Guillermo Bernai. DDS, MSDBogota, Colombia

The Effect of Finish LineForm and Luting Agent on

the Breaking Strength ofDicor Crowns

Rose Marie /lines, DOS. MSD'

David T. Brown, DDS. MS'

Carlos A. Munoz, DDS, MSD*

Charles /, Coodacre, ODS, MSD"

Indiana University School of Dentistry

Molar teeth were prepared to a standardized cylindric formusing a lathe. Dicor crowns were lufed on these preparationsusing zinc phosphate, glass-ionomer, and resin cements. Thecrowns were thermocycled and subjected to fracture loadingfo compare the effect of fhe different luting agents onfracture strength. Five finishing lines were used to determineif resin cement and associated bonding procedures couldcounteract the negative effect certain finish lines produce onall-ceramic crowns. When resin cement was used inconjunction with restoration and tooth precementationbonding treatments, the crowns were significantly strongerthan when glass-ionomer or zinc phosphate cements wereused. No difference in strength was found between the fivefinish lines when the crowns were cemented with resin andassociated bonding procedures. Int I Proslhodont1993;6:286-290.

S everal investigations have been reported'"^ con-cerning the effect of tooth preparation design

on the breaking strength of Dicor (Dentsply, York,PA) complete crowns. The following tooth prepara-tion designs were found to enhance all-ceramicrestoration strength: (1) a total occlusal conver-gence of 10 degrees; (21 a shoulder finish line with asharp axiogingival line angle; (3) a finish line withminimal cervical slope; and (4) a finish line withminimal variation in its occlusocervical locationaround the circumference of the tooth. FHowever,these studies did not determine if the use of resincement and associated bonding procedures couldcounteract the weakening effect of certain finishline forms.

'Associate Professor, Department of Proslhodootics."Professor and Chairman, Department of Prosthodontics.

Reprint requests: Dr Rose Marie Iones, Iridiaría University, Schoolof Dentistry, 1121 West Michigan Street, Indianapolis, Indiana46202.

The selection of a luting agent and/or restorationand tooth precementation bonding procedureshave been shown to affect the strength of all-ceramic restorations. Grossman and Nelson' fabri-cated 50 Dicor crowns to fit identically preparedextracted human teeth and found that crowns lutedusing zinc phosphate cement had significantlylower crushing strengths than crowns luted usingDicor light-activated resin cement.

Mclnnes-Ledoux et al' determined that surfacetreatment of enamei and dentin increased thebond strength of glass-ionomer cement and thatDicor iight-activated resin cement produced higherbond strengths than three different glass-ionomercements.

Duffin et al" tested the fracture resistance ofDicor crowns of varying thicknesses that had beenluted using different agents on extracted third mo-lars. The highest overall fracture resistance oc-curred when resin cement was used in combinationwith etched enamel and the ceramic thickness was0.5 mm.

Malament and Grossman- evaluated the clinicalperformance of Dicor crowns luted using a light-activated resin cement and bonding procedures.

The International lournal Ü I Prosthodortk 286

nal et al Finish Line Form and Luting Agents

Fig 1 Finishing line forms:A, shouider with a 0.5-mm round-ing of the axiogingival line angle:B, shoulder witii a 0 3-mm round-ing of the axiogingivai line angle:C, shoulder with a 90-degree ax-iogingival line angle; D, shoulderwith a 135-degree cervical slopeand sharp axiogingivai line angle:E, shoulder with a 0,4-mm-wideaxiogingivai groove. \n

0,5 mm 0,3 mm

A B

0,4 mm

E

One practitioner placed 397 crowns over 33months. The overall success rate for bonded Dicorcrowns was 98,7% and was significantly higher thanpreviously reported values for nonbonded tech-niques. The restorations had a mean axial wallthickness of 1,82 mm and a mean posterior occlusalthickness of 1,74 mm,

Thepurposesof fhis study were to determine: (1)if different luting agenfs produce variations in all-ceramic restoration strength and (2) if the use ofresin cement and associated bonding procedureswould counteract the negative effect of certainfinishing line designs.

Materials and Methods

One hundred five intact, extracted human maxil-lary first molars were cleaned and continuouslystored in distilled water to maintain their integrity.The teeth were prepared using five different finishline forms (Fig 1), To uniformly prepare each tooth,fhe rotational center was determined using acustom-made centering device (Tiber Industries,Bogota, Colombia) and then the teeth weremounfed in a cylindric acrylic resin (Formafray, KerrMfg, Romulus, Ml) base. Uniform cylindric toothpreparations were made using a lathe and diamondrotary instruments by reducing the external surfaceof the tooth into a cylindric form, then creating fhefinish lines shown in Fig 1, Finish line E was pre-pared using two different diamond instruments,

A total occlusal convergence angle of 10 degreeswas used in conjunction with a finish line depth of1,2 mm for all tooth preparations. Dimensional uni-formity was verified using digital calipers (MitutoyoDigimatic Caliper 0,01-100 mm, Code No, 500-350,Mitutoyo Corp, Japan) and a Boley gauge machinedso Ihat the calipers possessed a 10-degree conver-gence angle. The mean variation in tooth prepara-tion diameter and height were found to be 0,09 mmand 0,07 mm, respectively.

Fig Z Left, schematic diagram of ex-tracted tooth and iufed restoration withload being appiied through a bail bearingattached to Ihe moving head of an instrontesting machine Right, occlusal view ofrestoration showing three triangularndges that were brougiit mto simuitane-ous contact with the bali bearing prior totesting.

Impressions of the prepared teeth were madeusing a poly(vinyl siloxane) impression material(Reprosil, Caulk Dentsply, Milford, DE], and 105stone dies (Prima Rock, Whip Mix Corp, Louisville,KY) were made,

A complete contour master wax pattern withthree cusps was formed with an occlusal thicknessof 1.5 mm in the depth of the occlusal grooves and2,3 mm at the cusp tips. The axial wall thickness was1,2 mm cervicaily and increased to 1,6 mm at theocciusal surface. The triangular ridges were formedso that a 6-mm-diamefer stainless sleel ball bearingwould make simultaneous contact with the threeridges of the crowns during fracture testing (Fig 2),

287 The Internationai Iournal oí Prosihodoiitics

Finish Line Form and Luting Agents

A 4-mm-long, 10-gauge Y-shaped wax sprue formwas aftached to the wax pattern, and a polytvinylsiloxanel mold was fabricated to permit the forma-tion of duplicate wax patterns using the wax injec-tion technique described by Friedlander et al.^

Tbe axial and occlusal walls of each stone diewerecoatedwith two layers of Dicor die spacer thatwere found to range between 35 and 50 ixm.' Dielubricant (Slaycris Products, Portland, OR] was ap-plied, and the die was placed in the previouslyfabricated siiicone mold that was supported by astone matrix secured in position with an elasticband. Inlay wax (Kerr Mfg] was heated to ZSOF in awax-injection apparatus lAutrol, Pro-Craft, GFC,Carlstadt, N)], and 105 identical patterns were made.

The wax patterns were recovered from the mold,the margins were refined on each die, and Dicorcrowns were fabricated according to the manufac-turer's recommendations. Two layers of Dicorshading porcelain (D-C3] were applied and fired.

To verify that differences in restoration strengthcould be detected when different luting agentswere used with this tooth-restoration experimentalmodel, 15 crowns Imade for dies with a shoulderfinish line and 0.3 mm of axiogingival rounding; Fig1, finish line B) were luted using each of the follow-ing three materials: 11) zinc phosphate cement

(Fleck's Mizzy Inc, Cherry Hill, Nj]; I2) glass-ionomer cement (Fuji Glass lonomer 1 Luting Ce-ment, Scottsdale, AZ); or (3] visible light-activatedresin cement IDicor, Dentsplyl. The rounded inter-nal finish line was selected to facilitate fabricationof the specimens.

To determine if resin cement could counteractthe negative effect of certain finish line characteris-tics, 15 additional restorations were luted usingresin cement on teeth prepared with each of theother finishing lines shown in Fig 1. Dicor visiblelight-activated resin cement was used in conjunc-tion with the enamel, dentin, and restoration pre-cementation treatments listed in Table 1. The ce-mented crowns were placed in a humidor at 37°Cfor 7 days and then subjected to 2,500 thermalcycles between 7°C and 47°C with adwell timeof 30seconds at each temperature. The crowns werethen stored for 1 week at 3 7 ^ and 100% humidityand then tested to fracture.

Each crown was placed in a testing jig speciallyfabricated for this study. With the use of 0.04-mm-thick articulating paper (Bausch Articulating Paper,Dentrade Internat, San Francisco, CA], the crownswere aligned so the three triangular ridges werebrought into simultaneous contact with the 6-mmstainless steel ball bearing fitted into the moving

Table 1 Restoration and Tooth Pretreatment and Cementation Procedures UsingDicor Visible Li g ht-Activated Resin Cement

Restoration pretreatment Tooth pretreatment

Crown dried withcompressed air

Ammonium hitiuoride etchinggel applied to internal surtacefor 1 minute

Crown rinsed with water tor15 seconds

Crown air dried for 10seconds

0.1% silane Dicor couplingagent solution applied for 3minutes

Crown air dried tor 10seconds and set aside for 24hours

Tooth cieaned witti oil-freenontluoride prophylaxispaste, rinsed with water tor15 seconds, and dried withcompressed air for 10seconds

Enamel etched with GlumaEtchant (Coiumbus Dental,St Louis, MO) for 30 seconds

Tocth rinsed with water for 10seconds and then dried withcompressed air

Gluma 2 oleanser applied todentin for 30 seconds

Tooth rinsed lor 10 secondswith water and air driedGiuma 3 primer applied todentin for 30 seconds, tooththen dried with compressedair

Gluma 4 sealer applied toenamei and dentin, tooththen dried with compressed

Base and oataiyst mixed inequal portions tor 30seconds

Cement placed into thecrown and crown seatedusing firm finger pressure foexpress excess cement

Crown placed under 3.5-kgioad

Cement light-polymerized for30 seconds, moving iightwand over ali crown surfaces

Excess cement removed

Cement poiymerized tor anadditionai 2 minutes

I of Prosthodontics 288 Volume 6, Nur"!—

Finish Line Form and Luting Agents

head of a mechanical testing machine (InstronModel 1123, Instron Corp, Canton, MA) (Fig 2). Acrosshead speed of 1 mm/min was used and theload at fracture was recorded on tbe chart of thetesting machine.

The mean and standard deviation were calculatedfor each group and fhe resulfs subjecfed fo a two-way analysis of variance. Since differences amongmeans were found, a Newman-Keul's multiplecomparison test was performed to identify differ-ences between groups.

Results

Restorations luted using fbe visible l i gh t -activated resin cement and associated bonding pro-cedures were significantly stronger than fhoseluted using glass-ionomer or zinc phosphate ce-ments (Table 2). Similar restoration strengths wererecorded for botb glass-ionomer and zinc pbos-phate materials (Table 2). No differences in strengthwere found between tbe five finish line designswhen the light-activated resin cement and bondingprocedures were used (Table 3).

Table 2 Effect of Cement on Restoration StrengthUsing Finish Line With Shoulder and 0.3 mm ofAxiogingival Rounding (n = 15)'

BreakingCement strength (kg)

Visibie iight-activated resin

Glass-ionomer

Zinc phosphate

104.8098.33

21.2820.61

•Lislet) ir decreasing order of restoralion sirenglh.Values connected by vertical lirie are not signilicantiy ditrererl at (he95% confidence level (Newman-Keui's test).

Discussion

It appears that use of resin cement and/or one ormore of the associated procedures, such as etchingthe internal restoration surface or the surface treat-ment of the tooth, was able to counteract tbe nega-tive effect of a sloping finish line. Therefore, theform of the finish line does not appear to be asimportant when a resin lufing agent and bondingprocedures are used. However, it should be re-membered that other factors may have been re-sponsible for this finding and the same resultsmight not occur clinically. The cylindric machinedtooth preparation established a finishing line ofuniform depth on all surfaces. The finishing linewas located at the same occlusocervical levelaround the tooth's circumference, a factor found toincrease the strength of Dicor crowns,' eventhough this might be impossible to achieve clini-cally. Tbe flat machined occlusal surface may haveprovided additional restoration support. Tbe ma-cbined tooth preparations and stylized wax patternproduced a restoration with uniform thickness ofthe ceramic material axially and occlusally. Also, anoptimal total occlusal convergence angle^' of 10degrees was used on all the tooth preparations.

Additional research is needed to further clarifythe effect of the following factors upon restorationstrength when resin cements and bonding proce-dures are used : aspects of tooth preparation designsuch as total occlusal convergence and occlusocer-vical finish line location; etching of the internalceramic surtace; and the precementation treat-ments of the prepared tooth surfaces. Another as-pect thaf needs to be evaluated is the mechanismby which tbe resin cement improves the strength ofthe restoration. Finite element analysis modelinghas shown that when cements fill internal tiaws.

Table 3 Effect of Finish Une on Restoration Strength (n = 15)'

Group*

D

C

B

EA

Finish line.axiogingi^fal iihe angle form

135-degree cervical slope, sharpaxiogirigivai line angle90-degree shoulder sharpaxiogingivai line angleShouider with 0.3 mm of axiogingivalroundihgShoulder with axiogingival grooveShcuider with 0.5 mm of axiogihgivai

Cement-f

VLAR

VLAFt

VLAR

VLARVLAR

Breakinçstrength (kg)

148.47

145.20

142.73

141.00129.13

SD

21.11

21.50

21.68

26.56

29.57rounding

•Groups designated ,n accordance «it« Fig 1, iisted in decreasing order ot restoralion strerglh..

::c'tL^d''^y^raMrnVr:nct..nifican^test).

re6 . Number 3,1993 289 The International Journal of Prosthodontic

and Luting Agent5

such as voids in the crown, a significant improve-ment in tensile stress distribution results/

Conclusion

Dicor glass ceramic crowns were luted to humanmolar teeth that had been machined to standard-ized cylindric preparations. Three different lutingagents and five finishing lines were evaluated.Within the design of this study and under the con-ditions used, Dicor crowns luted using resin ce-ment and associated bonding procedures were sig-nificantly stronger than those luted usingglass-ionomcr or zinc phosphate materials. Also,variations in the finishing line form did not producesignificant differences in restoration strength whena resin cement and associated bonding procedureswere used.

References

1, Friedlander LD, Munoz CA, CoodacreQ, Doyle MC, MooreBK, Andres CJ, The effect of tooth preparation design on thebreaking strength of Dicor crowns: Part 1, Irtt ) Prosthodont1990;3:159-16a,

2, Doyle MG, Munoz CA, Goodacre C|, Friedlander LD, MooreBK, The effect of tooth preparation design on the breakingstrength ot Dicor crowns: Part 2, Int I Prosthodont 1990;3:241-24B.

3, Doyle MC, Coodacre CI, Muñoz CA, Andres C], The effect oftooth preparation design on the breaking strength of Dicorcrowns: Part 3. Int | Prosthodont 1990;3:327-340,

4, Grossman DC, Neison |W, The bonded Dicor crovi'n IIADRabstract B00|. | Dent Res 19B7;66¡special issuel:206,

5, Mclnnes-Ledoux PM, Ledoux WR, Weinberg R. A bondstrength study of luted castable ceramic restorations. ] DentRes1989;6a:e23-a25.

6, Duffin |L, Baies D|, Johnson CH. Fracture resistance of casta-ble ceramic crowns IIADR abstract 429|. | Dent Res 1989;6eispecial issue) :235,

7, Malament KA, Grossman DC. Clinicai appiication of bondedDicor crowns. Two year report |IADRabstract1S23|. J Dent Res1990;69lspecialissue):299,

6, Anusavice K|, Hoijatie B, Tensile stress in glass-ceramiccrowns: Effect of flaws and cement voids. Int J Prosthodont1992:5:351-358,

Literature Abstract _

A Seven-artd-a-Half-Year Survival Study ot Resin BondedFixed Partial Dentures

Tbe purpose of tbis study was to collect survivai data on resin bonded fixed partial dentures (RBFPDs)that were piaced under controiled clinical conditions and to investigate possible relationshipsbetween survival and retention type, cementation material, and five patient-related variables (location,abutment restorations, abutment mobility, occlusion, and anterior spatiai relation). No toothpreparations were carried out, except in premoiars and molars where guiding planes and occlusalrests were prepared. The 203 RBFPOs included in the clinical triai were considered to have survivedwhen no loss of retention was detected by the observers or by the patients. The survival rates after 7.5years were 75% for anterior RBFPDs and 44% for posterior RBFPDs. Etched metal RBFPDs weresignificantly more retentive than perforated RBFPDs. The survival rates were 7B% and 63%,respectively. With respect to cementation material, Clearfil F (Caves/Kuraray, Haarlem, TbeNetherlands) in combination with etched metal RGFPDs had tbe best overail survival rate. Maxillaryanterior R8FPDs were more susceptible to failure than mandibular anterior RBFPDs, The authorsconclude that the conservative approach of minimal tooth preparation can be successful in anteriorRBFPDs, However, the results seem to support tbe reported need for a design incorporating moreextensive tooth preparation in posterior RBrPDs.

Creugers NH|, Kayser AF, Van't Hof MA, / Dent Res 1992;71 (11): 1822-1825. HeferencFs: 35. Reprints; NH| Creugers,Oepartmeni of Oral Function and Prosthetic Dentistry, Dental Sctiool, tjriversity of t^iimegen. Philips vanLeydenlaan 25, 6525 EX Nijmegen, The Netfierlands. — Richard R. Seals, Ir, DDS, MEd, MS, DepartmenI ofPiosthodontics, The University of Texas Health Science Center al San Antonio, San Antonio, Texas

The internationai journal of Prosthodontics 290 Volumes, Number 3,1993