Veneering zirconium oxide restorations is common practice in many dental laboratories. But what influence does a different number of fires have on the thermal and mechanical properties of veneering ceramics? This was examined in more detail in a study at the LMU Munich.
Image (fromu): Test specimens fired 2, 4, 6, 8 and 10 times with increasingly noticeable edge rounding.
Justine Hensel, Munich
All-ceramic restorations have proven themselves and are becoming increasingly popular. They represent a mechanically reliable restoration with high stability and excellent biocompatibility. From an aesthetic point of view, there are also many advantages over metal-ceramic restorations. zirconia As a dental ceramic with the highest flexural strength (1st and 2nd generation = 900 - 1500 MPa) and fracture toughness, it is often used as a framework material. However, due to its crystalline structure, this zirconium oxide appears opaque and is not used monolithically, especially in the anterior tooth area. As is known from metal-ceramic restorations, veneering occurs. However, fractures within the veneering ceramic – “chipping” – have often been observed as a major factor in restoration failure.
Internal tensions seem to be of great importance because, as a brittle material, zirconium oxide is less able to compensate for these than the classic form of metal frameworks. It is therefore important to keep the overall stress level of the restoration as low as possible. These internal stresses are primarily thermal stresses that arise during the manufacture of the restoration and are “stored” in the material when it cools down. It can therefore be assumed that the thermal properties of the materials play a crucial role.
Aim of the investigation
When veneering all-ceramic restorations, several firing processes are usually necessary to adjust the shape and color accordingly. The aim of the study was to analyze the influence of a different number of fires on the thermal properties of the materials - thermal expansion coefficient (CTE) and glass transition temperature (Tg) - as well as the bond strength between the veneer and the zirconium oxide framework. Four veneering ceramics with different properties were used, including one that is intended for use on zirconium oxide and titanium and is leucite-free. Leucite is usually added to veneering ceramics as a crystalline phase so that the CTE can be controlled and adapted to the CTE of the framework material.
Tested materials and their composition as well as their thermal properties according to the manufacturer's information.
material and methods
The thermal properties (CTE and Tg) were examined after 2, 4, 6, 8 and 10 volumes, respectively. The CTE was also measured for the zirconium oxide framework material. The measurements were carried out with a dilatometer. The expansion of the test specimens caused by the heat could be recorded by induction by moving the measuring rod made of quartz glass. The bond strength was determined using Schwickerath crack initiation tests with the same veneering ceramics and zirconium oxide. The ceramic powder was applied to an area of 3 mm x 8 mm and fired. Here too, fires were fired 2, 4, 6, 8 and 10 times. Half of the test specimens were artificially aged in a thermocycler (5000 cycles, 5 °C/55 °C, 20 s). The adhesion strength was then tested.
Simplified representation of the test specimen holder of a universal machine for the 3-point bending test with the test specimen inserted
Results
Thermal properties
The leucite-free veneering ceramic (1) showed almost constant CTE values throughout the fires. For the other ceramics, there were fluctuations in values between firings. Tg was not affected by the number of fires. A significant correlation between CTE and Tg has been observed.
CTE mean values of the four veneering ceramics tested, with veneering ceramic (1) being leucite-free.
Bond strength
Only the strength values after 2 and after 10 firings differed significantly from each other, although somewhat lower results were achieved for the values of all veneering ceramics taken together after 10 firings. The highest strength values were measured for the leucite-free veneering ceramic (1). Artificial aging through thermocycling had no influence on the bond strength between the veneer and the framework.
The results were published in a publication Dental Materials submitted.