form
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`hydroxyapatite is not suitable as an implant. Furthermore,
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`the production of a dense walled but openly porous structure
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`with the correct ratio of Ca/P using hydroxyapatite would be
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`very challenging.
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`[0011] As far as the development of a porous hydroxya-
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`patite implant goes, White and Schors developed the
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`“Replamineform” process to duplicate the porous micro-
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`structure and interconnection found in natural corals. The
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`primary advantage of this process is that the pore sizes and
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`microstructure are uniform and controlled. Also, there is
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`complete interconnection between the pores. In terms of
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`mechanical properties, such porous materials are weaker
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`than their bulk counterparts. As surface area increases in
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`porous ceramics, effects of the environment on decreasing
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`strength become more predominant. In order to compensate
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`for strength degradation, these porous ceramics require bone
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`growth to stabilize the implant. So,
`in spite of excellent
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`biocompatibility, hydroxyapatite may not be the best choice
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`as a porous monolithic implant material because of its poor
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`mechanical properties and aging behavior.
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`[0012] Strategies for utilizing hydroxyapatite as a success-
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`ful implant material include adding other ceramic reinforce-
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`ments to hydroxyapatite, coating a biocompatible metal with
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`hydroxyapatite, or making hydroxyapatite/polymer compos-
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`ites. Coating a metallic substrate with hydroxyapatite has
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`several benefits. Coating provides stable fixation of the
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`implant by precision fit to the bone and minimizes adverse
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`reaction. Hydroxyapatite coatings decrease the release of
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`metal ions of the implant into the body and shield the metal
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`surface from environmental attack. Aporous hydroxyapatite
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`coating facilitates bone growth through a highly convoluted
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`interface. When pore sizes exceed 100 um, bone grows
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`through the channels of interconnected surface pores, thus
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`maintaining the bone’s vascularity and viability. Porosity
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`also helps provide a smooth blood supply to promote the
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`in-growth of connective tissues. Composition, crystal struc-
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`ture, and ultrastructure also affect implant-tissue interaction.
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`The hydroxyapatite coating should be approximately 40-200
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`pm thick to resist resorbability of hydroxyapatite, it should
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`be porous to facilitate bone growth, and it should not contain
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`impurities.
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`SUMMARY
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`[0013] The present invention was developed in an effort to
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`help provide a bioactive dental implant that will promote
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`regeneration of surrounding tissues. As part of this effort, we
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`discovered that microwave sintering could be used to pro-
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`duce new metallic structures particularly suited for use as
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`dental
`implants and other prosthesis. Accordingly, one
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`embodiment of the invention is directed to a solid part that
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`includes a metallic monolith having a dense core surrounded
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`by a porous periphery. In another embodiment, the solid part
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`includes a shaped metallic structure having a dense core
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`surrounded by a porous periphery characterized by a mul-
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`titude of interconnected pores. These parts are new in at least
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`two respects—the monolithic nature of the part and inter-
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`connectedness of the pores. Conventional methods used to
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`produce metal parts having a dense core and porous periph-
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`ery require multiple steps in which materials are added or
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`Page 11 of 15
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`removed and, hence, conventional processes do not and
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`cannot yield monolithic structures, nor do they result in
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`interconnected pores.
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`[0014] The preferred dental implant includes a bioactive
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`coating on a monolithic titanium or other suitable metallic
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`substrate having a dense core and a porous periphery. The
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`substrate provides strength while the bioactive coating
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`improves tissue response and bone growth.
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`[0015] Another embodiment of the invention is directed to
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`the use of microwave sintering to form the desired structure.
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`In this embodiment,
`titanium or another suitable metal
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`powder is compacted into the general shape desired and then
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`exposed to microwaves under conditions sufficient to trans-
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`form the compressed powder into a monolith having a dense
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`core surrounded by a porous periphery. If a bioactive coating
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`is desired, hydroxyapatite or other suitable bioactive coat-
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`ings may be applied by electrocrystallization.
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`DESCRIPTION OF THE DRAWINGS
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`[0016] FIG. 1 is a perspective view of a typical metallic
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`dental implant.
`[0017] FIG. 2 is a partial cut-away view showing a dental
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`implant after implantation.
`[0018] FIG. 3 is a schematic sectional representation of an
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`implant having a dense core surrounded by a porous periph-
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`ery and a bioactive coating.
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`[0019] FIG. 4a is a schematic representation of a micro-
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`wave furnace system.
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`[0020] FIG. 4b is a schematic representation of one pre-
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`ferred arrangement for a substrate during microwave sinter-
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`ing.
`[0021] FIG. 5 is a schematic representation of an electro-
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`crystallization system for applying a bioactive coating to a
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`substrate.
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`[0022] FIG. 6 is a photograph of a near net shape screw
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`shaped titanium substrate after microwave sintering.
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`[0023] FIGS. 7a and 7b are micrographs showing in
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`section the different porosity distributions of the dense inner
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`core and the porous periphery of a titanium substrate.
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`[0024] FIG. 8 is a higher magnification micrograph in
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`section showing the porosity distribution along the periph-
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`ery of a titanium substrate.
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`[0025] FIG. 9 is a micrograph of the surface of a titanium
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`substrate showing interconnected open pores.
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`[0026] FIG. 10 is a graph showing the distribution of the
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`pores as a function of distance from the surface.
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`[0027] FIG. 11 is an X-ray diffraction pattern of a
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`hydroxyapatite coating.
`[0028] FIG. 12 is a micrographic surface view of a
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`hydroxyapatite coating.
`[0029] FIG. 13 is a micrographic side view of a hydroxya-
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`patite coating.
`[0030] FIG. 14 is a series of micrographic maps of cal-
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`cium and phosphorous showing the extent of a hydroxya-
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`patite coating.
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`Page 11 of 15
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`

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`US 2002/0106611 A1
`
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`Aug. 8, 2002
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`
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`DETAILED DESCRIPTION
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`In conventional processing, slow heating rates and
`[0031]
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`a great deal of insulation are used to sinter metallic powders
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`to full density. We have discovered, however, utility in the
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`exactly opposite conditions, i.e., we intentionally use fast
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`heating rates and less insulation. The overall effect is a
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`gradient in temperature with an inverse sintered density
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`profile. This leads to the desired monolithic implant sub-
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`strate—a dense core surrounded by a porous periphery.
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`[0032] Since the processing techniques of the present
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`invention involve microwave energy a brief background of
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`microwaves is presented. Microwaves are electromagnetic
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`radiation with wavelengths ranging from 1 mm to 1 min free
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`space. In the United States, frequencies of 915 MHZ and
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`2.45 GHz are assigned to microwave operation. Microwaves
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`are believed to be reflected by metals, although this is not
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`always true. Certain minerals and ceramics absorb micro-
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`waves and become self-heated. Microwave absorption gen-
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`erates heat in-situ. Microwave energy is more energy effi-
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`cient than conventional heating. The power deposited into a
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`material
`is given by P=23'cfeoer
`tan 5E2 where f is the
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`frequency, 60 is the permittivity of the free space, er is the
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`relative permittivity, tan 5 is the loss tangent, and E is the
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`electric field. The desired porosity profile is achieved by
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`utilizing how microwaves are coupled in metal powders.
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`Due to the high electrical and thermal conductivity of
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`metallic powders, even though microwaves are coupled
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`within the central portion of a part, the heat generated is
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`quickly dissipated.
`[0033] FIG. 3 is a schematic sectional representation of
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`the preferred structure of one embodiment of the implant of
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`the present
`invention. Referring to FIG. 3,
`implant 10
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`includes a monolithic substrate 20 and a bioactive coating
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`22. Substrate 20 has a dense core 24 surrounded by a porous
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`outer region 26. Outer region 26 is also referred to herein as
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`the periphery of substrate 20. Monolithic substrate 20 pro-
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`vides reasonable strength while coating 22 enhances bioac-
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`tivity upon implantation to stimulate bone growth. Although
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`a coating 22 of hydroxyapatite or another suitable bioactive
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`material is preferred for enhanced bioactivity, the uncoated
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`monolithic substrate 20 with its dense core 24 and porous
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`periphery 26 could be used for the implant. Implant 10 refers
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`generally to any of various embodiments of the implant of
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`the present invention. Therefore, in the case of an uncoated
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`implant,
`the term “substrate” is not appropriate and the
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`monolithic structure would be referred to directly as the
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`implant.
`[0034] One innovation of the present invention arose from
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`our serendipitous discovery of a microwave sintering pro-
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`cess for making titanium with a dense core surrounded by a
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`porous periphery. One of our objectives was to fabricate near
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`net shape titanium dental
`implants with limited surface
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`porosities. Rather than machining titanium ingots, we pur-
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`sued a powder metallurgy approach. Creation of surface
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`porosities along with a dense central core by means of
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`microwave sintering provides adequate strength while facili-
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`tating bioactive coating penetration. This has not been
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`possible by conventional sintering in a furnace.
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`In one preferred process, the desired structure is
`[0035]
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`obtained by pressing and subsequently densifying commer-
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`cially pure titanium powder. Although the process will be
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`described with reference to the fabrication of the screw
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`Page 12 of 15
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`shaped titanium dental implant shown in FIG. 6, the process
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`may be used to produce other shapes and with other metallic
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`powders. Titanium powder is green compacted at about 20
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`Ksi in a mold of the desired size and shape in a cold isostatic
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`press. If the implant
`is to be constructed as a coated
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`substrate, then the mold will reflect the size and shape of the
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`uncoated substrate. If the implant is to be uncoated, then the
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`mold will more nearly reflect the final size and shape of the
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`implant.
`[0036] The preferred initial titanium particle size is less
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`than —325 mesh in order to enhance reactivity. Abinder may
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`be used as necessary or desirable to give some handling
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`strength to the titanium powder. The binder is removed after
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`pressing by slowly heating the implant to about 200° C. and
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`keeping it at that temperature for 1-1.5 hours. Heating the
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`implant in a vacuum furnace backfilled with argon (Ar) will
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`help prevent surface oxidation during removal of the binder.
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`[0037] The implant is densified by microwave sintering in
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`a microwave furnace system 30 such as the one shown
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`schematically in FIG. 4a. Referring to FIG. 4a, a typical
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`system 30 includes a magnetron 32 operatively coupled to a
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`furnace cavity 34 through a wave guide 36. Furnace con-
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`troller 38 coupled to forwarded power supply 40, reflected
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`power supply 42 and magnetron 32 controls the output of
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`magnetron 32 at the direction of controlling computer 44.
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`Magnetron 32 should have a variable power output from 0
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`to 3 kW at 2.45 GHz. It is desirable that the output of
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`magnetron 32 be set and stabilized using a feedback loop
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`with saturable reactors in the primary circuit of the high
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`voltage magnetron power supply. Preferably, furnace cavity
`34 is about 10 times the wave length of the electromagnetic
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`radiation, which has a wave length of about 12.5 cm. Hence,
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`cavity 34 is said to be “overmoded.” Arelatively large sized
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`cavity enhances the mixing of the incoming microwave
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`pattern with the reflected pattern. An optical pyrometer 46
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`measures the temperature of materials in cavity 34 and
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`provides temperature feedback to controller 38.
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`[0038] The atmosphere inside the vessel is controlled with
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`a vacuum pump 48 and a supply of argon or other insert gas
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`50. Vacuum pump 48 removes air from cavity 34 as neces-
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`sary or desirable while an inert gas is introduced into the
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`evacuated cavity from tank 50.
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`for
`[0039] FIG. 4b shows one preferred arrangement
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`substrate 20 during microwave sintering. Referring to FIG.
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`4b, substrate 20 is placed in cavity 34 in an aluminum oxide
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`(A1203) fiberboard box 60. The aluminum oxide fibers are
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`low density and insulating but not significantly absorptive at
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`the operating frequency. The box preferably also contains
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`strips of dense silicon carbide (SiC) to act as a susceptor 62.
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`Substrate 20 is surrounded by alumina fiber blankets 64.
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`Insulation 64 insulates substrate 20 against overheating from
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`direct contact with susceptor 62, prevents any diffusion of
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`carbon from the silicon carbide susceptor 62 into the metal-
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`lic substrate 20 and appears to be important in the creation
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`of an inverted temperature gradient in substrate 20.
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`[0040] For a titanium dental implant size substrate 20,
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`such as the one shown in FIG. 6, the substrate 20 is exposed
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`to microwaves at 1.0-2.5 kw for not more than 20 minutes
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`in a 10 cm><10 cm box 60. Substrate 20 is surrounded with
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`14-30 grams of insulation 64. In this arrangement, substrate
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`20 heats rapidly, about 50° C. per minute, to 1200° C.-1400°
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`C. Alumina fiber is the preferred insulator because it is a
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`Page 12 of 15
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`

`

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`US 2002/0106611 A1
`
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`Aug. 8, 2002
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`good insulator and does not couple microwave energy.
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`Microwaves are coupled in the interior of substrate 20 while
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`the surface dissipates the heat faster. Thus, an inverted
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`temperature gradient is generated in substrate 20 such that
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`the temperature in the core of substrate 20 is greater than the
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`temperature at the periphery. Porosity results from tempera-
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`ture differences between the core and the periphery. In
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`conventional sintering, products are usually denser at the
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`periphery because the periphery is heated first and most and
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`the core remains porous unless some pressure is applied
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`along with the heat. In microwave sintering, substrate 20
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`absorbs the microwave energy in the core first, to heat the
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`core, and then at the periphery. Heat is also dissipated faster
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`at the periphery than at the core. Less insulation allows
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`greater but controlled heat dissipation for greater porosity
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`while more insulation slows heat dissipation for a more
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`dense structure with fewer pores.
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`[0041] While it is expected that the values noted above for
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`the microwave sintering processing parameters may be
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`varied somewhat as necessary or desirable to accommodate
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`different size and shape substrates or different metallic
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`powders, the processing parameters should be set at values
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`that will create the inverted temperature gradient that allows
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`for the formation of the desired density variation within the
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`substrate.
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`[0042] After microwave sintering, substrate 20 is washed
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`in an ultrasonic bath, dried, and slightly etched in nitric acid
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`(HNO3) solution before the bioactive coating is applied.
`Etching cleans the pores to promote infiltration during
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`electrodeposition.
`is
`coating material
`[0043] The preferred bioactive
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`hydroxyapatite and the preferred form of electrodeposition
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`is electrocrystallization. In the literature, there are many
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`references for coating metallic substrates with hydroxyapa-
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`tite. These include plasma spraying, sol-gel, flame spraying,
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`chemical vapor deposition, sputtering, laser ablation, bio
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`mimicking, hydrothermal and electrodeposition. Plasma
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`spraying is
`the most common process for producing
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`hydroxyapatite coatings. Plasma spraying has advantages as
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`well as disadvantages. The advantages are: 1) plasma spray
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`produces high temperature at which hydroxyapatite particles
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`melt and form strong bonds to the substrate and, therefore,
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`no post annealing is needed; 2) plasma spray is a fast process
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`for building up layers; and 3) no protective atmosphere is
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`needed for plasma spraying hydroxyapatite coating. The
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`disadvantages of plasma spaying are: 1) prolonged high
`temperature exposure may alter the microstructure of the
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`substrate; 2) hydroxyapatite is unstable at high temperature
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`and may decompose to calcium oxide,
`tricalcium phos-
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`phates, and other phosphates; 3) because plasma spraying is
`a line-of—sight process, the coating