Retentive forces and vertical displacements of stress breaking
attachments for implant over dentures
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Daisuke Ozawa , Yasunori Suzuki , Hidekazu Osada , Kentaro Kono , Chikahiro Ohkubo
Department of Removable Prosthodontics, Tsurumi University School of Dental Medicine.
Division of Oral and Maxillofacial Implantlogy, Tsurumi University School of Dental Medicine.

Introduction
Numerous studies with reliable results on attachment systems for implant overdentures in themandible and maxilla have been published. Most attachments allow for rotational excusion, butcannot compensate for vertical displacement under the functional forces. There are extraordinary
differences in setting under a chewing load between the implant and mucosa under the denturebase. In addition, horizontal forces and rotational excusion 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 are applied to the implants. To protect implants fromexcessive forces, a few stress-breaking attachments have been manufactured.

Objective
This study evaluated the retentive forces and displacement of stress-breaking attachments after
repeated loads simulating masticatory function.

Materials and Methods
Four types of stress-breaking attachments (Fig.1), namely, a self-adjustment type magnet attachment(Magfit SX, Aichi Steel), a cushion type magnet attachment (Magfit IPS, Aichi steel), a locatorattachment(Locator, ZEST anchors), and three types of Stress-Breaking Ball (SBB) attachments(amount of displacement:0.3mm, 0.5mm, 0.7mm,GC corp.), were placed on the implants. Theimplants were embedded in a resin block using autopolymerized resin (Fig.2). To simulate thechewing cycles, a load of 5 kgf was repeatedly applied up to 50,000 using a loading apparatus(Fig.3). The retentive force was measured by means of tensile testing at a crosshead speed of 5mm/min. The vertical displacement of each female was measured under 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. The contact surface of attachment female wereexamined using a microscope (SZX, OLYMPUS).

Fig. 1 Schema of stress breaking attachment

Fig. 2 Schematic drawing of Specimens

Fig. 3 Cyclic loading testing machinefor tensile test

Results
The initial retentive force of the locater attachment was significantly greater than that of any otherattachments tested (P<0.05). After 10,000 cycles, the retentive force of the locator attachment
decreased to two-thirds of the initial one. There were no significant differences of the retentive
forces of both magnet and SBB attachments before and after loading (P<0.05) (Fig4). The verticaldisplacement of MagfitSX and SBB attachments showed slight decrease after loading. On the other
hand, there was little vertical displacement of the locator attachment irrespectively of the loadapplied. The vertical displacement of Magfit IPS was significantly decrease after 20,000 cycles, andthere was little vertical displacement after 40,000 cycles (Fig.5). No crack and no deterioration were
not observed in the O- ring rubber of the SBB attachment and these was no change of a diameter
ratio compared between initial and 50,000cycles (Fig.6). Liner scratches to many directions weredetected on the magnet and keeper surface of magnet attachments (Fig.7). The deformation wasobserved in the female silicone of the locator attachments (Fig.8). As for the Magfit IPS, built-in ballthat made by silicon recognized the deformation. It caused displacement decrease. (Fig.9)

Fig. 4 Changes of retentive forces of each attachment

Fig. 5 Vertical displacement of each attachment

Fig. 6 Magnification of O-ring rubber in the SBB attachment (×10)

Fig. 7 Magnification of keeper surface of magnet attachment structure (×20)

Fig. 8 Magnification of female silicone in the locator attachment (×10)

Fig.9 Magnification of the deformation built-in cushion of magnet attachment (×10)

Discussions
It is very difficult to determine the amount of the most suitable retentive force as the attachment foroverdentures. Generally, a single overdenture may require a retentive force of approximately 2 kg towithstand the chewing of sticky foods, and simultaneously, it should be easily removable by thewearers. 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 theretentive force after 10,000 cycles was greater than the other attachments, the locator attachment

would be effective to use the clinical application similar to the other attachments.
When a static load of axial direction were applied to the implant superstructure and the alveolar
mucosa, the displacement was approximately less than 5μm2)and 300μm3), respectively. The
stress-breaking attachment may compensate for this displacement difference in accuracy. The
vertical displacement of Magfit SX and SBB attachments showed slight decrease after repeatedloading. But these attachments may distribute the occlusal force equally between the alveolar ridge
and the implant.

Conclusions
The initial retentive force of the locater attachment was significantly greater than the otherattachments, but after 10,000 cycles, the retentive force of the locator attachment remarkablydecreased. There were no significant differences of the retentive forces and vertical displacements ofboth magnet and SBB attachment before and after repeated loading.

References
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