Orthodontics
Orthodontic questions test cephalometric interpretation, classification, and the mechanical or growth-based reason for a treatment decision.
Diagnosis
- SNA about 82 degrees, SNB about 80 degrees, ANB about 2 degrees; ANB above 4 degrees suggests skeletal Class II, below 0 degrees skeletal Class III.
- Angle classification uses first molars and canines; Class II division 1 has proclined incisors, division 2 retroclined central incisors with deep overbite.
- Space analysis: arch length discrepancy, with Tanaka-Johnston or Moyers prediction of unerupted canines and premolars.
Biomechanics and appliances
- Tooth movement is a periodontal ligament response: light continuous force for intrusion and translation, heavier force tips the tooth; the centre of resistance determines the movement type.
- Anchorage is maximum, moderate or minimum; temporary anchorage devices give absolute anchorage.
- Functional appliances modify growth, mainly during the pubertal spurt at cervical vertebral maturation stage 3.
- Headgear restricts maxillary growth; clear aligners suit mild to moderate crowding with good compliance.
Retention
Relapse risk persists; a fixed lingual retainer plus a removable retainer is common, and the periodontal fibre response explains early relapse.
Worked example
A 12-year-old has 10 mm overjet, ANB 7 degrees, Class II division 1 and cervical vertebral maturation stage 3. The ANB and the growth stage mean a functional appliance can reduce overjet through restraint and redirection of maxillary growth. A non-growing adult with the same overjet would need camouflage extractions or orthognathic surgery.
Traps
Reading ANB without considering soft tissue and incisor inclination; confusing dental and skeletal Class II; treating after the growth spurt; forgetting to localise an impacted canine before expansion.