You checked the insertion path, cleared the undercuts, and the crown still rocks or sits high at delivery. The next place to look is the margin line. In CAD, the margin line tells the design where the crown ends, so a blurry or stepped margin carries straight into the restoration you seat. This guide covers the two checks that decide whether a crown seats the first time, plus a short checklist to run before you finalize.

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The short version

1. Trace the margin on a high-resolution scan, zoomed in to 400%.
2. Keep the line sharp and continuous, with no blurry or stepped segments.
3. Rotate the model and check the entire finish line for undercuts.
4. Set the view where the whole margin is visible as the insertion axis.

Why does margin line quality affect crown fit?

The margin line is the boundary the CAD software designs the crown to. If that line is blurry or stepped, the crown edge is built to the wrong place, and the error shows up at delivery. A sharply defined margin gives the design an accurate finish line, so the crown can seat fully.

A poorly defined margin can lead to:

• Open contacts
• Crown rocking
• Extensive chairside adjustments
• Remake risk

First, rule out the insertion path

A crown can look perfect on screen and still fail to seat because of how it was set to go on. Before you rework the margin, check these three CAD mistakes. The examples below are from exocad.

Mistake 1: Designing without checking the insertion path

Switch to Expert Mode and open Set insertion direction before you design, so you know the path the crown will take onto the prep.

Mistake 2: Not looking for undercuts

Look for the red-highlighted areas on the prep. These are typically undercuts.

Mistake 3: Seeing the red zones and leaving them

Adjust the insertion path to reduce the red zones, then regenerate the crown base for a precise fit. The fix takes about 15 seconds, and it happens before the crown ever reaches CAM.

If the fit is still poor after that, move on to the margin line.

Step 1: Trace the margin at pixel-level detail

The accuracy of a margin line depends on two things: the resolution of the scan you trace it on, and how closely you zoom in while tracing. A low-resolution scan hides the finish line, and tracing from too far out turns a sharp edge into a guess.

What you should do

1. Start from high-resolution scan data, below 10 µm.
2. Zoom to 400% in your CAD software when tracing margins.
3. Trace a sharp, continuous margin line, with no blurry or stepped segments.

Pro tip: Small tracing errors at the margin become fit errors at delivery, so slow down where the finish line is hardest to read.

Step 2: Rotate the model to check for undercuts

An undercut can hide part of the finish line from the insertion direction. If the margin disappears in the set view, there's an undercut blocking full seating. The fix is to change the view until the whole margin line can be seen.

What you should do

1. Rotate the model to inspect the entire finish line for undercuts.
2. If any part of the margin disappears in the set view, treat it as an undercut that will block full seating.
3. Adjust the set view to an angle where all the margin lines are clearly visible, then set that view as the insertion axis.

Pro tip: If you can't see the whole margin from the insertion direction, the crown can't seat fully on it.

Quick checklist before you finalize the margin

• The scan data is high resolution (below 10 µm).
• The margin was traced at 400% zoom, with no blurry or stepped segments.
• The model was rotated and the entire finish line was checked for undercuts.
• The set view shows every part of the margin line clearly and is set as the insertion axis.

How does a clean margin improve crown seating?

A clean, sharp margin allows cement to flow evenly, so the crown seats completely. That is what turns a design that looks right on screen into a crown that fits in the mouth.

• Better crown fit
• Fewer chairside adjustments
• Higher patient satisfaction

Frequently asked questions

Why won't my crown seat fully?

In CAD, the two most common places to look are the insertion path and the margin line. An undercut along the insertion path blocks the crown from going all the way down, and a blurry or stepped margin line builds the crown edge to the wrong place. Check the insertion path first, then the margin.

What is a margin line in dental CAD?

The margin line is the traced outline of the finish line on the prepared tooth. It tells the CAD software where the crown ends. The crown edge is designed to that line, so its accuracy decides how well the crown meets the tooth.

How far should I zoom in when marking a margin line?

Zoom to 400% in your CAD software when tracing margins, on scan data with a resolution below 10 µm. At that zoom you can follow the finish line closely and keep the margin sharp and continuous, with no blurry or stepped segments.

How do I check for undercuts in exocad?

Switch to Expert Mode and open Set insertion direction. Red-highlighted areas are typically undercuts. Rotate the model until the red zones are reduced and the whole margin line is visible, then click "Set current view as insertion axis" and regenerate the crown base.

What causes open contacts or a rocking crown?

A poorly defined margin is one cause. Blurry or stepped margins lead to open contacts, crown rocking and more grinding at delivery. Tracing a sharp margin on a high-resolution scan and confirming the full finish line is visible from the insertion direction removes that cause before the crown is made.

Crowns that seat right the first time

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