CLINICAL RESEARCH

aPrivate pracbPrivate prac

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Effect of different cement types on monolithic lithiumdisilicate complete crowns with feather-edge preparation

design in the posterior region

Johannes H. Schmitz, DDS, PhDa and Massimiliano Beani, DDSb

ABSTRACTStatement of problem. Ideally, tooth preparation for complete crowns should require the removalof the smallest amount possible of sound tooth structure to maximize the strength of the remainingtooth. Some preparation designs, such as the feather-edge margin, are less invasive. However,limited data are available regarding monolithic lithium disilicate crowns for molars andpremolars with this type of margin geometry.

Purpose. The purpose of this retrospective study was to evaluate the clinical outcome and survivalof monolithic lithium disilicate crowns in the posterior region fabricated with feather-edge marginsand cemented either with conventional (glass ionomer) or resin self-etching cement in 2 privatepractices.

Material and methods. A total of 257 monolithic lithium disilicate restorations on posterior teeth(108 premolars, 149 molars) were placed in 158 patients. All teeth were prepared with feather-edgemargins and restored with single crowns. The modified California Dental Association (CDA) criteriawere used to clinically evaluate participants recalled between June and December 2014.

Results. The mean ±standard deviation follow-up time was 24 (±13.6; range: 6-75) months. Threecrowns were replaced during the follow-up period because of the bulk fracture of the material(98.83% survival rate). No other technical or biological failure was observed.

Conclusions. In this retrospective evaluation, monolithic lithium disilicate crowns with feather-edgemargins yielded clinical outcomes similar to those reported with other margin designs andmaterials. (J Prosthet Dent 2016;115:678-683)

Complete crowns in the pos-terior region are common inpatients with severe loss oftooth structure due to cariesor trauma and after endodon-tic treatment. Although castgold and metal ceramic resto-rations have proven theirlong-term reliability and dur-ability,1,2 clinicians and pa-tients are currently attractedto more natural-looking alter-natives. Restorations basedon yttrium-oxideestabilizedtetragonal zirconia polycrystals(Y-TZP) have become popularbecause of their excellent me-chanical properties, biocom-patibility, and good estheticswhen veneered with porcelain.Several studies of zirconia-

based restorations have shown acceptable survivalrates,3-6 but almost all also report chipping or veneeringmaterial fractures of the porcelain.7-9

Depending on its magnitude and location, a smallveneering material fracture may require a simpleadjustment of the restoration through polishing or repairwith composite resin. However, if the damage grosslyalters the esthetic appearance or leads to the loss ofocclusal or proximal contacts, the restoration may need tobe replaced. The use of veneering ceramics with a CTE

tice, Milan, Italy.tice, Aiello, Italy.

(coefficient of thermal expansion) as close as possible tothat of the zirconia core, anatomically supportingframeworks, as well as slow heating and cooling ratesduring the veneering process have been shown to reducethe incidence of chipping.10,11

Other solutions to this shortcoming include theelimination of esthetic layering by using monolithiccrowns made from a more translucent high-strengthmaterial.12-16 Instead of building up the porcelain inseveral layers and firing in multiple firing cycles, a single

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Clinical ImplicationsThe use of feather-edge preparations incombination with monolithic lithium disilicatesingle crowns on posterior teeth cemented witheither self-adhesive resin cement or glass ionomercement was associated with high survival ratesduring the observation period.

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material is used to fabricate the restoration. The estheticsof monolithic crowns can be individualized by usingstaining techniques.

A modified lithium disilicate material has been intro-duced (IPS e.max Press; Ivoclar Vivadent AG) whoseintrinsic mechanical and esthetic properties seem to beclinically suitable for the fabrication of single crowns onposterior single crowns.17,18 It is available both as an ingotthat can be pressed or as a solid block that can be milledwith computer-aided design and computer-aidedmanufacturing (CAD/CAM) technology (IPS e.max CAD;Ivoclar Vivadent AG). The pressable material has becomea popular dental restorative because of its precision, goodmechanical properties, and translucency.19

In vitro testing of this material suggested thatmonolithic lithium disilicate restorations can be morefatigue resistant than veneered zirconia.20-22 Clinicaltesting has also shown promising results in terms ofshort- to medium- term survival rates, esthetic outcome,and wear-friendliness to opposing enamel.12-16,23-25

Monolithic restorations made with this material can beeither adhesively or nonadhesively cemented, as clinicalstudies and in vitro testing have reported no relevantdifferences between the 2 modes of cementation.26-28

Traditional ceramic systems require adequate clear-ance to achieve proper esthetics and strength of therestoration. The typical recommendation for preparingteeth for ceramic crowns is a rounded shoulder, or heavychamfer, with a facial tooth reduction up to 2 mmconsidered appropriate.29-31 These deep-shoulder mar-gins, however, may compromise tooth strength andpulpal health because they require the removal of a largeamount of tooth structure.32 Reducing the thickness ofmonolithic lithium disilicate crowns to 1 mm does notseem to decrease the reliability of the restorations.21,22

Less extensive preparations, therefore, are possiblewhen using monolithic lithium disilicate. This is clinicallyrelevant especially for teeth with long clinical crownsbecause of their narrower diameter in the cervical region.In fact, preparations extending onto or beyond thecementoenamel junction already require the removal of asignificant amount of tooth structure to eliminate un-dercuts. In such situations, it would be desirable to pre-serve as much sound tooth structure as possible during

Schmitz and Beani

tooth preparation by using the most conservative prep-aration designs, such as a vertical margin geometry. Thistype of preparation design (named feather-edge or knife-edge) has no identifiable visible margin on the preparedtooth and is, generally, a less invasive alternative to ahorizontal margin (such as shoulder or chamfer).33-36 Inaddition to periodontally compromised teeth,37-39 otherclinical situations could also benefit from preserving amaximum of intact coronal and radicular tooth structure.For example, preserving the parallel walls of dentin thatextend in a coronal direction in endodontically treatedteeth provides a ferrule by reducing stresses within atooth after being encircled by a crown.40 Although metalceramic restorations fabricated on vertical margin ge-ometry usually require margins in metal, the use of zir-conia or lithium disilicate can improve the esthetic as wellas mechanical outcome.5,6,12-16

This nonrandomized retrospective clinical studyevaluated the clinical performance of pressable lithiumdisilicate single tooth restorations with feather-edgepreparations, cemented either with glass ionomer orself-adhesive resin based cement.

MATERIAL AND METHODS

A total of 257 monolithic lithium disilicate crowns withfeather-edge margins were cemented with 2 differentcements. The crowns were placed by 2 private dentalpractices between January 2009 and December 2014(Table 1) in a total of 168 patients. The distribution ofcrowns by tooth position is reported in Table 2. All res-torations were evaluated during follow-up appointmentsbetween January and July 2015. The clinical evaluation(Table 3) was performed according to the modified Cal-ifornia Dental Association (CDA) criteria.41

Both operators (J.H.S. and M.B.) followed similarclinical procedures for tooth preparation, using the samearmamentarium. Teeth were prepared with a vertical(feather-edge) margin. A reduction of at least 1 mm wasmade along the axial walls and approximately 0.3 mm atthe margins, with flame- shape diamond rotary in-struments (862.12, 862.16, 8862.12; Brasseler-Komet).The finish line was placed at gingival level or up to 1mm apical to the free gingiva. Abutments were alsoreduced by 1.5 mm at the occlusal surface, with aconvergence angle of approximately 6 to 10 degrees toprovide adequate strength for the restoration.

Interim crowns were fabricated and luted witheugenol-free zinc oxide eugenol cement (Temp Bond NE;Kerr Corp). Impressions were made approximately 2weeks after tooth preparation. A double displacementcord technique was used (Ultrapack; Ultradent ProductsInc), and impressions were made with a polyetherimpression material (Impregum Penta; 3M ESPE) with asingle viscocity procedure.42-44

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Table 1.Distribution of molars or premolars and number of patients with posterior crowns included

Year

Resin Cement Group Glass Ionomer Cement Group Both Groups

Premolar Molar n Patients Premolars Molars n Patients Premolar Molars n Patients

2009 1 0 1 1 0 1 2 0 2

2010 2 2 4 1 3 3 3 5 7

2011 5 7 10 8 11 15 13 18 27

2012 9 18 20 16 12 9 25 30 35

2013 19 30 30 24 26 27 43 56 27

2014 8 23 27 14 17 21 22 40 46

Total 44 80 92 64 69 76 108 149 168

Table 2.Mean outcome percentages for each group

ParameterResin Cement

GroupGlass Ionomer Cement

GroupBoth

Groups

Range 6-75 6-68 6-75

Mean no. ofmonths

23.1 25.5 24.0

SD (months) ±12.7 ±14.9 ±13.6

CSR (%) 99.2 98.5 98.8

CSR, cumulative survival rate.

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The clinical fit of the restorations was verified with asilicone material (Fit Checker Black; GC America Inc).Undesirable pressure spots on the intaglio surface wereremoved before cementation. Cementation was the onlyprocedure carried out differently by each clinician.Crowns (n=124) placed by J.H.S. were cemented withself-adhesive resin cement. The intaglio surface of thecrown was etched for 20 seconds with hydrofluoric acid,and a silane coupling agent (Clearfil Ceramic Primer;Kuraray Noritake Dental Inc) was then applied before theapplication of a self-etching resin-based cement (Rely-XUnicem 2; 3M ESPE). The tooth surface was cleaned withpumice on a rotary brush at low speed, and a 2%chlorhexidine solution (chlorhexidine digluconate 20%solution diluted 1:10 in distilled water; Caesar & LoretzGmbH) was applied before cementation. Isolation wasobtained with the split dam technique or with cotton rollswhenever appropriate. The resin cement was mixed ac-cording to manufacturer instructions, the crown wasseated, and the excess cement was light polymerized for5 seconds. Excess cement was removed, and cement waslight polymerized for an additional 60 seconds.

Crowns placed by the other operator (n=133 [M.B.])were cemented with conventional glass ionomer cement(Vivaglas; Ivoclar Vivadent AG). The intaglio surfaces ofthe crowns in this group, as well as the tooth surfaces,were cleaned with hydrogen peroxide before the appli-cation of the glass ionomer cement, which was mixedaccording to manufacturer’s instructions. The workingarea was isolated with cotton rolls, and aluminum foilwas applied over the seated crown to avoid contact withmoisture until the cement had set completely. Excesscement was removed at this stage. In both groups, aftercementation and removal of excess cement, the occlusalcontacts were evaluated and adjusted.

All patients were enrolled in a maintenance programevery 3 to 6 months according to their periodontal con-dition. During regular maintenance appointments be-tween June and December 2014, the crowns werevisually inspected for chips, cracks, or fractures, andmarginal integrity was evaluated with a sharp dentalexplorer (XP23/UNC12; Hu-Friedy). Data were gatheredaccording to the modified CDA criteria for color match,porcelain surface, marginal discoloration, and integrity

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and further evaluated with descriptive statistics. Patients’satisfaction was assessed using nominal scores (non-acceptable, acceptable, good, and excellent). In the caseof any mechanical complication, the restoration wasconsidered a failure.

RESULTS

Three of the crowns fractured and were classified asfailures: 1 failed during cementation in the resin cementgroup, 1 failed after 2 months in a patient in the glassionomer cement group, and 1 failed after 4 years ofservice in the glass ionomer group.

One mandibular molar crown in the resin cementgroup required endodontic treatment because of hy-persensitivity. The access cavity was restored withcomposite resin, and the crown remained functional; itwas not considered a failure. Therefore, at the time ofclinical evaluation, 254 of the initial 257 crowns wereavailable for evaluation (Table 2). No loss of retentionwas observed during the observation period (Table 1,mean ±SD of 24 ±13.6 months; range, 5-75 months).Clinical ratings of the monolithic crowns are reportedin Table 3. The color match was rated excellent for230 crowns, and the surface and anatomic form wererated excellent for 248 crowns; 6 crowns showed mi-nor wear and a dull appearance and were polishedchairside. A total of 245 crowns were rated excellentfor marginal discoloration and 252 for marginalintegrity. No surface chipping was noted. The colormatch was the lowest rating recorded, at 90.5%. Allalpha scores were approximately 97% or above, indi-cating no appreciable change in the crowns in theobserved sample. Representative crowns are shown inFigures 1-3.

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Figure 1. Monolithic crown on left mandibular second molar 39 monthsafter cementation.

Table 3. Frequency distribution of clinical ratings according to modified CDA criteria

Parameter

Modified CDA rating

A (%) B (%) C (%) D (%)

ResinCement

Glass IonomerCement Both

ResinCement

Glass IonomerCement Both

ResinCement

Glass IonomerCement Both 1 2 1+2

Color match 91.9 89.3 90.6 8.1 10.7 9.4 0 0 0 0 0 0

Surface 98.4 97.0 97.6 1.6 3.0 2.4 0 0 0 0 0 0

Marginal discoloration 97.6 95.5 96.4 2.4 4.5 3.6 0 0 0 0 0 0

Marginal Integrity 99.2 98.5 98.8 0.8 1.5 1.2 0 0 0 0 0 0

CDA, California Dental Association.

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DISCUSSION

In this study, monolithic lithium disilicate crownswere associated with high short-term success rates(close to 99%) during the observation period, com-parable with available data reported in otherstudies.15,16 In an effort to reduce the biological costwithout compromising the esthetic outcome of thecrown (in this case, biological cost was the removal ofsound tooth structure), a high-strength ceramic ma-terial was selected in combination with feather-edgeemargin geometry. This type of cervical margindesign has been shown to provide excellent marginalintegrity with veneered zirconia partial fixed dentalprostheses and crowns.5,6,12,14

Various investigators have shown that such verticalmargins bring the procedural advantages of easierimpression making, even for multiple abutments, andimproved marginal adaptation after cementation.33-36 Nodifferences in periodontal health have been demon-strated among different geometric patterns of margindesigns, and the crown margins in teeth restored withfeather-edge margins have influenced gingival conditionsno more than natural teeth in a sample of patients withperiodontal disease.37-39

Monolithic complete restorations in lithium disilicatehave shown higher in vitro fracture loads than hand-layered zirconia restorations, with bulk fracture of thematerial seen only at high stress levels.13,20-22 Clinicalreports suggest that this type of ceramic restoration isreliable,12-14 even at a reduced thickness.21,22

Cortellini et al12 reported use of monolithic lithiumdisilicate crowns with feather-edge margin design.Considering only crowns in the posterior region, the 99crowns cemented with a self-etching resin cementincluded in the study showed high survival rates, withfew technical complications. Valenti and Valenti14 re-ported using 70 posterior monolithic and bilayered singlelithium disilicate crowns cemented with a self-etchingresin cement, confirming the good clinical performanceof these restorations.

In the present study, one clinician cemented therestorations with conventional glass ionomer cementwhile the other used self-adhesive resin-based cement.No relevant clinical differences were found between the

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groups of restorations. Neither the type of cement northe location of the crown (premolar versus molar) influ-enced the survival rate of the crowns.

Although feather-edge preparations are not specifiedfor use with this material by the manufacturer, themanufacturer claims it can be pressed to form thin,preparationless veneers with a minimum thickness of 0.3mm, which is compatible with the current study’s crownthickness at the margin. A recently published in vitrostudy showed that lithium disilicate ceramic crownsbonded to abutment teeth prepared with this type ofmargin resulted in a fracture strength similar to that ofthose bonded on abutments with a horizontal finishline.13

The favorable results of in vitro13,20,21,23 and clinicalstudies12,14,15,21,22 suggest that the toughness and initialstrength of monolithic pressed lithium disilicate is suit-able for the fabrication of crowns in the posterior region.Nevertheless, there is a risk of subcritical crack formationand propagation, especially in the presence of moisture.Repetitive or long-term loading at low levels may causepreexisting subcritical defects within the restorative ma-terial to grow slowly until failure occurs.45,46 In theory,bulk fracture may occur even at a level of loadingthat was originally insufficient to cause failure of theprosthesis. In vitro studies have reported that a thicknessof 1.5 mm or greater (similar to the restoration thickness

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Figure 2. Crown on right maxillary first molar after 1 year. A, Occlusal view. B, Buccal view.

Figure 3. Crown on left maxillary second premolar after 10 months. A, Occlusal view. B, Buccal view.

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considered in this study) significantly increases thenumber of cycles needed for specimen failure.45,46 Asufficient restoration thickness in the occlusal region maybe helpful in preventing the onset of crack formation andpropagation. Unfortunately, limited clinical data areavailable regarding the medium to long-term survival ofthis type of restoration.16

In the present study, 254 monolithic crowns withfeather-edge preparations placed in 2 private dentalpractices gave favorable results, similar or superior toother short-term data reported for other margin designsand materials.

According to CDA evaluation criteria, the clinicalquality of all crowns was within the satisfactory range.Patient satisfaction with the crowns was also high. Nocaries were detected, and no adverse soft tissue reactionsaround the crowns were observed. Margin integrity wasrated excellent in virtually all crowns.

A few limitations of this retrospective study should beconsidered. Treatment was performed in 2 separate pri-vate practices by different clinicians following similarprotocols but with different cementation procedures. The

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same type of margin was prepared with identical rotaryinstruments. In the dental laboratories, the lost waxtechnique and same pressed ceramic systems were usedto fabricate the crowns. A direct comparison of the 2groups of restorations reported was, nevertheless,impossible because the number of crowns per patientand group were different and placed at different times.Both groups, however, showed a low failure rate. Allpatients were well motivated and complied with main-tenance appointments.

Results of the present study suggest that the clinicalperformance of monolithic lithium disilicate crowns withfeather-edge margins is similar to that reported withother margin designs, although it required less removalof tooth structure. Existing recommendations to avoidknife-edge margins for lithium disilicate restorationswere not supported by the study or by other clinical andin vitro studies.12,14 Despite such favorable and encour-aging results, longer observation periods and randomizedcontrolled trials are needed to compare the long-termeffectiveness of lithium disilicate crowns fabricated withdifferent marginal designs.

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CONCLUSIONS

Results from this retrospective evaluation suggest that formonolithic lithium disilicate, feather-edge margins yieldclinical outcomes similar to those reported with othermargin designs.

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Corresponding author:Dr Johannes H. SchmitzGalleria Buenos Aires, 1420124 MilanITALYEmail: jschmitz@tiscali.it

Copyright © 2016 by the Editorial Council for The Journal of Prosthetic Dentistry.

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