Dental implant materials. I. Some effects of preparative procedures on surface topography D. C. Smith,* R. M. Pilliar, and R. Chernecky Centre for Biomaterials, University of Toronto, 170 College Street, Toronto, Ontario, M 5 S 1Al The effect of different treatments for preparing implant materials was examined by scanning electron microscopy and by contact angle measurements. The materials examined were Ti6A14V alloy, Co-Cr-Mo alloy, A1203, and synthetic hydroxyapatite. Samples were prepared with solid or porous surfaces of these materials. These were detergent-cleaned and then either autoclaved (steam sterilization),radiation-
sterilized, nitric acid-etched, or plasmacleaned. The results of wettability studies indicated marked changes in surface energy corresponding to the different preparation methods, and differences in surface morphology were also observed. These differences could have significant consequences on in vivo implant behaviour as mediated by tissue-implant interactions.
IN TRODUCTION
As a result of the developments of the past few a variety of endosseous dental implant systems has become available with widely different designs, surface textures, and materials of construction. A few systems have received official sanction for clinical use but, in general, there is little scientific evidence for the safety and efficacy of the majority of systems available. Claims for osseointegration, fibroosteal integration, bone bonding, and bony ankylosis abound but there is little precise knowledge of the actual interface between implant and tissue and of the factors which influence host response and the long-term integrity of the implant system. More fundamental research is needed on both materials and design for rational progress5 It is well known, as discussed by Ratner et al.,7 that the surface chemistry, surface energy, and surface topogography govern the biological response to an implanted material. The tissue response to a dental (or surgical) implant may involve physical factors such as size, shape, surface texture, and relative interfacial movement as well as chemical factors associated with the composition and surface s t r ~ c t u r e . ” ~ has continued to accumulate that implanted metallic, ceramic, polymeric materials degrade in the body releasing constituent ions or components that may lead in the fullness of time to an adverse response.I3 *To whom correspondence should be addressed. Journal of Biomedical Materials Research, Vol. 25, 1045-1068 (1991) 0 1991 John Wiley & Sons, Inc. CCC 0021-9304/91/091045-24$4.00
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Cellular response to implant materials is affected also by adsorbed surface species that are a function of the surface composition and charge.l4-I7These adsorbed species may be, initially, contaminant films arising from preparative procedures that may result in low-energy surfaces and adverse responses.16Such films may be removed at least in part by cleaning procedures such as gas plasma (glow discharge) treatment or UV/~zone.’”’~ Surface contamination has not received great attention until quite recently in dental implantology research in spite of the importance to protein binding, thrombogenecity, and cell attachment.2’ The majority of studies on dental implants has not characterized in a precise manner the surface and bulk characteristics of the material being used. Further the cellular effects of the particular cleaning and sterilization systems have received little attention.16 For example, plasma cleaning of dental implants has been recommendedI6 but conflicting results as to bone apposition using different plasmas have been r e p ~ r t e d .Brunette3’ ~ ~ , ~ ~ has illustrated the pronounced effects of dental implant surface topography on cell behavior. Thus it appears essential in future studies to fully characterize the surface of materials before in v i m evaluation. We have previously demonstrated that significant topographical and surface compositional variations can occur in dental titanium implant^'^,'^ and in hydroxyapatiteZ5as a result of preparative procedures. Thus in the present work we have examined several currently used implant materials to assess these parameters prior to in vitro and in in vivo evaluation. In this report topographical factors and surface wetting are discussed. Subsequently, surface chemistry, and contamination data and later in vivo response will be presented.
MATERIALS A N D METHODS
The following materials were investigated: (a) Ti6Al4V ELI conforming to ASTM F-136 (Dynamet Corp.), (b) Co-Cr-Mo conforming to ASTM F-75 (Canox Ltd.), (c) single crystal alumina Al2O3(Kyocera, Kyoto) (AO), (d) dense hydroxyapatite (HA) (Sterling Winthrop Laboratories). Chemical analyses of these materials are given in Table I. Discs 3.5 mm in diameter and 1.5 mm thick were fabricated by cutting from rod stock with a low-speed diamond saw. The alloys and HA specimens TABLE I Composition of Implant Materials Composition
Material Ti A1 V Co-Cr-Mo A1203
HA
Ti Bal. A1 5.96; Trace: H, Y C o Bal. Cr 27.77; Fe 0.54; Ni 0.08 A1203 99.9 Ca, P, 0, H, Bal. Sr 0.05 Trace: 8, Si, Mn, Fe, Mg, Al, Cu
V 4.04;
Fe 0.184;
C 0.18;
Mo 5.38;
Mn 0.79;
Si 0.72;
N 0.1
DENTAL IMPLANT MATERIALS. I
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were finished by wet grinding to 600 grit on Si C paper. Porous-surfaced alloy discs of similar size were prepared by sintering spherical particles of 50-300-pm diameter onto the solid substrate as described elsewhere.26For the Co-Cr-Mo alloy powders (Nuclear Metals Inc.), a 1300°C 3-h sinter in uucuo (1 x mm Hg or better) followed by furnace cooling to room temperature was employed, whereas for the Ti6A14V powder (Nuclear Metals Inc.) 1250°C for 1 h in uucuo was used,I2 followed by a furnace cool to room temperature. The solid discs were subjected to the same heat treatment (simulating sintering) before finishing. After fabrication the specimens were washed in distilled water and airdried. This was the “as received” condition. Groups of three specimens for each material were subjected to the following cleaning regimens: (1)Retained in the ”as received” condition. (2) Washing in an ultrasonic cleaner (Branosonic 220) as follows: (a) Sonicated for 60 min in a 2% solution of Decon detergent (Decon Labs). (b) Then sonicated in deionized water for 2 min, three changes. (c) Air-dried in a closed container. (3) As in (2), followed by steam sterilization by autoclaving at 121°C for 30 min in an Ansco Laboratory Sterilizer. (4) As in (2),followed by radiation sterilization at 2.5 megarads. (5) As in (2), followed by treatment with 40% HNO, at RT for 60 min in the sonicator followed by similar rinsing with deionized water for 3 min, three changes (ASTM-F-86). (6) As in (2), followed by exposure to an argon plasma for 30 min at 0.3 mm Hg (Harrick Model PDC-3XG Plasma Cleaner). The specimens were examined in an ISI-60 scanning electron microscope equipped with a PGT-1000 (Princeton Gamma Tech) for EDX analysis. Asatomized (not sintered) powders of both alloys were examined in the SEM as well. The ceramic materials were coated with Au for SEM and carbon for energy dispersive analysis (EDX) in a Polaron sputter coater. For comparison specimens of titanium and alumina commercial dental implants were also scanned. Additional disc specimens were transferred to small sealed acid-washed polypropylene tubes for storage until contact angle measurement. Advancing water contact angles were determined by a sessile drop method.25In brief, 0.2 mL deionized distilled water was placed on the specimen surface and photographed at x25 on 35mm film using a Wild Heerburg Microscope 5-A with camera attachment. Contact angles were measured directly on the prints or using an Image Analyzer. Six determinations were made for each material/preparative procedure combination. RESULTS
The various cleaning procedures had little effect on the gross appearance of the disc specimens except for the porous alumina, which exhibited slight yellowing after plasma treatment for some specimens.
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SMITH, PILLIAR, AND C H E R N E C K Y
Scanning electron microscopy
Ti6Al4V The "as received (Procedure 1) and detergent-treated (2) surfaces of the solid alloy specimens appeared similar and were characterized by parallel scratches arising from the finishing technique with some small pits (Fig. 1A). The scratches were 0.1 to about 1 pm. After autoclaving (Procedure 3), some microstructural features were faintly visible (Fig. ZB). These were identified by the fact that the scratches traversed their boundaries, suggesting microstructural features. They resembled the alpha-colony structure typical of Ti6A14V alloy after beta-annealing (above 99OOC) and slow (furnace) cooling2' The nitric acid treatment (Procedure 5) revealed some areas in which the alpha-colonies were just apparent. Overall, there appeared to be general chemical attack of the ground surface, particularly along the surface scratches (Fig. 1C). The etched regions, when examined at the higher magnification, showed a faceted appearance. These probably corresponded to planes of close packing in the h.c.p. alpha-phase platelets. Back-scattered electron imaging was especially effective in revealing the alpha-platelets (Fig. 1D). The radiation sterilization resulted in little change to the surface observed after Procedure 2. The alpha-colony structure and alpha-platelets were evident also in the plasma-cleaned discs (Procedure 6; Fig. 2), although the scratches were not delineated, nor were the faceted surface features as evident as in the nitric-acid-etched samples. . Examination of the Ti6A14V alloy powders after sintering showed distinctive thermal-etched surfaces with all the treatments (Fig. 3A). That these features developed during the 1250°C vacuum treatment was clear from comparisons with the as-atomized loose powders (Fig. 3B). The sintering treatment resulted in substantial interparticle bonding (Fig. 3A), but there were deep fissures evident in some of the particle-particle neck regions (Fig. 3C).
Co-Cr-Mo Some surface scratches were observed on the solid Co-Cr-Mo specimens. They were far less pronounced than on the softer Ti alloy discs, however. The detergent-cleaned surfaces (Procedure 2) appeared similar to the "as received disc surfaces. The autoclaving treatment (Procedure 3), somewhat surprisingly, resulted in a clearer appearance of grain boundaries and other microstructural features (Fig. 4A). Small islands of, presumably, thicker oxide layer also were observed over this surface (Fig. 4B). The radiation sterilization treatment (Procedure 4) did not result in the appearance of any features, whereas the nitric-acid and plasma cleaning treatment did. Nitric-acid cleaning made grain boundaries visible (Fig. 4C). These boundaries, when viewed at higher magnification (Fig. 4D), showed features that appeared to represent the eutectic structure reported to form after a 1300°C furnace cool treatment for this alloy.26The plasma cleaning treatment appeared to make the underly-
DENTAL IMPLANT MATERIALS. I
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ing cellular or dentritic matrix morphology (Fig. 4E), or regions of nonmetallic inclusions (perhaps carbides), more distinct in some areas (Fig. 4F). Like the plasma-treated samples, those given the nitric-acid treatment also exhibited regions where patches of thicker (presumed) oxide were present and other regions where the alloy surface morphology was more distinct.
(B) Figure 1. Scanning electron micrograph of (A) an "as received (sinter annealed) Ti6A14V alloy surface. Surface scratches resulting from the 600-grit final grinding operation are visible (original magnification X1500). (B) Solid Ti6A14V alloy surface after detergent cleaning (2) and 121°C autoclaving (3) treatment showing the alpha-plate structure and a colony border (original magnification ~ 2 0 0 )(C) . After treatment 2 followed by nitric acid treatment ( 5 ) . (D) after treatment 5 (back-scattered electron imaging) showing a prior beta-grain boundary outlined by primary alpha (original magnification ~ 1 2 8 ) .
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(D)
Figure 1. (continued)
The differently treated sintered Co-Cr-Mo alloy, beads all looked similar. Regions of thicker oxide covering were observed (Fig. 5A) and evidence of thermal etching was seen (Fig. 5B). The neck regions were about as extensive as for the Ti6A14V system, but in contrast were more regular in shape. This would be expected for this alloy, in which liquid phase is known to occur during the sintering process.27
The single crystal alumina discs showed only faint striations on a generally featureless surface, but some areas of surface defects were noted (Fig. 6A).
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Figure 2. Solid Ti6A14V alloy surface prepared by plasma cleaning (original magnification x200).
(A)
Figure 3. Ti6A14V alloy powder: (A) sintered, showing thermal etching features (original magnification ~1500),(B) as atomized loose powder (original magnification X1500), (C) particle-particle neck zone showing deep fissures (original magnification ~ 1 0 0 0 ) .
Treatment with detergent (Procedure 2) increased the prominence of the striations and showed apparent surface prominences (Fig. 6B). The surfaces subjected to the sterilization (Procedures 3 and 4) revealed little change from the original. Nitric-acid etching developed small areas of localized attack having a granular appearance (Fig. 6C). Plasma treatment (Procedure 6) revealed areas of apparent uniform prominences over the surface at high magnification (Fig. 6D).
SMITH, I’ILLIAR, A N D CHERNECKY
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(C) Figure 3. (continued)
The porous alumina surface consisted of fused irregular grains with a rough surface (Fig. 7A). There was little difference in the appearance for the specimens subjected to Procedures 1-4. At higher magnification the polycrystalline structure was essentially unchanged (Fig. 7B).
Hydroxyapat ite The ”as received” surface had pronounced scratches with considerable attached debris (Fig. 8A). Sonication with detergent (Procedure 2) removed most of the surface debris. The sterilization procedures gave little change.
DENTAL IMPLANT MATERIALS. I
Figure 4. Solid Co-Cr-Mo alloy surface: (A) after autoclaving treatment (3) with grain boundaries evident (original magnification x ~ O O ) , (B) higher-magnification view of a showing patches of thicker oxide regions (original magnification X1500), (C) after nitric acid treatment showing rain boundaries (original magnification ~ 1 5 0 0 )(D) , higher-magnification view of a grain boundary region showing eutectic composition (MZ3Cb+ gamma-phase + s i g ~ n a - p h a s e ~(original ~) magnification X3000), (E) after plasma cleaning, showing regions of dendritic or cellular surface morphology that formed during particle solidification (original magnification X3000), (F) a nonmetallic inclusion in a plasma-cleaned surface (original magnification X1500).
1053
SMITH, PILLIAR, AND CHERNECKY
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(D)
Figure 4. (continued)
Only a brief exposure to nitric acid was needed to etch the surface and reveal the polycrystalline structure. In contrast, plasma cleaning resulted in localized erosion, but the ground surface was still largely present (Fig. 8B).
Dental implants The surfaces of these widely used clinical implants were quite different in topography, cutting debris and machining grooves being evident on the Ti
DENTAL IMPLANT MATERIALS. I
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(F) Figure 4. (continued)
implants (Fig. 9) porous plasma sprayed coating on the IT1 implant (Fig. 10) and a smooth surface with prominences on the A1203 implant (Fig. 11, cf. Fig. 6D).
Energy dispersive analysis EDS spectra obtained from the various specimens confirmed the major elements expected from the compositions in Table I. The only major difference
SMITH, PILLIAR, AND CHERNECKY
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(B) Figure 5. Typical Co-Cr-Mo surface appearance: (A) particle-particle neck region (original magnification X1500), (B) patches of thicker oxide (original magnification ~ 3 0 0 0 ) .
of note was the presence of Y in the porous alumina (Fig. 12), which was found to occur in all the specimens, irrespective of preparative procedure. The Y did not appear to be concentrated in any precise location. The influence of this element on implant surface properties is not known. Contact angle measurements The results of the surface wetting experiments are given in Table 11. The data indicate some effect of detergent cleaning in improving wettability of
DENTAL IMPLANT MATERIALS. I
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Figure 6. Single crystal A1203 disc surface: (A) "as received (original magnification x ~ O O ) , (B) detergent-cleaned (original magnification x ~ O O ) , (C) nitric-acid-treated surface (original magnification x1500), (D) plasmacleaned surface showing homogeneous distribution of prominences (original magnification ~ 5 0 0 0 ) .
the two metal surfaces, but less so for the ceramics. Plasma treatment appeared to have the most pronounced effect. Application of a general factorial design showed that significant differences existed between materials and treatments. A modified t test for multiple comparisons showed significant differences (at the 95% level) between procedures 1 and 2, 3 and 4 for the Ti alloy and the Co-Cr-Mo alloy but not among the latter three procedures. This suggests that the chemical cleaning by the detergent was effective in remov-
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(D) Figure 6. (continued)
ing surface contaminant and this was not improved by the other procedures. The changes in morphology previously discussed did not seem to affect the wetting behavior for either. In each case after Procedure 2 there seemed to a trend to higher contact angles after sterilization procedures or nitric acid treatment that may be due to either oxide modification or absorbed contaminant layers. Radiation sterilization (Procedure 4) appeared to give high values also for the alumina, whereas there was no significant difference between the "as received and Procedures 2,3, and 4. The basis for this effect of Procedure 4 is not clear at present although the effects of radiation on the
DENTAL IMPLANT MATERIALS. I
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(B) Figure 7. Porous A1203 surface: (A) "as received (original magnification x ~ O O ) , (B) detergent-cleaned (original magnification X1500).
plastic storage containers may have released volatile contaminants. There was also no significant difference among Procedures 1, 2, and 3 for the HA. Procedure 5 was significantly lower, probably because of the pronounced surface roughness introduced by acid etching as shown previously.25 The contact angles for the plasma-cleaned surfaces were obtained within a few minutes of the procedure. As is evident from Table 11, alumina and HA gave more wettable surfaces than the two alloys. All the values, however, are well below those obtained with the more commonly used preparative procedures, including simple water washing.
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(B) Figure 8. Solid hydroxyapatite surface: (A) "as received (original magnification ~ 2 0 0 ) ,(B) plasma cleaning treatment (original magnification ~ 1 5 0 0 ) . DISCUSSION
The SEM findings and the contact angle data demonstrate that the various preparative procedures result in differences in surface topography and surface energy. A marked change in contact angle especially for the alloys is induced by ultrasonic cleaning the "as received surfaces with an effective detergent. Since there are no pronounced changes in surface roughness but the grooves in the surfaces appeared cleaner in the SEM micrographs
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(B) Figure 9. Surface topography of Ti screw thread implant2*: (A) original magnification X70, (B) original magnification ~ 2 0 0 .
the data suggest that removal of contaminant layers and/or particulate is responsible. Little change in surface topography was produced by radiation sterilization after detergent cleaning but it was surprising that significant structure could be seen after autoclaving for the Ti- and Co-based alloys. This may have been due to chemical effects of the autoclave cycle. Doundoulakis” has found inorganic and organic contaminants on Ti surfaces subjected to steam sterilization. Some areas of contamination or reaction product were seen on the Co base alloy after Procedure 3 as previously reported also by Baier.29 Table I1 indicates a decreased wettability from Procedure 2.
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(B) Figure 10. Surface topography of plasma-sprayed Ti implant: (A) original magnification ~ 2 0 (B) , original magnification x1000.
This effect of autoclaving was not particularly evident on the porous surfaces of the two alloys, both of which had enhanced microstructural features due to the effect of thermal etching. As previously noted, a substantial fused neck region was evident between the Co base alloys spheres (Fig. 8) in contrast to the deep fissures in the same region of the porous-surfaced Ti alloy (Fig. 6). Thus there are substantial topographical differences between the two materials. The nitric acid passivation process had pronounced effects on the two alloys and (as shown previouslyz5) on the HA exposing microstructural features and regions of differential composition. The process produced only
DENTAL IMPLANT MATERIALS. I
(B) Figure 11. Surface topography of single crystal sapphire (A1203) implant: (A) original magnification ~ 2 0 (B) , original magnification X 500.
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Figure 12. EDX spectrum from porous A1203 sample, showing distinctive Y peaks.
slightly more wettable surfaces for the two alloys suggesting residual reaction products. The alumina was little affected though some surface prominences related to manufacturing processes were seen (cf Fig. 15). Plasma cleaning was clearly the most effective means of reducing water contact angle as suggested by Baier." It is evident that some surface erosion/ etching occurs during the process. None of the contact angles is zero though we have observedz5values
sterilized, nitric acid-etched, or plasmacleaned. The results of wettability studies indicated marked changes in surface energy corresponding to the different preparation methods, and differences in surface morphology were also observed. These differences could have significant consequences on in vivo implant behaviour as mediated by tissue-implant interactions.
IN TRODUCTION
As a result of the developments of the past few a variety of endosseous dental implant systems has become available with widely different designs, surface textures, and materials of construction. A few systems have received official sanction for clinical use but, in general, there is little scientific evidence for the safety and efficacy of the majority of systems available. Claims for osseointegration, fibroosteal integration, bone bonding, and bony ankylosis abound but there is little precise knowledge of the actual interface between implant and tissue and of the factors which influence host response and the long-term integrity of the implant system. More fundamental research is needed on both materials and design for rational progress5 It is well known, as discussed by Ratner et al.,7 that the surface chemistry, surface energy, and surface topogography govern the biological response to an implanted material. The tissue response to a dental (or surgical) implant may involve physical factors such as size, shape, surface texture, and relative interfacial movement as well as chemical factors associated with the composition and surface s t r ~ c t u r e . ” ~ has continued to accumulate that implanted metallic, ceramic, polymeric materials degrade in the body releasing constituent ions or components that may lead in the fullness of time to an adverse response.I3 *To whom correspondence should be addressed. Journal of Biomedical Materials Research, Vol. 25, 1045-1068 (1991) 0 1991 John Wiley & Sons, Inc. CCC 0021-9304/91/091045-24$4.00
SMITH, PILLIAR, A N D CHERNECKY
1046
Cellular response to implant materials is affected also by adsorbed surface species that are a function of the surface composition and charge.l4-I7These adsorbed species may be, initially, contaminant films arising from preparative procedures that may result in low-energy surfaces and adverse responses.16Such films may be removed at least in part by cleaning procedures such as gas plasma (glow discharge) treatment or UV/~zone.’”’~ Surface contamination has not received great attention until quite recently in dental implantology research in spite of the importance to protein binding, thrombogenecity, and cell attachment.2’ The majority of studies on dental implants has not characterized in a precise manner the surface and bulk characteristics of the material being used. Further the cellular effects of the particular cleaning and sterilization systems have received little attention.16 For example, plasma cleaning of dental implants has been recommendedI6 but conflicting results as to bone apposition using different plasmas have been r e p ~ r t e d .Brunette3’ ~ ~ , ~ ~ has illustrated the pronounced effects of dental implant surface topography on cell behavior. Thus it appears essential in future studies to fully characterize the surface of materials before in v i m evaluation. We have previously demonstrated that significant topographical and surface compositional variations can occur in dental titanium implant^'^,'^ and in hydroxyapatiteZ5as a result of preparative procedures. Thus in the present work we have examined several currently used implant materials to assess these parameters prior to in vitro and in in vivo evaluation. In this report topographical factors and surface wetting are discussed. Subsequently, surface chemistry, and contamination data and later in vivo response will be presented.
MATERIALS A N D METHODS
The following materials were investigated: (a) Ti6Al4V ELI conforming to ASTM F-136 (Dynamet Corp.), (b) Co-Cr-Mo conforming to ASTM F-75 (Canox Ltd.), (c) single crystal alumina Al2O3(Kyocera, Kyoto) (AO), (d) dense hydroxyapatite (HA) (Sterling Winthrop Laboratories). Chemical analyses of these materials are given in Table I. Discs 3.5 mm in diameter and 1.5 mm thick were fabricated by cutting from rod stock with a low-speed diamond saw. The alloys and HA specimens TABLE I Composition of Implant Materials Composition
Material Ti A1 V Co-Cr-Mo A1203
HA
Ti Bal. A1 5.96; Trace: H, Y C o Bal. Cr 27.77; Fe 0.54; Ni 0.08 A1203 99.9 Ca, P, 0, H, Bal. Sr 0.05 Trace: 8, Si, Mn, Fe, Mg, Al, Cu
V 4.04;
Fe 0.184;
C 0.18;
Mo 5.38;
Mn 0.79;
Si 0.72;
N 0.1
DENTAL IMPLANT MATERIALS. I
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were finished by wet grinding to 600 grit on Si C paper. Porous-surfaced alloy discs of similar size were prepared by sintering spherical particles of 50-300-pm diameter onto the solid substrate as described elsewhere.26For the Co-Cr-Mo alloy powders (Nuclear Metals Inc.), a 1300°C 3-h sinter in uucuo (1 x mm Hg or better) followed by furnace cooling to room temperature was employed, whereas for the Ti6A14V powder (Nuclear Metals Inc.) 1250°C for 1 h in uucuo was used,I2 followed by a furnace cool to room temperature. The solid discs were subjected to the same heat treatment (simulating sintering) before finishing. After fabrication the specimens were washed in distilled water and airdried. This was the “as received” condition. Groups of three specimens for each material were subjected to the following cleaning regimens: (1)Retained in the ”as received” condition. (2) Washing in an ultrasonic cleaner (Branosonic 220) as follows: (a) Sonicated for 60 min in a 2% solution of Decon detergent (Decon Labs). (b) Then sonicated in deionized water for 2 min, three changes. (c) Air-dried in a closed container. (3) As in (2), followed by steam sterilization by autoclaving at 121°C for 30 min in an Ansco Laboratory Sterilizer. (4) As in (2),followed by radiation sterilization at 2.5 megarads. (5) As in (2), followed by treatment with 40% HNO, at RT for 60 min in the sonicator followed by similar rinsing with deionized water for 3 min, three changes (ASTM-F-86). (6) As in (2), followed by exposure to an argon plasma for 30 min at 0.3 mm Hg (Harrick Model PDC-3XG Plasma Cleaner). The specimens were examined in an ISI-60 scanning electron microscope equipped with a PGT-1000 (Princeton Gamma Tech) for EDX analysis. Asatomized (not sintered) powders of both alloys were examined in the SEM as well. The ceramic materials were coated with Au for SEM and carbon for energy dispersive analysis (EDX) in a Polaron sputter coater. For comparison specimens of titanium and alumina commercial dental implants were also scanned. Additional disc specimens were transferred to small sealed acid-washed polypropylene tubes for storage until contact angle measurement. Advancing water contact angles were determined by a sessile drop method.25In brief, 0.2 mL deionized distilled water was placed on the specimen surface and photographed at x25 on 35mm film using a Wild Heerburg Microscope 5-A with camera attachment. Contact angles were measured directly on the prints or using an Image Analyzer. Six determinations were made for each material/preparative procedure combination. RESULTS
The various cleaning procedures had little effect on the gross appearance of the disc specimens except for the porous alumina, which exhibited slight yellowing after plasma treatment for some specimens.
1048
SMITH, PILLIAR, AND C H E R N E C K Y
Scanning electron microscopy
Ti6Al4V The "as received (Procedure 1) and detergent-treated (2) surfaces of the solid alloy specimens appeared similar and were characterized by parallel scratches arising from the finishing technique with some small pits (Fig. 1A). The scratches were 0.1 to about 1 pm. After autoclaving (Procedure 3), some microstructural features were faintly visible (Fig. ZB). These were identified by the fact that the scratches traversed their boundaries, suggesting microstructural features. They resembled the alpha-colony structure typical of Ti6A14V alloy after beta-annealing (above 99OOC) and slow (furnace) cooling2' The nitric acid treatment (Procedure 5) revealed some areas in which the alpha-colonies were just apparent. Overall, there appeared to be general chemical attack of the ground surface, particularly along the surface scratches (Fig. 1C). The etched regions, when examined at the higher magnification, showed a faceted appearance. These probably corresponded to planes of close packing in the h.c.p. alpha-phase platelets. Back-scattered electron imaging was especially effective in revealing the alpha-platelets (Fig. 1D). The radiation sterilization resulted in little change to the surface observed after Procedure 2. The alpha-colony structure and alpha-platelets were evident also in the plasma-cleaned discs (Procedure 6; Fig. 2), although the scratches were not delineated, nor were the faceted surface features as evident as in the nitric-acid-etched samples. . Examination of the Ti6A14V alloy powders after sintering showed distinctive thermal-etched surfaces with all the treatments (Fig. 3A). That these features developed during the 1250°C vacuum treatment was clear from comparisons with the as-atomized loose powders (Fig. 3B). The sintering treatment resulted in substantial interparticle bonding (Fig. 3A), but there were deep fissures evident in some of the particle-particle neck regions (Fig. 3C).
Co-Cr-Mo Some surface scratches were observed on the solid Co-Cr-Mo specimens. They were far less pronounced than on the softer Ti alloy discs, however. The detergent-cleaned surfaces (Procedure 2) appeared similar to the "as received disc surfaces. The autoclaving treatment (Procedure 3), somewhat surprisingly, resulted in a clearer appearance of grain boundaries and other microstructural features (Fig. 4A). Small islands of, presumably, thicker oxide layer also were observed over this surface (Fig. 4B). The radiation sterilization treatment (Procedure 4) did not result in the appearance of any features, whereas the nitric-acid and plasma cleaning treatment did. Nitric-acid cleaning made grain boundaries visible (Fig. 4C). These boundaries, when viewed at higher magnification (Fig. 4D), showed features that appeared to represent the eutectic structure reported to form after a 1300°C furnace cool treatment for this alloy.26The plasma cleaning treatment appeared to make the underly-
DENTAL IMPLANT MATERIALS. I
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ing cellular or dentritic matrix morphology (Fig. 4E), or regions of nonmetallic inclusions (perhaps carbides), more distinct in some areas (Fig. 4F). Like the plasma-treated samples, those given the nitric-acid treatment also exhibited regions where patches of thicker (presumed) oxide were present and other regions where the alloy surface morphology was more distinct.
(B) Figure 1. Scanning electron micrograph of (A) an "as received (sinter annealed) Ti6A14V alloy surface. Surface scratches resulting from the 600-grit final grinding operation are visible (original magnification X1500). (B) Solid Ti6A14V alloy surface after detergent cleaning (2) and 121°C autoclaving (3) treatment showing the alpha-plate structure and a colony border (original magnification ~ 2 0 0 )(C) . After treatment 2 followed by nitric acid treatment ( 5 ) . (D) after treatment 5 (back-scattered electron imaging) showing a prior beta-grain boundary outlined by primary alpha (original magnification ~ 1 2 8 ) .
SMITH, MLLIAR, AND CHERNECKY
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(D)
Figure 1. (continued)
The differently treated sintered Co-Cr-Mo alloy, beads all looked similar. Regions of thicker oxide covering were observed (Fig. 5A) and evidence of thermal etching was seen (Fig. 5B). The neck regions were about as extensive as for the Ti6A14V system, but in contrast were more regular in shape. This would be expected for this alloy, in which liquid phase is known to occur during the sintering process.27
The single crystal alumina discs showed only faint striations on a generally featureless surface, but some areas of surface defects were noted (Fig. 6A).
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Figure 2. Solid Ti6A14V alloy surface prepared by plasma cleaning (original magnification x200).
(A)
Figure 3. Ti6A14V alloy powder: (A) sintered, showing thermal etching features (original magnification ~1500),(B) as atomized loose powder (original magnification X1500), (C) particle-particle neck zone showing deep fissures (original magnification ~ 1 0 0 0 ) .
Treatment with detergent (Procedure 2) increased the prominence of the striations and showed apparent surface prominences (Fig. 6B). The surfaces subjected to the sterilization (Procedures 3 and 4) revealed little change from the original. Nitric-acid etching developed small areas of localized attack having a granular appearance (Fig. 6C). Plasma treatment (Procedure 6) revealed areas of apparent uniform prominences over the surface at high magnification (Fig. 6D).
SMITH, I’ILLIAR, A N D CHERNECKY
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(C) Figure 3. (continued)
The porous alumina surface consisted of fused irregular grains with a rough surface (Fig. 7A). There was little difference in the appearance for the specimens subjected to Procedures 1-4. At higher magnification the polycrystalline structure was essentially unchanged (Fig. 7B).
Hydroxyapat ite The ”as received” surface had pronounced scratches with considerable attached debris (Fig. 8A). Sonication with detergent (Procedure 2) removed most of the surface debris. The sterilization procedures gave little change.
DENTAL IMPLANT MATERIALS. I
Figure 4. Solid Co-Cr-Mo alloy surface: (A) after autoclaving treatment (3) with grain boundaries evident (original magnification x ~ O O ) , (B) higher-magnification view of a showing patches of thicker oxide regions (original magnification X1500), (C) after nitric acid treatment showing rain boundaries (original magnification ~ 1 5 0 0 )(D) , higher-magnification view of a grain boundary region showing eutectic composition (MZ3Cb+ gamma-phase + s i g ~ n a - p h a s e ~(original ~) magnification X3000), (E) after plasma cleaning, showing regions of dendritic or cellular surface morphology that formed during particle solidification (original magnification X3000), (F) a nonmetallic inclusion in a plasma-cleaned surface (original magnification X1500).
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(D)
Figure 4. (continued)
Only a brief exposure to nitric acid was needed to etch the surface and reveal the polycrystalline structure. In contrast, plasma cleaning resulted in localized erosion, but the ground surface was still largely present (Fig. 8B).
Dental implants The surfaces of these widely used clinical implants were quite different in topography, cutting debris and machining grooves being evident on the Ti
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(F) Figure 4. (continued)
implants (Fig. 9) porous plasma sprayed coating on the IT1 implant (Fig. 10) and a smooth surface with prominences on the A1203 implant (Fig. 11, cf. Fig. 6D).
Energy dispersive analysis EDS spectra obtained from the various specimens confirmed the major elements expected from the compositions in Table I. The only major difference
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(B) Figure 5. Typical Co-Cr-Mo surface appearance: (A) particle-particle neck region (original magnification X1500), (B) patches of thicker oxide (original magnification ~ 3 0 0 0 ) .
of note was the presence of Y in the porous alumina (Fig. 12), which was found to occur in all the specimens, irrespective of preparative procedure. The Y did not appear to be concentrated in any precise location. The influence of this element on implant surface properties is not known. Contact angle measurements The results of the surface wetting experiments are given in Table 11. The data indicate some effect of detergent cleaning in improving wettability of
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Figure 6. Single crystal A1203 disc surface: (A) "as received (original magnification x ~ O O ) , (B) detergent-cleaned (original magnification x ~ O O ) , (C) nitric-acid-treated surface (original magnification x1500), (D) plasmacleaned surface showing homogeneous distribution of prominences (original magnification ~ 5 0 0 0 ) .
the two metal surfaces, but less so for the ceramics. Plasma treatment appeared to have the most pronounced effect. Application of a general factorial design showed that significant differences existed between materials and treatments. A modified t test for multiple comparisons showed significant differences (at the 95% level) between procedures 1 and 2, 3 and 4 for the Ti alloy and the Co-Cr-Mo alloy but not among the latter three procedures. This suggests that the chemical cleaning by the detergent was effective in remov-
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(D) Figure 6. (continued)
ing surface contaminant and this was not improved by the other procedures. The changes in morphology previously discussed did not seem to affect the wetting behavior for either. In each case after Procedure 2 there seemed to a trend to higher contact angles after sterilization procedures or nitric acid treatment that may be due to either oxide modification or absorbed contaminant layers. Radiation sterilization (Procedure 4) appeared to give high values also for the alumina, whereas there was no significant difference between the "as received and Procedures 2,3, and 4. The basis for this effect of Procedure 4 is not clear at present although the effects of radiation on the
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(B) Figure 7. Porous A1203 surface: (A) "as received (original magnification x ~ O O ) , (B) detergent-cleaned (original magnification X1500).
plastic storage containers may have released volatile contaminants. There was also no significant difference among Procedures 1, 2, and 3 for the HA. Procedure 5 was significantly lower, probably because of the pronounced surface roughness introduced by acid etching as shown previously.25 The contact angles for the plasma-cleaned surfaces were obtained within a few minutes of the procedure. As is evident from Table 11, alumina and HA gave more wettable surfaces than the two alloys. All the values, however, are well below those obtained with the more commonly used preparative procedures, including simple water washing.
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(B) Figure 8. Solid hydroxyapatite surface: (A) "as received (original magnification ~ 2 0 0 ) ,(B) plasma cleaning treatment (original magnification ~ 1 5 0 0 ) . DISCUSSION
The SEM findings and the contact angle data demonstrate that the various preparative procedures result in differences in surface topography and surface energy. A marked change in contact angle especially for the alloys is induced by ultrasonic cleaning the "as received surfaces with an effective detergent. Since there are no pronounced changes in surface roughness but the grooves in the surfaces appeared cleaner in the SEM micrographs
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(B) Figure 9. Surface topography of Ti screw thread implant2*: (A) original magnification X70, (B) original magnification ~ 2 0 0 .
the data suggest that removal of contaminant layers and/or particulate is responsible. Little change in surface topography was produced by radiation sterilization after detergent cleaning but it was surprising that significant structure could be seen after autoclaving for the Ti- and Co-based alloys. This may have been due to chemical effects of the autoclave cycle. Doundoulakis” has found inorganic and organic contaminants on Ti surfaces subjected to steam sterilization. Some areas of contamination or reaction product were seen on the Co base alloy after Procedure 3 as previously reported also by Baier.29 Table I1 indicates a decreased wettability from Procedure 2.
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(B) Figure 10. Surface topography of plasma-sprayed Ti implant: (A) original magnification ~ 2 0 (B) , original magnification x1000.
This effect of autoclaving was not particularly evident on the porous surfaces of the two alloys, both of which had enhanced microstructural features due to the effect of thermal etching. As previously noted, a substantial fused neck region was evident between the Co base alloys spheres (Fig. 8) in contrast to the deep fissures in the same region of the porous-surfaced Ti alloy (Fig. 6). Thus there are substantial topographical differences between the two materials. The nitric acid passivation process had pronounced effects on the two alloys and (as shown previouslyz5) on the HA exposing microstructural features and regions of differential composition. The process produced only
DENTAL IMPLANT MATERIALS. I
(B) Figure 11. Surface topography of single crystal sapphire (A1203) implant: (A) original magnification ~ 2 0 (B) , original magnification X 500.
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Figure 12. EDX spectrum from porous A1203 sample, showing distinctive Y peaks.
slightly more wettable surfaces for the two alloys suggesting residual reaction products. The alumina was little affected though some surface prominences related to manufacturing processes were seen (cf Fig. 15). Plasma cleaning was clearly the most effective means of reducing water contact angle as suggested by Baier." It is evident that some surface erosion/ etching occurs during the process. None of the contact angles is zero though we have observedz5values
