Aoki et al.

Int Chin J Dent 2005; 5: 39-42.

Bond strength of denture base resin to castable Fe-Pt magnets
Takayuki Aoki, DMD, PhD,a Chikahiro Ohkubo, DMD, PhD,a Hiroyuki Kouno, DMD,aToshio Hosoi DDS, PhD,a Yasuhiro Tanaka, PhD,b and Ikuya Watanabe, DDS, PhDc
aDepartment of Removable Prosthodontics, Tsurumi University School of Dental Medicine, Yokohama,bJapan, Department of Developmental and Reconstructive Medicine, Nagasaki University GraduatecSchool of Biomedical Sciences, Nagasaki, Japan, and Department of Biomaterials Science, Baylor
College of Dentistry, Texas A&M University System Health Science Center, Dallas, TX, USA
Purpose: The purpose of this study was to evaluate the bond strengths of denture base resin to a magnetic Fe-Pt
alloy using four adhesive metal primers.
Materials and Methods: Disk patterns (7.5 mm diameter, 2.0 mm thick) were cast from a custom-made Fe-Pt
ingot (Fe-69.4wt%Pt-0.6wt%Nb) in a high-frequency centrifugal casting machine. Aging at 600°C wasperformed for five hours to obtain hard magnetic properties. After the adhesive surfaces were sandblasted with
50 µm Al2O3, they were primed with Metal Primer II (MP), Meta Fast Bonding Liner (BL), New Meta Color
INFIS Opaque Primer (IN), or Epricord Opaque Primer (EP). Denture base resin (Palapress Vario) was thenpolymerized according to the manufacturer’s instructions. After the specimens were immersed in 37°C water for
24 hours (Baseline, TC0), the shear bond strengths (n=7) were measured at a crosshead speed of 0.5 mm/minuteafter 10,000 thermal cycles (TC 10,000 at 4°C to 60°C). The data were statistically analyzed by ANOVA/Scheffé’s test (α=0.05).
Results: MP and EP had significantly higher bond strengths (p<0.05) than BL and IN at TC0. Thermal cycling
significantly (p<0.05) reduced the bond strengths using all the primers tested. However, the bond strength of EP
was slightly less after thermal cycling than for the other three primers (MP, BL, IN).
Conclusion: The results of this in vitro study indicate that EP provides superior long-term bond strength when acast Fe-Pt magnet is embedded in denture base resin. (Int Chin J Dent 2005; 5: 39-42.)
Clinical Significance: The use of Epricord Opaque Primer is recommended to obtain long-term clinicalreliability when the denture base resin is bonded to cast Fe-Pt magnet.
Key Words: cast Fe-Pt magnet, denture base resin, metal primer, bond strength.

Introduction
Dental magnetic attachments are frequently used to retain dental and maxillofacial prostheses1,2 because of
their excellent attractive force.

Most commercial dental magnetic attachments utilize rare-earth Fe14Nd2B

magnets, which is the strongest type of magnet with excellent magnetic properties (hard magnetic properties)
such as high maximum energy product and high coercive force (strength of magnet).3-5 Since the corrosion
resistance of Fe14Nd 2B magnets is very low, they must be covered with a corrosion-resistant magnetic stainlesssteel yoke cap.6 Recently, castable Fe-Pt magnetic attachments with high corrosion resistance were developed7,8
and clinically applied in sectional or collapsible removable dentures for patients with microstomia.9,10 These
reports indicated favorable results because of the custom-cast magnetic attachments made for individual patients.
In these cases, the Fe-Pt magnet or keeper was embedded in the denture base resin without a metal primer.
Although primers had the potential to enhance long-term clinical reliability, there was a little information about
them. Ohkubo et al.11 investigated the bond strengths of laboratory prosthetic composite to magnetic Fe-Pt alloy
using several metal primers. They reported that primers containing 10-methacryloxydecyl dihydrogen phosphate(MDP) and methacryloyloxyalkyl thiophosphate derivative (MEPS) were effective at obtaining high bondstrength and durability.
To ensure reliable prostheses without adhesive failure and microleakage, it is important to obtain a strong
bond between the denture base resin and the Fe-Pt magnetic alloys. This study examined the shear bond strength39

Aoki et al.

Int Chin J Dent 2005; 5: 39-42.

of a denture base resin to cast custom-made Fe-Pt magnets using several metal primers.

Materials and Methods
The materials used in this study are listed in Table 1. Plastic disk-shaped patterns (7.5 mm diameter, 2 mmthick) were prepared for casting a custom-made Fe-Pt magnetic alloy. The disk patterns were invested in moldrings with a magnesia-based investment material (Titavest CB, Morita, Japan). The burn-out procedure wasaccomplished according to the manufacturer's instructions, and the patterns were then cast from a custom-madeFe-Pt ingot (Fe-69.4wt%Pt-0.6wt%Nb) in a high-frequency centrifugal casting machine (Eagle; Jelenko,Armonk, NY, USA). After casting, the disks were vacuum-sealed in quartz-glass tubes and heat-treated at1,325°C for 45 minutes, and then quenched in ice water. Aging at 600°C was subsequently performed for 5
hours to obtain hard magnetic properties. Then the surfaces of the cast Fe-Pt alloy disks were polished with upto No. 400 SiC abrasive paper under water.
Table 1.

Materials used.

Material/Trade name CodeManufacturer

Code Composition

Magnetic Fe-Pt alloy

Custom-madeIshifuku Metal Industry, Tokyo, Japan
Nilaco, Tokyo, Japan
Hirano-Seizaemon, Tokyo, Japan

Fe-69.4wt%Pt-0.6wt%Nb
Pt: 99.5% pure
Fe: 99.99% pureNb: 99.9% pure

Denture base resin
Palapress Vario

Heraeus Kulzer, Inc. Hanau, Germany

Poly(methyl methacrylate)Primer
Metal Primer IIMeta Fast Bonding Liner
New Meta Color INFIS Opaque Primer
Epricord Opaque Primer

GC, Tokyo, JapanSun Medical, Moriyama, JapanSun Medical
Kuraray, Osaka, JapanMPBLIN
EP

MEPS
4-META
VTD
MDP

MEPS, Methacryloyloxyalkyl thiophosphate derivative; 4-META, 4-methacryloxyethyl trimellitate anhydride;
VTD, 6-(4-vinylbenzyl-n-propyl) amino-1,3,5-triazine-2,4-dithiol; MDP, 10-methacryloxydecyl dihydrogen phosphate.

The adhesive surfaces were sandblasted with 50 µm Al2O3 for 10 s at an emission pressure of 0.28 MPa with
the nozzle positioned approximately 10 mm away from the surface of the metal adherend. After the bonding
surfaces were air-abraded, they were primed with one of four primers: Metal Primer II (MP), Meta Fast Bonding
Liner (BL), New Meta Color INFIS Opaque Primer (IN), or Epricord Opaque Primer (EP) (Table 1). Thebonding area was defined by placing a piece of double-stick tape with a 4.8 mm diameter hole on the bonding
surface of each cast disc and then positioning a Teflon ring (2 mm thick) with a circular hole (6 mm in diameter)over the tape. Autopolymerizing PMMA resin (Palapress Vario, Heraeus Kulzer, Inc.) was poured inside theTeflon ring.

Polymerization was performed according to the manufacturer’s instructions, namely, in apressure-curing unit (Palamat, Heraeus Kulzer, Inc.) at 55°C and 200 kPa for 15 minutes. After removing theTeflon ring, all specimens were stored in water at 37°C for 24 hours (Baseline, TC0); they then underwent
10,000 thermal cycles (TC10,000) in water between 4°C and 60°C with a 1-minute dwell time. As a control,non-primed specimens were fabricated in the same manner. The shear bond strengths were measured on ascrew-driven mechanical testing machine (Model 5565, Instron Corp., Canton, MA, USA) at a crosshead speedof 0.5 mm/minute. The means and standard deviations of shear bond strength (n=7) were calculated andstatistically analyzed by ANOVA and the Scheffé’s test at a significance level of α=0.05.

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Int Chin J Dent 2005; 5: 39-42.

Results
Table 2 shows the shear bond strength results. MP and EP had significantly higher bond strengths (p<0.05)
than BL and IN at TC0. IN had the lowest bond strength among the other three primers, and there were no
significant differences (p>0.05) in bond strengths at TC0 without a primer (control).
significantly (p<0.05) reduced the bond strengths using all the primers tested.

Thermal cycling

However, the EP-primed

specimens had higher durability (only 16% decrease) after thermal cycling than did the other three primers (MP,BL, IN).
Table 2.

Mean shear bond strengths (MPa) of denture base resin to primed Fe-Pt magnet surfaces.

Primer (Functional monomer)

TC 0 (Baseline)
Mean SD Grouping

TC 10,000 (Thermocycled)
Mean SD Grouping

Control (No treatment)MP(MEPS)
BL(4-META)
IN(VTD)
EP(MDP)

4.1
17.6
10.8
4.3
16.6

1.1
10.4
1.7
0.8
13.9

0.8
0.8
3.1
0.9
1.6

cabca

1.0
2.2
0.9
0.5
2.2

cbcca

Decrease (%)
74
4184
81
16SD, One standard deviation. Similar lowercase letters indicate statistically equivalent values (Scheffé’s test, p>0.05).
MP, Metal Primer II; BL, Meta Fast Bonding Liner; IN, New Meta Color INFIS Opaque Primer; EP, Epricord Opaque Primer; TC,Thermal cycles.

Discussion
To investigate the bond strength and durability of denture base resin bonded to cast Fe-Pt magnetic alloy, thisstudy evaluated four metal primers with individual functional monomers (Table 1) purported by eachmanufacturer to react with precious and/or nonprecious metals. The Fe-Pt magnet used in the present study
contains a large amount of precious Pt (69.4 %) and nonprecious Fe (30%). The bond strength and durability ofdenture base resin bonded to cast Fe-Pt magnetic alloy was greater when MP and EP primers were used, and EP
in particular had the greatest durability (16% decrease). MP contains MEPS, which was intended for bonding toboth precious and nonprecious metals. EP contains MDP and was developed for bonding to nonprecious metals.
Our results were in agreement with a previous study by Ohkubo et al.11 who reported that the MDP- andMEPS-containing primers were effective at promoting high bond strength and durability of a prosthetic
composite bonded to Fe-Pt magnetic alloy. These primers might be effective at increasing the bond strength ofdenture base resin to Fe-Pt alloy surfaces partially covered with iron oxides. Although BL contains 4-META,
which was also intended for bonding to nonprecious metals, the bond strength and durability of specimens
primed with BL were not as good as those primed with EP (MDP) primer. Therefore, a MDP-containing primer
is more clinically reliable than a 4-META-containing primer when they are applied to a cast Fe-Pt magnet before
the denture base resin is bonded to the alloy. The bond strength of the specimens primed with IN was the lowest
at both baseline (TC0) and 10,000 thermocycles (TC 10,000). The IN used in this study contains an adhesivemonomer of VTD, which is claimed by the manufacturer to react with the gold in precious metal alloys such as
Type IV gold alloy and gold/silver/palladium alloys. Since IN was not an effective primer for the Fe-Pt magnets,
which contain a large amount of precious metal (69.4% Pt), the difference in the elemental composition of thealloy (gold/silver/palladium vs. platinum) may be one reason for the low bond strength of the denture base resin
to the Fe-Pt alloy.

41Aoki et al.

Int Chin J Dent 2005; 5: 39-42.

Conclusion
Of the metal primers employed in this study, Epricord Opaque Primer produced the greatest bond strength
both pre- and post-thermocycling of a denture base resin to cast Fe-Pt magnetic alloy.

Acknowledgment
This investigation was supported in part by Research Grant No. 15101 from the Magnetic Health Science Foundation,
Tokyo, Japan and by Grant-in-Aid 14370640 from the Japan Society for the Promotion of Science. Editorial assistance by
Mrs. Jeanne Santa Cruz is greatly appreciated.

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Mater 2001; 17: 217-20.
10. Watanabe I, Tanaka Y, Ohkubo C, Miller A. Application of cast magnetic attachments to sectional complete dentures for apatient with microstomia: A clinical report. J Prosthet Dent 2002; 88: 573-7.
11. Ohkubo C, Watanabe I, Tanaka Y, Hosoi T. Application of cast iron-platinum keeper to a collapsible denture for a patient withconstricted oral opening: A clinical report. J Prosthet Dent 2003; 90: 6-9.
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Reprint request to:
Dr. Ikuya Watanabe
Department of Biomaterials Science
Baylor College of Dentistry, Texas A&M University System Health Science Center
3302 Gaston Ave., Dallas, TX 75246, USA
Fax: +1-214-370-7001 E-mail: iwatanabe@bcd.tamhsc.edu
Received May 26, 2005. Revised June 7, 2005. Accepted June 8, 2005.
Copyright ©2005 by the Editorial Council of the International Chinese Journal of Dentistry.

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