Saudi Journal of Oral Sciences

: 2020  |  Volume : 7  |  Issue : 1  |  Page : 11--17

Effect of veneering material and technique on the fracture resistance of porcelain-veneered zirconia crowns

Yousif Essam Ezzat1, Mohammed A Al-Rafee2,  
1 Saudi Board in Prosthodontic Dentistry; College of Dentistry, Riyadh Elm University, Riyadh, Saudi Arabia
2 College of Dentistry, Riyadh Elm University, Riyadh, Saudi Arabia

Correspondence Address:
Dr. Yousif Essam Ezzat
Saudi Board in Prosthodontic Dentistry, College of Dentistry, Riyadh Elm University, Riyadh, Saudi Arabia. Ministry of Health
Saudi Arabia


Context: Different porcelain veneering materials and techniques are used for the fabrication of porcelain-veneered zirconia crowns. Aim: The aim of this study was to examine the effects of different veneering materials and techniques (layering or over pressing) on the fracture resistance of zirconia-based crowns. Materials and Methods: A prepared molar tooth was scanned using computer-aided design/computer-aided manufacturing technology to create a master metal die. The scanned dies was used to produce forty zirconia copings. The zirconia copings were divided into four groups (n = 10) based on the veneering technique used, as follows: over pressing using Cercon Ceram press (PR1), IPS e.max ZirPress (PR2), layering using IPS e.max Ceram (LR1), and VITAVM9 (LR2). All crowns were cemented using glass-ionomer cement and thermocycled for 3000 cycles, between 5°C and 55°C. They were then loaded using a universal testing machine (3.7-mm ball and 0.5-mm/min crosshead speed) until failure. One-way ANOVA with Bonferroni corrections for multiple comparisons was used for the statistical analyses. Results: The means and standard deviations for failure loads were 1420 ± 54 N, 1797 ± 31 N, 1698 ± 36 N, and 2120 ± 73 N for the PR1, PR2, LR1, and LR2 groups, respectively. The differences in failure loads were statistically significant (P < 0.05) among the different test groups. Failure was predominantly due to adhesive failure in the PR1 and PR2 groups, whereas core fracture occurred more often in the LRI and LR2 groups. Conclusion: The fracture resistance of zirconia-based crowns was affected by the veneering materials and techniques used.

How to cite this article:
Ezzat YE, Al-Rafee MA. Effect of veneering material and technique on the fracture resistance of porcelain-veneered zirconia crowns.Saudi J Oral Sci 2020;7:11-17

How to cite this URL:
Ezzat YE, Al-Rafee MA. Effect of veneering material and technique on the fracture resistance of porcelain-veneered zirconia crowns. Saudi J Oral Sci [serial online] 2020 [cited 2020 Feb 28 ];7:11-17
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Fixed dental restorations constructed from yttrium oxide-stabilized tetragonal zirconia polycrystals (Y-TZP) have become prominent due to their high mechanical properties, high strength using the fracture resistance test (900–1200 MPa), excellent biocompatibility, and good esthetics when veneered with porcelain.[1]

The success of dental restoration is determined by three main factors: esthetic value, resistance to fracture, and marginal adaptation.[2] For zirconia-based restorations, most clinical failures have been reported at the weakest part of the veneering glass ceramics.[3] A fracture in the veneering material forms a rough, sharp edge that requires readjustment in the form of polishing or repair, and a replacement may be necessary if the fracture affects the tooth's esthetic appearance or leads to functional impairments.[4],[5],[6]

Many factors can cause the chipping of the veneering layer, including the design of the zirconia core, support and thickness of the veneering layer,[7],[8] the firing protocol,[9],[10] the surface treatment of the zirconia core,[6],[11] the morphology of the finish line,[12] the adhesive forces between the core and the veneering layer,[13] a mismatch in the coefficients of thermal expansion between the core and the veneering layer,[10] the veneering technique used, and the type of veneering ceramic applied.[14]

Veneering of zirconia is commonly carried out using a conventional layering technique that involves compaction, firing, and glazing. It is also commonly conducted using the over-pressing technique, which requires forming a wax pattern for the veneering ceramic layer over the zirconia core, followed by spruing, investing, and using a piston to force a heated ceramic ingot into the mold. After the ceramic material cools and hardens, the investment is broken apart, and the recovered crown is stained and glazed in a conventional furnace.[15],[16]

There is controversy in the literature regarding the impact of the veneering material and the technique on the fracture resistance of zirconia copings. Some studies found no effect of veneering material or technique on the fracture resistance of zirconia crowns, whereas others reported superior fracture strength when a certain veneering technique was used.[6],[9],[13],[17],[18],[19],[20]

The present study aimed to evaluate the effects of different veneering materials and techniques (layering and press over) on the fracture strength of zirconia-based crowns.

 Materials and Methods

A prepared maxillary molar was selected from a demo model. The preparation had a deep chamfer finish line (135°, 1-mm thickness), a smooth continuous margin, no irregularities, a total convergence angle of 10°, an axial surface height of 5 mm, and 2 mm of occlusal reduction.

The demo model was scanned using a three-dimensional (3D) laser scanner (Smart Optical 3D scanner, Open Technology, Italy). The data were then uploaded into a software (Exocad software, Germany) to mill a master metal die (Metal Alloy, Mesa, Italy) in a milling machine (Roland, California, USA). Nine additional metal dies were milled to be used in load testing. After milling the metal dies, they were smoothed with a rubber wheel and polished with pumice in a lathe brush. Rouge was then applied to create a smooth polished surface such that no interference with the seating of the crowns would occur later.

The master metal die was scanned to produce a 3D virtual die [Figure 1] using a 3D-laser scanner (Smart Optical 3D Scanner, Open Technology, Italy). The data were then transferred into Exocad software (exocad GmbH, Darmstadt, Germany), and zirconia coping was designed with a thickness of 0.6 mm and 30-μm spacer [Figure 2]. Forty zirconia-based copings were milled and veneered with four different types of porcelain and two different veneering techniques. Accordingly, four study groups were created (n = 10), as shown in [Table 1]. The sample of ten specimens for each group achieved 85% power to detect the differences among the means with a significance level (α) of 0.05.{Figure 1}{Figure 2}{Table 1}

The forty copings were milled from presintered Y-TZP blanks (Kerox Dental Zircostar, Hungary) in a milling machine (Roland, California, USA) in the white state. After completion of the milling procedure, the zirconia copings were separated from the blanks, steam cleaned, and placed in the sintering furnace (E4K, Hungary). Sintering was carried out according to the manufacturer's instructions at 1450°C.

The forty copings were randomly divided into four equal groups according to the veneering technique and porcelain material to be applied [Table 1].

Two veneering techniques were performed as follows:

The press-over technique was used in the PR1 and PR2 groups. To achieve an identical veneering structure, the veneering layer was designed using the software (Exocad software, Germany). The veneering layer was designed with a 0.7-mm cervical thickness and 1.5 mm at the occlusal surfaces. Then, the Castable Wax (computer-aided design–computer-aided manufacturing Wax Blank, Bilkim, Turkey) was milled Then attached to the zirconium oxide frameworks. Each wax veneer was then sprued and invested. The created mold was Pressed using the appropriate pressable glass-ceramic ingots, IPS e.max ZirPress, Ivoclar Vivadent and Cercon Ceram Press, Degudent, Germany, for the PR1 and PR2 groups, respectively. Firing was performed in a proper ceramic furnace (Programat EP 3010; Ivoclar Vivadent, Liechtenstein) at a temperature specified in the manufacturer's instructions for the PR1 (940°C) and PR2 (910°C) groups. After recovering the restorations, they were finished with finishing burs (Nach ZTM Wolfgang Welsser) and silicon carbide paper (1400 and 2000 grit, 401Q; 3M) and then glazed according to the manufacturer's instructions. The temperature for glazing was 800°C for the PR1 group (glazed using Vita AKZENT Plus) and 725°C for the PR2 group (glazed using IPS e.max Ceram Glaze)-The layering technique was used for the LR1 and LR2 groups. To standardize the shape and the size of veneers in all groups, a silicone putty index (Dentona 1:1 gum, Germany) captured from the crowns processed in the PR1 and PR2 groups was used. Next, porcelain materials were applied to the zirconia copings of groups LR1 and LR2 (IPS e.max Ceram and VITAVM9, respectively) and were fired in a compatible ceramic furnace (Programat 700; Ivoclar Vivadent) according to the manufacturer's instruction. Firing was followed by glazing. LR1 maximum firing was conducted at 750°C, followed by glazing with IPS e.max Ceram Glaze at a temperature under 725°C. The firing cycle for LR2 was 910°C (Dentin 1), followed by 900°C (Dentin 2), and then glazing with Vita AKZENT Plus was performed at 900°C. The specimens were polished with silicon carbide paper (1400 and 2000 grit, respectively, 401Q; 3M) before glazing.

All specimens were thermocycled for 3000 cycles lasting 60 s each using thermocycler machine (THE-1100-SD Mechatronik GmbH). The specimens were submerged in a hot distilled water bath (55°C) for 20 s, were removed for 10 s, and then were submerged in a cold distilled water bath (5°C) for 20 s. Each of the two cycles were separated by 10 s.

The metal dies were then mounted vertically using a Dental Surveyor in a self-cured Acrylic Ortho resin (Orthoplast, Vertex Dental, Zeist, The Netherlands) within a customized cylindrical silicone mold such that the crown margin was 2 mm above the Ortho resin. The cylindrical silicone mold was 35 mm in diameter, which was based on the size of the holder in the masticatory machine that we used for the preloading tests. The metal dies were mounted using a cold-cure acrylic resin material.

Crowns were cemented on the metal die with glass-ionomer cement (Ketac™ Cem Aplicap™, 3M ESPE, Germany) according to the manufacturer's recommendations. To standardize the cement mix, Forty capsules were used for the forty crowns. The Aplicap activator activated one capsule for each crown for 2–4 s. The capsule was then mixed at approximately 4300 rpm in a high-frequency mixing device (Cap Mix™) for 10 s. Next, a thin coat of cement was applied inside of the restoration, and the restoration was seated on the metal die. During the setting of the cement, all crowns were loaded in the direction of insertion with a force of 15 N for 60 s. After cementation, the crowns were placed in a plastic container with water covering the bottom surface and a sealed lid to create a humid atmosphere to prevent desiccation of the luting cement.

In the second stage of aging, a three-point contact between the indenter and the occlusal surface of each crown was secured before loading. The crowns were cyclically preloaded in a wet environment for 10,000 cycles using loads between 30 and 300 N with a load profile in the form of a sine wave at 1 Hz that was parallel to the vertical plane.

After being stored in distilled water for 24 h, the specimens were fixed to the universal testing machine (Instron 5960; Instron, High Wycombe) in an upright position using a standard holder. The machine was recalibrated at the beginning of the test. The crowns were loaded until failure using a 3.7-mm diameter stainless steel ball indenter with a 0.5-mm/min crosshead speed [Figure 3].[17] Failure was defined as the occurrence of visible cracks in combination with load drops and acoustic events. Failure loads were recorded, and the means were calculated for each group. Failed crowns and fractured segments were inspected using a magnifier (Mobiloskop “s;” Renfert, Hilzingen, Germany), and failure modes were identified and recorded. Three sources of failure were identified: cohesive failures within the veneering layer, adhesive failures between the veneering layer and the core, and core failures.{Figure 3}

The Statistical Package for the Social Sciences (SPSS) for Windows (release 18, SPSS Inc., Chicago, IL, USA) was used for all the statistical analyses in this study. The Shapiro–Wilk tests were used to check for the normal distributions. The means and standard deviations (SD) of failure loads were presented and compared. One-way analysis of variance (ANOVA) with Bonferroni corrections was used to analyze the data. Statistical analysis of variance was applied to determine whether any differences existed among the groups. A statistically significant difference among the groups was indicated when P < 0.05.


The means and SD of the failure loads were calculated for each group [Table 2]. The LR2 group demonstrated the highest failure load values among the study groups, with a mean (±SD) of 2120.73 (±73.36) N and a maximum of 2241.02 N. The PR1 group demonstrated the lowest failure load values, with a mean (±SD) of 1420 (±53.88) N and a maximum of 1551.06 N.{Table 2}

One-way ANOVA showed statistically significant differences in the failure load between the study groups (P< 0.05). A Scheffe post hoc test showed statistically significant differences between all the study groups (P< 0.05) except between PR2 and LR1 (P > 0.05). The highest mean difference in failure load was noted between the LR2 and PR1 groups (700.39 N), and the smallest mean difference in failure load was noted between the PR2 and LR1 groups (99.43 N) [Table 3].{Table 3}

To further investigate the effect of veneering technique on the fracture loads, samples from the PR1, PR2, LR1, and LR2 groups were rearranged into two groups based on the veneering technique: a press-over group and a layering group (n = 20). The mean (±SD) failure load was higher with the layering technique (1909.39 ± 244.07 N) than that with the press-over technique (1608.91 ± 198.14 N). Moreover, independent sample t-tests indicated that this difference was statistically significant (P< 0.05) [Table 4].{Table 4}

Two-way cross tabulation showed that the PR1 and PR2 groups were more likely to demonstrate adhesive failures [Figure 4]. The LR1 and LR2 were more likely to demonstrate core fractures [Figure 5]. Fisher's exact tests showed that these associations were statistically significant (P = 0.000) [Table 5].{Figure 4}{Figure 5}{Table 5}


Stress patterns in the oral cavity are complicated and assessing the loads that a dental restoration must resist over time is difficult. Therefore, when testing a material clinically, shaped restorations should be evaluated under ecological conditions similar to those in the oral cavity, and the results should be compared with clinical data on the maximum loads that may occur in the oral cavity.[7]

In the present study, the specimens were shaped-like crowns. When bi-layered materials were used, they were layered as recommended by the manufacturers for restorations intended for clinical use. To simulate aging of the materials similar to that experienced in the oral environment, thermocycling and cyclic preloading in a wet environment were used; this combination of methods is accepted and commonly employed in in>-vitro studies.[7]

The results of the current study indicate that the fracture resistance of zirconia-based restorations varied between different veneering materials and techniques.

In previous studies, zirconia-based restorations have shown a wide range of single-cycle loads to failure (346–6263 N), which can be attributed primarily to the variability between loading protocols, including the abutment material, the luting cement, the diameter of the loading indenter, and the loading angle.[15],[16] In the present study, zirconia-based crowns demonstrated mean failure load values between 1357 and 2120.73 N, which is within the range reported by previous studies. These fracture loads exceeded the expected maximum occlusal loads in the oral cavity (446–1221 N).[21]

The press-over technique can be expected to introduce fewer flaws than the layering technique, resulting in superior physical properties as it is a more controlled technique than the layering technique, which is more sensitive and subject to variability in proper layering and firing procedures.[18] However, in the present study, the LR2 group showed the highest fracture loads than that of both the PR1 and PR2 groups. Similar results were reported by previous studies.[16],[19],[20] The significantly lower failure loads of the PR groups than those of the LR1 group in the present study can be attributed to the “one-shot” firing of the entire anatomy over zirconia copings with a uniform thickness in the PR specimens, which may have resulted in residual stress. Compressive stresses are generated within the veneer near the infrastructure and tensile stresses developing at the surface of the veneer, especially in large unsupported areas such as cusp tips.[20]

Consistent with previous studies, the LR groups in the present study demonstrated predominantly core failures (20 of 20), indicating a strong bond between the veneering layer and the zirconia core.[20],[22] On the other hand, the PR groups demonstrated more adhesive failures (10/20), suggesting a weaker core/veneer interface than that in the LR groups. This finding can be explained by the formation of tensile stresses at the inner surface of the veneering layer, resulting from the presence of a relatively weak intermediate layer,[20] which may explain the generally lower fracture loads of the PR groups than those of the LR groups.

In the current study, as in all in-vitro studies, some limitations should be considered. In-vitro studies may not precisely mimic natural circumstances. Therefore, the results obtained from in-vitro experiments may not fully or accurately predict the outcomes in natural dentition. For instance, metal dies do not have the same modulus of elasticity as natural teeth (218 GPa compared with 12 GPa, respectively). However, metal dies are used because of their known strength and standardization, which is not feasible with natural teeth.[19]


Within the limitations of this in-vitro study, the following can be concluded:

The zirconia veneering material and technique had a significant impact on the fracture resistance of zirconia-based crownsThe crowns fabricated with the layering veneering technique showed predominantly core failures, while those fabricated using the press-over veneering technique showed more adhesive failuresLayering technique showed the highest fracture resistance.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.


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