J . BIOMED. MATER. RES. SYMPOSIUM
NO. 6 , pp. 23-27 (1975)
Replamineform Porous Biomaterials for Hard Tissue Implant Applications EUGENE W. WHITE, JON N . WEBER, DELLA M. ROY and EDWIN L. OWEN, Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 and RICHARD T. CHIROFF and RODNEY A. WHITE, Orthopaedic Research Laboratory, Upstate Medical Center, The State University of New York, Syracuse, New York 13210
Summary By means of the newly developed Replamineform process, the unique pore microstructures found in the skeletal calcium carbonate of certain reef corals can be replicated or reproduced with high precision in a wide variety of materials suitable for hard tissue implant and prosthetic applications. The advantages of fabricating porous biomaterials with this method include closely controlled size of both the pore diameters and the diameters of the pore interconnections, and virtually complete interconnection of the uniformly spaced pores. These properties are of great importance in implant devices, because tissue ingrowth, the stimulation of new bone formation, the suppression of undesirable scar tissue, the inhibition of adverse body responses, and firm biological fixation of the implanted material all depend upon the nature of the pore-microstructure configuration. Replamineform preparation of AI,O,, TiO,, hydroxyapatite, silver, Co-Cr-Mo alloys, and polymers is described in detail, and the CharacteriLation procedures used to determine the physical and structural properties of their materials are discussed. A few of the routinely measured characteristics include ( I ) quantitative computerized SEM image analysis for determining the volume, size and shape distributions of the macro and microporosity and the grain size measurement of the solid; (2) nondestructive x-radiography of specimens to reveal any internal defects; (3) mechanical strength measurements of randomly selected specimens. Experimental results up to now clearly demonstrate the superiority of microstructures imparted to metals, ceramics, and polymers with the Replamineform process.
INTRODUCTION Our interest in microporous biomaterials developed during studies of unusual mechanical and physical properties associated with the internal pore structure configuration of skeletal calcium carbonate deposited by echinoderms and corals [ 11. Such microstructures have ideal qualities for implant purposes because of the remarkable uniformity of pore size and complete interconnection of the pores. Both the pore size, and size of interconnection, fall well within the ranges found to be suitable for op23
@ 1975 by John Wiley & Sons, Inc.
WHITE ET AL.
24
timum bone tissue ingrowth [2], [ 3 ] . Subsequently, techniques were developed [4], [ 5 ] to replicate the invertebrate skeletal microstructures in a wide range of the most promising materials, including metals, ceramics, and polymers. For hard tissue implant applications, the coral genus Porites appears to offer an ideal microstructural configuration. Its skeleton has pores with diameters in the range of 140--160 pm, and all of the pores are interconnected. The precursor carbonate is soft and easily machined into any conceivable geometry (plugs, screws, plates, pins, or custom-shaped devices) prior t o replication of the microstructure in another material. The general procedures are diagrammed in Figure 1. It is possible t o prepare implants which are wholly porous, or implants which have a solid core with contiguous porous overlay. A wide range of composite materials, furthermore, can be fabricated for special purposes. A description of the Replamineform process, a discussion of its advantages in relation to other pore-forming techniques, and an evaluation of modern biomaterials engineering technology have been published [6].
I
Select coral wtth destred microstructure
I
I
I exchange wq for c o 3
I j
Dilute HCI treatment t o remove carbonate
InYeSl wax negative
wkth slurry
I I
I
Cao at 800°C
with monomers
II
I
S t d e r for strength at 1450°C
I
I to remove carbomte
Burn off wax
form mold
Centrifugslly cast
I treatment t o remove CaO
1 replieas*
Calc~umphosphate "pasitlves"
Polymer "negatwe" ""egatlve" replicas
1. o-a1um,na
1. Hydrunyaptite
1. POlyurethane
1. Cr-Co-Mo Alloy
2. Tt0,
2. WhLtlockcte
2. SLIaStlC
2 . s,tuer
3. Methyl methacrylate
3. Tin
*Typical shrinkages 12-ZO'l
Fig. I . Replamineform procedure for preparing controlled pore structures in ceramics, polymers, and alloys. The ceramic, polymer, and alloy materials may be prepared in a completely porous configuration or as continguos porous overlay on solid core material
REPLAMINEFORM BIOMATERIALS FOR IMPLANTATION
25
Following the successful preparation of Replamineform materials, specifically those with the replicated microstructure of Porites, biocompatibility tests of the various implant materials were initiated as a cooperative investigation by the Materials Research Laboratory at The Pennsylvania State University and the Orthopaedic Research Laboratory of the Upstate Medical Center at Syracuse. I n these experiments, uniformly porous plugs were placed in the cancellous bone of the distal femora and proximal tibiae of large mongrel, adult dogs. Long-term tolerance studies demonstrated acceptance of these implants without any evidence of migration. The Replamineform implants stimulated new bone ingrowth but also induced regeneration of the excised articular cartilage. Three of the most promising materials were selected for further implant tests as part of the feasibility study begun in May 1973: alumina (a-Al,O,), titania (TiO,), and hydroxyapatite [Ca,,(PO,),(OH),]. Plugs of each were placed in the subchondral bone of the lateral condyles of the distal femora of adult male rabbits. The results of these studies are discussed by Chiroff et al. [7]. Preparation and Preimplantation Characterization of Hydroxyapatite Hydroxyapatite test specimens were prepared by hydrothermal conversion of 4 mm diam by 4 mm long cylinders of skeletal CaCO, from the coral Porites. Complete hydrothermal conversion of the aragonite (CaCO,) t o hydroxyapatite was accomplished by a 24 hr treatment in a fluid medium of (NH,),HPO, and H,O maintained at 300°C and 15,000 psi [8]. Specimens were also prepared with an impermeable cap on a cylinder which is dominantly porous. In this case pre-machined coral carbonate was reacted at high temperature and high pressure t o produce porous apatite. A slurry of apatite was then used to fill the pores in the top 0.5 mm of the implant. After drying, the samples were reacted at a high temperature to cause sintering and formation of a hard compact cap. After sintering, the specimens were boiled in water to regenerate any apatite that might have been dehydroxylated. The apatite formed by this process completely replaces Porites carbonate as was confirmed by both petrographic microscope and x-ray powder diffraction. Electron microprobe analyses show that the composition of the apatite is the same as that obtained for apatites that crystallized at higher temperatures and are stoichiometric, i.e., Ca:P = 10:6. SEM micrographs demonstrate the morphological integrity of resultant apatite. The infrared spectral data are characteristic of hydroxyapatites having
26
WHITE ET Al.
a small amount of carbonate substitution in the crystal structure. The specific positions of the carbonate absorption frequencies indicate that this is lattice COB=, substituted for columnar OH- and for PO: tetrahedra in the apatite structure, rather than admixed or incompletely reacted calcium carbonate [9]. This is a significant observation, as most experts believe that CO, is an important, though relatively small, constituent of the hydroxyapatite in bone. Magnesium, another minor or trace element present in the specimens synthesized from coral, is also believed t o have a significant role in bone mineral formation.
Preparation and Preimplant Characterization of a - A l z 0 3 The most satisfactory a-Al,O, replicas have been made using Alcoa A15SG alumina. This starting material has been thoroughly characterized by standard CESEMI procedures [lo], [ I I ] developed at The Pennsylvania State University. The median particle size (number count basis) is 0.35 pm, and the maximum and minimum sizes are 10 p m and 0.1 p m respectively. The size distribution is significantly bimodal. The particle mass distribution has also been determined; in this case the median size is 4.2 p m . Alcoa A-I5SG is 99.5+% Al,03 with the remainder composed primarily of SiO,, Na,O, and Fe,O,. Procedures for preparing Replamineform alumina were reported by Tarhay, et al [12]. Final firing conditions are at 1650°C for 3 hr. S E M micrographs confirm the high fidelity of replication. Linear shrinkage on firing is 12-13%. Both the macro and microporosity have been quantitatively characterized by quantitative S E M image analysis. Macro pores comprise 62 vol % with a median pore diam of 135 pm. The microporosity (porosity within the sintered a-Al,O,) is 5% with a median pore size of 0.7 p m . The maximum pore diam in the sintered area is 5.0 pm. Grain growth upon sintering is not excessive. Median grain size in the sintered a-A1203 is 2 p m compared with the 0.35 p m median size for the starting material. Maximum sintered grain size is 20 pm. Preparation of Replamineform a-Al,O, materials having a nonporous layer or a solid core proves to be quite straightforward. The one-step sintering yields a completely integral structure with no mechanical discontinuities at the porous-impervious cap interface, further confirming that the volume shrinkage within the Replamineform-porous materials is the same as for the sintering of a bulk alumina body.
Preparation of Replamineform TiO, The best starting materials for preparing the TiO, (rutile) implant test specimens has been found to be Baker reagent grade TiO, (anatase). I n
REPLAMINEFORM BIOMATERIALS FOR IMPLANTATION
27
this material, 50% of the particles are smaller than 0.1 pm. The material is 99.0% TiOz with a maximum of 0.023% trace metal content. Fired at 1425°C for 3 to 4 hr, this material undergoes a 29% linear shrinkage. Microporosity is less than that of the a-Al,O, but sintered grain size is larger. Reproducibility of the preparation is not at an acceptable level yet, but enough suitable specimens have been prepared of the completely porous configuration to carry out the implant test program.
Preparation of Replamineform Metals An important step has been taken toward simplifying the Replamineform work with metals. This results from the finding that the coral aragonite can be sintered to form C a O without destruction or degradation of the original Porites microstructure. Calcium oxide is an excellent investment material in that it is highly refractory (mp = 2580"C), yet rapidly dissolves in dilute HCI. Thus after casting the metal, the investment can be completely removed by merely immersing the cast metal in water at room temperature. This research is being supported by N S F Grant GH-38944. The assistance of Leo Tarhay, D. Dinger and E. J . Majzlik, J r . is gratefully acknowledged.
References [I] J. N . Weber, R. Greer, B. Voight, E. White, and R. Roy, J . Ultrastruct. Res., 26, 355 (1969). [2] S. F. Hulhert, et al., J . Biomed. Muter. Res., 4 , 433 (1970). [3] P. Predecki, J . E. Stephan, B. A . Auslaender, V. L. Mooney, and K. Kirkland, J . Biomed. Muter. Res.. 6, 375 (1972). [4] J . N. Weber, E. W. White, and J . Lebiedzik, Nature, 233, 337 (1971). [5] E. W. White, K. Mayberry, and G . G . Johnson, Jr., Pattern Recognition. 4 , 173 (1972). [6] J. N . Weber and E. W. White, Min. Sci. Eng., 5 , 151 (1973). [7] R. T. Chiroff, E. W. White, J. Weber, and D. Roy, J . Biomed. Muter. Res. Symp. No. 6 (part I), MH6 (1975). [8] D. M. Roy and S. Kurtossy Linnehan, Nature. 247, 220 (1974). [9] W . Eysel, D. Dinger, and D. M . Roy, Mat. Res. Bull, 9 , 35 (1974). [lo] R . A. White, J . N . Weber, and E. W . White, Science, 176, 922 (1972). [ I l l J. Lebiedzik, K. G . Burke, S. Troutman, G. G. Johnson, Jr., and E. W. White, in Scanning Electron Microscopy/l973, Chicago, Illinois, 1973, pp. 121- 127. [I21 L. Tarhay, W. R. Buessem and E. W. White, Ceram. Abstr., 52, 394 (1973).
NO. 6 , pp. 23-27 (1975)
Replamineform Porous Biomaterials for Hard Tissue Implant Applications EUGENE W. WHITE, JON N . WEBER, DELLA M. ROY and EDWIN L. OWEN, Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 and RICHARD T. CHIROFF and RODNEY A. WHITE, Orthopaedic Research Laboratory, Upstate Medical Center, The State University of New York, Syracuse, New York 13210
Summary By means of the newly developed Replamineform process, the unique pore microstructures found in the skeletal calcium carbonate of certain reef corals can be replicated or reproduced with high precision in a wide variety of materials suitable for hard tissue implant and prosthetic applications. The advantages of fabricating porous biomaterials with this method include closely controlled size of both the pore diameters and the diameters of the pore interconnections, and virtually complete interconnection of the uniformly spaced pores. These properties are of great importance in implant devices, because tissue ingrowth, the stimulation of new bone formation, the suppression of undesirable scar tissue, the inhibition of adverse body responses, and firm biological fixation of the implanted material all depend upon the nature of the pore-microstructure configuration. Replamineform preparation of AI,O,, TiO,, hydroxyapatite, silver, Co-Cr-Mo alloys, and polymers is described in detail, and the CharacteriLation procedures used to determine the physical and structural properties of their materials are discussed. A few of the routinely measured characteristics include ( I ) quantitative computerized SEM image analysis for determining the volume, size and shape distributions of the macro and microporosity and the grain size measurement of the solid; (2) nondestructive x-radiography of specimens to reveal any internal defects; (3) mechanical strength measurements of randomly selected specimens. Experimental results up to now clearly demonstrate the superiority of microstructures imparted to metals, ceramics, and polymers with the Replamineform process.
INTRODUCTION Our interest in microporous biomaterials developed during studies of unusual mechanical and physical properties associated with the internal pore structure configuration of skeletal calcium carbonate deposited by echinoderms and corals [ 11. Such microstructures have ideal qualities for implant purposes because of the remarkable uniformity of pore size and complete interconnection of the pores. Both the pore size, and size of interconnection, fall well within the ranges found to be suitable for op23
@ 1975 by John Wiley & Sons, Inc.
WHITE ET AL.
24
timum bone tissue ingrowth [2], [ 3 ] . Subsequently, techniques were developed [4], [ 5 ] to replicate the invertebrate skeletal microstructures in a wide range of the most promising materials, including metals, ceramics, and polymers. For hard tissue implant applications, the coral genus Porites appears to offer an ideal microstructural configuration. Its skeleton has pores with diameters in the range of 140--160 pm, and all of the pores are interconnected. The precursor carbonate is soft and easily machined into any conceivable geometry (plugs, screws, plates, pins, or custom-shaped devices) prior t o replication of the microstructure in another material. The general procedures are diagrammed in Figure 1. It is possible t o prepare implants which are wholly porous, or implants which have a solid core with contiguous porous overlay. A wide range of composite materials, furthermore, can be fabricated for special purposes. A description of the Replamineform process, a discussion of its advantages in relation to other pore-forming techniques, and an evaluation of modern biomaterials engineering technology have been published [6].
I
Select coral wtth destred microstructure
I
I
I exchange wq for c o 3
I j
Dilute HCI treatment t o remove carbonate
InYeSl wax negative
wkth slurry
I I
I
Cao at 800°C
with monomers
II
I
S t d e r for strength at 1450°C
I
I to remove carbomte
Burn off wax
form mold
Centrifugslly cast
I treatment t o remove CaO
1 replieas*
Calc~umphosphate "pasitlves"
Polymer "negatwe" ""egatlve" replicas
1. o-a1um,na
1. Hydrunyaptite
1. POlyurethane
1. Cr-Co-Mo Alloy
2. Tt0,
2. WhLtlockcte
2. SLIaStlC
2 . s,tuer
3. Methyl methacrylate
3. Tin
*Typical shrinkages 12-ZO'l
Fig. I . Replamineform procedure for preparing controlled pore structures in ceramics, polymers, and alloys. The ceramic, polymer, and alloy materials may be prepared in a completely porous configuration or as continguos porous overlay on solid core material
REPLAMINEFORM BIOMATERIALS FOR IMPLANTATION
25
Following the successful preparation of Replamineform materials, specifically those with the replicated microstructure of Porites, biocompatibility tests of the various implant materials were initiated as a cooperative investigation by the Materials Research Laboratory at The Pennsylvania State University and the Orthopaedic Research Laboratory of the Upstate Medical Center at Syracuse. I n these experiments, uniformly porous plugs were placed in the cancellous bone of the distal femora and proximal tibiae of large mongrel, adult dogs. Long-term tolerance studies demonstrated acceptance of these implants without any evidence of migration. The Replamineform implants stimulated new bone ingrowth but also induced regeneration of the excised articular cartilage. Three of the most promising materials were selected for further implant tests as part of the feasibility study begun in May 1973: alumina (a-Al,O,), titania (TiO,), and hydroxyapatite [Ca,,(PO,),(OH),]. Plugs of each were placed in the subchondral bone of the lateral condyles of the distal femora of adult male rabbits. The results of these studies are discussed by Chiroff et al. [7]. Preparation and Preimplantation Characterization of Hydroxyapatite Hydroxyapatite test specimens were prepared by hydrothermal conversion of 4 mm diam by 4 mm long cylinders of skeletal CaCO, from the coral Porites. Complete hydrothermal conversion of the aragonite (CaCO,) t o hydroxyapatite was accomplished by a 24 hr treatment in a fluid medium of (NH,),HPO, and H,O maintained at 300°C and 15,000 psi [8]. Specimens were also prepared with an impermeable cap on a cylinder which is dominantly porous. In this case pre-machined coral carbonate was reacted at high temperature and high pressure t o produce porous apatite. A slurry of apatite was then used to fill the pores in the top 0.5 mm of the implant. After drying, the samples were reacted at a high temperature to cause sintering and formation of a hard compact cap. After sintering, the specimens were boiled in water to regenerate any apatite that might have been dehydroxylated. The apatite formed by this process completely replaces Porites carbonate as was confirmed by both petrographic microscope and x-ray powder diffraction. Electron microprobe analyses show that the composition of the apatite is the same as that obtained for apatites that crystallized at higher temperatures and are stoichiometric, i.e., Ca:P = 10:6. SEM micrographs demonstrate the morphological integrity of resultant apatite. The infrared spectral data are characteristic of hydroxyapatites having
26
WHITE ET Al.
a small amount of carbonate substitution in the crystal structure. The specific positions of the carbonate absorption frequencies indicate that this is lattice COB=, substituted for columnar OH- and for PO: tetrahedra in the apatite structure, rather than admixed or incompletely reacted calcium carbonate [9]. This is a significant observation, as most experts believe that CO, is an important, though relatively small, constituent of the hydroxyapatite in bone. Magnesium, another minor or trace element present in the specimens synthesized from coral, is also believed t o have a significant role in bone mineral formation.
Preparation and Preimplant Characterization of a - A l z 0 3 The most satisfactory a-Al,O, replicas have been made using Alcoa A15SG alumina. This starting material has been thoroughly characterized by standard CESEMI procedures [lo], [ I I ] developed at The Pennsylvania State University. The median particle size (number count basis) is 0.35 pm, and the maximum and minimum sizes are 10 p m and 0.1 p m respectively. The size distribution is significantly bimodal. The particle mass distribution has also been determined; in this case the median size is 4.2 p m . Alcoa A-I5SG is 99.5+% Al,03 with the remainder composed primarily of SiO,, Na,O, and Fe,O,. Procedures for preparing Replamineform alumina were reported by Tarhay, et al [12]. Final firing conditions are at 1650°C for 3 hr. S E M micrographs confirm the high fidelity of replication. Linear shrinkage on firing is 12-13%. Both the macro and microporosity have been quantitatively characterized by quantitative S E M image analysis. Macro pores comprise 62 vol % with a median pore diam of 135 pm. The microporosity (porosity within the sintered a-Al,O,) is 5% with a median pore size of 0.7 p m . The maximum pore diam in the sintered area is 5.0 pm. Grain growth upon sintering is not excessive. Median grain size in the sintered a-A1203 is 2 p m compared with the 0.35 p m median size for the starting material. Maximum sintered grain size is 20 pm. Preparation of Replamineform a-Al,O, materials having a nonporous layer or a solid core proves to be quite straightforward. The one-step sintering yields a completely integral structure with no mechanical discontinuities at the porous-impervious cap interface, further confirming that the volume shrinkage within the Replamineform-porous materials is the same as for the sintering of a bulk alumina body.
Preparation of Replamineform TiO, The best starting materials for preparing the TiO, (rutile) implant test specimens has been found to be Baker reagent grade TiO, (anatase). I n
REPLAMINEFORM BIOMATERIALS FOR IMPLANTATION
27
this material, 50% of the particles are smaller than 0.1 pm. The material is 99.0% TiOz with a maximum of 0.023% trace metal content. Fired at 1425°C for 3 to 4 hr, this material undergoes a 29% linear shrinkage. Microporosity is less than that of the a-Al,O, but sintered grain size is larger. Reproducibility of the preparation is not at an acceptable level yet, but enough suitable specimens have been prepared of the completely porous configuration to carry out the implant test program.
Preparation of Replamineform Metals An important step has been taken toward simplifying the Replamineform work with metals. This results from the finding that the coral aragonite can be sintered to form C a O without destruction or degradation of the original Porites microstructure. Calcium oxide is an excellent investment material in that it is highly refractory (mp = 2580"C), yet rapidly dissolves in dilute HCI. Thus after casting the metal, the investment can be completely removed by merely immersing the cast metal in water at room temperature. This research is being supported by N S F Grant GH-38944. The assistance of Leo Tarhay, D. Dinger and E. J . Majzlik, J r . is gratefully acknowledged.
References [I] J. N . Weber, R. Greer, B. Voight, E. White, and R. Roy, J . Ultrastruct. Res., 26, 355 (1969). [2] S. F. Hulhert, et al., J . Biomed. Muter. Res., 4 , 433 (1970). [3] P. Predecki, J . E. Stephan, B. A . Auslaender, V. L. Mooney, and K. Kirkland, J . Biomed. Muter. Res.. 6, 375 (1972). [4] J . N. Weber, E. W. White, and J . Lebiedzik, Nature, 233, 337 (1971). [5] E. W. White, K. Mayberry, and G . G . Johnson, Jr., Pattern Recognition. 4 , 173 (1972). [6] J. N . Weber and E. W. White, Min. Sci. Eng., 5 , 151 (1973). [7] R. T. Chiroff, E. W. White, J. Weber, and D. Roy, J . Biomed. Muter. Res. Symp. No. 6 (part I), MH6 (1975). [8] D. M. Roy and S. Kurtossy Linnehan, Nature. 247, 220 (1974). [9] W . Eysel, D. Dinger, and D. M . Roy, Mat. Res. Bull, 9 , 35 (1974). [lo] R . A. White, J . N . Weber, and E. W . White, Science, 176, 922 (1972). [ I l l J. Lebiedzik, K. G . Burke, S. Troutman, G. G. Johnson, Jr., and E. W. White, in Scanning Electron Microscopy/l973, Chicago, Illinois, 1973, pp. 121- 127. [I21 L. Tarhay, W. R. Buessem and E. W. White, Ceram. Abstr., 52, 394 (1973).
