Int Poster J Dent Oral Med 2008, Vol 10 No 01, Poster 397
Dental magnetic systems as prosthetic attachments on retained roots
Carolin Ehring, OA Dr. med. dent. Arne F. Boeckler, Prof. Dr. med. dent. habil. Jürgen M. Setz
Martin-Luther-University Halle-Wittenberg, Poliklinik für Prothetik, Halle/Saale
March 9-12, 2005
83rd General Session & Exhibition of the IADR
Baltimore, Maryland, USA
Sensodyne-Poster-Studien-Award 2009 für das beste Poster 2008
Magnetic retention is a popular method of attaching removable prostheses
to retained roots 1, 2. Despite their advantages, which include ease of
cleaning, ease of placement for both dentist and patient, constant retention
with number of cycles, automatic reseating and reduced lateral stresses on
the roots, magnets have small attractive forces compared to other
prefabricated prosthetic attachments 1 - 4. Nevertheless, there is a strong
evidence that retention is of great importance for a patient's satisfaction
4, 5. Contemporary two different physical and technical concepts can be
found: mono systems consisting of a magnet and a corresponding keeper from
magnetizable alloy with an open (MO) or with a closed magnetic field (MC) 1.
The force produced by any two magnets is inversely proportional to the
square of the distance between them 6. Separation between magnet and keeper,
however caused, will result in a drastic reduction in the retention. MC's
produce the highest initial retention force. MO's are more voluminous and
providing a comparatively lower initial retention force. Nevertheless, the
release of the retention is not so noticeable when the magnets are separated
for a small distance. The retention provided would be quite close to that
claimed by the manufacturer as long as the magnet and the abutment remain in
contact. This condition may not be possible in the clinical situation. The
retention in function is very sensitive to distance. The point to be made,
therefore, is that the manufacture's claimed retention may not be what is
obtained clinically. Following the manufacturers' information recently
developed or improved products despite their small size should produce high
The aim of this study was to verify and to compare the initial retention
force and the force-distance relation of contemporary magnetic systems for
Material and Methods
11 products of different height and diameter were tested (Tab). All of
these retention systems consisted essentially of a magnetic assembly which
is incorporated into the prostheses and a corresponding keeper. In the
magnetic units rear earth magnets from NeFeB are embedded. To protect the
brittle rare earth magnets against corrosion they are incorporated into a
thin non magnetizable alloy casting (Ti or stainless steel). The keepers
consist of a magnetizable corrosion resistant alloy. They can be classified
into the cement-in keepers, the cement-on keepers and two types of cast
keepers-those cast entirely in magnetizable alloys and those where a
magnetizable keeper is incorporated into a non magnetizable alloy casting.
From each product or combination 5 specimens were tested in an adjusted and
computer navigated pull-testing machine (Z005, Zwick, Ulm, Germany). A
special non magnetizable holder for the keepers was locked onto the base of
the testing machine (Fig). To avoid tilting of the moving magnet it was
fixed on the tip of a special holder which was connected with the crosshead
by a nonflexible string. The crosshead speed was set at 20 mm/min (s=40 mm).
The breakaway force was the maximum force during the separation of the
magnet and the keeper when the magnet slowly moved away. The breakaway force
measurement was repeated ten times and the mean for each sample was used.
The results were descriptive and statistical analysed (H-/U-Test,
p0.05). The findings were compared with the manufacturers'
|Fig. 3 Force-distance-relation of magnetic attachments on implants
|Tab.1 Magnetic attachments for retained roots
||Tab.2 initial retention forces of the tested magnetic attachments
The highest initial retention force was 6.6 N. In a recently developed and
distinctly smaller specimen an initial force of 6.2 N were found (Fig). The
smallest initial breakaway force was measured with 1.4 N (Fig). Beside the
different initial forces the recorded force-distance relations according to
the respective type of magnetic system were characteristically for each of
the samples. The highest retention forces achieved the MC's followed by the
MO's. After a separation of 1 mm the remaining forces were reversed (Fig).
The MO's produced about 25 % of their initial force whereas the MC's showed
approx. 5 % of their initial breakaway forces. The value of the retention
force is depending on the dimension of the magnet unit. Therefore the
discrepancies of the recorded breakaway forces and the manufacture's claimed
retention were determinate (Tab or Fig). No product achieved the retention
that was claimed by the respective manufacturer. In one product there was
more than 90 % of the indicated retention. In 5 samples there were more than
70 % but in 4 products under 50 % of the indicated breakaway force.
|Fig.1 Tested specimens
||Fig.2 Non magnetizable holder and mounting
Within the limits of this study it could be drawn that there were
significant differences between the clinically important breakaway forces.
No product achieved the retention that was claimed by the respective
manufacturer. No product achieved the retention that was claimed by the
respective manufacturer. In the majority of cases the maximum retention
forces were found notable under the manufacturers' claimed retention.
Mono-systems with a closed magnetic field produced the highest breakaway
forces. Concerning the reseating forces the force-distance relations could
indicate a slightly advantage for the mono-systems with an open magnetic
field. These results should be taken into consideration when choosing
implant supported magnetic attachments for individual situations.
- Riley M, Walmsley A, Harris I. Magnets in prosthetic dentistry. J Prosth Dent 2001;86:137-142.
- Coca I,.Klimek K. Prospective clinical evaluation of retained abutment teeth with different partial dentures in the reduced dentition of the mandible. Zahn Prax 2002;5:252-254.
- Setz J, Seok H, Engel E. Retention of prefabricated attachments for implant stabilized overdentures in the edentulous mandible. J Prosth Dent 1998;80:323-329.
- Burns D, Unger J, Elswick R, Beck D. Prospective clinical evaluation of mandibular implant overdentures: part I. Retention, stability and tissue response. J Prosth Dent 1995;73:354-363.
- Burns D, Unger J, Elswick R, Giglio J. Prospective clinical evaluation of mandibular implant overdentures: part II. Patients' satisfaction and preference. J Prosth Dent 1995;73:364-369.
- Jackson T. The application of rare earth magnetic retention to osseointegrated implants. Int J Oral Maxillofac Implants 1986;1:81-92.
- Wisser W, Coca I, Ruppel-Schönewolf K. Design for root keepers in magnet retained overdentures. ZWR 1996;7:374-378.
This Poster was submitted by Dr. med. dent. Arne F. Boeckler.
OA Dr. med. dent. Arne F. Boeckler
Poliklinik für Zahnärztliche Prothetik
Grosse Steinstrasse 19
06108 Halle / Saale