Impression Trays in Dentistry

Impression Trays

• Impression tray: A receptacle or device used to carry impression material to the mouth, confining the material to the surfaces to be recorded, and controlling the impression material while it sets, forming the impression.
• Requirements for impression trays:
  • Rigid but not overly thick.
  • Retains its shape during construction and impression pouring.
  • Simple construction method for minimal time and cost.
  • Easily trimmed or thinned with burs, mounted stones, or scissors.
  • Smooth to prevent injury to oral tissues.
• Impression tray materials:
  • Special autopolymerizing acrylic resin.
  • Conventional autopolymerizing acrylic resin.
  • Thermoplastic resin sheets.
  • Thermoplastic shellac baseplate materials.
  • Vacuum or pressure-adapting devices.
• Autopolymerizing resin impression trays:
  • Easy to use and no special equipment required.
  • Can be thin but rigid.
  • Easily modified by grinding and smoothing to polish.
  • Two methods: sprinkle-on and finger-adapted dough.
• The sprinkle-on method:
  • Commonly used for acrylic resin baseplates, also for resin impression trays.
  • Dentist specifies amount and placement of relief based on the impression material (e.g. sharp bones, hyperplastic tissues).
• Procedure for creating an impression tray (using a cast):
  • Making an outline of the impression tray on the cast.
  • Blocking out severe undercuts.
  • Adapting a layer of baseplate wax to the cast for relief.
  • Placing tissue stops on the relief wax.
  • Painting a tinfoil substitute on the stone cast and the relief wax.
• Procedure for maxillary cast:
  • Scribe red line at the depth of the vestibule.
  • Scribe blue line 2mm above the red line.
  • This determines tray extension.
  • Block out undercut areas like the frenum, buccal surfaces of tuberosity, rugae, and flabby alveolar ridges.
• Mixing and curing the impression tray:
  • Sift powder polymer onto the cast and relief wax.
  • Saturate with liquid monomer until a 2mm thick uniform coating is created.
  • Cure under an inverted bowl or pressure pot.
• Finishing and Handling:
  • Incorporating a handle (4mm thick and 8mm long) from mixed dough resin.
  • Wetting the resin attachment point with liquid monomer aids in bonding.
  • Removing tray from the cast after setting.
  • Trimming with a bur.
  • Finishing and polishing to eliminate discomfort for the patient.
  • Storing the finished tray on the cast.
• Problem areas for the sprinkle-on method:
  • Difficulty controlling the thickness.
  • Tilting the cast during the process can compensate for the flow characteristics.
  • Severe undercuts pose breakage risk
  • Finger-adapted dough method is preferable for modified acrylic tray resins.
• Finger-adapted dough method:
  • Extensive use in making resin trays.
  • The resin tray material is shaped to a dough to be thinned and adapted to the cast.
  • Procedure:
    • Place the tray outline on the cast.
    • Block out the teeth with base plate wax.
    • Adapt a layer of base plate wax over the cast then trim.
    • Paint tinfoil on the cast and the relief wax.
    • Proportion the auto-polymerizing resin and mix it.
    • Immediately before reaching the dough stage, remove a portion of the resin mix and place in border area of cast and roll the resin.
    • Hand-adapt the material carefully to avoid over-thinning.
    • Remove excess tray material from the cast borders.
    • Form the excess material into handles. Smooth area around the handle, use liquid monomer to improve bonding.
    • Make handles small for shorter finish time.
    • Continue adaptation to prevent rebound.
    • Cure the tray on a bench or in a bowl/pressure pot.
    • Remove, trim, smooth and finish borders.
    • Use teeth that project out from the cast as handles.
    • Remove any block out
    • If required, perforate the tray for elastic impression material
    • Store on the cast.
    • Problem areas include difficulty maintaining tray thickness, overall bulk.
• Vacuum-adapted method:
  • Uses thermoplastic sheets.
  • Requires specialized equipment (vacuum or pressure machine) to adapt the sheet to the cast.
  • Different colors, thicknesses, and flexibility options available.
• Shell-ac method:
  • Obsolete due to material instability.
• Impression trays for immediate dentures:
  • Presence of teeth alters tray method (e.g. traditional vs. immediate)
  • Posterior teeth are sometimes removed to create a healing period to make an impression of the posterior region, making a denture, and then removing anterior teeth.
• Full-arch impression trays:
  • Three methods used (finger-adapted dough, vacuum, sprinkle-on)
• Custom posterior trays:
  • Another method to tray over posterior edentulous area of the case,
  • Another impression then is made over teh teeth and anterior vestibules with a stock tray
  • Can be made with finger-adapted dough, sprinkle-on, or vacuum-adapted methods
• Two-piece trays:
  • Method using two impression trays.
  • One part first over posterior edentulous cast, then the second stage fits over the first.
• Additional Procedures (general):
  • Placing outlines on casts
  • Beading outlines on casts with a sharp instrument for trimming,
  • Blocking wax where necessary for relief,
  • Utilizing tinfoil,
  • Mixing resin in a suitable container.
  • Adapting the material to the cast.