Prosthodontics

Viability of magnetic denture retainers: A 3-year case reportHilary P.Y.Thean, BDS, MScVStephen K. L. Khor. BDS /̂Poey-LIng Loh, BDS. MSD'

The purpose of this article is to report on ttie use of a nsodymiu m-iron-bo ron magnet enclosed in a laser-welded stainless steel casing as a viable method ot retention tcr removable partial dentures. This mode ofretention dispenses with unsightly clasps, at the same time providing direct tooth support via the overden-ture abtrtment. Magnetic attachments are simpler to incorporate into the prostitesis than mechanical attach-ments, which require springs, ccplanar surfaces, or screws. Overdentures of this nature have the addedadvantage of being more stable and retentive than conventional partial dentures, and they are easilyremoved and seated without the patient having to grapple with clasps and complex paths of insertion. Thisparticular magnetic attachment exhibited no deterioration after 3 years in function, and the results so farhave been promising. (Quintessence Int 2001,32:517-520)

Key words: magnets, neodymium-ircn-boron, overdentures. rare-earth magnet, removable partialdentures, retention

Tbe use of magnets for denture and overdentureretention is not new.̂ "= Many autbors have identi-

fied size, corrosion, and subsequent loss of magnetismas major limiting factors pertaining to the applicationof magnets in tbe intraoral environment.*"^

Gillings' split-pole samarium-cobalt magnets (Inno-vadent). introduced to the profession in tbe 1970s,were tbe beginning of a new era for intraoral mag-nets.̂ These magnets bave a closed-field system withan attractive breaking force of 250 to 400 gf and a sizesmall enougb to ailow incorporation into overden-ttu'es- Since then, other minimagnetic systems like theJackson (Solid State Innovations), Dyna (DynaEngineering), Hicorex (Hitachi Metals), Steco-Titan-magnetics (Steco-System-Technik), and Magfit (AichiSteel) have evolved,'" New magnetic materials withhigher coercivity bave also been introduced,"

Neodymium-iron-boron magnets are permanentmagnets with the capability of generating a magneticforce 16 times tbat of an equivalent-sized aluminum-nickel-cobalt magnet.'^ Dental magnets as small as3-4 X 2-4 X 1.5 nun claim to have an attractive force of400 gf; an example is the Magfit EX (Aichi Steel), Thisis a closed-field magnet employing the saiidwich-type

Assistant Protessof, Department ot Restorative Dentistry. Faculty o(

Dentistry, National University ot Singapore.

^Dental Otticer, Ministry of Healtti. Singapore.

Reprint requests: Hilary P. Y. Theafi, Facutty of DenSstry, National Univer-SFty ot Singapore, Lower Kent Ridge Road, Singapore 119074. E-mail:

design (Fig 1), and it has been recommended by themantifacturer for use in removable denture retention.

Unfortunately, rare-earth magnets like neodymium-iron-boron are susceptible to corrosion and have beensbown to break down in tbe moutb," In the oral envi-ronment, magnets and keepers are required to with-stand chemical erosion as well as mechanical wearfrom mastication and brusbing; various coating proce-dures have been employed to seal them off from thebarsb intraoral environment. For example, paryienecoatings and stainless steel casings sealed with epoxyresin, welding, or laser welding have been used-'*'^

The intraoral magnet remains nontoxic as long asits casing is not breached and the magnet does notcorrode. The external magnetic field of the magnetand keeper system is negligible, a mere 0.1 mT. whichis appro-ximately double that of the earth's magneticfield. When the denture is not worn, the intraoralcomponent is only the stainless steel keeper; hence, ithas no magnetic field around it-

Currently, magnetic retention is presented to thepafient as a last resort. This is probably because it isconsidered by many clinicians to be less predictabletban traditional modes of denture retenfion by metalclasps and precision attacbments."'^

A distinct advantage of magnetic retainers overmecbanical attachments is that they do not wear andfail with an increasing number of breakaway cycles andcan remain functional indefinitely as long as they donot corrode,'" Tbe amount of extraction force a mag-netic retainer can exert on the tootb is also self-limitingbecause it cannot exceed its maximum retention value

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for vertical displacement. Lateral forces are also lim-ited, as magnets shde on their keepers during denturemovement.''

Magnetic attachments are simple to incorporateinto the prosthesis, as the technique does not requireeomplex coplanar surfaces or springs and specialequipment, unhke many of the available mechanicalprecision attachments.^'"

As for size, magnets have now been reduced to anacceptable size for intraoral use. If these tiny magnetscan overcome problems of corrosion, loss of magnet-ism, and unpredictability over the long term, they willhe a viable option and may even become the retainerof choice for overdenture abutments.

The magnet used in this study was a Magfit EX 600,a neodymium-iron-boron magnet that claims to havean attractive breaking force of 600 gf. This magnet isof a sandwich design and is hermetically sealed bylaser welding within a stainless steel casing [Fig 1).The keeper is of magnetizable stainless steel with achromium-rich outermost layer to prevent corrosion.The technique involves making a dowel space in theoverdenture abutment and fabricating a custom-castdowel by an indirect method. The keeper is incorpo-rated into the dowel during the casting process. Thekeeper dowel is cemented pertnanently onto the tooth.The magnet is subsequently located on the keeperdowel and attached to the denture hy a pickup tech-nique employing autopolymerizing acrylic resin.

CASE REPORT

A 65-year-old man presented for treatment with a dis-lodged maxillary left first premolar post crown. Hisexisting removable partial denture had a retentiveclasp arm on the huccal aspect of this dislodgedcrown. The seeond premolar on the same side hadheen converted to an overdenture abutment when thecrown fractured on a previous occasion.

The first premolar was examined, caries removed,and the root canal therapy assessed. The existing rootcanal therapy was deemed to be acceptable, so thedowel space was prepared using Parapost (Whaledent)twist drills, and the coronal aspect of the root wasmodified to receive a casting. A complete-arch impres-sion was made, and the dowel was fabricated using theindirect technique. The complete-arch impression wasnecessary to locate the keeper face in a direction per-pendicular to the path of insertion of tbe denture. Theprefabricated stainless steel keeper was incorporatedinto the dowel and core wax-up and noble metal wascast onto it (Figs 2 and 3).

The keeper dowel was retrieved and polished, tak-ing care not to modify the exposed keeper surface. It is

important to retain the original flat surface andcbromium-rieh coating to prevent corrosion at a laterdate. There must be no undercut areas on the keeperdowel or root. The keeper dowel was cemented ontothe premolar root with glass-ionomcr luting media.After final set of the cement, the magnet was locatedon the keeper and picked up into the denture withautopolymerizing acrylic resin.

The existing denture was modified in the followingway. A C-clasp in the left first premolar region was cutoff, and a shell crown was fabricated and incorporatedin its place. A vent was made on the palatal plate of thedenture. The vent hole had two functions: to allowexcess acrylie resin to escape and to enable the clini-cian to check the precise loeation of the magnet on itskeeper. The pickup method involved placing fluidacryhc resin on the inside of the shell crown and seat-ing the denture into position. The patient wasinstructed to occlude gently until the resin had poly-merized completely The denture was then taken out ofthe mouth, and excess resin was removed with tungstencarbide burs and smoothed with silicon points (Fig 4).

Clinically, an adequate retentive force from themagnet could be detected. The patient was examined afew weeks later to determine whether the prosthesiswas sfill retentive and whether the magnet and keeperwere stifl in situ. Thereafter, the patient was reviewedannually. At the 3-year examination, the results wereencouraging. There was some plaque around the rootat the gingival area, but the tooth showed no signs ofcaries lesions, mobility, or periodontal pockets. Themagnet was in situ and exhibited no surface deteriora-tion (Fig 5). The keeper dowel was intact and demon-strated no signs of color change or corrosion (Fig 6),Clinically, the retentive force of the magnet did notshow any deterioration at all. Most gratifying of all,the patient was very pleased with the denture.

DISCUSSION

Corrosion of the keeper or magnet and the resultantloss of magnetism are considered the most importantlimiting factors in the use of magnets intraorally.^"''"Both samarium-cohalt and neodymium-iron-boronmagnets are prone to corrosion. Only the samarium-cobalt ones, however, proved cytotoxic when testedwith a millipore filter technique.'^ In another corro-sion study of neodymium-iron-horon intraoral mag-nets using multispecies biofilms, the authors con-cluded that tbe levels of corrosion seen implied thatintraoral magnets could pose a hazard to health if pro-tective coatings were breached.'"

Many authors have concluded that the best way ofpreventing deterioration of the magnet is to protect it in

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Hermetic seal bymicrolaser welding

•eper holder

Fig 1 Magfit EX magnet and keeper. (Modified and reprintedwitti permission from Aichi Steel.)

Fig 2 Keeper dowei casting

Rg 3 Keeper dowel casting on the working cast Fig 4 Removable partial denture after piacement of magnet.

Rg 5 Magnet in denture at 3'year foilow-up. Fig 6 Intraorai view of the keeper dowel at 3-year follow-up.

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some kind of casing.^'' Theoretically, as long as the cas-ing is not breacbed, the functional life of the magnet canbe prolonged indefinitely.'" Stainless steel casings havebeen used for some fime, but as tbe casing is not a con-tinuous piece, the joints are weak links. Walmsiey et aFand Riley et al* investigated magnets encased in tita-nium domes sealed witb epoxy resin. They reported thatcorrosion occurred either when moisture seepedthrough the seal or the dome broke down. No mentionwas made of the make or model of tbe magnet investi-gated, altbougb a description oí it was given. In anotberreport, Walmsiey and Frame'' noted tbat some of tbemagnets used in their study had to be replaced afteronly 1 year of clinical service. Noar et al" tested thedurability of parylene coatings for intraoral magnets andconcluded tbat tbe coating is unlikely to protect tbemagnet adequately from intraoral forces. The Magfit EXseries is encased in a stainless steel casing and is her-metically sealed by laser welding. At tbe 3-year exami-nation in tbe present study, tbere was no loss of magnet-ism, nor were there any detectable signs of deteriorationof the stainless steel casing. This result is encouraging.

The use of magnets as an aid to denture retentionhas been cautious. It has been viewed as a novelty andgreeted with skepticism. These feelings are notunfounded, as magnets have yet to prove themselves.When magnetic retention systems become more pre-dictable over the long term, they can be proposed as atreatment of choice for overdenture work rather thanjust as an experiment. Overdenture abutments that canprovide botb reliable retenfion and direct tooth supportcan dispose of unsightly clasps and reduce the size andextension of flanges in the prosthesis. Elderly or physi-cally impaired patients would probably find a magneticretainer simpler to use than having to struggle withclasps, guide planes, and awkward paths of insertion.

CONCLUSION

This method of denture retenfion is simple, affordable,requires no special skills, and makes use of existingequipment and clinical and laboratory tecbniques. Thelifcspan and viability of such a treatment modality islargely dependent on the ability of the magnetic sys-tem to maintain its integrity over fime with constantuse. So far, the results have been promising. Furtberwork needs to be carried out to test the durability ofthe stainless steel casing and laser-welded joints inboth clinical and in vitro studies.

ACKNOWLEDGMENTS

The authors are grateful to Aichi Steel Corporation (Japan) for spon-soring the magnets; Prof C. L. Chew for his encouragement; DrKeson B. C. Tan for introducing this magnet to the dental school; MrPeier l.im of National University Hospital for his lechnica) expertisein ftibritiiting the keeper dowels; and Dr M. L. Wong for his assis-tance in the preparation of the manuscript.

REFERENCES

1. Grimaldi RM. The use of magnetic force in prosthetic den-tistry. Dent Dig 1954;60:205-207.

2. Behrman S|. The implantation of magnets in the jaws to aiddenture retention. J Prosthet Dent 1960;10.807-814.

3. Gillings BRD. Intraradicular anchorage of overlay denturesusing cobalt rare earth magnets. Aust Soc Prosth Bull 1977;7:27-32.

4. Gillings BRD. Magnetic retenfion for complete and partialoverdentures. Part 1. ] Prosthet Dent 1981;45:484-491.

5. Gillings BRD. Magnetic retention for overdenfures. Parf 2.J Prosthet Dent 1983;49:607-618.

6. Angelini E, Pezzoli M, Zucchi F. Corrosion under static anddynamic conditions of alloys used for magnetic retention indentistry. J Prosthet Dent 1991;65:848-853.

7. Walmsiey AD, Brady CL, Smith PL, Frame JW. Magnetretained overdentures using fhe Astra dental implant sys-tem. Br Dent ) 1993;174:599-404.

8. Riley MA, Williams AJ, Speight )D, Walmsiey AD, HarrisIR. Investigafiorts into the failure of dental magnets. Int JProsfhodont 1999;12:249-254.

9. Walmsiey AD, Frame JW. Implant supported overdentures-The Birmingham experience. J Dent 1997;25(suppl l):43-47

10. Jackson TR, Healey KW. Rare earth magnetie attachments;The state of fhe art in removable prosthodonfics. Quintes-sence Inf I987;18;41-51.

11. Livingsfon JD. Supermagnefs. In: Driving Force-TheNafural Magic of Magnefs. Cambridge, MA' HarvardUniversify Press, 1996:54-56.

12. Vardimon AD, Graber TM, Drescher D, Bourauel C. Rareearth magnels and impacfion. Am J Orthod DentofacOrthop 1991;100:494-512.

13. Noar JH, Wahab A, Evans RD, Wojcik AG. The durabilityof parylene coatings on neodymium-iron-horon magnets,Eur] Orthod 1999;21:685-693.

14. Wang NH, von der Lehr WN. The direct and indirect fech-niques of making magnetically retained overdentures. JProsthet Dent 1991;65:n2-117

15. Davis DM, Packer ME. Mandihular overdentures stabilizedby Asfra Tech implants with either ball attachments or mag-nets: D-year results. Int J Prosthodonf 1999;12:222-229.

16. Bondemark L, Kurol J, Wennberg A. Orthodontic rare earthmagnets-ln vitro assessment of cytofoxicity. Br J Orthod1994;21:335-341.

17 Wilson M, Patel H, Kpendema H, Noar JH, Hunt NP,Mordan NJ. Corrosion of infra-oral magnefs hy multi-species biofilms in the presence and ahsence of sucrose.Biomaterials 1997; 18:53-57.

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