An impression tray in dentistry is a device used to hold and control impression material while creating a mold of the patient’s oral structures. This mold is then used to fabricate restorations like crowns, bridges, or dentures. While stock trays come in pre-made sizes, a custom impression tray is specially fabricated for an individual patient’s unique anatomy. Using a tray designed for a specific mouth ensures the impression material is confined to a uniform thickness, which is essential for achieving the high dimensional accuracy required for final restorations. This process reduces the volume of material needed and minimizes the potential for distortion, leading to a better fit for the final appliance.
The Use of Spaced Custom Impression Trays
The design of a spaced custom impression tray incorporates a controlled gap, or spacer, between the tray’s surface and the patient’s tissue. This spacer, often a layer of wax about 2 to 3 millimeters thick, is placed over the preliminary cast before the tray is constructed. The gap accommodates a sufficient bulk of impression material, which is necessary to prevent tearing and distortion.
Spaced trays are typically used with bulkier materials like polysulfide or alginate in final impression techniques. Because these materials have lower tear strength, they require uniform thickness to remain intact and dimensionally stable upon removal. The tray design often includes small perforations, or holes, to mechanically retain the impression material and allow excess material to escape.
This type of tray is often used when making final impressions for edentulous arches, meaning arches with no teeth. Maintaining a uniform layer of material helps manage the hydraulic pressure applied to the soft tissues during the impression process. Tissue stops are small areas where the spacer is removed, allowing the tray to contact the tissue and ensure stable seating while maintaining the intended space for the material.
The Use of Close-Fitting Custom Impression Trays
A close-fitting custom impression tray is designed to rest in near-direct contact with the underlying oral tissues, requiring no wax spacer or only minimal relief. Before fabrication, severe undercuts on the cast are blocked out with wax to ensure the tray can be removed without locking onto the model. This design is engineered for use with highly accurate, low-viscosity impression materials.
Materials such as vinyl polysiloxane (PVS) or polyether are used with close-fitting trays because they are effective in a very thin layer. The minimal space between the tray and the tissue confines the impression material to a thickness often less than one millimeter. This constraint allows for precise reproduction of minute surface details.
The close-fitting tray is the standard choice for fixed prosthodontics, which involves creating restorations like crowns and bridges. Its ability to capture precise marginal detail is critical, as the fit of the final restoration depends on an accurate record of the preparation finish line. These trays are also used for final impressions in dentulous arches, where the presence of teeth prevents the flexible tissue displacement seen in edentulous cases.
The Use of Relief and Window Custom Impression Trays
The relief and window custom impression tray is a modification of the close-fitting design, created for specific anatomical challenges. This tray features an area of extra space, or relief, or an opening known as a window, cut into the tray body. This specialized feature is needed when a patient has highly mobile or “flabby” soft tissue, often found in the anterior maxilla of long-term denture wearers.
When conventional impression techniques are used on flabby tissue, the hydraulic pressure from seating the tray and material can compress and distort the mobile ridge. This distortion leads to an inaccurate impression and a poorly fitting denture that lacks stability. The window in the tray is positioned directly over the mobile tissue area.
The window allows the flabby tissue to be recorded without excessive compression, as the impression material flows through the opening after the rest of the impression is seated. This technique ensures the final denture base will rest passively on the mobile tissue in its undistorted, natural position. This design manages localized areas of unsupported tissue, ensuring the stability and retention of the final prosthesis.