Chen et al.

Int Chin J Dent 2005; 5: 7-11.

Effect of surface treatment with commercial primers on tensile
bond strength of auto-polymerizing resin to magnetic stainlesssteel
Jian-rong Chen, DDS,a,b Kenji Oka, DDS, PhD,c Wei Hua, DDS,a and Tetsuo Ichikawa,DDS, PhDc
ab

The University of Tokushima Graduate School of Oral Science, Tokushima, Japan, Tonji University,cSchool of Oral Stomatology, Shanghai, P. R. China, and Department of Oral & Maxillofacial
Prosthodontics and Oral Implantology, The University of Tokushima Graduate School, Institute ofHealth Biosciences, Tokushima, Japan
Purpose: It often happens that magnets clinically detach from denture bases. To establish the appropriate
laboratory procedure to fix the magnet in the denture base, the tensile bond strength of magnetic steel inauto-polymerizing resin was measured under several conditions.
Materials and Methods: Magnetic stainless steel cylinders (AUM 20) of 4 mm diameter were prepared asspecimens. The cross section of the specimens was ground with silicon-carbide abrasive paper (#800) to create abondable surface. After grinding, half of the specimens were treated by air abrasion with 50 µm alumina. Bothsurfaces were treated with one of the three commercial primers: Metal Primer II (MPII), Metal Link Primer(MLP), and Meta Fast Bonding Liner (MFL). Auto-polymerizing resin was poured on the bonded surfaces ofthe metal. The tensile bond strength between the metal and resin was measured after the specimens were storedin water at 37˚C for 24 hours. Thermal cycle examinations, with immersion alternately in water baths at 4˚C and60˚C for 1 minute up to 2,000 times were conducted.
Results: The bond strength of sanded surfaces with primer treatment varied from 21.6 to 32.2 MPa and the value
increased significantly when primer treatments other than MPII were used. The bond strength of air-abradedsurfaces with primer treatment varied from 31.1 to 38.3 MPa and the value increased significantly with allprimer treatments. Bond strengths of all groups were significantly reduced by the application of thermal cycling(p<0.05). The bond strength of air-abraded surfaces with primer treatment varied from 3.6 to 4.2 MPa and thebond strength was significantly (p<0.05) larger than those in other groups.
Conclusion: The combination of air abrasion and primer application is the most effective procedure to fix themagnet. (Int Chin J Dent 2005; 5: 7-11.)
Clinical Significance: To solve the clinical problem of magnetic detachment, enhancement of bond strength isnecessary.
Key Words: air abrasion, magnetic attachment, magnetic stainless steel, primer, tensile bond strength.

Introduction
In the prosthodontic field, magnetic attachments that help to retain overlay dentures are being widely used andare becoming popular in the Asian and European regions.1 This device was originally used as a stud attachment
on the root cap and is becoming more widely applicable to dental implants and extracoronal attachments.2 Asianpeople in particular have poor bone morphology and magnetic attachments are very useful for
implant-supported/retained overlay dentures.
Magnetic attachments have several advantages: ease of dental laboratory and clinical work, and stability ofretention force. Magnetic attachments consist of two parts: a rare earth magnet covered by stainless steel and amagnetic steel keeper. The keeper is fixed to an abutment coping using either cast-to technique, cementing orwelding.3 The magnet is embedded in the denture base using auto-polymerizing resin in the mouth. However
the magnet often detaches from the denture base because of low bond strength between the resin and themagnetic stainless steel. Although various improvements have been reported: application of an undercut on themagnet, surface treatment of the magnet, and primer application on the magnet surface,4 limited information isavailable concerning bonding procedure between the magnet and denture base, especially as related to chemical

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Int Chin J Dent 2005; 5: 7-11.

bonding primers. In addition, the magnetic stainless steel around rare earth magnet, has also been improved
from SUS 316L (16%Cr-12%Ni-2%Mo-remaining Fe) to AUM 20 (19%Cr-2%Mo-0.2%Ti-remaining Fe).5 The
purpose of the present study was to evaluate bonding strength using commercial bonding primers on surface
treatments and to clarify the appropriate clinical procedure for fixing magnet to the denture base.

Materials and Methods
Magnetic stainless steel cylinders (AUM 20, 19%Cr-2%Mo-0.2%Ti-remaining Fe, Aichi Steel, Tokai, Japan)of 4 mm diameter were prepared as specimens. This material is the same material as the surface material of themagnet in the commercial magnetic attachment.
Table 1. Primers used in this study.
Trade name

Manufacturer

Lot number

Metal Primer II
Metal Link
Meta Fast Primer

GC Corp., Tokyo, Japan
0211131
Shofu Inc., Kyoto, Japan080304
Sun-Medical Co., Ltd., Moriyama, Japan 30301

Adhesive monomer

Abbreviation

MEPS
10-MDDT, 6-MHPA
4-META

MPII
MLP
MFL

MEPS, methacryloyloxyalkyl thiophosphate derivative; 10-MDDT, 10-methacryloxydecyl 6,8-dithiooctanate;
6-MHPA, 6-methacryloxyhexyl phosphonoacetate; 4-META, 4-methacryloyoxyethyl trimellitate anhydride.

Fig. 1.
Diagram of tensile bond testing.

As a bonded surface the cross section of the specimen was ground with 800 grit silicon-carbide abrasive paper.
After grinding, half of the specimens were treated by air abrasion with 50 µm alumina. The air pressure was 0.4
MPa and the distance of the nozzle from the bonded surface was 20 mm. Both surfaces were treated by one of
the three commercial primers: Metal Primer II (MPII, GC, Tokyo, Japan), Metal Link Primer (MLP, Shofu,Kyoto, Japan), and Meta Fast Bonding Liner (MFL, Sun-Medical, Moriyama, Japan). Information on theprimers is summarized in Table 1. Auto-polymerizing resin (Provinice, Shofu) was poured on the bonded
surfaces of the specimens surrounded by plastic tape as shown in Fig. 1. The specimens were stored in water at
37˚C for 24 hours. The tensile bond strength was measured using a universal testing machine (AG-100A,Shimadzu, Kyoto, Japan) at cross head speed of 0.5 mm/minute before and after thermocycling (cycled between
4˚C and 60˚C water with a 1-minute dwell time per bath for 2,000 cycles). Five test pieces were prepared ineach condition.
Bonded and debonded surfaces of specimens were also examined using a scanning electron and stereoscopic
microscopes. The values of each group were compared by analysis of variance (ANOVA) and Duncan multiple
range interval at 0.05 level.

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ResultsFig. 2 shows the bonded surface of the magnetic stainless steel by scanning electron microscope. Simple
continuous scratch lines were observed on the specimen surface before air abrasion, whereas more complicatedroughness was in the specimens after air abrasion.

Fig. 2. Pre-thermocycling bond strength.

a: sanding with #800 SiC paper

Fig. 3. Post-thermocycling bond strength.

b: air-abrasion with 50 µm alumina

c: alumina abrasion + priming (MPII)

Fig. 4. Scanning electron micrographs of the AUM 20 steel after surface preparations.
Fig. 3 shows the tensile bond strength between the magnetic stainless steel (AUM 20) and auto-polymerizing
resin before thermocycling. The tensile bond strength in air abraded surfaces without primer treatment was 19.9
MPa and the value was not significantly different from that without both air abrasion and primer treatment. The
bond strength of sanded surfaces with primer treatment varied from 21.6 to 32.2 MPa and the value increased
significantly for primers other than MPII. The bond strength of air abraded surfaces with primer treatment
varied from 31.1 to 38.3 MPa and the value increased significantly with all primer treatments.
Fig. 4 shows the tensile bond strength between the magnetic stainless steel (AUM 20) and theauto-polymerizing resin after thermocycling. Bond strengths of all groups were significantly reduced by theapplication of thermal cycling (p<0.05).

The bond strength of bonded surfaces with and without primer

treatment varied from 0 to 0.78 MPa and the bond strength was reduced by thermal stress. The bond strength of
air abraded surfaces with primer treatment varied from 3.6 to 4.2 MPa and the bond strength was significantly(p<0.05) larger than those in the other groups.
Cohesive failure regions were observed on a large area of debonded surfaces in most specimens before
thermocycling, whereas no cohesive failed resin regions were observed in specimens without primers afterthermocycling. Cohesive failure regions were observed on a part of debonded surface in specimens with air

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Int Chin J Dent 2005; 5: 7-11.

abrasion and primer treatment after thermocycling.

Discussion
It is a conventional procedure to fix magnets to denture bases in the mouth using auto-polymerizing resin (Fig.
5). This procedure is easy to fix and it is possible to position the magnet into the denture base with minimum
error. There are two categories of magnet fixation method: the chemical method and the mechanical method.
Bond strength of magnets either with a projection of 0.2 mm or with reverse tapered form of 15 degree in themechanical method shows high values of 186.82 and 353.92 N, respectively. Although these in vitro values are
satisfactory, it often happens in the long term that the magnet detached from the denture base due to thehygrothermal effect of the denture resin. In addition, it is important to enhance the bond strength between themagnet and denture base to prevent a fracture of the denture base.

a: bar-type attachment of implants

b: extracoronal attachment

Fig. 5. Application of magnetic attachment. Arrows; magnetic keepers. Arrowheads; magnets.
A push out test is commonly used as an evaluation of the bond strength between the auto-polymerizing resin
and the magnet, because this test simulates a clinical situation. In the present study, the tensile bond strength test
was used to clarify the influence of a chemical primer on bond strength.
In the literature, air abrasion with alumina has been mentioned as a standard laboratory procedure to enhancebond strength between resin and metal.6 However the present results show that no difference between air
abrasion and paper abrasion were found. In pouring auto-polymerizing resin onto the bonded metal surface, airabrasion treatment affects the bond strength less than the paper abrasion treatment.
On the other hand, it is well known that metal primer chemically enhances bond strength.7 Although the same
result was shown in the present study, it is reported that the effect of MEPS application has about 40-50 MPa
after thermocycling,8 i.e., thermal stress drastically decreased the strength except for those specimens that
underwent air abrasion treatment.

Thermal cycling revealed that the reduction in bond strength between

stainless steel (SUS 316) and auto-polymerizing resin after 20,000 cycles ageing was significant for all
specimens and the rate of reduction varied from 65.2% to 100%.5 The combination of air abrasion and primer
application is the most effective procedure to fix the magnet. Thermal cycling stress reduced less the bondstrength of specimens using both air abrasion and primer together less than any other procedure.
The mechanical effect of air abrasion with alumina is produced by the following factors: cleaning of bonded
surface, increase of bonded area, and interlocking of rough surface. The surface produced by abrasive paper ismonotonically scratched, whereas the surface produced by air abrasion with alumina is rough with complicated10Chen et al.

Int Chin J Dent 2005; 5: 7-11.

morphology. The mutually potential effect of this surface morphology with primer application may prevent theresin detaching from the metal surface.

Conclusion
It was concluded that the alumina air abrasive treatment with primer was effective for strengthening the tensile
bond strength of magnetic assemblies to auto-polymerizing resin.

References
1. Riley MA, Walmsley AD, Harris IR. Magnets in prosthetic dentistry. J Prosthet Dent 2001; 86: 137-42.
2. Walmsley AD, Frame JW. Implant supported overdentures -the Birmingham experience. J Dent 1997; 25 Suppl 1: S43-7.
3. Ishikawa M, Kashiwabara T, Ishida O, Ichikawa T. Installing magnetic keepers using LASER welding. J Prosthodont 2002;
11: 49-52.
4. Nakamura Y, Tanaka Y, Ishida T, et al. Experimental research on retention of magnetic assemblies to denture bases. Jpn
Soc Magnetic Appl Dent 1998; 7: 52-8.
5. Matsumura H, Tanaka T, Atsuta M. Effect of acidic primers on bonding between stainless steel and auto-polymerizing
methacrylic resins. J Dent 1997; 25: 285-90.
6. Kim-Hai N, Esquivel-Upshaw J, Clark AE. Surface treatments to improve bond strength in removable partial denture. GenDent 2003; 51: 402-4.
7. Yoshida K, Taira Y, Sawase T, Atsuta M. Effects of adhesive primers on bond strength of self-curing resin to cobalt-chromium
alloy. J Prosthet Dent 1997; 77: 617-20.
8. Yoshida K, Atsuta M. Effects of adhesive primers for noble metals on shear strengths of resin cements. J Prosthet Dent 1997;
25: 53-8.

Reprint request to:
Dr. Jian-rong Chen
Department of Oral & Maxillofacial Prosthodontics and Oral Implantology
The University of Tokushima Graduate School of Oral Science
3-18-15, Kuramoto, Tokushima 770-8504, Japan
Fax: +81-88-633-7461 E-mail: cjr@dent.tokushima-u.ac.jp
Received November 30, 2004. Revised December 10, 2004. Accepted December 20, 2004.
Copyright ©2005 by the Editorial Council of the International Chinese Journal of Dentistry.

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