KITZIS, MILLSTEIN, AND NATHANSON
is necessary to provide occlusal restorations that are in harmony on the articulator and intraoraUy. Using the field inspection gauge on properly functioning articulators fills this need. CLINICAL
SIGNIFICANCE
Articulators, when properly calibrated, can be used to transfer mounted casts with positional accuracy. REFERENCES 1. Millstein PL. Accuracy of laminated wax interocclusal wafers. J PROSTHET DENT 1985;54:574-7.
2. Instruction manual for Whip-Mix articulator. Louisville, Ky: Whip-Mix Corp, 1974. 3. Impressions. The occlusion, TMS and implant specialists. Product Showcase. Anaheim, Calif.' Denar Corp, September, 1988;vol 2, issue 2. 4. Needles JW. Mandibular movements and articulator design. J Am Dent Assoc 1923;10:927-35. 5. Millstein PL. A simplified method for ~esting the accuracy of interocclusal recording media. J PROSTHETDENT 1981;46:107. 6. Millstein PL. Determination of split cast accuracy [Abstract]. J Dent Res 1986;65:250. Reprint requests to: DR. PHILIP L. MILLSTE1N 15 LANGDONST. CAMBRIDGE,MA 02138
Corrosion under static and dynamic conditions of alloys used for m a g n e t i c retention in dentistry E. A n g e l i n i , Ph.D.,* M. P e z z o l i , D.D.S.,** a n d F. Z u c c h i , D.D.S.***
Politecnico, Torino, Torino, Italy; University of Torino, Torino, Italy; and University of Ferrara, Ferrara, Italy In recent y e a r s m a g n e t i c retention has g a i n e d i n c r e a s i n g popularity in dental practice. This i n v e s t i g a t i o n compared the corrosion r e s i s t a n c e of the palladiumcobalt f e r r o m a g n e t i c alloy (constituent of the k e e p e r c e m e n t e d on the abutment teeth) coupled with the s a m a r i u m - c o b a l t m a g n e t s embedded in the r e m o v a b l e part of the denture. The b e h a v i o r of three couples (cobalt-palladium, cobalt-palladium/ titanium, and cobalt-palladium/palladium) has been studied. The magnets, b e c a u s e of their poor corrosion resistance, are e n c a p s u l a t e d in various materials. To s i m u l a t e clinical conditions, c h a r a c t e r i z e d by the continuous m o v e m e n t of the k e e p e r with respect to the magnet, the e x p e r i m e n t s w e r e conducted in artificial s a l i v a under intermittent and continuous wear. (J PROSTHET DENT 1991;65:848-53.)
H
i
g
h
strength, high intrinsic coercivity samariumcobalt (Sm-Co) magnets are used with increasing frequency in overdentures and removable partial dentures. 1-4 The denture retention system usually includes one or more magnets embedded in the removable part of the denture coupled with a ferromagnetic keeper cemented on the abutment teeth. The matching magnet-keeper presents a similar action from a biomechanical point of view. 5, 6 Bare magnets, however, tend to corrode and wear in the oral cavity. The maximum loss due to wear and corrosion has been observed to be approximately 5 mg per year de-
This research was financially supported by the M.P.I. (Ministero della Pubblica Istruzione), Rome, Italy. *Associate Professor of Chemistry, Politecnico, Torino. **Professor of Restorative Dentistry, Clinica Odontostomatologics, University of Torino. ***Professor of Dental Materials, Centro Studi Corrosione Dacco, University of Ferrara. 10/1/25535
848
pending on the magnet configuration and size. 7 This loss is less than a thousandth of the recommended maximum ingested levels of samarium and cobalt per year. 7, 8 An insulation of the magnet from the oral cavity to resist this tendency has been suggested. Presently, a tentative coating of the magnets with acrylic resin has been universally substituted by their encapsulation in stainless steel, titanium (Ti), or palladium (Pd). With these new coatings, problems of corrosion and wear can arise because of the continuous movement of the keeper with respect to the magnet in the oral cavity. The corrosion properties of the various materials have been studied separately and in static conditions by several authors.9-11 Vrijoeff et al. 12 investigated the behavior of three Pd-Co ferromagnetic casting alloys by using standard potentiodynamic techniques. The alloys all showed good corrosion resistance in the potential range of the oral environment from -100 to 300 mV(SCE). Pezzoli et al. 13 examined the electrochemical behavior of a Pd-Co alloy and three caps (AISI 316, Ti, and Pd) in artificial saliva and in Ringer's
JUNE 1991 VOLUME65 NUMBER S
M A G N E T I C R E T E N T I O N IN D E N T I S T R Y
V (SCE) __
,m~.~_ -_._ _
~_--.,m:--.-m~
-0.1
- 0.2 -0.3
-0.4 0
,
i
i
,
5
10
15
20
, 25 days
Fig. 2. Short circuit potentials versus time of immersion in artificial saliva at 37 ° C, in static conditions, for three couples: (m) Pd/Pd-Co; (0) stainless steel/Pd-Co; ( • ) Ti/ Pd-Co.
V(SCE) -0.1 -0.2 -03
-0.4 -0.5 -0.6
Fig. 1. Apparatus used for wear tests; (1) electric toothbrush, (2) magnet embedded in epoxy resin provided with electric contact, (3) Pd-Co sample embedded in epoxy resin, fixed at bottom of Plexiglas cell.
solution. In artificial saliva good corrosion resistance was evidenced for all materials; Ti and Pd in particular showed a behavior typical of a passive material. Ringer's solution, which is more reactive because of its high chloride content, does not allow the passivation of Pd-Co alloy. This study compared the corrosion resistance of these materials in conditions that may better simulate clinical situations: for example, by coupling the Pd-Co ferromagnetic keeper with the magnets encapsulated with the three alloys (stainless steel, Ti, and Pd) and by moving the magnets with respect to the keeper. MATERIAL
AND METHODS
The specimens used in this investigation were a Pd-Co ferromagnetic alloy whose chemical composition (weight % ) as determined by atomic absorption spectrometry is Pd 41.5%, Co 56.5%, Ga 2.0%; and Sm-Co magnets cylindrically shaped, 4 mm in diameter, encapsulated in stainless steel, titanium, and palladium caps (Dyna Engineering, Bergon Op Zoom, The Netherlands). The corrosion behavior of three different couples (Pd-
THE J O U R N A L OF P R O S T H E T I C D E N T I S T R Y
0
5
10
15
20
25
days
Fig. 3. Short circuit potentials versus time of immersion in artificial saliva at 37 ° C, under continuous wear, for three couples: (.) Pd/Pd-Co; (0) stainless steel/Pd-Co; (&) Ti/ Pd-Co.
Co/stainless steel, Pd-Co/Ti, and Pd-Co/Pd) has been studied. All of the experiments, in static and in dynamic conditions, were performed with a custom-built apparatus (Fig. 1). It consists of an electric toothbrush, in which the brush has been substituted by a magnet previously embedded in epoxy resin. Only the bottom of the cap was exposed to the active medium, and it was provided with an electric contact and a copper wire fixed on the opposite side. By means of a transformer and a variable rheostat, the oscillations of the sample attached to the apparatus were reduced to two per second. To simulate the conditions of the oral cavity, the magnet applied friction against a sample of Pd-Co alloy fixed at a bottom of a Plexiglas cell that contained the electrolyte. The Pd-Co cast specimens, 20 × 10 mm, were embedded in epoxy resin and provided with an electric contact. The surface of the sample was prepared with emery paper and diamond paste. It was ultrasonically degreased with acetone. The electrolyte was thermostated at 37 ° C in artificial
849
ANGELINI, PEZZOLI, AND ZUCCHI
V{SCE}
-0.2
b
-0.3 -0.4 -0.5 -0.6 I
|
I
0 1 2 3 4 ; 0 1 2 3 4
I
days
Fig. 4. Short circuit potentials versus time of immersion in artificial saliva at 37 ° C during intermittent wear tests for (a) Ti/Pd-Co; and (b) stainless steel/Pal-Co. saliva. The composition of the saliva was potassium chloride, 1.47g; sodium bicarbonate, 1.25 g; potassium thiocyonate (KSCN), 0.52 g; NaH2P04 • H20, 0.19 g, and distilled water up to 1 L, buffered at pH 6.7 with lactic acid. In long-term experiments the electrolyte was changed once a week. The characterization of the materials was performed in three different conditions: one static and two dynamic, as follows. S t a t i c condition. The various couples (Pd-Co/encapsulated magnets) were simply immersed in artificial saliva for 30 days and the short circuit potentials were continuously recorded. D y n a m i c , u n d e r continuous w e a r . The magnets applied a continuous friction against the Pd-Co alloy (two passages per second) for 30 days and the short circuit potentials were continuously recorded. D y n a m i c , u n d e r i n t e r m i t t e n t w e a r . The friction was applied for 12 hours followed by rest periods of 12 hours, for a total immersion period of 30 days, during which time the short circuit potentials were continuously recorded. Additional information on the electrochemical behavior was obtained by recording polarization curves on all of the materials not coupled. The electrodes were immersed for 30 minutes in the active solution. Starting from a potential 0.15 V lower than the free corrosion potential, a scanning in the positive direction was performed, with a scan rate of 0.5 mV/sec. All the electrochemical measurements were repeated three times. Microhardness measurements were made with a Leitz Duromet instrument (Leitz, Rockleigh, N.J.) with a 10 g load.
RESULTS The short circuit potentials versus time of immersion in artificial saliva for the coupled specimens are seen in Fig. 2. In the first days of immersion, the potentials decreased until a minimum, between the second and the third day of permanence in the electrolyte, successively a slow increase could be observed and, after 15 days, the potential values
850
Fig. 5. Surface of titanium cap of magnet after continuous wear test. A, Original magnification × 25. B, Original magnification × 250.
stabilized near 0.0 V(SCE), independent of the material of the caps. The influence of continuous wear is evident in Fig. 3, where the short circuit potentials for the various coupled specimens are reported versus days of immersion and number of wear cycles. After fluctuations in the first days, analogous to those observed in static conditions, the short circuit potentials stabilized at values near -0.4 V(SCE), more active than the others previously observed. For a better understanding of this phenomenon, tests were carried out under intermittent wear. An example in Fig. 4 is reported as the results obtained during the first days of immersion on couples Pd-Co/Ti and Pd-Co/stainless steel. During wear the potential shifted in the negative direction, as observed in Fig. 3, whereas during rest periods, an abrupt increase was followed by slow nobilitation until the wear started again.
J U N E 1991
VOLUME 65
NUMBER 6
MAGNETIC RETENTION IN DENTISTRY
F i g . 6. Surface of the palladium cap of a magnet after continuous wear test. A, Original magnification × 25. B, Original magnification x 250.
F i g . 7. Surface of stainless steel cap of magnet after continuous wear test. A, Original magnification x 25. B, Original magnification × 250.
In Figs. 5 through 7, the magnets with the caps of the three materials examined are shown after the continuouswear experiments. The deepest mechanical abrasion signs are visible on the palladium caps, whereas the titanium and stainless steel caps are abraded to a lesser extent. These results are in accordance with the hardness measurements performed on the magnets: for palladium a value of 175 HV, for t i t a n i u m a value of 199 HV, and for stainless steel a value of 234 HV. Dark corrosion products are visible on sinkings of the specimens. These abrasions are superficial as confirmed by the measurements of thickness on the cross sections of the samples s u b m i t t e d to continuous wear. Even in the palladium caps no variation of thickness was detected with respect to the as-received specimens. Useful information on the behavior of the various alloys under study may be obtained also from electrochemical
tests as polarization curves. The polarization curves recorded in artifical saliva, t h e r m o s t a t e d at 37 ° C, for all of the materials under study are shown in Fig. 8. T i t a n i u m and palladium show an anodic polarization curve typical of passive materials, the passive current density is approximately 1 #A/cm 2, with potential values near +0.9 V(SCE). The stainless steel shows a well-defined pitting potential at + 0.5 V(SCE). W i t h the P d Co ferromagnetic alloy, the anodic current density does not exceed values near 1 ttA/cm 2 until + 0.4 V(SCE), then slowly increases with the shift of potential in the positive direction.
THE JOURNAL OF PROSTHETIC DENTISTRY
DISCUSSION In the oral cavity, the couples magnet-keeper present in a magnetically retained denture experiences biomechanical stresses during mastication, swallowing, or other func-
851
ANGELINI, PEZZOLI, AND ZUCCHI
V(.SCE)
1.0
,/
0.6
o,,,......o,.~#
//
02
/./1
.
Clinical implications
-0.2 101
10 =
103,
104
10s Iogi (nAIcrr~
F i g . 8. Polarization curves in artificial sa]iva at 37 ° C for Pd-Co ( - - - - - - ) ; Pd ( .... ); T i ( - - - - ) ; stainless steel ( . . . . . . ).
tions as teeth grinding. With time, these stresses may dramatically affect the corrosion resistance of the material in contact, in particular the caps, which are thin (100/~m) so as not to cause a sharp decrease of the magnetic retention strength. The simulation of wear cycles in artificial saliva, as described here, allows a tentative quantification of this phenomenon. During the long-term static immersions, the short-circuit potentials of the coupled Pd-Co/caps reach, after several days, a value similar to all of the samples, appreciably nobler than the initial one. Continuous wear hinders the formation of a passivating film, as evidenced by the highly negative potentials reached by the samples after some fluctuations in the first days' immersion. The inhibition of the passivating capacity of the surfaces under study was particularly evident in the intermittent wear cycles, where, in the first days' immersion, the beginning of wear caused a sudden decrease of nearly 200 millivolts of the short circuit potential of the couples. Successively, the passivating effort became increasingly depressed, the potential differences between static and dynamic conditions tended toward zero, and finally the potentials stabilized at values between -0.5 and -0.6 V(SCE). The polarization curves recorded in artificial saliva evidenced, for some elements under study, the tendency to passivation in static conditions. This phenomenon was particularly evident for titanium and palladium, whereas Pd-Co alloy had no tendency to passivation. Polarization curves allow comparison of the behavior of the alloys in the first moments of immersion. They are not used to predict the long-term behavior of a material
852
although they produce information about its active or passive behavior. It may be concluded that in the corrosion tests, performed in a limited time range to compare the behavior of different materials, no strong corrosive attacks were observed on the Pd-Co samples or on the caps. However, with longer immersion times in the oral fluids, comparable with those of the in vivo performance of an overdenture or a removable partial denture, the corrosive effects may increase dramatically until the perforation of the caps release noticeable amounts of metal ions in the oral cavity.
Several studies deal with the possible harm for the patient associated with the release of metallic ions. 1417 The toxicity of elements such as cadmium and nickel is well known. However, concern must also be given to the presence of elements such as Zn, Cu, Sn, and Ag, whose introduction in the body with soluble corrosion products can cause biologic unbalances and allergic phenomena. Ions may migrate in the material used for the reconstruction of teeth (resin or porcelain) and cause unesthetic discoloration, or migration into the neighboring soft tissues causing gingival pigmentation and the taste of metal in the saliva. Corrosion tests in dynamic conditions may help in the selection of materials for the manufacture of corrosion-resistant dental prostheses for improved clinical results.
SUMMARY AND CONCLUSIONS The corrosion resistance of a Pd-Co ferromagnetic alloy and of three types of caps (stainless steel, Ti, and Pd) for permanent Sm-Co magnets was investigated in artificial saliva under dynamic conditions. Wearing has a deleterious effect on the corrosion resistance of the materials, because it hinders the formation of a continuous oxide layer and consequently the passivation of the surface. In the experiments, performed in a limited time range, strong corrosive attacks were not observed. However, longer immersion times in the oral fluids, comparable with those of the in vivo performance of an overdenture or a removable partial denture may dramatically affect the resistance of the Pd-Co keeper and especially the caps.
REFERENCES 1. Winkler S, Pearson MH. The effectiveness of embedded magnets in complete dentures during speech and mastication: a cineradiographic study. Dent Dig 1967;73:118-29. 2. Becket JJ. Permanent magnets. Sci Am 1970;32:223-6. 3. Sasaki H, Kinouchi Y. Application of cobalt samarium alloy magnets in prosthetic restorations. Hotetsu Rinsho (Practice in orthodontics) 1976;9:77. 4. Kinouchi Y, Ushita T, Tsutsui H, Yoshida Y, Sasaki H, Miyazaki Y. Pd-Co dental casting ferromagnetic alloy. J Dent Res 1981;60:50-8. 5. Pezzoli M, Highton R, Caputo AA, Matyas J. Magnetizable abutment crowns for distal-extension removable partial dentures. J PROSTHET DENT 1986;55:475-80. 6. Highton R, Caputo AA, Pezzoli M, Matyas J. Retentive characteristics
JUNE 1 9 9 1
VOLUME 65
NUMBER 6
MAGNETIC RETENTION IN DENTISTRY
7. 8.
9. 10. 11.
12.
of different magnetic systems for dental application. J PROSTHETDENT 1986;55:104-6. Gillings B. Magnetic retention units for overlay dentures. Procedure Manual, Oct, 1978. Kinouchi Y, Ushita T, Tsutsui T, Yoshida Y, Sasaki H, Miyazaki T. Studies on the Sin-Co magnet as a dental material. J Dent Res 1979;58:1597-1606. Glanz PO. Intraoral behaviour and biocompatibility of gold versus non precious alloys. J Biol Buccale 1984;12:3-16. Palaghias G. Oral corrosion and inhibition processes. Swed Dent J 1985;suppl 30:1-72. Bessing C, Bergman M, Thoren A. Potentiodynamic polarization analysis of low-gold and silver-palladium alloys in three different media. Dent Mater 1987;3:153-9. Vrijhoeff MMA, Mezger PR, Van Der Zel JM, Greener EH. Corrosion of ferromagnetic alloys used for magnetic retention of overdentures. J Dent Res 1987;66:1456-9.
13. Pezzoli M, Angelini E, Zucchi F, Re G. Comportamento elettrochimico di tre accoppiamenti magnetici impiegati per la ritenzione protesica. Minerva Stomatol 1989;38:605-9. 14. Bergman B, Bergman M, Soremark R. Dissolution and uptake of cadmium from dental solder alloy implants. Scand J Dent Res 1977;85: 623-8. 15. Fisher AA. Contact dermatitis. Philadelphia: Lee & Febiger, 1967;16. 16. Wood JF. Mucosal reaction to cobalt chromium alloy. Br Dent J 1974;136:423-9. 17. Leventine AV, Bettley FR. Sensitivity to metal dental plate. Proc R Soc Med 1974;67:1007-13. Reprint requests to: DR. EMMAANGELINI POLITECNICODI TORINO C.SO DUCADEGLIABRUZZI24 I10129 TORINO,ITALY
A v a i l a b i l i t y o f JOURNAL b a c k i s s u e s , 1 9 8 6 - 1 9 9 0 Back issues of THE JOURNAL OF PROSTHETIC DENTISTRY are available for purchase from the publisher, Mosby-Year Book, Inc., at a cost of $6.50 per issue. (Foreign postage is not included.) The following quantity discounts are available: 25% off on quantities of 12 to 23, and one third off on quantities of 24 or more. Please write to Mosby-Year Book, Inc., Subscription Services, 11830 Westline Industrial Drive, St. Louis, MO 63146-3318, or call (314)453-4351 for information on availability of particular issues for that period from 1978 to 1989. If unavailable from the publisher, photocopies of complete issues are available from University Microforms International, 300 N. Zeeb Rd., Ann Arbor, MI 48106, (313)761-4700.
THE J O U R N A L OF P R O S T H E T I C D E N T I S T R Y
853
is necessary to provide occlusal restorations that are in harmony on the articulator and intraoraUy. Using the field inspection gauge on properly functioning articulators fills this need. CLINICAL
SIGNIFICANCE
Articulators, when properly calibrated, can be used to transfer mounted casts with positional accuracy. REFERENCES 1. Millstein PL. Accuracy of laminated wax interocclusal wafers. J PROSTHET DENT 1985;54:574-7.
2. Instruction manual for Whip-Mix articulator. Louisville, Ky: Whip-Mix Corp, 1974. 3. Impressions. The occlusion, TMS and implant specialists. Product Showcase. Anaheim, Calif.' Denar Corp, September, 1988;vol 2, issue 2. 4. Needles JW. Mandibular movements and articulator design. J Am Dent Assoc 1923;10:927-35. 5. Millstein PL. A simplified method for ~esting the accuracy of interocclusal recording media. J PROSTHETDENT 1981;46:107. 6. Millstein PL. Determination of split cast accuracy [Abstract]. J Dent Res 1986;65:250. Reprint requests to: DR. PHILIP L. MILLSTE1N 15 LANGDONST. CAMBRIDGE,MA 02138
Corrosion under static and dynamic conditions of alloys used for m a g n e t i c retention in dentistry E. A n g e l i n i , Ph.D.,* M. P e z z o l i , D.D.S.,** a n d F. Z u c c h i , D.D.S.***
Politecnico, Torino, Torino, Italy; University of Torino, Torino, Italy; and University of Ferrara, Ferrara, Italy In recent y e a r s m a g n e t i c retention has g a i n e d i n c r e a s i n g popularity in dental practice. This i n v e s t i g a t i o n compared the corrosion r e s i s t a n c e of the palladiumcobalt f e r r o m a g n e t i c alloy (constituent of the k e e p e r c e m e n t e d on the abutment teeth) coupled with the s a m a r i u m - c o b a l t m a g n e t s embedded in the r e m o v a b l e part of the denture. The b e h a v i o r of three couples (cobalt-palladium, cobalt-palladium/ titanium, and cobalt-palladium/palladium) has been studied. The magnets, b e c a u s e of their poor corrosion resistance, are e n c a p s u l a t e d in various materials. To s i m u l a t e clinical conditions, c h a r a c t e r i z e d by the continuous m o v e m e n t of the k e e p e r with respect to the magnet, the e x p e r i m e n t s w e r e conducted in artificial s a l i v a under intermittent and continuous wear. (J PROSTHET DENT 1991;65:848-53.)
H
i
g
h
strength, high intrinsic coercivity samariumcobalt (Sm-Co) magnets are used with increasing frequency in overdentures and removable partial dentures. 1-4 The denture retention system usually includes one or more magnets embedded in the removable part of the denture coupled with a ferromagnetic keeper cemented on the abutment teeth. The matching magnet-keeper presents a similar action from a biomechanical point of view. 5, 6 Bare magnets, however, tend to corrode and wear in the oral cavity. The maximum loss due to wear and corrosion has been observed to be approximately 5 mg per year de-
This research was financially supported by the M.P.I. (Ministero della Pubblica Istruzione), Rome, Italy. *Associate Professor of Chemistry, Politecnico, Torino. **Professor of Restorative Dentistry, Clinica Odontostomatologics, University of Torino. ***Professor of Dental Materials, Centro Studi Corrosione Dacco, University of Ferrara. 10/1/25535
848
pending on the magnet configuration and size. 7 This loss is less than a thousandth of the recommended maximum ingested levels of samarium and cobalt per year. 7, 8 An insulation of the magnet from the oral cavity to resist this tendency has been suggested. Presently, a tentative coating of the magnets with acrylic resin has been universally substituted by their encapsulation in stainless steel, titanium (Ti), or palladium (Pd). With these new coatings, problems of corrosion and wear can arise because of the continuous movement of the keeper with respect to the magnet in the oral cavity. The corrosion properties of the various materials have been studied separately and in static conditions by several authors.9-11 Vrijoeff et al. 12 investigated the behavior of three Pd-Co ferromagnetic casting alloys by using standard potentiodynamic techniques. The alloys all showed good corrosion resistance in the potential range of the oral environment from -100 to 300 mV(SCE). Pezzoli et al. 13 examined the electrochemical behavior of a Pd-Co alloy and three caps (AISI 316, Ti, and Pd) in artificial saliva and in Ringer's
JUNE 1991 VOLUME65 NUMBER S
M A G N E T I C R E T E N T I O N IN D E N T I S T R Y
V (SCE) __
,m~.~_ -_._ _
~_--.,m:--.-m~
-0.1
- 0.2 -0.3
-0.4 0
,
i
i
,
5
10
15
20
, 25 days
Fig. 2. Short circuit potentials versus time of immersion in artificial saliva at 37 ° C, in static conditions, for three couples: (m) Pd/Pd-Co; (0) stainless steel/Pd-Co; ( • ) Ti/ Pd-Co.
V(SCE) -0.1 -0.2 -03
-0.4 -0.5 -0.6
Fig. 1. Apparatus used for wear tests; (1) electric toothbrush, (2) magnet embedded in epoxy resin provided with electric contact, (3) Pd-Co sample embedded in epoxy resin, fixed at bottom of Plexiglas cell.
solution. In artificial saliva good corrosion resistance was evidenced for all materials; Ti and Pd in particular showed a behavior typical of a passive material. Ringer's solution, which is more reactive because of its high chloride content, does not allow the passivation of Pd-Co alloy. This study compared the corrosion resistance of these materials in conditions that may better simulate clinical situations: for example, by coupling the Pd-Co ferromagnetic keeper with the magnets encapsulated with the three alloys (stainless steel, Ti, and Pd) and by moving the magnets with respect to the keeper. MATERIAL
AND METHODS
The specimens used in this investigation were a Pd-Co ferromagnetic alloy whose chemical composition (weight % ) as determined by atomic absorption spectrometry is Pd 41.5%, Co 56.5%, Ga 2.0%; and Sm-Co magnets cylindrically shaped, 4 mm in diameter, encapsulated in stainless steel, titanium, and palladium caps (Dyna Engineering, Bergon Op Zoom, The Netherlands). The corrosion behavior of three different couples (Pd-
THE J O U R N A L OF P R O S T H E T I C D E N T I S T R Y
0
5
10
15
20
25
days
Fig. 3. Short circuit potentials versus time of immersion in artificial saliva at 37 ° C, under continuous wear, for three couples: (.) Pd/Pd-Co; (0) stainless steel/Pd-Co; (&) Ti/ Pd-Co.
Co/stainless steel, Pd-Co/Ti, and Pd-Co/Pd) has been studied. All of the experiments, in static and in dynamic conditions, were performed with a custom-built apparatus (Fig. 1). It consists of an electric toothbrush, in which the brush has been substituted by a magnet previously embedded in epoxy resin. Only the bottom of the cap was exposed to the active medium, and it was provided with an electric contact and a copper wire fixed on the opposite side. By means of a transformer and a variable rheostat, the oscillations of the sample attached to the apparatus were reduced to two per second. To simulate the conditions of the oral cavity, the magnet applied friction against a sample of Pd-Co alloy fixed at a bottom of a Plexiglas cell that contained the electrolyte. The Pd-Co cast specimens, 20 × 10 mm, were embedded in epoxy resin and provided with an electric contact. The surface of the sample was prepared with emery paper and diamond paste. It was ultrasonically degreased with acetone. The electrolyte was thermostated at 37 ° C in artificial
849
ANGELINI, PEZZOLI, AND ZUCCHI
V{SCE}
-0.2
b
-0.3 -0.4 -0.5 -0.6 I
|
I
0 1 2 3 4 ; 0 1 2 3 4
I
days
Fig. 4. Short circuit potentials versus time of immersion in artificial saliva at 37 ° C during intermittent wear tests for (a) Ti/Pd-Co; and (b) stainless steel/Pal-Co. saliva. The composition of the saliva was potassium chloride, 1.47g; sodium bicarbonate, 1.25 g; potassium thiocyonate (KSCN), 0.52 g; NaH2P04 • H20, 0.19 g, and distilled water up to 1 L, buffered at pH 6.7 with lactic acid. In long-term experiments the electrolyte was changed once a week. The characterization of the materials was performed in three different conditions: one static and two dynamic, as follows. S t a t i c condition. The various couples (Pd-Co/encapsulated magnets) were simply immersed in artificial saliva for 30 days and the short circuit potentials were continuously recorded. D y n a m i c , u n d e r continuous w e a r . The magnets applied a continuous friction against the Pd-Co alloy (two passages per second) for 30 days and the short circuit potentials were continuously recorded. D y n a m i c , u n d e r i n t e r m i t t e n t w e a r . The friction was applied for 12 hours followed by rest periods of 12 hours, for a total immersion period of 30 days, during which time the short circuit potentials were continuously recorded. Additional information on the electrochemical behavior was obtained by recording polarization curves on all of the materials not coupled. The electrodes were immersed for 30 minutes in the active solution. Starting from a potential 0.15 V lower than the free corrosion potential, a scanning in the positive direction was performed, with a scan rate of 0.5 mV/sec. All the electrochemical measurements were repeated three times. Microhardness measurements were made with a Leitz Duromet instrument (Leitz, Rockleigh, N.J.) with a 10 g load.
RESULTS The short circuit potentials versus time of immersion in artificial saliva for the coupled specimens are seen in Fig. 2. In the first days of immersion, the potentials decreased until a minimum, between the second and the third day of permanence in the electrolyte, successively a slow increase could be observed and, after 15 days, the potential values
850
Fig. 5. Surface of titanium cap of magnet after continuous wear test. A, Original magnification × 25. B, Original magnification × 250.
stabilized near 0.0 V(SCE), independent of the material of the caps. The influence of continuous wear is evident in Fig. 3, where the short circuit potentials for the various coupled specimens are reported versus days of immersion and number of wear cycles. After fluctuations in the first days, analogous to those observed in static conditions, the short circuit potentials stabilized at values near -0.4 V(SCE), more active than the others previously observed. For a better understanding of this phenomenon, tests were carried out under intermittent wear. An example in Fig. 4 is reported as the results obtained during the first days of immersion on couples Pd-Co/Ti and Pd-Co/stainless steel. During wear the potential shifted in the negative direction, as observed in Fig. 3, whereas during rest periods, an abrupt increase was followed by slow nobilitation until the wear started again.
J U N E 1991
VOLUME 65
NUMBER 6
MAGNETIC RETENTION IN DENTISTRY
F i g . 6. Surface of the palladium cap of a magnet after continuous wear test. A, Original magnification × 25. B, Original magnification x 250.
F i g . 7. Surface of stainless steel cap of magnet after continuous wear test. A, Original magnification x 25. B, Original magnification × 250.
In Figs. 5 through 7, the magnets with the caps of the three materials examined are shown after the continuouswear experiments. The deepest mechanical abrasion signs are visible on the palladium caps, whereas the titanium and stainless steel caps are abraded to a lesser extent. These results are in accordance with the hardness measurements performed on the magnets: for palladium a value of 175 HV, for t i t a n i u m a value of 199 HV, and for stainless steel a value of 234 HV. Dark corrosion products are visible on sinkings of the specimens. These abrasions are superficial as confirmed by the measurements of thickness on the cross sections of the samples s u b m i t t e d to continuous wear. Even in the palladium caps no variation of thickness was detected with respect to the as-received specimens. Useful information on the behavior of the various alloys under study may be obtained also from electrochemical
tests as polarization curves. The polarization curves recorded in artifical saliva, t h e r m o s t a t e d at 37 ° C, for all of the materials under study are shown in Fig. 8. T i t a n i u m and palladium show an anodic polarization curve typical of passive materials, the passive current density is approximately 1 #A/cm 2, with potential values near +0.9 V(SCE). The stainless steel shows a well-defined pitting potential at + 0.5 V(SCE). W i t h the P d Co ferromagnetic alloy, the anodic current density does not exceed values near 1 ttA/cm 2 until + 0.4 V(SCE), then slowly increases with the shift of potential in the positive direction.
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DISCUSSION In the oral cavity, the couples magnet-keeper present in a magnetically retained denture experiences biomechanical stresses during mastication, swallowing, or other func-
851
ANGELINI, PEZZOLI, AND ZUCCHI
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.
Clinical implications
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F i g . 8. Polarization curves in artificial sa]iva at 37 ° C for Pd-Co ( - - - - - - ) ; Pd ( .... ); T i ( - - - - ) ; stainless steel ( . . . . . . ).
tions as teeth grinding. With time, these stresses may dramatically affect the corrosion resistance of the material in contact, in particular the caps, which are thin (100/~m) so as not to cause a sharp decrease of the magnetic retention strength. The simulation of wear cycles in artificial saliva, as described here, allows a tentative quantification of this phenomenon. During the long-term static immersions, the short-circuit potentials of the coupled Pd-Co/caps reach, after several days, a value similar to all of the samples, appreciably nobler than the initial one. Continuous wear hinders the formation of a passivating film, as evidenced by the highly negative potentials reached by the samples after some fluctuations in the first days' immersion. The inhibition of the passivating capacity of the surfaces under study was particularly evident in the intermittent wear cycles, where, in the first days' immersion, the beginning of wear caused a sudden decrease of nearly 200 millivolts of the short circuit potential of the couples. Successively, the passivating effort became increasingly depressed, the potential differences between static and dynamic conditions tended toward zero, and finally the potentials stabilized at values between -0.5 and -0.6 V(SCE). The polarization curves recorded in artificial saliva evidenced, for some elements under study, the tendency to passivation in static conditions. This phenomenon was particularly evident for titanium and palladium, whereas Pd-Co alloy had no tendency to passivation. Polarization curves allow comparison of the behavior of the alloys in the first moments of immersion. They are not used to predict the long-term behavior of a material
852
although they produce information about its active or passive behavior. It may be concluded that in the corrosion tests, performed in a limited time range to compare the behavior of different materials, no strong corrosive attacks were observed on the Pd-Co samples or on the caps. However, with longer immersion times in the oral fluids, comparable with those of the in vivo performance of an overdenture or a removable partial denture, the corrosive effects may increase dramatically until the perforation of the caps release noticeable amounts of metal ions in the oral cavity.
Several studies deal with the possible harm for the patient associated with the release of metallic ions. 1417 The toxicity of elements such as cadmium and nickel is well known. However, concern must also be given to the presence of elements such as Zn, Cu, Sn, and Ag, whose introduction in the body with soluble corrosion products can cause biologic unbalances and allergic phenomena. Ions may migrate in the material used for the reconstruction of teeth (resin or porcelain) and cause unesthetic discoloration, or migration into the neighboring soft tissues causing gingival pigmentation and the taste of metal in the saliva. Corrosion tests in dynamic conditions may help in the selection of materials for the manufacture of corrosion-resistant dental prostheses for improved clinical results.
SUMMARY AND CONCLUSIONS The corrosion resistance of a Pd-Co ferromagnetic alloy and of three types of caps (stainless steel, Ti, and Pd) for permanent Sm-Co magnets was investigated in artificial saliva under dynamic conditions. Wearing has a deleterious effect on the corrosion resistance of the materials, because it hinders the formation of a continuous oxide layer and consequently the passivation of the surface. In the experiments, performed in a limited time range, strong corrosive attacks were not observed. However, longer immersion times in the oral fluids, comparable with those of the in vivo performance of an overdenture or a removable partial denture may dramatically affect the resistance of the Pd-Co keeper and especially the caps.
REFERENCES 1. Winkler S, Pearson MH. The effectiveness of embedded magnets in complete dentures during speech and mastication: a cineradiographic study. Dent Dig 1967;73:118-29. 2. Becket JJ. Permanent magnets. Sci Am 1970;32:223-6. 3. Sasaki H, Kinouchi Y. Application of cobalt samarium alloy magnets in prosthetic restorations. Hotetsu Rinsho (Practice in orthodontics) 1976;9:77. 4. Kinouchi Y, Ushita T, Tsutsui H, Yoshida Y, Sasaki H, Miyazaki Y. Pd-Co dental casting ferromagnetic alloy. J Dent Res 1981;60:50-8. 5. Pezzoli M, Highton R, Caputo AA, Matyas J. Magnetizable abutment crowns for distal-extension removable partial dentures. J PROSTHET DENT 1986;55:475-80. 6. Highton R, Caputo AA, Pezzoli M, Matyas J. Retentive characteristics
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7. 8.
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of different magnetic systems for dental application. J PROSTHETDENT 1986;55:104-6. Gillings B. Magnetic retention units for overlay dentures. Procedure Manual, Oct, 1978. Kinouchi Y, Ushita T, Tsutsui T, Yoshida Y, Sasaki H, Miyazaki T. Studies on the Sin-Co magnet as a dental material. J Dent Res 1979;58:1597-1606. Glanz PO. Intraoral behaviour and biocompatibility of gold versus non precious alloys. J Biol Buccale 1984;12:3-16. Palaghias G. Oral corrosion and inhibition processes. Swed Dent J 1985;suppl 30:1-72. Bessing C, Bergman M, Thoren A. Potentiodynamic polarization analysis of low-gold and silver-palladium alloys in three different media. Dent Mater 1987;3:153-9. Vrijhoeff MMA, Mezger PR, Van Der Zel JM, Greener EH. Corrosion of ferromagnetic alloys used for magnetic retention of overdentures. J Dent Res 1987;66:1456-9.
13. Pezzoli M, Angelini E, Zucchi F, Re G. Comportamento elettrochimico di tre accoppiamenti magnetici impiegati per la ritenzione protesica. Minerva Stomatol 1989;38:605-9. 14. Bergman B, Bergman M, Soremark R. Dissolution and uptake of cadmium from dental solder alloy implants. Scand J Dent Res 1977;85: 623-8. 15. Fisher AA. Contact dermatitis. Philadelphia: Lee & Febiger, 1967;16. 16. Wood JF. Mucosal reaction to cobalt chromium alloy. Br Dent J 1974;136:423-9. 17. Leventine AV, Bettley FR. Sensitivity to metal dental plate. Proc R Soc Med 1974;67:1007-13. Reprint requests to: DR. EMMAANGELINI POLITECNICODI TORINO C.SO DUCADEGLIABRUZZI24 I10129 TORINO,ITALY
A v a i l a b i l i t y o f JOURNAL b a c k i s s u e s , 1 9 8 6 - 1 9 9 0 Back issues of THE JOURNAL OF PROSTHETIC DENTISTRY are available for purchase from the publisher, Mosby-Year Book, Inc., at a cost of $6.50 per issue. (Foreign postage is not included.) The following quantity discounts are available: 25% off on quantities of 12 to 23, and one third off on quantities of 24 or more. Please write to Mosby-Year Book, Inc., Subscription Services, 11830 Westline Industrial Drive, St. Louis, MO 63146-3318, or call (314)453-4351 for information on availability of particular issues for that period from 1978 to 1989. If unavailable from the publisher, photocopies of complete issues are available from University Microforms International, 300 N. Zeeb Rd., Ann Arbor, MI 48106, (313)761-4700.
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