2-Basics of Dental Impressions, Working Casts, and Removable Dies Study Guide

LECTURE 2: BASICS OF MAKING THE IMPRESSION, WORKING CAST AND DIES

Lecturer: Prof. Dr. Nikola Gigovski

THE DENTAL IMPRESSION

After the preparation of the natural tooth crown is finished and the abutment is prepared, the next phase is taking the impression.

Definition of an Impression: An impression is a precise negative (copy) of the situation inside the patient’s mouth. It serves as a mold to create a positive replica.

Scope and Requirements of the Impression: The impression can capture the entire jaw or just a specific part, but for fixed prosthodontic restorations, it must always include:

  • The prepared tooth (or multiple teeth).

  • The edentulous space (in the case of bridges).

  • At least 11 to 22 adjacent teeth.

  • A portion of the gingival tissue.

Definition of a Cast: By pouring the impression (the negative), a cast working model (the positive) is obtained. This is an accurate replica of the intraoral condition.

IMPRESSION TRAYS AND MATERIALS

Impression taking is performed using a tray and impression material.

Components of the Tray:

  1. The body.

  2. The handle.

Tray Characteristics: Trays used for fixed prosthodontic restoration impressions are deeper compared to those used for edentulous alveolar ridges.

Tray Shapes by Jaw:

  • Lower Jaw Tray: Horse-shoe shaped; covers the teeth and alveolar ridge while providing space for the tongue.

  • Upper Jaw Tray: Covers the teeth and alveolar ridge, as well as the entire hard palate.

Classification of Trays by Size:

  • Full

  • Half

  • Quarter

  • Bite registration trays

  • Manufacturers mentioned: ALEER LIU, TONGDA

Primary Impression Materials:

  • Elastomers (for primary impressions and antagonists).

  • Alginate.

CLINICAL AND TECHNICAL REQUIREMENTS

From the perspective of the dental technician, the impression must meet the following criteria:

  • It must be free of defects.

  • It must have a clearly defined and visible preparation margin.

BITE REGISTRATION

Bite registration impressions can be taken with or without a tray.

Advantages of Bite Registration Impressions:

  • Antagonists are recorded simultaneously.

  • Economical and cost-effective due to minimal material usage.

Disadvantages of Bite Registration Impressions:

  • Cannot be mounted in an articulator.

  • Does not allow for the simulation of protrusive and lateral movements.

  • Morphology and degree of abrasion of opposing teeth cannot be assessed.

  • Potential for deformation during pouring because the connection between the oral and vestibular parts is thin and weak, making it prone to bending under the weight of gypsum.

IMPRESSION PROCESSING AND TIMING

Cleaning and Disinfection Protocol:

  1. Rinsing: The impression is first rinsed with running water to remove saliva, mucus, and blood residue.

  2. Disinfection: It is immersed in a disinfectant solution.

  3. Final Rinse: It is rinsed again after disinfection.

Preparation for Pouring: The impression should remain moist. This moisture acts as a barrier to prevent gypsum from sticking to the impression material and facilitates the flow of the mixed gypsum.

Critical Timing: IMPORTANT: The impression must be poured with gypsum (dental stone/plaster) between 20 minutes20\,\text{minutes} and 2 hours2\,\text{hours} after it is taken. This window allows for the stabilization of deformations that may have occurred during removal from the oral structures.

TYPES OF CASTS AND MATERIALS

Materials:

  • Dental Stone (Gypsum, Plaster).

  • Types: Alabaster, Hard, Extra Hard.

Cast Classification:

  1. Anatomical Cast: Used as an antagonist during fabrication. Usually poured with regular white alabaster dental stone.

  2. Study Cast: Used for analysis, case evaluation, and treatment planning for both jaws. Poured with alabaster gypsum. In fixed prosthodontics, these must be mounted on an articulator.

  3. Working Cast: Obtained from an impression taken after tooth preparation. The restoration is directly waxed and fabricated on this model. It must be poured using hard or extra-hard gypsum (dental stone).

POURING THE WORKING CAST

Step-by-Step Pouring Process:

  1. Mix extra-hard gypsum.

  2. Apply a small amount of gypsum to the most prominent area (distal part of the palate).

  3. Gently tap the impression so the gypsum flows into the lumen of the recorded teeth. This displaces water and air, preventing porosities.

  4. Requirement: The teeth and an equivalent of 22 to 3 mm3\,\text{mm} of the alveolar extension must be filled using extra-hard gypsum.

  5. Technical Enhancements: Better results are achieved using a vacuum mixer for the gypsum and a vibrator during the pouring process.

WORKING CASTS WITH REMOVABLE DIES

Modern laboratory fabrication (waxing, carving) requires "master" casts with removable working dies.

Why Removable Dies?

  • Provides easy access for modeling the artificial crown.

  • Allows precise control of the gingival margin, particularly on the mesial and distal sides where space is limited.

  • Essential for specific wax coping methods, such as the ADAPTA SYSTEM (using plastic foil) or the dipping method in molten wax.

METHODS FOR CREATING REMOVABLE WORKING DIES

1. Metal Dowel Pins (Ney-type Pins)

These are conical metal pins, flattened on one side. The head is grooved for gypsum retention. They may or may not include an alignment needle.

  • Placement with Alignment Needle: The pin is inserted into the center of the lumen of the prepared tooth impression vertically. Multiple pins must be parallel.

  • Placement without Alignment Needle: The pin is glued to a horizontal wire above the midpoint of the lumen. The grooved head must not touch the bottom of the lumen.

  • Timing: Pins must be placed before pouring the gypsum.

  • Retention: Retention metal rings with serrated edges are placed halfway into the gypsum (except around the pins). If rings are unavailable, small gypsum mounds can be used.

  • Isolation: The gypsum surface must be smooth and coated with an isolating agent once hardened.

  • Markers: A small wax ball is placed on the tip of the pin (or a wax strip for multiple pins) to help locate the pin in the base later.

  • Pouring the Base: The second phase involves pouring the base (22 to 3 cm3\,\text{cm} high) using white alabaster or a mix of hard gypsum, up to the level of the wax balls.

  • Finishing: After hardening, the cast is trimmed using a gypsum trimmer before cutting the dies.

2. Double Metal Pins

Recommended for crowns with milled components. They offer higher precision and stability after repeated removal/reinsertion.

3. Di-Lock Tray Model System (Working Cast Without Pins)

This system uses a plastic base with grooves on the bottom and inside surfaces.

  • Four protrusions on the bottom ensure consistent positioning.

  • A central opening allows compressed air to separate the model from the base.

  • The impression and plastic base are filled with gypsum and merged. Once set, air separates them, and the dies are cut.

4. Root Extension with Metal Pin (Without Separation)

  • Prepared teeth are outlined with a plastic strip.

  • Lumens and a small part of the ridge are filled with hard gypsum.

  • Metal pins are inserted into soft gypsum.

  • Once hardened, these are removed and the extension is trimmed with a milling machine into a shortened conical root shape.

  • The preparation margin is refined to leave a shoulder of approximately 0.2 mm0.2\,\text{mm}.

  • A longitudinal groove is made on the root extension to ensure alignment.

  • The die is isolated, returned to the impression, and the entire tray is poured to complete the cast.

5. Galvanization (Electroforming)

Highly accurate and strong casts made by metallizing the impression surface via electrolytic metal deposition.

  • Conductivity: Surfaces are coated with a conductive suspension (silver, copper, nickel, and aqueous colloidal graphite).

  • Process: Requires a bath, anode, cathode, electrolyte, and transformer. The process takes 88 to 12 hours12\,\text{hours}.

  • Coverage: Only the working area (abutment, adjacent teeth, and surrounding gingiva) is metallized.

  • Thickness: The resulting metal layer is approximately 0.3 mm0.3\,\text{mm}.

WORKING CAST WITH A MOVABLE PERIODONTIUM

This method simulates the surrounding gingiva to allow better control over the gingival margin shape, which is critical for preventing inflammatory reactions.

Fabrication Process:

  1. On the gypsum cast, parts around the abutment (gingival margin, interdental papilla, marginal gingiva 33 to 5 mm5\,\text{mm} down, and part of the ridge) are removed by grinding.

  2. A section of the original impression (containing the abutment and adjacent teeth) is cut out with a scalpel.

  3. New, different impression material is applied to this section (e.g., if the original was silicone, the new material is polyether to prevent bonding).

  4. The section is placed back on the ground-down working cast until it sets.

  5. The resulting "movable gingiva" is elastic and can be removed/repositioned.

SEPARATION OF THE REMOVABLE DIE

After pouring and opening the cast, the die must be sectioned.

  • Tools: A thin saw blade.

  • Technique: Cuts are made mesial and distal to the abutment, stopping at the base of the working cast.

  • Parallelism: The two cutting lines must be parallel or slightly converging as they go downward. If they diverge, the die will be "stuck."

  • Removal:     * For Ney pins: Remove wax markers and apply pressure from the back of the base.     * For Di-lock: The die is extracted directly from the base.