Tweed Analysis
The Tweed analysis was developed by Dr. Charles H. Tweed, who observed that his most stable and esthetically pleasing treatment outcomes shared a common trait: the lower incisors sat upright over their basal bone rather than tipped forward. Building on Downs' earlier work, Tweed formalized this observation into a simple diagnostic triangle built from three reference planes — the Frankfort Horizontal, the mandibular plane, and the long axis of the lower incisor.
Because the three angles of this triangle always sum to exactly 180°, the analysis doubles as an internal accuracy check: if a traced case does not add up, a measurement error is indicated. This simplicity, combined with its clear clinical target for incisor position, has kept the Tweed triangle a staple of orthodontic diagnosis, particularly for Class II cases and extraction decisions.
What It Evaluates
Tweed's method evaluates the vertical divergence of the face (via the mandibular plane's relationship to the Frankfort Horizontal) together with the angulation of the lower incisor relative to both the skull base and its own supporting jawbone. In doing so, it tells the clinician not just whether a patient is a vertical or horizontal grower, but where the lower incisor needs to end up for treatment to be stable — a key input when deciding whether a case needs extractions or interproximal reduction to make room for uprighting.
Learn more: Lower Incisor Position in Different Malocclusions and Facial Patterns
Key Measurements
- FMA (Frankfort-Mandibular Plane Angle) — the angle between the Frankfort Horizontal and the mandibular plane, describing vertical facial divergence. Normal value: 25° (clinical range approximately 20°-30°). Higher values indicate a more vertical (hyperdivergent) growth pattern; lower values indicate a more horizontal (hypodivergent) pattern.
- FMIA (Frankfort-Mandibular Incisor Angle) — the angle between the Frankfort Horizontal and the long axis of the lower incisor, describing the incisor's angulation relative to the skull base. Normal value: 65° (clinical range approximately 60°-70°).
- IMPA (Incisor-Mandibular Plane Angle) — the angle between the long axis of the lower incisor and the mandibular plane, describing the incisor's angulation relative to its own jawbone. Normal value: 90° (clinical range approximately 85°-95°).
- Triangle identity — FMA + FMIA + IMPA always equal 180°, since the three lines form a closed triangle; this relationship is used to verify that a cephalometric tracing was measured correctly.
Learn more: FMA and the Tweed Triangle — Mandibular Plane Definition Explained
Clinical Interpretation
Tweed's original guidance was to aim for an FMIA near 65°, adjusted for the patient's individual FMA — a high-FMA (vertical grower) patient typically needs a somewhat lower FMIA target to keep the incisor from further tipping forward, while a low-FMA (horizontal grower) patient can tolerate a slightly higher FMIA. A high IMPA (incisor tipped forward relative to the mandible) combined with a low FMIA is a classic sign of dental crowding or protrusion that dictates extraction or significant uprighting to reach a stable, upright incisor position. Population studies have also shown that these "normal" values shift meaningfully by ethnicity — for example, some populations show a naturally higher IMPA and lower FMIA than the original Caucasian-based norms — which is a reminder to weigh Tweed's numbers alongside the patient's overall facial balance and soft-tissue profile rather than as a rigid universal target.
Learn more: Cephalometric Profile of Bangladeshis: Tweed's Analysis