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Department of Dental Materials and Prosthodontics, São José dos Campos, Brazil. .... of monolithic zirconia systems elimi- ... 3D inLab; Sirona Dental) according.
The International Journal of Periodontics & Restorative Dentistry © 2016 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.

901

Fracture Strength, Failure Types, and Weibull Characteristics of Three-Unit Zirconia Fixed Dental Prostheses After Cyclic Loading: Effects of Veneering and Air-Abrasion Protocols Fernanda Campos, DDS, MSc1 Rodrigo O.A. Souza, DDS, PhD2 Marco A. Bottino, DDS, PhD3 Mutlu Özcan, DDS, DMD, PhD4 The required connector dimension for zirconia fixed dental prostheses (FDPs) may be a clinical limitation due to limited space in the occlusogingival direction. Using no veneering in the gingival regions of the pontics and connectors may solve this problem. This study evaluated the mechanical durability of zirconia FDPs with and without veneering in the gingival area of the connectors and pontics and subsequent airabrasion of this region with different protocols. Models were made of resin abutments (diameter = 6 or 8 mm, height = 6 mm, 6 degrees convergence) and embedded in polyurethane resin (distance = 11 mm). Zirconia frameworks were milled and randomly distributed by veneering (veneering of the entire framework [VEN] or no veneering at gingival regions of the pontic and connector [NVEN]) and by air-abrasion (Al2O3/SiO2, 30 µm; or 45 µm Al2O3). FDPs were adhesively cemented and subjected to mechanical cycling (1,200,000 cycles, 200 N, 4 Hz, with water cooling). Specimens were tested until fracture (1 mm/min), and failure modes were classified. Data (N) were subjected to one-way analysis of variance in two sets, Tukey test (α = .05) and Weibull analysis. While veneering did not significantly affect the results (VEN: 1,958 ± 299 N; NVEN: 1,788 ± 152 N; P = .094), air abrasion did (P = .006), with the worst results for the groups conditioned with 45 µm Al2O3 (SiO2: 1,748 ± 273 N; Al2O3: 1,512 ± 174 N). The NVEN group demonstrated the highest Weibull modulus (12.8) compared with the other groups (5.3–7.2). Fractures commonly initiated from the gingival side of the connector. Veneering of the gingival region of the connectors and pontics in zirconia FDPs did not diminish the fracture strength, but air-abrasion of this area with 45 µm Al2O3 decreased the results. Int J Periodontics Restorative Dent 2016;36:901–908. doi: 10.11607/prd.2524

PhD in Prosthodontics, São Paulo State University, São José dos Campos Dental School, Department of Dental Materials and Prosthodontics, São José dos Campos, Brazil. 2Adjunct Professor, Federal University of Rio Grande do Norte, Department of Dentistry, Division of Prosthodontics, Natal, Brazil. 3Professor, São Paulo State University, São José dos Campos Dental School, Department of Dental Materials and Prosthodontics, São José dos Campos, Brazil. 4Professor and Head of Dental Materials Unit, University of Zürich, Center for Dental and Oral Medicine, Clinic for Fixed and Removable Prosthodontics and Dental Materials Science, Zürich, Switzerland. 1

Correspondence to: Dr Rodrigo O.A. Souza, Federal University of Rio Grande do Norte (UFRN), Department of Dentistry, Av Salgado Filho, 1787, Lagoa Nova, Natal/RN, CEP: 59056-000, Brazil. Email: [email protected]  ©2016 by Quintessence Publishing Co Inc.

The use of zirconia-based ceramics for the fabrication of crowns and frameworks of fixed dental prostheses (FDPs) has increased over the years due to the increased use of computer-aided design/computerassisted manufacture (CAD/CAM) procedures and the acceptable optical and mechanical properties of the material.1–4 Zirconia is polymorphic, meaning that it has three main phases that are stable at different temperature ranges (monoclinic, up to 1,170°C; tetragonal, from 1,170°C to 2,370°C; and cubic, above 2,370°C).5 The phase transformation associated with volumetric expansion can occur under mechanical, thermal, or chemical stresses.6 Typically, clinical procedures during surface conditioning of zirconia or veneering initiate phase transformation at different levels. An airborne particle abrasion (air-abrasion) process used to increase the bond strength of the resin cement to zirconia7–11 may induce phase transformation on the ceramic surface,12,13 creating a compressive stress layer.5,14 According to some studies, this transformation could increase the resistance of this material in this area.14,15 However, airabrasion may also generate microcracks on the ceramic surface that could decrease the overall fracture resistance.5,16 The type and size of particles used for air-abrasion could

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902 also affect the fracture resistance, in that larger particles promote microcracks in the material that can lead to critical failure and smaller particles, mainly alumina particles coated by silica, were reported to increase the mechanical resistance of the zirconia ceramic, even after mechanical fatigue.15,16 Phase transformation also occurs during the veneering procedure due to the exposure of the zirconia framework to temperature changes and moisture.17 Since the gingival region of the FDP connectors is the region most susceptible to fracture,18 depending on the system manufacturers suggest minimal diameter of connectors in the transverse region of between 9 and 16 mm2 to maintain the resistance of zirconia FDPs.19 Clinically, this minimum FDP connector size increases after veneering. Increased connector dimensions after veneering may then limit the ability to clean the FDP, eventually causing periodontal problems. This may also impair esthetics in anterior regions, limiting the indications for zirconia FDPs.1 Thus, a zirconia FDP connector that is not veneered at the gingival region would be smaller in the transverse section. Introduction of monolithic zirconia systems eliminates the need for veneering, but to increase optical outcome occlusal or buccal areas of the FDP are either veneered or glazed. The objective of this study therefore was to evaluate the mechanical durability of zirconia FDPs with and without veneering in the gingival area of the connectors and pontics and subsequent air-abrasion of this region with different proto-

cols. The hypotheses tested were that veneering procedures and air-abrasion protocols on the gingival region of the connectors and pontics would increase the fracture strength of FDPs.

Materials and methods Specimen preparation

Abutments of glass-fiber-filled epoxy resin (National Electrical Manufacturers Association [NEMA] grade G10; Accurate Plastics) (n = 80) were milled to represent second premolars (base diameter = 6 mm) and second molars (base diameter = 8 mm).20 These preparations had the characteristics of full crown abutments with 6 mm height with rounded angles, a large-chamfer finish line, and convergence angle of 6 degrees. To reproduce the distance between the abutment teeth for a missing first molar, the prepared dies were embedded in polyurethane resin (F16 Resin, Axson) with a standard distance of 11 mm from each preparation base, and a model was obtained. The models were scanned (InEos Blue, Sirona Dental), and the frameworks were virtually generated using the accompanying software (Cerec 3D inLab; Sirona Dental) according to the manufacturer’s recommendations. Zirconia frameworks (IE) (n = 40) were machined (Cerec inLab MC XL; Sirona Dental) from zirconia blocks (Vita In-Ceram 2000 YZ Cubes, Vita Zahnfabrik) and then sintered according to the firing protocol recommended by the manufacturer

(ZYrcomat T, Vita Zahnfabrik). Zirconia frameworks had an occlusal wall thickness of 0.7 mm, an axial wall thickness of 0.5 mm, and an area of 9 mm2 for the transverse section of the connector. The frameworks were either entirely or partially veneered with a veneering ceramic (Vita VM9, Vita Zahnfabrik) by an experienced dental technician. The veneering ceramic was uniformly placed around the external area of the retainers, connectors, and pontics and sintered (Vacumat 6000 MP, Vita-Zahnfabrik). The axial and occlusal wall thicknesses were then measured with a micrometer (KG Sorensen) with an accuracy of 1.0 ± 0.1 mm to control the veneering thickness. In partial veneering, the gingival region of the pontics and the connectors were not veneered with ceramic. The area with no veneering was controlled using a previously prepared silicon model. The FDPs were then divided into four groups according to the veneering and air-abrasion factors: (1) VEN (control): veneering on the entire framework; (2) NVEN: no veneering of the gingival region of the pontic and connectors; (3) SiO2-NVEN + air-abrasion with 30-µm alumina particles coated with silica (CoJet Sand, 3M ESPE) in the gingival area of the connectors and pontics (3.5 bar, 20 seconds, distance: 10 mm); and Al2O3-rNVEN + air-abrasion with 45-µm alumina particles (Polidental) in the gingival area of connectors and pontics (3.5 bar, 20 seconds, distance: 10 mm). During air-abrasion, the FDPs were protected by a resin device

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903

Table 1

Materials used in this study

Type

Brand

Manufacturer

Zirconia ceramic block

Vita In-Ceram 2000-YZ Cubes

Vita Zahnfabrik

Veneering ceramic

Vita VM9

Vita Zahnfabrik

Alumina particle coated with silica

CoJet Sand

3M ESPE

Alumina particle

Aluminum oxide

Polidental

10% hydrofluoric acid gel

Porcelain Conditioner

Dentsply

Resin cement

Panavia F2.0

Kuraray

Silane

Clearfil Porcelain Bond Activator Clearfil SE Bond (Primer)

Kuraray

Glass fiber-reinforced epoxy resin

G10 Resin (Epoxyglass)

Accurate Plastics

developed specifically for this study that allowed air-abrasion to be performed only at the gingival region of connectors and pontics. Table 1 details the materials used in this study.

Cementation of the FDPs

The abutment resin dies were treated with 10% hydrofluoric acid gel (Porcelain Conditioner, Dentsply) for 1 minute, rinsed with water, and dried with compressed air. The silane (Clearfil Bond Activator and Clearfil SE Bond, Kuraray) was applied to the preparation with a microbrush.20 After 5 seconds, a gentle air stream was applied, and the preparation was left to dry for 60 seconds. The adhesive system (ED Primer; Kuraray) was then applied, and after 60 seconds gently air-dried. The zirconia FDPs were ultrasonically cleaned in water for 5 minutes before cementation. Dualpolymerized resin cement (Panavia F2.0, Kuraray) was mixed for 20 sec-

onds and placed on the intaglio surfaces of each framework. The FDPs were placed on the models under a constant load of 750 g. The specimens were then photopolymerized (Radii-Cal, SDI) for 40 seconds from four directions (occlusal, buccal, lingual, cervical). The cemented FDPs were then stored in water for 24 hours at 37°C in darkness.

Cyclic loading and fracture test

The FDPs were subjected to mechanical cycling (ER-11000 Plus, Erios) under a load of 200 N, at a frequency of 4 Hz, for 1,200,000 cycles. The force was applied at the center of the occlusal region of the pontic. An acetate stopper was placed between the piston and the pontic to avoid stress concentration in this area. The specimens were tested until fracture in a universal testing machine (DL1000, EMIC) at a crosshead speed of 1 mm/min using a load cell of 1,000 Kgf.

Failure type analysis

The failure patterns were classified according to the macroscopic onset of fracture: in the connector (Type 1), in the pontic (Type 2), or in the retainer (Type 3). The specimens were then cleaned with isopropyl alcohol in ultrasonic bath for 10 minutes, and failure surfaces were observed under an optical microscope (×25) (Discovery V20, Carl Zeiss) to verify possible failure initiation points. The specimens with the most significant failures were coated with gold and viewed by scanning electron microscopy (SEM) (×30–×1,000) (Inspect S50, FEI).

Statistical analysis

The fracture strength (N) data obtained were subjected to statistical analysis (MINITAB for Windows 16.1/2010, Minitab) using one-way analysis of variance (ANOVA) in two sets, according to the independent variables of veneering and

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904

Table 2 Fracture strength and Weibull distribution values Fracture strength (mean ± SD [N])

Group VEN

σ0

m (CI)

1,958 ± 299

5.3 (4–7)

σ0.05

2,262.73

σ0.01

r

1,299.23

958.21

0.897

NVEN

1,788 ± 152

12.8 (7.3–22.6)

1,855.55

1,473.72

1,298.69

0.996

SiO2

1,748 ± 273

7.2 (4.3–11.9)

1,858.58

1,232.97

984.34

0.978

Al2O3

1,512 ± 174

5.7 (2.8–11.6)

1,556.8

701.6

0.942

930.54

The estimation method used was least squares. m = Weibull modulus; CI = confidence interval; σ0 = characteristic strength; σ0.05 = probability of failure at 5%; σ0.01 = probability of failure at 1%.

Results 99

%

90 80 70 60 50 40 30 20 10 5 3 2

0 3,

00

0 2,

00

0 50 1,

1, 0 00 0

0

90

80

70

0

1 N Shape 5.3536 12.8919 7.2375 5.7717

Scale 2262.73 1855.55 1858.58 1556.80

Corr 0.897 0.996 0.978 0.942

F 10 10 10 10

C 0 0 0 0

VEN NVEN SiO2 AI2O3

Fig 1  Weibull plot for tested groups. Corr = correlation value; F = failed; C = censured.

air-abrasion. Multiple comparisons were made with Tukey test. Maximum likelihood estimation without a correction factor was used for two-parameter Weibull distribution, including the Weibull modulus, scale (m) and shape (0), to interpret predictability and reliabil-

ity of ceramic durability (Minitab Software version 14) together with Bonferroni test:21 ln ln

1 1 − F(σc)

= m ln σc − m ln σ0

P < .05 was considered to be statistically significant in all tests.

All specimens survived 1,200,000 cycles, and none exhibited signs of defects after mechanical cyclic loading. For the groups VEN (1,958 ± 299 N) and NVEN (1,788 ± 152 N), where veneering of the gingival region of connectors and pontics was analyzed, the mean fracture strength was statistically similar (df = 1; F = 3.13; P = .094) (one-way ANOVA). For the NVEN group, when airabraded with SiO2 (1,748 ± 273 N) no statistically significant difference was observed (df = 2; F = 6.25; P > .05) but air-abrasion with Al2O3 (1,512 ± 174 N) significantly decreased the results (P < .05). The NVEN group demonstrated the highest Weibull modulus (m) compared with the other groups (Table 2, Fig 1). Failure analysis showed Type 1 being the most prevalent (38 specimens). One specimen showed a Type 2 and one a Type 3 failure. For most of the specimens, failures were located on the tensile side of the loading point at the gingival side of one of the connectors (Fig 2). All specimens remained luted on the models after fracture test.

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905 Discussion The results of this study suggest that lack of veneering ceramic on the gingival region of the pontics and connectors of a zirconia FDP does not decrease the fracture strength and reliability of zirconia FDPs. Thus, the hypothesis that the veneering procedure would increase the fracture strength of FDPs was rejected. However, the possibility of increasing the fracture strength of the zirconia FDP by air-abrasion of the nonveneered region was negated. This region is particularly critical in the FDPs as the tensile stresses produced by occlusal loads are concentrated here.18,22 The material that is under tensile stresses—in this case, the veneering material—seems to guide the mode of failure of the bilayered zirconia FDPs.18,23,24 In that regard, some studies recommend not veneering the gingival region of the pontics and the connectors, since in such a design this region could act as a monolithic zirconia body.25,26 According to some authors, the veneering process could affect the mechanical properties of the zirconia ceramic,17 and covering the FDP with a feldspathic ceramic could be beneficial as it could fill the surface flaws of the framework, generating compressive stresses and increasing the resistance of the system. However, successive firing cycles during the veneering ceramic application could promote the transformation phase,27 leading to degradation. Likewise, the veneering process by the layering technique is subject to human performance inconsistencies that could result in structural defects

Fig 2  SEM micrographs of failure types. (a)The most prevalent type of fracture was located at the tensile side of the loading point at the gingival aspect of one of the connectors. (b–f) Characteristics such as hackle lines (arrows) were observed, indicating the origin of failure. (g) Origin of the fracture in the framework.

a

b

c

d

e

g

f

g

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906 and air bubbles at the interface between the veneer and the framework.28,29 In this study, the m values indicated this difference in terms of reliability of the entirely or partially veneered groups. On the contrary, the fracture strength values did not show significant difference, possibly because the test used was unable to provide rupture values for the failure of the veneer layer.25,26 Despite some studies indicating that air-abrasion could increase fracture strength values by forming a compressive layer and leading from tetragonal to monoclinic phase transformation,5,14,17 the results of the present study did not verify this statement. The air-abrasion particle size and type could influence this strengthening process,16 as air-abrasion with 110-µm alumina particles in particular decreases the reliability of zirconia.30 This deterioration would be due to the stress concentration in the small flaws caused by the impact of particle deposition. Air-abrasion with 50-µm alumina particles could generate defects of approximately 4 µm, decreasing the mechanical stability of zirconia.31 If these small flaws are exposed to a wet environment and cyclic loads, the structure becomes brittle.30 Moreover, the type of particle used can greatly influence this process.32 Alumina particles coated with silica are softer and more regular than alumina alone and could cause less damage and consequently, minor surface degradation.31 Furthermore, air-abrasion with these particles could round or eliminate the surface irregularities resulting from machining, increas-

ing the fatigue behavior and survival probability of zirconia ceramic.33 This was partially confirmed in the present study since the air-abraded group with alumina-coated silica particles did not show decreased fracture strength values. The fracture strength values found in this study (1,512–1,958 N) were similar to those of other studies in which zirconia frameworks were used for FDPs (1,700–1,900 N).2,17,24 Nonetheless, it is difficult to compare the fracture strength values of this study directly with those generated in other studies due to variations in dimensions of the FDPs. Factors such as connector dimension and shape,19 framework design,34 fatigue regimen,33 distance between abutments,2 abutment resilience,35 and abutment material20 could strongly affect the mechanical properties of FDPs in in vitro studies. In terms of abutment material, studies that used stainless steel,2,17 human teeth,36 and epoxy resin as in this study obtained similar fracture load values (1,700–1,960 N). On the contrary, in comparison with the load-to-fracture results of some other studies,17,19,35–37 abutment resilience could have a stronger effect on the results. For example, Tinschert et al37 used a nickel-chromium alloy as an abutment material, without resilience, and reported a loadto-fracture mean value of 2,300 N. In other studies where stainless steel was used as abutment material for abutments without mobility, a fracture strength of 1,900 N was obtained.2,17 In the study by Rosentritt et al35 where human teeth were used as resilient abutments, the fracture

strength values were about 1,140 N. The effect of abutment material certainly needs further research. From a clinical perspective, the fact that similar values were obtained for FDPs with and without veneering in the gingival area of connectors and pontics is noteworthy when the use of monolithic zirconia FDPs with smaller connectors are considered for short abutments. However, the possibility of exposure to the oral environment of zirconia without veneering is still being questioned. Although some studies have shown that zirconia accumulates less biofilm than titanium,38–40 previous studies have demonstrated that the phase transformation resulted from the low-temperature degradation when zirconia is exposed to the oral environment.41,42 Fatigue testing of dental ceramics shows great variation in the literature. Studies on in vitro FDP systems used cycling times ranging from 100 to 28 × 106.43 It has been previously reported that 2 × 106 cycles correspond to approximately 4 years of normal occlusal and masticatory activity.44 The load applied also showed variations between 5 and 100 N.43 The results of this study need to be verified using a more sophisticated method where dynamic loading is practiced in water with offset sliding loads.45 Thus, additional in vitro studies using other evaluation methods, such as analysis of lifetime and finite element analysis, and longitudinal clinical studies are needed to predict the clinical behavior of zirconia FDPs with and without veneering surveyed in this study.

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907 Conclusions Zirconia three-unit FDPs with or without veneering of the gingival region of connectors and pontics showed similar fracture strength, although nonveneered FDPs demonstrated higher Weibull moduli (m) compared with the other groups. Air-abrasion of the gingival area of connectors and pontics with 45-µm alumina decreased the fracture resistance of zirconia FDPs.

Acknowledgments This study was supported by FAPESP Grants (Fundação de Amparo à Pesquisa do Estado de São Paulo) (Process #2010/14355-7 and #2010/19126-6) and is based on a Master’s thesis submitted to the University Estadual Paulista (UNESP), Institute of Science and Technology (ICT), São José dos Campos/ SP, Brazil as part of the requirements for the MSc degree. The authors reported no conflicts of interest related to this study.

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