Orthodontics
What this domain tests
Orthodontics tests growth, classification, space analysis, biomechanics and retention. The exam asks for diagnosis and treatment sequencing rather than appliance details.
Core rules
- Angle classification: Class I molar relationship; Class II when the mandibular first molar is distal; Class III when it is mesial. Skeletal class is assessed cephalometrically, commonly with ANB.
- Normal overjet and overbite are about 2-3 mm. Increased overjet raises trauma risk.
- The maxilla grows until about 12-14 years; mandibular growth continues later. Treatment timing depends on skeletal maturity.
- Space analysis in the mixed dentition may use Moyers prediction; arch length discrepancy determines extraction versus non-extraction decisions.
- Anchorage is maximum, moderate or minimum; extraoral or skeletal anchorage may be needed.
- Light continuous forces are preferred; force magnitude, duration and center of resistance determine tooth movement.
- Retention addresses relapse from gingival fibres, growth and tongue thrust; fixed or removable retainers are selected with a review plan.
Worked example
A 12-year-old has Class II division 1 malocclusion with 8-mm overjet, competent lips and no crowding. Cephalometric ANB is 6 degrees. The problem is skeletal and dental; treatment may need growth modification or camouflage depending on remaining growth. Extracting premolars without assessing skeletal pattern would be poor sequencing.
Traps
- Do not treat Class III early without confirming growth status; some cases worsen.
- Ectopic canines need radiographic localisation, not just observation.
- Retainer wear is not indefinite for every case; follow the retention plan.
- Do not confuse overjet with overbite.