In vitro resistance of reinforced interim fixed partial dentures

Peter Pfeiffer, Prof Dr Med Dent,a and Lars Grube, Dr Med Dentb

School of Oral Medicine, University of Cologne, Cologne, Germany

Statement of problem. Comprehensive restorative dental treatment often necessitates the use of interim fixedpartial dentures (FPDs) with high stiffness, especially in long-span restorations or areas of heavy occlusal stress.Purpose. This in vitro study evaluated the fracture load of interim FPDs made with various materials and spanlengths.Material and methods. Groups (n � 3) of interim FPDs were fabricated with prosthodontic resin materialson 2 abutments with 3 different pontic widths of 3 units (12 mm), 4 units (19 mm), and 5 units (30 mm). Thefollowing materials were tested: (1) a thermoplastic polymer (Promysan Star), (2) Promysan Star with a veneeringcomposite (Vita Zeta), (3) a nonimpregnated polyethylene fiber reinforced resin (Ribbond) with a veneeringcomposite (Sinfony), (4) an impregnated fiber reinforced composite system (Targis/Vectris), and (5) a conven-tional polymethyl methacrylate, Biodent K�B, (control group). After 5000 thermocycles in 2 water baths at 5°and 55° C, the FPDs were temporarily fixed with a provisional cement on the corresponding abutments andsubjected to 3-point bending until fracture by a universal testing machine. Statistical analysis consisted of ananalysis of variance (ANOVA, 1-way, 2-way) and Bonferroni-Dunn’s multiple comparisons post hoc analysis fortest groups (� � .05).Results. Fracture resistance (N) differed significantly for 3 (mean: 640 � 146 N), 4 (626 � 229 N), and 5 unit(658 � 98 N) Targis/Vectris FPDs compared with the corresponding Promysan (284 � 21 N to 125 � 73 N),Biodent K�B (247 � 91 N to 218 � 85 N), and Promysan/Vita Zeta (95 � 15 N to 82 � 6 N) groups (P � .05).Significant differences were obtained for the 4 and 5 unit Targis/Vectris FPDs compared with the Sinfony/Ribbond FPDs (281 � 25 N � 252 � 74 N) for the corresponding pontic spans.Conclusion. Within the limitations of this in vitro study the impregnated fiber reinforcement may considerablyenhanced the fracture resistance of interim FPDs of different span lengths. (J Prosthet Dent 2003;89:170-4.)

CLINICAL IMPLICATIONS

This in vitro study showed a significant improvement in the fracture resistance of interim FPDsreinforced with impregnated fibers compared to conventional polymethylmethacrylate (PMMA)restorations.

During the fabrication of cast fixed restorations,tooth protection with interim restorations after prepara-tion is mandatory. Normally, short-term interim resto-rations are fabricated. Comprehensive dental treatment,including the treatment of temporomandibular disor-ders or change in the vertical dimension of occlusion,however, often necessitates the use of interim fixed par-tial dentures (FPDs) yielding high stability, especially inlong-span restorations or areas of heavy occlusal stress.

The fracture resistance of interim FPDs can be in-creased by reinforcing them with fibers.1-5 Varioustypes of fiber reinforcements have been investigatedfor use with dental polymers.6-8 Glass fibers were eval-uated to improve mechanical properties of denturebase polymers.9-12 Glass fibers improved fatigue resis-tance and demonstrated good esthetic properties.Carbon/graphite fibers were also tested as reinforce-ment of denture bases and implant-supported

FPDs.13-15 Carbon fibers placed perpendicular to thedirection of applied stress produced the most favor-able combination of increased resistance to bendingand flexural fatigue.14 It has been shown that theproduction of properly oriented fibers that are wellcentered within the resin was technically difficult andyielded less predictable property improvements thanresulted from randomly dispersed fibers.14

Other types of fibers evaluated include organic fiberssuch as aramid fibers16,17 and ultrahigh molecularweight polyethylene fibers (UHMWP).18,19 PMMAspecimen reinforced with Kevlar fibers showed signifi-cantly greater fracture resistance than the control group(PMMA resin).17 Studies revealed that the impactstrength of PMMA resin was improved by untreatedUHMWP fibers.18,19 Poor adhesion of UHMWP fi-bers has been improved by various types of electro-chemical “plasma” treatment.18,19 However, this typeof surface treatment has not increased the strength ofthe composite compared with that obtained with un-treated fibers.18,19

aAssociate Professor, Department of Prosthetic Dentistry.bResearch Assistant, Department of Prosthetic Dentistry.

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Factors that relate to the strength of the fiber com-posite are quantity of fibers in the polymer matrix, ori-entation of fibers, fiber impregnation, and adhesion offibers to the polymer matrix.10,20-24 Accurately placedand oriented preimpregnated glass fibers increased frac-ture resistance.7,25 Continuous unidirectional fibers pro-vided the highest strength and stiffness for the compos-ite, but only in the direction of the fibers.7 Woven fibersreinforced the polymer in 2 directions.7 If the fibers wereoriented randomly as in a fiber mat, the mechanicalproperties were the same in all directions.7

The aim of this in vitro study was to determine thefracture resistance of interim FPDs made of differentmaterials and span lengths on the basis of the hypothesisthat different materials and span lengths may result indifferent fracture resistance.

MATERIAL AND METHODS

A metal cast (nickel-chrome-alloy, Wiron 88; Bego,Bremen, Germany), was used for the fabrication of theinterim FPDs. The mesial abutment tooth was a pre-pared maxillary premolar (height: 5 mm), while a pre-pared maxillary molar (height: 5 mm) represented thedistal abutment. The preparation finish line was a circu-lar shoulder (1 mm) with rounded internal axiogingivalline angle.26 The buccolingual and mesiodistal conver-gence angle was 10 degrees. Tooth diameters at theinternal line angle of the circular shoulder were 3.5 mm(mesiodistal) and 6.5 mm (buccolingual) for the premo-

lar. Tooth diameters for the molar were 6 mm and 7.5mm, respectively. The final abutment preparations ofthe master casts fabricated were identical. The 2 prepa-rations of the original cast were duplicated, cast withdental stone (Moldano; Heraeus Kulzer, Dormagen,Germany), and secured in the required distances of 12mm, 19 mm, and 30 mm for all the other casts necessaryfor the investigation. With this method, standard castsfor 3-, 4- and 5-unit FPDs were designed (Fig. 1). Asilicone mold (Protesil; Krupp Medizintechnik, Essen,Germany) was then fabricated for each of the 3 casts.These silicone molds were then used to reproduce addi-tional master casts, which were fabricated from cast ma-terial (Epoxy-Die; Ivoclar Dental, Ellwangen, Germa-ny). Resin teeth (Frasaco, Tettnang, Germany) wereused to fabricate the interim FPDs of different spanlengths (n � 3). Once the FPDs were adjusted to aver-age tooth sizes with wax,27 the modified resin teeth wereduplicated. The impressions were filled with wax (Pre-pon; Heraeus Kulzer). This way, it was ensured that the3 FPDs of 1 series demonstrated equal dimensions. The3-unit FPD to replace a molar demonstrated a ponticspan of 12 mm.27 The 4-unit FPD replaced 1 premolar

Table I. Materials tested for fabrication of 3-, 4-, and 5-unit interim FPDs with different span lengths

FPD material Manufacturer

Biodent K�B Plus (PMMA material, control group) Dentsply De Trey, Dreieich, GermanyPromysan Star (polyester-based thermoplastic resin) Pedrazzini Dentaltechnologie, Munich, GermanyPromysan Star/Vita Zeta (polyester-based thermoplastic resin/veneering

composite)Pedrazzini Dentaltechnologie, Munich, Germany;

Vita Zahnfabrik, Bad Sackingen, GermanyRibbond/Sinfony (non-impregnated polyethylene fiber reinforced resin/

veneering composite)Ribbond Inc, Seattle, Wash.; Espe, Seefeld, Germany

Vectris/Targis (impregnated polyethylene fiber reinforced resin/veneeringcomposite)

Ivoclar Dental, Ellwangen, Germany

Fig. 1. Master cast for fabricating 3-unit interim FPD.Fig. 2. Experimental arrangement to test fracture resistanceon specimen of 5-unit FPD on flexibly mounted supports.Pontic width span (w) was 12 mm for 3-unit, 19 mm for4-unit, and 30 mm for 5-unit FPDs. Pontic height was h1 �4.3 mm, h2 � 7.0 mm and h3 � 4.3 mm for all FPDs. Ponticwidth, buccally/lingually, was 5.3 mm at h1 and h2 and 8.5mm at h3.

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and 1 molar over a span of 19 mm, whereas the 5-unitFPD replaced 1 premolar and 2 molars with the span of30 mm. Pontic height was h1 � 4.3 mm, h2 � 7.0 mm,and h3 � 4.3 mm for all FPDs (Fig. 2). Pontic widthbuccally/lingually, was 5.3 mm at h1 and h2 and 8.5 mmat h3. The waxed FPDs were subsequently cast in a nick-el-chrome-alloy (Wiron 88; Bego) and finished. Thecasts were used as a pattern for fabricating resin FPDs ofdifferent pontic spans. The FPDs were cast in a nickel-chrome-alloy to ensure exact and reproducible dimen-sions of the resin FPDs.

The fabrication of the 9 FPDs per material required 3silicone molds of the 3-, 4- and 5-unit master casts andthe corresponding nickel-chrome FPDs. First, 3 siliconemolds for each material were poured using epoxy resindie material (Epoxy-Die; Ivoclar Dental). To enhancethe handling properties, the epoxy resin casts were sawcut. Three master casts each were used to fabricate theFPDs of different materials and pontic widths. In orderto achieve as exact a reproduction as possible of thestandard FPDs, a duplicating procedure of the nickel-chrome FPDs with silicone molds (Silaplast; Detax, Et-tlingen, Germany) was used. The FPDs were then fab-ricated according to the manufacturer’s instructions(shade Vita A3) by refilling the silicone molds (Table I).The light-activated Targis/Vectris materials have beendeveloped as definitive restorations. Targis/Vectris wasalso recommended for long-term interim FPDs.24 Vac-uum light polymerizing unit (Vectris VS1; Ivoclar Den-tal) and light polymerizing unit (Targis Power; IvoclarDental) were used for the light polymerization of thefiber reinforced Targis/Vectris restorations. The Pro-mysan Star FPDs (Table I) were fabricated in a dentallaboratory authorized by the manufacturer. The FPDswere stored in air at room temperature (21°C) for 24hours. All FPDs were subjected to a thermocycling byimmersing them in 2 water baths at 5° and 55°C, respec-tively, for 5000 cycles with a dwell time of 30 seconds ineach bath.29

The specimens were then attached to the supportabutments for the bending test. For that purpose, 3molds (Protesil; Krupp Medizintechnik) were filled withwax. The lower part of the wax dies was given a wedge-like shape (buccolingual) to simulate the flexibility of theperiodontium,28 and cast with a base metal alloy (Wiron

88; Bego) (Fig. 2). The FPDs were luted on the dieswith a provisional cement (Temp Bond; Kerr, Karlsruhe,Germany). The cement was allowed to set at room tem-perature (21°C). The FPDs were subjected to 3-pointbending until fracture with a universal testing machine(model 1120; Zwick, Ulm, Germany). The experimen-tal arrangement consisted of the metal base, the FPDscemented on the support abutments, and the crossheadof the testing machine (Fig. 2).

Force was applied perpendicular to the center of theFPD. The center was marked at the midpoint of thepontic width. The support abutments were exactly po-sitioned at the reference markings on the ground plateto ensure loading at a similar location. The load wasapplied to the FPDs by a steel ram placed in the cavity inthe middle fossa (3- and 5-units, Fig. 2) and on themesial cusps of the first molar (4-units).

To prevent tensile peaks, a thin tin foil of 0.5 mm wasplaced between the loading point and the FPD. The testspecimens were continuously loaded at a crossheadspeed of 1 mm per minute. After a load drop of 25%, thetest was terminated. The maximum force (N) at fracturewas recorded.

Statistical analysis of the results was carried out with a3 (width spans) by 5 (materials) 2-way analysis of vari-ance (ANOVA, ��.05) to evaluate interaction effectsbetween these 2 independent variables. One-wayANOVA and Bonferroni-Dunn’s multiple comparisonspost hoc analysis of the load values were conducted forthe test groups (��.05).

RESULTS

Two-way ANOVA revealed a significant effect ofbrand of polymer on the fracture resistance (P � .05).No significant effect was obtained for the pontic width(P �.05). No interaction between these variables wasfound (P �.05). Bonferroni-Dunn’s post hoc analysisshowed statistical differences of fracture resistance(P �.05) between groups (Table II). Fracture resistance(N) differed significantly for 3, 4, and 5 unit Targis/Vectris FPDs compared with the corresponding Pro-mysan, Biodent K�B and Promysan/Vita Zeta groups(P �.05) (Table II). Significant differences were ob-tained for the 4 and 5 unit Targis/Vectris FPDs com-

Table II. Mean fracture resistance (N) of interim FPDs of different materials and span lengths; aFracture near abutmentsupport, bFracture in middle of pontic area, cChipping of veneer recorded for each specimen (n � 3)

Span length

Biodent K�B Promysan StarPromysan Star/Vita

Zeta Ribbond/Sinfony Vectris/Targis

Mean SD Mean SD Mean SD Mean SD Mean SD

12 mm 247.0abb 90.9 284.1abb 20.8 95.4ccc 14.9 429.2ccc 55.5 640.2ccc 146.319 mm 197.4aab 31.5 291.9abb 57.2 83.0ccc 17.6 280.7ccc 25.5 625.9ccc 229.130 mm 218.2abb 85.4 124.5aab 73.4 82.0ccc 6.1 252.4ccc 73.9 658.0ccc 98.4

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pared with the Sinfony/Ribbond FPDs for the corre-sponding pontic spans. The fracture loads were similarfor different pontic spans of the FPDs (P �.05).

The mean fracture resistance values for the PromysanStar groups did not differ significantly from the corre-sponding Biodent K�B control groups (P � .05). Meanfracture resistance was significantly lower for the PromysanStar/Vita Zeta combination compared to both fiber rein-forced resin composite FPDs. Statistically significant differ-ences with the fracture resistance for the correspondingBiodent groups were not noted (P �.05).

The failure of the Targis/Vectris FPDs could be at-tributed to the chipping of the Targis veneers off theframework (Table II). Examination of the PromysanFPDs veneered with Vita Zeta also showed that the earlyfailure of the restorations could be attributed to theveneering material chipping off the Promysan frame-work. In the fractured Sinfony/Ribbond FPDs, the Rib-bond framework remained intact. All fractures devel-oped within the veneering composite.

DISCUSSION

Abutment teeth are not rigidly anchored in the max-illa or mandible. For that reason, flexible mounting wasselected for the testing of FPDs, which was meant tosimulate the flexibility of the periodontium.28 A previ-ously used experimental arrangement included immo-bile supports.9 Consequently, the support abutmentsdid not move during bending, which resulted in thedeflection being limited. The fracture resistance valuesmeasured in this type of investigation are generallyhigher than those determined with supports that allowtheir movement.29

In this in vitro study axial forces were applied to thecenter of the occlusal pontic area. Clinically axial forces,in addition to lateral forces and fatigue loading on in-terim FPDs, should be considered. These may have anadditional effect on the mechanical properties of FPDs.Aging processes, such as alternate thermal stress, me-chanical stress, and wear, weaken FPDs. No standard ofthe International Organization for Standardization(ISO) exists for the bending tests of interim FPDs withregard to such aging processes. However, thermocy-cling, as was performed in the present study, has becomecommon for other ISO standards.28 Another aspect thatmay lead to different fracture resistance values is the typeof cementation and the materials used. The cementationmethod for Targis/Vectris FPDs, which have been de-veloped as definitive restorations, was performed inmost investigations according to the principles of theadhesive technique.9 For this study the long-term in-terim restorations were luted by use of a provisionalcement for the sake of creating test conditions similar tothe clinical procedures. Therefore a provisional cement

was selected, which most probably resulted in the de-creased fracture resistance values.

The fracture resistance values determined by the var-ious investigators were recorded under different mea-surement criteria. These criteria were either initial crack-ing noises that were interpreted as crack development,or a drop in the load by an absolute or relativeamount.8,13 For this investigation the maximum forceon fracture was determined. Fracture appeared close tothe abutment, as well as in the middle of the pontic area.Location of the fracture varied in the same group, thusno relation of fracture pattern with fracture resistance ofthe materials could be derived. The failure of a restora-tion is determined by the weakest link in a chain. In thisstudy the weakest link was the bond between the mate-rials used. In all Targis/Vectris FPDs the Vectris frame-work remained intact. These FPDs failed because of theTargis Ceromer chipping or cracking. It should be men-tioned, however, that in the clinical situation, minorchipping can be repaired intraorally.30

As far as the bending test with Sinfony and Ribbond,the weakest link was also the inadequate bond, since theRibbond fibers did not tear in any of the situations.2,6

For Promysan and Vita Zeta, however, the bond wasinadequate to such an extent that the fracture resistancevalues were extremely low.

The results achieved with impregnated fibers (Vec-tris) were better than those achieved with nonimpreg-nated fibers (Ribbond). Studies revealed that the correctorientation of the fibers as in fiber mats increased thefracture resistance.7,10,14 With its largely automated pro-cessing steps, the Vectris System maintains the fiber ori-entation. With respect to the results found in literatureand those of this study, several factors may result in anincrease in the fracture resistance of FPDs fabricatedfrom glass-fiber reinforced materials systems. In addi-tion to the type of fibers used, the quantity of fibers, fibertoughness, fiber orientation, and the type of impregna-tion may substantially influence the fracture resis-tance.11,12,20-24 Therefore preimpregnated fiber systemswith defined fiber concentrations, and carefully deter-mined, coordinated material combinations, which arerecommended by the corresponding manufacturers,should be used. In spite of the automated processingsteps of the fiber reinforced Vectris system light initiatedpolymerization of the veneering material onto theframework remains critical. It is difficult to bond thecomposites, Vectris and Targis, to each other, becausethe composite component of the Vectris fiber frame-work is cross-linked and highly polymerized.24 Thismay, aside from the technique sensitivity of the labora-tory processing, explain the large standard deviations ofthis system. Strength of the Ribbond/Sinfony FPDsmay be influenced by the positioning and adaptation ofthe Ribbond fibers, therefore processing according tothe manufacturer’s instructions is mandatory. The poly-

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merizing process of thermoplastic Promysan FPDs isautomated, whereas Biodent K�B FPDs are fabricatedwithout automated processing, which may lead to largerstandard deviations.

In this in vitro study significant differences were ob-tained between different brands of resin materials usedfor provisional FPDs. To overcome the limitations of thein vitro tests interim FPDs reinforced with impregnatedfibers must be evaluated for stability in the oral environ-ment.

CONCLUSIONS

Within the limitations of this study, the followingconclusions may be made:

1. The interim FPDs of different pontic length, rein-forced with impregnated fibers (Vectris/Targis) showedhigher fracture resistance (mean: 626-658 N) than res-torations with nonimpregnated fibers (Ribbond/Sin-fony, 252-429 N), as well as thermoplastic polymer(Promysan Star, 125-284 N) and conventional PMMA(Biodent K�B, 197-247 N) FPDs.

2. Fracture resistance of interim FPDs reinforcedwith impregnated fibers (Targis/Vectris) was not af-fected by pontic spans (12, 19, 30 mm).

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DEPARTMENT OF PROSTHETIC DENTISTRY

SCHOOL OF ORAL MEDICINE

UNIVERSITY OF COLOGNE

KERPENER STR. 3250931 COLOGNE

GERMANYFAX: 49-221-478-6722E-MAIL: peter.pfeiffer@uni-koeln.de

Copyright © 2003 by The Editorial Council of The Journal of ProstheticDentistry.

0022-3913/2003/$30.00 � 0

doi:10.1067/mpr.2003.29

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