Downs Analysis
The Downs analysis, published by Dr. William B. Downs in 1948, was the first cephalometric analysis developed in the United States and one of the earliest systematic methods in orthodontics. Downs derived his norms from lateral cephalograms of twenty adolescents (ages 12-17) with excellent, naturally occurring occlusions, using the Frankfort Horizontal plane — a line from Porion to Orbitale — as his primary reference instead of the internal skull-base lines used by some later analyses.
The analysis defines ten measurements, split evenly between five skeletal and five dental parameters, and remains historically significant as the foundation many later analyses (including Tweed's) built upon.
What It Evaluates
Downs' method evaluates the skeletal pattern of the face (how convex or straight the profile is, and how the jaws are rotated relative to the skull base), the position and angulation of the incisors relative to their supporting jaws, and how the skeletal and dental patterns interact — for example, whether upright or proclined incisors are compensating for an underlying skeletal discrepancy.
Learn more: Downs Analysis — Skeletal and Dental Evaluation Framework
Key Measurements
- Facial Angle — the angle between the Frankfort Horizontal and the facial plane (Nasion-Pogonion), describing overall facial convexity and mandibular projection. Normal value: 87.8° ± 3.6°.
- Angle of Convexity — the angle at point A between the Nasion-A line and the A-Pogonion line, indicating how convex or concave the profile is. A larger value indicates a more convex (Class II-leaning) profile.
- A-B Plane Angle — the angle between the A-B line and the facial plane, summarizing the anteroposterior relationship between the maxilla and mandible.
- Mandibular Plane Angle — the angle between the mandibular plane (Gonion-Menton) and the Frankfort Horizontal, describing vertical facial divergence. Normal value: 21.9° ± 5°.
- Y-Axis — the angle between the Sella-Gnathion line and the Frankfort Horizontal (or Sella-Nasion line, depending on the reference used), describing the general direction of facial growth.
- Incisor Mandibular Plane Angle (IMPA) — the angle between the lower incisor's long axis and the mandibular plane, describing lower incisor upright-ness. This measurement was later adopted and popularized as one leg of Tweed's diagnostic triangle.
- Interincisal Angle — the angle between the long axes of the upper and lower incisors. Normal value: 135.4° ± 5.8°.
Learn more: Cephalometric Tracing — Downs Measurement Definitions
Clinical Interpretation
An orthodontist uses the facial angle and angle of convexity together to gauge the skeletal sagittal pattern much like Steiner's ANB or Ricketts' convexity: a smaller facial angle or a larger convexity angle points toward a Class II-leaning skeletal pattern, while the reverse suggests Class III. The mandibular plane angle and Y-axis are read together to judge vertical growth direction — steep values suggest a vertical (hyperdivergent) grower prone to open bites, while flatter values suggest a horizontal (hypodivergent) grower prone to deep bites. Because IMPA describes lower incisor uprightness independent of the skeletal pattern, comparing it against the facial angle helps distinguish a true skeletal problem from one where the teeth have already dentally compensated for it — a distinction that shaped Tweed's later refinement of this same measurement.
Learn more: Down's Analysis — Video Lecture