Preparing Teeth for Zirconia Crowns: What the Lab Looks For

Zirconia is forgiving in many ways, but the prep still sets the ceiling for the result. When a prep is even, smooth and clearly captured, we can design a crown that fits well and looks right. When it is not, we spend our time working around it. Here is what we look for when a zirconia case arrives.

Follow the material manufacturer's guidance

Each zirconia product has its own requirements for minimum thickness, and those requirements differ between high-strength and more translucent formulations. Before you prep, check the instructions for use for the material you plan to prescribe. If you are not sure which zirconia we will use for a case, call and ask. We can tell you what the material needs.

Adequate and even reduction

Reduction that is even all the way around matters as much as the total amount. A prep that is generous on one wall and thin on another forces a compromise. Either the crown is thin where it is under-reduced, or we add bulk that changes the contour.

A few things help:

• Reduce following the anatomy of the tooth rather than flattening it
• Check functional cusps, which are often under-reduced
• Watch the axial walls, especially on the lingual of lower molars
• Use a reduction guide if you want a quick check against the planned contour

Smooth, continuous margins

A clean margin is the single most important line on the die. We design the crown edge to that line, so any irregularity shows up in the fit.

We look for a margin that is:

• Smooth and continuous, without ledges or lips
• Free of unsupported enamel
• Defined in a consistent form all the way around

Follow the material manufacturer's guidance on margin design. Zirconia generally works well with the margin forms the manufacturer specifies, and a clean, even margin will serve better than a perfect form that is ragged in places.

Rounded internal angles

Sharp internal line angles are hard to reproduce. Milling burs have a fixed diameter, so a sharp corner in the prep becomes a rounded corner in the crown. That mismatch can hold the crown up and keep it from seating.

Rounding the internal angles and the occlusal-axial transitions lets the intaglio match the prep closely. It also spreads stress more evenly through the restoration.

Clear margins in the scan or impression

We can only design to what we can see. For digital cases, that means the margin is visible all the way around, without tissue, blood or saliva covering it. Retraction and moisture control make the biggest difference. Review the scan before you send it, rotate it, and rescan any area where the margin is unclear.

Our guide to sending digital scans to a dental lab covers what we need in a digital case.

Check clearance before you finish

Have the patient close and check clearance in centric and through excursions. A clearance check in the mouth takes moments. Finding the problem on the model means either a thinner crown than the material allows or an adjustment to the opposing tooth.

When space is limited

Sometimes you cannot reduce further without compromising the tooth. When that happens, tell us on the Rx. Note where space is tight and whether you are open to adjusting the opposing tooth. We can let you know if the case falls below the material's minimum and talk through options before we mill, rather than after.

Learn more about our zirconia crowns and bridges, or read how zirconia compares in our zirconia vs. e.max guide. If you want a second look at a prep before you send it, call us at 781.853.0777 or submit a case online.