Impression Trays and Casts

Impression Trays

Definitions

  • Dental Impression: A negative replica of intraoral anatomy, used to produce a positive replica (cast).

  • Cast: A positive replica of the intraoral anatomy.

  • Impression Tray: A tool to carry, confine, and control impression material.

Types of Impressions

  • Primary Impression: Made on the first visit using a stock tray to create a primary/diagnostic cast.

  • Secondary (Final) Impression: Made on a custom tray to create a final/master cast.

Types of Casts

  • Preliminary (Diagnostic) Cast: From a preliminary impression.

  • Final (Master) Cast: From a final impression.

Objectives of Impression Making

  1. Retention

  2. Stability

  3. Support

  4. Aesthetics

  5. Preservation of residual alveolar ridge and soft tissue

Impression Tray Components

  • Body: Floor and flanges.

  • Handle: L-shaped extension from the body's midline.

Impression Tray Requirements

  • Rigid but not too thick

  • Extends 2mm from the vestibule

  • Shape retention

  • Smooth, rounded borders

  • Handle doesn't interfere with lips

  • Modifiable

Stock Trays

Definition

Impression trays that carry and support impression material and are ready-made in various shapes and sizes.

Specifications

  • Used for multiple patients (primary impressions).

  • Made of materials like aluminum, stainless steel, tin, brass, or plastic.

Factors in Selection

  1. Type of impression material.

  2. Arch size.

  3. Arch form (round, square, taper).

  4. Coverage of anatomical landmarks.

  5. Sufficient space for impression material (4-5mm).

Types of Stock Trays

  1. According to teeth presence:

    • Dentulous: Flat floor, high flanges.

    • Edentulous: Rounded floor, short flanges.

    • Partially Edentulous: Flat floor/high flanges anteriorly, rounded floor/short flanges posteriorly

  2. According to arch size: Small, medium, large or Numbered

  3. According to arch shape: Full arch, part of arch, round, square, or V-shaped.

  4. According to tray material: Metal or non-metallic (plastics).

  5. According to impression material:

    • Plain (Non-perforated): For compound materials.

    • Perforated: For alginate and rubber base materials.

Stock Tray Modifications

  • Bending

  • Cutting flanges

  • Adding wax or compound

Custom Trays

Definition

Trays made on preliminary casts for making final impressions.

Requirements

  • Rigidity

  • Dimensional accuracy

  • Easy to modify/trim

  • Accepts border tracing material

Advantages Over Stock Trays

  • Accurate fit to patient's arches.

  • Reduces impression material bulk.

  • More comfortable for the patient.

Materials

  • Thermoplastic Materials: Shellac Base Plate, Plastic Sheets, Modeling Compound

  • Resins: Self Cure, Heat Cure, Light Cure

  • Metals: Casted, Swaged

  • Old Dentures

Custom Tray Techniques

  1. Mucostatic (non-pressure)

  2. Muco-compression (pressure)

  3. Selective pressure

Common Faults in Impression Making

  1. Poor tray selection

  2. Insufficient/excessive material

  3. Incomplete seating

  4. Excessive seating pressure

  5. Incorrect tray position

  6. Obstruction by oral structures

Cast Adaptation for Self-Cure Resin Trays

  • Close Fit Tray: Shorter from base of sulcus 2mm

  • Spacer (Shim) Tray: Used in mucostatic technique; shim thickness typically 2 mm with tissue stops.

Primary Impression Pouring (Primary Cast)

  • Plaster is mixed to a thick consistency and placed into one corner of the impression and allowed to flow around the impression to avoid trapping of air bubbles.

  • A putty of plaster is placed on a glass slab, filled impression is inverted on it. The plaster is pulled up around the impression with a plaster spatula or knife and then smoothed.

Final Impression Pouring (Master Cast)

  • Conventional Method: Same as primary cast.

  • Wax Boxing Method:

    • Impression is beaded using 4mm roll of beading wax which placed around the impression 3mm. below border & parallel to it.

    • Tongue space of lower impression is sealed by adapting a sheet of base plate wax 3-4 mm. below the border of the impression.

    • Sidewall 12mm of wax is built against beading wax to make a box into which stone is poured.

  • Plaster Boxing Method:

    • Plaster and pumice is mixed, placed on a glass slab and the impression is seated into it with the fitting surface upward.

Advantages of Boxing:

  • Produces a container into which stone can be poured.

  • It allows preservation of the borders of the impression.

  • Allows use of a mounting plate (permits master cast to be repositioned accurately on the articulator after the denture has been cured.

  • Allows vibration to get rid of air bubbles.

  • Boxing produce a dense accurate master cast of a pre- determined thickness.

  • Save stone material

Dental Cast Requirements

  • Outer & inner vestibule roll should be complete with dimensions of (2- 3mm depth & $2-3mm width)

  • The cast Edge extending out from this roll (land area), should be of $3- 4mm width

  • The side walls of a cast should be vertical.

  • Cast base should not less than 10 mm at the thinnest portion.

  • Base is trimmed so the plane of edentulous ridge is parallel to the base.

  • The tongue space on a mandibular cast should be flat & smooth.

  • Notches should be added to ease mounting on articulator