What Is Tweed Analysis?
Tweed analysis — more precisely, the Tweed Triangle — is a cephalometric method developed by Charles H. Tweed in the 1940s and 1950s to define the ideal position of the lower incisor within the facial skeleton. It is based on a simple geometric observation: the Frankfort Horizontal plane, the mandibular plane, and the long axis of the lower incisor form a triangle on the lateral cephalogram. The three angles of that triangle — FMA, FMIA, and IMPA — are interdependent by definition, always summing to 180°.
Tweed's core argument was that long-term orthodontic stability and facial balance depend on the lower incisor being properly upright over basal bone. A lower incisor that is excessively proclined may resolve crowding in the short term but creates instability, soft-tissue strain, and potential periodontal compromise over time. Tweed used the triangle to define how much incisor retraction was needed and, consequently, whether extraction of lower premolars was indicated.
While contemporary orthodontics draws on a broader range of diagnostic frameworks — Steiner analysis, Ricketts, McNamara, and others — the Tweed Triangle remains a practical and widely taught tool for lower incisor assessment, particularly in practices where incisor position and extraction decisions are central diagnostic questions. It is one of the core analyses included in cephalometric analysis software across the market.
Plane Angle
Incisor Angle
Plane Angle
Tweed Norms and Clinical Ranges
Tweed established his norms primarily from a sample of untreated individuals with excellent facial balance and stable dentitions. The three angles are not independent — because they form a closed triangle, fixing any two determines the third. This mathematical constraint is both the elegance and the limitation of the system: the entire framework pivots on accepting FMA as a fixed skeletal baseline and working backwards to define the required FMIA and IMPA.
| Measurement | Norm | Clinical Range | High Value Indicates | Low Value Indicates |
|---|---|---|---|---|
| FMA | 25° | 20°–30° | Hyperdivergent — steep MP, open bite tendency, long LAFH | Hypodivergent — flat MP, deep bite tendency, short LAFH |
| FMIA | 65° | 60°–70° | Retroclined — lower incisor tipped back, upright or retruded | Proclined — lower incisor tipped forward, dentoalveolar compensation |
| IMPA | 90° | 85°–95° | Proclined — incisor leaning forward relative to mandibular plane | Retroclined — incisor upright or tipped back into mandible |
Landmarks and Planes Required
Tweed analysis requires three planes, each defined by two landmarks. Accurate identification of these six points — particularly Porion, Orbitale, and the apex of the lower incisor — is essential. Errors at any one landmark propagate into all three angle measurements simultaneously, because of their shared dependence.
Frankfort Horizontal (FH Plane)
Mandibular Plane (MP)
Long Axis of the Lower Incisor
FMA-Adjusted Treatment Goals
One of Tweed's most clinically important contributions was the recognition that the ideal FMIA target should vary with the patient's FMA. A patient with a steep mandibular plane (high FMA) cannot tolerate as much lower incisor proclination as a patient with a flat mandibular plane (low FMA) — the soft tissue drape, lip support, and smile arc all change with the vertical pattern. Tweed proposed the following adjustment framework:
Low FMA Below 20°
Hypodivergent pattern. Mandibular plane is flat. More proclination of the lower incisor is compatible with facial balance. Target FMIA may be adjusted downward to ~60°. Non-extraction is more frequently viable even with moderate crowding.
Average FMA 20°–30°
Normodivergent pattern. Tweed's original norms apply directly. Target FMIA of 65° and IMPA of 90°. Extraction decisions driven by the degree of proclination and the space required to achieve that FMIA goal.
High FMA Above 30°
Hyperdivergent pattern. Steep mandibular plane. Lower incisor proclination is poorly tolerated — soft tissue response, periodontal risk, and open bite tendency all argue for uprighting. Target FMIA adjusted upward to ~70°. Extraction strongly indicated in most cases.
This FMA-adjusted framework means Tweed analysis is not a single fixed target but a sliding-scale diagnostic model. The FMIA goal is set first based on FMA, and the degree of lower incisor change required to reach that goal determines whether extraction is indicated. If the required retraction exceeds approximately 4 mm at the crown, extraction is generally the preferred path.
Tweed Analysis and Extraction Decisions
Tweed's original motivation for developing the triangle was specifically to create a rational basis for extraction decisions — an issue that dominated orthodontic debate throughout the mid-twentieth century. His approach was systematic: measure FMA to characterise the vertical pattern, set the FMIA target accordingly, calculate the required change in lower incisor position, and extract if that change cannot be achieved non-extractively.
The Diagnostic Sequence
The working sequence in Tweed treatment planning runs as follows. First, measure the current FMA to identify the skeletal vertical pattern. Second, set the treatment target FMIA based on the FMA value — 65° for average FMA, adjusted up or down by 5° for high or low FMA respectively. Third, measure the current FMIA to quantify how far the lower incisor sits from the goal. Fourth, calculate the space needed to retract the lower incisor sufficiently to reach the target FMIA. If that space requirement exceeds what arch expansion or interproximal reduction can provide, extraction of lower first or second premolars is indicated.
Where Tweed Analysis Fits in Contemporary Practice
Modern orthodontic diagnosis rarely relies on a single cephalometric analysis. Tweed's framework is most useful as a focused lower incisor assessment tool within a broader diagnostic workup. It pairs well with Steiner's analysis — which approaches the lower incisor through its relationship to the NB line — and with Downs' analysis, which adds the A–B plane and facial angle to the vertical and incisor picture. Running Tweed alongside these analyses in a tool like BCeph gives a multi-framework view of the same clinical question.
The core insight remains clinically valid: the mandibular plane angle is a skeletal baseline that the orthodontist cannot substantially change, and the lower incisor position must be planned relative to it. A patient with FMA 35° who presents with IMPA 105° has a significantly different prognosis and treatment plan than one with FMA 20° and IMPA 100° — even though both have proclined lower incisors. Tweed's triangle makes that distinction measurable and communicable.
Running Tweed Analysis in BCeph
BCeph includes Tweed Triangle analysis as a built-in free module. The workflow follows standard cephalometric analysis procedure: load your lateral cephalogram, place the required landmarks, and the three angles are calculated automatically with normative comparison.
The landmarks required for Tweed in BCeph are Po (Porion), Or (Orbitale), Go (Gonion), Me (Menton), L1 (lower incisor tip), and the lower incisor root apex. If you have already placed these landmarks for another analysis — Downs, for example, shares several of the same points — BCeph carries them over automatically. Switching on Tweed will highlight only the landmarks that still need placement.
Results are displayed with normative ranges and a colour-coded deviation flag for each measurement. The 180° sum is verified automatically. PDF report export includes Tweed values alongside any other analyses you have activated for the same case — useful for case documentation and referral letters.
Tweed analysis in BCeph is completely free, with no subscription and no per-case charge. Patient images never leave your browser — there is no cloud upload. See how BCeph compares to other tools in the full cephalometric software comparison.