DMD Orthodontics 2 — Questions and Answers
Question 1: Which functional appliance is a fixed device that positions the mandible forward to correct a Class II malocclusion in a growing patient by altering condylar growth?
- Hawley retainer
- Herbst appliance (Correct answer)
- Rapid palatal expander (Hyrax)
- Lingual arch
Correct answer: Herbst appliance
The Herbst appliance is a fixed functional device that holds the mandible in a protruded position continuously, stimulating condylar adaptation and correcting Class II skeletal discrepancy in growing patients.
Question 2: The ANB angle in cephalometric analysis directly represents which skeletal relationship?
- Inclination of the maxillary incisors to the cranial base
- Vertical skeletal pattern
- Anteroposterior relationship between the maxilla and mandible (Correct answer)
- Mandibular plane angle
Correct answer: Anteroposterior relationship between the maxilla and mandible
The ANB angle (SNA minus SNB) reflects the anteroposterior discrepancy between the maxillary (A point) and mandibular (B point) bases; the norm is approximately 2°.
Question 3: A patient presents with a unilateral posterior crossbite and an apparent midline shift. Upon opening, the mandibular midline coincides with the maxillary midline. What is the most likely etiology?
- True skeletal mandibular asymmetry
- Functional shift caused by a premature occlusal contact (Correct answer)
- Bilateral maxillary constriction
- Condylar hyperplasia on the non-crossbite side
Correct answer: Functional shift caused by a premature occlusal contact
When the midlines align on opening but deviate in centric occlusion, it indicates a functional shift where a premature contact deflects the mandible laterally into a unilateral crossbite position.
Question 4: The Hyrax rapid palatal expander achieves skeletal expansion of the maxilla primarily through which mechanism?
- Dentoalveolar tipping of the posterior teeth
- Opening of the midpalatal suture to produce true skeletal widening (Correct answer)
- Stretching of the periodontal ligament fibers
- Resorption of the palatal cortical bone
Correct answer: Opening of the midpalatal suture to produce true skeletal widening
The Hyrax RPE opens the midpalatal suture through heavy orthopedic forces, producing true skeletal expansion of the maxillary arch — most effective before suture fusion in adolescence.
Question 5: In orthodontic mechanics, 'anchorage' is best defined as:
- The elastic force used to move teeth
- Resistance to unwanted or reactive tooth movement (Correct answer)
- The retentive phase following active treatment
- The bond strength of brackets to enamel
Correct answer: Resistance to unwanted or reactive tooth movement
Anchorage refers to the resistance offered by teeth, bone, or extraoral structures to the reactive (unwanted) forces generated during orthodontic tooth movement.
Question 6: Which bracket slot dimension is characteristic of the standard edgewise appliance and allows full three-dimensional control of tooth position including torque?
- 0.018 × 0.025 inch slot
- 0.022 × 0.028 inch slot (Correct answer)
- 0.018 × 0.018 inch slot
- 0.014 × 0.014 inch slot
Correct answer: 0.022 × 0.028 inch slot
The standard edgewise bracket uses a 0.022 × 0.028 inch rectangular slot, which accommodates large rectangular wires that allow precise torque and tip control in three planes of space.
Question 7: In orthodontic treatment planning, which factor most strongly contraindicates the use of functional appliances for skeletal correction?
- Mild crowding of the lower arch
- A Class II skeletal discrepancy of 4 mm ANB
- Completion of skeletal growth (closed epiphyses, stable cephalometrics) (Correct answer)
- Presence of all four first premolars
Correct answer: Completion of skeletal growth (closed epiphyses, stable cephalometrics)
Functional appliances rely on redirecting active skeletal growth; once growth is complete, they cannot produce significant skeletal change, making orthognathic surgery the appropriate alternative.
DMD Orthodontics 2
Which functional appliance is a fixed device that positions the mandible forward to correct a Class II malocclusion in a growing patient by altering condylar growth?