Denture base strength influence of its thickness surrounding magnetic
assembly -Examination by the model experimentsT. Kanazawa1, Y. Umekawa1,2, T. Ishii1, A. Tateno1, E. Nagai1,2, K. Ohtani1,2, K. Suda1
and T.Ishigami1,2
1

Department of Partial Denture Prosthodontics, Nihon University School of Dentistry
Division of Clinical Research, Dental Research Center, Nihon University School of Dentistry

2Introduction
Denture fracture is frequently reported in during long-term use of a magnetic overdenture. Thereare not so much studies about effect between thickness of denture base resin around magnetic
assembly and its fracture. 1) In this study the effect between thickness and strengths of denture basein overdenture was examined.

Objective
Experiments are performed using two types of bending test (modified 3-point bending test andmodified 4-point bending test) to simplified overdenture models.

Materials and Methods
1.

Materials
a. Coping models
Fig 1 shows the coping model made of the stainless steel (TOKYO GIKEN, Inc). The coping
models have two difference shape types. Model A has dome shape (dome) and model B has
trapezoidal shape (trapezoidal).
Dome is 6.0mm in diameter and 2.0mm in height. Trapezoidal is 6.0mm in the diameter of bottom,4.5mm in diameter of surface, and 2.0mm in height.

Fig1
b. Bending specimens
Rectangular solid bending specimens were fabricated by heat-curing resin (ACRON, GC). The
shape of specimens is 64.0 mm in width, 10.0 mm in length, and 2.5, 3.0 and 3.5mm in thickness(Fig 2,3). 2) Polymerized resin blocks were shaped by using semi-automatic polishing machine(Doctor Lap ML180, NARUTO) and silicon carbide paper.
According the shape and the number of concave portion, four specimen groups were fabricatedand named type A,B,C and D. Five specimens were randomly assigned to each group.
Type A had one hemisphere concave part in center. Type B had one concave frustum of circularcone part in center. Type C had two hemisphere concave surfaces in the position of 9.5mm and39.5mm from the left end of the major axis. Type D had two concave surfaces of circular cone partsin same position as type C.

Fig2

Fig3

2.

Methods
Each specimen used for the experiments was soaked in the water of 37℃ for 48 hours. Thebending test utilized a universal testing machine (EZ-test, SHIMADZU). The loading plunger
according to JIS T6501 was used for the experiments. 3) The experiments were conducted on twokinds (modified three-point bending test and modified four-point bending test). The modified
three-point bending test (3PB) was assumed for experiment 1, and the modified four-point bending
test (4PB) was assumed to experiment 2.
a.
Modified three-point bending test (3PB)
Type A or B was loading until fracture from both sides on a coping model. (Fig4) Coping model
attached to under part in EZ-test. Type A is on top of dome model. Similarly, type B is on top of
trapezoidal model. Loading plunger attached the crossing head part in EZ-test by the distance
30.0mm between fulcrums. Bending test was done a crosshead speed of 5.0 mm/min. Bending
strength of the specimen measured in the maximum loading point. And it was calculated the averagevalue.
Acquire data did statistical analysis by critical region 5%. It was used to two-way ANOVA andTukey-Kramer test for statistical analysis.
b.
Modified four-point bending test (4PB)
Type C or D was loaded until fracture on same kind of two coping models. (Fig5) On the rightcoping model it loaded from both sides. On the left coping model it loaded from one side only. It
was assuming unilateral free-end in this state. The bending test did at the distance between fulcrums
30.0mm and a crosshead speed of 5.0 mm/min. The statistical analysis was similar experiment 1.

Fig4

Fig5

Results
Fig 6 shows the result of experiment 1. Bending strength increased intentionally in both models oftype A and B whenever the thickness of the specimen increased. As a result of two-way ANOVA, it
was not admitted for the alternating cropping. Moreover, there was not a significant difference by thedifference to coping shape.
Fig 7 shows the result of experiment 2. It’s not fractured in 1.5mm (type C and D) in the loading
areas of EZ-test. Therefore, the value was not able to calculate. In the comparison between 0.5mm
and 1.0mm (type C and D), there were significant differences in the bending strength, more strength
for more thickness. As a result of two-way ANOVA, it was not admitted to the interaction. Moreover,there were significant differences to the coping shape.

Fig6

Fig7

Fig 8 shows to the appearance of fracture from the underside in 1.0mm specimen (type A, B, Cand D). In type A, It was shown that a lot of the fracture from the slightly position displaced to one
side from the deepest in concave portion. In type B, It was shown that a lot of the fracture occurredfrom the position that hit a near edge of concave portion. In type C and D, It was shown that there
was a lot of the fracture from the middle of both coping models. This corresponds to a part of the leftindenter of loading plunger.

Fig8Discussions
In this bending tests, the thickness of the resin plate is increased the flexural strength increases,
the difference between the coping no effect. In the case of one coping, many of the specimens werefracture in coping mounting portion. In the case of two copings, many of the specimens werefracture at the mid-point of the coping. This result is different from the clinical reports that often
fracture of the coping mounting portion.

Conclusions
This time of specimen, consideration by the three-dimensional finite element method is also based,
is a simplified shape. 4) For future, based on the review by this model experiment andthree-dimensional finite element method, it is necessary to consider the experiment in line with moreclinical. For fracture of resin base on coping, the studies are needed in terms of thickness.

References
1. N. Nozaki, Y. Tanaka, Y. Dezaki, et al: Effect of a Magnetic attachment on the Bending Strengthof the Acrylic Resin Denture Base, J J Mag Dent, 8(1), 63-68, 1999.
2. E. Kishimoto, K. Ogata, K. kawahara: The Effect of the Overdenture Abutment Teeth onToothbrushing : Evaluation with a Simulator, J Dent Hlth, 47, 132-138, 1997.
3. JIS T6501: Acrylic denture base resins, 2012.
4. M. Obayashi T. Ohyama, S. Nakabayashi, et al: The thickness of the resin base needed to the topof copings -Examination by the three-dimensional finite element method- ,2015