Long-term fixation of a modified magnet assembly to the denture baseusing soft resins
S. Okayama, Y. Suzuki, H. Shimpo, C. Ohkubo
Department of Removable Prosthodontics, Tsurumi University School of Dental Medicine
Abstract
Introduction:
Special care must be taken during the fixation of magnetic assembly because the denture may become
impossible to remove from the abutment teeth or implant due to the PMMA resin’s hardening within theundercut around the keeper. The aim of this study was to investigate the durability of fixation strengths andattractive forces of magnetic assemblies to denture bases using soft resins for long term use.
Materials and Methods:
Magnetic assemblies with different undercut and three resins (experimental resin, temporary filling resin,and conventional PMMA resin) as fixation materials to magnetic assembly were prepared in this study.
Tensile testing was performed to evaluate the durability of fixation strength and the attractive force ofmagnetic assembly after repeated insertion/removal fatigue testing. The data of fixation strengths andattractive forces were analyzed using a two-way ANOVA, Tukey’s multiple comparison and t-test(α=0.05).
Results and Discussion:
Magnet assembly was detached using experimental resin during fatigue testing. Temporary filling resin andconventional PMMA resin demonstrated constant attractive forces without removing the magnetic
assembly after repeated insertion/removal fatigue testing until 50,000 cycles. When PMMA resin andtemporary filling resin integrated to modified magnet assembly, it might be suggested to less detachable
from denture bases for long term.

Introduction
The magnetic assembly has been directly fixed to the denture base with autopolymerized polymethyl
methacrylate (PMMA) resin using the brush-on technique after the magnetic assembly was placed on thekeeper of the abutment tooth or implant. However, special care must be taken during the fixation because
the denture may become impossible to remove from the abutment teeth or implant due to the PMMA
resin’s hardening within the undercut around the keeper. Modified magnetic assembly and new fixationmaterials for wearing magnetic assembly to the denture base have been investigated to resolve the serious
problems. In this study, modified magnetic assembly and fixation materials were evaluated by fatigue
testing for their durability.

Objective
The aim of this study was to investigate the durability of fixation strengths and attractive forces ofmagnetic assemblies to denture bases using many types of fixation materials.
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 undercut wing (wing diameter [undercut]:4.5 mm [0.5 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).

Modified magnetic assemblyControl

PHYSIO MAGNET 35

3.5 mm

4.5 mm

3.5 mmUndercut(0.5 mm)
Fig. 1

Magnetic assemblies modified by adding undercut wing
(wing diameter [undercut]: 4.5 mm [0.5 mm])

An experimental resin (70% polyethylene glycol dimethacrylate 23G and 30% MMA in the monomer,
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, GCCorp., Ltd., Tokyo, Japan) were selected as fixation materials. Magnetic assemblies were bonded to thelower jig using a cyanoacrylate adhesive (ARON ALPHA, Toagosei Co., Ltd., Tokyo, Japan) for tensile
testing (Fig. 2). For fatigue testing repeated insertion/removal, the keeper was mounted in the lower jig, andthe 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 and poured
into the housing in the upper jig (Fig. 2). Tensile testing was performed to evaluate the fixation strengthand the attractive force of the magnetic assembly using resins after repeated insertion/removal fatigue
testing up to 50,000 cycles (Figs. 3 and 4). Tensile strengths were measured by an autography (EZ-S 200N,Shimadzu, Kyoto, Japan) 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 a t-test (α=0.05).

Resin housing
Fixation material
Magnetic assembly
KeeperCyanoacrylate adhesive
For tensile testingFig. 2

Fig. 3For insertion/removal fatigue testing
Fixation of the magnetic assembly for each test

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

Results
Figure 5 shows the fixation strengths of magnetic assemblies using three fixation materials, both initialand after 50,000 cycles. Fixation strength of temporary filling resin indicated appropriate potential withslight decrease. Modified magnetic assembly 4.5 mm was detached from lower jig and fixation strength
data could not be measured.
Figures 6 and 7 show the changes in the attractive forces of the magnetic attachments when using
fixation materials with and without the undercut wing, respectively. Experimental resins were detachedfrom magnet assembly during fatigue testing. With and without the undercut wing, attractive forces of both
temporary filling resin and conventional PMMA resin showed the sufficient attractive forces (4-5 N) andthere were no significant differences between control and 4.5 mm undercut after repeated insertion/removal
fatigue testing until 50,000 cycles (p>0.05).

Fig. 5

Fig. 6

Fixation strengths of magnetic assemblies using fixation materials

Changes of attractive forces to the magnetic assembly without
undercut (Control)Fig. 7

Changes of attractive forces to the magnetic assembly with a 4.5-mm
undercut

Discussion
Regardingtheexperimental

resin,

polyethylene

glycol(PEG)

dimethacrylate

23G

andpolybuthylmethacrylate were influenced for flexibility and expanded the polymerization time. All themagnetic assemblies were removed from the housing during up to 50,000 insertion/removal cycles. Thus,
the mechanical property of the experimental resin should be improved for rigid fixation. The fixation
strengths of magnetic assemblies using temporary filling resin without undercuts, showed approximately 50
N after 50,000 insertion/removal motions. Temporary filling resin and conventional PMMA resin
demonstrated a constant attractive force (approximately 4 to 5 N) without removing the magnetic assembly.
From the results of fatigue test, the try to use of temporary filling resin would be recommended as apermanent fixation material similar to conventional PMMA resin.

Conclusions
Using experimental resin, magnet assembly was detached during fatigue testing, its mechanicalproperty should be improved. When temporary filling resin was integrated to the magnetic assembly with
undercut, it might be suggested the less detachment from denture base for long term similar to using ofconventional PMMA resin.

References
1. Okayama S, Suzuki Y, Shimpo H, Ohkubo C: Fixation of magnet assembly to denture base usingalternative resins, Dent Mater J, 34(3), 364-70, 2015.
2. S. Okayama, H. Shimpo, Y. Suzuki, C. Ohkubo: Fixation of a modified magnet assembly to denturebase using alternative resins, J J Mag Dent, 24(2), 23-27, 2015.