Dentofacial Orthopedics and Orthognathic Surgery
Domain 5: Dentofacial Orthopedics and Orthognathic Surgery
The exam tests orthopedic force systems in growing patients and the surgical-orthodontic workflow in adults. Expect questions on facemask, expansion, chin cup, surgical VTO, and stability.
Core rules
- Maxillary protraction with a facemask applies anterior and inferior force to the maxilla; it is most effective before mid-puberty in Class III with maxillary hypoplasia. Force is commonly 300-500 g per side, worn 12-16 hours daily.
- Rapid maxillary expansion is orthopedic in children and mainly dental in adults; in adults, surgically assisted RPE is needed for transverse deficiency.
- Chin cup therapy is controversial; it may restrain mandibular growth but has limited evidence and relapse risk.
- Orthognathic surgery requires a surgical VTO, model surgery or digital planning, and a splint. Common procedures: Le Fort I osteotomy for maxillary advancement or impaction; bilateral sagittal split osteotomy for mandibular advancement or setback; genioplasty for chin position.
- Stability: maxillary advancement is relatively stable; mandibular setback has more relapse; large advancements and counterclockwise rotations increase relapse risk.
- Timing: wait for growth completion, confirmed by serial cephalograms or hand-wrist films, before definitive surgery.
Worked example
A 22-year-old woman has Class III malocclusion, negative overjet 4 mm, maxillary hypoplasia, and a concave profile. Cephalometrics: SNA 76°, SNB 84°, ANB -8°, SN-MP 32°. The VTO shows that Le Fort I advancement of 4 mm and BSSO setback of 3 mm achieve positive overjet and improve the profile. The reasoning includes decompensation: presurgical orthodontics removes lower incisor compensation to allow the planned skeletal correction. The surgical plan must account for nasal changes and upper lip support after maxillary advancement. The patient is warned about relapse and the need for retention.
Traps
- Operating before growth is complete in a patient with remaining mandibular growth.
- Planning surgery from the cephalometric tracing alone without model or digital surgery.
- Forgetting presurgical decompensation; the occlusion will not fit at surgery.
- Ignoring the soft tissue response and the patient's expectations.