Prosthodontics

Preinipregnated, fíber-reinforced prostheses.Part I. Basic rationale and complete-coverage andintracoronal fixed partial denture designsMartin A. Freilich, DDS*/Jacqueline P. Duncan, DMD, MDSc*/Jonathan C. Meiers, DMD, MS**/A. Jon Goldberg. PhD***

This is the first of two articles describing the development and use of a continuous fiber-reinforced com-posite as a framework for the fabrication of fixed partial dentures and splints. The chemical compositionand physical structure of the fiber-reinforced composite, along with the progression and development of avariety of fiber-reinforced composite materials, are discussed. Criteria for case selection, tooth prepara-tion, and the clinical and laboratory procedures required for partial- or complete-coverage fixed partialdenture fabrication and delivery are described. (Quintessence Int 1998;29:689-696}

Key words; complete-coverage prosthesis, fiber-reinforeed eomposite, fixed prosthesis, framework,partial-cove rage prosthesis, prei m preg nation

Clinical relevance

A new fiber-reinforced composite provides thepotential for fabrication of a metal-free, ceramic-free prosthesis with potential for long-term dnrabil-ity and excellent esthetics.

Traditional dental restorative composites consist ofparticles such as quartz or giass contained within a

resinous matrix. These particulate composites are com-monly used to restore defects in a single tooth or as aveneer material for a tooth or prosthesis, hut they arerarely used alone to make final complete-coveragecrowns and fixed partial dentures.

This article describes a new technology that usesbundles of long glass fibers preimpregnated with a resinmatrix to make frameworks for fixed prostheses. Thisfiher-reinforced composite (FRC] framework replacesthe classic metal framework of a porcelain-fused-to-

'Deparrmenl of Proilhodontics, University of Conneclieul. Sehool ofDental Medicine, Farmingloii, Conneclieul.

** Depanmenl of Restoralive Dentislry and Endodontology, Universjiyof Connecticut, Sehool of Dental Medieine, Farmington, Conneeticul.

*** Depanmeiil of Ptosrh odontic s and Center for Biomaterials, Universityof Connecticut, School of Denial Medicine, Farminglon, Conneclicut.

Reprint requests: Dr Martin A. Freilich, Associate Professor, Deparl-meni of Proslhodontics. University of Connecticut, School of DenialMedicine 263 Farmingion Avenue, Farminglon, Connecticut 06030-1615.E-mail: FREILICH@NSO2.UCHC.EDU

metal prosthesis, while a particulate composite appliedover this FRC substructure corresponds to the porcelainapplied in a traditional restoration. The FRC frameworkprovides strength and rigidity beneath the outer layerof particulate composite. This iwo-phase polymer pros-thesis combines the best characteristics of the fiher-reinforced composite (strength and rigidity) with thoseof the particulate composite (wear resistance and esthet-ics) to provide an alternative to all-ceramic or porce-lain-fused-to-metal restorations.

The fiber-reinforced composites described in thisarticle are different from most of the commercially avail-able Fiber-reinforcing materials. These othei- materials arecomposed of woven glass or polyethylene fihers that arehand impregnated with a composite or unfilled resin bythe dentist or laboratory technician. In contrast, the fibersand the resinous matrix of this new preimpregnated FRCare coupled during the manufacturing process. This tech-nique results in fibers that are uniformly impregnatedwith matrix.' A magnified cross section of the longfiher-reinforced composite bonded to a particulate com-posite is shown in Fig 1. It has been shown that, underthree-point loading conditions, preirnpregnated. unidirec-tional FRC is able to support two to three times the loadand exhibits up to 10 times the flexure modulus of somewoven FRCs that require hand impregnation.-

The mechanical properties of earlier preimpregnatedFRC formulations have undergone extensive testing.^-'One of these early formulations was a thermoplasticmaterial consisting of S2 glass fihers and a polycarbon-ate matrix." This material had been used to make

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Fig 1 Scanring electron micrograph of a Fig 2a First-generation liber-reinforced Fig 2b First-generation tiber-reinforcedfiber-reinlorced composite composite fixed parliaf denture. composite fixed partial denture replacing

the mandibular first molar.

botided prostheses,' splints, atid retainers^ in human re-search subjects (Figs 2a and 2b). Over the course of al-most a decade of clinical follow-up, these preimpreg-nated polycarbonate FRC prostheses and applianceshave never exhibited catastrophic structural failure.However, while the polycarbonate FRC demonstratedhigh stretigth and other excellent mechanical properties,it had undesirable handling characteristics and anopaque appearance and did not bond well to enamel orother resin composite materials.

The improved FRC formulation described here islight- and heat-polymerized and contains S2 glass fibersthat are preimpregnated with a bis-GMA matrix. It ex-hibits the same excellent physical properties of the ear-lier polycarbonate FRC but handles with much less dif-ficulty. Mechanical tests of this new material haveshown that it has up to seven times the strength of par-tieulate composite. It is also much more rigid than par-ticulate composite,'-"

This newer formulation also has improved opticalproperties and is not opaque. Consequently, no addi-tional coat of masking material has to he placed overthe FRC. This allows a relatively thin (approximately0,5-mm) layer of paniculate composite to be placedover the FRC substructure while maintaining an estheticappearance.

Advances in resin composite technology haveenhanced and supplemented the FRC technology. Someof these improved materials include Sculpture (Jeneric/Pentron), Artglass (Kulzer/Jelenko), Poly(mer)glass(Kulzer/Jelenko), Targis (Ivoclar/ Williams), Ceromer(Ivoclar/Williams), and belleGlass HP (belle de StClaire/Kerr), These products employ new polymer for-mulations, improved filler particle distribution andloading, and polymerization with intense light, vacuum,and heat. All of these factors have improved their wearresistance and increased elasticity, which, in tum, hasresulted in increased impact and fracture resistance,'^-'^This new generation of composite materials, used as theoverlay or veneer providing the anatomic shape and

contour over the FRC framework, provides the potentialfor a metal-free and ceramic-free fixed partial denturewith long-term durability and service.

Complete-coverage prostheses

Case selection

There are several indications for selecting a fiber-reinforced polymer prosthesis: (!) the need for an opti-mal esthetic result; (2) the desire for a metal-free, non-porcelain prosthesis (especially important for those withmetal allergies); (3) the desire for ease of fabrication inthe laboratory (versus porcelain-fused-to-metal designs);(4) the desire to decrease wear of the opposing dentition(versus porcelain): and (5) the desire to bond the pros-thesis retainer to the abutment teeth.

These materials can be used anywhere in the mouthwhere esthetics is important. The lack of metal andopaque materials allows for good translucency and avery natural appearance of the prosthesis. This naturalappearance at the cervical aspect of the prosthesis re-tainer eliminates the need for the dentist to hide marginssuhgingivally, where they may create periodontal prob-lems for the patient. The supragingival margins of thispolymer prosthesis hlend in easily with the nonpreparedtooth structure apical to the tooth preparation finishline, just as the overall prosthesis blends in with the ad-jacent natural teeth.

The use of resin composite luting materials that bondto the intetnal aspect of the polymer prosthesis retainersand to the dentin and enamel of the abutment teeth pro-vides improved retention of the prosthesis. This featuremay provide critical retention of a polymer prosthesison abutment teeth that cannot be made to exhibit opti-mum geometric retention form.

Contraindications for selecting an FRC prosthesis in-clude (Ij the inability to maintain good fluid control,such as in patients exhibiting chronic or acute gingivalinflammation or when margins would be placed deeply

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TABLE 1 Case selection for fiber-reinforced prostheses

Indications

Patients who desire an optimalesthetic result

Patients who require or i3esire am eta i-free prosthesis

Cases in which ease of fabrication in thelaboratory is desired

Patients In whom it is desirabie todecrease wear to opposing teeth

Patients in whom it is desirabie fo usean adhesive iuting technique

Contraindications

Palients in whom it is impossibie tomaintain fluid control

Pafients who require a iong-spanprosthesis

Patients wilh parafunctionai habitsPatients who have unglazed porcelain

in the opposing dentitionPatients who abuse aicohoi

into the sulcus; (2) the need for a long-spati prosthesis,ie, two or more pontics; (3) patients who exhibit para-functional habits; (4) the presence of unglazed porcelainor removable partial denture frameworks that wouldoppose the prosthesis; and (5) patients who continue toabuse alcoholic substances. Case selection criteria aresummaiized in Table 1,

The use of adhesive cetnentation techtiiqnes requiresthat the operator be able to maintain a contamination-freefield. Rubber dam isolation is ideal and should be usedwhenever possihle. At this time, fixed partial denturesthat replace tnore than one tooth are not recommendedhecause there is a lack of documentation regarding theahility of the material to support greater edentulousspans. Increased susceptibility to wear or fracture mayoccur in patients who exhibit bruxism or clenching, andstuface degradation of the particulate composite is likelyto he a problem in alcoholics. Because there are no clini-cai data to substantiate how the FRC prosthesis wouldperform when subjected to these conditions, it should nothe considered as the treatment of choice at this time.

Clinical and laboratory procedures

Tooth preparation. Tooth preparations made for a rein-forced polymer prosthesis should provide adequatespace for the FRC substructure and the covering panic-ulate composite. Features that clinicians must be awareof are (]) adequate amount of tooth reduction; (2) mar-ginal configuration; (3) proximal step placement onaxia] walls adjacent to edentulous space; and (4) lingualstep placement on anterior abutments.

Chamfer preparations with minimally tapered axialwalls and smooth, continuous finish lines (with a 90- to120-degree cavosurface angle) are recommended. Addi-tionally, a 1.2- to 1,3-mm axial reduction on the facialand lingual surfaces and a minimum of 1,5 mm of oc-clusa! reduction are required for adequate materialthickness. These preparation guidelines are advocated atpresent; there are no clinical data or in vitro studies that

have Indicated which type of finish line, the shoulder orchamfer, is more desirable.

Additional preparation features include proximalsteps as well as an occiusai isthmus. The proximal .stepsshould he 2,0 to 3,0 mm wide and need be no more than1,0 tnm deep. They are prepared on the edentulous sideof the coronal half of the axial walls of the abutmentteeth. The isthmus is a shallow channel (0,5 mm deep)and 2,0 to 3.0 mm wide) that is prepared on the occiusaisurface of the ahutment teeth. These features createaddilional room for the FRC substructure. The proximalbox allows for sufficient material at the connector areaand also provides a positive stop for the technician toplace the pontic FRC support. The occiusai isthmus al-lows for a continuous I-heam configuration of FRC overeach abutment tooth and across the edentulous space.

An ideal complete-coverage preparation on a poste-rior tooth is shown in Fig 3, Anterior tooth preparationsshould exhihit a step or double-shoulder eonfigurationon the lingual surface so that the dental laboratory tech-nician may avoid creating a retainer with an overcon-toured lingual axial surface (Fig 4),

Working casis and dies. Final impressions, dies, andworking casts should he made with conventional meth-ods and materials. A second pour of the final impressionis made so that the dies of both abutment teeth can beleft in one solid segment or solid cast for use while theFRC substructure is made. The first pour of the impres-sion is cut into individual dies, allowing the technicianeasy access to all retainer margins, A thin coat of a rub-ber separating material should he painted on the dies towithin 1,0 mm of the finish line. An additional non-spaced separating material should then be placed overthe remaining die surface.

Prosthesis fabrication. It is important for the dentistto he familiar with the design features of an FRC pros-thesis in order to be able to fully appreciate tooth prepa-ration requirements and to critically evaluate fiber-rein-forced frameworks and prostheses made hy the dentallahoratory. Fabrication of an FRC prosthesis involves

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Posterior FRC Tooth Preparation

Fig 3 Abutment conligutation atter preparation for a compiete-coverage retainer on a posterior tootb.

Anterior FRC Tooth Preparation

snouldeinfHpchamrerflnlihll(1,5-1,5 mm)

Fig 4 Abutment configuration atter preparation for a connplete-coverage retainer an an anterior tootri

four steps: íl¡ fabrication of the coping, (2) placementof the pontic bar FRC connector, (3) development of theFRC enveloping substructure, and (4) addition of theparticulate composite overlay,

A thin "coping" of opacious body particulate com-posite (Sculpture, Jeneric/Pentron) is adapted to the dies(Fig 5a), The coping includes a cervicai collar that isplaced on the a\ial wall. This collar helps to ensure thatthe FRC is contained above the cervical third of theaxial walls during its placement. A notch is also placedin each of the copings at the midproximal level of theaxial surfaces that face the edentulous area (Fig 5b¡,This notch corresponds to the proximal step placed inthe tooth preparation and will .stabilize the FRC materialwhen it is placed between the two copings. The com-pleted copings are removed from the trimmed dies andplaced on the second-pour, solid die cast.

The FRC (FibreKor, Jeneric/Pentron) is availablefrom the manufacturer in long strips, measuring 3,0 mmor 6,0 mm in width and 0,3 mm in thickness, that can becut to the desired length with ceramic scissors. A bar ofFRC is then formed by combining five to seven strips ofthe 6,0-mm-wide FRC, cut to the appropriate inlerabut-ment length, A small amount of Special Liquid, a bis-GMA gel supplied by Che manufacturer, is placed intothe notches of the copings to enhance the bonding be-tween the oxygen-inhibited layer of the coping and theunpolymerized FRC, The connecting bar is then placedand bonded into the notches of the composite copingswith light polymerization (Fig 5c), The positioning ofthe bar must leave enough space between it and the op-posing tooth to allow adequate thickness of external par-ticnlate composite and good gingival embrasure form,

A long, single strip of 3,0-mm-wide FRC is thenbonded to one end of the already polymerized ponticbar (Fig 5d), The long strip is then adapted and light

polymerized continuously along the bar and around theaxial surfaces of the copings. This is done in a stepwisefashion; only one segment of the FRC strip at a time isplaced in the desired position and then selectively poly-merized and bonded. When the entire strip is adaptedand bonded to the copings, the first portion of the sub-structure is completed.

Additional strips of FRC are cut to size, placed, andbonded to the buccal, lingual, and cervical surfaces of theFRC bar that already spans the edentulous area, A contiti-uous occlusal strip is bonded to the occlusal surface ofone coping, over the occlusal aspect of the PRC in theedentulous area, and across to the occlusal aspect of thesecond coping. This stepwise construction of the FRCsubstructure results in the creation of a miniature ponticcomposed of bonded and light-polymerized layers ofFRC, some of which arc continuous with the FRC thatwas bonded to and around the abutment tooth copings.

An important characteristic of the fiber-reinforcedpolymer prosthesis framework is its single-unit construc-tion. Although the framework is made in layers, begiti-ning with the opacious body paniculate composite, alllayers retain their oxygen-inhibited external surface priorto the placement of the subsequent composite layer.There is no modification made to each of the compositelayers once they are polymerized, maintaining the in-tegrity of the oxygen-inhibited layer and preventingthese layers from being contaminated with grindings,dust, grease, or debris. This assures a unified pro.sthesiswithout boundaries between the various layers and with-out areas of potential weakness or separation within thefiber reinforcement. If boundaries are created, the sub-structure would have the potential to fail at loads that arelower than the component materials are ultimately ableto withstand. The long fibers of the pontic area are thesame fibers that encircle the axial walls or cover the oc-

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Fig 5a Thin coping ot opacious Dûdy par-ticulate composite adapted to the die.

Fig 5b Corr.pposite coping.

leied, ligh!-pülymerizeú com- Fig 5c Bar ot nultiple layers ol fiber-rein-forced composite spanning the pontic re-gion and bonding the abutments together.

Fig 5d Continuous strip ot tiber-reinforcedcomposite bonded to one end of the ponticbar and then wrapped around the axial sur-faces of the copings while being poiynier-izect segmentaliy.

Fig Se Occiusal uiew of the completedfiber-reinforced composite framework.

Fig 5f Lingual view of the completed fiber-reinforced composite framework.

FigSg Completed prosthesis. Fig 5h Compieied prosthesis Fig 5i Instruments used to shape, finish,and polish the fiber-reintorced compositeprosthesis in the dental laboratory.

Fig 5j Faciai view QI the compieted pros-thesis at deiivery.

Fig 5k Linguai view of the completed pros-thesis at delivery.

Fig 51 Instruments used to shape, finish,and poiish the liber-reinlorced compositeprosthesis in the dentai opeiatory.

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clusai surfaces of each of the abutment teeth. The com-pleted FRC substnicture is shown in Figs 5e and 5f.

The FRC substructure exhibits an oxygen-inhibiledlayer on its external surface allowing ior ihe bonding ofthe outer layer of particulate composite (Sculpture), Thisouter layer of composite is built incrementally, allowingfor the placement of cervical colors, translucent cover-ings, and a variety of customization where indicated. Acompleted three-unit prosthesis is shown in Figs 5g and5h. After final light polymerization, shaping, fini.shing,and polishing, the fixed partial denture is placed in anoven at IIO'C and a 29-inch vacuum for 15 minutes tomaximize strength and other physical characteristics.

Figure 5i shows instruments (fine tungsten carhidccutter, Diacomp composite finishing kit, flexible dia-mond disk, and diamond polishing paste. Brasseler)used to shape, finish, and polish the fiber-reinforcedprosthesis in the dental lahoratory.

Prosthesis delivery. As with any delivery, the dentistmust check proximal contacts, occlusion, anatomic form,and shade and make any necessary adjustments. Proxi-mal contacts can he added by using a hybrid restorativecomposite after the surface is roughened and an unfilledresin is placed on the FRC's overlay particulate com-posite. The shade of the prosthesis should be assessedusing a water-soluble try-in paste that corresponds to theluting resin composite shade selected. Minor adjustmentscan be obtained by selecting darker or lighter lutingresins. The translucency of the FRC prosthesis allows theluting composite to play a role in the final shade.

Luting an FRC prosthesis involves those proceduresthat accompany any bonded restorative procedure: isola-tion of the abutment teeth; treatment of the inner surfaceof the FRC prosthesis retainers; and treatment of theabutment teeth. First, the internal surfaces of the retain-ers are sandblasted with 50-|jm aluminum oxide and thentreated with a bonding agent (Bond It. Jeneric/Pentron).Concurrently, the abutment teeth are etched with 37%phosphoric acid, rinsed, and lightly dried (not desiccated)and treated with a dentin bonding system (Bond It).

The sandblasted, primed FRC prosthesis is thendelivered with a low-vi.scosity, hyhrid resin compositeluting material (Lute-It. Jeneric/Pentron). This lutingmaterial will form a unified structure, linking the insideof the retainers to the etched enamel and hybridizeddentin of the abutment teeth. The excellent esthetic re-sult can be seen in Figs 5j and 5k. Note the close matchin shade and translucency to the adjacent natural tooth.The margins are supragingival, yet this tooth-prosthesisjunction is difficult to see because of the close matchbetween the natural tooth and the FRC prosthesis.

Figure 51 shows the adjusting, finishing, and polish-ing instruments (fine tungsten carbide cutter, Diacomp

composite finishing kit, multifluted carbide finishingburs, and diamond polishing paste. Brasseler) used bythe dentist at chairside prior to delivery.

Intracoronal prostheses

Case selection

The FRC partial-coverage prosthesis allows a more con-servative design when the abutment teeth are unrestoredor have modest intracoronal restorations. When an im-plant is not possible, an etched-metal (Maryland) pros-thesis is the only other conservative, fixed treatment al-ternative. However, these prostheses are becoming lessfavored hecause of problems with dehonding, graying ofabutment teeth caused by metal show-through, andovercontoured retainers. The advantages discussed forthe complete-coverage hridge (esthetics, metal-freeframework, ease of laboratory fabrication, and use of anadhesive cementation technique) remain relevant.

Clinical and laboratory procedures

Tooth preparation. The preparation design can incor-porate an existing cavity preparation, as long as thewalls are made divergent. Abutment teeth with no exist-ing restorations are prepared with a Class II compositeinlay design with a short (occlusogingival) proximalstep (Fig 6). There is no purpose in preparing a fully ex-tended proximal box, because the FRC cannot be placedapical to the contact area and maintain adequate emhra-sure form. Only particulate composite would be used iofill the portion of the box apical to the contact, and thiswould provide no benefit to the overall restoration.

This point is clearly demonstrated in Fig 7a. In thiscase, the existing restoration necessitated the box evi-dent on the distal surface of the premolar. Ohserve theextent of the FRC pontic bar in relation to the contactarea. The particulate composite apical to FRC does notadd to the structural integrity of the overall prosthesis.

Prosthesis fabrication. As for the complete-cover-age prosthesis, dies and working casts should be fabri-cated. The rubber die spacer is not placed, only the lu-bricant. The framework design is less complicated thanfor complete-coverage restorations hecause the intra-coronal design eliminates the need for circumferentialwrapping of the FRC around the axial walls of theabutment teeth.

A thin layer of body particulate composite is placedon the floor of the preparation on the dies and is lightpolymerized (Fig 7b). Six to seven FRC strips are cut tosize and tben placed over the particulate layer withineach preparation and across the edentulous space (Fig

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Intracoronal FFtC Tooih Preparation

proilnIBl itap

Fig 6 Preparation design for an intracoronal prostttesis.

Fig 7a The liber-reinlorced composite is Fig 7b Tiiin layer of opacious body partie- Fig 7c Multiple layers of fiber-reinforcednot placed below tlie proximal contact area,thus ttie proximal tooth preparation shouldnot extend apical to tiie contact area unlessa previous restoration or caries dictatessuch placement.

ulate oomposite adapted to the die. composite bonded together to span thepontic region and connect the abutments.

Fig 7d Completed prosthesis Fig 7e Facial «iew of lhe completed pros- Fig 7f Lingual view ot the oompleted pros-thesis at delivery thesis at delivery

7c). The FRC is polymerized, and then an additional 12to 15 strips of FRC are added to the buccal, lingual, andcervical aspects to create a miniature pontic shape. Thecompleted anatomic form of the pontic and retainers isdeveloped with paniculate composite. The final pros-thesis can be seen in Fig 7d,

Prosthesis delivery. Delivery proceeds with verifica-tion of marginal fit, occlusion, and shade. Water-solubleshades of the luting composite may be used, as recom-mended with complete-coverage prostheses. Afteradjustments are completed, the teeth should be isolated

with a rubber dam. The abutments are then etched andcoated with a dentin primer/adhesive. The inner aspectsof the inlay retainers are lightly sandblasted with 50-[imaluminum oxide and then coated with primer/adhesivefollowed by a dual-cure luting composite.

After light polymerization and removal of the rubberdam, the occlusion is reconfirmed. Any final adjust-ments can be made with 30-fiuted carbide finishing bursand paper disks. Rubber polishing points can be usedfor the final finish. The esthetic final result can be seettin Figs 7e and 7f.

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Conclusion

This article describes a glass fiber-teinforced compositethat can be used in the fabricaiion of fixed prostheses.This prosthesis consists of an external particulate com-posite combined with an internal FRC substructure ihatis honded to abuimeni teeth requiring a complete- orpartial-coverage restoration. Over the past 8 years,investigators at the University of Connecticut haveplaced almost 100 FRC splints, retainers, and fixed par-tial dentures of variotis designs and resin matrix mater-ials. Although the earlier experimental FRC formula-tions experienced some problems with bonding,handling, and esthetics, none of the original FRC appli-ances or prostheses has ever exhihited catastrophicstructural failure.

In vitro testing to define the mechanical properties ofa new light- and heat-polymerized, fiber-reinforcedcomposite continues, along with the comprehensiveclinical evaluation of various complete- and partial-coverage designs of fiber-reinforced polymer fixed par-tial dentures. At this time, however, there are no long-term clinical data regarding the overall success of theFRC prosthesis.

Acknowledgments

We wish to thank Ms Diane Kosis, MPH, Coordinator of the Univer-sity of Connecticut Clmical Dental Research Cetiter, for her invalu-able assistance. We also wish to thank Ms Shirley Carrolla for herexcellent laboratory support.

This research was supported by the National Institute of DentalResearch, NÏH Research Grant DE-09l2ñ. Connecticut Innovations,Inc, and Jeneric/Penrron, Inc,

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