Retentive forces and displacement of new stress breaking
magnet attachment
Mana Torii, Takuya Waki, Daisuke Ozawa, Yasunori Suzuki,
Chikahiro Ohkubo
Dept. of Removable Prosthodontics, Tsurumi Univ. Sch. of Dent.
Med.
Introduction
Numerous studies with reliable results on attachment systems
for implant overdentures in the mandible and maxilla have been
published. Most attachments allow for rotational excursion, butcannot compensate for vertical displacement under the functionalforces. There are extraordinary differences in settling under achewing load between the implant and mucosa under the denturebase. In addition, horizontal forces and rotational excursion are
also applied to the implants depending on the occlusal contact,location, and numbers of implants in the dental arch. Therefore,
excessive and harmful occlusal forces would be applied to theimplants. To protect implants from excessive forces, a few
stress-breaking attachments have been manufactured.
Objective
This study evaluated the retentive forces and displacement ofstress-breaking attachments after repeated loads simulating
masticatory function.

Materials and MethodsSchema of Stress-Breaking Magnetic (SBM) attachment is shown inFig. 1. The SBM attachment consists of housing unit including thesteel spring (SUS304: height 2 mm) and magnetic assembly (HyperSlim 3513, NOMAX, diameter: 4 mm undercut: 1 mm), can provide
the displacement of 0.5 mm.

Fig. 1 Schema of SBM attachent

Four stress-breaking attachments (Fig. 2), namely, a self-adjustment
type magnetic attachment (Magfit SX, Aichi Steel), a cushion typemagnetic attachment (Magfit IPS, Aichi steel), a locator attachment(Locator, ZEST anchors), three types of stress breaking ball (SBB)
attachments (amount of displacement: 0.3 mm, 0.5 mm, 0.7 mm, GCcorp.), were placed on the implants. The implants were embedded in aresin block using autopolymerized resin (Fig. 3). To simulate thechewing cycles, a load of 5 kgf was repeatedly applied up to 50,000
cycles using a loading apparatus (Fig. 4). The retentive force wasmeasured by means of tensile testing at a crosshead speed of 5

mm/min. The vertical displacement of each female was measured
under vertical load of 5 kgf. These measurements were repeated for
10,000 cycles. The mean values were analyzed using 1-way ANOVA
followed by the Tukey’s test at a significance level of α=0.05.

Fig. 2 Schema of four stress breaking attachments

Fig. 3 Schematic
drawing of specimen

Fig. 4 Cyclic loading testing machinefor tensile testResults
The initial retentive force of the locater attachment wassignificantly greater than those of the other attachments tested(P<0.05). After 10,000 cycles, the retentive force of the locator
attachment decreased to two-thirds of the initial one. There wereno significant differences of the retentive forces of two magnetic
attachments, SBB and SBM attachments between before and afterloading (P<0.05) (Fig. 5).

Fig. 5 Changes of retentive forces of each attachment

The vertical displacement of Magfit SX, SBB and SBM
attachments showed slight decrease after loading (Fig. 6). On theother hand, there was little vertical displacement of the locator
attachment irrespectively of the load applied. The verticaldisplacement of Magfit IPS was significantly decrease after
20,000 cycles, and there was little vertical displacement after
40,000 cycles.

Fig. 6 Changes of vertical displacement of seven attachments

Discussions

It is very difficult to determine the amount of the most suitableretentive force as the attachment for overdentures. Generally, a single
overdenture may require a retentive force of approximately 2 kg towithstand the chewing of sticky foods, and simultaneously, it shouldbe easily removable by the wearers. One retainer may require
retentive force of approximately 300~1,000 g [1]. After 10,000
cycles, the retentive force of the locator attachment decreased to
two-thirds of the initial one. But the retentive force after 10,000 cycles
was greater than the other attachments, the locator attachment would
be effective to use for the case needed larger retention. When a staticload of axial direction is applied to the implant superstructure and thealveolar mucosa, the displacement was approximately less than 5 µm
[2] and 300µm [3], respectively. The stress-breaking attachment maycompensate for these displacement differences in accuracy. The
vertical displacement of Magfit SX, SBB, and SBM attachmentsshowed slight decrease after repeated loading. However, theseattachments may distribute the occlusal force equally between thealveolar ridge and the implant. The possibility of clinical application
was suggested that the SBM attachment should be miniaturized.

Conclusions
Although the initial retentive force of the locater attachment wassignificantly greater than the other attachments, it remarkably
decreased after 10,000 cycles. There were no significant differences of
the retentive forces and vertical displacements of two magnetic
attachments, SBB and SBM attachment between before and after
repeated loadings.

References[1.] Muraishi E. Retentive forces and fitting accuracy of repaired akers
clasps using laser welding. Tsurumi Univ Dent J. 2010;36:53-65.
[2.] Chavez H, Ortman LF, DeFranco R.L. Medige J. Assessment oforalimplant mobility. J Prosthet Dent 1993 ;70:421-426.
[3.] Watt DM, MacGregor AR. Support. Designing Complete
Dentures. London:W.B.S and ers.co, 1976:24-34.