BlODEGRADATION OF RESTORATIVE METALLIC SYSTEMS LINDA C. LUCAS* JACK E. LEMONS+
*Department of Biomedical Engineering Division of Orthopaedic Surgery The University of Alabama at Birmingham Birmingham, Alabama 35294
+
Adv Dent Res 6:32-37, September, 1992
Abstract—Metallic materials utilized for the construction of intra-oral and implant dental restorations include a wide range of relatively pure metals and multicomponent alloys. Basic corrosion and biodegradatioh properties of these alloys have been studied by both in vitro and in vivo techniques. These property characteristics have been shown to be dependent on composition and metallurgical state, combinations within a construct, surface conditions, mechanical aspects of function, and the local and systemic host environment. The susceptibility of these metallic materials to various forms of biodegradation will be presented, with emphasis on corrosion.
This manuscript is published as part of the proceedings of the NIH Technology Assessment Conference on Effects and Sideeffects of Dental Restorative Materials, August 26-28,1991, National Institutes of Health, Bethesda, Maryland, and did not undergo the customary journal peer-review process.
32
M
aterials used for the construction of intra-oral dental restorations/prostheses and dental implants include a wide range of metals and alloys. Due to the large number of alloys currently being used, these alloys will be placed into the following two groups: the noble and semi-noble gold/palladium/silver alloys, and the non-noble or base metals and alloys, which will include the nickel-, copper-, iron-, and cobalt-based alloys, nickel-titanium alloys, and titanium and titanium-based alloys.
TYPES OF ALLOYS USED Noble and Semi-noble Alloys The gold-based alloys have been used longer than any of the other alloys and are referred to as noble alloys, based upon their electrochemical properties. Applications for these alloys include crowns, fixed partial dentures, inlays, bar and attachment connectors, and porcelain-fused-to-metal (PFM) restorations. The alloys contain gold, silver, copper, platinum, palladium, and zinc (Tesk, 1986). The corrosion resistance of the alloys is due to the high thermodynamic stability of the gold in the alloys (Marek, 1986). Although the alloys are considered to have good corrosion resistance, metallic ions are released to the surrounding environment. This observation has been reported iri both in vitro and in vivo studies (Marek, 1986; Gettlem&n et al.9 1980; Sarkar et aL, 1979). The palladium-based and silver-palladium alloys are used for a variety of applications, including PFM restorations, inlays, crowns, fixed partial dentures, abutments of several designs, and removable partial dentures. Many of these alloys have been recently developed as a means of reducing the cost while improving some of the advantageous physical/mechanical properties. Vaidyanathan and Prasad (1981) evaluated the corrosion characteristics of these alloys. In general, alloys with the higher concentrations of palladium and lower silver concentrations exhibited lower corrosion current densities and an increased resistance to sulfide tarnish. Electrochemical corrosion analyses have been used to evaluate the corrosion properties of the various alloys that are contained in this group (Lane et aL, 1985). For these studies, several electrolytic conditions have been used; however, a trend can be observed with respect to the corrosion properties, as is demonstrated in Fig. 1 for a gold-based alloy. The composition of the alloy is provided in the Table. As the potential applied to the gold-based alloy is increased, the measured current also increases. There is an absence of a clearly defined active or passive region on the curve, indicating that the alloy does not rely on the formation of a passive oxide surface for protection from corrosion. Polarization curves for some other noble and semi-noble alloys follow this same trend. Although the current density magnitudes exhibited by these alloys are relatively low, there is a finite release of metallic ions
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VOL.
6
BlODEGRADATION OF METALLIC SYSTEMS
from these materials. Sarkar et al (1979,1983) have reported the dissolution of copper from the gold-based alloys and silver from the silver-palladium alloys. Clearly the ability of these alloys to form multiphase structures that are dependent upon laboratory processing can significantly contribute to the rate and amount of the various metallic ions released to the host environment. Because of the long-term and multifactorial technical aspects of alloy utilization in dentistry, most dentists and supporting dental laboratories prefer the noble and semi-noble alloys. Therefore, these systems have been extended from the restoration of teeth to the restoration of dental implant devices. Because of the historical aspects, the known longevity and biocompatibility profiles, and the basic inertia of the dental field, these alloys should continue to be utilized for a significant portion of metallic-based dental restorations. Base Metal/Alloys The higher costs of materials have encouraged the development of various base-alloy compositions. These alloys are used for the fabrication of crowns, fixed partial dentures, inlays, abutments for implants, removable partial dentures, solders, orthodontic appliances, and implants (Tesk, 1986). Major components of these alloys include either nickel, cobalt, copper, iron, or titanium. In contrast to the noble and semi-noble alloys, these metals are not as thermodynamically stable, and a major aspect of their corrosion resistance is related to the formation of a thin, protective oxide film (passive film) oil the surface of the metal. If the oxide film is disrupted, then the metal or alloy must repassivate in order for the material to be protected. Thus, the stability and the ability of the passive film to re-form are important considerations when the overall corrosion properties of these alloys are being determined. The nickel-based alloys have been studied extensively, and electrochemical corrosion analyses have proven to be a useful method for the evaluation of corrosion properties. Chromium is added to the nickel-based alloys to improve the alloys' 900
LU ^j 600
Artificial Salin€f
Sallv .. ^ « - - Modulay (MO) "°-Trlndlum(TR) -°• ° - " Duracast MS (MS) ~^~ ~ Goldent(QD) ~°-
800 700 600
>=-
500
g0)
300
jl
200
.2
I ioo
QL -100
-200 -300 -400 -500 10'
10
f
10°
10 f
10*
105
Current Density
Fig. 1—Anodic and cathodic polarization curves for Modulay®, Trindium®, Duracast MS®, and Goldent® in saline and saliva solutions (± S.D.). From Johansson et al., 1989b. ability to form a protective oxide film on the surface. It has been suggested that a chromium content from 16-27% will provide an optimum corrosion resistance for the nickel-based alloys, while the addition of manganese and molybdenum will also further enhance the corrosion resistance (Pourbaix, 1984; 900
Lltecast (saline) Lltecast (artificial saliva)
Lltecast-B (saline) Lltecast-B (artificial saliva)
•p 600 UJ O 300
300
-3
I
33
1
°
"S -300
-300
QL
-600
10 3
10*
10 1
10°
10'
10'
103
Current Density (jiA/cm2) Fig. 2—Anodic and cathodic polarization curves for Litecast® in saline and artificial saliva solutions (± S.D.). From Johansson et al., 1989b.
-600
103
101
10°
10f
10*
103
Current Density (^A/cm2) Fig. 3—Anodic and cathodic polarization curves for Litecast-B® in saline and artificial saliva solutions (± S.D.). From Johansson et al., 1989b.
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ADV DENT RES SEPTEMBER 1992
LUCAS & LEMONS
34
TABLE CHEMICAL COMPOSITION OF THE ALLOYS USED IN THE STUDY, WEIGHT% Cu
Al
Fe
81.6
8.3
3.9
Trindium
87.0
11.0
Goldenf
76.0
6.5
Alloy Duracast MSa b
Zn
12.0
Mn
Au
Ag
Pd
Ni
1.2
4.1
1.0
1.0
0.5
5.0
d
Cr
Mo
Ga,In
Litecast
68.5
15.5
14.0
Litecast-Bd
77.5
12.5
4.0
8.0 Modulay* * D u r a c a s t , Inc., Brazil. b Trindium Corp. of America, Sauquoit, New York, USA. c AJE Comercio E Represenacoes Ltda., Brazil. d J.F. Jelenko Co., Armonk, New York, USA. e Williams Gold, Buffalo, New York, USA. (From Johansson et al, 1989b.) Brune, 1986). Alloys with lower chromium content may not be able to develop oxide films adequate for corrosion resistance (Sarkar and Greener, 1973). Electrochemical corrosion techniques have been used to show that nickel-chromium alloys do corrode in physiological solutions such as balanced salt, proteinaceous, artificial saliva, human saliva, and artificial sweat solutions (Lee et al, 1985; Lucas et al, 1991; Randin, 1988; Johansson et al, 1989a; Cow'mgionetal, 1985). Specifically, some of the nickel-based alloys have been shown to be susceptible to pitting and/or crevice corrosion phenomena. Cyclic anodic polarization curves have demonstrated characteristic hysteresis behavior, which indicates that, once the oxide film on the alloys has been disrupted, the alloys have difficulty repassivating. Cyclic anodic and cathodic polarization curves are provided in Figs. 2 and 3 for two commercial alloys (Litecast and Litecast-B). The two alloys were evaluated in two different electrolytes, a buffered saline solution and an artificial saliva. Compositions of the two alloys are provided in the Table. The hysteresis loop exhibited by Litecast-B (Fig. 3) indicates that this alloy is more susceptible to pitting corrosion, whereas the lack of hysteresis for Litecast (Fig. 2) predicts that this alloy should not be as susceptible to pitting corrosion. Litecast-B is a berylliumcontaining alloy. Several studies have reported that beryllium forms a nickel-chromium-beryllium eutectic phase, which is susceptible to preferential corrosion (Lee et al, 1985; Hero et al, 1987). Johansson et al (1989b) evaluated the surfaces of Litecast and Litecast-B crowns placed in dogs for one year. The Litecast crowns did not exhibit any significant signs of corrosion; however, the Litecast-B crowns revealed an etched surface showing the corrosion of preferential phases and thus a disruption of the oxide film. The literature clearly reports on the susceptibility of nickel-based alloys to localized corrosion phenomena. The phases formed by the minor alloying elements have been shown to have a significant effect on the corrosion properties of the nickel-based alloys. The base-metal alloy group (nickel-chromes) has evolved because of relative cost reductions per unit of restoration
77.0
14.0
Be Others
1.7
1.0
(density and element cost) and advances in technology of the past half-century. Advantageous physical and mechanical properties have sometimes been the most important considerations. The base-metal alloys continue to occupy a significant portion of the dental alloy marketplace, and this situation is expected to continue. Copper-based Alloys
A series of copper-based alloys has been introduced for the fabrication of crowns and bridges. These alloys contain copper concentrations as high as 87%, with alloying additions of aluminum, zinc, nickel, cobalt, and manganese. The corrosion properties of these alloys have been evaluated by several investigators. German (1985) reported that two different aluminum-bronze alloys were significantly less resistant to corrosion and tarnish than was a control gold-based alloy. Stoffers et al (1987) evaluated the corrosion and tarnish resistance of the copper alloy MS (Table) and concluded that the alloy exhibited poor tarnish and corrosion resistance, and the use of this alloy in prosthetic dentistry was questionable. Lucas et al (1988) evaluated the corrosion properties of a series of copper alloys in both buffered saline and artificial saliva environments. Polarization curves and composition for the copper-based alloys in the two electrolytes are shown in Fig. 1 and the Table, respectively. Lucas etal (1988) concluded that the artificial saliva electrolyte was less corrosive for this alloy group; however, high current magnitudes were observed for the copper-based alloys at all potentials evaluated. Clearly the in vitro corrosion studies predict significantly higher corrosion rates for the copper-based alloys. The in vitro corrosion rates of the copper-based alloys can be as much as 1000 times greater than those for the gold-based alloy, Modulay (Table). Laboratory animal studies have also been used for evaluation of the corrosion properties of the copper alloys. Crowns fabricated from various copper-based alloys were placed in dogs and retrieved over a period of one year. Upon retrieval, the surfaces of the crowns were evaluated by light and scanning
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VOL.
6
BlODEGRADATION OF METALLIC SYSTEMS
electron microscopy. The copper crowns exhibited significant surface alterations. Corrosion products adhered to the copper alloys, selected copper crowns exhibited an etched appearance, and pitting was observed (Johansson et ai, 1989b). Gingival tissues adjacent to the crowns were analyzed by atomic absorption analyses. Elevated copper levels were measured in the gingival tissue adjacent to selected copper-alloy crowns (Burns and Lucas, 1989; Soileau et ai, 1990). Histological evaluations of the gingival tissue revealed a localized chronic inflammatory process, a predominate lymphocytic infiltration in the submucosa of the gingiva, and a proliferation of the junctional epithelial tissue for some of the copper-based alloys (Hao, 1990). Nickel-Titanium Alloys
The nickel-titanium alloys have been used in orthodontic applications. Although the compositions can vary, usually the composition of these alloys is about 50% nickel and 50% titanium. Several investigators have reported on the corrosion properties of these alloys. Rondelli etal. (1988) evaluated the corrosion properties of an equi-atomic NiTi alloy. The polarization curves for the NiTi alloy demonstrated a higher pitting potential than that found for 316 stainless steels. However, when the potentiostatic scratch test was used, the stainless steel exhibited a higher pitting potential, indicating that the NiTi alloy had a slightly lower localized corrosion resistance than did the stainless steels. Sarkar et al. (1983) evaluated the corrosion properties of a NiTi alloy. After the corrosion tests, the alloy's surfaces revealed pitting. The pits contained crystalline corrosion products richer in titanium, indicating that there may have been a selective dissolution of nickel from the alloy surface. Geis-Gerstorfer and Weber (1988) reported that a NiTi alloy showed rapid breakdown of passivity, with increasing chloride concentrations in unbuffered solutions. However, in buffered solutions, little differences were observed. In general, studies indicate a concern over the susceptibility of NiTi alloys to pitting and/or crevice corrosion. However, the surface condition and electrolyte composition play significant roles in the corrosion properties of this alloy group. Cobalt-based Alloys
Cobalt-based alloys are used for the fabrication of removable partial dentures and selected surgical implants (root forms such as the Lew screw, blades, and subperiosteals). Alloying additions for alloys used for partial dentures include chromium, molybdenum, nickel, silicon, tungsten, manganese, and iron. These alloys also contain some amounts of carbon. The composition of cobalt-based alloys used for implant applications is according to ASTM F75. The cobalt-based alloys have good corrosion resistance, due in part to the chromium oxide passive film that is formed on the alloy surface. In vitro corrosion studies conducted by Mueller and Greener (1970) involving static conditions revealed no evidence of pitting corrosion. Syrett and Wing (1978a), utilizing cyclic polarization curves, observed that neither as-cast nor annealed Vitallium® alloy demonstrated hysteresis behavior. They concluded that the cobalt-based alloys should not be susceptible to pitting or
35
crevice corrosion. Other investigators, studying the susceptibility of the cobalt alloy to pitting corrosion under in vitro static conditions, have shown the alloy not to be susceptible to localized corrosion (Cahoone/a/., 1975; Lucas et aL, 1982). However, when the alloy is either severely cold-worked or subjected to fretting or cyclic loading conditions, pitting corrosion has been observed in vitro (Syrett and Wing, 1978b; Cohen, 1962). The in vivo literature related to the susceptibility of Co-Cr-Mo to localized corrosion is mixed. Syrett and Davis (1978) observed no crevice corrosion after implantation times of up to two years in dogs and rhesus monkeys. Galante and Rostoker (1972) implanted crevice-type samples in rabbits for 12 months, and although no severe corrosion was observed, signs of "single pits" in the crevice regions were observed. In general, the cobalt-based alloys have exhibited good corrosion resistance. Iron-based Alloys
The iron-based alloys used in dentistry for the most part are the stainless steels. These materials are used for the fabrication of orthodontic appliances, temporary crowns, magnetic connectors and clips, and dental implants. The primary alloying elements include chromium, nickel, and molybdenum. The corrosion resistance of these alloys results in part from the formation of a protective oxide film. However, under certain conditions, the stainless steels are known to be susceptible to pitting and crevice corrosion. These forms of corrosion have been observed under in vitro electrochemical testing (Griffin, 1979; Williams, 1981). Cyclic anodic polarization curves demonstrate the characteristic hysteresis behavior. Analyses of retrieved stainless devices support the electrochemical testing in that crevice and pitting corrosion have been observed on numerous devices. The literature also documents the susceptibility of the stainless steel alloys to intragranular and galvanic corrosion phenomena when the alloys are subjected to selected environmental conditions (Griffin, 1979; Williams, 1981). Titanium and Titanium-based Alloys
Commercially pure titanium and titanium alloys are used for dental implants and are evolving for crown and bridge applications. A commercially pure titanium is essentially a dilute titanium-oxygen alloy. The most common titanium alloy used for dental applications is Ti-6A1-4V. Titanium is one of the most corrosion-resistant materials used for biomedical applications. The oxide that forms on titanium provides the corrosion resistance under static conditions, and it has often been reported that titanium is not susceptible to pitting and/or crevice corrosion phenomena. However, it should be pointed out that the oxide film is not sufficiently stable to prevent galling and seizing under loading conditions (Williams, 1981). Thus, under these conditions, the titanium oxide can be removed, resulting in the release of metallic debris and ions. These properties could represent a limitation related to some dental applications.
GALVANIC CORROSION Galvanic corrosion can occur when different alloys are placed in direct contact within the oral cavity or within tissues.
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LUCAS & LEMONS
36
Concern has been expressed associated with the coupling of selected restorative materials as well as implant materials with various alloys used for restorative procedures. Electrochemical corrosion analyses have been used to predict the possibility for enhanced corrosion of one of the alloys in the galvanic couple. The alloy in the couple that would experience enhanced corrosion would be the less noble or more active alloy. For example, data from dental implant and restorative material combinations—where nickel-based alloys were used for crowns, bridges, connectors, and other attachments—support a possible problem related to galvanic interactions. This could be critical if the crown or bridge had a subgingival finish line with a metallic zone in contact with the tissue, and the implant was made of either titanium or a cobalt alloy. Galvanic coupling under these conditions could exist if the implant connector came into contact with the nickel alloy at some location with the abutment construct. One important aspect of this phenomenon is that the biodegradation products could be increased at the adjacent gingival region, where dynamic interrelated processes could occur. Also, coupling could result in an electropositive local environment along the crown, abutment, or implant interface which itself could directly influence tissue conditions, especially the resorption of bone. Certainly, for conditions of osteointegration, this would be undesirable. In all cases, galvanic coupling of alloy systems that could result in enhanced biodegradation should be avoided (Lemons etal, 1992).
SUMMARY Metallic systems have been in use in dentistry for more than 100 years. In general, most applications have achieved a significant level of clinical success with large numbers of patients utilizing metallic restorations for most of their adult lives. This history, the evolved technology for producing highquality cost-effective prostheses, and the continued desire for "permanent" restorations strongly support continued use within the profession. Corrosion and biodegradation phenomena have been analyzed in detail and laboratory and clinical research data correlated for many of the alloy-based devices. Because of concerns about biodegradation, product-based reactions, and host biocompatibility profiles, systems are selected where environmental reactions are minimized. This trend is expected to continue. The future should include some new metallic materials. Emphasis will most likely be placed on surface modifications and composite metallic-ceramic or metallicpolymeric systems. REFERENCES Brune D (1986). Metal release from dental biomaterials. Biomaterials 7:163-175. Burns JK, Lucas LC (1989). Atomic absorption analyses in a canine copper dental alloy study (abstract). / Dent Res 68:322. Cahoon JR, Bandyopadhya R, Tennese L (1975). The concept of protection potential applied to the corrosion of metallic orthopaedic implants. J Biomed Mater Res 9:259-264. Cohen J (1962). Corrosion testing of orthopaedic implants. /
ADV DENT RES SEPTEMBER 1992
Bone Joint Surg 44(A):307-316. Covington JS, McBride MA, Slagle WF, Disney AL (1985). Quantization of nickel and beryllium leakage from base metal casting alloys. / Prosthet Dent 54:127-136. Galante J, Rostoker W (1972). Corrosion-related failures in metallic implants. Clin Orthop Rel Res 86:237-244. Geis-Gerstorfer J, Weber D (1988). Corrosion resistance of the implant materials contimet 35, memory, and vitallium in artificial physiological fluids. Int J Oral Maxillofac Implants 3:135-140. German RM (1985). Evaluation of two aluminum-bronze dental casting alloys (Corrosion Conference). Callaway Gardens (GA). Gettleman L, Cocks FH, Darmiento LA, Levine PA, Wright S, and Nathanson D (1980). Measurement of in vivo corrosion rates in baboons and correlation with in vitro tests. J Dent Res 59:689-707. Griffin CD (1979). An in vitro electrochemical corrosion study of coupled surgical implant materials (MSE Thesis). Birmingham (AL): University of Alabama. Hao SQ (1990). Reaction of dental tissues to copper, nickel and gold based alloy crowns in dogs (MS Thesis). Birmingham (AL): The University of Alabama. Hero H, Valderhaug J, J0rgensen RB (1987). Corrosion in vivo and in vitro of a commercial NiCrBe alloy. Dent Mater 3:125-130. Johansson BI, Lemons JE, Hao SQ (1989a). Corrosion of dental copper, nickel, and gold alloys in artificial saliva and saline solutions. Dent Mater 5:324-328. Johansson BI, Lucas LC, Lemons JE (1989b). Corrosion of copper, nickel and gold dental casting alloys: an in vitro and in vivo Study. J Biomed Mater Res 23:349-361. Lane J, Lucas L, Lacefield W, O'Neal JO, Lemons JE (1985). In vitro corrosion evaluations of Pd-base alloys (abstract). JDent Res 64:318. Lee J, Lucas L, O'Neal J, Lacefield W, Lemons J (1985). In vitro corrosion analyses of nickel-base alloys (abstract). / Dent Res 64:317. Lemons JE, Lucas LC, Johansson BI (1992). Intraoral corrosion, resulting from coupling dental implants and restorative metallic systems. / Oral Implantol (in press). Lucas LC, Buchanan RA, Lemons JE, Griffin CD (1982). Susceptibility of Surgical Cobalt-based Alloy to Pitting Corrosion. / Biomed Mater Res 16:799-810. Lucas LC, Dale P, Buchanan R, Gill Y, Griffin D, Lemons JE (1991). In vitro vs. in vivo corrosion analysis of two alloys. J Invest Surg 4:13-21. Lucas LC, Lemons JE, O'Neal J, Joshi U (1988). In vitro and in vivo corrosion analysis of copper alloys (abstract). JDent Res 67:141. MarekM(1986). Corrosion in a biological environment. In: Biocompatibility, toxicity, and hypersensitivity to alloy systems used in dentistry. Ann Arbor (MI): The University of Michigan School of Dentistry, 103-122. Mueller HJ, Greener EH (1970). Polarization studies of surgical materials in Ringer's solution. J Biomed Mater Res 4:29-41. Pourbaix M (1984). Electrochemical corrosion of metallic
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biomaterials. Biomater 3:122-134. Randin JP (1988). Corrosion behavior of nickel-containing alloys in artificial sweat. J Biomed Mater Res 22:649-666. Rondelli G, Vicentini B, Cigada A (1988). Shape memory alloy as a human body implant material. Trans Third World Biomater Cong 11:169. Sarkar NK, Fuys RA, Stanford JW (1979). The chloride corrosion of low-gold casting alloys. J Dent Res 58:568575. Sarkar NK, Greener EH (1973). In vitro corrosion resistance of new dental alloys. Biomater Med Dev Artif Org 1:121129. Sarkar NK, Redmond W, Schwaninger B, Goldberg AJ (1983). The chloride corrosion behaviour of four orthodontic wires.
JOralRehabil 10:121-128. Soileau RM, Lucas LC, Gantenberg JB (1990). Metallic ion release and distribution from copper-based dental alloys (abstract). / Dent Res 69:264. Stoffers K, Strawn S, Asgar K (1987). Evaluation of properties of MS dental casting alloy (abstract). J Dent Res 66:205.
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Syrett BC, Davis EE (1978). Crevice corrosion of implant alloys—a comparison of in vitro and in vivo studies. American Society for Testing and Materials Meeting, Kansas City, MO. Syrett BC, Wing SS (1978a). Pitting resistance of new and conventional orthopedic implant materials—effect of metallurgical condition. Corrosion 34(A): 138-145. Syrett BC, Wing SS (1978b). An electrochemical investigation of fretting corrosion of surgical implant materials. Corras/o« 34:379-386. Tesk JA (1986). The role of structure and composition on the physical and biological properties. In: Biocompatibility, toxicity and hypersensitivity to alloy systems used in dentistry. Ann Arbor (MI): The University of Michigan School of Dentistry, 3-15. Vaidyanathan TR, Prasad A (1981). In vitro corrosion and tarnish analysis of the Ag-Pd binary system. / Dent Res 60:707-715. Williams DF (1981). Biocompatibility of clinical implant materials. Vol.1. Boca Raton (FL): CRC Press.
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*Department of Biomedical Engineering Division of Orthopaedic Surgery The University of Alabama at Birmingham Birmingham, Alabama 35294
+
Adv Dent Res 6:32-37, September, 1992
Abstract—Metallic materials utilized for the construction of intra-oral and implant dental restorations include a wide range of relatively pure metals and multicomponent alloys. Basic corrosion and biodegradatioh properties of these alloys have been studied by both in vitro and in vivo techniques. These property characteristics have been shown to be dependent on composition and metallurgical state, combinations within a construct, surface conditions, mechanical aspects of function, and the local and systemic host environment. The susceptibility of these metallic materials to various forms of biodegradation will be presented, with emphasis on corrosion.
This manuscript is published as part of the proceedings of the NIH Technology Assessment Conference on Effects and Sideeffects of Dental Restorative Materials, August 26-28,1991, National Institutes of Health, Bethesda, Maryland, and did not undergo the customary journal peer-review process.
32
M
aterials used for the construction of intra-oral dental restorations/prostheses and dental implants include a wide range of metals and alloys. Due to the large number of alloys currently being used, these alloys will be placed into the following two groups: the noble and semi-noble gold/palladium/silver alloys, and the non-noble or base metals and alloys, which will include the nickel-, copper-, iron-, and cobalt-based alloys, nickel-titanium alloys, and titanium and titanium-based alloys.
TYPES OF ALLOYS USED Noble and Semi-noble Alloys The gold-based alloys have been used longer than any of the other alloys and are referred to as noble alloys, based upon their electrochemical properties. Applications for these alloys include crowns, fixed partial dentures, inlays, bar and attachment connectors, and porcelain-fused-to-metal (PFM) restorations. The alloys contain gold, silver, copper, platinum, palladium, and zinc (Tesk, 1986). The corrosion resistance of the alloys is due to the high thermodynamic stability of the gold in the alloys (Marek, 1986). Although the alloys are considered to have good corrosion resistance, metallic ions are released to the surrounding environment. This observation has been reported iri both in vitro and in vivo studies (Marek, 1986; Gettlem&n et al.9 1980; Sarkar et aL, 1979). The palladium-based and silver-palladium alloys are used for a variety of applications, including PFM restorations, inlays, crowns, fixed partial dentures, abutments of several designs, and removable partial dentures. Many of these alloys have been recently developed as a means of reducing the cost while improving some of the advantageous physical/mechanical properties. Vaidyanathan and Prasad (1981) evaluated the corrosion characteristics of these alloys. In general, alloys with the higher concentrations of palladium and lower silver concentrations exhibited lower corrosion current densities and an increased resistance to sulfide tarnish. Electrochemical corrosion analyses have been used to evaluate the corrosion properties of the various alloys that are contained in this group (Lane et aL, 1985). For these studies, several electrolytic conditions have been used; however, a trend can be observed with respect to the corrosion properties, as is demonstrated in Fig. 1 for a gold-based alloy. The composition of the alloy is provided in the Table. As the potential applied to the gold-based alloy is increased, the measured current also increases. There is an absence of a clearly defined active or passive region on the curve, indicating that the alloy does not rely on the formation of a passive oxide surface for protection from corrosion. Polarization curves for some other noble and semi-noble alloys follow this same trend. Although the current density magnitudes exhibited by these alloys are relatively low, there is a finite release of metallic ions
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VOL.
6
BlODEGRADATION OF METALLIC SYSTEMS
from these materials. Sarkar et al (1979,1983) have reported the dissolution of copper from the gold-based alloys and silver from the silver-palladium alloys. Clearly the ability of these alloys to form multiphase structures that are dependent upon laboratory processing can significantly contribute to the rate and amount of the various metallic ions released to the host environment. Because of the long-term and multifactorial technical aspects of alloy utilization in dentistry, most dentists and supporting dental laboratories prefer the noble and semi-noble alloys. Therefore, these systems have been extended from the restoration of teeth to the restoration of dental implant devices. Because of the historical aspects, the known longevity and biocompatibility profiles, and the basic inertia of the dental field, these alloys should continue to be utilized for a significant portion of metallic-based dental restorations. Base Metal/Alloys The higher costs of materials have encouraged the development of various base-alloy compositions. These alloys are used for the fabrication of crowns, fixed partial dentures, inlays, abutments for implants, removable partial dentures, solders, orthodontic appliances, and implants (Tesk, 1986). Major components of these alloys include either nickel, cobalt, copper, iron, or titanium. In contrast to the noble and semi-noble alloys, these metals are not as thermodynamically stable, and a major aspect of their corrosion resistance is related to the formation of a thin, protective oxide film (passive film) oil the surface of the metal. If the oxide film is disrupted, then the metal or alloy must repassivate in order for the material to be protected. Thus, the stability and the ability of the passive film to re-form are important considerations when the overall corrosion properties of these alloys are being determined. The nickel-based alloys have been studied extensively, and electrochemical corrosion analyses have proven to be a useful method for the evaluation of corrosion properties. Chromium is added to the nickel-based alloys to improve the alloys' 900
LU ^j 600
Artificial Salin€f
Sallv .. ^ « - - Modulay (MO) "°-Trlndlum(TR) -°• ° - " Duracast MS (MS) ~^~ ~ Goldent(QD) ~°-
800 700 600
>=-
500
g0)
300
jl
200
.2
I ioo
QL -100
-200 -300 -400 -500 10'
10
f
10°
10 f
10*
105
Current Density
Fig. 1—Anodic and cathodic polarization curves for Modulay®, Trindium®, Duracast MS®, and Goldent® in saline and saliva solutions (± S.D.). From Johansson et al., 1989b. ability to form a protective oxide film on the surface. It has been suggested that a chromium content from 16-27% will provide an optimum corrosion resistance for the nickel-based alloys, while the addition of manganese and molybdenum will also further enhance the corrosion resistance (Pourbaix, 1984; 900
Lltecast (saline) Lltecast (artificial saliva)
Lltecast-B (saline) Lltecast-B (artificial saliva)
•p 600 UJ O 300
300
-3
I
33
1
°
"S -300
-300
QL
-600
10 3
10*
10 1
10°
10'
10'
103
Current Density (jiA/cm2) Fig. 2—Anodic and cathodic polarization curves for Litecast® in saline and artificial saliva solutions (± S.D.). From Johansson et al., 1989b.
-600
103
101
10°
10f
10*
103
Current Density (^A/cm2) Fig. 3—Anodic and cathodic polarization curves for Litecast-B® in saline and artificial saliva solutions (± S.D.). From Johansson et al., 1989b.
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ADV DENT RES SEPTEMBER 1992
LUCAS & LEMONS
34
TABLE CHEMICAL COMPOSITION OF THE ALLOYS USED IN THE STUDY, WEIGHT% Cu
Al
Fe
81.6
8.3
3.9
Trindium
87.0
11.0
Goldenf
76.0
6.5
Alloy Duracast MSa b
Zn
12.0
Mn
Au
Ag
Pd
Ni
1.2
4.1
1.0
1.0
0.5
5.0
d
Cr
Mo
Ga,In
Litecast
68.5
15.5
14.0
Litecast-Bd
77.5
12.5
4.0
8.0 Modulay* * D u r a c a s t , Inc., Brazil. b Trindium Corp. of America, Sauquoit, New York, USA. c AJE Comercio E Represenacoes Ltda., Brazil. d J.F. Jelenko Co., Armonk, New York, USA. e Williams Gold, Buffalo, New York, USA. (From Johansson et al, 1989b.) Brune, 1986). Alloys with lower chromium content may not be able to develop oxide films adequate for corrosion resistance (Sarkar and Greener, 1973). Electrochemical corrosion techniques have been used to show that nickel-chromium alloys do corrode in physiological solutions such as balanced salt, proteinaceous, artificial saliva, human saliva, and artificial sweat solutions (Lee et al, 1985; Lucas et al, 1991; Randin, 1988; Johansson et al, 1989a; Cow'mgionetal, 1985). Specifically, some of the nickel-based alloys have been shown to be susceptible to pitting and/or crevice corrosion phenomena. Cyclic anodic polarization curves have demonstrated characteristic hysteresis behavior, which indicates that, once the oxide film on the alloys has been disrupted, the alloys have difficulty repassivating. Cyclic anodic and cathodic polarization curves are provided in Figs. 2 and 3 for two commercial alloys (Litecast and Litecast-B). The two alloys were evaluated in two different electrolytes, a buffered saline solution and an artificial saliva. Compositions of the two alloys are provided in the Table. The hysteresis loop exhibited by Litecast-B (Fig. 3) indicates that this alloy is more susceptible to pitting corrosion, whereas the lack of hysteresis for Litecast (Fig. 2) predicts that this alloy should not be as susceptible to pitting corrosion. Litecast-B is a berylliumcontaining alloy. Several studies have reported that beryllium forms a nickel-chromium-beryllium eutectic phase, which is susceptible to preferential corrosion (Lee et al, 1985; Hero et al, 1987). Johansson et al (1989b) evaluated the surfaces of Litecast and Litecast-B crowns placed in dogs for one year. The Litecast crowns did not exhibit any significant signs of corrosion; however, the Litecast-B crowns revealed an etched surface showing the corrosion of preferential phases and thus a disruption of the oxide film. The literature clearly reports on the susceptibility of nickel-based alloys to localized corrosion phenomena. The phases formed by the minor alloying elements have been shown to have a significant effect on the corrosion properties of the nickel-based alloys. The base-metal alloy group (nickel-chromes) has evolved because of relative cost reductions per unit of restoration
77.0
14.0
Be Others
1.7
1.0
(density and element cost) and advances in technology of the past half-century. Advantageous physical and mechanical properties have sometimes been the most important considerations. The base-metal alloys continue to occupy a significant portion of the dental alloy marketplace, and this situation is expected to continue. Copper-based Alloys
A series of copper-based alloys has been introduced for the fabrication of crowns and bridges. These alloys contain copper concentrations as high as 87%, with alloying additions of aluminum, zinc, nickel, cobalt, and manganese. The corrosion properties of these alloys have been evaluated by several investigators. German (1985) reported that two different aluminum-bronze alloys were significantly less resistant to corrosion and tarnish than was a control gold-based alloy. Stoffers et al (1987) evaluated the corrosion and tarnish resistance of the copper alloy MS (Table) and concluded that the alloy exhibited poor tarnish and corrosion resistance, and the use of this alloy in prosthetic dentistry was questionable. Lucas et al (1988) evaluated the corrosion properties of a series of copper alloys in both buffered saline and artificial saliva environments. Polarization curves and composition for the copper-based alloys in the two electrolytes are shown in Fig. 1 and the Table, respectively. Lucas etal (1988) concluded that the artificial saliva electrolyte was less corrosive for this alloy group; however, high current magnitudes were observed for the copper-based alloys at all potentials evaluated. Clearly the in vitro corrosion studies predict significantly higher corrosion rates for the copper-based alloys. The in vitro corrosion rates of the copper-based alloys can be as much as 1000 times greater than those for the gold-based alloy, Modulay (Table). Laboratory animal studies have also been used for evaluation of the corrosion properties of the copper alloys. Crowns fabricated from various copper-based alloys were placed in dogs and retrieved over a period of one year. Upon retrieval, the surfaces of the crowns were evaluated by light and scanning
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BlODEGRADATION OF METALLIC SYSTEMS
electron microscopy. The copper crowns exhibited significant surface alterations. Corrosion products adhered to the copper alloys, selected copper crowns exhibited an etched appearance, and pitting was observed (Johansson et ai, 1989b). Gingival tissues adjacent to the crowns were analyzed by atomic absorption analyses. Elevated copper levels were measured in the gingival tissue adjacent to selected copper-alloy crowns (Burns and Lucas, 1989; Soileau et ai, 1990). Histological evaluations of the gingival tissue revealed a localized chronic inflammatory process, a predominate lymphocytic infiltration in the submucosa of the gingiva, and a proliferation of the junctional epithelial tissue for some of the copper-based alloys (Hao, 1990). Nickel-Titanium Alloys
The nickel-titanium alloys have been used in orthodontic applications. Although the compositions can vary, usually the composition of these alloys is about 50% nickel and 50% titanium. Several investigators have reported on the corrosion properties of these alloys. Rondelli etal. (1988) evaluated the corrosion properties of an equi-atomic NiTi alloy. The polarization curves for the NiTi alloy demonstrated a higher pitting potential than that found for 316 stainless steels. However, when the potentiostatic scratch test was used, the stainless steel exhibited a higher pitting potential, indicating that the NiTi alloy had a slightly lower localized corrosion resistance than did the stainless steels. Sarkar et al. (1983) evaluated the corrosion properties of a NiTi alloy. After the corrosion tests, the alloy's surfaces revealed pitting. The pits contained crystalline corrosion products richer in titanium, indicating that there may have been a selective dissolution of nickel from the alloy surface. Geis-Gerstorfer and Weber (1988) reported that a NiTi alloy showed rapid breakdown of passivity, with increasing chloride concentrations in unbuffered solutions. However, in buffered solutions, little differences were observed. In general, studies indicate a concern over the susceptibility of NiTi alloys to pitting and/or crevice corrosion. However, the surface condition and electrolyte composition play significant roles in the corrosion properties of this alloy group. Cobalt-based Alloys
Cobalt-based alloys are used for the fabrication of removable partial dentures and selected surgical implants (root forms such as the Lew screw, blades, and subperiosteals). Alloying additions for alloys used for partial dentures include chromium, molybdenum, nickel, silicon, tungsten, manganese, and iron. These alloys also contain some amounts of carbon. The composition of cobalt-based alloys used for implant applications is according to ASTM F75. The cobalt-based alloys have good corrosion resistance, due in part to the chromium oxide passive film that is formed on the alloy surface. In vitro corrosion studies conducted by Mueller and Greener (1970) involving static conditions revealed no evidence of pitting corrosion. Syrett and Wing (1978a), utilizing cyclic polarization curves, observed that neither as-cast nor annealed Vitallium® alloy demonstrated hysteresis behavior. They concluded that the cobalt-based alloys should not be susceptible to pitting or
35
crevice corrosion. Other investigators, studying the susceptibility of the cobalt alloy to pitting corrosion under in vitro static conditions, have shown the alloy not to be susceptible to localized corrosion (Cahoone/a/., 1975; Lucas et aL, 1982). However, when the alloy is either severely cold-worked or subjected to fretting or cyclic loading conditions, pitting corrosion has been observed in vitro (Syrett and Wing, 1978b; Cohen, 1962). The in vivo literature related to the susceptibility of Co-Cr-Mo to localized corrosion is mixed. Syrett and Davis (1978) observed no crevice corrosion after implantation times of up to two years in dogs and rhesus monkeys. Galante and Rostoker (1972) implanted crevice-type samples in rabbits for 12 months, and although no severe corrosion was observed, signs of "single pits" in the crevice regions were observed. In general, the cobalt-based alloys have exhibited good corrosion resistance. Iron-based Alloys
The iron-based alloys used in dentistry for the most part are the stainless steels. These materials are used for the fabrication of orthodontic appliances, temporary crowns, magnetic connectors and clips, and dental implants. The primary alloying elements include chromium, nickel, and molybdenum. The corrosion resistance of these alloys results in part from the formation of a protective oxide film. However, under certain conditions, the stainless steels are known to be susceptible to pitting and crevice corrosion. These forms of corrosion have been observed under in vitro electrochemical testing (Griffin, 1979; Williams, 1981). Cyclic anodic polarization curves demonstrate the characteristic hysteresis behavior. Analyses of retrieved stainless devices support the electrochemical testing in that crevice and pitting corrosion have been observed on numerous devices. The literature also documents the susceptibility of the stainless steel alloys to intragranular and galvanic corrosion phenomena when the alloys are subjected to selected environmental conditions (Griffin, 1979; Williams, 1981). Titanium and Titanium-based Alloys
Commercially pure titanium and titanium alloys are used for dental implants and are evolving for crown and bridge applications. A commercially pure titanium is essentially a dilute titanium-oxygen alloy. The most common titanium alloy used for dental applications is Ti-6A1-4V. Titanium is one of the most corrosion-resistant materials used for biomedical applications. The oxide that forms on titanium provides the corrosion resistance under static conditions, and it has often been reported that titanium is not susceptible to pitting and/or crevice corrosion phenomena. However, it should be pointed out that the oxide film is not sufficiently stable to prevent galling and seizing under loading conditions (Williams, 1981). Thus, under these conditions, the titanium oxide can be removed, resulting in the release of metallic debris and ions. These properties could represent a limitation related to some dental applications.
GALVANIC CORROSION Galvanic corrosion can occur when different alloys are placed in direct contact within the oral cavity or within tissues.
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Concern has been expressed associated with the coupling of selected restorative materials as well as implant materials with various alloys used for restorative procedures. Electrochemical corrosion analyses have been used to predict the possibility for enhanced corrosion of one of the alloys in the galvanic couple. The alloy in the couple that would experience enhanced corrosion would be the less noble or more active alloy. For example, data from dental implant and restorative material combinations—where nickel-based alloys were used for crowns, bridges, connectors, and other attachments—support a possible problem related to galvanic interactions. This could be critical if the crown or bridge had a subgingival finish line with a metallic zone in contact with the tissue, and the implant was made of either titanium or a cobalt alloy. Galvanic coupling under these conditions could exist if the implant connector came into contact with the nickel alloy at some location with the abutment construct. One important aspect of this phenomenon is that the biodegradation products could be increased at the adjacent gingival region, where dynamic interrelated processes could occur. Also, coupling could result in an electropositive local environment along the crown, abutment, or implant interface which itself could directly influence tissue conditions, especially the resorption of bone. Certainly, for conditions of osteointegration, this would be undesirable. In all cases, galvanic coupling of alloy systems that could result in enhanced biodegradation should be avoided (Lemons etal, 1992).
SUMMARY Metallic systems have been in use in dentistry for more than 100 years. In general, most applications have achieved a significant level of clinical success with large numbers of patients utilizing metallic restorations for most of their adult lives. This history, the evolved technology for producing highquality cost-effective prostheses, and the continued desire for "permanent" restorations strongly support continued use within the profession. Corrosion and biodegradation phenomena have been analyzed in detail and laboratory and clinical research data correlated for many of the alloy-based devices. Because of concerns about biodegradation, product-based reactions, and host biocompatibility profiles, systems are selected where environmental reactions are minimized. This trend is expected to continue. The future should include some new metallic materials. Emphasis will most likely be placed on surface modifications and composite metallic-ceramic or metallicpolymeric systems. REFERENCES Brune D (1986). Metal release from dental biomaterials. Biomaterials 7:163-175. Burns JK, Lucas LC (1989). Atomic absorption analyses in a canine copper dental alloy study (abstract). / Dent Res 68:322. Cahoon JR, Bandyopadhya R, Tennese L (1975). The concept of protection potential applied to the corrosion of metallic orthopaedic implants. J Biomed Mater Res 9:259-264. Cohen J (1962). Corrosion testing of orthopaedic implants. /
ADV DENT RES SEPTEMBER 1992
Bone Joint Surg 44(A):307-316. Covington JS, McBride MA, Slagle WF, Disney AL (1985). Quantization of nickel and beryllium leakage from base metal casting alloys. / Prosthet Dent 54:127-136. Galante J, Rostoker W (1972). Corrosion-related failures in metallic implants. Clin Orthop Rel Res 86:237-244. Geis-Gerstorfer J, Weber D (1988). Corrosion resistance of the implant materials contimet 35, memory, and vitallium in artificial physiological fluids. Int J Oral Maxillofac Implants 3:135-140. German RM (1985). Evaluation of two aluminum-bronze dental casting alloys (Corrosion Conference). Callaway Gardens (GA). Gettleman L, Cocks FH, Darmiento LA, Levine PA, Wright S, and Nathanson D (1980). Measurement of in vivo corrosion rates in baboons and correlation with in vitro tests. J Dent Res 59:689-707. Griffin CD (1979). An in vitro electrochemical corrosion study of coupled surgical implant materials (MSE Thesis). Birmingham (AL): University of Alabama. Hao SQ (1990). Reaction of dental tissues to copper, nickel and gold based alloy crowns in dogs (MS Thesis). Birmingham (AL): The University of Alabama. Hero H, Valderhaug J, J0rgensen RB (1987). Corrosion in vivo and in vitro of a commercial NiCrBe alloy. Dent Mater 3:125-130. Johansson BI, Lemons JE, Hao SQ (1989a). Corrosion of dental copper, nickel, and gold alloys in artificial saliva and saline solutions. Dent Mater 5:324-328. Johansson BI, Lucas LC, Lemons JE (1989b). Corrosion of copper, nickel and gold dental casting alloys: an in vitro and in vivo Study. J Biomed Mater Res 23:349-361. Lane J, Lucas L, Lacefield W, O'Neal JO, Lemons JE (1985). In vitro corrosion evaluations of Pd-base alloys (abstract). JDent Res 64:318. Lee J, Lucas L, O'Neal J, Lacefield W, Lemons J (1985). In vitro corrosion analyses of nickel-base alloys (abstract). / Dent Res 64:317. Lemons JE, Lucas LC, Johansson BI (1992). Intraoral corrosion, resulting from coupling dental implants and restorative metallic systems. / Oral Implantol (in press). Lucas LC, Buchanan RA, Lemons JE, Griffin CD (1982). Susceptibility of Surgical Cobalt-based Alloy to Pitting Corrosion. / Biomed Mater Res 16:799-810. Lucas LC, Dale P, Buchanan R, Gill Y, Griffin D, Lemons JE (1991). In vitro vs. in vivo corrosion analysis of two alloys. J Invest Surg 4:13-21. Lucas LC, Lemons JE, O'Neal J, Joshi U (1988). In vitro and in vivo corrosion analysis of copper alloys (abstract). JDent Res 67:141. MarekM(1986). Corrosion in a biological environment. In: Biocompatibility, toxicity, and hypersensitivity to alloy systems used in dentistry. Ann Arbor (MI): The University of Michigan School of Dentistry, 103-122. Mueller HJ, Greener EH (1970). Polarization studies of surgical materials in Ringer's solution. J Biomed Mater Res 4:29-41. Pourbaix M (1984). Electrochemical corrosion of metallic
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biomaterials. Biomater 3:122-134. Randin JP (1988). Corrosion behavior of nickel-containing alloys in artificial sweat. J Biomed Mater Res 22:649-666. Rondelli G, Vicentini B, Cigada A (1988). Shape memory alloy as a human body implant material. Trans Third World Biomater Cong 11:169. Sarkar NK, Fuys RA, Stanford JW (1979). The chloride corrosion of low-gold casting alloys. J Dent Res 58:568575. Sarkar NK, Greener EH (1973). In vitro corrosion resistance of new dental alloys. Biomater Med Dev Artif Org 1:121129. Sarkar NK, Redmond W, Schwaninger B, Goldberg AJ (1983). The chloride corrosion behaviour of four orthodontic wires.
JOralRehabil 10:121-128. Soileau RM, Lucas LC, Gantenberg JB (1990). Metallic ion release and distribution from copper-based dental alloys (abstract). / Dent Res 69:264. Stoffers K, Strawn S, Asgar K (1987). Evaluation of properties of MS dental casting alloy (abstract). J Dent Res 66:205.
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Syrett BC, Davis EE (1978). Crevice corrosion of implant alloys—a comparison of in vitro and in vivo studies. American Society for Testing and Materials Meeting, Kansas City, MO. Syrett BC, Wing SS (1978a). Pitting resistance of new and conventional orthopedic implant materials—effect of metallurgical condition. Corrosion 34(A): 138-145. Syrett BC, Wing SS (1978b). An electrochemical investigation of fretting corrosion of surgical implant materials. Corras/o« 34:379-386. Tesk JA (1986). The role of structure and composition on the physical and biological properties. In: Biocompatibility, toxicity and hypersensitivity to alloy systems used in dentistry. Ann Arbor (MI): The University of Michigan School of Dentistry, 3-15. Vaidyanathan TR, Prasad A (1981). In vitro corrosion and tarnish analysis of the Ag-Pd binary system. / Dent Res 60:707-715. Williams DF (1981). Biocompatibility of clinical implant materials. Vol.1. Boca Raton (FL): CRC Press.
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