Tooth preparation for metal-free restorations

Tooth preparation is the first stage to producing a world-class dental restoration.

The rewards for getting preparations right are significant and ultimately benefit the patient in terms of the longevity, comfort and fit of their crown or bridge.

The ideal preparation

• No undercuts
• No feathered edges
• No gutter shoulders
• No parallel walls
• No 90° shoulders
• No sharp corners

Avoiding undercuts

If a tooth is prepped with an undercut (as shown right), any coping that is made as a replica of the prep will be unable to drop over the preparation and sit flush with the margin line.

Undercuts can be overcome by locally increasing the cement gap between the preparation and the coping, but this often compromises marginal adaptation.

Example preparation with undercuts

Side view	Top view
Red areas above margin line are undercuts.	Green margin line is obscured by overhanging material above.

Avoiding sharp corners

Sharp corners on the top of preparations (as shown right) acts as stress points which may lead to failures.

If the coping is made as a replica of the preparation, it will also have a sharp internal corner, which will increase the bending stresses on this part of the coping under biting forces.

The solution is simple to break the sharp edge.  It is not necessary to produce a perfectly rounded shoulder.

Example preps with sharp corners

Side view	Top view
Sharp edge at top of preparation will act as a stress point.	If the coping replicates this preparation, it will have a sharp internal corner.