Orthodontics and Dentofacial Orthopaedics: Growth, Ceph and Biomechanics
Orthodontics and Dentofacial Orthopaedics
This domain tests cephalometric interpretation, Angle's classification, the biology of tooth movement, anchorage and appliance selection, and retention.
Core rules
- Cephalometric norms: SNA 82° ± 2, SNB 80° ± 2, ANB 2° ± 2. ANB above 4° suggests a skeletal Class II pattern, below 0° a skeletal Class III.
- Angle's classification is defined by the first molar relationship and confirmed by canines; Class I has the mesiobuccal cusp of the upper first molar in the buccal groove of the lower.
- Orthodontic force causes PDL compression, hyalinisation and a lag phase, followed by undermining resorption; light continuous force favours direct resorption and faster movement.
- Anchorage is the resistance to unwanted reaction force; loss shows as proclination of anterior teeth or mesial drift of posteriors. Extraoral appliances, palatal coverage and temporary anchorage devices conserve anchorage.
- Growth modification works best in the prepubertal patient: functional appliances and headgear for Class II, facemask for Class III maxillary deficiency.
- Retention is lifelong in principle; relapse arises from periodontal fibre recoil, occlusal factors and continued growth, so supracrestal fibreotomy and fixed retainers are used.
Worked example
A tracing gives SNA 84°, SNB 78°, ANB 6°. SNA above the norm means the maxilla is protrusive; SNB below the norm means the mandible is retrusive; ANB of 6° confirms a skeletal Class II relationship. Because both jaws contribute, the treatment implication is that masking the discrepancy by extraction alone will not correct the skeletal base in a growing patient, whereas a functional appliance may, since the mandible has growth potential remaining. The reasoning step is to always interpret ANB as a relationship, then check which jaw is responsible.
Traps
- Reading ANB alone and concluding maxillary protrusion without checking SNA and SNB separately.
- Applying functional appliances after the pubertal growth spurt.
- Confusing anchorage loss with active tooth movement.