Guillermo Bernai. DDS, MSD Bogota, Colombia

Rose Marie /lines, DOS. MSD'

David T. Brown, DDS. MS' The Effect of Finish Line Carlos A. Munoz, DDS, MSD* Form and Luting Agent on the Breaking Strength of Charles /, Coodacre, ODS, MSD" Dicor Crowns Indiana University School of

Molar teeth were prepared to a standardized cylindric form using a lathe. Dicor crowns were lufed on these preparations using , glass-ionomer, and resin cements. The crowns were thermocycled and subjected to fracture loading fo compare the effect of fhe different luting agents on fracture strength. Five finishing lines were used to determine if resin cement and associated bonding procedures could counteract the negative effect certain finish lines produce on all-ceramic crowns. When resin cement was used in conjunction with restoration and tooth precementation bonding treatments, the crowns were significantly stronger than when glass-ionomer or zinc phosphate cements were used. No difference in strength was found between the five finish lines when the crowns were cemented with resin and associated bonding procedures. Int I Proslhodont 1993;6:286-290.

everal investigations have been reported'"^ con- The selection of a luting agent and/or restoration S cerning the effect of tooth preparation design and tooth precementation bonding procedures on the breaking strength of Dicor (Dentsply, York, have been shown to affect the strength of all- PA) complete crowns. The following tooth prepara- ceramic restorations. Grossman and Nelson' fabri- tion designs were found to enhance all-ceramic cated 50 Dicor crowns to fit identically prepared restoration strength: (1) a total occlusal conver- extracted human teeth and found that crowns luted gence of 10 degrees; (21 a shoulder finish line with a using zinc phosphate cement had significantly sharp axiogingival line angle; (3) a finish line with lower crushing strengths than crowns luted using minimal cervical slope; and (4) a finish line with Dicor light-activated resin cement. minimal variation in its occlusocervical location Mclnnes-Ledoux et al' determined that surface around the circumference of the tooth. FHowever, treatment of enamei and dentin increased the these studies did not determine if the use of resin bond strength of glass-ionomer cement and that cement and associated bonding procedures could Dicor iight-activated resin cement produced higher counteract the weakening effect of certain finish bond strengths than three different glass-ionomer line forms. cements. Duffin et al" tested the fracture resistance of Dicor crowns of varying thicknesses that had been luted using different agents on extracted third mo- lars. The highest overall fracture resistance oc- curred when resin cement was used in combination 'Associate Professor, Department of Proslhodootics. with etched enamel and the ceramic thickness was "Professor and Chairman, Department of . 0.5 mm. Malament and Grossman- evaluated the clinical Reprint requests: Dr Rose Marie Iones, Iridiaría University, School of Dentistry, 1121 West Michigan Street, Indianapolis, Indiana performance of Dicor crowns luted using a light- 46202. activated resin cement and bonding procedures.

The International lournal ÜI 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, shoulder with a 135-degree cervical slope and sharp axiogingivai line angle: E, shoulder with a 0,4-mm-wide axiogingivai groove.

0,5 mm 0,\n3 mm 0,4 mm A B E

One practitioner placed 397 crowns over 33 months. The overall success rate for bonded Dicor crowns was 98,7% and was significantly higher than previously reported values for nonbonded tech- niques. The restorations had a mean axial wall thickness of 1,82 mm and a mean posterior occlusal thickness 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 of resin cement and associated bonding procedures would counteract the negative effect of certain finishing line designs.

Materials and Methods

One hundred five intact, extracted human maxil- lary first molars were cleaned and continuously Fig Z Left, schematic diagram of ex- stored in distilled water to maintain their integrity. tracted tooth and iufed restoration with The teeth were prepared using five different finish load being appiied through a bail bearing line forms (Fig 1), To uniformly prepare each tooth, attached to Ihe moving head of an instron testing machine Right, occlusal view of fhe rotational center was determined using a restoration showing three triangular custom-made centering device (Tiber Industries, ndges that were brougiit mto simuitane- Bogota, Colombia) and then the teeth were ous contact with the bali bearing prior to mounfed in a cylindric acrylic resin (Formafray, Kerr testing. Mfg, Romulus, Ml) base. Uniform cylindric tooth preparations were made using a lathe and diamond rotary instruments by reducing the external surface Impressions of the prepared teeth were made of the tooth into a cylindric form, then creating fhe using a poly(vinyl siloxane) impression material finish lines shown in Fig 1, Finish line E was pre- (Reprosil, Caulk Dentsply, Milford, DE], and 105 pared using two different diamond instruments, stone dies (Prima Rock, Whip Mix Corp, Louisville, A total occlusal convergence angle of 10 degrees KY) were made, was used in conjunction with a finish line depth of A complete contour master wax pattern with 1,2 mm for all tooth preparations. Dimensional uni- three cusps was formed with an occlusal thickness formity was verified using digital calipers (Mitutoyo of 1.5 mm in the depth of the occlusal grooves and Digimatic Caliper 0,01-100 mm, Code No, 500-350, 2,3 mm at the cusp tips. The axial wall thickness was Mitutoyo Corp, Japan) and a Boley gauge machined 1,2 mm cervicaily and increased to 1,6 mm at the so Ihat the calipers possessed a 10-degree conver- occiusal surface. The triangular ridges were formed gence angle. The mean variation in tooth prepara- so that a 6-mm-diamefer stainless sleel ball bearing tion diameter and height were found to be 0,09 mm would make simultaneous contact with the three and 0,07 mm, respectively. ridges 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 form (Fleck's Mizzy Inc, Cherry Hill, Nj]; I2) glass- was aftached to the wax pattern, and a polytvinyl ionomer cement (Fuji Glass lonomer 1 Luting Ce- siloxanel mold was fabricated to permit the forma- ment, Scottsdale, AZ); or (3] visible light-activated tion of duplicate wax patterns using the wax injec- resin cement IDicor, Dentsplyl. The rounded inter- tion technique described by Friedlander et al.^ nal finish line was selected to facilitate fabrication Tbe axial and occlusal walls of each stone die of the specimens. werecoatedwith two layers of Dicor die spacer that To determine if resin cement could counteract were found to range between 35 and 50 ixm.' Die the negative effect of certain finish line characteris- lubricant (Slaycris Products, Portland, OR] was ap- tics, 15 additional restorations were luted using plied, and the die was placed in the previously resin cement on teeth prepared with each of the fabricated siiicone mold that was supported by a other finishing lines shown in Fig 1. Dicor visible stone matrix secured in position with an elastic light-activated resin cement was used in conjunc- band. Inlay wax (Kerr Mfg] was heated to ZSO^F in a tion with the enamel, dentin, and restoration pre- wax-injection apparatus lAutrol, Pro-Craft, GFC, cementation treatments listed in Table 1. The ce- Carlstadt, N)], and 105 identical patterns were made. mented crowns were placed in a humidor at 37°C for 7 days and then subjected to 2,500 thermal The wax patterns were recovered from the mold, cycles between 7°C and 47°C with adwell timeof 30 the margins were refined on each die, and Dicor seconds at each temperature. The crowns were crowns were fabricated according to the manufac- then stored for 1 week at 37^ and 100% humidity turer's recommendations. Two layers of Dicor and then tested to fracture. shading porcelain (D-C3] were applied and fired. To verify that differences in restoration strength Each was placed in a testing jig specially could be detected when different luting agents fabricated for this study. With the use of 0.04-mm- were used with this tooth-restoration experimental thick articulating paper (Bausch Articulating Paper, model, 15 crowns Imade for dies with a shoulder Dentrade Internat, San Francisco, CA], the crowns finish line and 0.3 mm of axiogingival rounding; Fig were aligned so the three triangular ridges were 1, finish line B) were luted using each of the follow- brought into simultaneous contact with the 6-mm ing three materials: 11) zinc phosphate cement stainless steel ball bearing fitted into the moving

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

Restoration pretreatment Tooth pretreatment

Crown dried with Tooth cieaned witti oil-free Base and oataiyst mixed in compressed air nontluoride prophylaxis equal portions tor 30 paste, rinsed with water tor seconds 15 seconds, and dried with compressed air for 10 seconds Ammonium hitiuoride etching Enamel etched with Gluma Cement placed into the gel applied to internal surtace Etchant (Coiumbus Dental, crown and crown seated for 1 minute St Louis, MO) for 30 seconds using firm finger pressure fo express excess cement Crown rinsed with water tor Tocth rinsed with water for 10 Crown placed under 3.5-kg 15 seconds seconds and then dried with ioad compressed air Crown air dried for 10 Gluma 2 oleanser applied to Cement light-polymerized for seconds dentin for 30 seconds 30 seconds, moving iight wand over ali crown surfaces 0.1% silane Dicor coupling Tooth rinsed lor 10 seconds Excess cement removed agent solution applied for 3 with water and air dried minutes Giuma 3 primer applied to Cement poiymerized tor an dentin for 30 seconds, tooth additionai 2 minutes then dried with compressed air Crown air dried tor 10 Gluma 4 sealer applied to seconds and set aside for 24 enamei and dentin, tooth hours then dried with compressed

I of Prosthodontics 288 Volume 6, Nur"!— Finish Line Form and Luting Agents

head of a mechanical testing machine (Instron Discussion Model 1123, Instron Corp, Canton, MA) (Fig 2). A crosshead speed of 1 mm/min was used and the It appears that use of resin cement and/or one or load at fracture was recorded on tbe chart of the more of the associated procedures, such as etching testing machine. the internal restoration surface or the surface treat- The mean and standard deviation were calculated ment of the tooth, was able to counteract tbe nega- for each group and fhe resulfs subjecfed fo a two- tive effect of a sloping finish line. Therefore, the way analysis of variance. Since differences among form of the finish line does not appear to be as means were found, a Newman-Keul's multiple important when a resin lufing agent and bonding comparison test was performed to identify differ- procedures are used. However, it should be re- ences between groups. membered that other factors may have been re- sponsible for this finding and the same results might not occur clinically. The cylindric machined Results tooth preparation established a finishing line of uniform depth on all surfaces. The finishing line Restorations luted using fbe visible light- was located at the same occlusocervical level activated resin cement and associated bonding pro- around the tooth's circumference, a factor found to cedures were significantly stronger than fhose increase the strength of Dicor crowns,' even luted using glass-ionomer or zinc phosphate ce- though this might be impossible to achieve clini- ments (Table 2). Similar restoration strengths were cally. Tbe flat machined occlusal surface may have recorded for botb glass-ionomer and zinc pbos- provided additional restoration support. Tbe ma- phate materials (Table 2). No differences in strength cbined tooth preparations and stylized wax pattern were found between tbe five finish line designs produced a restoration with uniform thickness of when the light-activated resin cement and bonding the ceramic material axially and occlusally. Also, an procedures were used (Table 3). optimal total occlusal convergence angle^' of 10 degrees was used on all the tooth preparations.

Table 2 Effect of Cement on Restoration Strength Additional research is needed to further clarify Using Finish Line With Shoulder and 0.3 mm of the effect of the following factors upon restoration Axiogingival Rounding (n = 15)' strength when resin cements and bonding proce- Breaking dures are used : aspects of tooth preparation design Cement strength (kg) such as total occlusal convergence and occlusocer- vical finish line location; etching of the internal Visibie iight- ceramic surtace; and the precementation treat- activated resin ments of the prepared tooth surfaces. Another as- 104.80 21.28 Glass-ionomer pect thaf needs to be evaluated is the mechanism 20.61 Zinc phosphate 98.33 by which tbe resin cement improves the strength of •Lislet) ir decreasing order of restoralion sirenglh. the restoration. Finite element analysis modeling Values connected by vertical lirie are not signilicantiy ditrererl at (he has shown that when cements fill internal tiaws. 95% confidence level (Newman-Keui's test).

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

Finish line. Breakinç Group* axiogingi^fal iihe angle form Cement-f strength (kg) SD VLAR 148.47 21.11 D 135-degree cervical slope, sharp axiogirigivai line angle VLAFt 145.20 21.50 C 90-degree shoulder sharp axiogingivai line angle 142.73 21.68 B Shouider with 0.3 mm of axiogingival VLAR roundihg 141.00 26.56 E Shoulder with axiogingival groove VLAR A Shcuider with 0.5 mm of axiogihgivai VLAR 129.13 29.57 rounding

•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- References ment in tensile stress distribution results/ 1, Friedlander LD, Munoz CA, CoodacreQ, Doyle MC, Moore BK, Andres CJ, The effect of tooth preparation design on the Conclusion breaking strength of Dicor crowns: Part 1, Irtt ) Prosthodont 1990;3:159-16a, Dicor glass ceramic crowns were luted to human 2, Doyle MG, Munoz CA, Goodacre C|, Friedlander LD, Moore molar teeth that had been machined to standard- BK, The effect of tooth preparation design on the breaking strength ot Dicor crowns: Part 2, Int I Prosthodont 1990; ized cylindric preparations. Three different luting 3:241-24B. agents and five finishing lines were evaluated. 3, Doyle MC, Coodacre CI, Muñoz CA, Andres C], The effect of Within the design of this study and under the con- tooth preparation design on the breaking strength of Dicor ditions used, Dicor crowns luted using resin ce- crowns: Part 3. Int | Prosthodont 1990;3:327-340, ment and associated bonding procedures were sig- 4, Grossman DC, Neison |W, The bonded Dicor crovi'n IIADR abstract B00|. | Dent Res 19B7;66¡special issuel:206, nificantly stronger than those luted using 5, Mclnnes-Ledoux PM, Ledoux WR, Weinberg R. A bond glass-ionomcr or zinc phosphate materials. Also, strength study of luted castable ceramic restorations. ] Dent variations in the finishing line form did not produce Res1989;6a:e23-a25. significant differences in restoration strength when 6, Duffin |L, Baies D|, Johnson CH. Fracture resistance of casta- a resin cement and associated bonding procedures ble ceramic crowns IIADR abstract 429|. | Dent Res 1989; 6eispecial issue) :235, were used. 7, Malament KA, Grossman DC. Clinicai appiication of bonded Dicor crowns. Two year report |IADRabstract1S23|. J Dent Res 1990;69lspecialissue):299, 6, Anusavice K|, Hoijatie B, Tensile stress in glass-ceramic crowns: Effect of flaws and cement voids. Int J Prosthodont 1992:5:351-358,

Literature Abstract _

A Seven-artd-a-Half-Year Survival Study ot Resin Bonded Fixed 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 relationships between survival and retention type, cementation material, and five patient-related variables (location, abutment restorations, abutment mobility, occlusion, and anterior spatiai relation). No tooth preparations were carried out, except in premoiars and molars where guiding planes and occlusal rests were prepared. The 203 RBFPOs included in the clinical triai were considered to have survived when no loss of retention was detected by the observers or by the patients. The survival rates after 7.5 years were 75% for anterior RBFPDs and 44% for posterior RBFPDs. Etched metal RBFPDs were significantly more retentive than perforated RBFPDs. The survival rates were 7B% and 63%, respectively. With respect to cementation material, Clearfil F (Caves/Kuraray, Haarlem, Tbe Netherlands) in combination with etched metal RGFPDs had tbe best overail survival rate. Maxillary anterior R8FPDs were more susceptible to failure than mandibular anterior RBFPDs, The authors conclude that the conservative approach of minimal tooth preparation can be successful in anterior RBFPDs, However, the results seem to support tbe reported need for a design incorporating more extensive 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 van Leydenlaan 25, 6525 EX Nijmegen, The Netfierlands. — Richard R. Seals, Ir, DDS, MEd, MS, DepartmenI of Piosthodontics, The University of Texas Health Science Center al San Antonio, San Antonio, Texas

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