ABO Analysis

The ABO Analysis is the cephalometric evaluation protocol codified by the American Board of Orthodontics (ABO) for its clinical certification exam. It is not a single new set of landmarks invented by the Board; rather, it is a curated combination of well-established cephalometric measurements (drawn largely from the Steiner, Downs, and Tweed traditions — SNA, SNB, ANB, FMA, incisor angulations, and the E-line) that the ABO requires every candidate to trace and report in a consistent, standardized way.

Its clinical purpose is to give a concise, reproducible snapshot of a patient's skeletal pattern, dental compensation, and soft-tissue profile so that treatment outcomes can be judged against recognized norms. Because it is the reference protocol used in board certification, it has become a de facto standard that many orthodontic practices use for case documentation and quality review, independent of certification itself.

What It Evaluates

The ABO Analysis looks at the lateral cephalogram (side-view skull X-ray) and answers three practical questions: Where are the jaws positioned relative to the skull base and to each other (skeletal pattern)? Are the incisors tipped forward, backward, or well-compensated for that skeletal pattern (dental compensation)? And how does the vertical growth pattern and soft-tissue profile look in relation to accepted norms? All measurements are traced relative to the Frankfort Horizontal plane, and craniofacial superimpositions are performed on the anterior cranial base so that pre- and post-treatment films can be compared consistently.

Learn more: The Evolution of the Cephalometric Superimposition Techniques from the Beginning to the Digital Era: A Brief Descriptive Review (PMC)

Key Measurements

  • SNA — Angle from Sella to Nasion to Point A; describes how far forward or back the maxilla sits relative to the cranial base. Norm: approximately 82° ± 3°.
  • SNB — Angle from Sella to Nasion to Point B; describes how far forward or back the mandible sits relative to the cranial base. Norm: approximately 78° ± 3°.
  • ANB — The difference between SNA and SNB; summarizes the anteroposterior relationship between maxilla and mandible. Norm: approximately 2°, with values above ~4° suggesting a Class II skeletal tendency and values below ~0° suggesting a Class III tendency.
  • SN-MP / FMA — Angle between the mandibular plane and the anterior cranial base (SN) or Frankfort Horizontal (FMA); reflects the vertical growth pattern. Norm: FMA approximately 25° ± 4°; higher values indicate a more vertical (long-face) growth pattern, lower values a more horizontal (short-face) pattern.
  • U1-SN / U1-NA — Angular and linear position of the upper central incisor relative to the SN line and Point A; shows whether the upper incisor is proclined/retroclined and protruded/retruded relative to the maxilla. Norm: approximately 102° (angular) and 4 mm (linear).
  • L1-MP / L1-NB (IMPA) — Angular and linear position of the lower incisor relative to the mandibular plane and Point B; shows lower incisor compensation. Norm: approximately 90–95° (angular) and 4 mm (linear).
  • E-line (Ricketts esthetic plane) — Distance from the lower lip to a line drawn from the tip of the nose to the tip of the chin; describes soft-tissue lip projection.

Learn more: Key Cephalometric Values and Their Normal Ranges (Ortho Analyser)

Clinical Interpretation

An orthodontist reads the ABO tracing as a chain of relationships rather than isolated numbers. First, SNA and SNB establish where each jaw sits independently; ANB then combines them into a single skeletal classification (Class I, II, or III tendency). Next, FMA (or SN-MP) tells the clinician whether the case has room to work with vertically — a high FMA case tolerates less mechanical extrusion and is more prone to open bite, while a low FMA case tolerates more vertical control. Finally, the incisor angulations (U1-SN, U1-NA, L1-MP, L1-NB) reveal whether the dentition is already compensating for the underlying skeletal discrepancy — for example, retroclined lower incisors masking a true Class II skeletal pattern. The E-line measurement rounds out the picture by connecting these hard-tissue findings to the visible soft-tissue profile, which matters for both esthetic planning and case acceptance.

Learn more: Cephalometric Analysis 2 (Skeletal Sagittal Analysis via Steiner's Analysis)

References