Impressions - Undercuts

Definition

Desirable vs. undesirable undercuts

Mosby's Dental Dictionary defines an undercut as "the portion of a tooth that lies between its height of contour and the gingiva, only if that portion is of less circumference than the height of contour."

Undercuts exist because of natural tooth morphology and contours. Aligners rely on these naturally-occurring desirable undercuts for normal retention.

Undesirable undercuts, however, are too retentive, making it difficult to remove impressions or aligners.

With undesirable undercuts, impression material can get locked in the patient's mouth because the material fills in between the teeth. When the impression hardens, there isn't enough flex to remove the impression. 

Causes

Some common causes of undesirable undercuts include:

• Gingival recession whereby a notch exists at the cementoenamel junction
• Under restored dental implants and bridges  
• Anomalous tooth morphology
• Overcontoured restorations
• Black triangles

Solutions

One of the most common techniques for dealing with undesirable undercuts is to block them out, try the following:

• Before taking impressions, fill in the undesirable undercuts with a soft material that will not bond to the impression (like wax). This will make the impression easier to remove, and also eliminate those undercuts from the resulting 3D model, making the aligners easier to remove as well.
• Do not allow the wax used to fill an undercut to obscure any actual anatomy, such as tissue or teeth.

Another technique is to trim the medium body material:

• Between the first and second steps of a two-step impression, use a knife to trim away the impression 2 millimeters above the gingival in the medium body material. This will ensure that the only material covering the undercuts is the flexible light body material, allowing the impression to be removed easily.