CHARACTERIZATION OF A NEW POROUS TANTALUM
`BIOMATERIAL FOR RECONSTRUCTIVE ORTHOPAEDICS
`
`JD Bobyn PhD, SA Hacking MEng, SP Chan MD, K-K Toh MD, JJ Krygier CET, M Tanzer MD
`Jo Miller Orthopaedic Research Laboratory, Division of Orthopaedics, McGill University, Montreal, Quebec, Canada
`
`A SCIENTIFIC EXHIBIT AT THE 1999 ANNUAL MEETING OF THE
`AMERICAN ACADEMY OF ORTHOPAEDIC SURGEONS, ANAHEIM, CALIFORNIA
`
`INTRODUCTION
`PROBLEM: A need exists for a structural porous biomaterial for solving implant problems in reconstructive orthopaedics.
`Porous polymers and porous ceramics do not generally have adequate mechanical properties.1-4 Conventional porous
`metals such as sintered beads, fibre metal, and plasma spray have limited porosity and are useful as coatings. However,
`they are not suitable for use as bulk structural materials for implant fabrication.5-11
`
`SOLUTION: A highly porous tantalum biomaterial with unique physical, mechanical, and tissue ingrowth properties has
`recently been developed.12-16 Potential applications include bone augmentation, fracture fixation, spine and joint fusion,
`tumor management, joint replacement, and soft tissue repair.1
`
`Scanning electron micrograph (SEM)
`of a plasma spray porous surface
`
`SEM of a sintered bead porous
`surface
`
`SEM of a fibre metal porous
`surface
`
`SEM of a porous tantalum structure
`with interconnected porosity
`
`POROUS TANTALUM - FABRICATION
`• A polymer with a porous structure is reduced at high temperature to vitreous carbon
`• The struts of the carbon network serve as a template for coating with tantalum
`• Pure tantalum is progressively deposited onto the struts using vapor deposition techniques (40-50 µm thickness)
`• The volume porosity is 75-80%, 2-3 fold greater than with conventional porous materials
`• The structure consists of a regular array of highly interconnected pores, similar to trabecular bone
`• The porous structure can be formed into complex implant shapes before or after tantalum deposition
`
`SEM of vitreous carbon substrate
`
`SEM of x-section of porous tantalum. The struts
`appear white; the black inside the struts is carbon
`
`SEM of human trabecular bone
`
`Page 1 of 7
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`ZIMMER EXHIBIT 1022
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`

`

`POROUS TANTALUM - PHYSICAL PROPERTIES
`• Tantalum is a highly biocompatible, corrosion resistant, strong, and tough metallic element (A.N. = 73) 17,18
`• Tantalum has greater than a 50 year history of use as an implant material in humans 19-33
`• The average full pore diameter of the porous tantalum is 547 ± 52 µm
`• Adjacent full pores are completely interconnected by multiple smaller-sized portals
`• The average two-dimensional porosity (full pores and portals combined) is 430 ± 270 µm 34
`• The vapor deposition process results in a microtexture on the struts due to tantalum crystal growth
`
`Portals
`Portals
`
`Full Pore
`Full Pore
`
`SEM of pores with connecting struts and portals
`
`Tantalum within the periodic
`table of elements
`
`Higher magnification SEM illustrating strut microtex-
`ture. Inset - crystal struture of deposited tantalum
`
`CONCLUSIONS: The material has a porous structure resembling trabecular bone. The pore size is ideal for vascularized
`tissue ingrowth. The high volume porosity enables extensive tissue infiltration and strong attachment strength. The strut
`microtexture is conducive to bone formation. 35-38
`
`POROUS TANTALUM - MECHANICAL PROPERTIES
`
`CoCr 210
`
`Cortical Bone 130-150
`
`Titanium 110
`
`Other Porous Metals 20-150
`
`Cortical Bone ~ 15
`
`Porous tantalum ~ 3
`
`Subchondral bone ~ 2
`
`Trabecular bone ~ 0.1
`
`Porous tantalum ~ 50-80
`
`Trabecular bone ~ 10-50
`
`Porous Ceramics ~ 3-30
`
`Porous Polyethylene ~ 3-5
`
`Elastic modulus (GPa) of various materials
`
`Compressive strength (MPa) of various porous materials
`
`High ductility during compressive testing
`
`0.88
`
`0.74
`
`0.50
`
`cortical cancellous
`Porous tantalum on
`
`cancellous
`Beads on
`
`High torsional deformation without fracture
`Ultimate torsional strength = 40-60 MPa39
`
`4-point fatigue bend test - no load (top)
`Plastic deformation under high load (bottom)
`Tensile endurance limit @ 106 cycles = 18-20 MPa 39
`
`Coefficient of friction of porous
`tantalum and sintered beads against bone 40
`
`CONCLUSIONS: The strut/pore geometry creates a structure with a high strength/weight ratio. The mechanical proper-
`ties vary with the thickness of tantalum deposition; greater thickness provides greater strength. 39 The material displays
`high ductility - struts deform plastically in compression, tension, and torsion without brittle failure. The strength character-
`istics are superior to porous polymers and ceramics and comparable to other porous metals of higher density. The low
`modulus is conducive to load transfer without stress shielding problems. The higher coefficient of friction against bone
`increases initial stability during implantation. 40
`
`Page 2 of 7
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`

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`APOROUS TANTALUM - BONE INGROWTH - TRANSCORTICAL IMPLANTS
`
`STUDY PROTOCOL
`
`• Transcortical canine implants
`• Study @ 2,3,4,8,16,52 weeks
`• Mechanical push-out tests
`• Undecalcified thin section histology
`• Quantitative backscattered
`scanning electron microscopy
`
`Transcortical implants - 5x10 mm
`
`Femoral implantation - lateral cortices
`
`4 weeks - implanted drill hole, longitudinal section
`
`SEM of bone ingrowth @ 6 weeks
`
`8 weeks - implanted drill hole, longitudinal section
`
`Bone ingrowth @ 16 weeks (paragon stain)
`
`SEM of implant cross section @ 52 weeks
`
`Bone formation within a single pore
`@ 52 weeks (paragon stain)
`
`> 18.5
`
`~ 7-9
`
`Tantalum
`Porous
`
`Beads
`Sintered
`
`75
`
`65
`
`47
`
`12
`
`8
`
`2 3 4 16 52
`
`Weeks
`
`% filling of porosity with new bone
`
`8 weeks - empty drill hole, longitudinal section
`
`Interface shear strength (MPa) @ 4 weeks
`sintered beads versus porous tantalum
`
`CONCLUSIONS: Porous tantalum filled rapidly with new bone in the transcortical model. The majority of the available porosity was
`bone ingrown by 8 weeks after surgery. The healing of new bone into porous tantalum was comparable to bone healing within empty
`drill holes. New bone formed in apposition to the tantalum struts; apposition increased with postoperative time. At 16 and 52 weeks
`haversian remodelling occurred within the pores. Because of the high volume porosity, more bone formed at the interface compared
`with other porous materials.41-44 This resulted in a faster rate of development of high fixation strength.
`
`Page 3 of 7
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`POROUS TANTALUM - BONE INGROWTH - ACETABULAR CUPS
`
`STUDY PROTOCOL
`
`• Fully loaded canine total hip model
`• Nonmodular porous tantalum cups
`• Study periods @ 6, 12, 24 weeks
`• Undecalcified thin section histology
`• Quantitative backscattered
`scanning electron microscopy
`
`Canine acetabular cup with compression
`molded ployethylene liner
`
`Compression molded polyethylene (grey)
`into porous tantalum (white struts)
`
`Backscattered SEM at 6 weeks. Compression
`molded polyethylene (appears black under SEM)
`fills the inner half of the porous tantalum
`
`Backscattered SEM at 12 weeks illustrating
`uniform trabecular bone ingrowth
`
`Backscattered SEM at 24 weeks illustrating bone
`ingrowth to maximum depth
`
`High resolution contact radiograph illustrating
`uniform contact and stable interfaces @ 24 weeks
`
`Histologic coronal section @ 24 weeks. Bone
`ingrowth and density is greater at periphery
`(paragon stain)
`
`HIgher magnification of bone ingrowth region
`@ 24 weeks
`
`16.8
`
`16.5
`
`acetabular trabecular bone density
`acetabular trabecular bone density
`
`17.7
`
`~ 18
`
`6 12 24
`Weeks Implanted
`
`Undecalcified histology @ 24 weeks (paragon stain)
`New bone has formed in the dome region. The
`region of interest (blue) is expanded at right.
`
`Histology without (left) and with (right) polarized light.
`Bone ingrowth on top, molded polyethylene at bottom,
`pink stained striated fibrous tissue filling the non-bone
`ingrown pores in between
`
`Overall % bone ingrowth @ 6, 12 & 24 weeks
`compared to acetabular trabecular bone density
`
`CONCLUSIONS: Acetabular cup fixation was reproducible using a non-modular implant with a compression molded poly-
`ethylene liner. 45-48 Bone ingrowth was maximized by 6 weeks, with little change at 12 and 24 weeks. Bone often formed
`to the maximum depth within the pores, up to the compression molded polyethylene. Gap healing in the dome region was
`common. The density of bone ingrowth was comparable to surrounding acetabular trabecular bone. Fibrous tissue filled
`most of the regions without bone ingrowth, possibly representing a barrier to fluid and particulate material.
`
`Page 4 of 7
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`

`POROUS TANTALUM - SOFT TISSUE ATTACHMENT
`
`Porous tantalum soft tissue implant
`
`Mechanical peel test - vascularity of tissue is evident
`
`STUDY PROTOCOL
`
`• Rectangular canine implants
`• Paraspinal subcutaneous sites
`• Study periods @ 4, 8, 16 weeks
`• Mechanical peel tests
`• Thin section histology
`
`89
`
`70
`
`Porous Tantalum
`61
`
`28
`
`13
`
`Sintered Beads
`11
`
`4 weeks 8 weeks 16 weeks
`
`Atttachment strength (g/mm) of porous tantalum and
`beaded porous implants at 4, 8 & 16 weeks
`
`Histologic section at 4 weeks showing complete ingrowth
`of fibrous tissue through the implant (basic fuschin stain)
`
`Vascularized fibrous tissue ingrowth at 16 weeks -
`nutrient vessel with contained erythrocytes
`
`CONCLUSIONS: Vascularized fibrous tissue ingrowth occurred rapidly throughout porous tantalum. The mechanical
`attachment strength was 3-5 fold greater than with sintered beaded porous surfaces.49,50 This was probably due to the
`higher volume porosity, an interconnected pore size that better supported vascularization, and a surface roughness that
`caused greater soft tissue adherence.
`
`POROUS TANTALUM - CLINICAL APPLICATIONS
`
`Non-modular acetabular cup - compression molded PE*
`
`Non-modular tibial component - compression molded PE*
`
`Porous tantalum “allograft” and custom knee implant †
`
`Porous tantalum spacer for knee reconstruction*
`
`Porous tantalum support dowel for avascular necrosis†
`Porous tantalum washer for soft tissue reconstruction†
`* This device is intended to be used in the United States as described in the product’s labeling † This device is not cleared by the FDA for distribution in the United States
`
`• Medical Research Council of Canada
`
`ACKNOWLEDGEMENT
`
`Page 5 of 7
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`REPRINTS
`Please address reprint requests to: J. Dennis Bobyn, PhD, Jo Miller Orthopaedic Research Laboratory, Montreal General Hospital, McGill University,
`1650 Cedar Avenue, Montreal, Quebec, Canada H3G 1A4. email: jdbobyn@canada.com
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