Page 26 - DT Vol 15 No 3

26 I aesthetics DENTAL TECHNOLOGY, JULY-SEPTEMBER 2025

REQUIREMENTS FOR ZIRCONIUM OXIDE MULTILAYER CASE II: A CLASSIC FOR MICROSTRATIFICATION
The choice of a particular zirconium oxide therefore requires a thorough
evaluation. Zirconium oxide must meet certain criteria as a base material,
including:
• Excellent balance between opacity and translucency (colour harmony)
• Reliable processing properties (milling and sintering)
• Consistently good results (reproducibility)
• Stable material properties (durability)
Figure 1 clarifies the significance of microstratification. The initial situation
is always the abutment, sometimes more or less discoloured. The general rule
is that the space for a ceramic restoration is limited. This is followed by the
base (multilayered zirconium oxide), which is encased in a thin layer of veneer.
To be successful with this technique, a “colour-safe” foundation is essential.
Working with a zirconium oxide that does not possess this property creates
problems. With such a thin coating layer, it is difficult to compensate for colour Fig 11
inconsistencies (e.g., grey shades, incorrect colour distribution) (Figure 1).
This results in a problem that previously went unnoticed: we are working in
the dark when the components of multilayer zirconium oxide do not conform
and the colour does not match the classical reference. The great advantage of
microstratification is that 80–90% of the zirconium oxide’s defining factors
are predetermined. The aesthetic success of the entire restoration relies on
zirconium as it is the basis of both shape and colour.

THE COMMUNICATION OF COLOUR
The choice of colour for zirconia multilayer represents a challenge for all of
us. Whether we like it or not, we usually work on the basis of the Vita colour
scale, which has been used for decades. It is an old reference for innovative Fig 12
materials, and its shortcomings are well known. Fig 11-12: Zirconium oxide base (ArgenZ HT+ Multilayer) prepared for
There are good alternatives for proper communication of colour. microstratification. Approximately 80-90% of the restoration is defined by the
Nevertheless, it is to be expected that the majority of communication about zirconium oxide
colours will continue to take place through the classical colour scale. This
also means that zirconia oxides should theoretically match the colour
scale. In practice, this is a problem in the daily life of many laboratories.
Analysis of several polished crowns made of zirconium oxide shows that in
many cases the result has little in common with the reference colour of the
conventional scale. Manufacturers do not seem to maintain uniformity in
the development of zirconium oxides. This demonstrates once again that a
high level of expertise is needed to produce a “colour-safe” zirconia.
As a laboratory, we want to be able to trust the manufacturer. An A2
blank should result in an A2 restoration, and we should not have to work “in
the middle of nowhere” during finishing (e.g., with microstratification). A
zirconium oxide that is truly suitable for daily use should be able to reliably Fig 13: After microstratification (Initial One Sqin, GC) we focused on layering
reproduce every colour on the colour scale. completely during the aesthetic finish
INDIVIDUALITY IN THE CHOICE OF UNIQUENESS
The variety of natural tooth colours and the demand for naturalness are
the basis of our work. We would like to work according to the standardized
procedures, but the results must not be standard. In our laboratory, a method
of chromatic communication or colour matching has been established.
It is the basis upon which we select zirconium oxide, ensuring that the
corresponding colour is a perfect match.
THE COLOUR INDEX IN ZIRCONIUM OXIDE
We work with an individual colour model, the chromatic index, based on the
original colours of zirconium oxide. This is our tuning tool. At first, it may Fig 14a
seem expensive, but the result justifies the “investment.” The production of the
chromatic index is a great advantage for processing zirconia. The procedure Fig 14a-b: Finished
is simple and rigorous: a crown is milled from the original material, sintered, Fig 14b restoration after
and coated with glaze without the need for major machining. Please note: cementation
since each tooth has different volume ratios which affect the base colour, it
is necessary to create a colour index for each anterior tooth, premolar, and
molar. The result is a complete index of colours for all cases. Experience
shows that the initial outlay is amortized over a short period of time.
Colour matching without patients in the dental laboratory often comes
with vague instructions such as “A3 please, but a little clearer!”—probably
every ceramist has heard this statement from dentists. It creates only a
mental idea of the desired colour. To select the corresponding zirconium
oxide, the reference (Vita A3 colour scale) is kept next to the individual
colour sample (our index of colours, Figure 2) in zirconium oxide, and a
comparison (colour matching) is made.
Interesting, and at the same time worrisome, is the observation that many
multilayer zirconium oxides have almost nothing in common with the colour