D e t e r m i n i n g the a c c u r a c y of articulator i n t e r c h a n g e a b i l i t y G a r y D. K i t z i s , D.M.D.,* P h i l i p L. M i l l s t e i n , D . M . D . , M.S.,** a n d D a n N a t h a n s o n , D . M . D . , M.S.D.*** State University of New York at Stony Brook, School of Dental Medicine, Stony Brook, N.Y.; and Boston University, Goldman School of Graduate Dentistry, Boston, Mass. T w e l v e articulators (Denar D5-A and Mark II) w e r e calibrated using the D e n a r Field Inspection Gage device. Silicone interocclusal records and the split cast method w e r e used to test the positional accuracy of mounted casts transferred from one articulator to another. The i n t e r c h a n g e a b i l i t y w a s performed by three independent observers. The results indicate that calibrated articulators can be used to transfer c a s t s b e t w e e n articulators w i t h o u t a loss of positional accuracy. (J PROSTHET DENT 1991;65:845-8.)
D e n t a l plaster is commonly used to mount maxillary and m a n d i b u l a r dental casts in occlusion on dental articulators. Positional accuracy between opposing casts is established so t h a t the dental occlusion can be analyzed and treated. 1 Until recently, casts m o u n t e d in occlusion on one articulator could not be transferred to another. The reason was t h a t no two articulators were built exactly alike. 2 This m e a n t t h a t articulated casts could not be sent to a dental laboratory without an accompanying articulator. Expensive mailing costs, coupled with the need for several dental articulators and the t h r e a t of loss or breakage in the mails, deterred communication between dentists and the dental laboratory. Recent claims indicate t h a t m o u n t e d casts can now be sent to a dental laboratory without an articulator and can be positioned with occlusal accuracy on a similar articulator. 3 Similar articulators can now be optically calibrated with a field inspection gauge to accept m o u n t e d casts from other articulators. By using interchangeable articulators there is less expense, less chance for breakage, and there is enhanced communication between dentist and dental technician. Articulators are precise instruments t h a t are calibrated by the manufacturer. Using a special alignment device, articulators can now be calibrated by the clinician. This is a new and innovative idea. T h e procedure involves the use of a Field Inspection Gage (Denar Corp., Anaheim, Calif.), which is an optical alignment device used to calibrate articulators and laboratory relators. A laboratory relator is a type of articulator t h a t functions as a hinge instrument for mounting casts and finalizing an occlusion. T h e dentist
makes diagnostic casts and a centric relation recording and then makes a pantograph of the occlusion. For diagnostic purposes, the pantograph is transferred to an articulator onto which the study casts are eventually mounted. Subsequently, the m o u n t e d casts are positioned on a lab relator for transfer to the dental laboratory, where they are placed on a calibrated, fully adjustable articulator. The restoration is now made in accordance with the patient's condylar settings as prescribed by the dentist. T h e restoration is again placed on the lab relator on which the casts were originally mounted, and the position of maximal intercuspation is finalized. In this manner the lab relator functions as an intermediary unit between the dentist and laboratory, thereby reducing the need for many articulators. Sometimes, however, it is more practical to use another articulator, either fully or semiadjustable, rather than a lab relator. There is a greater degree of versatility when tile articulator on which the casts were originally m o u n t e d can be used as a lab relator for finalizing the occlusion. To determine the accuracy of this system (for which there are no reports in the literature), a study was designed to test the hypothesis t h a t dental casts can be transferred from one articulator to similar articulators with positional accuracy. Testing included the use of a polyvinyl siloxane interocclusal record for positioning the casts and the split cast method to determine the accuracy of the mounting. The Denar D 5-A fully adjustable and M a r k II semiadjustable articulators (Denar Corp.) were studied. (Hereafter, these articulators are referred to as "D 5-A" and " M a r k II.") MATERIAL
*Assistant Professor, Department of Periodontology, State University of New York at Stony Brook, School of Dental Medicine. **Associate Clinical Professor, Department of Biomaterials, Boston University, Goldman School of Graduate Dentistry. *** Professor and Chairman, Department of Biomaterials, Boston University, Goldman School of Graduate Dentistry. 10/1/24227
THE JOURNAL OF PROSTHETIC DENTISTRY
AND
METHODS
The Denar Field Inspection Gage is an optical gauge used to calibrate Denar articulators and lab relators. According: to the manufacturers, the gauge allows the direct transfer of mounted casts between calibrated instruments with positional accuracy. T h e gauge consists of an upper m e m b e r t h a t is an optical microscope and a lower m e m b e r containing a stage with reticle and cross hairs for calibration. T h e
845
KITZIS, MILLSTEIN, AND NATHANSON
F i g . 1. Denar Field Inspection Gage instrument. Upper member is an optical microscope and the lower m e m b e r contains a stage with reticle and cross hairs for calibration.
features allow measurements and adjustments of the articulator members in three dimensions (Fig. 1).
Articulator calibration When calibrating the articulators, the following procedures were observed. The condylar controls on the D 5-A articulator were set to a protrusive p a t h of 30 degrees and a progressive side shift of 5 degrees. The immediate side shift, top wall adjustment, and rear wall a d j u s t m e n t were all set to 0 degrees and the vertical axis a d j u s t m e n t of intercondylar width was set at 53 m m from the center. The settings on the Mark II articulator were set the same way except for the vertical axis, which was 55 mm. The difference was due to minor differences in construction between the D 5-A and the M a r k II articulators with respect to the location and access to the calibration screws on the atticulator.
Articulator adjustment The first a d j u s t m e n t involved the use of a heavy interbow pin t h a t was used in place of the incisal pin and table and t h a t set the anterior height of the articulator (Fig. 2). Next, the upper and lower members of the field gauge were attached to their respective articulator parts, making certain t h a t all contacting mounting surfaces were immaculate. After mounting the gauge, the accuracy of the calibration dials on the sides of the upper scope m e m b e r was
846
F i g . 2. The Inspection gauge mounted on an articulator. Note heavy interbow pin used in place of incisal pin and table.
confirmed. If the dials did not match the calibration standard etched on the back of the gauge, they were rotated until they were aligned, thus zeroing the gauge (Figs. 3 and 4). The vertical heights on the condyles were then adjusted by alternately turning the left and right condylar height adjusting screws until the dials read zero (Fig. 5). Once the vertical calibration was set, the horizontal calibration was performed. The horizontal a d j u s t m e n t screws were loosened, and the upper member of the articulator was moved in two dimensions until the reticle and cross hairs, as viewed through the scope, overlapped. When calibrating the articulators, it was imperative for the centric holding spring on the D 5-A and the centric latch on the M a r k II to be in good working order. When these retainers were worn or not used, it was extremely difficult to maintain the condyles against the posterior and superior walls of the upper member of the articulator while making the adjustments with the field gauge.
Articulator testing After calibration, a mandibular cast was mounted in occlusion on an articulator where the maxillary cast was prepared for a split cast using "V" notches. 4, 5 An interocclusal record of the interocclusal space with no opposing tooth contact was made using Express (3M Co., St. Paul, Minn.), which is a vinyl siloxane impression material t h a t has been shown to be accurate. 6 The maxillary cast was separated
J U N E 1991
V O L U M E 65
NUMBER 6
ARTICULATOR I N T E R C H A N G E A B I L I T Y
F i g . 3. Posterior view of the Inspection gauge with calibration standards for articulator etched into back of gauge.
from the mounting stone and the casts were then transferred to other calibrated articulators. The mandibular cast was first attached to the lower m e m b e r of an articulator, the interocclusal record was positioned on the m a n d i b u l a r cast, and the maxillary cast was placed into the occlusal record. The split cast mounting attached to the upper m e m b e r of the articulator was then lowered into position until the "V" projections on the mounting were fully seated into the notches in the maxillary cast. This method of testing was appropriate because the split cast method is an acceptable method in prosthetic dentistry verifying jaw position records. It is a precise method when the interocclusal records are accurate and dimensionally stable. Articulator
evaluation
Articulator interchangeability was performed by three i n d e p e n d e n t observers and was recorded as a fit and successful transfer or as not fitting. A successful transfer of casts from one articulator to another d e m o n s t r a t e d a precise fit between the maxillary cast grooves and the projections of the split cast mounting. If the grooves and projections did not fit precisely, the transfer was not considered successful. Three evaluators, all of whom were dentists, rehearsed before the study using calibrated and uncalibrated articulators to recognize transfers of mounted casts t h a t did and did not fit. RESULTS
F i g . 4. Accuracy of the calibration dials on the sides of the upper scope member is confirmed by rotating them until they match the calibration s t a n d a r d etched on the back of the gauge (see Fig. 3).
AND
DISCUSSION
The evaluation of the 12 articulators indicated a 100% transferability among them. The results d e m o n s t r a t e d t h a t the calibrated articulators used in this study allowed the successful transfer of m o u n t e d casts without the loss of positional accuracy. In clinical terms, this system provides
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
F i g . 5. Vertical heights on the condyles are adjusted so the dials~ read zero.
an economical means for the use of fully adjustable articulators. :Furthermore, the use of a precise recording material, such as Express, enables both clinician and technician to check and corroborate articulator calibrations. This procedure is important, because calibrating a damaged articulator may create the false assumption of precise transferability when in fact there is none. Positional inaccuracy of the casts would not be apparent, which could result in a breakdown in communication between dentist and dental technician and lead to the inability to properly fill occlusal prescriptions. The need for like calibration of articulators and a simple means for checking such calibrations
847
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
D e n t a l plaster is commonly used to mount maxillary and m a n d i b u l a r dental casts in occlusion on dental articulators. Positional accuracy between opposing casts is established so t h a t the dental occlusion can be analyzed and treated. 1 Until recently, casts m o u n t e d in occlusion on one articulator could not be transferred to another. The reason was t h a t no two articulators were built exactly alike. 2 This m e a n t t h a t articulated casts could not be sent to a dental laboratory without an accompanying articulator. Expensive mailing costs, coupled with the need for several dental articulators and the t h r e a t of loss or breakage in the mails, deterred communication between dentists and the dental laboratory. Recent claims indicate t h a t m o u n t e d casts can now be sent to a dental laboratory without an articulator and can be positioned with occlusal accuracy on a similar articulator. 3 Similar articulators can now be optically calibrated with a field inspection gauge to accept m o u n t e d casts from other articulators. By using interchangeable articulators there is less expense, less chance for breakage, and there is enhanced communication between dentist and dental technician. Articulators are precise instruments t h a t are calibrated by the manufacturer. Using a special alignment device, articulators can now be calibrated by the clinician. This is a new and innovative idea. T h e procedure involves the use of a Field Inspection Gage (Denar Corp., Anaheim, Calif.), which is an optical alignment device used to calibrate articulators and laboratory relators. A laboratory relator is a type of articulator t h a t functions as a hinge instrument for mounting casts and finalizing an occlusion. T h e dentist
makes diagnostic casts and a centric relation recording and then makes a pantograph of the occlusion. For diagnostic purposes, the pantograph is transferred to an articulator onto which the study casts are eventually mounted. Subsequently, the m o u n t e d casts are positioned on a lab relator for transfer to the dental laboratory, where they are placed on a calibrated, fully adjustable articulator. The restoration is now made in accordance with the patient's condylar settings as prescribed by the dentist. T h e restoration is again placed on the lab relator on which the casts were originally mounted, and the position of maximal intercuspation is finalized. In this manner the lab relator functions as an intermediary unit between the dentist and laboratory, thereby reducing the need for many articulators. Sometimes, however, it is more practical to use another articulator, either fully or semiadjustable, rather than a lab relator. There is a greater degree of versatility when tile articulator on which the casts were originally m o u n t e d can be used as a lab relator for finalizing the occlusion. To determine the accuracy of this system (for which there are no reports in the literature), a study was designed to test the hypothesis t h a t dental casts can be transferred from one articulator to similar articulators with positional accuracy. Testing included the use of a polyvinyl siloxane interocclusal record for positioning the casts and the split cast method to determine the accuracy of the mounting. The Denar D 5-A fully adjustable and M a r k II semiadjustable articulators (Denar Corp.) were studied. (Hereafter, these articulators are referred to as "D 5-A" and " M a r k II.") MATERIAL
*Assistant Professor, Department of Periodontology, State University of New York at Stony Brook, School of Dental Medicine. **Associate Clinical Professor, Department of Biomaterials, Boston University, Goldman School of Graduate Dentistry. *** Professor and Chairman, Department of Biomaterials, Boston University, Goldman School of Graduate Dentistry. 10/1/24227
THE JOURNAL OF PROSTHETIC DENTISTRY
AND
METHODS
The Denar Field Inspection Gage is an optical gauge used to calibrate Denar articulators and lab relators. According: to the manufacturers, the gauge allows the direct transfer of mounted casts between calibrated instruments with positional accuracy. T h e gauge consists of an upper m e m b e r t h a t is an optical microscope and a lower m e m b e r containing a stage with reticle and cross hairs for calibration. T h e
845
KITZIS, MILLSTEIN, AND NATHANSON
F i g . 1. Denar Field Inspection Gage instrument. Upper member is an optical microscope and the lower m e m b e r contains a stage with reticle and cross hairs for calibration.
features allow measurements and adjustments of the articulator members in three dimensions (Fig. 1).
Articulator calibration When calibrating the articulators, the following procedures were observed. The condylar controls on the D 5-A articulator were set to a protrusive p a t h of 30 degrees and a progressive side shift of 5 degrees. The immediate side shift, top wall adjustment, and rear wall a d j u s t m e n t were all set to 0 degrees and the vertical axis a d j u s t m e n t of intercondylar width was set at 53 m m from the center. The settings on the Mark II articulator were set the same way except for the vertical axis, which was 55 mm. The difference was due to minor differences in construction between the D 5-A and the M a r k II articulators with respect to the location and access to the calibration screws on the atticulator.
Articulator adjustment The first a d j u s t m e n t involved the use of a heavy interbow pin t h a t was used in place of the incisal pin and table and t h a t set the anterior height of the articulator (Fig. 2). Next, the upper and lower members of the field gauge were attached to their respective articulator parts, making certain t h a t all contacting mounting surfaces were immaculate. After mounting the gauge, the accuracy of the calibration dials on the sides of the upper scope m e m b e r was
846
F i g . 2. The Inspection gauge mounted on an articulator. Note heavy interbow pin used in place of incisal pin and table.
confirmed. If the dials did not match the calibration standard etched on the back of the gauge, they were rotated until they were aligned, thus zeroing the gauge (Figs. 3 and 4). The vertical heights on the condyles were then adjusted by alternately turning the left and right condylar height adjusting screws until the dials read zero (Fig. 5). Once the vertical calibration was set, the horizontal calibration was performed. The horizontal a d j u s t m e n t screws were loosened, and the upper member of the articulator was moved in two dimensions until the reticle and cross hairs, as viewed through the scope, overlapped. When calibrating the articulators, it was imperative for the centric holding spring on the D 5-A and the centric latch on the M a r k II to be in good working order. When these retainers were worn or not used, it was extremely difficult to maintain the condyles against the posterior and superior walls of the upper member of the articulator while making the adjustments with the field gauge.
Articulator testing After calibration, a mandibular cast was mounted in occlusion on an articulator where the maxillary cast was prepared for a split cast using "V" notches. 4, 5 An interocclusal record of the interocclusal space with no opposing tooth contact was made using Express (3M Co., St. Paul, Minn.), which is a vinyl siloxane impression material t h a t has been shown to be accurate. 6 The maxillary cast was separated
J U N E 1991
V O L U M E 65
NUMBER 6
ARTICULATOR I N T E R C H A N G E A B I L I T Y
F i g . 3. Posterior view of the Inspection gauge with calibration standards for articulator etched into back of gauge.
from the mounting stone and the casts were then transferred to other calibrated articulators. The mandibular cast was first attached to the lower m e m b e r of an articulator, the interocclusal record was positioned on the m a n d i b u l a r cast, and the maxillary cast was placed into the occlusal record. The split cast mounting attached to the upper m e m b e r of the articulator was then lowered into position until the "V" projections on the mounting were fully seated into the notches in the maxillary cast. This method of testing was appropriate because the split cast method is an acceptable method in prosthetic dentistry verifying jaw position records. It is a precise method when the interocclusal records are accurate and dimensionally stable. Articulator
evaluation
Articulator interchangeability was performed by three i n d e p e n d e n t observers and was recorded as a fit and successful transfer or as not fitting. A successful transfer of casts from one articulator to another d e m o n s t r a t e d a precise fit between the maxillary cast grooves and the projections of the split cast mounting. If the grooves and projections did not fit precisely, the transfer was not considered successful. Three evaluators, all of whom were dentists, rehearsed before the study using calibrated and uncalibrated articulators to recognize transfers of mounted casts t h a t did and did not fit. RESULTS
F i g . 4. Accuracy of the calibration dials on the sides of the upper scope member is confirmed by rotating them until they match the calibration s t a n d a r d etched on the back of the gauge (see Fig. 3).
AND
DISCUSSION
The evaluation of the 12 articulators indicated a 100% transferability among them. The results d e m o n s t r a t e d t h a t the calibrated articulators used in this study allowed the successful transfer of m o u n t e d casts without the loss of positional accuracy. In clinical terms, this system provides
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
F i g . 5. Vertical heights on the condyles are adjusted so the dials~ read zero.
an economical means for the use of fully adjustable articulators. :Furthermore, the use of a precise recording material, such as Express, enables both clinician and technician to check and corroborate articulator calibrations. This procedure is important, because calibrating a damaged articulator may create the false assumption of precise transferability when in fact there is none. Positional inaccuracy of the casts would not be apparent, which could result in a breakdown in communication between dentist and dental technician and lead to the inability to properly fill occlusal prescriptions. The need for like calibration of articulators and a simple means for checking such calibrations
847
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
