Alginate Impressions & Diagnostic Casts

Discussion Diagnostic casts are a supplement to the oral examination. A. Correct B. Incorrect

Background Information A recurring requirement in the dental office is for study models or diagnostic casts of patient’s teeth and adjacent tissues. Various requirements necessitate these models, the most common being for diagnostic purposes. Models are also required in fabricating crowns and dies for prosthodontic appliances.

A diagnostic cast is an accurate replication of the anatomic form of the maxillary or mandibular dental arch showing the relationships of the remaining teeth and the surrounding tissues. Diagnostic casts are a supplement to the oral examination, but in some ways they can reveal more.

Diagnostic casts permit inspection from perspectives that are impossible to obtain when looking in the patient’s mouth. Diagnostic casts permit extended observations and comparasions far beyond the patient’s endurance for holding his/her mouth open and are available for study during the patient’s absence.

Diagnostic casts are used in educating the patient about his/her dental needs and corresponding treatment plan.

Discussion An alginate impression is a ____________ mold of the teeth. A. Positive B. Negative

Three steps of obtaining diagnostic casts An alginate impression is made directly in the patient’s mouth producing a negative mold. The impression is poured in dental stone, producing a positive cast. The cast is inspected for defects, repaired if possible, and trimmed to the proper size and shape on a model trimmer.

Physical Properties of Alginate Alginate is an impression material that is supplied in powder form and mixed with water. When set, the material is a flexible gel resembling rubber. Its most important characteristic is its ability to rebound from stresses.

Composition of Alginate Alginate is classified as irreversible hydrocolloid. Hydrocolloid because it consists of particles of a gelatinous (colloidal) state in water (hydro) and irreversible because once it has jelled it cannot be returned to a liquid solution.

The alginate particles are compromised of: Salts of alginic acid, a product of marine kelp An activator, usually calium sulfate A retarder such as sodium phosphate An inert filler to reduce stickiness, produce a smooth texture and add strength Flavoring

Packaging of Alginate Bulk form, the most popular Preweighted package, the more desirable choice when temperature change and moisture are of concern. All types of alginate packaging must be stored in a cool, dry place.

Alginate is sensitive to higher temperature and moisture contamination. Those variables cause the alginate to lose strength during mixing and be more apt to distort upon removal from the mouth.

Mixing Alginate Fluff or shake the powder to distribute the powder particles evenly Use the specific measuring devices (water & powder) provided by the manufacturer for mixing Follow the manufacturer’s direction regarding the ratio of water to powder. Use exact measurements

After the water and powder have been measured, place the water in a clean dry bowl. Sift the powder into the water. Adding the powder to the water ensures the powder particles are wet evenly. If mixed in reverse (the water is added to the powder) the chemical reaction will start early with some particles setting faster than others.

Mix the alginate for the specific amount of time and using a stiff spatula “swipe” the alginate mass against the sides of the bowl to avoid entrapment of air in the mix.

Discussion Care should be taken in shaking a “dustless” can of alginate. A. Correct B. Incorrect

Setting Time of Alginate Since alginate reacts or gels chemically, temperature is a major factor in the setting time. The colder the temperature of the water the longer it takes to set; conversely, the higher the water temperature the faster it sets. Ideal water temperature is 68 degrees or room temperature.

Discussion So… increasing the temperature of the water used to mix alginate material will: A. Decrease the setting time B. Increase the setting time

When ideal conditions exists a fast-setting type of alginate will gel in 1 to 2 minutes and a normal type gels in 2 to 4.5 minutes. The fast-setting type is the most widely used.

Dimensional Stability Alginate has a tendency, after it sets to lose (syneresis) or absorb (imbibition) water, depending on the atomosheric conditions surrounding it.If conditions are dry it loses water & shrinks; if immersed in water, it imbibes moisture or swells.

Advantages of Alginate It makes an accurate impression It allows for undercuts The process is not time consuming It is easy to work with, has good viscosity & is low in adhesive qualities It causes no great pain to the patient Inexpensive to the dentist Requires little armamentarium

Alginate Impression Procedure Assemble the armamentarium Patient drape Impression trays Flexible rubber bowl Stiff spatula Saliva ejector Alginate of choice Utility wax Disinfecting agent

Seat & drape the patient Operator should wear appropriate PPE Instruct patient of procedure Position the patient upright & inspect mouth Lubricate the patient’s lips Select the appropriate trays

Install the wax Mold the wax to the vestibule Have patient rinse mouth Measure water & test temperature Fluff alginate in can & measure into cup Prepare the alginate Load tray (always taking the mandibular arch first)

Semi-dry the teeth & mucosa Position & seat the tray Remove excess alginate Hold the tray in place for the prescribed time Remove the tray by breaking the seal at the periphery then giving a quick pull

Remove remaing pieces of alginate from mouth & allow patient to rinse Rinse the impression Evaluate the impression Trim away excess alginate Take wax registration bite Disinfect impression

Label impression & store in humidor Repeat procedure for opposite arch Pour impressions immediately after

Criteria for Selecting Appropriate Trays Tray clears all tissues by at least 4 mm. Tray is long enough to cover part of retromolar pads or tuberosity Tray sides fall at least 4 mm short of periphal turn Tray sides do not grossly depress any frenum Patient should not feel pain or excessive pressure

Evaluation Criteria All detail is reproduced, including the complete peripheral turn and a portion of the retromolar pads or tuberosities Detail is sharp, not blurred or indistinct Free of voids in critical areas Free of large folds of alginate No areas where alginate has pulled away from the tray

Free of rips and tears except in interproxinal areas Alginate thoroughly covers the tray, no tray visible through alginate Free of bulges or depressions that indicate a sub-surface bubble Alginate is smooth, not sponge-like