Development of Implant Magnetic Keepers– Study on the screw loosening by repeated loadH. Nagai, T. Iwai, Y. Nakamura, H. Kumano, K. Yoshihara, K. Hayashi, Y. Takada 1, Yu. Ohno,
Y. Tanaka
Department of Removable Prosthodontics, School of Dentistry, Aichi-Gakuin University1
Division of Dental Biomaterials, Tohoku University Graduate School of Dentistry

Introduction
Implant materials and techniques have markedly improved in recent years. An implant overdenture withmagnetic attachments has excellent retention mechanism, and has been drawing attention as one of theimplant treatments.
We investigated attractive force and surface treatment methods to develop safe and practical magnetic
implant system, and devised an ideal prototype keeper. Mechanical analysis was performed using
three-dimensional finite element method to assess stress-bearing ability of this implant system for further
optimization of a keeper.
A magnetic attachment is incorporated in the overdenture in the present implant system. External force isapplied to an implant keeper during mastication due to the characteristics of an implant overdenture. Theabutment was fixed to a fixture using an abutment screw, and the implant keeper was fixed on the top. The
prototype implant keeper was fixed to an abutment by a screw structure of a keeper.
The axial force in the tensile direction in a bolt created by tightening the screw and compressive force in atightened substance are combined and balanced when there is no external force. However, these forces
decrease by the application of external force, resulting in loosening of a screw. Loosening of a keeper
during functioning may cause impairment in function as an attachment, and increase a risk of accidental
ingestion and aspiration of a keeper.

Objective
Repeated load was applied on a keeper of the implant to investigate loosening at the junction between akeeper and abutment during function.

Materials and Methods
1.Materials
An abutment and a keeper that fit into the fixture of commercially-available implant system (GENESIO,GC, Tokyo) were devised. Magnetic stainless steel SUSXM 27 was used for a prototype implant keeper,
6Al-4V-Ti alloy was used for a special jig, and pure titanium (JIS IV) was used for a fixture. The head of aGENESIO system abutment screw was shortened by the thickness of a keeper. A fixture and a special
abutment were fixed with abutment screws according to the GENESIO system, and a special abutment andprototype implant keeper were fixed by a screw consisting of the inner part of the abutment and outersurface of a keeper. An implant keeper was designed to be completely stored in the abutment to avoid
deformity and edge roll-off due to long-term use (Fig. 1).

Keeper(SUSXM 27)
Abutmentscrew(6Al-4V-Ti)

Abutment(6Al-4V-Ti)

Fixture(pure titanium)

Figure 1. The structure of a prototype implant system1) Prototype implant keeper
An implant keeper was designed in the cylindrical shape to match the size of a commercially-available
magnetic attachment (GIGAUSS D600, GC, Tokyo). A keeper had a hexagonal screw hole with 1.3 mm indiameter and 0.8 mm in depth, and screw structure was added to the outer surface of a keeper. The hole
center model of D 600 was designed 3.8 mm x 1.2 mm in diameter, and the standard of the screw structure
was Class M 3.5 x 0.35 (Fig. 2).

Figure 2. Prototype implant keeper2) Experimental abutment
An abutment was designed to fit a prototype implant keeper. The size of a experimental abutment was 4.4mm x 3.7 mm in diameter.
2.Methods
The lower plate simulating the environment that the prototype implant was placed into the bone, and theupper plate simulating an overdenture on the top were fabricated. The accelerated study was performed byapplying repeated loads on the upper and lower plates to measure loosening of a keeper.

1) Lower plate
A square plate was fabricated using base resin (ACRON MC, GC, Tokyo), and 25 abutments werefixed concentrically from the center of the place using self-curing resin (UNIFAST, GC, Tokyo). The
upper surfaces of abutments were adjusted to be on the same surface. Implant keepers were fixed on
the abutments secured on the plate using a torque gauge (FTD50CN2-S, TOHNICHI, Tokyo). Torquevalue was changed to 1, 2, 3, 4, 5, 10, 15, 20, and 25 N, and 5 implant keepers were fixed at eachtorque value (Fig. 3 and 4).

Figure 3. Torque gauge

Figure 4. Lower plate2) Upper plateAlthough magnetic attachments were supposed to be incorporated into the upper plate reproducing anoverdenture, keepers (GIGAUSS D600, GC, Tokyo) were conveniently used instead of magnetic structurein the present study. The upper plate was fixed to the lower plate to make a pair using the UNIFAST (Fig.
5).

Figure 5. Upper plate
3) Repeated loads

(1) Loading
The upper plate was placed over the lower plate, and the load was applied to the upper plate so that
repeated load was applied indirectly to the implant keeper. Only a keeper of the upper plate assuming amagnetic assembly was set to be in contact with an implant keeper fixed in the lower plate, followed byload application (Fig. 6, 7).

Figure 6.The upper plate was placed over the lower plate

Figure 7. Repetition load
(2) Loading condition
A tap tester devised in our department was used for the load test (Fig. 8). A 20 kgf load was repeatedly
applied 500,000 times at a cycle of 0.8 seconds.

Figure 8.Tap tester devised in our department
4) Measurement method
The torque values for fixing an implant keeper and for removing it after repeated loading were compared.
The difference of the torque values for fixation and removal of an implant keeper was calculated, and it wasset as the loosening value of a screw.

Results
Although there was no detachment of a keeper from the abutment after repeated loading of 500,000 times,
loosening of a screw was confirmed in all samples. The torque value was decreased by 70.0% at 1N fixed
torque value, 18.0% at 2N, 11.4% at 3N, 5.5% at 4N, 3.6% at 5.10N, 6.1% at 15N torque value before
loading, 6.4% at 20N, and 9.8% at 25N (Table. 1).
Table.1 Loosening of a keeper after the load test

Discussion
It is said that a screw has a proper torque. The proper torque value is a value when a screw is tightenedwith small rate of decrease in the torque value. The results of the present study showed that the rate ofdecrease in the torque value was the smallest when a keeper was fixed with 5 – 10 N torque value,suggesting that the proper torque value was around 5 – 10 N.
Some keepers fixed with over 20 N torque values showed surface bounce. It is considered that the marginof a screw hole bounced during fixation of a keeper to an abutment with the large torque value due to soft
magnetic stainless-steel keeper. The adsorption face is distorted, resulting in a decrease in magnetic
attraction of magnetic attachment.
The results of the present study suggested that the proper torque value to fix a prototype implant keeper
with fine screw structure of M 3.5 x 0.35 using hexagonal screw hole (1.3 mm in diameter and 0.8 mm indepth) is 5 – 10 N.

Conclusion
Repeated load was applied to a prototype implant keeper 500,000 times, which is equivalent to thenumber of chewing strokes per year, to investigate loosening of a keeper screw, and the following results
were obtained:
1. 500,000 times repeated loading caused loosening of a screw in all samples despite the size of thetorque value.
2. The proper torque value to fix a prototype implant keeper was 5.10 N.

References
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