Stress Analysis of Implant Keepers with Different Retaining
Methods
K. Hayashi, R. Kanbara, Y. Nakamura, Yo. Ohno, K. Shiraishi, K. Shoji, H. Kumano,
Y. Takada*, Yu. Ohno, Y. Tanaka
Department of Removable Prosthodontics, School of Dentistry, Aichi-Gakuin University*Division of Dental Biomaterials, Tohoku University Graduate School of Dentistry

Introduction
Restorative implant materials and technique have continued to evolve and improve. Implant
overdentures with magnetic attachment are drawing attention due to their excellent retentive
mechanism.
A keeper corresponding a magnetic assembly is secured to the implant to exert the function of anattachment.
In the screw-retaining method, a keeper is mechanically secured to an implant using a retaining
screw. The benefit of this method is that a keeper can be removed by operators, and, therefore,
suitable for the maintenance. However, since a keeper is mechanically fixed using a screw, safety
should be considered in the long-term intraoral performance of this implant. There have been
several reports on clinical problems such as a fracture of an abutment screw. It is extremely
important to understand mechanical influence of keeper fixture methods on implants to investigate
the optimal retaining method.

ObjectiveThe purpose of the present study was to investigate mechanical influence of keeper retaining
method with screws using the three-dimensional finite element method.

Analysis methods
1. Analysis model
Figure 1 shows the keeper retaining methods of 2 types of keepers.
The methods include “fixation structure A” in which a fixture and an abutment are fixed withabutment screw and keeper unit, and “fixation structure B” in which a fixture and an abutment arefixed with abutment screw, and a keeper is separately fixed with an abutment on top.
Finite element models of “fixation structure A” (Model A) and “fixation structure B” (Model B)
were constructed using a general purpose finite element pre-post processor (Patran 2010, MSC
software). These FEM models are shown in Fig. 2.
The detailed size of each model is shown in Figures 3 and 4. The screw structure followed thesame standard, and the pitch of the screw was 2 mm.
The element type designation was three-dimensional tetrahedral primary element. A total nodal
point and total element count of Model A were 36,931 and 211,512, respectively, and those of Model
B were 38,759 and 222,615, respectively.
The cortical bone surrounding a fixture was constructed for the convenience of analysis.

Fig. 1.

The keeper retaining methods of 2 types of keepers.

Fig. 2.

Finite element models

Fig. 3.

The detailed size（Model A）

Fig. 4.

The detailed size（Model B）

2. Analysis condition
An elastic stress analysis was performed using a general purpose finite element program (Marc2010, MSC software). A workstation DELL PRECISION T 7400 (DELL) was used throughout thestudy.
1) Boundary condition
A complete constraint was applied to the inferior and lateral surfaces of the cortical bone in theX, Y, and Z directions (Fig. 5).
Two loading conditions (LC 1 and 2) were applied. A 800 N load was applied to the uppersurface of a keeper in the vertical direction of the implant long axis in LC 1. A 800 N load wasapplied to the upper surface of a keeper in the 45 °direction of the implant body long axis in LC2.
A contact condition was applied between each structure except for a fixture and the corticalbone.

Fig. 5.

Fix condition

2) Mechanical properties
Table 1 shows material constants used in the present study. Soft magnetic stainless steel
SUSXM 27 was used as a keeper material of a magnetic attachment, and the abutment screw ofModel B was made of Ti-6AL-4V. Material constants of a fixture and abutment were set
according to the previous references.
Table. 1.

Mechanical Properties

Analysis results
1. Stress distribution
Stress evaluation of LC1 and 2 was performed using Von-Mises stress. The evaluation wasconducted for the surface and longitudinal plane of an abutment screw.
Stress evaluation of LC2 was performed using principal stress. The evaluation was conducted
for the surface of an abutment screw.

1)Von-Mises stress
Figure 6 shows the overview of LC1. Although there was no significant mechanical difference inan abutment screw between Models A and B, a minor stress relaxation was observed in Model B.
Figure 7 shows the overview of LC2. A larger stress concentration in the neck and center parts ofan abutment screw was observed in Model A compared with Model B.

Fig. 6.

Von-Mises stress (LC1)

Fig. 7.

Von-Mises stress (LC2)

2)Principle stress
Figure 8 shows the minimum principle stress distribution chart. The higher compressive stressconcentration was observed in the corner and neck parts of an upper abutment screw in Model A
compared with Model B.
Figure 9 shows the maximum principle stress distribution chart. The higher tensile stressconcentration was observed in the neck part in Model A compared with Model B.

Fig. 8.

The minimum principle stressFig. 9.

The maximum principle stress

2. Stress values (maximum and minimum principle stress)
Figure 10 shows the minimum and maximum principle stress values of LC2. The measurement
point excludes the point adjacent to the loading point. Minimum and maximum principle stressvalues were higher in Model A compared with Model B.

Fig. 10.

The minimum and maximum principle stress values

Discussion
1. Stress distribution
The main purpose of the present study was to investigate the mechanical influence of thedifference in the fixation method of a magnetic implant keeper on a load. The inner structures oftwo analysis models were faithfully constructed according to the size shown in Fig. 2.
Compared to “fixation structure A” in which a fixture and an abutment are directly fixed withabutment screw and keeper unit, “fixation structure B” is physical fixation of an abutment screw
and a keeper using screw structure. Therefore, an appropriate setting is necessary regarding theinterfacial positional relationship. An abutment screw and a keeper of “fixation structure B” wereconstructed on the model in the contacted position without space to evaluate the influence of thedifference in the two fixation methods of a magnetic implant keeper on a load.
It is considered that the analysis model constructed in the present study can accurately evaluate
the difference in keeper fixation structure.
2. Analysis condition
Boundary condition significantly affects the analysis results. The appropriate boundary condition
for the present study was carefully investigated. The cortical bone was constructed around theimplant body, and a complete constraint was applied around the cortical bone.
The functional force applied to the implant overdenture includes component force in differentdirections. Therefore, a load was applied perpendicular and 45°to the keeper adsorption face toefficiently transfer stress to an abutment screw. A load larger than estimated occlusal force wasapplied.
A contact element that can reproduce the contact condition between objects was applied on theinterface of fixture, abutment, abutment screw and keeper.
We focused on the difference in materials of soft magnetic stainless steel (SUSXM27) used as akeeper material of magnetic attachment and titanium material (Ti-6AL-4V) used as an abutment
screw material to set mechanical property value of each analysis model component. Experimental
data of these materials are shown in Table 2. The mechanical influence between materials needs tobe considered when these materials are used for the implant inner structure. SUSXM27 was usedfor a keeper and abutment screw unit in Model A, whereas SUSXM27 was used for a keeper, andTi-6AL-4V was used for an abutment screw in Model B.
3. Analysis result
Figure 6-10 shows obtained results. A significant difference was observed in stress distribution oftwo keeper fixation structures during a load application in 45°compared with a load application in
the vertical direction. A load was applied in the long axis direction of the implant body during aload application in the vertical direction. Since a substantial load was placed on the inner abutment,
there was little influence of the difference in fixation structures and materials on abutment screws
for each model. In contrast, a tilted load was applied in the long axis direction of the implant bodyduring a load application in 45°direction. A substantial load was placed on the mutual inner screw
structure of an abutment and a fixture, and bending stress was placed against an abutment screw
located in the center. Therefore, there was a significant influence of the difference in fixation
structures and materials on abutment screws for each model.
For stress evaluation during a load application in 45°direction, Stress evaluation of was performed
using principle stress that evaluates compressive and tensile stress, and Von-Mises stress thatindicates various stress as scalar. A significant influence of the difference in fixation structures andmaterials was observed between Model A and Model B regarding the principle stress evaluation(Fig. 8 – 10). Regarding abutment screw materials (Tables 1 and 2), SUSXM27 has lower yieldstress compared with Ti-6Al-4V, and principle stress has a significant influence during fracture anddeformation of objects. These results suggest that Model B has a higher safety against fracture anddeformation compared with Model B considering the repetitive stress application in the mouth.

Table. 2.

Experimental data

Conclusions
The mechanical evaluation was performed regarding the keeper fixation using the screw retaining
method. Mechanical evaluation was conducted using three-dimensional finite element method, andthe following conclusions were drawn.
1. A significant stress relaxation was observed regarding compression and tensile stress duringload application in 45°direction by separating an abutment and keeper.
2. For abutment screw materials, a higher mechanical safety was suggested in Ti-6Al-4V
compared with soft magnetic stainless steel (SUSXM27).

References
1. Morooka M., Ishikawa T., Ito T. and Fukuti A.：Frequent fractures of titanium root-form implantand structure and handling – case reports –，Quintessence DENTAL Implantology，15(3)，2008.
2. Iwai T.,Yoshihara K.,NakamuraY., et al, Study of Prototype Implant Magnetic Keeper with TiNCoatings by Reactive Ion Plating, The Journal of the Japanese Society of Magnetic Applicationsin Dentistry. 2011
3. Japanese Industrial Standard Committee： Titanium and titanium alloys －Rods and bars,
Japanese Standards Association, 2007
4. M.J.Donachie,Jr ：Titanium and titanium alloys －Rods and bars, UCHIDA ROKAKUHOPUBLISHING CO, 1993