Orthodontics and Dentofacial Orthopedics
This domain tests growth assessment, cephalometric and space analysis, biomechanical reasoning and retention planning. Examiners reward answers that tie the appliance to the diagnosis and to the patient's growth status.
Diagnosis
- Cephalometric norms: SNA about 82 degrees, SNB about 80 degrees, ANB about 2 degrees. Class II skeletal pattern means ANB above 2; Class III means a negative ANB.
- Growth assessment uses cervical vertebral maturation staging; peak mandibular growth typically occurs around the CS3 stage, which is the window for functional appliances and face mask protraction.
- Space analysis includes arch length discrepancy, Bolton tooth size analysis and mixed dentition prediction.
Mechanics
- Anchorage is classified as maximum, moderate or minimum; temporary anchorage devices, headgear and transpalatal arches provide maximum anchorage.
- The moment-to-force ratio determines whether a tooth tips, translates or controls root torque.
- Class II correction options: functional appliance, Forsus, Herbst, headgear, premolar extraction, and orthognathic surgery in adults.
- Class III in a growing patient: rapid maxillary expansion with face mask protraction; in adults, camouflage or surgery.
Retention
Fixed lingual canine-to-canine retainers, Essix and Hawley appliances. Relapse arises from periodontal fibre reorganisation, late growth and tongue posture.
Worked example
A nine-year-old has an anterior crossbite with a narrow maxilla and a cephalometric ANB of minus 2 degrees. This is a growing Class III with maxillary transverse deficiency. Treatment is rapid maxillary expansion followed by face mask protraction during the growth spurt. The same patient as an adult would need camouflage or orthognathic surgery, so timing is the decisive variable.
Traps
- Extracting premolars before locating an impacted canine.
- Missing an ectopic first permanent molar.
- Applying heavy forces that cause root resorption and anchorage loss.
- Planning no retention at all.