Orthodontics and Dentofacial Orthopedics
What this domain tests
Growth and development, cephalometric analysis, classification of malocclusion, biomechanics, and the principles of appliance selection and retention.
Core rules
- Angle's classification uses the first permanent molar relationship: Class I (mesiobuccal cusp of the upper first molar in the buccal groove), Class II (mesiobuccal cusp mesial to the groove), Class III (distal to the groove).
- Skeletal pattern is described by SNA (maxilla to cranial base, mean 82°), SNB (mandible, mean 80°) and ANB (difference, mean 2°). An ANB above 4° suggests a skeletal Class II; a negative ANB suggests Class III.
- Space analysis: arch length discrepancy, Moyers' mixed dentition prediction, and the leeway space (about 1.5 mm per side in the upper and 2.5 mm per side in the lower arch from the primary molars to the premolars).
- Anchorage is the resistance to unwanted tooth movement; it is classified as maximum, moderate or minimum, and is conserved by methods such as transpalatal arches, headgear, or mini-implants.
- Retention is lifelong in principle; the periodontal and dentogingival fibres remodel slowly.
Worked example
A patient appears Class I in the molar region but has proclined upper incisors and an overjet of 9 mm. Reasoning: the molar relationship alone does not define the case. Check SNA, SNB and ANB; if ANB is above 4°, this is a skeletal Class II with a dental Class I camouflage. Treatment differs fundamentally from a Class I bimaxillary case.
Traps
- Classifying malocclusion by the canine relationship when the question specifies Angle's molar classification.
- Forgetting that the leeway space is larger in the lower arch.
- Treating a skeletal discrepancy with fixed appliances alone and expecting a stable outcome.