Multiple firings of veneered zirconia restorations: Influence on thermal and mechanical properties

Multiple firings of veneered zirconia restorations: Influence on thermal and mechanical properties

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What influence does a different number of firings of veneering ceramics on a zirconia framework have on the thermal and mechanical properties of these materials? Within an investigation at the LMU Munich, this question was examined in more detail.

The picture shows (from top to bottom): specimens fired 2, 4, 6, 8 and 10 times with increasingly visible edge rounding.

Justine Hensel, München

All-ceramic restorations have become increasingly popular among both patients and dentists, as they represent a mechanically reliable restoration with high stability and excellent biocompatibility, and also offer advantages over metal-ceramic restorations in terms of esthetics.

Zirconia is a dental ceramic with a high flexural strength (1st and 2nd generation with 900 -1500 MPa) and fracture toughness and is therefore popular as a framework material. However, zirconia of the first two generations appears opaque due to its crystalline structure and is not used monolithically in the anterior region.

The veneering procedure is well known from metal-ceramic restorations and has shown good mechanical properties so far. However, the development of a fracture within the veneering ceramic, known as „chipping“, has often been observed as a major factor in the failure of a restoration.

The development of internal stresses seems to be of great importance, which zirconia as a brittle material is less able to compensate for than the classic metal frameworks. Therefore, it is important to keep the overall stress state of the restoration as low as possible. These residual stresses are primarily thermal stresses that are generated during the fabrication of the restoration and are „stored“ during cooling. Consequently, it can be assumed that the thermal properties of the materials play a decisive role.

Aim of the investigation 

Since the dental technician may have to perform several firings during the fabrication of veneered all-ceramic restorations due to repairs and changes in shape or shade, the aim of the investigation was to analyze the influence of a different number of firings on the thermal properties of the materials: the coefficient of thermal expansion (CTE) and the glass transition temperature (Tg), and also on the bond strength between the veneer and the zirconia framework. For this purpose, four veneering ceramics with different properties were used, including one leucite-free material that is intended for use on both zirconia and titanium.

Leucite is usually added to the veneering ceramics as a crystalline phase, as this allows the CTE to be controlled and adapted to the CTE of the framework material.

Zirkonoxid

Examined materials and their composition and thermal properties according to the manufacturers´ specifications. 

Materials and Methods

The thermal properties (CTE and Tg) were investigated after 2, 4, 6, 8 and 10 firings, respectively. The CTE was also measured for the zirconia framework material.

The measurements were performed with a dilatometer. The expansion of the specimens caused by heat was registered by induction by moving the measuring rod made of quartz glass.

Bending tests were performed using Schwickerath crack initiation tests with the same veneering ceramics and zirconia. The ceramic powder was applied to a 3 mm x 8 mm area and fired. Again, firing was performed 2x, 4x, 6x, 8x and 10x.

Half of the specimens were artificially aged in the thermocycler (5000 cycles, 5°C/55°C, 20s).

Then the bond strength was tested.

Zirkonoxid-Restaurationen
Simplified illustration of the specimen holder for the universal machine for the 3-point-bending test with an inserted specimen. 

Results

Thermal properties

The leucite-free veneering ceramic (1) showed almost constant CTE values along the firings, while the other ceramics showed variations in values between firings. Tg was not affected by the number of firings. A significant correlation was observed between CTE and Tg.

Zirkonoxid-Restaurationen

WAK-Mittelwerte der jeweils getesteten vier Verblendkeramiken, wobei Verblendkeramik (1) leuzitfrei ist.

Bond strength

Only the bond strength values after 2 and after 10 firings differed significantly, with lower results obtained for the values of all veneering ceramics taken together after 10 firings. The highest bond strength values were measured for the leucite-free veneering ceramic (1). Artificial aging by thermocycling showed no effect on the bond strength between the veneer and the framework.

The results presented here are based on the following investigation:

Hensel J, Reise M, Liebermann A, Buser R, Stawarczyk B. Impact of multiple firings on thermal properties and bond strength of veneered zirconia restorations. J Mech Behav Biomed Mater 2022;128(4):105134.