`.
`
`
`United States Patent
`
`
`Kaplan
`
`
`
`
`[19]
`
`IIIIIIIllllllllllllllllllllIllllllllllllllllllllllllIllllllllllllllllllllll
`U8005282861A
`
`
`
`[11] Patent Number:
`
`
`
`[45] Date of Patent:
`
`
`
`5,282,861
`
`
`Feb. 1, 1994
`
`
`
`
`
`fill ,
`
`1
`5’
`
`
`
`75
`
`
`
`
`[56]
`
`[54] OPEN CELL TANTALUM STRUCI‘URES
`
`
`
`
`
`FOR CANCELLOUS BONE IMPLANTS AND
`
`
`
`
`'
`
`
`
`
`CELL AND TISSUE RECEPTORS
`luv to : Richard B. K 1
`, B
`I
`
`
`
`1
`I
`up u ever y
`Calif.
`en r
`
`[73] Assignee: mmet’ Pacoima, Calif
`
`
`
`
`
`.:
`21
`l.
`,
`8
`
`
`
`
`850 11
`] App No
`I
`
`
`
`
`
`[22] Filed:
`MEL 11, 1992
`,
`_
`[51]
`Int. Cl.5 .......................... A61F 2/28, A61F 2/02,
`
`
`
`
`
`
`
`
`
`
`
`
`A61F 2/54; AOIN 1/02
`
`
`
`
`
`
`[52] US. Cl. ........................................ 62632/31/6gé6222/71/12;
`
`
`[58] Field of Search ................. 623/16 11 667427/2-
`
`
`
`
`
`
`
`
`433 173’ 201.1, 2022, 206, 207
`
`
`
`
`
`/
`
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`
`
`7/1972 Bokros et al. ........................... 623/2
`3,677,795
`
`
`
`
`
`
`3992 725 11/1976 Homsy
`623/11
`
`
`
`
`
`7/1933 Ehrnford .....
`4,392,828
`433/201.1x
`4,457,984 7/1984 Otani et a1.
`....................... 433/201.1
`
`
`
`
`
`
`
`
`
`4/1988 Graves, Jr. et a1.
`.............. 623/16 X
`4,737,411
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`4,775,598 10/1988 Jaeckel ................................ 428/550
`
`
`
`
`
`7/1989 von Recum et a1.
`.. 427/2 X
`.
`4,846,834
`
`
`
`
`
`
`
`
`
`7/1991 Ducheyne ............. 623/16
`5,030,233
`5,152,794 10/1992 Davidson .............................. 623/16
`
`
`
`
`FOREIGN PATENT DOCUMENTS
`
`
`
`
`
`
`9/1982 United Kingdom .................. 623/16
`2093701
`
`
`
`Primary Examiner—David Isabella
`.
`Assistant Examiner—Dinh X. Nguyen
`
`
`
`
`
`
`
`Attorney, Agent, or Firm—Charles H. Schwartz;
`Ellsworth R. Roston
`
`
`
`
`[57]
`ABSTRACT
`A tantalum open cell structure is formed by chemical
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`“PM “13°51‘10“ °nt° *1 ”mula‘ed carbon ‘0“ “’3'
`
`
`
`
`
`
`
`strate. Tantalum has a long history of use as an implant
`material, in both bone and soft tissue. This lightweight,
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`strong, porous structure, mimicking the microstructure
`of natural cancellous bone, acts as a matrix for the in-
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`corporam" 0f b°“e’ prov‘dmg Opt‘mal Pemeab‘hty
`
`
`
`
`
`
`
`
`and a high surface area to encourage new bone in-
`
`STOWth‘
`
`
`
`
`
`18 Claims, 4 Drawing Sheets
`
`
`
`
`
`
`100
`
`
`
`{00
`
`I02
`
`102
`
`Page 1 of 11
`
`ZIMMER EXHIBIT 1013
`
`Page 1 of 11
`
`ZIMMER EXHIBIT 1013
`
`

`

`US. Patent
`
`
`
`Feb. 1, 1994
`
`
`
`
`» Sheetlof4
`
`
`
`
`V
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`
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`5,282,861
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`
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`[00
`
`
`
`[02
`
`F/G.
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`
`/
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`Page 2 of 11
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`Page 2 of 11
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`

`

`US. Patent
`
`
`
`Feb. 1,1994
`
`
`
`
`Sheet 2 of 4
`
`
`5,282,861
`
`
`
`[00
`
`
`F/G. 2
`
`Page 3 of 11
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`Page 3 of 11
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`

`

`US. Patent
`
`
`
`Feb. 1, 1994
`
`
`
`Sheet 3 of 4
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`
`5,282,861
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`
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`[04
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`
`
`104
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`F/G. 3
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`Page 4 of 11
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`Page 4 of 11
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`

`

`US. Patent
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`
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`Feb. 1, 1994
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`
`
`Sheet 4 of 4
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`
`5,282,861
`
`
`
`FIG. 4
`
`GASES IN
`
`
`(H2) (CI2)
`
`
`
`2/4
`
`
`REACT/0N
`
`CHAMBER
`
`200
`
`
`
`
`
`TANTALUM
`
`”5 7’“
`
`2’0
`
`
`
`RES/537g?
`205
`
`
`
`
`
`CHLOR/NATION
`
`
`
`CHAMBER
`202
`
`
`
`
`
`
`GRAPH/ TE
`I
`
`_ — SUSCEPTOR
`
`
`(HOT WALL FURNACE)
`[II/A
`I/I/I/IA
`
`O 204
`O
`INDUCTION
`
`
`II/I
`HEATING COIL —>O
`
`
`
`
`
`
`208
`/[
`_//1
`//////
`
`W
`
`
`
`
`
`00000
`
`7/
`
`
`
`
`
`222
`
`
`
`EXHAUST
`
`
`
`
`
`O O O O
`
`
`FOAM SUBSTRATE
`2/2
`
`Page 5 of 11
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`Page 5 of 11
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`

`

`1
`
`
`
`5,282,861
`
`OPEN CELL TANTALUM STRUCTURES FOR
`
`
`
`
`
`CANCELLOUS BONE IMPLANTS AND CELL AND
`
`
`
`
`
`TISSUE RECEPTORS
`
`
`
`
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`
`
`
`
`
`
`
`
`2
`
`
`
`
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`Bone ingrowth into the voids of a porous material
`
`
`
`
`
`
`
`
`provides ideal skeletal fixation for the permanent im~
`
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`
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`
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`plants used for the replacement of bone segments lost
`due to any number of reasons, or in total joint prosthe-
`
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`
`
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`ses. Biological compatibility, intimate contact with the
`
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`surrounding bone, and adequate stability during the
`
`
`
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`
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`early period of bone ingrowth have been identified as
`
`
`
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`
`
`important requirements, along with proper porosity.
`
`
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`The optimal porous material should have good crack
`
`
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`resistance, particularly under impact, and a compliance
`
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`comparable to that of bone. The material should also
`
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`make the manufacture of implants of precise dimensions
`
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`easy, and permit the fabrication of either thick or thin
`coatings on load-bearing cores.
`
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`One prerequisite for successful ingrowth is that the
`
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`implant be placed next to viable bone. In fact, the pres-
`ence of bone within the implant has become presump-
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`tive evidence of osteoconductive properties: that is, the
`
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`
`
`ability of bone to grow into a porous structure when the
`
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`structure is placed next to bone. Initially, the cells that
`
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`interface the implant convert to bone, then the front of
`regenerated bone progresses into the implant. This pro—
`
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`cess is known as osseointegration, meaning the achieve-
`
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`ment of direct contact between living bone and implant.
`
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`The research, development, and manufacture of syn-
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`thetic porous implants having the physical properties
`
`
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`
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`required to promote bone ingrowth have proved to be a
`
`
`
`
`
`
`major endeavor. Implants with porous surfaces of me-
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`
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`tallic, ceramic, polymeric, or composite materials have
`been studied extensively over the last two decades. A
`
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`significant early advance in this area was made with the
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`development of “replamineform” materials, so termed
`
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`because they replicate actual life forms. These materials
`are based on the three-dimensional microstructure of
`
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`certain marine invertebrates (best represented by corals
`and echinoids), which is uniform and completely per-
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`meable. The replamineform process utilizes the inverte-
`brate microstructure as a template to make porous
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`structures of other materials.
`
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`The most commonly used substance for porous bio-
`
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`materials is calcium hydroxyapatite (HA), which is the
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`
`
`largest chemical constituent of bone. Other nonmetallic
`
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`materials frequently used in porous form for implants
`include the ceramics tricalcium phosphate (TCP), cal-
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`cium aluminate, and alumina, carbon; various polymers,
`
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`including polypropylene, polyethylene, and polyoxy-
`methylene (delrin); and ceramic-reinforced or -coated
`
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`polymers. Unfortunately, ceramics, while strong, are
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`very brittle and often fracture readily under loading;
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`and polymers, while possessing good ductility, are ex-
`tremely weak. The very nature of these materials can
`
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`restrict their clinical dental and orthopedic applications.
`
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`Metals, on the other hand, combine high strength and
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`good ductility, making them attractive candidate mate
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`
`
`rials for implants (and effectively the most suitable for
`
`
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`
`
`load-bearing applications). Many dental and orthopedic
`implants contain metal, most often titanium or various
`
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`
`
`
`alloys such as stainless steel or vitallium (cobalt-chromi-
`
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`
`
`um-molybdenum). Ceramic-coated metals are also used,
`
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`
`
`
`typically HA or TCP on titanium. Additionally, a large
`
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`
`
`variety of metals are used internally in biomedical com-
`
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`
`ponents such as wire, tubing, and radiopaque markers.
`Many existing metallic biomaterials, however, do not
`
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`
`
`easily lend themselves to fabrication into the porous
`structures that are most desirable for bone implants.
`
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`
`
`These materials (e.g. stainless steel, cobalt-based alloys)
`exhibit the necessary properties and biocompatibility as
`
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`
`
`
`
`
`
`
`long as only a smo‘oth, bulk shape in a metallurgically
`
`BACKGROUND OF THE INVENTION
`
`
`
`The need for a cancellous bone substitute and/or cell
`
`
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`
`
`
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`
`
`
`
`and tissue receptive material is significant. For example,
`
`
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`
`cancellous autografts provide a porous framework
`within which revascularization occurs and against
`
`
`
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`
`
`which new bone is layered, and also provide a popula-
`
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`
`
`tion of osteoprogenitor cells and a complement of bone
`
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`growth-inducing factors. Grafting, however, requires
`surgery to obtain the material, and a viable substitute is
`
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`
`
`desirable. It is here that the concept of artificial biocom-
`
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`
`patible implants becomes of interest. Extensive studies
`over the last two decades have shown that to duplicate
`
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`
`the success of cancellous grafts, an implant should serve
`
`
`
`
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`
`
`as a porous framework. Indeed, early research demon-
`
`
`
`
`
`
`
`strated that an interconnected porous material is toler-
`
`
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`
`
`
`ated by the body, and encourages new bone growth,
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`better than the same material in solid form.
`
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`The replacement of diseased, destroyed, or degenera-
`tive bone and tissues consumes time and financial re-
`
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`
`
`sources from a large segment of the surgical commu-
`
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`
`
`nity, in both medicine and dentistry. Clinical and scien-
`
`
`
`
`
`tific work is directed at facilitating regeneration of tis-
`sues in affected patients so that normal biomechanical
`
`
`
`
`
`
`
`and physiologic functions can resume. In some patients,
`
`
`
`
`
`
`
`full restoration of function with normal tissue is achiev-
`
`
`
`
`
`
`
`able, while in others, prostheses are biologically at-
`
`
`
`
`
`
`
`tached to restore function. The specialty science de-
`
`
`
`
`
`
`
`voted to the study of substances utilized for implants in
`
`
`
`
`
`
`
`medicine and dentistry, biomaterials,
`is a young field
`
`
`
`
`
`
`that has taken tremendous strides in the last 20 years.
`
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`
`
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`
`Over the same period, dental implantology has evolved
`from early attempts by a few enthusiasts to a fully rec-
`
`
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`
`
`
`
`
`
`ognized branch of dentistry.
`
`
`
`
`
`
`
`Although indispensable for survival, the body’s natu-
`ral defense mechanisms, by which materials identified
`
`
`
`
`
`
`
`as nonself are rejected, have been the nemesis of sur-
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`geons using prostheses or implantable devices. It
`is
`necessary to minimize the rejection mechanism as much
`
`
`
`
`
`
`45
`as possible. Certain biomaterials have been identified
`
`
`
`
`
`
`
`having apparently limited reactions to the body’s de-
`
`
`
`
`
`
`
`
`fense mechanisms. These materials can be placed on a
`
`
`
`
`
`
`continuum that extends from relatively chemically reac-
`
`
`
`
`
`
`
`tive to completely nonreactive or passive. Generally,
`
`
`
`
`
`
`
`the more nonreactive the material is in vivo, the better
`
`
`
`
`
`
`
`
`
`the performance that can be expected.
`
`
`
`
`
`
`
`
`
`
`Matching the requisite biomechanical requirements
`
`
`
`
`
`
`
`
`for an implant with the environment of surrounding
`tissues has been a formidable challenge. Significant
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`progress was made in resolving this problem in the early
`19705, when the importance of porosity was first recog-
`
`
`
`
`
`
`
`nized. Later work showed that certain physical parame-
`
`
`
`
`
`
`
`
`ters of the porosity affect the type of tissue and the rate
`
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`
`
`
`
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`
`
`
`
`
`
`
`
`
`of ingrowth. The degree of interconnectivity and the
`nominal pore size were found to be critical factors in
`
`
`
`
`
`
`
`
`determining the success of an implant. Maximum inter-
`
`
`
`
`
`
`connectivity, or the absence of “dead ends”, was found
`
`
`
`
`
`
`
`to facilitate ingrowth. These studies showed that pore
`
`
`
`
`
`
`
`
`
`
`
`
`
`
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`
`
`sizes less than 10 um prevent ingrowth of cells; pore
`sizes of 15-50 pm encourage fibrovascular ingrowth;
`
`
`
`
`
`
`
`pore sizes to 50—150 um result in osteoid formation; and
`
`
`
`
`
`
`
`
`
`
`
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`
`
`
`pore sizes of greater than 150 um facilitate the ingrowth
`of mineralized bone.
`
`
`
`
`
`
`
`
`
`
`50
`
`55
`
`65
`
`Page 6 of 11
`
`Page 6 of 11
`
`

`

`5,282,861
`
`
`4
`
`
`
`
`
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`
`
`lum (atomic number 73, atomic weight 180.95, density
`
`
`
`
`
`
`16.68 g/cm3) is a transition element (periodic group
`
`
`
`
`
`
`
`VB), a highly refractory (melting point 2996° C.),
`strong, ductile metal with excellent oxidation and cor-
`
`
`
`
`
`
`
`
`’ rosion resistance. These properties led to its early inves-
`
`
`
`
`
`
`
`
`tigation, in both animal and human experiments, as a
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`potential human implant material. Early evidence of
`excellent tissue acceptance, combined with low corro-
`
`
`
`
`
`
`
`sion, has led to the use of tantalum as a surgical implant
`
`
`
`
`
`
`
`
`material and its use in a variety of applications, includ-
`
`
`
`
`
`
`ing pacemaker electrodes, wire, foil and mesh for nerve
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`repair, cranioplasty plates, contrast media for airwave
`
`
`
`
`
`
`radiographic studies, radiopaque markers for following
`bone growth, ligation clips, and more recently on an
`
`
`
`
`
`
`
`
`
`
`
`
`experimental basis in femoral endoprostheses.
`The crystal structure of tantalum is body-centered
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`cubic, giving it excellent ductility due to the six possible
`slip planes. It is so corrosion-resistant that it resists the
`
`
`
`
`
`
`
`
`attack of most chemical agents; tantalum pacemaker
`
`
`
`
`
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`
`electrodes have exhibited excellent corrosion resistance
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`both in vitro and in vivo. This inertness likely accounts
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`for the good tissue compatibility of the base metal as
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`well, whereas a noble metal such as gold, though con-
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`sidered corrosion-resistant, is not sufficiently biocom-
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`patible due to its catalytic surface.
`Comparative studies have demonstrated that tanta-
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`lum does not inhibit cell growth and indeed becomes
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`tightly enveloped by new osseous tissue soon after im-
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`plantation, whereas dental gold and cobalt-based alloys
`can inhibit cell growth and cause bone resorption. With
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`tantalum, osseous ingrowth has been demonstrated
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`right up to and into implants. Complete, strong, long-
`term osseointegration has been demonstrated with tan-
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`talum implants in both dental and orthopedic applica-
`tions, under both unloaded and heavily loaded condi-
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`tions, for implantation periods as long as eight to twelve
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`years.
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`In addition, tantalum has an elastic modulus close to
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`that of bone, much closer than any of the other high-
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`strength metals and alloys commonly used for implants;
`this too may well contribute to the favorable reaction
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`with bone. With its greater ductility, excellent corro-
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`sion resistance, good workability, and demonstrated
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`biocompatibility, tantalum clearly can be regarded as an
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`excellent alternative to the metals and alloys presently
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`in use and under development for bone implants.
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`FIG. 1 is a perspective view of an open cell tantalum
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`structure constructed in accordance with the present
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`invention;
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`FIG. 2 is an enlarged view of the surface of the tanta-
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`lum structure of FIG. 1;
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`FIG. 3 is a detailed view of small sections of the
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`material of FIGS. 1 and 2; and
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`FIG. 4 is illustrative of one method of making the
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`tantalum structure of the present invention.
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`DESCRIPTION OF THE PREFERRED
`
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`EMBODIMENT
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`Cancellous, or spongy, bone is composed of a porous
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`space-frame structure formed of open spaces defined by
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`interconnected trabeculae, oriented along lines of prin-
`cipal stresses. At the microstructural level, the trabecu-
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`Iae are composed of layers of lamellar bone. Cancellous
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`bone has anisotropic mechanical properties, i.e. differ-
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`ent structural behavior along different orientations.
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`Along the axis of the major channels, cancellous bone
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`3
`perfect state is needed. The machining or other treat-
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`ment needed to obtain a porous or surface-textured
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`shape for interlocking with skeletal tissue can have a
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`detrimental effect on the properties and biocompatibil-
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`ity, and can even result in material failure. For example,
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`the hexagonal crystal structure of titanium makes it
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`susceptible to cracks and fractures, as has been seen in
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`the case of dental implants. Some porous metallic mate-
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`rials (e.g. flame- or plasma-sprayed titanium, porous
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`sintered powder metallurgy materials) do not match the 10
`structure of cancellous bone sufficiently well to ensure
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`successful ingrowth and integration. Also, most metals
`and alloys currently in use are subject to some degree of
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`corrosion in a biological environment. Finally, the high
`densities of metals can make them undesirable from a 15
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`weight standpoint.
`'
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`SUMMARY OF THE INVENTION
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`New materials are enabling the design of innovative,
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`and increasingly biocompatible, replacements for dam- 20
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`aged human tissues. In the present invention, reticulated
`open cell carbon foam is infiltrated with tantalumby the
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`chemical vapor deposition (CVD) process. It should be
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`noted that niobium, which has similar chemical and
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`mechanical properties to tantalum, may also be used as 25
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`well as appropriate alloys of tantalum and niobium. For
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`example, other metals such as niobium, hafnium and/or
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`tungsten could be alloyed with the tantalum or hafnium
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`and/or tungsten with niobium to change modulus and-
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`/or strength. Therefore, any reference to tantalum 'is 30
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`not meant to be an exclusion of other metals.
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`The carbon foam is infiltrated by chemical vapor
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`deposition (CVD). The resulting lightweight, strong,
`porous structure, mimicking the microstructure of natu-
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`ral cancellous bone, acts as a matrix for the incorpora— 35
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`tion of bone or reception of cells and tissue. The pores
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`of the matrix are connected to one another to form
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`continuous, uniform channels with no dead ends. This
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`intricate network of interconnected pores provides op-
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`timal permeability and a high surface area to encourage 40
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`cell and tissue ingrowth, vascularization, and deposition
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`of new bone.
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`The result is a new biomaterial that, when placed next
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`to bone or tissue, initially serves as a prosthesis and then
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`functions as a scaffold for regeneration of normal tis- 45
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`sues. The new biomaterial fulfills the need for an im-
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`plant modality that has a precisely controllable shape
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`and at the same time provides an optimal matrix for cell
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`and bone ingrowth. Additionally, the physical and me-
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`chanical properties of the porous metal structure can be 50
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`specifically tailored to the particular application at
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`hand. This new implant offers the potential for use in
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`alveolar ridge augmentation, periodontics, and orthog-
`nathic reconstruction. As an effective substitute for
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`autografts, it will reduce the need for surgery to obtain 55
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`those grafts. It is useful in orthopedic applications as
`well.
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`The present invention may also be used for tooth
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`replacement because of the ability to induce tissue and
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`bone growth even in the face of mildly infectious condi- 60
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`tions. For example, an artificial tooth can be joined to
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`an open cell tantalum stem and positioned in an appro-
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`priately sized hole in the jaw. The gum is allowed to
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`rest against the artificial tooth and some of the stem to
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`form a seal.
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`Tantalum was selected as the material of choice based
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`on its good mechanical properties, excellent corrosion
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`resistance, and demonstrated biocompatibility. Tanta-
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`5
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`65
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`Page 7 of 11
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`Page 7 of 11
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`5
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`5
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`exhibits elastic behavior with sudden brittle failure at
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`ultimate load in tension. When loaded with a tensile
`force whose line of action is skewed with respect to the
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`channel axis of the bone, the stress-strain curve is para-
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`_ bolic with plastic deformation and greater energy ab-
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`sorption. It is therefore stiffer (has higher tensile and
`compressive moduli) but fails at a lower strain when
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`loaded parallel to the predominant spicular direction
`than when loaded in other directions. These properties
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`are important because they serve to absorb shock and 10
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`distribute load in the vicinity of the articular surfaces of
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`joints.
`Any material to be used as a substitute for cancellous
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`bone must therefore allow elastic deformation and load
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`distribution. In addition, the material must not produce 15
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`load concentrations, particularly if placed close to the
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`underlying surface of articular cartilage, which might
`increase the local stresses on the articular surface and
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`lead to wear and damage of the surface.
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`Cancellous bone demonstrates remodeling behavior 20
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`according to Wolff‘s Law: that is, with the form being
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`given, bone adapts to the loads applied to it. The con-
`verse is also true, and equally important: where loads
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`are not applied, bone tends to resorb. An implant, then,
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`must distribute stresses throughout its structure,
`the 25
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`ingrowing bone, and the surrounding bone in order to
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`avoid bone resorption and weakening caused by stress
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`shielding.
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`The density of cancellous bone is 0.7 g/cm3; its tensile
`modulus 0.2—0.5 GPa; tensile strength 10—12 MPa; and 30
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`strain to failure 5-7%. Compared to cortical bone, can-
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`cellous bone is é-i as dense (indicating its porous na-
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`ture); l/lO—l/ZO as stiff; and five times as ductile. The
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`mechanical properties of the two types, though, actu-
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`ally represent a continuum, reflecting the behavior of a 35
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`relatively uniform material (bone) modified by differ-
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`ences in density and structure.
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`Based on experiments with hydroxyapatite implants,
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`ingrowth and maturation of new bone are more rapid
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`from a cancellous bone region than from cortical bone, 40
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`with the tissue-implant interface reaching peak shear
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`strength in dogs in 8 weeks. The process may take
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`longer in humans, with remodeling still possible up to 2
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`years postoperation. Inadequate device designs may
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`produce continued stress shielding remodeling as long 45
`as 9—10 years postoperation.
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`Materials for osseous, or bone, implants must be rigid
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`and stress-resistant, while avoiding self-concentration
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`of stresses that result in stress shielding. Also, osseous
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`implants should ideally reside in the bone without inter- 50
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`fering with bone remineralization, the natural process
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`by which the body replenishes bone. The implant
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`should be able to be precisely shaped and placed for
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`optimal interface and performance. Finally, non-resorp-
`tion would be a beneficial quality for implants used in 55
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`load-bearing applications, and/or those in which com-
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`plete bone ingrowth is not possible.
`Critical to the performance of a porous implant is the
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`completeness of its interconnectivity. This is essential
`because constrictions between pores and isolated, dea- 60
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`dend pockets can limit vascular support to ingrowing
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`tissues; ischemia of the ingrowing bone cells results in
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`failure of the implant. Incomplete vascularization or a
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`reduction in the neovascularity also makes an implant
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`vulnerable to bacterial colonization. Implants lacking 65
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`completely interconnected porosity-can also result in
`aberrant mineralization, stress shielding,
`low fatigue
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`strength, and/or bulk displacement.
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`Page 8 of 11 7
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`5,282,861
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`6
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`The open cell metal structure of the present invention
`
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`offers highly interconnected, three-dimensional poros-
`ity that is uniform and consistent, a structure exception-
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`ally similar to that of natural cancellous bone. In this
`
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`way it is superior to other porous metallic implant mate-
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`rials, whose “porosity” is artificially produced via some
`form of surface treatment that does not result in a truly
`
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`complete, open porosity. Examples of these methods
`
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`include macroscopic porous coatings (e.g. metal micro-
`spheres or wires sintered or otherwise attached to a
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`bulk surface); microscopic surface porosity (e.g. metal
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`powder particles flame- or plasma-sprayed onto a bulk
`surface); and controlled surface undulations machined
`
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`into a bulk surface.
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`Although certain porous ceramic materials do offer
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`full porosity (e.g. the replamineform process for hy-
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`droxyapatite), they have properties inferior to metals as
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`discussed previously. The open cell metal structure is
`osteoconductive, like other porous implants. Also, it is
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`entirely biocompatible, based on the demonstrated bi-
`
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`ocompatibility of tantalum.
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`Allowing full mineralization is another extremely
`
`
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`
`
`
`important property required of bone substitute materi-
`als. The highly organized process of bone formation is a
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`complex process and is not fully understood. There are,
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`however, certain prerequisites for mineralization such
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`as adequate pore size, presumably larger than 150 pm
`with interconnect size in the range of 75 pm. A pore
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`diameter of 200 um corresponds to the average diame-
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`ter of an osteon in human bone, while a pore diameter of
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`500 um corresponds to remodeled cancellous bone. The
`open cell metal structures of the present invention can
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`be fabricated to virtually any desired porosity and pore
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`size, and can thus be matched perfectly with the sur-
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`rounding natural bone in order to provide an optimal
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`matrix for ingrowth and mineralization. Such close
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`matching and flexibility are generally not available with
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`other porous implant materials.
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`One concern with an implant must be the potential
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`for stress shielding. According to Wolff‘s law, bone
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`grows where it
`is needed (that is, where there is a
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`stress). Stress on a bone normally stimulates that bone to
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`grow. With an implant, it is primarily the stress/strain
`field created in the tissue around an implant that con-
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`trols the interface remodeling. Stress shielding occurs
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`when an overly stiff implant carries stresses that were
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`previously applied to the bone in that area; it can result
`
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`in inhibition of mineralization and maturation of the
`
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`ingrowing bone, and/or the resorption of existing natu-
`ral bone.
`
`
`An implant, then, must distribute stresses throughout
`
`
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`
`
`
`
`its structure, the ingrowing bone, and the surrounding
`
`
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`bone in order to avoid bone resorption and weakening
`
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`
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`caused by stress shielding. Because metals are stronger
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`than natural bone, this would seem to be a concern with
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`a metallic implant in that the implant would itself focus
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`and bear direCtly the majority of local loads and stresses
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`that would ordinarily be placed on the bone, thus de-
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`priving both the existing and new bone of those forces
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`which, in effect, help keep it at optimal strength.
`The unique structure and properties of the open cell
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`metal structures of the present
`invention, however,
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`avoid this drawback altogether. The deposited thin
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`films operate as an array within the porous metal body,
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`contributing their exceptional mechanical properties to
`the structure at large. One result of this effect is that
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`imposed loads are distributed throughout the body. In
`the case of a open cell metal bone implant, stresses are
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`Page 8 of 11
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`distributed into both the ingrowing new bone and the
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`surrounding existing bone as well, thereby providing
`both the old and new bone with the normal, healthy
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`forces they require.
`In fact, with the ability to finely tailor the open cell
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`metal structure’s properties during the fabrication pro-
`' cess, an implant can be designed to distribute stresses in
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`a given direction(s), depending on the needs of the
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`specific application at hand. The bonding of regener-
`ated bone to the implant also helps to transfer stresses
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`directly to the bone in and around the implant; this
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`sharing of biofunction is a consequence of the compos-
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`ite nature of the implant/bone structure. The advantage
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`of these metal structures over other porous implant
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`materials is especially strong in this area. Ceramics lack
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`sufficient mechanical properties to begin with, and no
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`current implant material, either ceramic or metallic,
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`possesses the unique properties of the metal structure as
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`described here.
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`In the present invention, useful lightweight refrac-
`tory structures are made by the chemical vapor deposi-
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`tion (CVD) of a small amount of metallic material such
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`as tantalum or niobium (or combination of these materi-
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`als with other materials to form alloys) into a reticulated
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`(porous) vitreous carbon foam. The density of the resul-
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`tant body is purposely maintained at substantially below
`full density, resulting in a structure with extremely
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`favorable properties. The basic approach involves the
`use of a low-density carbon foam, w