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CLINICAL SCIENCE
aAdjunct lectbAssociate lecSenior lectudSenior lectu
THE JOURNA
A prospective evaluation of zirconia anterior partial fixeddental prostheses: Clinical results after seven years
Maria Fernanda Solá-Ruíz, DMD, PhD, MD,a Rubén Agustin-Panadero, DMD, PhD,b
Antonio Fons-Font, DMD, PhD, MD,c and Carlos Labaig-Rueda, DMD, PhD, MDd
ABSTRACTStatement of problem. Because of the high mechanical strength of zirconium dioxide, the metal infixed partial prostheses can now be replaced. However, the material is susceptible to aging orhydrothermal degradation and to chipping of the feldspathic veneer.
Purpose. The purpose of this prospective study was to evaluate the survival (without failure) andsuccess (survival without any complication or failure) rate and clinical efficacy of anterior zirconiapartial fixed dental prostheses.
Material and methods. Twenty-seven anterior partial fixed dental prostheses of 3 to 6 units werefabricated. All participants were examined after 1 month and 6 months, then annually for 7 years.
Results. Three partial fixed dental prostheses failed and had to be removed: 2 because of secondarycaries, which increased failure significantly (P=.001) and 1 because of severe chipping. Six partialfixed dental prostheses had complications: 2 debonded, 3 had chipping, and 1 had periapicalpathology. All veneer porcelain fractures occurred in 6-unit fixed partial prostheses (P=.002). Theclinical success rate was 88.8% after the 7-year follow-up.
Conclusions. The clinical behavior of partial fixed dental prostheses with a zirconium dioxide corein the anterior region provides an adequate medium-term survival rate. The main cause of failurewas secondary caries. The most frequent complication was chipping, which was directly related tothe number of units of the prosthesis. (J Prosthet Dent 2015;-:---)
Since the end of the 20thcentury, high-strength ce-ramics have come into use toreplace the metal in fixed res-torations, a development thathas led to high expectations.1
Among the ceramics used inthese new systems, zirconiumdioxide (zirconia) has been themain focus of research becauseit offers a range of propertiesthat make it suitable for usein dentistry: biocompatibility,high fracture resistance, lowthermal conductivity, resis-tance to corrosion, and atotally crystalline microstruc-ture.1,2 Yttrium-stabilized zir-conium dioxide is suitable for
optical applications because of its high refraction index,its low absorption coefficient, and its high opacity in thevisible and infrared spectra.3 Its grain size and the dis-tribution of different grain sizes, the pressure methodand conditions, and different additives all determine thetranslucency of a restoration.3 In spite of the material’shigh fracture resistance, chipping of the feldspathic por-celain veneer of zirconia fixed dental prostheses duringmastication is a frequent problem.4 This complicationgenerates some uncertainty as to the long-term perfor-mance of the material in dental restorations.5 Anurer, Department of Buccofacial Prosthetics, Faculty of Medicine and Dencturer, Department of Buccofacial Prosthetics, Faculty of Medicine and Derer, Department of Buccofacial Prosthetics, Faculty of Medicine and Dentirer, Department of Buccofacial Prosthetics, Faculty of Medicine and Denti
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inherent problem of the material is a phenomenonknown as spontaneous aging, hydrothermal degradation,or low-temperature degradation.6,7 These factors changeits crystalline phase from tetragonal to monoclinic, whichincreases the volume (4% to 5%) of the crystals causingthe loss of their mechanical properties and the appear-ance of microcracks or macrocracks.6-9
In spite of these setbacks, the survival rates of zirconiafeldspathic fixed partial dental prostheses (FDPs) aregreater than those of lithium disilicate-based core ce-ramics10 and similar to those of metal ceramic prostheses,
tistry, University of Valencia, Valencia, Spain.ntistry, University of Valencia, Valencia, Spain.stry, University of Valencia, Valencia, Spain.stry, University of Valencia, Valencia, Spain.
1
Figure 1. Chipping of veneer ceramic in maxillary central incisors offixed dental prosthesis with zirconia core.
Clinical ImplicationsZirconia fixed partial dental prostheses offer arecommendable alternative for replacing teeth inthe esthetic zone.
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which have survival rates of 97% and 99% after 5 years,respectively.11-14
Most published research has analyzed the behavior ofzirconia restorations in the posterior zone.15,16 Chippingor delamination has been defined as the fracture of theveneer ceramic, and a high rate varying between 6% and28% has been confirmed over a period of 3 to 10 years.16-33 These are high values compared to the 4% fracture ratedemonstrated by conventional metal ceramic restorationsover 10 years.34
According to Heintze and Rousson,14 chipping, canbe classified by severity and by the treatment required forrepair, as follows: grade 1, small surface chipping, withtreatment being polishing the restoration surface; grade2, moderate surface chipping, with treatment being usinga resin composite repair system (Fig. 1); and grade 3,severe veneer ceramic chipping exposing the zirconiumdioxide core, with treatment being replacing thedamaged prosthesis. Literature reviews such as those byHeintze and Rousson,14 Anusavice,35 and Raigrodski,36
show that the most frequent types of zirconia-basedfixed dental prostheses chipping are grades 1 and 2,which do not involve restoration failure.
Several factors have been identified that can influencethe incidence of chipping, as follows: (1) residual tensiondue to differences in the thermal expansion coefficientsof the core and the veneer materials; (2) poor wettabilityof the core by the veneer ceramic37; (3) compressionresulting from firing the porcelain38,39; (4) the protocol forheating and cooling the veneer and core37; (5) trans-formation of the zirconium dioxide crystal phase at thecore-porcelain veneer interface caused by thermal in-fluences or load forces40; (6) formation of inherent defectsduring processing41; (7) veneer ceramic applicationtechnique (stratification/injection)41-43; (8) Inadequatethickness of veneer ceramic44; and (9) occlusal trauma.45
The aim of this study was to evaluate the success andsurvival rates and biological and/or mechanical compli-cations of zirconium dioxide FDPs in the anterior regionover a 7-year follow-up.
MATERIAL AND METHODS
Twenty-seven participants (14 women and 13 men) agedbetween 30 and 65 years took part in the study, whichwas carried out in the Prosthodontics and OcclusionDepartment at the University of Valencia betweenJanuary 1, 2005, and January 1, 2006, with latest
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evaluation being on January 1, 2014. The ethical board forclinical trials of the University of Valencia approved thestudy protocol, and all participants gave their informedconsent to take part.
Inclusion criteria were the need to replace 1 or 2anterior teeth (central or lateral incisors), indicating theplacement of FDPs of between 3 and 6 units, periodon-tally healthy abutment teeth, no signs of either resorptionor periapical pathology, stable occlusion, and naturalteeth in the antagonist arch.
Individuals requiring a fixed partial prosthesis of morethan 2 pontics orwith poor oral hygiene, a high incidence ofcaries, active periodontal disease, or bruxismwere excluded.
Prosthodontic proceduresThree clinicians (S.R.M.F., A.P.R., F.F.A.) with experiencein fixed prosthodontics prepared the abutment teeth tomeet the following parameters: occlusal and/or incisalreduction of 1.5 to 2 mm; axial reduction of 1 to 1.5 mmwith a 10-degree included convergence angle, and acircular chamfer or shoulder of 1 mm. Particular attentionwas paid to rounded line angles (Figs. 2, 3). The color ofeach abutment tooth and adjacent teeth was identifiedwith a shade guide (Vita shade guide, Vita Zahnfabrik).Interim restorations were fabricated from polymethylmethacrylate (AcryLux C&B; Ruthinium Group, DentalManufacturing Spa) and cemented with eugenol-freeinterim cement (Temp Bond NE; Kerr Corp).
Definitive impressions were made with polyvinylsiloxane impression material (Exaflex; GC America Inc) ina stock perforated stainless steel tray (Zhermack; BadiaPolesine). Impressions of the opposing arch were madewith irreversible hydrocolloid impression material(Orthoprint; Zhermack) and intermaxillary relations wereregistered in wax (X-hard; Miltex).
The FDPs were fabricated with a computer-aideddesign and computer-aided manufacturing (CAD/CAM)
Solá-Ruíz et al
Figure 3. Abutment tooth preparation.Figure 2. Patient before treatment with existing fixed dental prosthesisand tooth wear.
Figure 4. A, B, Zirconia framework evaluated intraorally to ensure adequate fit.
Figure 5. Fixed dental prosthesis cemented.
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system (Lava; 3M ESPE). They had a connector surfacearea of 7 mm2; a uniform coping thickness of 0.5 mm wasused for all prostheses to standardize the study protocol.All internal frameworks were evaluated in the mouth toensure an adequate fit (Fig. 4). The veneer ceramic usedwas Lava Ceram (3M ESPE). Before bonding, the internalsurfaces of the prosthetic framework were treated byairborne-particle abrasion with a tribochemical silicacoating with 30 mm Al2O3 particles (CoJet; 3M ESPE). Alayer of silane (Monobond; Ivoclar Vivadent) wasapplied. The teeth were also treated with 35% ortho-phosphoric acid, followed by application of the dentinadhesive (NT Prime Bond; 3M ESPE). All the FDPs werebonded with a resin cement (Multilink; Ivoclar Vivadent)(Fig. 5).
Clinical follow-upThe 27 participants were examined by 2 clinicians whohad not been involved in treating them, at 1 month afterrestoration, after 6 months, and thereafter annually for 7years. The clinical parameters analyzed were loss of
Solá-Ruíz et al
vitality or infection of the abutment teeth (cold test andperiapical radiographs), secondary caries, debonding,fracture of the prosthesis core, and chipping of the veneerceramic. Both clinicians evaluated the prostheses inde-pendently. The parameters were such that assessment
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Figure 6. A, Incisal edge chipping level 1. Maxillary right canine. B, Chipping corrected with intraoral polishing.
Table 1. Complication details in prospective evaluation after 7 years
Complication Complication Present n Incidence (%)
Chipping No 23 85.2
Yes 4 14.8
Core fracture No 27 100
Yes 0 0
Debonding No 25 92.6
Yes 2 7.4
Secondary caries No 25 92.6
Yes 2 7.4
Endodontic No 26 96.3
Yes 1 3.7
1.0
Survival Function0
0.8
0.6
0.4
0.2
0.0
0.00 2.00 4.00 6.00
Complication time8.00 10.00
Surv
ival
Figure 7. Probability of survival without complications until end offollow-up period.
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was objective in all prostheses, so reliability testing wasnot thought to be necessary. Nevertheless, if divergencesdid occur between their findings, the lower value wasused for analysis.
Statistical analysis was performed with software(Statistical Package for the Social Sciences; IBM SPSSStatistics) applying initial descriptive and bivariate ana-lyses, the Kruskal-Wallis test, the Mann-Whitney test,and Kaplan-Meier survival analysis (a=.05).
RESULTS
The 27 participants received 27 FDPs: ten 3-unit, ten 4-unit, two 5-unit, and five 6-unit FDPs. All participantscompleted the 7-year follow-up, and no appointmentswere missed. After 7 years of monitoring, 3 completerestoration failures had occurred requiring removal (one3-unit and one 4-unit FDP failure because of secondarycaries and one 6-unit FDP failure because of irreparablechipping). The survival rate of the zirconia core restora-tions was 88.8% after the 84-month follow-up (95%confidence interval [CI] 70.8 to 97.7).
The complications observed were classified as bio-logical (secondary caries, pulp affectation) or mechanical(fracture of the core or veneer ceramic, debonding).Biological complications involved 2 FDPs (one 3 unit, one
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4 unit) with secondary caries (7.4%) in the abutmentteeth, requiring replacement of the prosthesis at the 3-year follow-up and one 4-unit FDP (3.7%) with anendodontic problem with periapical lesions after 2.5years; however, after periapical surgery, there was noneed to replace the prosthesis. Mechanical complicationsinvolved 4 FDPs with chipping (14.8%), all of them 6-unit FDPs: one was replaced after 3 years and theothers were corrected by polishing and intraoral repair(Fig. 6). Two FDPs (one 4 unit and one 6 unit) debondedafter 7 and 6 months, respectively. None of the FDPs hadfracture of the internal zirconia cores (Table 1). When thetype of complication was related to the FDP’s number ofunits (applying the Kruskal-Wallis test), a statisticallysignificant relation was identified involving chipping(P=.002). Five 6-unit FDPs showed a higher incidence ofchipping than those with shorter spans (P<.001, Mann-Whitney). No identified differences were found be-tween 3-unit and 4-unit spans (P=1.000, MW), with nullincidence of chipping. When the causes of restorationfailure were analyzed, the only statistically significant
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1.0
3 unitsType FPD
4 units5 to 6 units
3 units-censored4 units-censored5 to 6 units-censored
0.8
0.6
0.4
0.2
0.0
0.00 2.00 4.00 6.00
Complication time8.00 10.00
Surv
ival
Figure 8. Survival (without failure) according to FDP number of units.
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factor (Mann-Whitney test) was secondary caries(P=.001). All FDPs that presented with secondary cariesin the abutment teeth had to be removed.
The Kaplan-Meier survival test estimated an accu-mulated survival rate of 8 years and 5 months (95% CI7.6 to 9.0). The critical moment for survival was aroundthe 3-year mark, given that a restoration that had sur-vived 3 years maintained a constant probability of sur-viving the follow-up period (with a value of 0.9). Themean survival time without any incidence of complica-tions (success) was 6.83 years (95% CI 5.6 to 8.1). If by 3years no complication had occurred, there was a highprobability that none would appear thereafter (Fig. 7).When success was related to the FDP (Kaplan-Meiertest), the probability of some complication occurringincreased in relation to the number of units of the FDP(Fig. 8). For 6-unit FDPs, complications occurred bothsoon and frequently. For these restorations, the timetaken to stabilize survival probability was 2 years, butthereafter the probability of survival was only 0.42.
DISCUSSION
Little research has been published on the clinicalbehavior of anterior FDPs with zirconia cores. For thistype of restoration, the most frequent complication ischipping of the veneer porcelain.15-34 Factors that influ-ence chipping are related to a series of variables, whichrange from the varying thermal behavior after firing andcooling the substrate and the overlay ceramic to insuffi-cient wettability of the substrate in relation to the veneerporcelain during construction, which can produce insuf-ficient homogeneity at the veneer/core interface.3,4,37
One of the most widely studied ceramic veneerphenomena that might be related to ceramic veneerchipping, is aging or low-temperature degradation.Zirconium dioxide’s hydrothermal transformation wasdiscovered by Kobayashi et al,46 who observed aslow, progressive, spontaneous transformation of the
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tetragonal to monoclinic phase associated with thedegradation of the material’s mechanical properties. Thisphenomenon is promoted by the presence of water orvapor and by the temperature caused by the reactions ofwater and the crystalline lattice. The susceptibility ofzirconia to aging depends on factors such as grain size,presence of residual stress, or the quantity, type, anddistribution of the stabilizing element; the larger thegrain size or the greater the residual stress or the lowerthe quantity of stabilizing element, the greater the ag-ing.46 This is also a phenomenon that is accelerated bythe presence of water vapor.47 Of particular relevancewas the news reported in various American orthopedicjournals of the fracture-after only a short time-of 400Zircone Prozyr (3Y TZP) femoral prostheses made fromzirconia in 2001 to 2002. These had to be removed andled to a marked reduction in the use of the material andthe closure of the supplier of the prosthesis.1 Numerousresearch articles have shown how the kinetics of thetetragonal-monoclinic transformation, which generallyappears to be linked to chemical composition, is accel-erated as temperature rises.38-40 The activation energiesmeasured vary between 70 and 110 kcal/mol. This evi-dence has important technological consequences becauseof zirconia’s low thermal conductivity (2.5 W/mK),whereby the surface treatment of zirconium dioxide linesby abrasion can cause notable temperature rises locallythat can initiate phase transformation.46
Crisp et al17 analyzed the behavior of 13 FDPs (3- and4-unit prostheses), but they did not observe any com-plications after a 12-month follow-up, nor did Tinschertet al18 in an analysis of 15 anterior FDPs (3- to 10-unitprostheses) over 38 months. These results are not com-parable with the present study given the small numbersof FDPs and the shorter follow-up periods.
Schmitter et al19 monitored 30 FDPs (4- to 7-unitprostheses) over 25 months, distributed in both theanterior and posterior regions; among the anterior FDPs, 1had endodontic problems and 2 debonded among theposterior FDPs, 1 had a fracture of the internal core, and 1had veneer chipping. Edelhoff et al20 analyzed thebehavior of 21 FDPs (3- to 6-unit prostheses) over 39months, of which only 4 were placed in the anterior re-gion, with 1 incidence of a loss of pulpal vitality in 1abutment tooth.
Almost all published research of FDP behavior dealswith restorations placed in the posterior region.21-33 Themost common mechanical complication was chipping ofthe veneer ceramic, although there is controversy be-tween researchers as to its incidence. Some authorsreport that chipping occurs in 3% to 6% of posteriorFDPs,17-19,22,23 whereas others state that it is 9% to15%,8,13,20,24-29 and some report that it is as high as 19%to 28%.21,29-31 However, several other authors havefound no mechanical complications among the FDPs
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monitored.32,33 The present study found that 14.8% ofthe restorations had chipping, all of them longer, 6-unitFDPs. None of the FDPs in the present study had frac-ture of the zirconia cores.
As for biological complications, the present studyfound an incidence of 11.1% (2 participants [7.4%]with secondary caries, and 1 [3.7%] endodontic prob-lem) over the 84-month follow-up period. Again, thereis controversy between authors over the incidence ofthese complications, which range between 1.5% to5.5% 36,18-20,21,23,27,28,30 and 10.1% to 20% 26,31,29 overclinical observation periods of between 50 and 84months. The latter studies concur more closely withthe present one, both in the number of biologicalcomplications and in the follow-up period.
When the causes of mechanical restoration failure areanalyzed, according to studies with 3- to 5-year follow-up, success rates vary between 88.8% and100%,6,12,17,18,20,22,25,30,32,33 while for failure due to bio-logical complications, the success rate decreases to73.9%,11,21,23,27,29,31 findings that agree with the presentstudy, in which the incidence of caries increased FDPfailure (P=.001, Mann-Whitney test).
In the studies discussed previously, most FDPs weremade with ceramic veneer over a zirconia substrateapplied in layers with a stratification technique. Chris-tensen and Ploeger41 stated that veneer ceramics thatcontain leucite and that are applied with a pressingtechnique have markedly better resistance to chippingcompared to stratification. However, studies by Choiet al42 refute this theory, and a study by Ishibe et al43
comparing the pressing technique and layered ceramicveneers failed to identify significant differences in zirco-nia or metal substrates.
Recently, monolithic zirconia restorations, which havegood optical and mechanical qualities and are not sus-ceptible to chipping, have become popular.48 Longitudi-nal long-term prospective studies are needed to confirmthe performance of these restorations, although theiresthetics limits their use to the posterior region.
All the FDPs in our study were cemented with astandard technique. To date, consensus has not beenreached as to the ideal cementation technique. Never-theless, most authors recommend airborne-particleabrading the internal structure with 30 mm silica oxideparticles at a pressure of 200 kPa from a distance of 2 cmfor 10 seconds and then cementing with adhesive andcomposite resin.49,50 No immediate damage attributed toairborne-particle abrasion has been observed that com-promises zirconia’s fatigue resistance. Zhang et al51
indicated that abrasion by aluminum oxide particles ofup to 50 mm increased surface resistance, while airborne-particle abrasion with aluminum oxide particles of 120mm significantly weakened the structure by increasingsurface roughness.
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CONCLUSIONS
Within the limitations of this study (in particular thesmall sample size), the conclusions are as follows:
1. Anterior FDPs with zirconium dioxide frameworksshowed a success rate of 88.8% within the 7-yearfollow-up.
2. The most frequent cause of failure was secondarycaries (P=.001), which was related to the number ofunits of the FDP.
3. The most frequent complication was chipping of theveneer ceramic.
4. All chipping occurred in longer, 5- or 6-unit FDPs(P=.002).
5. The critical period for the occurrence of mechanicaland biological complications was between the firstand the third year.
6. More long-term longitudinal studies are required toconfirm the behavior of this type of restoration inthe oral environment.
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Corresponding author:Dr Maria Fernanda Solá-RuizFaculty of Medicine and DentistryUniversity of ValenciaC/ Gascó Oliag, N� 146010 ValenciaSPAINEmail: [email protected]
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