Resistance to maxillary premoiar fractures after restorationof Class II preparations with resin composite or ceromer

Claudia Regina Buainain de Freitas, DMD, MSVMaria Isabel Serra Miranda, DMDVMarcelo Ferrarezi de Andrade, DDS, MS, PhDWictor Humberto Orbegoso Flores, DDS, MS, PhDVLuis Geraldo Vaz, DDS, MS, PhD^/Norberto Catanzaro Guimaraes, DDS, MS, PhD*

Objective: The aim of this study was to evaiuate the resistance fo fracture of intact and restored humanmaxillary premolars. Method and materials: Thirty noncarious human maxiiiary premclars, divided intoIhree groups of 10. were submitted to mechanical tests to evaluate their resistance to fracture. Group 1consisted of intact teeth. Teeth in group 2 received mesio-occlusodistal cavity preparations and were re-stored with direct resin composite restorations. Teeth in group 3 received m es io-occluse dista I cavity prepa-rations and were restored with ceromer inlays piaced with the indirect technique. After restoration, teethwere stored at 3 7 X for 24 hours and then thermocycled for 500 cycles at temperatures of 5'C and 55°C.Results: Statistical analysis revealed that group 3 (178.765 kgf) had a significantiy greater maximum rup-ture load than d[d group 1 (120,040 kgf).There was no statistically significant difference between groups 1and 2 or between groups 2 and 3. Conclusion: Class II cavity preparations restored with indirect ceromerinlays offered greater resistance to fracture than did intact teeth. The fracture resistance of teeth restoredwith resin composite was not significantly different (rom that of either the ceromer or intact teeth.(Quintessence int 2002:33:589-594)

Key words: ceromer. Class II restoration, fracture, polymer, resin composite

CLINICAL RELEVANCE: Both resin composites and

ceromers reinforce tooth structure.

The ¡OSS of dental structure because of caries, oreven tbe cavity preparation itself, ivill weaken tbe

tooth. Teeth witb large restorations can suffer cuspal

'Private practice. Araraquara, Sao Paulo, Brazil.

^Assistant Professor, Department of Operative Dentistry, Scliooi ofDentistry at Araraquara, State University of Sao Paulo, Araraquara, Sao

Pauio, Brazil.

^Assistant Professor. Department of Dental Materials and Piosthetics,SchoQl of Dentistry at Araraquara, State University of Sào Paulo,

Araraquara. Sao Paulo, Brazil.

'Assistant Professor, Deparlment of Dental Materials and Prostfietics,Sctiool of Dentistry at Araraquara, State Universily of Sao Paulo,Aiaraquara, Sào Pauio. Brazil.

Reprint requests; Dr Marcelo Ferrarezi de Andrade, Department ofOperative Dentstry, Schcoi of Dentistry at Araraquara, State University ofSao Paulo, Rua Humaiatá, 1680, 14801-903 Araraquara, Sao Pauio, Brazil.E-maii: restaura@foar.unesp.br

fractures. Tbe anatomy of tbe posterior teetb facilitatestbe deflection of tbese fractures under tensile stress.The cuspal inclination of maxillary premolars is muchgreater than that of maxillary' molars, thus resulting indifferent fracture resistance patterns in these teeth.Vale' evaluated the resistance of teeth submitted tocavity preparations and concluded that the greater thepreparation width, the lower the fracture resistance ofthe tooth. Other studies have corroborated these con-cepts.'-' However, Jagadish and Yogesh, using severalrestorative materials, reported tbat the greatest frac-ture resistance was presented by. in descending order,teeth restored with resin composite, teeth restoredwitb glass-ionomer cement, healthy (intact, noncari-ous) teeth, teeth with adhesive amalgam restorations,and teeth that were prepared but not restored.

Because of their reported ability to reinforce weak-ened dental structure, and because they offer excellentesthetic quality, important physical properties, and easeof manipulation, resin composites have had great accep-tance in the dental market.* To strengthen the present es-thetic tendency, tbe so-called ceromers, or glass poly-mers, which are hybrid resin composites reinforced withceramic filaments, have heen introduced on the market.'*

Quimes589

• de Freitas et al

.e clock. .J to mea-lïiacromet-ng pressurelootb to be

trom an appli-

Fig 1 Maxiilary premolar embedded m a vertical position m apolyvinyl chlonde ring.

METHOD AND MATERIALS

Extracted noncarious, intact human maxillary premolarteeth were used in this study. The teeth were cleanedby scaling and fixed in 10% neutral formalin for 72hours. Only teeth with dimensions within the averageestablished by Galan^ were accepted for the study. Allteeth were evaluated in a stereoscopic loupe, and thosethat presented fracture or cracks were rejected.

Thirty teeth selected for the study were randomlydivided into three groups of 10. Teeth were embeddedup to the cervix, 1 mm below the eementoenameljunction, in polyvinyl chloride rings that were 0.75inches in diameter and 15 tnm in height. Self-curingacrylic resin was used to keep the exposed crown in avertical position (Fig 1). After they were embedded inthe rings, the teeth were kept in distilied water untilthe mechanical tests were completed.

The following experimental groups were defined:

1. Group 1 consisted of noncarious, intact teeth(control).

2. Group 2 consisted of noncarious teeth in whichmesio-oeclusodistal cavity preparations were cre-ated and restored directly with resin composite(Filtek Z250, 3M).

3. Group 3 consisted of noncarious teeth in whichmesio-occlusodistal cavity preparations were createdand restored indirectly with ceromer inlays (Solidex,Shofu) that were cemented with dual-curing cement(Enforce, Dentsply).

Each specimen, except those In group 1, received aClass II mesio-occlusodistal cavity preparation. Sothat the cavities wouid have uniform dimensions, thepreparation device described hy Sa and Gabrielli'« wasused. The device consists of a microscope adapted

with the following components; a Lto control pressure placed on tbe tu;sure the depth of the cavity prepara'rie screw to control the applicationto achieve the desired cavity deptiprepared, coupled to a mobile platfoiance-fixing system; a turbine-fixing device with regu-iating screws to ensure a perpendicular position of themill over the tooth surfaee to be prepared; screws withprecision of 0.1 mm to move the fixing system andplatform to the left or right and anteriorly or posteri-orly to define the mesiodistai and occlusogingival cav-ity dimensions, respectiveiy; and vernier scales toallow the fixation of reference points to control the di-mension of the tnesiodistai and occlusogingival walls(Fig 2).

The cavity preparation was started with an occlusalaccess cavity created with a No. 4137 diamond point(KG Sorensen). The buccal and lingual walls followedthe sideways tilt from the diamond point, remainitigslightly divergent. The occlusal eavity had a depth of1.5 mm and a width corresponding to the diameter ofthe point used, equivalent to 2.5 mm. The walls werefinished with a gingival margin trimtTier and chisel forenamel.

The construction of the proximal boxes followed thesame criteria used for the occlusal box. The buccal andlingual walls of the proximal boxes were quickly diver-gent to occlusal because of the point tilt 4137. The axialwalls converged gently from gingival to occlusal andformed a rounded angle with the pulpal wall. The buc-coiinguai dimension of the proximal box was 4.0 mtrt,on average, close to one third the width of tbe tooth,and the depth was approximately 1.0 mm.

Cavity preparations in group 2 were restored with aresin eomposite. All cavity preparations were cleanedwith a 0.2% chlorhexidine solution. Ali exterior mar-gins and the interior of the preparation were condi-tioned with 37''/o phosphoric acid for 15 seconds. Alayer of a simplified fourth-generation adhesive agent(Opti-Bond Solo Plus, Kerr/Sybron) was applied tothe previously conditioned enamel and to the dentinalsurfaces before the insertion of resin composite. Allthe materials were manipulated according to theirmanufacturer's instructions. The restorations wereplaced in an incremental insertion technique thatDavidson" has indicated is ideal for light-curing resincomposites. The finishing and pohshing were carriedout with silicone points and polishing paste.

The cavities in group 3 were prepared in accor-dance with tbe procedures described for group 2 andrestored with previously fabricated ceromer inlays.'^To promote internal cleansing, tbe inlays were condi-tioned witb 37% phosphoric acid for 1 minute, wasbedand dried. Tbis was followed by the application of the

590 obere. 2002

de Fretias et al -

Fig 2 (left) Device for uniform tooth preparation, developed by

Fig 3 (below) Fracture device used in the tests. Noie the wedgeshape and the straight aciive end. which is I ,T,m Ehick and 7 mm•.vide

adhesive agent {Opti-Bond. Solo Plus), After tbe ma-nipulation and placement of dual-ctiring cement ontbe interior of the inJay, the inlay was adapted to thetooth, Photopolymerization was carried out tor 40 sec-onds with an apphance (KM 200. DMC Equipment)previously cahhrated to a light intensity hetween 500and 600 mW/cm^,

Teeth were stored in deionized water until mechan-ical testing. Teeth ft-om groups 2 and 3 were subjectedto 500 thermocycles in water at temperatures of 5°Cand SS^C, with a dwell time of 15 seconds at eachtemperattire.

The specimens were subjected to a compressiveaxial load in a mechanical testing appliance (MaterialTest System, model 810, MTS Systems) operated at 0,5mm/min. The device had a wedge shape with astraight, active end that was 1 mm thick and 7 mmwide, thus concentrating the force at the center of theocclusal surfaces of the teeth (Figs 3 and 4), The frac-ttire resistance values were recorded in (kgf) at themoment of fracture. The data were submitted to statis-tical analysis.

RESULTS

Analysis of variance indicated that the treatment fac-tor was significant to the adopted level (P < ,05; Table1). This means that the null hypothesis of equalityamong the means had to be rejected; ie, it was likely,Í0 a 95% probability, that at least one of the means

Fig 4 Stress directed at the center of Ihe occlusal surface of thetooth during Iracture testing.

differed from the others. To identify which meanswere different, Tukey's test was used to compare thevalues two by two (Table 2), To perform this test theminimimi significant difference was calculated; the re-sult was a value of 50,545,

Teeth restored with ceromer inlays withstood a sig-nificantly greater fracture load than did intact teeth (P< ,05), No statistically significant difference was foundhetween intact and resin composite-restored teeth orbetween resin composite-restored and ceromer-re-stored teeth. The mean fracture resistances of groups 1to 3 are compared in Fig 5,

Quintessec 591

• de Freilas et al

TABLE 1

Resource

TreatmentHesidueTotal

'Stallstically s

Analysis of variance

df

22729

griifican

SS MS

17356.703 8678,35256959.35 2tO9,6174316,05

(P<.05).

F

4.11*

P

,027

TABLE 2 Mean tnaximutn load to failure (kgf)

Group Mean SD

1 Healthy teetha Resin composite3Cerorner

120.040^145.275='178.765"

20,26556,17446,149

SO = slandard deviation.Means assigned the same letter are not significantly ditferent.

200180160140120

604020 I—

0 Heaithyteeth

(group 1)

Resincomposite(group 2)

Ceromer(group 3)

Fig 5 Mean fracture resistances ot groups 1 to 3.

DISCUSSION

This study examined fracture resistance in maxillary pre-molars, the anatomic shape of which creates a tendencyfor separation of their cusps during mastication. Thecuspal incline in this type of tooth is much greater thanthat in maxillary molars and can result in a different pat-tern of fracture resistance for these teeth. It has alsobeen reported that the incidence of fracture is greater inmaxillary premolars than in mandibular premolars.^'

Cuspal separation rarely occurs in noncarious, in-tact teeth because of the presence of the puip chamherroof and marginal ridges, which can he consideredtooth-reinforcing structures. This highlights the impor-tance of prevention and early diagnosis of caries le-sions before they involve the marginal chamher,

The general effect of tnesio-occlusodistal intracoro-nal cavity preparations is the creation of long cusps.Thus, the restorative material used must not only re-place the lost tooth structure but also increase the frac-ture resistance of the tooth and promote effective mar-ginal sealing.

This research evaluated the resistance to compres-sion of three experimental groups. The results indicatedthe capacity shown by the material to support verticaltension, vital in areas of high masticatory effort.Occiusal forces among patients with good occlusionhave heen reported to he an average of 21 to 49 kg,*"

Numerous materials have been used as substitutesfor natural dental tissues. Amalgam, for instance, hasmore than 100 years of use in posterior restorations.However, hecause it docs not adhere to dental structure,amalgam does not compensate for the loss of fractureresistance. Bell et al'^ concluded that cuspal fractures inamalgam restorations result from the fatigue caused hycrack diffusions subjected to repeated loading.

For these reasons, the use of adhesive materials hasbeen considered useful for tooth reinforcement. Sincetheir arrival in the 1960s, resins have undergone greatchanges. Denehy and Torney,' in 1976, were the firstauthors to propose the use of adhesive materials to re-inforce the dental structure and to offer support forenamel altered hy cavity preparations.

The use of resin cotnposite in restorations reinforcesdental stiffness. It has been suggested that the adhesivenature of composite binds the cusps and decreasestheir flexion, which is the main cause of fractures inamalgam restorations,'' However, the tooth-restorationinterface suffers elastic stresses generated hy the con-traction of the material during polymerization; al-though these stresses can be dissipated by cuspal move-ment, in this clinical situation the adhesive junctioncan fail." It is known that the cuspal deflection in aweakened cusp can open the tooth-restoration inter-face, leading to marginal leakage and secondary caries.

The results ohtained in group 2 indicated that theposterior resin composite tested has good potential ascusp-reinforcing material. The present study confirmsprevious studiesi'*- ^ that found that the greatest resis-tance was presented by cavities restored with resincomposite. However, in the present study, certjmer in-lays bonded with dual-curing adhesive cement provedto offer more effective resistance to tooth fracture.

Both group 2 (resin composite) and group 3(cerotner) offered fracture resistance values that weresuperior to those ohtained in group 1 (intact and non-carious), !n this aspect, the present study corroboratesresearch hy Jagadish and Yogesh^ and McCuUock andSmith, "- ^ who reported superior fracture resistancevalues with the use of resin composite. This restorativematerial, when bonded to dentin hy an adhesive sys-tem, seems to increase the intrinsic strength of teeth.

592 ober 8. 2002

• de Frenas et al

In group 1 (intact teeth), after the maximum loadwas applied, fracture of the palatal cusp next to thecementoenamel junction was noticeable. In somecases in group 1. this line of fracture extended up tothe root- In group 2, the rupture occurred initially inthe marginal ridge before cuspai fracture occurred; inmost cases the palatal cusp fractured. In group 3. thefracture occtured through the interface of the buccalor lingual wall with the restoration.

The mean fracture resistance values of teeth re-stored with resin composite were not significantly dif-ferent from those of intact teeth {P > ,05), but themean values of indirectly restored teeth were signifi-cantly greater than those of intact teeth (P < ,05).These results can probably be attributed to the compo-sition of the ceromer. These materials combine thehigh technology of the latest generation of resin com-posites with the advantages of ceramic. They combinethe features of the inorganic phase (ceramic), such asdurability, resistance to abrasion, and stability, withthe features of the organic phase (resin), such as thegreatest polymerization capacity and an effective junc-tion with the resinous cement.-^

In this study, the ceromer Solidex was used. It iscomposed of about 53"o ceramic filaments, 25''.'ocopolymer with multifunctional resin, and 22% pho-toacfivated conventional resin,'- It exhibits wear simi-lar to that of tooth structure, the color stability is fa-vorable because of the reduction of water sorption,and its modulus of elasticity is close to that of dentin, -*The resin composite used in this study, Filtek Z250.has an inorganic component zirconium and silica rep-resenting eO b of total volume {without silane). It hasan average particle size of 0,19 to 3.3 pm. The organicpart consists of bisphenol glycidyl methacryiate. ure-thane dimethacrylate, and bisphenol ethyl methacry-iate (specifications obtained from manufacturer).

Individual variations in the morphology amongteeth, including cuspai angulation, enamel friabiilty,intrinsic weaioiess, small variations in size, and varia-tions in the device contact level with a main groovedtiring the test can contribute to deviations in fracturebehavior.

Results obtained in teeth restored with resin com-posite and fractured right after preparation do notconsider the long-term adhesive stability.

There are many differences between fractures thatocctir ciinicaliy and those induced by in vitro testingappliances. The forces created intraorally during masti-cation vary in magnitude, speed, and direction, whilethe forces appiied to the teeth in this study were con-stant in speed and direction and increased continuotislyuntil the fracture occurred. Additional research must becarried out to confirm the ability of ceromers and resincomposites to reinforce weakened tooth structure.

CONCLUSION

1, Teeth restored with ceromer inlays (178.765 kgf) ex-hibited significantly greater resistance to fracturethan did noncarious, intact teetb (120,040 kgl),

2. No statisfically significant differences in fracture re-sistance were found between groups 1 and 2 or be-tween groups 2 and 3,

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