Impression Materials Chapter 14

Impressions Impression material is used primarily to reproduce the form of the teeth in a negative reproduction. This includes existing restorations, hard and soft tissues, and preparations. Impression materials are also used to fabricate prostheses and other oral devices.

Types of Materials Several types of impression materials are available. Typically, a material must be in a moldable or plastic state that can adapt to the teeth and tissues. An impression tray is used to transport the material to the mouth. The material then sets to a semisolid or plastic state.

Impression Trays Impression trays are used to carry impression materials to the mouth. They are used to support the material once it is placed into the mouth. Trays are rigid to prevent distortion of the material once it is removed from the mouth. Stock trays or custom-made trays may be used.

Impression Trays (cont’d) Some trays are made of metal, others are made of acrylic or plastic. Impression trays may cover the whole arch or just a segment of the arch. A check-bite or triple tray is used to take a registration of the patient’s bite, that is, to show how the teeth come together.

Hydrocolloids A colloid is a glue-like material composed of two or more substances in which one substance does not go into solution but is suspended within another substance. Hydrocolloids are water-based colloids that function as elastic impression materials. Hydrocolloids used in dentistry include reversible and irreversible hydrocolloids.

Irreversible Hydrocolloid Also called alginate hydrocolloid or just alginate Most widely used impression material in dentistry Easy to manipulate Requires no special equipment Reasonably accurate

Uses Mouth guards Diagnostic models Preliminary impressions for working models Opposing models Custom fluoride trays Bleaching trays Diagnostic models Partial denture frameworks Repair of broken prostheses Fabrication of provisional restorations

Problems One of the reasons that alginate is not used for permanent prostheses is that it does not flow well. Alginate does not capture the fine detail necessary for accurate appliances. It is too thick to flow into the embrasures or to accurately show margins of preparations.

Composition The main active ingredient in alginate is potassium or sodium alginate. It accounts for approximately 15% to 20% of the powder content. It is produced from seaweed derivatives. Other components include Calcium sulfate dihydrate Potassium sulfate Trisodium phosphate

Composition (cont’d) When alginate powder is mixed with water, calcium sulfate dihydrate reacts with sodium alginate to form calcium alginate. Calcium alginate is insoluble and causes the solution of mixed powder and water to gel. Typically, the chemical reaction is rapid, but alginate manufacturers add a retardant to slow down the process.

Working Time Regular-set alginates have a working time of 2 to 3 minutes. Fast-set alginates have a working time of 1½ to 2 minutes. Working time starts when the materials—powder and liquid—are joined. The longer one takes to mix the material, the less time one has to load and seat the tray.

Setting Time Regular-set alginates take from 2 to 5 minutes to set completely. Fast-set alginates take from 1 to 2 minutes to set completely. Setting time may be adjusted by altering water temperature. The impression is left in the mouth beyond the appearance of set to reduce tearing and distortion.

Deformation Alginate will be compressed when it is removed from undercuts in the mouth. The greater the compression, the more likely it is that the alginate will be permanently deformed. The ideal thickness of the impression is 2 to 4 mm. Thin alginate deforms and tears easily.

Removal When removing the impression from the mouth, one should use a rapid snap movement to decrease deformation. The impression must be kept wet until the assistant is ready to pour up the impression in dental plaster (gypsum). Impressions also must be disinfected.

Stability The impression must be kept wet. Once removed from the mouth, the impression should be rinsed and disinfected. If the impression loses too much moisture, it will shrink and become distorted.

Tear Alginate mixed with too much water will be weaker and more likely to tear. Thin sections are also prone to tear. Slow removal from the mouth may cause the material to tear.

Impression Making The objectives during impression making are To reproduce oral structures with acceptable accuracy To maintain proper infection control To maintain patient comfort

Tray Selection Stock trays work well with alginate because they leave plenty of room for the alginate. If the tray is too small, tray burn or incomplete reproduction could occur. If the tray is too large, fitting it into the patient’s mouth will be difficult.

Tray Selection (cont’d) A properly selected tray will cover all of the teeth and will extend into the facial and lingual vestibules without impinging on tissues. For a mandibular impression, the tray will extend over the retromolar area. In the maxilla, the tray should cover the maxillary tuberosity and extend into the hamular notch.

Tray Selection (cont’d) Stock trays do not always fit every person. Sometimes the trays must be modified. They can be heated to make them wider or narrower. Wax can be added to the rims to extend the tray.

Adhesive Even though most trays have retention holes in them, tray adhesive should be used to help keep the material in place. Adhesive should cover the entire inside of the tray right up to the borders. Be careful not to get the adhesive on you or your patient because it does not come off easily. Do not cross-contaminate the adhesive.

Dispensing Material Manufacturers supply measuring devices with their alginate. Material may be provided in bulk (can) or may be individually packaged. The material settles and becomes compact, so you will have to “fluff” the material. Measure and dispense the water. Add powder to the water.

Load the Tray Scoop out the mixture and add it to the tray, starting at the posterior and working toward the anterior. Smooth the surface of the mixture with wet fingers, and remove excess material.

Seating the Tray Once the tray is loaded, the operator should take some of the alginate from the bowl and wipe it across the occlusal surfaces. Always take the lower impression first. Stand in front of the patient and insert the tray, first one side and then the other, into the patient’s mouth.

Seating the Tray (cont’d) Place the front of the tray first, and work to the posterior. Have the patient raise and extend the tongue and then relax the tongue. Mold the borders by gently pulling on the lips and cheeks in an upward motion. Leave in place about 1 minute beyond setting. Remove quickly.

Seating the Tray (cont’d) When seating the maxillary tray, stand behind the patient with arms around the back of the chair. Seat the anterior of the tray first and then the posterior. Have the patient tilt the head forward. Muscle trim the borders by gently pulling down on the cheeks and lips. Remove the impression quickly.

Criteria for Clinically Acceptable Impressions All teeth and alveolar processes recorded Peripheral roll and frenum included No large voids or tray burn Good detail No debris No distortion Palatal vault recorded Retromolar area or tuberosity recorded

Disinfection Dental impressions should be considered contaminated. After the impression is removed from the mouth, rinse with cool water to remove debris. Place the impression in a plastic bag and then spray with a disinfectant. Seal bag and let set for 30 minutes before pouring.