In Vitro Wear Behavior of Zirconia Opposing Enamel: A Systematic Review Sheila Pestana Passos, DDS, MSc, PhD,1 Ysidora Torrealba, DDS, MSc,1 Paul Major, DDS, MSc,1 Bernie Linke, DDS, MSD, FRCD(C),1 Carlos Flores-Mir, DDS, MSc, PhD,1 & John A. Nychka, MEng, PhD1,2 1 2
School of Dentistry, University of Alberta, Edmonton, Canada Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Canada
Keywords Tooth wear; zirconia; enamel; antagonist. Correspondence Dr. Sheila P. Passos, School of Dentistry, University of Alberta, 11405-87 Ave. NW, Room# 5083C, Edmonton Clinic Health Academy (ECHA), Edmonton, AB T6G 1C9, Canada. E-mail: [email protected] The authors deny any conflicts of interest. Accepted November 13, 2013 doi: 10.1111/jopr.12167
Abstract Purpose: The aim of this systematic review was to assess enamel wear on teeth opposing zirconia restorations and to evaluate factors related to the wear of natural teeth opposing zirconia restorations. Materials and Methods: Five electronic databases were searched through May 2013 without limitations. The terms “antagonist*,” “enamel,” “wear,” and “zirconi*” were used. Titles and abstracts were initially screened, and those that fulfilled the inclusion criteria were selected for a full-text assessment. Studies that evaluated only the material wear were not included. Results: The database search strategy retrieved 142 potentially eligible studies. After the duplicate studies were removed, 62 studies were obtained. Titles and abstracts that fulfilled the inclusion criteria were selected for a full-text assessment (25). Seven laboratory studies met the inclusion criteria. In addition, reference lists from the finally selected studies were also screened. Conclusions: There was a large variation in relation to wear test method quantification, applied force, lateral movement, number and frequency of cycles, number of specimens, and enamel specimen preparation. In all studies, enamel wear rates were lower against polished zirconia. Differences in the test methods did not allow for comparisons of wear rates among the studies. Clinical Significance: Polishing the surface is recommended for a full-contour zirconia restoration because polished zirconia presents favorable wear behavior opposing natural teeth.
High-strength substructures of zirconia-based restorations have been widely used because of their excellent mechanical properties. They are usually veneered by dental porcelain to achieve an adequate esthetic result; however, chips and fracture of the veneer1-9 and high wear of antagonist teeth10,11 have been reported. For these reasons, new CAD/CAM technologies and translucent zirconia materials have been introduced to produce nonveneered zirconia restorations.12,13 As a result, reduced chipping events and reduced wear of the restoration material and antagonist enamel have been reported.14-25 The abrasiveness of dental zirconia ceramic opposing natural enamel has been considered.12-20 A few in vitro studies investigated the wear resistance and abrasive potential of yttriastabilized tetragonal zirconia polycrystalline (Y-TZP) against opposing human enamel.14-20 According to their results, zirconia monolithic crowns can be used against enamel with less antagonistic wear than porcelain.15,16,18-20 The surface characteristics of dental ceramics also affect the wear process. Occlusal intraoral adjustments are usually
required to obtain an adequate occlusal contact. This procedure results in a rough surface after removing the glaze layer or the smoothness of polished surfaces, which affects the wear behavior of the ceramic restorations if it is not well repolished.17,24 An adequate finishing procedure on the ceramic surface will contribute to minimizing the wear of the ceramic restoration and its natural antagonist.14,16,17,19,20 As no clinical studies have evaluated the behavior of monolithic zirconia ceramic as an antagonist occlusal material, the aims of this study were (1) to systematically review the literature for in vitro studies that quantify the wear of enamel opposing zirconia ceramics, and (2) to evaluate factors related to the enamel wear opposing zirconia ceramics.
Materials and methods The PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) Statement26 was used as a reporting template as much as possible.
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Table 1 Search strategy Database
Dates of coverage
MEDLINE (OvidSP)
1950 to May 2013
PubMed (NLM)a
1950 to May 2013
EMBASE (OvidSP)
1974 to May 2013
Cochrane (Wiley)
To the fourth quarter of 2011
Scopus (Elsevier)
1960 to May 2013
a
Keywords “antagonist∗ ” OR “enamel” AND “wear” AND “zirconi∗ ” Same search strategy as MEDLINE (OvidSP). MeSH terms: dental enamel, tooth wear, zirconium dioxide Same search strategy as MEDLINE (OvidSP) Same search strategy as MEDLINE (OvidSP) “antagonist∗ ” OR “enamel” AND “wear.” Within results, “zirconi∗ ” was added.
The search terms were related to the MeSH terms.
Data collection
A systematic search of electronic databases was conducted using MEDLINE (OvidSP), PubMed (NLM—National Library of Medicine), EMBASE (OvidSP), Cochrane Library (Wiley), and Scopus (Elsevier) up to May 2013. The terms used were “antagonist*,” “enamel,” “wear,” and “zirconi*”. Specific search strategies for each electronic database are outlined in Table 1. A different search strategy was employed with the Scopus database. First, the terms “antagonist*”, “enamel,” and “wear” were used, and within the results the term “zirconi*” was added. In that way, more articles were found. No limits were applied during the electronic searches. Criteria for selection of studies
All titles revealed by the electronic search were screened according to the following inclusion criteria: 1. In vitro studies; 2. use of Y-TZP ceramic; 3. evaluation of the antagonist’s enamel.
reviewers was sought. Based on their final decision, all potentially relevant articles were retrieved in full. Conference abstracts such as IADR abstracts were not selected per se because they do not present the detailed test method, but an attempt was made to retrieve their full articles if they were already published. Reference lists from the selected studies were also screened. Data extraction
The studies were analyzed with regard to the wear-mimicking device and wear method, its testing parameters, ceramic preparation, finishing technique, antagonist’s enamel wear, and zirconia wear. When information was unavailable or limited, authors were contacted to obtain missing information. Methodological data among the selected studies
A customized systematic evaluation protocol (Table 2) was created to contrast the study methodology of the selected studies.
Results Study selection
Seven14-20 laboratory studies were included in the systematic review (Fig 1). Among the 25 studies initially considered in the second selection stage, a few studies were eliminated after inclusion and exclusion criteria were applied. In vitro studies that did not analyze the wear27 or did not have enamel as antagonist25,28-34 or evaluated only the material wear not measuring the enamel structure wear21-24 were excluded. Articles that did not use zirconia ceramic35,36 or that evaluated veneered zirconia37 were also not included in the review. One study13 was not included because Y-TZP was not the type of zirconia evaluated. Only one manuscript from the second stage was not assessed because of the language.38 The online search was complemented by manual search through the references of the selected articles; however, none were found. Study characteristics
The selected studies were published between 2010 and 2013. The experimental parameters, zirconia ceramic system, sample size, wear method, and device differed among the seven selected studies. Risk of bias within studies
In addition to the inclusion criteria, the following exclusion criteria were applied:
All articles did not report some experimental conditions, which might affect the wear mechanism. Results of individual studies
1. Veneered zirconia specimens; 2. absence of enamel wear evaluation. A search of the relevant titles/abstracts was conducted. Where a potentially relevant title without a listed abstract was available, the full article was later assessed to select the studies. This first stage was conducted by two independent reviewers. In cases of discrepancies, a consensus agreement between two 2
1. Experimental design. Forces ranged from 4 to 49 N, and the number of cycles from 3000 to 1,200,000 cycles. Water was used as the predominant liquid medium. Only one study used artificial saliva.19 2. Enamel antagonist. Regarding the antagonist response, human molars and premolars were used. The enamel was prepared differently among the studies. The enamel
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6. Enamel preparation
4. Number of specimens (N) 5. Enamel antagonist
3. Wear evaluation
2. Experimental design
1. Wear device/method
Experimental conditions
The tips of the cusps were polished (diamond rotary instrument)
Mandibular molar mesiobuccal cusps
Alabama: dual-axis simulator, vertical and horizontal movement 10 N, 0.33 Hz, 2-mm sliding, 200,000, 400,000 cycles 33% glycerine, and 66% distilled water 3D-Profilometer. Records volume loss by overlapping the 3D surfaces before and after the wear test 8
Janyavula et al14
Table 2 Articles included in the review
3D-Profilometer. Records volume loss by overlapping the 3D surfaces before and after the wear test 20 Maxillary premolars buccal cusps
SEM by analyzing wear scars
Premolar buccal surfaces
3D-Profilometer. Records volume loss by overlapping the 3D surfaces before and after the wear test 10 Maxillary and mandibular premolar cusps Nonstandardized
3D laser scanner. Records the maximal vertical loss and the mean vertical loss
16 First or second maxillary molar cusps Nonstandardized
3D-Profilometer. Records the vertical loss by overlapping the 3D surfaces before and after the wear test 6 Maxillary molar mesiobuccal cusps The tips of the cusps were rounded to a spherical shape
3D-surface analyzer. Records the vertical loss by overlapping the 3D surfaces before and after the wear test 12 First mandibular molar mesiobuccal cusps Non-standardized
Carborondum sand paper from 180 to 1500 mesh, 1 µm diamond sand paper
(Continued)
1200-grit abrasive paper
CS 4.8: dual-axis simulator, vertical and horizontal movement 5 kg weight, 0.8 Hz, 0.3 mm sliding, 240,000 cycles water 5°/55°C UMT-2: microfriction and wear testing apparatus 4 N, 2 Hz, 1 mm sliding, 5000 cycles, artificial saliva
CS 4.8: dual-axis simulator, vertical and horizontal movement 49 N, 0.8 Hza , 0.3 mm slidinga , 300,000 cycles heat circulation 5°/55°C
Ivoclar: dual-axis simulator, vertical and horizontal movement 5 kg weight, 1.6 Hz, 0.7 mm sliding, 120,000 cycles, demineralized water
Zurich: controlled masticator comprised occlusal loading 49 N, 1.7 Hz, 120,000, 240,000, 640,000 and 1,200,000 cycles, water 5°/50°C
Zurich: controlled masticator comprised occlusal loading 49 N, 1.7 Hz, 3000 cycles, water 5°/50°C
1
Jung et al20
Wang et al19
Kim et al18
Mitov et al17
Stawarczyk et al16
Mormann et al15
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3
4
Polished,glazed, polished, then reglazed
FCZ glaze
11. Glazed material
Flat Control (veneering porcelain and enamel),polished zirconia,glazed zirconia, polished then reglazed
8. Specimen shape 9. Sample grouping
10. Zirconia finishing technique
Monolithic zirconia
Janyavula et al14
7. Zirconia system
Experimental conditions
Table 2 Continued
NA
Flat Control (human enamel), zirconia, lithium disilicate, leucite glass ceramic, feldspathic ceramic, hybrid ceramic, nanocomposite CAD/CAM block, direct nanocomposite, experimental composite, unfilled PMMA, microfilled acrylate polymer Polished
inCoris TZI
Mormann et al15
Polished with 600-grit SiC, polished with 1200-grit SiC
NA
Polished, fine-grit diamond, coarse-grit diamond, glazed
Vita Akzent VM9
Glazed with a glaze ceramic, glazed with a glaze spray, manually polished, mechanically polished Glaze ceramic: Glaze Zirox with Stain LiquidGlaze spray: ZENOStar Magic
Cuboidal Three monolithic zirconia, one lithium disilicate, one feldspathic porcelain
Prettau, Lava, Rainbow
Kim et al18
Flat Control (polished leucite-reinforced glass ceramic), four different finishing procedures for zirconia: polished, fine-grit diamond, coarse-grit diamond, glazed
Everest ZH
Mitov et al17
Flat Control (monolithic base alloy), veneered zirconia, glazed zirconia with a glaze ceramic, glazed zirconia with a glaze spray, manually polished zirconia, mechanically polished zirconia
ZENOTEC Zr Bridge transluzent
Stawarczyk et al16
NA
Polished,rough
3Y-TZP (from powder doped with 3 mol% Y 2 O3 ) Hemisphere Polished zirconia, rough zirconia, hot-forged lithium disilicate glass ceramics, silicate-based veneer porcelain
Wang et al19
(Continued)
Glazing of Zirkonzahn Prettau
Polished, polished with glazing
Cuboidal Polished feldspar, polished zirconia, polished zirconia with glazing
Prettau
Jung et al20
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b
Data obtained by contacting the author (not reported in the article). E, enamel; M, material; >statistically significant more wear. c Data obtained according to the different masticatory cycles (120,000; 240,000; 640,000; and 1,200,000).
a
14. SEM analysis
Glazed with a glaze ceramic: 51.7 to 118 µm.c Glazed with a glaze spray: 24.5 to 62.2 µm.c Manually polished: 14.3 to 27.3 µm.c Mechanically polished: 14.7 to 28 µm.c
25.5 µm
Polished + 200,000 cycles: 0.11 (0.04) mm3 . Glazed + 200,000 cycles: 0.87 (0.21) mm3 . Polished then reglazed + 200,000 cycles: 0.59 (0.1) mm3 Polished + 400,000 cycles: 0.21 (0.05) mm3 Glazed + 400,000 cycles: 1.18 (0.2) mm3 Polished then reglazed + 400,000 cycles: 0.88 (0.12) mm3
13. Enamel antagonist wear Vertical substance loss (µm) Volumetric substance loss (mm3 ) Wear scars widths (µm)
E: glazed with a glaze ceramic > glazed with a glaze spray = polished M: glazed > polished
Stawarczyk et al16
NA
Mormann et al15
E: polished then reglazed = glazed > polished. No difference for the different cycles M: polished then reglazed = glazed > polished
Janyavula et al14
12. Wear resultsb
Experimental conditions
Table 2 Continued
E: coarse-grit diamond > polished. No difference among polished, fine-grit diamond and glazed. M: no measurable wear observed. Except for the glazed group. Polished: 171.74 (121.68) µm
Mitov et al17
0.04 (0.02) mm3
E: no difference between 600-grit and 1200-grit for all zirconia systems used M: not reported.
Kim et al18
Enamel wear scars widths: rough > polished Rough zirconia material: deep furrows
x
Glazed: 0.078 (0.063) mm3 Polished: 0.031 (0.033) mm3
E: glazed > polished M: not reported.
E: rough > polished M: very small damage in both surface treatments.
Enamel wear values were not determined. Wear scars widths: Rough: 300 to 400 µm Polished: 200 to 300 µm
Jung et al20
Wang et al19
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MEDLINE
PubMed
EMBASE
Cochrane
Scopus
40 studies
38 studies
27 studies
9 studies
58 studies
Full-contour zirconia restorations have been recently added to the market as alternative restorations. For that reason, no in vivo study reporting the enamel wear against zirconia is available, and there are only a few in vitro studies. The aim of this systematic review was to evaluate the enamel wear opposing zirconia ceramics.
After removing duplicates (62) Studies excluded from the first selection stage (37)
Experimental design
Potentially relevant studies (25)
Studies excluded from the second selection stage (18)
Studies relevant to the topic (7 articles) References of selected full-text articles (0) Studies included (7)
Figure 1 Schematic study of the selection process.
surface was only polished14 or ground and polished in a spherical16 or a flat way,19 whereas for the other four studies, no treatment was performed on the enamel cusps.15,17,18,20 3. Zirconia specimen. All but two studies18,20 used a different zirconia ceramic system. Most studies prepared the zirconia ceramic specimens in a flat shape. Hemisphere19 and cuboidal18 shape specimens were also used. In addition, different surface treatments were performed by most studies. 4. Finishing technique. Three studies observed more enamel wear for glazed specimens,14,16,20 whereas no difference between glazed and polished specimens was observed by one study.17 The remaining study did not include glazed specimens in its investigation.15,18,19 5. Wear values. One study19 did not determine the wear values (µm, mm3 ). The wear was evaluated by SEM, measuring the width of wear scars. In addition, one study listed the wear values obtained only from the polished specimens.17 Risk of bias across studies
A meta-analysis could not be performed, as the test parameters differed from one study to the other.
Discussion Dental wear is a complex process, which occurs over time and involves chemical and mechanical wear. Clinical investigations are essential to assess the wear process of dental materials. The ceramic material itself strongly affects the wear behavior; however, intraorally, several other factors influence the complex wear interaction between tooth enamel and opposing substrates. These factors are the abrasive nature of food, chewing behavior along with parafunctional habits and neuromuscular forces, as well as the antagonistic material (thickness, roughness, fracture toughness, hardness).20,39-41 6
Among the seven selected studies, each study used different wear-mimicking devices. Therefore, there was a great variety of simulator settings. Different wear experimental settings determine different wear mechanisms. Variation in relation to the applied force, the number and frequency of cycles, lateral movement, the number of specimens, and enamel specimen preparation were identified. For that reason, it is recommended to use at least two configurations of parameters to determine material wear.42 The same loading force, either 5 kg or 49 N, was applied by five included studies.43-45 This represents an average value of biting forces of occlusal contacts.46 In contrast, one study19 applied only 4 N, which was not sufficient to provide a clinical simulation. Two studies15,19 used 3000 or 5000 cycles, which seem insufficient to measure wear on zirconia specimens. For example, dental composite materials exhibited about 41% of the final wear after 8.3% of the total number of cycles by the Ivoclar chewing wear method.45 Approximately 1 year of chewing in oral function can be simulated by 240,000 to 250,000 loading cycles.47,48 The other five selected studies performed from 120,000 to 1,200,000 cycles. In one study16 different cycle numbers were evaluated, and showed that wear rate increases with the increasing number of cycles. In contrast, according to the results of one selected study, no difference was observed when 200,000 and 400,000 cycles were evaluated.14 Another point to consider is that the wear simulator should perform a lateral movement. Among the seven studies, the sliding range was 0.3 to 2 mm. One study16 that did not perform lateral movements during the test exhibited much lower enamel wear than a study that used the Ivoclar method (0.7 mm sliding).17 Wear of a dental composite increases eight times with 0.7 mm sliding.45 In addition, different intermedium fluids such as water, demineralized water, and artificial saliva were used. No study considered the corrosive wear, as no chemical reaction layer was placed between the substrates. It has been reported that acidulated phosphate fluoride alters the topography of some glass ceramic surfaces49 as well as other abrasive media.50 For that reason, the surface zirconia degradation by corrosion needs to be identified, as it plays an important role in the wear phenomenon. Enamel antagonist
Among the included studies, four used natural molars, and the remaining studies used natural premolars. The variation in wear data among the studies can also be attributed to the enamel specimen preparation, and morphological and structural differences of natural enamel as well. The physiological enamel wear rate, as evaluated from in vivo measurement is from 30 to 40 µm
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per year.51 Different enamel shapes were evaluated among the included studies, which is a critical factor because as the shape changes from a cusp to a flat surface there will be less force per area and consequently less wear (provided the wear mechanism does not change). Heintze et al52 reported more variation of the enamel wear results for the prepared cusps (Table 2; enamel preparation) than nonstandardized cusps, which is related to the enamel thickness. The authors reported that the higher the enamel thickness, the lower the wear of the prepared enamel cusps. Therefore, nonstandardized enamel cusps should be the set-up to measure the wear rate. Zirconia specimen shape
Hemisphere19 and cuboidal18 ceramic specimens instead of flat ceramic specimens were used by two studies. Only one study did not determine the enamel wear rates by vertical15-17 and volumetric14,18,20 wear. The study that evaluated hemisphere specimens assessed the wear by measuring the wear scar widths, and only 5000 cycles were performed. For that reason, it was not possible to compare the enamel antagonist against flat and hemisphere specimens. A cuboidal specimen shape was also used in one of the studies.18 Natural tooth antagonist wear rate more than doubled for flat Empress specimens compared to Empress inlays. On the other hand, it was the opposite for the restorative material wear.52 Therefore, flat ceramic specimens overestimate the enamel wear compared to anatomically formed specimens, and crown specimens exhibit less restorative material wear than flat ceramic specimens. Nevertheless, the findings may not be applicable to zirconia, because there is no report investigating zirconia specimen shape on the antagonist and material wear rates. This issue also needs to be clarified for zirconia material, because it is essential to standardize the antagonist shape for comparisons. Finishing technique
The enamel antagonist wear can be accelerated according to the ceramic surface characteristics.14,16,17,20 Only one study15 included in this review did not evaluate the surface finishing of zirconia including different glazing techniques, mechanical polishing, and manually polishing. The wear is more affected by the surface structure, roughness, and environmental factors.53 Among the selected studies, only one study17 reported the specimen roughness, which was 0.006 µm after polishing procedure. It has been reported that during the wear process the glaze layer is removed, and the underlying ceramic surface is usually not polished, thus upon glaze removal the rough ceramic surface accelerates enamel wear; however, two included studies14,20 observed that even when the surface under the glaze layer was polished (1200-grit abrasive paper), the glazed specimens presented more antagonist wear than the polished specimens. Among the four included studies that evaluated polished and glazed specimens, three14,16,20 observed that polished zirconia showed the lowest wear rate on natural tooth antagonist. For the remaining study,17 no difference among polished, fine-grit, and glazed was observed. It seems standardized enamel affects wear results for glazing.17 In contrast, nonstandardized enamel shows no difference for glazing.
Enamel Wear Caused by Zirconia Ceramics
Specimens glazed with a glaze spray (ZENOStar Magic) showed no difference of enamel wear compared to polished specimens.16 However, a specimen glazed with a glaze ceramic presented more enamel wear than polished and glazed with a glaze spray specimens. There is no standard glazing zirconia technique available for zirconia restorations in the literature. Mechanical properties of Y-TZP ceramic
Y-TZP ceramic has superior mechanical properties compared to other ceramics because of its phase transformation toughening mechanism that prevents further crack propagation in the material.54 Such a mechanism involves the transformation from tetragonal to monoclinic phase at the crack tip and around the crack by localized compressive stresses.55 This stress-induced phase transformation is a major characteristic of zirconia; however, the long-term stability of Y-TZP in the presence of water is limited by the continuing transformation from the tetragonal to monoclinic phase, which could raise low-temperature degradation (LTD). LTD is an aging phenomenon caused by the excessive progression of spontaneous tetragonal-to-monoclinic transformation in the presence of water.56 LTD not only results in undesirable transformation, but also loss of strength and surface degradation, which alters the surface roughness. As a result, wear resistance is compromised. Therefore, the effect of the LTD should be considered for the evaluation of the abrasiveness of polished zirconia over time. Nevertheless, the long-term wear behavior of full-contour zirconia restorations is still unknown.
Conclusions In summary, according to the included studies in this review, polished zirconia specimens showed a favorable wear behavior opposing natural teeth. Enamel pathological wear opposing zirconia full-coverage restorations has not yet been confirmed. It is important to assess the enamel wear behavior opposing zirconia using clinically relevant surface contours as well as to investigate the long-term stability and abrasiveness of polished zirconia. The studies agreed that the polishing of zirconia surface favors lower enamel wear rates.
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5. Schmitter M, Mussotter K, Rammelsberg P, et al: Clinical performance of extended zirconia frameworks for fixed dental prostheses: two-year results. J Oral Rehabil 2009;36:610-615 6. Crisp RJ, Cowan AJ, Lamb J, et al: A clinical evaluation of all-ceramic bridges placed in UK general dental practices: first-year results. Br Dent J 2008;205:477-482 7. Edelhoff D, Florian B, Florian W, et al: HIP zirconia fixed partial dentures—clinical results after 3 years of clinical service. Quintessence Int 2008;39:459-471 8. Molin MK, Karlsson SL: Five-year clinical prospective evaluation of zirconia-based Denzir 3-unit FPDs. Int J Prosthodont 2008;21:223-227 9. Tinschert J, Schulze KA, Natt G, et al: Clinical behavior of zirconia-based fixed partial dentures made of DC-Zirkon: 3-year results. Int J Prosthodont 2008;21:217-222 10. Etman MK, Woolford M, Dunne S: Quantitative measurement of tooth and ceramic wear: in vivo study. Int J Prosthodont 2008;21:245-252. Erratum in: Int J Prosthodont 2008;21:508 11. Esquivel-Upshaw J, Rose W, Oliveira E, et al: Randomized, controlled clinical trial of bilayer ceramic and metal-ceramic crown performance. J Prosthodont 2013;22:166-173 12. Kern M: Clinical long-term survival of two-retainer and single-retainer all-ceramic resin-bonded fixed partial dentures. Quintessence Int 2005;36:141-147 13. Ghazal M, Kern M: The influence of antagonistic surface roughness on the wear of human enamel and nanofilled composite resin artificial teeth. J Prosthet Dent 2009;101: 342-349 14. Janyavula S, Lawson N, Cakir D, et al: The wear of polished and glazed zirconia against enamel. J Prosthet Dent 2013;109:22-29 15. M¨ormann WH, Stawarczyk B, Ender A, et al: Wear characteristics of current aesthetic dental restorative CAD/CAM materials: two-body wear, gloss retention, roughness and Martens hardness. J Mech Behav Biomed Mater 2013;20:113-125 16. Stawarczyk B, Ozcan M, Schmutz F, et al: Two-body wear of monolithic, veneered and glazed zirconia and their corresponding enamel antagonists. Acta Odontol Scand 2013;71:102-112 17. Mitov G, Heintze SD, Walz S, et al: Wear behavior of dental Y-TZP ceramic against natural enamel after different finishing procedures. Dent Mater 2012;28:909-918 18. Kim MJ, Oh SH, Kim JH, et al: Wear evaluation of the human enamel opposing different Y-TZP dental ceramics and other porcelains. J Dent 2012;40:979-988 19. Wang L, Liu Y, Si W, et al: Friction and wear behaviors of dental ceramics against natural tooth enamel. J Eur Ceram Soc 2012;32:2599-2606 20. Jung YS, Lee JW, Choi YJ, et al: A study on the in-vitro wear of the natural tooth structure by opposing zirconia or dental porcelain. J Adv Prosthodont 2010;2:111-115 21. Preis V, Behr M, Kolbeck C, et al: Wear performance of substructure ceramics and veneering porcelains. Dent Mater 2011;27:796-804 22. Preis V, Hahnel S, Kolbeck C, et al: Wear performance of dental materials: A comparison of substructure ceramics, veneering ceramics, and non-precious alloys. Adv Eng Mater 2011;13:B432-B439 23. Rosentritt M, Preis V, Behr M, et al: Two-body wear of dental porcelain and substructure oxide ceramics. Clin Oral Investig 2012;16:935-943 24. Preis V, Behr M, Handel G, et al: Wear performance of dental ceramics after grinding and polishing treatments. J Mech Behav Biomed Mater 2012;10:13-22 25. Beuer F, Stimmelmayr M, Gueth J, et al: In vitro performance of full-contour zirconia single crowns. Dent Mater 2012;28:449-456
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26. Moher D, Liberati A, Tetzlaff J, et al: Preferred reporting Items for systematic reviews and meta-analyses: the PRISMA statement. BMJ 2009;339:b2535,332-336 27. Rosentritt M, Kolbeck C, Handel G, et al: Influence of the fabrication process on the in vitro performance of fixed dental prostheses with zirconia substructures. Clin Oral Investig 2011;15:1007-1012 28. Albashaireh ZS, Ghazal M, Kern M: Two-body wear of different ceramic materials opposed to zirconia ceramic. J Prosthet Dent 2010;104:105-113 29. Murthy AK, Komvopoulos K, Brown SD: Processing and characterization of multi-layered wear-resistant ceramic coatings. J Eng Mater Technol, Trans. ASME 1990;112: 164-174 30. Medevielle A, Th´evenot F, Tr´eheux D: Wear resistance of stabilized zirconias. J Eur Ceram Soc 1995;15: 1193-1199 31. Kontos L, Schille C, Schweizer E, et al: Influence of surface treatment on the wear of solid zirconia. Acta Odontol Scand 2013;71:482-487 32. Preis V, Behr M, Hahnel S, et al: In vitro failure and fracture resistance of veneered and full-contour zirconia restorations. J Dent 2012;40:921-928 33. Preis V, Weiser F, Handel G, et al: Wear performance of monolithic dental ceramics with different surface treatments. Quintessence Int 2013;44:393-405 34. Sabrah AH, Cook NB, Luangruangrong P, et al: Full-contour Y-TZP ceramic surface roughness effect on synthetic hydroxyapatite wear. Dent Mater 2013;29:666-673 35. Kato K, Kakuta K, Ogura H: Effect of material and configuration of antagonist on combined wear. Dent Mater J 2005;24: 368-376 36. Park J, Pekkan G, Ozturk A: Friction and wear behavior of selected dental ceramics. Surf Rev Lett 2009;16:653-661 37. Silva NR, Thompson VP, Valverde GB, et al: Comparative reliability analyses of zirconium oxide and lithium disilicate restorations in vitro and in vivo. J Am Dent Assoc 2011;142 Suppl 2:4S-9S 38. Chen S, Li G: Wear resistance and hardness of dental prosthetics materials versus native teeth. J Clin Rehabil Tissue Eng Res 2011;15:511-514 39. Johansson A, Kiliaridis S, Haraldson T, et al: Covariation of some factors associated with occlusal tooth wear in a selected high-wear sample. Scand J Dent Res 1993;101:398-406 40. Kim SK, Kim KN, Chang IT, et al: A study of the effects of chewing patterns on occlusal wear. J Oral Rehabil 2001;28:1048-1055 41. Mair LH, Stolarski TA, Vowles RW, et al: Wear: mechanisms, manifestations and measurement. Report of a workshop. J Dent 1996;24:141-148 42. Heintze SD, Barkmeier WW, Latta MA, et al: Round robin test: wear of nine dental restorative materials in six different wear simulators–supplement to the round robin test of 2005. Dent Mater 2011;27:1-9 43. Krejci I, Lutz F, Gautschi L: Wear and marginal adaptation of composite resin inlays. J Prosthet Dent 1994;72:233-244 44. Krejci I, Mueller E, Lutz F: Effects of thermocycling and occlusal force on adhesive composite crowns. J Dent Res 1994;73:1228-1232 45. Heintze SD: How to qualify and validate wear simulation devices and methods. Dent Mater 2006;22:712-734 46. Gibbs CH, Mahan PE, Lundeen HC, et al: Occlusal forces during chewing and swallowing as measured by sound transmission. J Prosthet Dent 1981;46:443-449
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47. DeLong R, Sakaguchi RL, Douglas WH, et al: The wear of dental amalgam in an artificial mouth: a clinical correlation. Dent Mater 1985;1:238-242 48. Sakaguchi RL, Douglas WH, DeLong R, et al: The wear of a posterior composite in an artificial mouth: a clinical correlation. Dent Mater 1986;2:235-240 49. Ccahuana VZ, Ozcan M, Mesquita AM, et al: Surface degradation of glass ceramics after exposure to acidulated phosphate fluoride. J Appl Oral Sci 2010;18:155-165 50. Heintze SD, Faouzi M, Rousson V, et al: Correlation of wear in vivo and six laboratory wear methods. Dent Mater 2012;28:961-973 51. Xu HH, Smith DT, Jahanmir S, et al: Indentation damage and mechanical properties of human enamel and dentin. J Dent Res 1998;77:472-480
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52. Heintze SD, Cavalleri A, Forjanic M, et al: Wear of ceramic and antagonist–a systematic evaluation of influencing factors in vitro. Dent Mater 2008;24:433-449 53. Oh WS, Delong R, Anusavice KJ: Factors affecting enamel and ceramic wear: a literature review. J Prosthet Dent 2002;87:451-459 54. Garvie RC, Hannink RH, Pascoe RT: Ceramic steel? Nature 1975;258:703-704 55. Christel P, Meunier A, Heller M, et al: Mechanical properties and short-term in-vivo evaluation of yttrium-oxide-partiallystabilized zirconia. J Biomed Mater Res 1989;23:45-61 56. Kosmac T, Oblak C, Jevnikar P, et al: The effect of surface grinding and sandblasting on flexural strength and reliability of Y-TZP zirconia ceramic. Dent Mater 1999;15:426-433
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School of Dentistry, University of Alberta, Edmonton, Canada Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Canada
Keywords Tooth wear; zirconia; enamel; antagonist. Correspondence Dr. Sheila P. Passos, School of Dentistry, University of Alberta, 11405-87 Ave. NW, Room# 5083C, Edmonton Clinic Health Academy (ECHA), Edmonton, AB T6G 1C9, Canada. E-mail: [email protected] The authors deny any conflicts of interest. Accepted November 13, 2013 doi: 10.1111/jopr.12167
Abstract Purpose: The aim of this systematic review was to assess enamel wear on teeth opposing zirconia restorations and to evaluate factors related to the wear of natural teeth opposing zirconia restorations. Materials and Methods: Five electronic databases were searched through May 2013 without limitations. The terms “antagonist*,” “enamel,” “wear,” and “zirconi*” were used. Titles and abstracts were initially screened, and those that fulfilled the inclusion criteria were selected for a full-text assessment. Studies that evaluated only the material wear were not included. Results: The database search strategy retrieved 142 potentially eligible studies. After the duplicate studies were removed, 62 studies were obtained. Titles and abstracts that fulfilled the inclusion criteria were selected for a full-text assessment (25). Seven laboratory studies met the inclusion criteria. In addition, reference lists from the finally selected studies were also screened. Conclusions: There was a large variation in relation to wear test method quantification, applied force, lateral movement, number and frequency of cycles, number of specimens, and enamel specimen preparation. In all studies, enamel wear rates were lower against polished zirconia. Differences in the test methods did not allow for comparisons of wear rates among the studies. Clinical Significance: Polishing the surface is recommended for a full-contour zirconia restoration because polished zirconia presents favorable wear behavior opposing natural teeth.
High-strength substructures of zirconia-based restorations have been widely used because of their excellent mechanical properties. They are usually veneered by dental porcelain to achieve an adequate esthetic result; however, chips and fracture of the veneer1-9 and high wear of antagonist teeth10,11 have been reported. For these reasons, new CAD/CAM technologies and translucent zirconia materials have been introduced to produce nonveneered zirconia restorations.12,13 As a result, reduced chipping events and reduced wear of the restoration material and antagonist enamel have been reported.14-25 The abrasiveness of dental zirconia ceramic opposing natural enamel has been considered.12-20 A few in vitro studies investigated the wear resistance and abrasive potential of yttriastabilized tetragonal zirconia polycrystalline (Y-TZP) against opposing human enamel.14-20 According to their results, zirconia monolithic crowns can be used against enamel with less antagonistic wear than porcelain.15,16,18-20 The surface characteristics of dental ceramics also affect the wear process. Occlusal intraoral adjustments are usually
required to obtain an adequate occlusal contact. This procedure results in a rough surface after removing the glaze layer or the smoothness of polished surfaces, which affects the wear behavior of the ceramic restorations if it is not well repolished.17,24 An adequate finishing procedure on the ceramic surface will contribute to minimizing the wear of the ceramic restoration and its natural antagonist.14,16,17,19,20 As no clinical studies have evaluated the behavior of monolithic zirconia ceramic as an antagonist occlusal material, the aims of this study were (1) to systematically review the literature for in vitro studies that quantify the wear of enamel opposing zirconia ceramics, and (2) to evaluate factors related to the enamel wear opposing zirconia ceramics.
Materials and methods The PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) Statement26 was used as a reporting template as much as possible.
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Table 1 Search strategy Database
Dates of coverage
MEDLINE (OvidSP)
1950 to May 2013
PubMed (NLM)a
1950 to May 2013
EMBASE (OvidSP)
1974 to May 2013
Cochrane (Wiley)
To the fourth quarter of 2011
Scopus (Elsevier)
1960 to May 2013
a
Keywords “antagonist∗ ” OR “enamel” AND “wear” AND “zirconi∗ ” Same search strategy as MEDLINE (OvidSP). MeSH terms: dental enamel, tooth wear, zirconium dioxide Same search strategy as MEDLINE (OvidSP) Same search strategy as MEDLINE (OvidSP) “antagonist∗ ” OR “enamel” AND “wear.” Within results, “zirconi∗ ” was added.
The search terms were related to the MeSH terms.
Data collection
A systematic search of electronic databases was conducted using MEDLINE (OvidSP), PubMed (NLM—National Library of Medicine), EMBASE (OvidSP), Cochrane Library (Wiley), and Scopus (Elsevier) up to May 2013. The terms used were “antagonist*,” “enamel,” “wear,” and “zirconi*”. Specific search strategies for each electronic database are outlined in Table 1. A different search strategy was employed with the Scopus database. First, the terms “antagonist*”, “enamel,” and “wear” were used, and within the results the term “zirconi*” was added. In that way, more articles were found. No limits were applied during the electronic searches. Criteria for selection of studies
All titles revealed by the electronic search were screened according to the following inclusion criteria: 1. In vitro studies; 2. use of Y-TZP ceramic; 3. evaluation of the antagonist’s enamel.
reviewers was sought. Based on their final decision, all potentially relevant articles were retrieved in full. Conference abstracts such as IADR abstracts were not selected per se because they do not present the detailed test method, but an attempt was made to retrieve their full articles if they were already published. Reference lists from the selected studies were also screened. Data extraction
The studies were analyzed with regard to the wear-mimicking device and wear method, its testing parameters, ceramic preparation, finishing technique, antagonist’s enamel wear, and zirconia wear. When information was unavailable or limited, authors were contacted to obtain missing information. Methodological data among the selected studies
A customized systematic evaluation protocol (Table 2) was created to contrast the study methodology of the selected studies.
Results Study selection
Seven14-20 laboratory studies were included in the systematic review (Fig 1). Among the 25 studies initially considered in the second selection stage, a few studies were eliminated after inclusion and exclusion criteria were applied. In vitro studies that did not analyze the wear27 or did not have enamel as antagonist25,28-34 or evaluated only the material wear not measuring the enamel structure wear21-24 were excluded. Articles that did not use zirconia ceramic35,36 or that evaluated veneered zirconia37 were also not included in the review. One study13 was not included because Y-TZP was not the type of zirconia evaluated. Only one manuscript from the second stage was not assessed because of the language.38 The online search was complemented by manual search through the references of the selected articles; however, none were found. Study characteristics
The selected studies were published between 2010 and 2013. The experimental parameters, zirconia ceramic system, sample size, wear method, and device differed among the seven selected studies. Risk of bias within studies
In addition to the inclusion criteria, the following exclusion criteria were applied:
All articles did not report some experimental conditions, which might affect the wear mechanism. Results of individual studies
1. Veneered zirconia specimens; 2. absence of enamel wear evaluation. A search of the relevant titles/abstracts was conducted. Where a potentially relevant title without a listed abstract was available, the full article was later assessed to select the studies. This first stage was conducted by two independent reviewers. In cases of discrepancies, a consensus agreement between two 2
1. Experimental design. Forces ranged from 4 to 49 N, and the number of cycles from 3000 to 1,200,000 cycles. Water was used as the predominant liquid medium. Only one study used artificial saliva.19 2. Enamel antagonist. Regarding the antagonist response, human molars and premolars were used. The enamel was prepared differently among the studies. The enamel
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6. Enamel preparation
4. Number of specimens (N) 5. Enamel antagonist
3. Wear evaluation
2. Experimental design
1. Wear device/method
Experimental conditions
The tips of the cusps were polished (diamond rotary instrument)
Mandibular molar mesiobuccal cusps
Alabama: dual-axis simulator, vertical and horizontal movement 10 N, 0.33 Hz, 2-mm sliding, 200,000, 400,000 cycles 33% glycerine, and 66% distilled water 3D-Profilometer. Records volume loss by overlapping the 3D surfaces before and after the wear test 8
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Table 2 Articles included in the review
3D-Profilometer. Records volume loss by overlapping the 3D surfaces before and after the wear test 20 Maxillary premolars buccal cusps
SEM by analyzing wear scars
Premolar buccal surfaces
3D-Profilometer. Records volume loss by overlapping the 3D surfaces before and after the wear test 10 Maxillary and mandibular premolar cusps Nonstandardized
3D laser scanner. Records the maximal vertical loss and the mean vertical loss
16 First or second maxillary molar cusps Nonstandardized
3D-Profilometer. Records the vertical loss by overlapping the 3D surfaces before and after the wear test 6 Maxillary molar mesiobuccal cusps The tips of the cusps were rounded to a spherical shape
3D-surface analyzer. Records the vertical loss by overlapping the 3D surfaces before and after the wear test 12 First mandibular molar mesiobuccal cusps Non-standardized
Carborondum sand paper from 180 to 1500 mesh, 1 µm diamond sand paper
(Continued)
1200-grit abrasive paper
CS 4.8: dual-axis simulator, vertical and horizontal movement 5 kg weight, 0.8 Hz, 0.3 mm sliding, 240,000 cycles water 5°/55°C UMT-2: microfriction and wear testing apparatus 4 N, 2 Hz, 1 mm sliding, 5000 cycles, artificial saliva
CS 4.8: dual-axis simulator, vertical and horizontal movement 49 N, 0.8 Hza , 0.3 mm slidinga , 300,000 cycles heat circulation 5°/55°C
Ivoclar: dual-axis simulator, vertical and horizontal movement 5 kg weight, 1.6 Hz, 0.7 mm sliding, 120,000 cycles, demineralized water
Zurich: controlled masticator comprised occlusal loading 49 N, 1.7 Hz, 120,000, 240,000, 640,000 and 1,200,000 cycles, water 5°/50°C
Zurich: controlled masticator comprised occlusal loading 49 N, 1.7 Hz, 3000 cycles, water 5°/50°C
1
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3
4
Polished,glazed, polished, then reglazed
FCZ glaze
11. Glazed material
Flat Control (veneering porcelain and enamel),polished zirconia,glazed zirconia, polished then reglazed
8. Specimen shape 9. Sample grouping
10. Zirconia finishing technique
Monolithic zirconia
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7. Zirconia system
Experimental conditions
Table 2 Continued
NA
Flat Control (human enamel), zirconia, lithium disilicate, leucite glass ceramic, feldspathic ceramic, hybrid ceramic, nanocomposite CAD/CAM block, direct nanocomposite, experimental composite, unfilled PMMA, microfilled acrylate polymer Polished
inCoris TZI
Mormann et al15
Polished with 600-grit SiC, polished with 1200-grit SiC
NA
Polished, fine-grit diamond, coarse-grit diamond, glazed
Vita Akzent VM9
Glazed with a glaze ceramic, glazed with a glaze spray, manually polished, mechanically polished Glaze ceramic: Glaze Zirox with Stain LiquidGlaze spray: ZENOStar Magic
Cuboidal Three monolithic zirconia, one lithium disilicate, one feldspathic porcelain
Prettau, Lava, Rainbow
Kim et al18
Flat Control (polished leucite-reinforced glass ceramic), four different finishing procedures for zirconia: polished, fine-grit diamond, coarse-grit diamond, glazed
Everest ZH
Mitov et al17
Flat Control (monolithic base alloy), veneered zirconia, glazed zirconia with a glaze ceramic, glazed zirconia with a glaze spray, manually polished zirconia, mechanically polished zirconia
ZENOTEC Zr Bridge transluzent
Stawarczyk et al16
NA
Polished,rough
3Y-TZP (from powder doped with 3 mol% Y 2 O3 ) Hemisphere Polished zirconia, rough zirconia, hot-forged lithium disilicate glass ceramics, silicate-based veneer porcelain
Wang et al19
(Continued)
Glazing of Zirkonzahn Prettau
Polished, polished with glazing
Cuboidal Polished feldspar, polished zirconia, polished zirconia with glazing
Prettau
Jung et al20
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b
Data obtained by contacting the author (not reported in the article). E, enamel; M, material; >statistically significant more wear. c Data obtained according to the different masticatory cycles (120,000; 240,000; 640,000; and 1,200,000).
a
14. SEM analysis
Glazed with a glaze ceramic: 51.7 to 118 µm.c Glazed with a glaze spray: 24.5 to 62.2 µm.c Manually polished: 14.3 to 27.3 µm.c Mechanically polished: 14.7 to 28 µm.c
25.5 µm
Polished + 200,000 cycles: 0.11 (0.04) mm3 . Glazed + 200,000 cycles: 0.87 (0.21) mm3 . Polished then reglazed + 200,000 cycles: 0.59 (0.1) mm3 Polished + 400,000 cycles: 0.21 (0.05) mm3 Glazed + 400,000 cycles: 1.18 (0.2) mm3 Polished then reglazed + 400,000 cycles: 0.88 (0.12) mm3
13. Enamel antagonist wear Vertical substance loss (µm) Volumetric substance loss (mm3 ) Wear scars widths (µm)
E: glazed with a glaze ceramic > glazed with a glaze spray = polished M: glazed > polished
Stawarczyk et al16
NA
Mormann et al15
E: polished then reglazed = glazed > polished. No difference for the different cycles M: polished then reglazed = glazed > polished
Janyavula et al14
12. Wear resultsb
Experimental conditions
Table 2 Continued
E: coarse-grit diamond > polished. No difference among polished, fine-grit diamond and glazed. M: no measurable wear observed. Except for the glazed group. Polished: 171.74 (121.68) µm
Mitov et al17
0.04 (0.02) mm3
E: no difference between 600-grit and 1200-grit for all zirconia systems used M: not reported.
Kim et al18
Enamel wear scars widths: rough > polished Rough zirconia material: deep furrows
x
Glazed: 0.078 (0.063) mm3 Polished: 0.031 (0.033) mm3
E: glazed > polished M: not reported.
E: rough > polished M: very small damage in both surface treatments.
Enamel wear values were not determined. Wear scars widths: Rough: 300 to 400 µm Polished: 200 to 300 µm
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Wang et al19
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MEDLINE
PubMed
EMBASE
Cochrane
Scopus
40 studies
38 studies
27 studies
9 studies
58 studies
Full-contour zirconia restorations have been recently added to the market as alternative restorations. For that reason, no in vivo study reporting the enamel wear against zirconia is available, and there are only a few in vitro studies. The aim of this systematic review was to evaluate the enamel wear opposing zirconia ceramics.
After removing duplicates (62) Studies excluded from the first selection stage (37)
Experimental design
Potentially relevant studies (25)
Studies excluded from the second selection stage (18)
Studies relevant to the topic (7 articles) References of selected full-text articles (0) Studies included (7)
Figure 1 Schematic study of the selection process.
surface was only polished14 or ground and polished in a spherical16 or a flat way,19 whereas for the other four studies, no treatment was performed on the enamel cusps.15,17,18,20 3. Zirconia specimen. All but two studies18,20 used a different zirconia ceramic system. Most studies prepared the zirconia ceramic specimens in a flat shape. Hemisphere19 and cuboidal18 shape specimens were also used. In addition, different surface treatments were performed by most studies. 4. Finishing technique. Three studies observed more enamel wear for glazed specimens,14,16,20 whereas no difference between glazed and polished specimens was observed by one study.17 The remaining study did not include glazed specimens in its investigation.15,18,19 5. Wear values. One study19 did not determine the wear values (µm, mm3 ). The wear was evaluated by SEM, measuring the width of wear scars. In addition, one study listed the wear values obtained only from the polished specimens.17 Risk of bias across studies
A meta-analysis could not be performed, as the test parameters differed from one study to the other.
Discussion Dental wear is a complex process, which occurs over time and involves chemical and mechanical wear. Clinical investigations are essential to assess the wear process of dental materials. The ceramic material itself strongly affects the wear behavior; however, intraorally, several other factors influence the complex wear interaction between tooth enamel and opposing substrates. These factors are the abrasive nature of food, chewing behavior along with parafunctional habits and neuromuscular forces, as well as the antagonistic material (thickness, roughness, fracture toughness, hardness).20,39-41 6
Among the seven selected studies, each study used different wear-mimicking devices. Therefore, there was a great variety of simulator settings. Different wear experimental settings determine different wear mechanisms. Variation in relation to the applied force, the number and frequency of cycles, lateral movement, the number of specimens, and enamel specimen preparation were identified. For that reason, it is recommended to use at least two configurations of parameters to determine material wear.42 The same loading force, either 5 kg or 49 N, was applied by five included studies.43-45 This represents an average value of biting forces of occlusal contacts.46 In contrast, one study19 applied only 4 N, which was not sufficient to provide a clinical simulation. Two studies15,19 used 3000 or 5000 cycles, which seem insufficient to measure wear on zirconia specimens. For example, dental composite materials exhibited about 41% of the final wear after 8.3% of the total number of cycles by the Ivoclar chewing wear method.45 Approximately 1 year of chewing in oral function can be simulated by 240,000 to 250,000 loading cycles.47,48 The other five selected studies performed from 120,000 to 1,200,000 cycles. In one study16 different cycle numbers were evaluated, and showed that wear rate increases with the increasing number of cycles. In contrast, according to the results of one selected study, no difference was observed when 200,000 and 400,000 cycles were evaluated.14 Another point to consider is that the wear simulator should perform a lateral movement. Among the seven studies, the sliding range was 0.3 to 2 mm. One study16 that did not perform lateral movements during the test exhibited much lower enamel wear than a study that used the Ivoclar method (0.7 mm sliding).17 Wear of a dental composite increases eight times with 0.7 mm sliding.45 In addition, different intermedium fluids such as water, demineralized water, and artificial saliva were used. No study considered the corrosive wear, as no chemical reaction layer was placed between the substrates. It has been reported that acidulated phosphate fluoride alters the topography of some glass ceramic surfaces49 as well as other abrasive media.50 For that reason, the surface zirconia degradation by corrosion needs to be identified, as it plays an important role in the wear phenomenon. Enamel antagonist
Among the included studies, four used natural molars, and the remaining studies used natural premolars. The variation in wear data among the studies can also be attributed to the enamel specimen preparation, and morphological and structural differences of natural enamel as well. The physiological enamel wear rate, as evaluated from in vivo measurement is from 30 to 40 µm
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per year.51 Different enamel shapes were evaluated among the included studies, which is a critical factor because as the shape changes from a cusp to a flat surface there will be less force per area and consequently less wear (provided the wear mechanism does not change). Heintze et al52 reported more variation of the enamel wear results for the prepared cusps (Table 2; enamel preparation) than nonstandardized cusps, which is related to the enamel thickness. The authors reported that the higher the enamel thickness, the lower the wear of the prepared enamel cusps. Therefore, nonstandardized enamel cusps should be the set-up to measure the wear rate. Zirconia specimen shape
Hemisphere19 and cuboidal18 ceramic specimens instead of flat ceramic specimens were used by two studies. Only one study did not determine the enamel wear rates by vertical15-17 and volumetric14,18,20 wear. The study that evaluated hemisphere specimens assessed the wear by measuring the wear scar widths, and only 5000 cycles were performed. For that reason, it was not possible to compare the enamel antagonist against flat and hemisphere specimens. A cuboidal specimen shape was also used in one of the studies.18 Natural tooth antagonist wear rate more than doubled for flat Empress specimens compared to Empress inlays. On the other hand, it was the opposite for the restorative material wear.52 Therefore, flat ceramic specimens overestimate the enamel wear compared to anatomically formed specimens, and crown specimens exhibit less restorative material wear than flat ceramic specimens. Nevertheless, the findings may not be applicable to zirconia, because there is no report investigating zirconia specimen shape on the antagonist and material wear rates. This issue also needs to be clarified for zirconia material, because it is essential to standardize the antagonist shape for comparisons. Finishing technique
The enamel antagonist wear can be accelerated according to the ceramic surface characteristics.14,16,17,20 Only one study15 included in this review did not evaluate the surface finishing of zirconia including different glazing techniques, mechanical polishing, and manually polishing. The wear is more affected by the surface structure, roughness, and environmental factors.53 Among the selected studies, only one study17 reported the specimen roughness, which was 0.006 µm after polishing procedure. It has been reported that during the wear process the glaze layer is removed, and the underlying ceramic surface is usually not polished, thus upon glaze removal the rough ceramic surface accelerates enamel wear; however, two included studies14,20 observed that even when the surface under the glaze layer was polished (1200-grit abrasive paper), the glazed specimens presented more antagonist wear than the polished specimens. Among the four included studies that evaluated polished and glazed specimens, three14,16,20 observed that polished zirconia showed the lowest wear rate on natural tooth antagonist. For the remaining study,17 no difference among polished, fine-grit, and glazed was observed. It seems standardized enamel affects wear results for glazing.17 In contrast, nonstandardized enamel shows no difference for glazing.
Enamel Wear Caused by Zirconia Ceramics
Specimens glazed with a glaze spray (ZENOStar Magic) showed no difference of enamel wear compared to polished specimens.16 However, a specimen glazed with a glaze ceramic presented more enamel wear than polished and glazed with a glaze spray specimens. There is no standard glazing zirconia technique available for zirconia restorations in the literature. Mechanical properties of Y-TZP ceramic
Y-TZP ceramic has superior mechanical properties compared to other ceramics because of its phase transformation toughening mechanism that prevents further crack propagation in the material.54 Such a mechanism involves the transformation from tetragonal to monoclinic phase at the crack tip and around the crack by localized compressive stresses.55 This stress-induced phase transformation is a major characteristic of zirconia; however, the long-term stability of Y-TZP in the presence of water is limited by the continuing transformation from the tetragonal to monoclinic phase, which could raise low-temperature degradation (LTD). LTD is an aging phenomenon caused by the excessive progression of spontaneous tetragonal-to-monoclinic transformation in the presence of water.56 LTD not only results in undesirable transformation, but also loss of strength and surface degradation, which alters the surface roughness. As a result, wear resistance is compromised. Therefore, the effect of the LTD should be considered for the evaluation of the abrasiveness of polished zirconia over time. Nevertheless, the long-term wear behavior of full-contour zirconia restorations is still unknown.
Conclusions In summary, according to the included studies in this review, polished zirconia specimens showed a favorable wear behavior opposing natural teeth. Enamel pathological wear opposing zirconia full-coverage restorations has not yet been confirmed. It is important to assess the enamel wear behavior opposing zirconia using clinically relevant surface contours as well as to investigate the long-term stability and abrasiveness of polished zirconia. The studies agreed that the polishing of zirconia surface favors lower enamel wear rates.
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