To leverage the high aesthetic potential and balanced mechanical properties of these types of zirconia, it is essential that the restorations be processed under ideal conditions, especially during sintering. Unwanted optical effects that may otherwise occur and suitable measures to help avoid them are summarised below.

White spots on the surface

White spots on a restoration’s surface are usually indicators of contaminated alumina sintering beads or the use of the wrong instruments for surface modification and sprue removal. The effect is avoidable through replacement of the sintering beads as soon as they show any signs of discoloration, as well as the exclusive use of fine-grit diamond instruments for adjustments prior to sintering.

Blue-grey appearance and low chroma

A blue-grey appearance and low chroma may be attributed to mineral residues from dipping liquids in the chamber. They are effectively removed with the aid of a decontamination program selected from the furnace menu and run after inserting several residual pieces of a highly translucent white zirconia blank. In order to prevent the occurrence of a greyish appearance in new restorations, it is recommended that a decontamination program be performed at least once per month.

Green or yellowish discoloration

If a restoration appears to be green or yellowish, it is likely that the furnace is equipped with molybdenum disilicide (MoSi₂) heating elements, which might need regeneration or replacement. Regeneration firing, which involves a rapid heating rate and a long firing phase at approximately 1,450 °C, may solve the problem for a while. A better strategy, however, is the use of a furnace with silicon carbide heating elements, which are highly resistant to ageing and do not cause discoloration.

Variations in translucency, chroma and pigmentation

Variations in the translucency, chroma or pigmentation of restoration surfaces are often due to sintering temperature deviations from the recommended temperature curve. The only way to solve this issue is temperature calibration—usually carried out with the aid of TempTABs or process temperature control rings. They are placed into the furnace on a sintering tray and processed by running a calibration cycle. After sintering, the tab or ring diameter is determined and assigned to a temperature on a conversion table. If the determined temperature deviates from the temperature displayed on the furnace, the latter value is adjusted.

General recommendations

Prevention is better than troubleshooting. Therefore, it is essential to:

• adhere to the manufacturer’s sintering protocols;
• remove the dust from the sintering chamber and heating elements with a soft brush before each use;
• replace alumina sintering beads at least once per month;
• use only fine-grit diamond instruments for pre-sintering adjustments;
• use a furnace with silicon carbide heating elements, or run regeneration cycles for MoSi₂ elements;
• run a decontamination program with decontaminating powder or white zirconia residues at least once per month; and
• calibrate the temperature on a monthly basis.

With these simple measures, it is possible to maximise the potential of Kuraray Noritake’s KATANA Zirconia multilayered series.

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