Tweed Triangle
A cephalometric diagnostic triangle formed by the Frankfort Horizontal Plane, the mandibular plane, and the long axis of the lower incisor, used to assess lower incisor angulation and facial balance.
Full Definition
The Tweed Triangle, described by Charles Tweed in the 1950s, comprises three angular measurements: FMIA (Frankfort Mandibular Incisor Angle), FMA (Frankfort Mandibular Plane Angle), and IMPA (Incisal Mandibular Plane Angle), which together sum to 180°. The triangle is used to determine the correct inclination of the lower incisors relative to the facial skeleton, and to guide extraction versus non-extraction decisions. Ideal values for a well-balanced face are approximately FMIA 68°, FMA 25°, and IMPA 87°. Editors should note that 'Tweed Triangle' is a proper noun and should be capitalised; individual angles within the triangle (FMIA, FMA, IMPA) should be presented as uppercase abbreviations.
Usage
Usage note: Always capitalise 'Tweed Triangle' as a proper noun. When reporting values, list all three angles (FMIA, FMA, IMPA) and confirm they sum to 180°.
In Context
- "Analysis of the Tweed Triangle revealed an FMIA of 58°, indicating excessive lower incisor proclination and supporting the extraction plan." — Cephalometric analysis report
- "The manuscript incorrectly labelled the FMA as the FMIA; the editor corrected this within the Tweed Triangle description." — Manuscript review note