Fixation of modified magnet assembly to denture base using alternative resins
S. Okayama, H. Shimpo, Y. Suzuki, C. Ohkubo
Department of Removable Prosthodontics, Tsurumi University School of Dental Medicine

Introduction
Magnetic attachments have been widely used as stud attachments for root- and implant-retained
overdenture rehabilitation. In general, the magnetic assembly has been directly fixed to the denturebase with autopolymerized polymethyl methacrylate (PMMA) resin using the brush-on techniqueafter the magnetic assembly is placed on the keeper of the abutment tooth or implant. However,special care must be taken during the fixation because the denture may become impossible toremove from the abutment teeth or implant due to the PMMA resin’s hardening within the undercut
around the keeper. In this study, the magnetic assembly was modified to add several undercuts toimprove mechanical retention.
Objective
The aim of this study has been to investigate the fixation strengths of magnetic assemblies todenture bases using alternative resins rather than conventional PMMA autopolymerized resin
without housing.
Materials and Methods
To evaluate the effectiveness of mechanical retention in this study, a commercially available
magnetic assembly (PHYSIO MAGNET 35, Neomax, Gunma, Japan; diameter: 3.5 mm; thickness:
0.8 mm; attractive force: approximately 5.5 N) was modified by adding three different undercut
wings [wing diameter (undercut): 4.5 mm (0.5 mm), 4.8 mm (0.65 mm), and 5.5 mm (1.0 mm)]. A
conventional magnetic attachment (PHYSIO MAGNET 35, Neomax, Gunma, Japan) of the same
size without undercut wings was compared as a control (Fig. 1).
The materials selected for the retaining magnet as fixation material, an experimental resin (70%Polyethylene

glycoldimethacrylate

23G

and30%

MMAinthemonomer,

20%

Polybuthylmethacrylate and 80% PMMA in the polymer), a temporary filling resin (Dura Seal,Reliance Dental Mfg Co., Worth, IL, USA), and an autopolymerized PMMA resin (UNIFAST III,GC Corp., Ltd, Tokyo, Japan) were used. Magnetic assemblies were bonded to the lower jig using acyanoacrylate adhesive (ARON ALPHA, Toagosei Co., Ltd, Tokyo, Japan) for tensile testing. For
testing repeated insertion/removal, the keeper was mounted in the lower jig, and the magnetic

assembly was placed on the keeper without a cyanoacrylate adhesive. After the polymers andmonomers of the fixation materials were mixed, they were applied to the magnetic assembly andpoured into the housing in the upper jig (Fig. 2). Tensile testing was performed to evaluate thefixation strength and the attractive force of magnetic assembly using resins after repeatedinsertion/removal testing up to 10,000 cycles (Fig. 3, 4). The tensile strengths were measured using
an autography at a crosshead speed of 1.0 mm/min. The data of fixation strengths and attractive
forces were analyzed using a two-way ANOVA, Tukey’s multiple comparison and t-test (α=0.05).
Modified magnetic assemblyControl

PHYSIO MAGNET 35

3.5 mm

3.5 mm

4.5 mm, 4.8 mm, 5.5.mm
Undercut(0.5 mm, 0.65 mm, 1.0 mm)

Fig. 1

Magnetic assemblies modified by adding three different undercut wings[wing diameter (undercut): 4.5 mm (0.5 mm), 4.8 mm (0.65 mm), and 5.5 mm (1.0mm)]

Resin housing
Fixation material
Magnetic assembly
KeeperCyanoacrylate adhesive

For tensile testingFor insertion/removal testingFig. 2 Fixation of magnetic assembly for each test

Fig. 3

Autography used for tensile testingFig. 4Insertion/removal testing machine

Results
Figure 5 shows the fixation strengths of magnetic assemblies using permanent fixation material,both initially and after 10,000 cycles. The fixation strengths of a temporary filling resin (Dura Seal)
with a 4.5-mm undercut wing and conventional PMMA resin could not be measured because themagnetic assembly was separated from the lower jig without failure between the magnetic assembly
and the fixation resin.
Figures 6 and 7 show that the change in the attractive forces of the magnetic attachments whenusing fixation materials with and without the undercut wing, respectively. Without the undercut
wing, the magnetic assembly was removed in one of 5 experimental resin specimens. There were no
significant differences in the attractive forces with and without undercuts (p>0.05). Although theinitial attractive force of the experimental resin was similar to the others both with and without theundercut wing, a remarkable decrease was shown at 1,000 cycles, keeping constant attractive forcesat 2,000 cycles.

Fig. 5 Fixation strengths of magnetic assemblies using fixation materials

Fig. 6The change of attractive forces to magnetic assembly without
undercut using fixation materials

Fig. 7The change of attractive forces to magnetic assembly with 4.5-mm
undercut using fixation materials
Discussion

Regarding the experimental resin, polyethylene glycol (PEG) dimethacrylate 23G andpolybuthylmethacrylate were prepared for flexibility and expanded the polymerization time. The
magnetic assembly was removed from the housing in one of five specimens that used theexperimental resin during insertion/removal cycles of up to 10,000. Thus, the mechanical property ofthe experimental resin should be improved for rigid fixation if the experimental resin is used as apermanent fixation material. The fixation strengths of magnetic assemblies using temporary filling
resin, even without undercut, showed approximately 50 N even after 10,000 insertion/removalmotions. Until 10,000 insertion/removal motions, temporary filling resin and conventional PMMA
resin demonstrate a constant attractive force (approximately 4 to 5 N) without removing themagnetic assembly. In conclusion, using a temporary filling resin can be recommended as apermanent fixation material similar to conventional PMMA resin.
Conclusions
By adding undercut wings to the magnetic assembly, fixation strengths tended to be increased
when fixation materials were used. Although the experimental resins demonstrated satisfactory
fixation strengths, improving the mechanical strengths is necessary because the magnetic assembly
was removed from the housing in one of five specimens. Temporary filling resin showed comparable
fixation strengths to conventional resins; thus, they could be used as permanent fixation materials formagnetic attachments used in this study.