Prosthodoníics

Improved retention for bonded cast metal rests: a case reportAlton M. Lacy*

This article describes improvements in retention of resin-bonded, cast metal rest sealsthai utilize cast pins for additional stabilization and resistance to intraoral forces ofdislodgment. This method combines the classic design of pin-ledge retainers with theresin-bonded concept of retention to provide a reliable and stable, but conservative,method of altering tooth form to support removable partial dentures. (Quintessence Int}99I;22:439-^42.)

Introductíon

Teeth that serve as abutments or indirect retainers forremovable partial dentures often require alterations incontour to accommodate clasps and rests.' Althonghcustom-contoured crowns are often placed to fulfillthis need, it is often more convenient and more con-servative to attach a rest seat to the tooth than toprepare the entire tooth.'

Composite resin may be conveniently bonded to en-amel to alter contour, but free-standing resin has lim-ited durability and low fracture strength. The princi-ples of re sin-bonded (Maryland) bridges may be usedto wax rests, cast them in nickel-chromium (Ni-Cr)alloy, and sandblast and adhesively bond the rests toetched enamel with composite resin cements that arespecifically adhesive to these alloys.̂ "* Cast metal pro-vides the strength necessary to support a dentureframework, but often must be retained on the toothunder adverse forces of occlusal loading.'' In additionthere are to successful cases, reports of debonding ofresin-bonded retainers for "Maryland" bridges in theliterature.*""

Seto and Caputo'^ have shown that placement of ahorizontal groove along the lingual bonding surface

• Associate Professor, Deparlmeni of Resiuralivc Dentistry, Uni-vershy of California. San Francisco, School of Dentistry, 707Parnassus Avenne, San Francisco, California 94143-0758.

of a cingulum rest improves retention of bonded rests.Wiltshire'^ has described improvements in retainer de-sign for resin-bonded bridges and retainers that utilizecast pins (which penetrate the dentinoenamel junctionapproximately 1 mm) in addition to resin-retainedetched metal. The pins are readily cast as part of themetal retainer and are effective in resisting the forcesof dislodgment. Wiltshire" reported no incidence ofatraumatic debonding, and this has also been my chn-ical experience with no known cases of retainer dis-lodgment when such a design has been employed inresin-bonded prothèses. Application of these princi-ples thereby allows the clinician to alter preparationdesign simply and provide improved retention for res-in-bonded rests.

Case report

The patient is a saxophone player who had lost hismaxillary left central incisor and right central and lat-eral incisors in an accident. His maxillary arch wasprepared for a rotational path removable partial den-ture requiring cingulum rests on the right canine andleft lateral incisor. Previous efforts to make such restssimply by enamel contouring provided inadequatesupport, and the addition of acid-etch-retained com-posite resin had resulted in fracture of the compositeresin within a few weeks of placement of the dentures(Fig 1).

The teeth were prepared by removing approximately0.5 mm of enamel from the lingual surface in the areaof the proposed rest, and pinholes were placed in thecingulum area of each tooth, first with a No. 1 round

Ouintessence International Volume 22, Number 6/1991 439

Prosthodontics

Fig 1 Preoperative oanine abutment. Fig 2 Prepared lingual surface with two pinholes pene-trating approximately 1 mm into dentin.

Fig 3 Waxup of rest seat on refractory die. Fig 4 Refractory die and waxup sprued for investment.

bur to penetrate the enamel, followed by a No. 170Lcarbide bur that penetrated roughly I tnm into thedentin (Fig 2).

An impression was taken with polyvinylsiloxaneimpression material; the pinhole was first filled withlow-viscosity material usitig a lentulo spiral on a low-speed handpiece. The impression was then poured ina refractory die stone and secondarily in conventionaldie stone. The cingulum rest was waxed directly ontothe refractory die without special regard for filling thepinhole with wax (Fig 3).

After completion of the waxitp, the tooth segmentwas cut from the refractory cast, a wax sprue-fortnerwas attached, and the unit was soaked in water andthen reinvested in the same phosphate-bonded invest-

ment (Fig 4). The rest was cast in Ni-Cr alloy byconventional techniques but altered lo provide rela-tively low expansion. After divestment ofthe castitig,the rest was sandblasted with 50-)im aluminum oxideon its internal surface, and the external surface waspolished. A schematic illustration ofthe rest assemblyis shown in Fig 5.

The lingual enamel was cleaned with pumice andetched with 50% phosphoric acid gel for 20 seconds,then rinsed and dried. No special precautions wererequired to prevent contact of the acid with the dentinin the pinhole, since the remaining dentinal thicktiesswas great and the surface area of expo .̂cd dentin wasrelatively small. The rest was bonded in place usingPanavia OP resin (Kuraray), providing adhesion to

440 Quintessence International Volume 22, Number 6/1991

Prosthociontics

Figo Schematic diagram of rost position on prepared Fig 6 Rest cemented intc place,tooth.

Fig 7 Bilateral placement ot rests on the canine and lat-eral incisor.

Fig 8 Rest-supported, rotational path partial denture inplace.

hoth the etched enamel and the sandblasted Ni-Cralloy (Fig 6), The opaque resin prevented "show-through" of the dark metal when the tooth was viewedfrom the facial aspect. In this case, a similar rest wasplaced on the lateral incisor (Fig 7), and a rotationalpath removable partial denture was fabricated to re-store the lost anterior teeth (Fig 8).

Conclusion

The simple inclusion of a cast pin adds substantialresistance to dislodgment from shear, tensile, and tor-quing forces. The rest becomes an analogue of thewell-known pin-ledge retainer whieh, while rarely usedin contemporary practices for fixed partial prostheses.

nonetheless has established a favorable reputation asa conservative and reliable retainer. Augmentation ofmechanical retention with the aeid-etching techniqueand adhesive, msoluble cetnenting resins provides theelements for long-term success with this cast rest.

References1. Aciidemy of Denture Prostlietics: Principles, concepts, and prac-

tices in prosthodontics—1989./i'ra,îr;iff DenM989;6t: BS-109,

2. Lyon HE: Resin bonded etched metal rest seats, J Prostiiel Dentt<)S5;53:366-368,

3. Abassi J. ßertolotti RL, Lacy AM, et al: Bond strengths ofporcelain repair monomers, / Denl Res 198E;57:233 (abstr No.

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Prosthodontics

Myers ML, Baiïilay 1, Cooper LF, et al: Comparison of elec-troL:hemical etching and adhesive resins for resin-bonded re-tainers. J Dent Res 1987;66:t98 (abstr No 736).Liao R, Grolman K, Thompson V, et al: Adhesive resins: tensilebond to tnetal atid resin thickness effects. J Denl Res1986:65:237 (abstr No. 6t2).

Thornpson VP, Grolman KM, Liao R: Bonding of adhesiveresins to various nonprecious alloys. / Dent Res 1985;64:314(abstr No t258).

Seto BG, Caputo AA: Stress analysis of resin bonded cinguiumretainers. J Dm/ Res 1986;65:312 (abstr No. 1283),Williams VD, Thayer KE, Denehy GE, et al: Cast metal, rcsitibonded prostheses: a 10-year retrospeetive study. J Prosthet Dent1989:16:436-441.

Thompson VP, Wood M: Etched casting bonded retainer recalls:results at 3-5 years. J Dent Res 19R6;65:3tl (abstr No, 1282).Mohl G, Mehra R: Clinical evaluation of etched metat resinbunded bridges. J Dent Res 19B6;65:311 (abstr No. 1278).

It. Crispin ßJ, f-isher DW, Avera S: Etched metal bonded resto-rations: three years of clinical foltow-iip. J Dent Res 198fi;<í5:312[abstrNo. 1285).

12, Wiltshire WA, FerreiraMR, Nel JC: Clinical évaluation of resiti-bonded bridges at 1-3 years../ Dent Res 1987;66:]99 (abstr No.740),

1Î. Al-Shammery AR, Ibraheem HS: Acid etch bridge — 36 casereports in 38 months fotlow-up. / Dent Res t988;67:306 (abstrNo. 1545).

14. Gratton DR, McConnell RJ, Hussey DL: Combining conven-tional and resin bonded bridge retainers, a clinicat assessment,JDent Res 1989;68:9iy (abstr No. 425).

15. Verzijden CWGJM, Creugers NHJ: Survival rate of resin-bond-ed bridges: a 5-year evaluation. / Dent Res 1989:68:920 (abstrNo. 426).

16. Seto BG, Caputo AA: Bond strength of some resin bondedcinguium rest seats. / Dent Res 1987;66:199 (abstr No. 738).

17. Wiltshire WA: Resin bonded fixed partial dentures utilizing ad-ditional pin retention. Quintes.-ience Int 1986; 17:343-347. O

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