Impression

A dental impression is a negative replica of the teeth, gums, and surrounding oral structures. When filled with dental stone or plaster, the impression produces a positive model (cast) that the lab technician uses to fabricate prosthetics, appliances, and restorations.

Why Impressions Matter

The impression is the foundation of every lab-fabricated prosthetic. Its accuracy directly determines the fit of the final product. A distorted, incomplete, or poorly captured impression leads to a prosthetic that doesn’t seat properly, requires excessive adjustment, or needs to be remade entirely.

For removable prosthetics in particular — dentures, partials, night guards — the impression must capture not just the teeth but the full tissue anatomy: the vestibular depth, the palatal contour, the retromolar pad area, and the mylohyoid ridge.

Impression Materials

Alginate (Irreversible Hydrocolloid)

The most widely used impression material for removable prosthetics. Alginate is affordable, easy to mix, and comfortable for patients. However, alginate impressions are dimensionally unstable — they shrink as they dehydrate and swell if submerged in water. Alginate impressions should be poured in stone within 30 minutes, or wrapped in a damp paper towel and sealed in a plastic bag if shipping to the lab.

Polyvinyl Siloxane (PVS)

A more accurate and dimensionally stable impression material. PVS can be shipped to the lab without dimensional change, making it ideal for cases that will spend time in transit. It’s available in multiple viscosities (heavy body for tray loading, light body for wash impressions) and captures finer detail than alginate.

Polyether

Similar in accuracy to PVS, polyether is rigid when set and captures excellent detail. It’s less commonly used for removable prosthetics than PVS but remains a reliable option.

Digital Impressions

Intraoral scanners capture the tooth and tissue surfaces digitally, eliminating the need for physical impression material. The scan data is sent electronically to the lab. While digital impressions are increasingly used for fixed restorations, their adoption for complete denture fabrication is still evolving due to the challenges of capturing soft tissue anatomy.

Common Impression Errors

• Pulls or tears — The material tore when removing the tray, distorting the anatomy. Usually caused by inadequate undercut management or premature removal.
• Bubbles — Air trapped between the material and the tissue creates voids in the impression that translate to bumps on the cast. Proper technique and material mixing minimize bubbles.
• Insufficient coverage — The impression doesn’t capture the full extent of the tissue-bearing area. The lab cannot fabricate proper denture flanges from an impression that stops short.
• Distortion from premature removal — Removing the tray before the material has fully set results in dimensional inaccuracy.
• Dehydration (alginate) — An alginate impression that sits unwrapped for even 15–20 minutes can shrink enough to compromise accuracy.

Tips for Better Impressions

1. Select the correct tray size — it should cover the entire area of interest with 3–4mm clearance
2. Use custom trays for final impressions in denture cases
3. Border mold custom trays to capture the vestibular depth
4. Pour alginate impressions immediately or wrap them properly for shipping
5. Include notes on the Rx about the impression material and any concerns about quality