Dental Impressions and Special Trays Quiz

Impression Trays

• Impression trays are devices used to hold impression materials during procedures to create a cast of the mouth.
• Different types of impression trays exist, classified by arch (upper, lower) and type (stock, custom).
• Stock trays are pre-fabricated, while custom trays are made for a specific patient.

Impression

• An impression is a negative reproduction of an object, which is then used to create a positive likeness (cast) of the object.
• In dental prosthetics, an impression is a negative registration of the entire denture-bearing area.
• Impressions are taken to create a positive form of the oral tissues (cast).

Cast

• A cast is a positive reproduction of the form of the upper or lower arch tissues.
• It serves as a model for creating denture bases or other dental restorations.

Impression Tray Parts

• Handle: The handle of the tray does not interfere with the lip.
• Body: Consisting of a floor and flanges.

Types of Impression Trays

• Arch Related:
  • Upper tray: Features a palatal part and a vault.
  • Lower tray: Includes a lingual flange.
• Impression Type:
  • Stock trays: Pre-fabricated trays used for preliminary impressions.
  • Custom trays: Individually made for the patient, allowing for a more detailed and accurate impression and preventing errors.

Objectives of Impression

• 1. Preservation of Remaining Tissues:

  • Pressure is directly reflected in the denture base, potentially damaging soft tissues and causing bone resorption. Proper techniques minimize this impact.

• 2. Support:

  • Support is a crucial quality, opposing forces that dislodge the denture. Comprehensive coverage is important for stable support and distributing applied forces across a wide area.

• 3. Retention:

  • Resistance is needed to dislodge the denture, effectively keeping it firmly in place. Forces working against insertion (away from tissue) must be resisted.

• 4. Stability:

  • Resistance to movements (horizontal and rotational) trying to dislodge the denture.

• 5. Aesthetics:

  • Border thickness is adjusted according to residual ridge loss to maintain a natural facial contour.

Preliminary Impression

• An impression taken in a stock tray to make a study cast that is subsequently used to create a custom tray.

Study (Diagnostic) Cast

• Formed from a primary impression, serving for diagnosis and fitting.

Final Impression

• An impression produced by a custom tray, for detailed reproduction of the tissues enabling accurate construction of the master cast, which in turn serves as a basis for the denture construction.

Master Cast (Definitive)

• A cast created from a final impression to fabricate the prosthesis.

Factors Affecting Stock Tray Selection

• Presence or absence of natural teeth
• Impression material type
• Size of arch
• Material of tray fabrication (metal, plastic)

Stock Tray Requirements

• Rigid but not overly thick, to maintain shape.
• Adequate space between the ridge and the tray for comfortable insertion.
• Flanges that are sufficiently angulated to aid handling without distorting the lip contour.
• Short of vestibular extension, avoiding contact or interference with the vestibule.
• Rounded, smooth borders, preventing injury to the surrounding tissues.
• Suitable for modifications, if needed.

Stock Tray Modifications

• Bending: Metal trays can be bent with specialized tools.
• Cutting: Metal trays can be trimmed to properly fit a patient.

Addition of Moldable Material

• Moldable materials (wax, green compound) accommodate irregularities by filling areas for more accurate support.
• This process can create under-extension flanges in high palatal areas, increase tray support, and define better seal areas to prevent excess material from flowing back into the throat.

Advantages of Custom (Special) Trays

• Economical due to reduced material for impression.
• Comfortable, for patient and minimizes tissue displacement.
• Allows for accurate adaptation to the vestibules, increasing denture retention.
• More accurate impressions.

Requirements of Custom Trays

• Perfectly adapted to the primary cast and patient's mouth.
• Needs rigidity and stability.
• Easily removable from the cast.
• Does not react with impression material.
• Simulates the final denture's shape and size.
• Border molding, extends 2mm short of the sulcus, adds rigidity, is a suitable colour for the patient's mouth, fits all anatomical landmarks.

Steps of Custom Tray Construction

• Determining the sulcus and creating the periphery.
• Creating the tray outline short of the vestibular depth.
• The tray should be outlined in such a way as to accommodate the anatomical landmarks, for example, the maxillary posterior border is determined from the hamular notches, passing posterior to the fovea palatine. The entire retromolar pad should be included within the outline.
• Adapting the proper base-plate wax thickness and trimming.

Materials used for Custom Tray Construction

• Auto polymerizing (chemical cured) acrylic resin: Two techniques include finger adapted dough method and sprinkle-on acrylic method.
• Visible light-cure resin (VLC): Supplied in lightproof packages; molded to desired shape then exposed to high-intensity light.
• Vacuum-form vinyl polysiloxane sheets: A common method to construct custom trays.
• Pre-existing dentures (if applicable):

Zinc Oxide Eugenol Impression

• Used for taking impressions of the oral structures.

Digital Impression

• Taking an optical impression using digital capture units (intra-oral scanners). Images are captured and converted into a 3D model on a computer. The digital impression is a valid alternative to traditional impression trays.