Orthodontics and Dentofacial Orthopedics
This domain tests growth and development, diagnosis with classification and cephalometry, biomechanical planning, appliance selection and the management of retention - the chain that turns a malocclusion into a stable result.
Growth and development
The maxilla grows by sutural apposition and surface remodelling; the mandible grows mainly by condylar endochondral ossification with remodelling of the ramus and posterior displacement. Cervical vertebral maturation stages tell you whether a growth spurt is imminent - the window in which functional appliances work best. Leeway space in the mixed dentition (about 1.5 mm per side in the mandible and 0.9 mm in the maxilla) and the E space are key to predicting crowding.
Diagnosis
Angle's classification describes first permanent molar relationships as Class I, II or III, with division 1 or 2 for Class II incisor inclination. Cephalometric values commonly quoted include SNA around 82 degrees, SNB around 80 degrees and ANB around 2 degrees, with incisor-to-maxillary and incisor-to-mandibular plane angles in the mid-100s and low-90s respectively - always interpret against the local normative values used in your training. Space analysis quantifies arch length discrepancy.
Biomechanics and appliances
Optimal tooth movement depends on the moment-to-force ratio delivered: translation needs a higher ratio than controlled tipping, which needs more than uncontrolled tipping. Anchorage - maximum, moderate or minimum - must be planned before extraction decisions. Options include fixed pre-adjusted edgewise appliances, clear aligners for suitable cases, functional appliances (Twin Block, Herbst), extraoral traction, protraction facemask and rapid palatal expansion in growing patients.
Worked example
An 11-year-old presents with Class II division 1, 10 mm overjet, a deep overbite and no dental crowding. Reasoning: the profile and overjet are skeletal as well as dental. With a maturating cervical vertebral stage indicating imminent growth, a functional appliance is used during the spurt to restrain maxillary growth and advance the mandible, followed by fixed appliances for detailing. If the patient were non-growing, camouflage or orthognathic surgery would be considered instead.
Traps that cost marks
- Planning extractions without an anchorage assessment.
- Relying on ANB alone when the skeletal pattern is borderline or when the reference points are hard to identify.
- Ignoring retention: relapse risk is lifelong, so bonded retainers and removable retainers must be planned from the outset.
- Applying heavy forces which cause hyalinisation and undermine tooth movement rather than accelerate it.