Dental Materials and Impression Techniques Quiz

Impression Making Part 2

• Impression materials are categorized as non-elastic, elastic, and non-elastic.
• Non-elastic materials are primarily used for removable dental prosthetics. Hydrocolloids, like reversible (Agar) and irreversible (Alginate), are examples. Addition silicone is common for crown and bridge impressions.
• Elastic materials include hydrocolloids (reversible and irreversible), synthetic elastomers (polysulphides, polyether, silicones), and condensation and addition silicone.
• Reversible hydrocolloids, introduced in 1925, are made of agar, borax, and water. The material can be reversed and reused by heating it. initial cost is for the apparatus and water-cooled impression trays needed. Patients may find this type less comfortable due to the bulky cooling apparatus and long setting time. Immediate pouring is necessary to avoid distortion. The material is fluid which allows for fine detail but it is low in tear resistance. Agar-agar is liquid between 50-100°C, turning to gel below 50°C.
• Irreversible hydrocolloids were introduced in 1941, made of soluble salt of alginic acid. This produces a calcium alginate gel. Flavors are often added to make them more palatable to patients. They are cheaper than elastomers, but easily distorted and have low tear resistance. They are mainly used to make study models and Cr&Br, only for the opposing arch.

Impression Materials: Elastic Materials

• Polysulphides: Introduced in the 1950s, made of polysulfide, sulfur, and lead dioxide. This gives it a brown color. It has an unpleasant odor but is the least expensive type of elastomer. Reactions produce water as a byproduct which causes shrinkage (0.5% - 1.5%). It has a long setting time (around 10 minutes) and should be poured within 1 hour to reduce distortion. The less volume, the more accurate the impression.
• Polyether: Introduced in the 1960s. It's made of unsaturated polyether, plasticizer, and filler. Setting time is 3-6 minutes. It's costly but does not produce byproducts. It can absorb moisture from the air and swell. For this reason, it needs to be poured within 1 hour. It features high accuracy and tear resistance, but also has a bitter taste.
• Silicones: First silicone impression materials were introduced in the 1950s. Two types exist: condensation and addition silicone. Condensation silicone (lab putty): used for making copies of dentures. It is hydrophobic and needs a dry field. It is more difficult to pour with gypsum stone. The pouring needs to be done within one hour, using a surfactant. Good tear resistance and excellent elastic recovery/detail reproduction. Must be mixed with gloves. Addition Silicone (PVS): introduced in the 1970s, also known as polyvinyl siloxane. It is made from silicone polymer, filler, and cross-linking agent(s), and activator. It's more expensive than condensation silicone but is dimensionally stable. Setting time is 4-6 minutes (variable). Less affected by temperature and humidity. Polymerization shrinkage is 0.3%-0.6%, and additional setting time is possible. It produces Ethyl alcohol.

Impression Making: Tray Selection

• Stock trays are low-cost and readily available. They feature larger quantities, potentially rigid or partially rigid materials (e.g., metal, plastic), and generally standard sizes. They are difficult to adapt.
• Custom trays take longer to create but are more tailored to individual patients' needs. Fabrication needs laboratory procedures, smaller quantities, and are rigid acrylic materials. They are adjustable with acrylic burs and sizes are tailored to the patient's specific primary cast.

Impression Techniques

• Two-stage technique: Putty and wash
• One-stage technique: Double mix.
• Monophase technique: Single mix.

Two-Stage Technique

• The putty is used to modify the stock tray into a close fitting one.
• The next step is using a lighter and more flowable wash material. The wash material is placed inside the tray and re-seated in the mouth. This technique is also referred to as putty and wash.
• Steps: select and verify the stock tray intraorally, apply tray adhesive to the tray, isolate prepared teeth, and apply retraction cord, mix equal amounts of base and catalyst of the heavy body, mix into a homogenous color, form a cylinder shape, place this cylinder in the tray. Avoid overfilling. Seat intraorally and hold still until fully polymerized. Once set remove from the mouth. Trim excess material with a blade. Create venting channels, use a straight, low-handpiece to remove undercuts, and test the tray intraorally to confirm easy reseating. Variations include using polyethylene as a spacer or taking the first impression prior to tooth preparation.
• Steps for two stage technique include checking the stock tray intraorally, and applying tray adhesive, isolating prepared teeth, and applying retraction cord, mixing heavy body material, forming a cylinder shape, placing it in the tray, avoiding overfilling, seating intraorally, holding, removing from mouth, trimming excess material, creating venting channels, using a handpiece to remove undercuts, checking tray reseating. Options are using a spacer and taking an initial impression before tooth prep.

One-Stage Technique

• Both heavy and light body impression materials are in the tray.
• Retraction cord is removed just prior to taking the impression as a single shot impression.
• This technique is faster than the two-stage process and can be used with custom or stock trays.
• Steps for one-stage technique include tray selection, applying tray adhesive, mixing heavy and light body materials, injecting light body over prepared teeth while the assistant loads the tray with heavy body, gently seats the tray, holds until set, and inspects for accuracy.

Monophase Technique

• One material (medium body) is used for a one-step impression.
• Custom made trays are used.
• Impression is seated and held until fully set.
• Accuracy is checked after removal.

Acceptable Impression Features

• Continuous finish line reproduction without gingival interference.
• Absence of tears, voids, or bubbles.
• At least 1 mm of uninterrupted cuff of material beyond the finish line.
• No mixing streaks.
• No gap between the impression and the tray.
• No exposure of the tray surface.

Errors

• Two-Stage Technique: Light body failure to cover the entire impression during seating can result in an imperfect master model, leading to inaccurate occlusal final prosthesis.
• One-Stage Technique: Poor blending of putty and wash materials can lead to horizontal crease lines. This is sometimes caused by differences in viscosity between the heavy and light putty, or improper setting time of the wash on the prepared teeth. Failure to seat the impression properly and improper injection technique of the light body can lead to a poor result.
• General: Inadequate moisture control resulting in shiny mesial prep margins, improper injection of light body resulting in voids, insufficient retraction cord or premature tray removal before material sets, drag marks in undercut areas, or tray separation from adhesive application or excessive pressure during removal.

Disinfection

• Rinse dental impressions under running water to remove debris.
• Immerse in the appropriate disinfectant for the manufacturer's recommended time.
• Avoid spray disinfectants; they reduce disinfection effectiveness and can cause inhalation hazards.
• Rinse impressions before sending to the lab.
• Alginate and polyether impressions can be temporarily covered in damp gauze. Elastomeric impressions can be immersed in the disinfectant solution. Many disinfectants are available commercially (e.g., chlorine, chlorhexidine, glutaraldehyde, phenolics).

References

• Contemporary Fixed Prosthodontics by Rosentiel, Land, and Fujimoto (chapter 14).