Analyse de Tweed et triangle FMA, IMPA, FMIA en orthodontie
Comprendre l’analyse de Tweed, FMA, IMPA et FMIA, valeurs de référence, lecture de la divergence et lien avec l’encombrement et la décision d’extraction.
Steiner’s chevrons: free step-by-step calculator
Enter the patient’s ANB, U1-NA, L1-NB and Pog-NB: the four chevrons are built before your eyes, with each formula explained, up to the incisor repositioning to carry over into the Steiner box.
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The principle of Steiner’s chevrons
The prognostic part of the Steiner analysis looks for the incisor position to aim for at the end of treatment. Each step is summarized by a chevron: a V pointing to the right, with ANB at the top, U1-NA and L1-NB on its arms and Pog-NB at the bottom.
- Chevron 1, the problem: the patient’s initial measurements.
- Chevron 2, the compromise: estimate the end-of-treatment ANB, then read the incisor position that Steiner considers acceptable for this ANB.
- Chevron 3, esthetics: according to Holdaway, the lower incisor lines up with the chin (L1-NB = final Pog-NB, growth included).
- Chevron 4, the solution: the average of chevrons 2 and 3, which reconciles function and esthetics.
- Chevron 5, individualized: if space is still lacking after balancing the Steiner box.
Steiner’s acceptable compromises
For each degree of ANB above 2°, the upper incisor moves back by 1° and 1 mm and the lower incisor moves forward by 1° and 0.25 mm: the incisors compensate for the discrepancy between the jaws. The table is only defined between −1° and 8°.
| Final ANB | U1-NA (°) | U1-NA (mm) | L1-NB (°) | L1-NB (mm) |
|---|---|---|---|---|
| −1° | 25° | 7 mm | 22° | 3.25 mm |
| 0° | 24° | 6 mm | 23° | 3.5 mm |
| 1° | 23° | 5 mm | 24° | 3.75 mm |
| 2° (ideal) | 22° | 4 mm | 25° | 4 mm |
| 3° | 21° | 3 mm | 26° | 4.25 mm |
| 4° | 20° | 2 mm | 27° | 4.5 mm |
| 5° | 19° | 1 mm | 28° | 4.75 mm |
| 6° | 18° | 0 mm | 29° | 5 mm |
| 7° | 17° | −1 mm | 30° | 5.25 mm |
| 8° | 16° | −2 mm | 31° | 5.5 mm |
Equivalent formulas: U1-NA (°) = 24 − ANB; U1-NA (mm) = 6 − ANB; L1-NB (°) = 23 + ANB; L1-NB (mm) = 3.5 + 0.25 × ANB.
The Holdaway ratio
Holdaway relates the position of the lower incisor to the prominence of the chin: ideally, L1-NB equals Pog-NB. A difference of 1 to 2 mm remains esthetic, 3 mm is barely acceptable, and from 4 mm in favor of the incisor, uprighting it becomes essential for the profile.
Frequently asked questions
What are Steiner’s chevrons?
They are V-shaped diagrams that summarize, at each step of the reasoning, ANB, the incisor position (U1-NA and L1-NB, in millimeters) and chin prominence (Pog-NB). They are used to set the incisor position to aim for at the end of treatment.
Why is the final ANB equal to ANB / 2 + 1?
This is the estimate taught when conditions are favorable: remaining growth, anterior rotation, possible orthopedic treatment and good cooperation. If these conditions are lacking, the estimate is weighted downward. Without growth, orthodontics alone does not change ANB: the initial ANB is kept.
Why does the compromise stop between −1° and 8°?
Steiner’s compromise describes incisor positions that compensate for a moderate skeletal discrepancy. Beyond this range, dental compensation alone is no longer enough: skeletal correction should be discussed.
How do I estimate chin growth?
The usual guideline over 2 years of treatment: about +1 mm of Pog-NB in girls and +2 mm in boys, more with anterior rotation or if the initial Pog-NB is already large, and no growth in adults. The tool suggests this value, which you can change.
What is incisor repositioning used for?
Incisor repositioning (IR) converts the movement of the lower incisor into millimeters of arch length: (final L1-NB − initial L1-NB) × 2, because both sides of the arch are involved. It is added to crowding and the curve of Spee in the Steiner box to decide on possible extractions.
Where do I find the initial measurements?
On the lateral cephalogram: our free cephalometric analysis tool calculates ANB, U1-NA and L1-NB from the landmarks you place, and can pass these values on to this page.
Learning tool: it applies the usual rules of the Steiner analysis and does not replace the practitioner’s judgment.