The Influence of the Screw Hole on the Implant Magnetic Keeper Attachment Surface

T. Iwai, H. Kumano, Y. Nakamura, K. Shoji, T. Masuda, R. Kanbara, T. Miyata, Y. Ohno, Y. Tanaka

Removable Prosthodontics, School of Dentistry, Aichi-Gakuin University

Introduction

Restorative implant materials and techniques have continued to evolve and adapt to new design needs. The improved variety of magnetic attachments available for implant overdentures has also shown adaptive improvements in design.

A subtle but important difference in magnetic attachment design is the method of attachment of an implant magnetic keeper by cementation or screw retention. While cementation may not affect a magnetic keeper’s magnetic potential, the physical differences in keeper retaining screw designs may affect keeper surface magnetic attraction potentials. 

 A screw hole is located under the fixed keeper in one design method, and cementation may preclude retrievable access. A screw hole in the keeper method may permit retrievability and easy maintenance . However, the effect upon magnetic potential by the screw hole in the keeper center is unknown. of  The screw hole keeper design may adversely affected the magnetic circuit thus resulting in a diminished magnetic force potential (Fig. 1).

 

Fig. 1　Fixation method of Implant Keepr

 

Objevtive

A preliminary investigation was prepared. Custom designed test implant keepers with varying screw holes access dimensions were prepared based on commercially available keepers to investigate the influence and effect of the access screw hole on magnetic keeper attractive force.

Materials and Methods

1.    Materials

GIGAUSS D 600 keeper samples with holes at different locations were prepared. (Figures 2 and 3). The locations of holes were the center (center-hole model) and the keeper lateral surface (side-hole model). The holes were round with ψ1.1, 1.3, and 1.6 mm in diameter and 1.0 mm in depth in the center-hole model. The lateral side-hole model has axially symmetrical 4 holes with 0.85 x 0.05 x 0.2 mm in size. A GIGAUSS D 600 keeper without a hole was used as a control. The total number of sample groups was five.

Fig. 2　Experimental samples

 

Fig. 3　Internal structure of Experimental samples

 

2.    Methods

Attractive force measurements between sample keepers and GIGAUSS D 600 magnetic assemblies (Fig. 4) were performed, and results were compared. A custom-made jig and mold (Reference) were used to hold and support the samples during testing measurements. Pull tests were measured 10 times for each sample at 5 mm / min crosshead speed using a compact table-top universal testing machine (EZ test, SHIMAZU) (Fig. 5).

Fig. 4　Gigauss D 600　（GC ）

 

Fig. 5  A compact table-top universal tester machine

 

3.    Statistical analysis

Average attractive forces of samples were calculated based on the obtained measurement at separation. One-way analysis of variance and multiple comparison using Sheffe’s test were performed at the significance level of 5%. Statistical analysis software (Dr. SPSS II for Windows standard version, SPSS) was used for the analysis.

Results

Table 1 shows the results of measurement analysis. A decrease in the retention force was observed in the center-hole and side-hole models compared with the control model. A significant difference was observed between each sample.

A decrease in the retention force was also observed with increasing diameter screw hole size in the center-hole model. Attractive force of Gigauss D 600 without a hole was 500.1 gf. In contrast, the attractive force decreased by 13% (433.4 gf) at 1.1 mm screw hole, 15% (422.8 gf) at 1.3 mm screw hole, and 18% (409.6 gf) at 1.6 mm screw hole.

The attractive force of the side-hole model decreased by 10% (447.1 gf).

 

Fig. 6 The Influence of the Screw Hole on the Implant Magnetic Keeper Attachment Surface

 

Discussion

 A decrease in the attractive force was confirmed in implant magnetic keepers with screw holes. The influence of screw hole location on attractive force was smaller when a screw hole was located aside from the center. However, a decrease in the attractive force in the center-hole model may be reduced to the same level as the side-hole model by decreasing the size of a hole.

Tanaka et al. reported a difficulty in obtaining required retentive force of a magnetic attachment. The ideal desired retention force level was 500 gf retentive force based on conventional mechanical retainers. The actual retentive force measurement of a prototype implant keeper based on GIGAUSS D 600 was 450 gf. This retention force level is considered insufficient for use as a denture retainer.

Screw retention of an implant keeper allows easy maintenance, and, therefore, considered feasible in clinical practice. Further studies are required to investigate optimal screw hole configuration and fixation method between a keeper and implant body, and to develop a prototype implant keeper with attractive force more than 500 gf.

Conclusion

Prototype implant keepers with different screw holes were prepared. Attractive force measurements and magnetic field analysis were performed using GIGAUSS D 600 to compare an influence of the screw hole configuration and location on attractive force, and the following results were obtained.

1.       Screw holes of any shape affected the attractive force of a magnetic attachment.

2.       The influence of screw hole location on attractive force was smaller when a screw hole was located aside from the center.

3.       In the center-hole model, the influence of a screw hole decreased with decreased diameter size of the screw access hole.

References

1.　Y.kiuchi:Magnetic Devices for Retaining Dental Prostheses,Bioinstrumentation and Biosensors　(Ed.D.L.Wise),pp.145−164,1991(Marcel　Dekker,Inc)

2. Okuno,O.,Ishikawa,S.,Iimuro,F.T.et al.:Development of sealed cup yoke type dental magnetic　attachment,Dental Materials Journal,10(2):172-184,1991.

3.  Y.Terao, Y.Nakamura, T.Ishida, A.Ando, H.Nakamura, et : Measuring Methods of the Attravtive Force of Magnetic Attachment  16(2)  :14~19, 2007.