CDT Orthodontic Appliance Construction 3 — Questions and Answers
Question 1: Which material is used to invest wire components before processing acrylic for a removable appliance to protect them from displacement?
- Plaster of Paris slurry
- Sticky wax and baseplate wax (Correct answer)
- Zinc phosphate cement
- Alginate
Correct answer: Sticky wax and baseplate wax
Sticky wax and baseplate wax are used to secure wire components to the model in their correct positions so they remain stable when the acrylic dough is applied.
Question 2: A lower lingual arch space maintainer is soldered to which tooth type to maintain arch length after premature primary tooth loss?
- Mandibular central incisors
- Mandibular first premolars
- Mandibular first or second permanent molars (Correct answer)
- Mandibular canines
Correct answer: Mandibular first or second permanent molars
The lower lingual arch is soldered to bands on the mandibular first or second permanent molars, which serve as stable anchor teeth to prevent mesial drift.
Question 3: When constructing a Nance palatal arch, the acrylic button is placed to contact which anatomical area?
- The labial frenum
- The rugae area of the anterior hard palate (Correct answer)
- The soft palate
- The buccal mucosa adjacent to the molars
Correct answer: The rugae area of the anterior hard palate
The Nance button rests on the rugae of the anterior hard palate, using tissue contact as anchorage to resist mesial movement of the upper molars.
Question 4: What is the gauge range most commonly used for stainless steel wire when fabricating the labial bow of a Hawley retainer?
- 0.3–0.4 mm (26–28 gauge)
- 0.7–0.8 mm (19–21 gauge)
- 0.5–0.6 mm (22–24 gauge) (Correct answer)
- 1.0–1.2 mm (17–18 gauge)
Correct answer: 0.5–0.6 mm (22–24 gauge)
The labial bow of a Hawley retainer is typically constructed from 0.5–0.6 mm (22–24 gauge) stainless steel wire, balancing flexibility for passive retention with sufficient rigidity.
Question 5: Which finishing step is essential before delivering a removable orthodontic appliance to the clinician to prevent soft tissue irritation?
- Sandblasting the acrylic surface
- Polishing with pumice and rag wheel to a smooth, glossy finish (Correct answer)
- Etching the wire components with acid
- Steam cleaning the entire appliance
Correct answer: Polishing with pumice and rag wheel to a smooth, glossy finish
Polishing the acrylic with pumice followed by a high-gloss polish removes surface scratches that would harbor bacteria and irritate the mucosal tissues.
Question 6: The Z-spring incorporated into a removable orthodontic appliance is used primarily to accomplish which tooth movement?
- Intrusion of a single tooth
- Labial tipping of a palatally positioned incisor (Correct answer)
- Rotation correction of a canine
- Distalization of a molar
Correct answer: Labial tipping of a palatally positioned incisor
The Z-spring (or double cantilever spring) applies a labially directed tipping force to a single tooth, most often used to correct a palatally displaced incisor.
Question 7: When using self-curing (cold-cure) acrylic for orthodontic appliance repair or additions, what mixing ratio by volume is generally recommended?
- 3 parts powder : 1 part liquid
- 1 part powder : 3 parts liquid
- Equal parts powder and liquid
- 2 parts powder : 1 part liquid (Correct answer)
Correct answer: 2 parts powder : 1 part liquid
A 2:1 powder-to-liquid ratio by volume produces a mix with appropriate consistency and physical properties for self-curing orthodontic acrylic.
CDT Orthodontic Appliance Construction 3
Which material is used to invest wire components before processing acrylic for a removable appliance to protect them from displacement?