To determine whether the thickness, processing technique, and cooling protocol of veneer ceramic influence the flexural strength of a bilayer ceramic system.
Materials and methods
Sixty-four bar-shaped specimens (20 mm × 4 mm × 1 mm) of yttria-stabilized tetragonal zirconia (Vita In-Ceram YZ, Vita) were fabricated (ISO 6872) and randomly divided into 8 groups ( n = 8) according to the factors “processing technique” (P – PM9 and V – VM9), “thickness” (1 mm and 3 mm), and “cooling protocol” (S – slow and F – fast). The veneer ceramics were applied only over one side of the bar-shaped specimens. All specimens were mechanically cycled (2 × 10 6 cycles, 84 N, 3.4 Hz, in water), with the veneer ceramic under tension. Then, the specimens were tested in 4-point bending (1 mm/min, load 100 kgf, in water), also with the veneer ceramic under tension, and the maximum load was recorded at first sign of fracture. The flexural strength ( σ ) was calculated, and the mode of failure was determined by stereomicroscopy (30×). The data (MPa) were analyzed statistically by 3-way ANOVA and Tukey’s test ( α = 0.05).
ANOVA revealed that the factor “thickness” ( p = 0.0001) was statistically significant, unlike the factors “processing technique” ( p = 0.6025) and “cooling protocol” ( p = 0.4199). The predominant mode of failure was cracking.
The thickness of the veneer ceramic has an influence on the mechanical strength of the bilayer ceramic system, regardless of processing technique and cooling protocol of the veneer ceramic.
Metal-free systems are widely used for the fabrication of partial and total ceramic restorations, due to their excellent esthetics , biocompatibility , and clinical success, as reported in the literature . However, some failures have been reported from clinical studies, such as fracture of the crown , fracture of the connectors , secondary caries and endodontic problems , and delamination and chipping of the veneer ceramic . Among the reported failures, fractures of the veneer ceramic (chipping and delamination) in crowns with zirconia infrastructure were the most common , varying according to the clinical trials: 8% after 37 months , 15.2% after 5 years , 20% after 31 months , and 30% after 5 years .
Factors that can cause these fractures include: the processing technique of the veneer ceramic , its thickness , as well as the cooling protocol adopted in the final cycle of ceramic sintering . Recent studies have investigated the influence of laboratory steps and the inherent properties of the materials, in attempts to improve our understanding of the causal factors related to clinically reported cohesive failures in the veneer ceramic , since the percentages of veneer ceramic chipping vary among laboratory prostheses, even when the materials used are identical . Moreover, as stated by Mainjot et al. , “The temperature distribution in a sample is complex, depending on materials’ thickness, cooling rate, materials’ thermal properties, the presence of a mesh-tray as a support for the firing procedure, and on temperature distribution within the furnace”. All of these factors influence the stress state of zirconia-based restorations. Thus, if there is a preponderant factor related to the occurrence of failures, it must be acknowledged.
For vitreous ceramic materials, the glass transition temperature (Tg) and values above it are critical for the formation of residual thermal stress, because the CTE values are higher at these temperatures . It is important that the ceramics be cooled below the Tg, so that the residual tensile stress formed in the inner layers of the ceramic is balanced by the superficial compressive stress, which can be twice the tensile stress . Moreover, during the cooling process, when different ceramic materials with low thermal conductivity are used in a restoration, a large temperature difference occurs between the layers of these materials, and high levels of residual stresses may arise, especially with rapid cooling .
In a study of analytical calculations, Swain observed a directly proportional relationship between cooling protocol and thickness of veneer ceramic, i.e. , the more rapid the cooling and the greater thickness of the veneer ceramic, the higher the residual thermal stress generated inside the veneer ceramic. Often, laboratory technicians do not respect the maximum amount of veneer ceramic that may be applied over the infrastructure, and they do not adhere to the cooling protocol recommended by the manufacturer. According to Choi et al. , cooling is often accelerated to shorten the time required for fabrication of prosthetic restorations in the laboratory.
Despite the residual thermal stress formed at the interface of all-ceramic restorations, not producing visible flaws, premature fractures in the veneer ceramic of restorations can occur when these are subjected to masticatory loads .
The veneering ceramic can be applied over the Y-TZP framework by the stratified technique, in which layers of powder and liquid mixture are applied and sintered, or by the pressed technique, in which a veneering ceramic is injected over the framework that is included in the investing material. The stratified technique is considered more sensitive because of the consecutive application of veneering ceramic layers and the successive cycles of sintering and cooling , in addition to the greater possibility of the incorporation of pores . In the pressed technique, the occurrence of pores is reduced , hence reducing the incidence of failures in the restorations, making this a more reliable process .
The objective of this study was to evaluate the influence of processing technique and thickness of the veneer ceramic under different cooling protocols on the flexural strength of bilayer ceramic specimens. The hypotheses investigated were that: (1) the slower cooling would increase the flexural strength of the bilayer system, (2) the increased thickness would decrease the strength, and (3) the application of veneer ceramic by the pressed technique would have better results in terms of flexural strength when compared with conventional techniques.
Materials and methods
The materials used in this study, as well as their brand names and manufacturers, are shown in Table 1 .
|Material||Brand name||Manufacturer||Chemical composition a||Batch number|
|Y-TZP ceramic||Vita In-Ceram YZ (15.5 × 19 × 39 mm)||Vita Zahnfabrik, Bad Säckingen, Germany||Zirconia powder: Al 2 O 3 (67%) ZrO 2 (33%) Ce-stabilized zirconia glass powder: Al 2 O 3 (14–18%), SiO 2 (14–18%), B 2 O 3 (11–15%), TiO 2 (2–7%), La 2 O 3 (25–30%), CeO 2 (6–10%), CaO (4–8%), ZrO 2 (1–4%), Y 2 O 3 (2–6%)||28,070|
|Feldspathic ceramic||Vita VM9, Dentin 2M2||Vita Zahnfabrik, Bad Säckingen, Germany||SiO 2 (60–64%), Al 2 O 3 (13–15%), K 2 O (7–10%), Na 2 O (4–6%), TiO 2 (<0.5%), CeO 2 (<0.5%), ZrO 2 (0–1%), CaO (1–2%), B 2 O 3 (3–5%), BaO (1–3%), SnO 2 (<0.5%), Mg, Fe and P oxides (<0.1%)||21,740|
|Feldspathic ceramic||Vita PM9, Opaque 0M2P-O||Vita Zahnfabrik, Bad Sackingen, Germany||SiO 2 (62–67%), Al 2 O 3 (16–19%), K 2 O (6–8%), Na 2 O (5–8%), B 2 O 3 (1–3%)||22,520|
|Bonding agent||Vita VM9 Effect Bonder||Vita Zahnfabrik, Bad Säckingen, Germany||Powder: SiO 2 (47–51%), Al 2 O 3 (10–15%), K 2 O (5–8%), Na 2 O (3–5%), CeO 2 (10–13%), ZrO 2 (5–8%), CaO (1–2%), B 2 O 3 (3–5%), BaO (0.5–1.5%), TiO 2 (<0.5%), SnO 2 (<0.5%), Mg, Fe and P oxides (<0.1%) liquid: containing ethanol (2.5–10%) e sodium hydroxide (2.5%)||15,800|