Cephalometrics and Imaging
What the exam tests
Landmark identification, standard analyses, and when to move from 2D imaging to CBCT or other modalities, plus superimposition for growth and treatment change.
Core rules
- Landmarks: Sella, Nasion, A-point, B-point, ANS, PNS, porion, orbitale, gonion, menton, gnathion, pogonion; errors at porion and orbitale are the most common.
- Steiner norms: SNA 82°±2, SNB 80°±2, ANB 2°±2, SN–mandibular plane 32°, U1–NA 22° and 4 mm, L1–NB 25° and 4 mm, interincisal 131°.
- Tweed: FMA 25°, IMPA 90°, FMIA 65°; Downs, Ricketts, McNamara, and Jarabak provide alternative frameworks.
- Wits appraisal measures A-point and B-point projected onto the occlusal plane; norms near 0 mm (slightly negative in females) and it is less affected by SN inclination than ANB.
- Superimposition: cranial base (Björk) for overall change, maxilla on palatal plane, mandible on symphysis and canal.
- CBCT indications: impacted canines, airway, asymmetry, root resorption, TAD placement, surgical planning; justify dose against benefit.
Worked example
SNA 82°, SNB 82°, ANB 0°, but the patient has a concave profile, anterior crossbite, and Wits −5 mm. Reasoning: ANB is normal because N-point and SN orientation mask the AP discrepancy; the negative Wits confirms a skeletal Class III relationship relative to the occlusal plane. Plan should address the skeletal base, not just incisor camouflage.
Traps
- Quoting norms without age, sex, or ethnic context.
- Forgetting that ANB depends on N-point position and SN inclination.
- Tracing rotated or poorly positioned radiographs.
- Requesting CBCT without a clinical question.