EFDA - Expanded Function Dental Auxiliary Impression Materials and Techniques Questions and Answers — Questions and Answers
Question 1: An EFDA is taking a final impression for a single crown. Upon removing the impression, they notice a "drag" or "pull" distortion near the margin of the prepared tooth. What is the most likely cause of this specific error?
- Premature removal of the tray from the mouth.
- Moisture contamination from saliva or blood.
- The impression material began to set while the tray was being seated. (Correct answer)
- The tray was removed with a slow, teasing motion.
Correct answer: The impression material began to set while the tray was being seated.
A "drag" or "pull" occurs when the impression material has started its setting reaction but is disturbed before it is complete. Seating the tray while the material is already partially set forces the material to flow under pressure, causing internal stress and distortion.
Question 2: Which of the following impression materials is classified as an irreversible hydrocolloid and is primarily used for diagnostic casts, preliminary impressions for custom trays, and opposing arch models?
- Alginate (Correct answer)
- Polyether
- Polyvinyl siloxane (PVS)
- Reversible hydrocolloid (Agar)
Correct answer: Alginate
Alginate is an irreversible hydrocolloid material. Its ease of use and lower cost make it ideal for applications where high precision is not the primary requirement, such as study models (diagnostic casts) and preliminary impressions.
Question 3: An EFDA has just taken an alginate impression. To ensure maximum dimensional accuracy before pouring the model, what is the most appropriate next step?
- Leave the impression on the counter to air dry for 15 minutes.
- Submerge the impression in a bowl of warm water until it can be poured.
- Wrap the impression in a dry paper towel to absorb excess moisture.
- Disinfect the impression, rinse, and pour it immediately or store it in 100% humidity. (Correct answer)
Correct answer: Disinfect the impression, rinse, and pour it immediately or store it in 100% humidity.
Alginate impressions are highly susceptible to dimensional changes. They can lose water and shrink (syneresis) or absorb water and swell (imbibition). The best practice is to pour the impression immediately after disinfection. If pouring must be delayed, storing it in a sealed bag with a damp paper towel (100% humidity) is the next best option to minimize distortion.
Question 4: A dentist requires a highly accurate and dimensionally stable final impression for a three-unit bridge. The material must have excellent tear strength and allow for multiple pours of the model if needed. Which material best fits this description?
- Polysulfide
- Polyvinyl siloxane (PVS) (Correct answer)
- Alginate
- Zinc oxide-eugenol (ZOE)
Correct answer: Polyvinyl siloxane (PVS)
Polyvinyl siloxane (PVS), also known as addition silicone, is known for its superior dimensional stability, high accuracy, and excellent tear strength. Its stability allows impressions to be poured long after they are taken and enables multiple accurate casts to be made from a single impression.
Question 5: When an EFDA dispenses material for a bite registration, what is the primary goal of this procedure?
- To record the occlusal relationship between the maxillary and mandibular arches in centric occlusion. (Correct answer)
- To create a detailed model of the patient's gingival contours.
- To serve as a mold for fabricating a provisional restoration.
- To capture the undercuts of the teeth for designing a removable partial denture.
Correct answer: To record the occlusal relationship between the maxillary and mandibular arches in centric occlusion.
A bite registration is taken to accurately record how the patient's upper and lower teeth occlude (come together). This record is essential for the dental laboratory to correctly mount the stone casts on an articulator, ensuring the final prosthesis will have the correct bite.
Question 6: An EFDA is assisting with a final impression using a polyether material. Which of the following is a key characteristic of this material that must be considered when handling the final impression?
- It is highly hydrophobic and repels all moisture.
- It has a very long setting time, often exceeding 15 minutes.
- It is very rigid when set and can be difficult to remove from the mouth, especially around undercuts. (Correct answer)
- It must be poured within 5 minutes due to poor dimensional stability.
Correct answer: It is very rigid when set and can be difficult to remove from the mouth, especially around undercuts.
Polyether impression materials are known for their rigidity and stiffness once set. While this property contributes to their accuracy, it can make tray removal challenging, especially in patients with significant undercuts, long teeth, or periodontal involvement. Care must be taken during removal to avoid torquing the tray or causing patient discomfort.
