The hallux valgus angle is formed by drawing one line through the centre of the first metatarsal and another through the centre of the proximal phalanx of the big toe. The angle between these lines describes how far the big toe has deviated relative to the first metatarsal.
This measurement is useful because it provides a reproducible way to describe the deformity, compare both feet and follow change over time. It also contributes to procedural planning if surgery is being considered.
However, the hallux valgus angle is not a complete diagnosis. It does not fully describe rotation of the first metatarsal, sesamoid position, joint congruity, arthritis, first-ray instability or the condition of the lesser toes. Two patients with similar hallux valgus angles may require different treatment because the rest of their examination and X-rays differ.
The angle also does not reliably predict pain. Some people have a marked deformity with little discomfort, while others have substantial symptoms with a smaller apparent angle. For this reason, treatment should not be recommended merely because a numerical threshold has been crossed.
When discussing imaging, it is usually more helpful to explain the overall pattern rather than focus on one number. The key questions are whether the joint is healthy, whether the deformity is flexible and correctable, how the sesamoids and first metatarsal are positioned, and whether associated problems are present.