Multilayer zirconia: positioning the restoration in the disc
How to position restorations in a multilayer zirconia disc: shade and strength gradients, vertical position, nesting, connectors in bridges and sintering.
Multilayer zirconia discs give a natural shade gradient without layering ceramic, and they are now standard for monolithic crowns and bridges. The same restoration, though, can come out of the furnace looking very different depending on where it was placed in the disc. This post explains how to position restorations in a multilayer zirconia disc, what the layers mean and the rules that matter for bridges.
What the layers are
A multilayer disc is built from layers with different pigment content, so the shade moves from a lighter, more translucent incisal zone to a more chromatic cervical zone. In many products the layers also differ in translucency and flexural strength, with stronger material in the cervical area and more translucent material at the top. The manufacturer’s data sheet describes the layers, their thickness and what each is suited for.
Read that data sheet for every disc type. Two discs with similar names can have very different gradients.
Positioning a single crown
The vertical position decides how much of each layer the crown takes. Placed high, the crown gets more of the translucent incisal zone. Placed low, it becomes more chromatic overall. For a typical anterior crown, the aim is to have the incisal third in the incisal layers and the cervical third in the dentine layers. Short posterior crowns usually need less translucency and can sit lower.
The CAM software shows the layers during nesting, and the position can be adjusted there without touching the design. A short note of the positions that worked for each disc type and shade saves trial and error later.
Bridges and connectors
Bridges add a constraint. Where the layers differ in strength, the manufacturer usually states which layers are suitable for connectors and for long-span bridges. The connectors have to sit in those layers, and that can decide the vertical position more than the shade does. Connector size and minimum thickness come from the material instructions as well, summarized in zirconia minimum thickness and connector size.
Nesting and the sintering shrinkage
Zirconia is milled oversized and shrinks during sintering. The CAM applies the enlargement factor stated for each disc batch, so the batch code has to be entered correctly. Place restorations far enough from the disc edge and from each other, as the CAM and disc instructions state, and keep long bridges in an orientation that the sintering instructions allow.
From design to furnace
The design itself follows the usual monolithic workflow in monolithic zirconia crowns in exocad. Sinter with the program the manufacturer states for the disc, because faster programs can change translucency and shade. When the result is still not what was expected, compare position, sintering program and batch before blaming the disc.
Leftover areas of a disc can often take the next small job, as long as they sit at the right height. Reusing partially milled zirconia discs explains how to record them and nest new work into them.
Frequently asked questions
Why does the position in a multilayer disc matter?
Multilayer discs change shade, and in many products also translucency and strength, from one layer to the next. The vertical position of the restoration in the disc decides how much of each layer ends up in the incisal edge, the body and the cervical area.
Where should bridge connectors sit in a multilayer disc?
In discs that combine layers of different strength, the manufacturer usually states which layers are suited to connectors in bridges. Connectors are placed in the stronger layers when the instructions require it, which can limit how high the bridge can be positioned.
Can I move a restoration up or down in the disc after design?
Yes, in the CAM nesting step. Most CAM software lets you shift the restoration vertically inside the disc and shows the layers, so the shade transition can be adjusted without changing the design.