Single-Unit Crown– Dental Impressions

Analog vs. Digital Impressions

Analog impression materials for crowns largely use elastomeric materials – vinylpolysiloxane (VPS), polyether or polysulfide. These contain a base and catalyst and are available in multiple viscosities. For digital impressions, an intra-oral digital scanner is used. Which method is most frequently used? In short, it’s still analog impressions*. Plus, alginate substitutes are often used for pre-operative impressions by clinicians who do take digital impressions. An analog impression is needed when:

• The peripheral soft tissue must all be captured to enable fabrication of a removable denture that provides for a peripheral seal and retention.
• Deeper subgingival preparations are involved, and complete moisture control is difficult to achieve.
• A patient has a small mouth and/or limited opening and cannot accommodate a large digital scanner, or the procedure would be difficult.
• A digital impression may not capture all the needed occlusal information for a complex restorative case.
• Scanning and exporting/transmission constraints may preclude use of a given digital scanner in full mouth rehabilitation cases involving extensive implant treatment.

Critical Details needed in an Impression for a Well-fitting Crown

For a well-fitting crown with no internal, marginal, or external deficiencies, the impression must capture all areas of the preparation, the adjacent and opposing teeth, and the adjacent soft tissue and the sulcus. Adjacent and opposing teeth must be captured to deliver a crown with accurate proximal width and shape, and an accurate, functioning occlusion. The sulcus, preparation margins and area of the tooth up to 2 mm apical to the preparation margin are critical. Accurately capturing these areas is essential for accurate margins, a good emergence profile, and to maintain periodontal health.

Common Errors with Analog Impressions and How to Avoid These

Poor input = poor output, just as with computers! In analog impressions, common errors include bubbles, voids, tears, drags and pulls, dimensional changes, lack of ‘blending’ of materials with different viscosities, and contamination. Fortunately, these errors are avoidable!  

Bubbles and voids occur when air or fluids are trapped in the impression material. Here’s how to avoid these issues:

1.  When using an air syringe to disperse wash material against a preparation, do so in a careful, quick manner.
2.  Keep the small dispensing tip in the sulcus while applying wash material along the entire circumference of the preparation.
3.  Continue delivering wash material when removing the dispensing tip from the sulcus.
4.  Apply wash material to adjacent teeth.  
5.  Ensure adequate moisture control.

Tears can occur when insufficient wash material is present, the material inherently has poor tear strength, or it did not reach full tear strength because it was prematurely removed before the final set. Properly retracting soft tissue to allow for a sufficient bulk of impression material, choosing a wash material with high tear strength, and removing the impression only after the final set solves these issues. Lastly, the tray must be able to retain the impression during removal from the mouth to avoid it tearing away from the tray.

Drags and Pulls are caused by incorrect material and tray usage. To avoid these issues:

1.  Do not adjust the tray position intraorally once inserted.
2.  Don’t be tempted to remove an impression before the final set has been achieved – it will distort.  
3.  Select a suitable tray. Options for single-unit crown procedures include stock triple trays, quadrant trays, rigid full arch trays and custom trays. Only use triple or quadrant trays if the patient has a stable opposing bite, as otherwise tray movement will occur. When using triple trays, make sure the tray size allows the patient to bite in centric occlusion. 

Lack of ‘blending’ of wash and tray materials is another common error. Depending on the cause, this issue can be solved by making sure the tray is the correct size, having your assistant prepare the tray so that it is ready for you to insert as soon as your wash material application is completed, using the same manufacturer’s brand for both the wash and tray materials, and selecting a wash material with a longer working time so that it has not already started setting when the tray is inserted.

Dimensional changes can occur when a tray is too flexible, causing it to deform during insertion and rebound when the impression is removed – pulling the impression in with the tray such that it becomes too narrow bucco-lingually. This is particularly relevant for triple trays. Other reasons include using an inferior impression material with poor elastic recovery or poor stability. The solution is to make sure the tray is rigid enough and offers retentive features, to always use a tray adhesive and to choose a high-quality impression material.

If details are missing: Make sure the tray fits and is large enough to allow flow, while also containing the impression material to push it into the areas that need to be captured. There must also be sufficient impression material present. It should offer good flow and adaptation (viscosity), hydrophilicity so that it contacts the tissues and flows well even if the site is moist, and wettability that allows the material to ‘wet’ tissues, achieve close contact with them and capture all the details. Make sure the tray does not touch the teeth/adjacent soft tissue to avoid the tray showing through the impression.

Contamination with sulfur from latex-containing materials such as gloves or rubber dam can result in material failing to set. Contamination with dried-on fluid (e.g., blood) or astringents (e.g., peroxide) while impressions are being taken results in inaccuracies. To resolve these issues, use latex-free products, and make sure preparations are clean and free of debris.

It’s clear that choosing a high-quality material contributes greatly to clinical success by providing for easy and accurate mixing and delivery; suitable wettability, viscosity, and hydrophilicity; suitable working and setting times; and a rigid, rubbery impression with excellent dimensional stability. All of this takes precise chemistry! Of course, the impression material manufacturer’s instructions for use should always be followed for successful outcomes.