Magnets in prosthetic dentistry

Melissa Alessandra Riley, BMedSc, PhD,a Anthony Damien Walmsley, BDS, MSc, PhD,b and Ivor Rex
Harris, BSc, PhD, DScc
The University of Birmingham and St. Chad’s Queensway, Birmingham, United Kingdom
Magnetic retention is a popular method of attaching removable prostheses to either retained roots
or osseointegrated implants. This review chronicles the development of magnets in dentistry and
summarizes future research in their use. The literature was researched by using the Science
Citation Index and Compendex Web from 1981 to 2000. Articles published before 1981 were
hand researched from citations in other publications. Articles that discussed the use of magnets in
relation to prosthetic dentistry were selected. (J Prosthet Dent 2001;86:137-42.)

M agnets have generated great interest within

dentistry, and their applications are numerous. The 2
resistance to demagnetization, high magnetization,
and better resistance than Nd-Fe-B-type magnets to
main areas of their use are orthodontics1,2 and remov- temperature and corrosion.7 This material is still under
able prosthodontics. The reason for their popularity is development, but it is expected to become available
related to their small size and strong attractive forces; for medical and dental applications in the near future.
these attributes allow them to be placed within pros- Additional information on magnetic materials and
theses without being obtrusive in the mouth. Despite their applications may be found in articles by Harris8
their many advantages, which include ease of cleaning, and Harris and Williams.9
ease of placement for both dentist and patient, auto-
TYPES OF MAGNETISM
matic reseating, and constant retention with number
of cycles, magnets have poor corrosive resistance with- Magnetic materials may be termed either “soft”
in oral fluids and therefore require encapsulation (easy to magnetize or demagnetize) or “hard” (able to
within a relatively inert alloy such as stainless steel or retain magnetic properties and be made into perma-
titanium. When such casings are breached, contact nent magnets). Whether a material is hard or soft
with saliva rapidly brings about corrosion and loss of depends on whether it retains its magnetic properties
magnetism. after the removal of an applied magnetic field.
This review chronicles the development of magnets Every atom is a magnet because electrons orbit its
in dentistry and critically reviews their current status in nucleus and, as moving charges, produce a magnetic
removable prosthodontics. field. However, most electrons are paired, and the
equal and opposite fields cancel out. In some atoms
MAGNETIC MATERIALS
such as Fe, Ni, and Co, there are unpaired electrons
Over the last century, significant advances have been that create a tiny magnetic field. In a magnetic mater-
made in the development of magnetic materials; these ial, a large portion of these atoms align in small regions
advances have been quickly transferred into dental appli- called “domains.” In an unmagnetized state, the ori-
cations. The main magnetic material used is the rare entation of these domains is random and no overall
earth material neodymium iron boron (Nd-Fe-B),3,4 magnetization is experienced.
which is the most powerful commercially available mag- On the application of a magnetic field (H), the
net material. Other materials used include the RE alloy domains align and thereby produce an overall magne-
samarium cobalt (Sm-Co).5,6 Before the development tization in the specimen, which will reach a saturation
of rare earth magnets, Alnicos—alloys based on alu- point (Ms). Magnetically soft materials require only
minum, cobalt, and nickel—were the main materials in small fields to reach saturation, whereas magnetically
use, although cobalt platinum (Co-Pt) magnets also hard materials require large fields to reach saturation.
existed.7 When the applied field is removed, a permanent mag-
Samarium iron nitride is a promising new candidate net or hard material retains much of the magnetization
for permanent magnet applications because of its high or remanence (Br). This magnetization in the speci-
men is reduced to zero by the application of an equal
but opposite field to the magnetization in the speci-
aRecent PhD graduate, School of Metallurgy and Materials, The
men. The value of H at this point is the intrinsic
University of Birmingham.
bProfessor, School of Dentistry, St. Chad’s Queensway. coercivity (iHc). If the applied field is reversed
cProfessor, School of Metallurgy and Materials, The University of between the same positive and negative limits, a sym-
Birmingham. metrical loop called a hysteresis loop is traced out.

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THE JOURNAL OF PROSTHETIC DENTISTRY RILEY, WALMSLEY, AND HARRIS

Fig. 1. Improvements in (BH)max with time (from Harris and Williams4).

For a permanent magnet, it is the maximum energy fields present while the denture is in place. The meth-
product, (BH)max, that gives an indication of its power. ods of corrosion protection have also improved.15
The larger this value, the greater the flux produced by a Improvements in magnetic materials have allowed
magnet of a given volume. The development of various smaller and more powerful magnetic attachments to
magnetic materials and improvements in energy prod- be produced from Sm-Co and Nd-Fe-B alloys.
uct ([BH]max) over the last century are shown in
IMPLANTATION OF MAGNETS
Figure 1. Additional information on magnetism may be
found in the text by Jiles.7
Magnet repulsion
The first recorded use of magnets in prosthetic den-
REMOVABLE PROSTHODONTICS
tistry involved using the repulsion of like poles of
Various devices such as springs, suction cups, clips, magnets to maintain and improve the seating of com-
and studs all have been used to retain complete and plete dentures.16 The magnetic material used was an
removable partial dentures within the mouth.10 Alnico type that has been discontinued in dental appli-
Magnets also have been used for this purpose because cations because of the large bulk necessary for magnet
they are easy to incorporate into a denture and can strength. The magnets were embedded in molar
simplify both clinical and technical procedures. regions in the bases of complete dentures so that the
However, there are limitations to their use; these lim- like poles were orientated toward each other. As the
itations are related mainly to their low corrosion patient closed his or her jaws together, mutual repul-
resistance within the mouth.11,12 sion of the like poles of the magnets seated the denture
The first attempts at using magnets to retain den- against the alveolar ridges.17 However, the constant
tures involved implanting them within the jaw13,14; repelling force promoted resorption of bone in the
problems ensued because of the large size of the mag- alveolar ridge, and the seating effect fell dramatically
nets and the inadequate forces that they provided. As when the jaws were apart and the need for the seating
material technology improved, smaller magnets were effect was at its greatest.
made that could be incorporated into retained roots
Magnet attraction
with similar units built into the denture. Later devel-
opments included the replacement of the root magnet The use of the attractive force between 2 magnets
with a soft magnetic material that is magnetized while for denture retention was reported in the early
the denture is in place but returns to a demagnetized 1960s.13,14 These first attempts were made with
state on removal of the denture. Alnico V and both rectangular and cylindrical PMMA-
In the last 20 years, the design of magnetic attach- coated magnets, which were surgically implanted in
ments has changed to reduce the external magnetic the mandible of an edentulous patient. This trial

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RILEY, WALMSLEY, AND HARRIS THE JOURNAL OF PROSTHETIC DENTISTRY

showed that, because of the distance between the 2 cemented into a prepared cavity in the root surface,
magnets, they provided inadequate force to aid den- and a similar magnet was placed in the denture. The
ture retention. The introduction of smaller, stronger technique was modified to prevent corrosion of the
Co-Pt magnets allowed continuation of clinical tri- magnets in the oral environment22 with the use of a
als.14 Unfortunately, several disadvantages were cast gold coping to cover the magnet; whether this was
associated with Co-Pt magnets, including their high successful is unclear.
cost, limited availability, and difficult fabrication. It Soft magnetic root keepers: Various studies have been
was also found that the implanted magnet migrated carried out on the effects of magnetic fields and mag-
through the bone and tissues until it became exposed netic materials with conflicting results.18,23-33 The
in the oral cavity.18 The procedure was eventually details of this work are beyond the scope of this article,
abandoned because of the high costs involved and but there is nothing to suggest that adverse clinical
poor success rates. effects have occurred after 40 years of magnetic applica-
With the introduction of the powerful magnet mate- tions within medicine and dentistry. However, because
rial Sm-Co, the use of implanted magnets to aid denture of fears over the effects of magnetic fields on the soft tis-
retention was investigated again.19 These magnets could sues, a soft magnetic material, Pd-Co-Ni alloy, was
be produced in dimensions approximately one fifth of developed for use in the root face.34 Three alloys were
the Co-Pt magnets and still provide the same force. investigated as replacements for the root element com-
Because of the susceptibility of the magnets to corro- ponent: Pd-Co, Pd-Co-Cr, and Pd-Co-Ni. After
sion, a proplast coating (polytetrafluoroethylene assessment of the magnetic and physical properties and
[PTFE] and pyrolytic graphite) was used. Experiments corrosion resistance, the Pd-Co-Ni alloy was found to
were carried out on dogs to establish whether proplast be the most suitable.35 However, it was also shown that
could be used as an effective coating for Sm-Co mag- Pd-Co-Pt alloys are the most corrosion-resistant.36 The
nets in the in vivo environment. The study concluded advantage of these alloys is that the root element pos-
that the coating provided corrosion protection if there sesses no permanent magnetic properties; thus, no
were no faults or damage to the coating during surgical magnetic fields are experienced within the oral envi-
placement. Proplast is no longer used as a coating mate- ronment once the dentures are removed. Other soft
rial, but PTFE is used as the binder in polymer-bonded magnetic materials used for root keepers have includ-
magnets.7 However, these are unsuitable for long-term ed magnetic stainless steels, Permendur (an alloy of
use within the body because diffusion of moisture iron and cobalt37), and chromium-molybdenum
through the polymer results in inadequate corrosion alloys.38
protection of the magnet material. Such alloys have been cast to form a root coping or
pre-formed into a keeper with or without a screw
Section summary
thread for cementation into the root or attachment to
Early attempts at using magnets for denture reten- an implant.39 The cast copings have been cemented
tion were unsuccessful, mainly because of the large size and, in some situations, cross-pinned into the root to
of magnets at that time and the inadequate forces that avoid loss of the keeper should breakdown of the
they provided. However, since the introduction of rare adhesive occur.40 Although there have been fears over
earth magnets such as Sm-Co5,6 and Nd-Fe-B,3,4 it has the effects of magnetic fields on human tissues, open-
become possible to produce magnets with small field systems are commonly used in both denture
enough dimensions to be used in dental applications retention and orthodontic applications today.
and still provide the necessary force. This negates the
Closed-field systems
need to implant the materials; consequently, interest in
using magnets for denture retention has once again Many commercial systems are now of the closed-
increased, as is demonstrated by the number of clinical field type; these attempt to reduce the magnetic field
reports on this subject. effects in the oral cavity. The magnetic attachments
incorporate soft magnetic materials (such as ferritic or
CONVENTIONAL USE OF MAGNETS
martensitic stainless steel or a Pd-Co-Ni alloy) that
Open-field systems connect the 2 poles of a magnet so the external field is
The first reported use of magnets for the retention shunted through the path of less resistance, reducing
of overdentures took place in the 1960s20 with the external fields in situ. This is demonstrated in Figure 2,
rehabilitation of a patient with a cleft lip and palate. which shows the differences in the external magnetic
The magnetic Co-Pt alloy was used to produce crowns fields experienced with open- and closed-field systems.
for 3 remaining teeth with cast Co-Pt also built into Attachment of closed-field magnets is more effi-
the denture. This was soon followed by the technique cient because both the north and south poles can be
of cementing magnets within retained roots for the used for attachment to the keeper (in open-field sys-
retention of overdentures.21 An Sm-Co magnet was tems, only one pole is used) and the keepers can

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THE JOURNAL OF PROSTHETIC DENTISTRY RILEY, WALMSLEY, AND HARRIS

A B
Fig. 2. For simple, cylindrical, open-field magnet encased in nonmagnetic housing, magnet-
ic field is experienced outside magnetic specimen (A). However, when soft magnetic
materials are used for encapsulation (for example, in cup design [B]), magnetic flux is con-
tained within encapsulation material and channeled into root keeper component.

contain the magnetic flux. Although these systems Clinical usage

generally provide a higher retentive force than a simi-
larly sized open-field system, the retention reduces Magnetic attachments have most commonly been
rapidly with increasing separation.41,42 The first used for the retention of mandibular overdentures.
closed-field design was the split pole design,43 which Many authors have described procedures for the use
consisted of 2 magnets arranged with opposite poles of magnets in this application,20,47-49 and patients
adjacent to each other. A soft magnetic keeper was have reported a high degree of satisfaction with
attached to the top of the magnets, and a similar keep- their dentures.50 There has been renewed interest in
er was built into the root. using magnetic attachments for the provision of
Comparisons of the forces provided by paired mag- mandibular overdentures with osseointegrated
nets, single magnets and soft magnetic material, and implants.15,51 The implant-supported overdenture
reversed and nonreversed poles have been performed. consists of an implant-supported keeper and a mag-
Paired magnets provided a greater breakaway force net that is built into the denture. Two to 4 implants
than a single magnet with a soft magnet keeper. A may be used, and these are placed in the anterior
reversed split pole system, as designed by Gillings,43 region of the mouth and spaced as widely as possi-
provided a greater force than a nonreversed split pole ble to provide maximum support and stability. The
design.44 magnets may be used as attachments on freestand-
Since then, other commercial systems have come ing implants or in combination with a bar
into use, and the designs of these systems have attachment. A bar attachment spans the implants;
evolved.35 Various designs exist that are based on cir- the magnets are placed in contact with the bar
cular and rectangular assemblies. A magnet sandwich rather than individual keepers on implants.51 Many
design has been shown to work well,45 although the clinical reports demonstrate the successful use of
amount of retention provided by this design depends magnetic attachments with implant-supported over-
on the thickness of the side plates and the base.46 denture systems. 52-60 Magnets have been used in
Finite element analysis (FEA) has been used to both mandibular and maxillary implant-supported,
improve the design of these attachments to maximize full-arch bar, fixed-detachable prostheses.51
the force that they provide.38 FEA is able to show
CORROSION
magnetic flux distributions within a design and also
give information on contact forces and the force sep- The main problem associated with the use of
aration characteristics of magnetic systems. A magnets as retentive devices is corrosion by oral flu-
closed-field design consisting of a magnet in a cup, ids.36,61-63 Both Sm-Co and Nd-Fe-B are extremely
which in turn is placed in an outer cup (Fig. 2), pro- brittle and susceptible to corrosion, especially in
vides a higher retention force than a simple open-field chloride-containing environments such as saliva. The
system that incorporates a similarly sized magnet. corrosion products from rare earth magnets also
However, a circular closed-field sandwich-type design have been shown to have cytotoxic effects in in vitro
provides a greater amount of retention still. If the tests. 28,64 Therefore, magnetic materials must be
keeper materials are made ellipsoidal, then retention securely separated from the oral fluids before use in
will increase further.38 dental applications.

140 VOLUME 86 NUMBER 2

RILEY, WALMSLEY, AND HARRIS THE JOURNAL OF PROSTHETIC DENTISTRY

Although some current magnet assemblies are ported, is able to plastically deform inward. Clinically,
encapsulated in stainless steel or titanium,15 some this is observed as a groove down the center of the
devices fail after only approximately 18 months in clin- magnet face.60
ical use because of corrosion and loss of retention
FUTURE IMPROVEMENTS
provided by the attachment.60,65 The buildup of cor-
rosion products may also result in discoloration of the The lifetime of dental magnetic attachments
denture teeth.66 Corrosion of magnetic attachments depends on several factors, but the main problem is
may occur by 2 different mechanisms11: (1) break- the inadequate protection of the encapsulation materi-
down of the encapsulating material, and (2) diffusion als; once they are breached, rapid corrosion of the
of moisture and ions through the epoxy seal between internal magnet occurs. Improvements in sealing tech-
can and magnet. niques (namely, laser welding) have resulted in more
Both Nd-Fe-B and Sm-Co5 magnets corrode rapidly effective sealing of magnet encapsulations. However,
in saliva, and the presence of bacteria has been shown to further work is required to find more corrosion- and
increase the corrosion of Nd-Fe-B magnets.67,68 wear-resistant encapsulation materials.
Various methods have been used to try to eliminate the
SUMMARY
problem of corrosion; these involve encapsulating or
coating the magnets for use intraorally. Titanium and Magnets provide a useful method for attaching
stainless steel are the most common materials used for dental prostheses to either retained roots or osseoin-
encapsulation of dental attachments,15 but polymeric tegrated implants. Magnetic technology is constantly
materials also have been used in both prosthodontic and improving: currently available magnets based on
orthodontic applications.19,28 However, continual wear Nd-Fe-B are small (which allows them to be incorpo-
of the encapsulating material leads to exposure of the rated into dentures) and have attractive forces that
magnet60; this has been shown to occur clinically.59,60 enable them to provide retention. The major research
The wear takes the form of deep scratches and gouges question that has not been solved is the problem of
on the surface caused by wear debris and other particles corrosion. When in contact with saliva, magnets cor-
that become trapped between the 2 surfaces.11 The rode and experience subsequent loss of magnetism.
excessive wear of the magnet may be due to the abrasive Encapsulating materials such as stainless steel are
nature of the titanium-nitride-coated soft magnetic root effective but susceptible to wear. Magnets therefore
keeper that is used with some implant systems. have a relatively short life, although more research is
The pitting corrosion of stainless steel occurs required to help the clinician determine their poten-
because of the corrosive oral environment; similar cor- tial lifespan within the mouth. The development of
rosion has been observed in different systems.11,12 To samarium-iron-nitride may offer better resistance to
overcome the problems associated with the use of den- corrosion, and its introduction into prosthodontics
tal magnets, it appears that different encapsulating will be viewed with much enthusiasm.
materials or surface coatings are required. In industry,
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