Zirconia minimum thickness and connector size for CAD design
Zirconia minimum thickness and connector size from manufacturer instructions: why translucent zirconia needs more, example values, and how to check in exocad.
Zirconia is not one material with one set of rules. A highly translucent multilayer disc and a classic high-strength zirconia can look alike in the CAM, yet their instructions for use ask for different wall thicknesses, connector sizes and bridge spans. Ivoclar lists failure to observe them among the processing errors that may compromise the success of the restoration. This post explains why the zirconia minimum thickness and connector size vary, shows real values from two manufacturers, and describes how to check them in exocad.
Why the values differ between zirconias
Zirconias are usually grouped by their yttria content, often written as 3Y, 4Y or 5Y. More yttria gives more translucency and less strength. Manufacturers compensate for the lower strength of the translucent grades with larger minimum thicknesses, larger connectors and stricter limits on bridge span, pontics and cantilevers.
Multilayer and gradient discs add another variable: the layers do not all have the same properties. Some manufacturers therefore specify where in the disc height the connector must be placed.
The consequence for design is simple. The minimum values belong to the specific product, and the material in the CAD settings has to match the disc that goes into the mill.
Example: Ivoclar IPS e.max ZirCAD
The IPS e.max ZirCAD instructions for use (Rev. 1, October 2023) list minimum layer thicknesses and connector dimensions per product. For fully anatomical restorations:
| Product | Crown, anterior | Crown, posterior | 3-unit bridge, anterior | 3-unit bridge, posterior |
|---|---|---|---|---|
| ZirCAD MT and MT Multi | 0.8 mm | 1.0 mm | 1.0 mm, 12 mm² | 1.0 mm, 16 mm² |
| ZirCAD Prime Esthetic | 0.8 mm | 1.0 mm | 1.0 mm, 12 mm² | 1.0 mm, 16 mm² |
| ZirCAD Prime | 0.8 mm | 1.0 mm | 1.0 mm, 9 mm² | 1.0 mm, 9 mm² |
| ZirCAD LT and MO | 0.4 mm | 0.6 mm | 0.6 mm, 7 mm² | 0.6 mm, 9 mm² |
The same document limits MT, MT Multi and Prime Esthetic to bridges of at most three units with one pontic. Prime, LT and MO allow bridges with up to two pontics, with larger connectors for longer spans: for Prime, 12 mm² posteriorly on bridges of four or more units and 12 mm² on cantilever bridges with one pontic. Ivoclar describes 12 mm² as a connector 4 mm high and 3 mm wide. For Prime, Prime Esthetic and MT Multi, the connector cross-section must sit in the dentin area of the disc.
Example: Kuraray Noritake KATANA Zirconia
The KATANA Zirconia technical guide follows the same logic. For anterior 2- to 3-unit bridges it asks for at least 12 mm² in the translucent multilayer UTML and STML grades, and at least 7 mm² in ML and HT. UTML and STML are not suitable for cantilevered pontics. ML and HT are limited to two pontics within a bridge, with at least 12 mm² where two pontics connect, and a cantilever is limited to one pontic with at least 12 mm².
The guide also warns against adjusting the connector cross-section with a diamond disc. The disc leaves sharp notches that can start cracks and lead to bridge failure. Connector size has to be right in the design, with rounded embrasures.
These tables are examples. Manufacturers update their instructions, and each product has its own. Check the current instructions for use of the exact zirconia you mill before setting up a new material.
Setting and checking connectors in exocad
In the exocad connector step, the minimum connector size can be defined either as a cross-sectional area or as height and width, with a slider for the value. The software tries to enforce that minimum when it generates the connectors. When space does not allow it, the connector is shown in red and needs manual work, for example moving it or changing its docking points. Hovering the mouse over a connector shows its measurements.
In the Free tab, connectors can be shaped by control points. If a connector ends up substantially thinner than the material parameters specify, exocad marks the thin area in magenta, and a connector with magenta areas should not be exported. The same applies to walls: check occlusal and axial thickness against the material minimum after the final free-forming, since that step often removes material.
The defaults behind these checks come from the material configuration, so it is worth reviewing those values against the instructions for use when you set up a new zirconia. The monolithic zirconia workflow covers the rest of the crown design, and the exocad cement gap settings article explains the parameters inside the crown.
When the space is not enough
Sometimes the vertical space between gingiva and occlusal plane does not allow the connector size the material requires. A connector thinner than the instructions allow is not a solution. The alternatives are a stronger zirconia grade with smaller minimums, a different design such as a framework with veneering, or a clinical change agreed with the dentist. Raise it before the case is milled, with the measured values, so the clinician can decide.
Sources
- Ivoclar: IPS e.max ZirCAD instructions for use, Rev. 1, October 2023
- Kuraray Noritake: KATANA Zirconia technical guide
- exocad wiki: Generating connectors
Frequently asked questions
What is the minimum thickness of a monolithic zirconia crown?
It depends on the zirconia. In the Ivoclar IPS e.max ZirCAD instructions for use, fully anatomical MT and Prime crowns need 0.8 mm anteriorly and 1.0 mm posteriorly, while LT and MO allow 0.4 mm and 0.6 mm. Always use the values of the material you mill.
What connector size does a zirconia bridge need?
Values range from about 7 mm² to 16 mm² depending on the material, the region and the number of units. For example, Ivoclar requires 16 mm² for posterior 3-unit bridges in IPS e.max ZirCAD MT, and 9 mm² in IPS e.max ZirCAD Prime.
Why does translucent zirconia need thicker connectors?
Higher translucency zirconias are less strong than the classic high-strength zirconias. Manufacturers compensate with larger minimum dimensions and tighter limits on bridge span and pontics.
How do I check connector size in exocad?
In the connector step, the minimum size can be set as a cross-sectional area or as height and width, hovering over a connector shows its measurements, and exocad marks connectors that are too thin in red or magenta.