Hallux valgus and over-pronation
Hallux valgus (a bunion) is a foot deformity in which the first metatarsal (the long bone behind the great toe) deviates medially and the hallux (great toe) deviates laterally.
Remember
In hallux valgus, over-pronation (the foot rolls inwards as it takes weight) shifts load towards the inner border of the foot, and the first metatarsal drifts inwards while the great toe is pushed outwards. It is a contributor, not the sole cause.
The first metatarsal deviates medially, the great toe deviates laterally, and the head of the first metatarsal stands out as a bump on the inner side of the foot. The word valgus gives the direction of the toe: it points laterally, towards the lesser toes. Varus is the opposite, with the toe pointing medially, and it does not describe a bunion.
The usual mechanical background is over-pronation. Pronation means the foot rolls inwards as it takes weight. When it is excessive, more weight passes through the medial (inner) side of the foot than the lateral (outer) side. That extra medial loading goes with the first metatarsal drifting inwards while the great toe is pushed outwards. A simple memory aid: weight falls on the inside, so the big toe leans out.
Over-pronation is only one contributor. Hallux valgus is multifactorial (several causes act together), and poorly fitting shoes and inherited foot structure also matter. It develops gradually and is most common in middle-aged women.
Management begins with wide shoes, low heels, soft soles, weight loss if overweight and simple painkillers. Surgery is reserved for severe, persistent pain.
Key facts
- Deformity
- the first metatarsal deviates medially, the great toe deviates laterally and the head of the first metatarsal is prominent.
- Mechanism
- over-pronated feet transmit more weight through the medial side of the foot than the lateral side.
- Valgus
- in hallux valgus the great toe points laterally, towards the lesser toes.
- Site
- the bump forms at the first metatarsophalangeal joint, the joint at the base of the great toe where the first metatarsal meets the first bone of the toe.
- Cause
- multifactorial, so pronation is a contributor and not the sole cause.
- Typical patient
- a middle-aged woman, with the change developing gradually.
Exam traps
- Supination, where the foot tilts onto its outer edge so that load shifts laterally, is the opposite of the pronation linked to a medial bunion, so loading the outer border of the foot does not explain hallux valgus.
- A high-arched (cavus) foot is not the mechanical background of a bunion. It is linked to nerve and muscle disorders, and in a large population study a cavus foot was not associated with hallux valgus.
- Loss of cartilage at the base of the great toe is osteoarthritis (called hallux rigidus). It causes pain and stiffness and can sit alongside a bunion, but it does not explain the sideways drift of the toe.
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