Chapter 46 Notes

>The three classifications of impressions taken in dental procedures consist of preliminary, final, and occlusal, also referred to as a (bite) registration. >type of dental material selected by the dentist to take these impressions will depend on what will be constructed from the impression.

>Preliminary impressions are taken by the dentist or by the EFDA and are used to create a reproduction of the teeth and surrounding tissues. >preliminary impression is used for the following reasons: •Diagnostic models •Custom trays •Provisional coverage •Orthodontic appliances •Pretreatment and posttreatment records

>Final impressions are taken by the dentist and are used to produce the most accurate reproduction of the teeth and surrounding tissue. >A final impression provides the dentist and the dental laboratory technician with essential information needed for the creation of indirect restorations, partial or full dentures, and implants.

>Bite registrations are taken by the dentist or the EFDA to produce a reproduction of the occlusal relationship of the maxillary and mandibular teeth when the mouth is occluded. >This provides the dentist and the laboratory technician with an accurate registration of the patient's centric relationship between the maxillary and mandibular arches.

>Impression trays are used to hold the impression material within the mouth. These trays must be sufficiently rigid to (1) carry the impression material into the oral cavity (2) hold the material close to the teeth (3) avoid breaking during removal (4) prevent warping the completed impression. >Impression trays are of two basic types: stock trays and custom trays.

Stock trays are manufactured in several ways and are available in a range of sizes and styles.

>Impression trays are selected for size by trying the tray in the patient's mouth before taking the impression. >The correct tray accomplishes the following: •Is comfortable for the patient •Extends slightly beyond the facial surfaces of the teeth •Extends approximately 2 to 3 mm beyond the third molar, retromolar, or tuberosity area of the arch •Is sufficiently deep to allow 2 to 3 mm of material between the tray and the incisal or occlusal edges of the teeth

>Impression trays are supplied in one of the following ways: •Quadrant trays, which cover half of an arch •Section trays, which are used to cover the anterior portion of the arch •Full-arch trays, which cover the entire arch >An impression tray is also characterized by whether the surface of the tray is perforated or smooth. >perforated surface tray is used, the impression material oozes through the holes of the tray, creating a mechanical lock to hold the material in place. >smooth surface tray is used, there is no mechanical lock, so the interior of the tray is painted or sprayed with an adhesive to hold the impression material securely in the tray.

>the depth or length of the tray can be extended by adding utility wax to the border of the tray. >This addition might be necessary if the tray does not completely cover the posterior teeth >For a patient with an unusually high palate, softened utility wax can be added to the palate area of the impression tray.

>A custom tray is constructed to fit the mouth of a specific patient. >The dental assistant or the laboratory technician constructs the custom tray in the laboratory from a diagnostic model made from a preliminary impression of the arch before the dentist has prepared the teeth.

>important that an adhesive be dry before the impression material is loaded into the tray. >If the adhesive has not had time to dry, the impression will pull away from the tray, causing deformity in the impression. >Paint the tray with the adhesive at least 15 minutes before use >Specific adhesives used with specific impression materials are as follows: •VPS adhesives (blue)—used with polyvinyl siloxane (polysiloxane) and polyether impression materials •Rubber base adhesives (brown)—used with rubber base impression materials •Silicone adhesives (orange-pink)—used with silicone impression materials

>Hydrocolloid (hydro- meaning water and colloid meaning a gelatinous substance) is a material used to obtain preliminary and final impressions. >Depending on the type of hydrocolloid, the physical change from sol (solution) to gel (solid) can be irreversible (changed by chemical factors) or reversible (changed by thermal factors).

>Hydrocolloid impression materials that cannot return to the sol state after they become a gel are termed irreversible hydrocolloids. >Alginate is the irreversible hydrocolloid most widely used for preliminary impressions.

>The main components of alginate include the following: •Potassium alginate, derived from seaweed, is used as a thickening agent. This ingredient is used in some ice creams for the same reason. •Calcium sulfate reacts with the potassium alginate to form the gel. •Trisodium phosphate is added to slow down the reaction time for mixing. •Diatomaceous earth is a filler that adds bulk to the material. •Zinc oxide adds bulk to the material. •Potassium titanium fluoride is added so as not to interfere with the setting and surface strength of the product used when making the model.

>Hydrocolloid impression materials have two physical phases. First, in the sol (solution) phase, the material is in a liquid or semi-liquid form. >Second, in the gel (solid) phase, the material is semisolid, similar to a pudding dessert. The gel strength of hydrocolloid is not as great as that of elastomeric impression materials.

>important for alginate to be sufficiently strong to resist tearing when the impression is being removed from the patient's mouth. >strength of the material will continue to increase even after it appears to be set. >leaving the impression in the mouth for the full length of time recommended by the manufacturer is important toward achieving maximum strength.

>Containers about the size of a coffee can are the most common form of packaging. >Premeasured, individual packages are more expensive than other types of packaging, but they save time by eliminating the need for measurement of the powder. >The material is also supplied with flavoring and changes color when set. >Alginate can deteriorate very quickly if exposed to elevated temperatures and moisture, causing the material to not set or to set too rapidly. >The shelf life of alginate is approximately 1 year. >Most alginate impression materials must be \"poured up\" within 1 hour of taking the impression—a requirement dictated by the environment. Because so much of the material is made from water, a slight change in the surroundings can distort the impression and cause dimensional change. >If an alginate impression is stored in water or in a soaked paper towel, the alginate will absorb the additional water and expand. This condition is called imbibition. >If an alginate impression remains in the open air, moisture will evaporate from the material, causing it to shrink and distort. This condition is called syneresis.

>Alginate is available in two settings: normal set and fast set. The types of alginate refer to the working time and the setting time. Normal-set alginate has a working time of 2 minutes and a setting time of up to 4 1/2 minutes after mixing. Fast-set alginate has a working time of 1 1/4 minutes and a setting time of 1 to 2 minutes. >Working time is the time allowed for mixing the alginate, loading the tray, and positioning the tray in the patient's mouth. >Setting time is the time required for the chemical action to be completed, after which the impression is ready to be removed from the patient's mouth. >No difference in the completed impression has been noted between the two setting types of alginates. >The decision as to which type to use is based on time-related factors, such as the following: •Difficulty in seating the impression tray (normal set allows more time for insertion and placement of the tray) •Whether or not the operator is working alone (normal set allows the operator more time to mix the alginate, load the impression tray, and seat the tray in the mouth) •Patients with a severe gag reflex (fast set allows the tray to be removed from the mouth much sooner)

>Room temperature water (21° C/70° F) is best to use when mixing alginate. >using cooler water will lengthen the setting time. >Warmer water will shorten the setting time of the procedure.

>It is important to accurately measure the alginate powder and the water to be mixed. To help ensure accuracy, the manufacturer supplies a plastic scoop for dispensing the bulk powder and a plastic cylinder for measuring the water. >ratio 1:1 (powder and water) >•An adult mandibular impression generally requires two scoops of powder and two measures of water. •An adult maxillary impression generally requires three scoops of powder and three measures of water.

>The most common mixing technique used for alginate materials is manual spatulation with a rubber bowl and beavertail wide spatula. >An additional way that is available is the use of an alginator, which is a rubber flexible bowl attached to an electrical component with a low or high speed.

It is important for the EFDA to be competent in mixing the alginate, loading the tray, and keeping the patient comfortable while taking the impression.

>Before an impression is taken, the procedure must be explained to the patient to ensure his or her comfort. >The patient needs to know the following: •The material will feel cool, there is no unpleasant taste, and the material will set quickly. •Breathing deeply through the nose will help the patient relax and be more comfortable. •The patient can use some type of hand signal to communicate any discomfort.

An acceptable alginate impression must meet specific criteria as follows: •The impression tray should be centered over the central and lateral incisors. •There is a complete \"peripheral roll,\" which includes all the vestibular areas. •The tray is not overseated (pushed down too far), which would result in exposure of areas of the impression tray. •The impression is free from tears or voids (holes). •Sharp anatomical detail of all teeth and soft tissues is provided. •The retromolar area, lingual frenum, tongue space, and mylohyoid ridge are reproduced in the mandibular impression. •The hard palate and tuberosities are recorded in the maxillary impression.

Impressions of Edentulous Arches Taking an alginate impression of an edentulous arch differs from taking other alginate impressions in two ways: (1) The height of the teeth is missing (2) it is important to include more extensive tissue details. >An edentulous tray is used to take this impression. This tray is not as deep as other trays used for alginate impressions. >attaching sticky wax or similar material to the edges will modify the borders of the tray. This modification allows border molding, also known as muscle trimming, to achieve closer adaptation of the edges of the impression of the tissues in the mucobuccal fold.

>Hydrocolloid impression materials that change physical states from a sol to a gel and then back to a sol are called reversible hydrocolloids. >A change in temperature causes the reversible hydrocolloid material to transform from one physical state to another. >Hysteresis is a term used to describe the change in temperature that causes the reversible hydrocolloid material to transform from one physical state to another. >Reversible hydrocolloid material is approximately 85 percent water and 13 percent agar. >The following three compartments maintain water at three different temperatures: For the reversible hydrocolloid to change from one consistency to another, a specialized conditioning bath is used. The following three compartments maintain water at three different temperatures (Fig. 46-9): •The first bath is for liquefying the semi-solid material. Immersing the tube of material in a special water bath called a hydrocolloid conditioner at 212° F (100° C) liquefies the material. After liquefying, the preset thermostat automatically cools the temperature to 150° F (65.5° C). •The second bath becomes a storage bath that cools the material, readying it for the impression. At this temperature, the tubes are waiting for use. •A third temperature in a separate bath is kept at 110° F (44° C) for tempering the material after it has been placed in the tray.

The material needs enough viscosity to keep it from flowing out of the tray when dispensed at 150° F (65.5° C). An impression tray is filled and then immersed in the 110° F (43.3° C) bath for a few minutes to further increase viscosity and reduce the temperature to a comfortable level for the patient.

>The syringe material is liquefied by placing it in the same 150° F (65.5° C) storage bath as the tray material. Because it requires easy flow, the syringe material is never tempered at 110° F (43.3° C). As the small stream flows through the dispensing needle, it cools to a comfortable temperature. >If tempered, the syringe material would be too thick for accurate placement and would gel too quickly, preventing proper displacement of contaminating fluids and bubbles.

>The dentist and the dental assistant must coordinate the following steps to ensure that the impression will be as accurate as possible: •A stock water-cooled tray is selected, making sure that the tray does not impinge on any of the teeth or soft tissues. •Plastic stops are placed in the tray, to help keep the tray from sticking to the teeth. •Tubing is connected to the tray and to the water outlet for drainage. •The material is liquefied and is moved to the storage bath (second bath). •The light-bodied material is placed in the syringe; the heavy-bodied material is placed in the tray and is moved to the tempering bath (third bath). •The light-bodied material is expressed around the prepared tooth, and the dentist seats the tray. •A stock water-cooled tray is selected, making sure that the tray does not impinge on any of the teeth or soft tissues. •Plastic stops are placed in the tray, to help keep the tray from sticking to the teeth. •Tubing is connected to the tray and to the water outlet for drainage. •The material is liquefied and is moved to the storage bath (second bath). •The light-bodied material is placed in the syringe; the heavy-bodied material is placed in the tray and is moved to the tempering bath (third bath). •The light-bodied material is expressed around the prepared tooth, and the dentist seats the tray.

Elastomeric impression materials are used when an accurate impression is essential. The term elastomeric indicates \"having elastic or rubber-like qualities.\"

Elastomeric impression materials are self-curing and are supplied as a base and a catalyst. The base is supplied as a paste in either tube, a cartridge with syringe tip, or as putty in a jar. The catalyst, also known as the accelerator, is packaged as a paste in a tube, in a cartridge, or as a liquid in a bottle with a dropper top.

Elastomeric impression materials generally are supplied in three forms: light-bodied, regular, and heavy-bodied.

Light-bodied material, also referred to as syringe-type or wash-type material, is used because of its ability to flow into and around the details of the prepared tooth. A special syringe, or extruder, is used to apply light-bodied material immediately around the prepared teeth.

Regular and heavy-bodied materials, also referred to as tray-type materials, are much thicker than light-bodied impression materials and are used to fill the tray. >Their stiffness helps to force the light-bodied material into close contact with prepared teeth and surrounding tissues to ensure a more accurate impression and details of a preparation.

•The material selected depends on the dentist's preference and the type of impression required for the procedure. •The dentist prepares the tooth (or teeth) for the impression. •The light-bodied material is prepared, loaded into the syringe, and transferred to the •The light-bodied material is prepared, loaded into the syringe, and transferred to the dentist. •The dentist places the light-bodied material over and around the prepared teeth and onto the surrounding tissues. •The heavy-bodied material is prepared, loaded into the tray, and transferred to the dentist. •When the impression material has reached final set, the impression is removed and is inspected for accuracy. •The impression is disinfected, placed in a biohazard bag, labeled, and readied for the laboratory technician.

>The curing reaction (polymerization), as the elastomeric material changes from a paste into a rubber-like material, begins as soon as the base and the catalyst are brought together. >The change occurs in a three-stage process: initial set, final set, and final cure. 1) Initial set results in stiffening of the paste without the appearance of elastic properties. The material may be manipulated only during this first stage. The mix must be completed within the limited working time specified by the manufacturer. 2) Final set begins with the appearance of elasticity and proceeds through a gradual change to a solid, rubbery mass. The material must be in place in the mouth before the elastic properties of the final set begin to develop. 3) Final cure occurs within 1 to 24 hours. Only slight dimensional change is noted in the details of the impression during this time.

When working with an elastomeric material in a paste system, timing is the most important reason for having the equipment and material prepared and ready to mix when the dentist signals the go-ahead. Make sure that you have enough material placed out for the type of tray you are using

>Automix systems are designed by manufacturers to complete the mixing process for the procedure. >The unique automix system device provides a homogeneous mix with the appropriate amount of material without waste. >The extruder is used to automatically mix and dispense elastomeric impression materials. >The unit can be used with light-bodied or heavy-bodied material and is operated with a trigger-like handle. >The extruder is loaded with dual cartridges that consist of a tube of catalyst and a tube of base material.

Several factors make this process easier: •Controlled dispensing—No excess material remains on the mixing pad. •Infection control—Less time is spent cleaning and disinfecting guns and cartridges. •Versatility—A variety of impression materials can be mixed and dispensed from this unit.

Putty impression material provides all the benefits of true putty, including a higher consistency and insertion force than are seen with heavy-bodied materials. Because the material is kneaded in the palms, putty is already warm when it is seated in the mouth; it helps warm up the syringe material, actually accelerating setting time without reducing the intraoral working time.

The four types of elastomeric impression materials most often used in dental practice are polysulfide, polyether, silicone, and polysiloxane (polyvinyl siloxane). Although these are similar in some aspects, each material has slightly different properties and characteristics. Of particular concern with these materials are dimensional stability and permanent deformation. •Dimensional stability is the ability of the material to keep its shape after it has been removed from the mouth. •Deformation is the ability of the material to resist permanent change caused by stresses during removal from the mouth. •Permanent deformation means that the material was changed and will not regain its previous shape. >The material is supplied as a two-paste system: the base and the catalyst. >Disadvantages of rubber base material are its strong odor and the fact that it can easily stain clothing. >Polysulfide material has relatively long working and setting times.

>Polyether impression material provides better mechanical properties than polysulfide and less dimensional change than silicone. >the set material is quite stiff, a third component called a thinner (or body modifier) is included. The thinner is added to the mix to reduce the thickness of the mix and improve the finished impression. >Polyether material is supplied as a two-paste system (the base and the catalyst) and as cartridges. The tubes are not the same size; however, when equal lengths are dispensed, the correct amount of each material is delivered.

>Condensation silicone is a material that is odor-free, nonstaining, and relatively easy to mix. >The deformation is much less than that of polysulfide, but the dimensional stability is superior. >Condensation silicone materials are supplied with the base as a paste in a tube, and the catalyst as a liquid in a bottle or a smaller tube of paste, cartridges, and putty.

>Polysiloxane (polyvinyl siloxane) materials have high dimensional stability and low tear resistance, making them easy to handle when taking final impressions. The material has no taste or odor, which makes it acceptable to patients. >Polysiloxane impression material is available in light-bodied, regular, and heavy-bodied forms. It is supplied in cartridges and in putty consistencies for single-impression or double-impression techniques.

>to having an accurate impression of the prepared teeth, the dentist and the laboratory technician must obtain an accurate registration of the normal centric relationship of the maxillary and mandibular arches. >This relationship is recorded as the occlusal registration, which is most often referred to as the bite registration.

>A wax bite registration is used to show the occlusal relationship of the maxillary and mandibular teeth. >useful when the diagnostic casts are trimmed. The easiest technique employs a softened baseplate wax.

>One of the most popular materials used for bite registrations is polysiloxane. The material is supplied as a paste system and as cartridges. >Polysiloxane provides the following benefits for the dentist and the laboratory technician: •The material is fast-setting. •The paste has no resistance to biting forces. •The paste has no odor or taste for the patient. •Polysiloxane material gains dimensional stability over time. •The material is convenient to use.

When a more durable material is needed for a bite registration, a zinc oxide-eugenol (ZOE) material may be selected. ZOE paste has little to no resistance to bite closure and is a fast-setting material. The material is supplied in a paste system, which is dispensed onto a paper pad, mixed, and placed onto a gauze tray for the patient to bite into.