What Is the ABO Cephalometric Analysis?
The American Board of Orthodontics (ABO), founded in 1929, is the oldest specialty board in dentistry and the certifying body for orthodontics in the United States. Cephalometric measurement has long been part of the diagnostic records the Board expects a candidate to present and defend, and over time a recognisable core set of skeletal, dental, and soft-tissue values became associated with that standard of case documentation.
The ABO analysis in BCeph implements that core set as 10 measurements: five skeletal values that locate the jaws sagittally and vertically, four dental values that describe incisor inclination and position, and one soft-tissue value for the lower lip. It is deliberately compact — every measurement earns its place by answering a distinct diagnostic question, which is exactly what makes it useful as a board-style summary of a case.
Built from analyses you already know. The ABO set is not a new measurement system so much as a curated selection of the most decision-relevant values from the classic analyses. SNA, SNB, ANB, and the incisor distances mirror Steiner; FMA and IMPA mirror Tweed; the lower-lip-to-E-plane value follows Ricketts' esthetic line. In BCeph these share a single tracing, so the ABO values are identical to the source modules.
Skeletal Measurements (5)
The five skeletal measurements place the maxilla and mandible in both the sagittal and the vertical plane — the two dimensions that drive the skeletal diagnosis.
| Measurement | Definition | Norm | Clinical meaning |
|---|---|---|---|
| SNA | Angle at Nasion between lines S–N and N–A | 82° ± 2° | Maxillary position relative to the cranial base. >84° = prognathic maxilla; <80° = retrognathic maxilla. |
| SNB | Angle at Nasion between lines S–N and N–B | 80° ± 2° | Mandibular position relative to the cranial base. >82° = prognathic mandible; <78° = retrognathic mandible. |
| ANB | Arithmetic difference: SNA − SNB | 2° ± 2° | Sagittal jaw relationship — the primary Class I/II/III skeletal classifier. |
| SN-MP | Angle between the S–N line and the mandibular plane (Go–Me) | 32° ± 5° | Vertical growth pattern referenced to the cranial base. >37° = hyperdivergent; <27° = hypodivergent. |
| FMA | Frankfort-mandibular plane angle (Po–Or to Go–Me) | 25° ± 4° | Vertical growth pattern referenced to the Frankfort horizontal — cross-checks SN-MP. |
Reading ANB for skeletal classification
ANB is the single most consequential skeletal value in the set. It captures the sagittal discrepancy between the apical bases and maps directly to the skeletal classification of the malocclusion:
Normal sagittal jaw relationship. Malocclusion, if present, is primarily dental in origin.
Maxilla positioned anteriorly relative to the mandible, or the mandible is retrognathic. Severity scales with ANB.
Mandible positioned anteriorly relative to the maxilla, or the maxilla is retrognathic. Negative values indicate increasing severity.
Why the ABO set carries two vertical measurements
The ABO analysis is one of the few compact sets that reports both SN-MP and FMA. They measure the same mandibular plane but against different references — the Sella-Nasion line and the Frankfort horizontal respectively — so they rarely move in perfect lockstep. When they agree, the vertical diagnosis is secure. When they disagree, the cause is usually a tilted reference plane (a steep or flat anterior cranial base, or a cant in the Frankfort plane), and that disagreement is itself a useful flag to verify landmark placement before committing to a high- or low-angle treatment mechanics decision.
Dental Measurements (4)
The four dental values describe the inclination and the anteroposterior position of the upper and lower incisors — the data that drives torque prescriptions and the extraction versus non-extraction decision.
| Measurement | Definition | Norm | Clinical significance |
|---|---|---|---|
| U1 to SN | Angle of the upper incisor long axis to the S–N line | 102° ± 2° | Upper incisor inclination relative to the cranial base. >104° = proclined; <100° = retroclined. |
| U1 to NA (mm) | Distance from the upper incisor tip to line N–A | 4 mm ± 2 mm | Upper incisor anteroposterior protrusion. Complements the angular reading. |
| L1 to MP (IMPA) | Incisor-mandibular plane angle: L1 long axis to Go–Me | 90° ± 5° | Lower incisor inclination over basal bone. >95° = proclined; <85° = retroclined. |
| L1 to NB (mm) | Distance from the lower incisor tip to line N–B | 4 mm ± 2 mm | Lower incisor protrusion. Read with IMPA to separate tipping from bodily position. |
IMPA is the lower-arch anchor. The incisor-mandibular plane angle is the value the ABO set shares with the Tweed triangle. Because the lower incisor sits at the centre of nearly every extraction decision, IMPA is read against the mandibular plane angle: a high-angle (hyperdivergent) patient tolerates far less lower-incisor proclination than a low-angle patient before the lip profile and periodontium are compromised.
Soft Tissue (1): Lower Lip to E-Plane
The single soft-tissue measurement closes the loop between the skeletal-dental diagnosis and the facial profile the patient actually presents with.
| Measurement | Definition | Norm | Clinical significance |
|---|---|---|---|
| Lower Lip to E-Plane | Perpendicular distance from the lower lip (Li) to the Ricketts esthetic line (Pronasale–soft-tissue Pogonion) | −2 mm ± 2 mm | Lower-lip prominence. A positive value places the lip ahead of the E-line (protrusive); a negative value places it behind. Falling lip support is a key check before planning incisor retraction. |
The E-plane reading is the reality check on the dental plan. Retracting proclined incisors improves the numbers, but if the lower lip already sits behind the E-line, further retraction risks flattening the profile. Reading the lower lip alongside U1/L1 position keeps the soft-tissue outcome in view from the first appointment.
How the ABO Analysis Guides Treatment Planning
A complete picture in ten numbers
The strength of the ABO set is coverage per measurement. In ten values it answers the four questions every diagnostic work-up must resolve: Where are the jaws sagittally? (SNA, SNB, ANB) What is the vertical pattern? (SN-MP, FMA) Where are the incisors? (U1 to SN, U1 to NA, IMPA, L1 to NB) and How is the profile? (lower lip to E-plane). That breadth is why a board-style summary favours this combination over any single classical analysis.
Skeletal versus dental aetiology
As with Steiner, the pivotal decision the ABO set informs is whether a malocclusion is skeletal or dental. A Class II molar relationship with an ANB of 2° points to a dental problem amenable to mechanics; the same molar relationship with an ANB of 7° signals a skeletal Class II that calls for growth modification in a growing patient or surgical consideration in an adult. Layering the vertical readings on top refines the mechanics — a hyperdivergent skeletal Class II is managed very differently from a hypodivergent one.
Documenting change
Because the ABO set is the lens through which board-style case presentation is framed, it is also a natural template for documenting treatment change. Recording the same ten values before and after treatment makes the skeletal, dental, and soft-tissue response explicit and comparable — exactly the evidence a case write-up needs.
Limitations and When to Supplement the ABO Analysis
The ABO set is a summary, not an exhaustive analysis, and a few limitations follow from that:
- No facial-height ratios. The set reports vertical angles (SN-MP, FMA) but not the posterior-to-anterior face-height ratio. For a fuller vertical work-up, supplement with Björk-Jarabak.
- Cranial-base dependence. SNA, SNB, ANB, SN-MP, and U1 to SN all reference the Sella-Nasion line, so an atypical anterior cranial base biases them together. The Wits appraisal provides a cranial-base-independent sagittal cross-check.
- One soft-tissue value. The lower lip to E-plane is a single profile data point. For a complete soft-tissue assessment add the Holdaway analysis or the full E-line analysis.
- Population norms. The normative values reflect predominantly white North-American samples. For patients of other ancestries, interpret deviations against population-appropriate norms.
BCeph runs the ABO set alongside Steiner, Tweed, Wits, Björk-Jarabak, Holdaway and nine other modules from a single set of landmarks — so every one of these cross-checks is one click away in the same session.