Hallux valgus, commonly known as a bunion, develops when multiple factors gradually shift the big toe toward the smaller toes, pushing the joint at the base of the big toe outward. There is no single cause. Instead, a combination of inherited foot structure, biomechanical imbalances, footwear habits, and sometimes underlying disease drives the deformity over months or years.
Genetics and Foot Structure
The strongest predictor of hallux valgus is family history. First-degree relatives of someone with bunions have a significantly higher risk of developing the condition themselves compared to the general population. What gets inherited isn’t the bunion itself but the foot shape that makes one likely: a wider forefoot, a longer first metatarsal bone, hypermobile joints, or ligaments that stretch more easily than average.
These inherited traits create a mechanical environment where the big toe joint is under uneven pressure with every step. Over time, that imbalance compounds. The big toe drifts laterally, the metatarsal head becomes more prominent on the inner edge of the foot, and the soft tissues around the joint remodel to accommodate the new alignment. This is why bunions tend to run in families even when relatives wear very different shoes.
Flat Feet and Overpronation
Flat feet (pes planus) are one of the clearest biomechanical risk factors. Data from the Framingham foot study, which examined over 5,700 participants, found that people with flat feet had about 60% higher odds of developing hallux valgus compared to those with normal arches. Even after adjusting for age, sex, and body weight, the association held, with flat-footed individuals still showing roughly 20% higher odds.
The mechanism is straightforward. When the arch collapses during walking, the foot rolls inward more than it should. This overpronation places extra stress on the inner side of the forefoot, particularly the first metatarsophalangeal joint. That repeated loading gradually destabilizes the joint, allowing the big toe to drift. People with flexible flatfoot who spend long hours on their feet are especially vulnerable because the cumulative stress on that joint is enormous over years.
Footwear
Narrow, pointed shoes and high heels don’t cause hallux valgus on their own, but they accelerate and worsen the deformity in people who are already predisposed. High heels shift body weight forward onto the forefoot, compressing the toes into the tapered toe box. This forces the big toe into a valgus position for hours at a time, stretching the inner ligaments and tightening the outer structures.
Population studies consistently show that hallux valgus is more common in shoe-wearing societies than in barefoot ones, and more common in women than men. In a study of community-dwelling older adults, 58% of women had hallux valgus compared to 25% of men. While some of that gap reflects hormonal differences in ligament laxity, decades of wearing constrictive footwear play a substantial role. That said, bunions absolutely occur in people who have never worn heels, which underscores that shoes are a contributor, not the root cause.
Inflammatory and Connective Tissue Conditions
Rheumatoid arthritis is strongly linked to hallux valgus. The disease causes chronic inflammation inside joints, which gradually destroys the ligaments and soft tissue that hold the big toe in proper alignment. Synovitis (inflammation of the joint lining) loosens the supporting structures, while joint erosion destabilizes the bones themselves. The result is a bunion that often progresses faster and becomes more severe than one caused by mechanical factors alone.
Other conditions that affect connective tissue or joint stability can have a similar effect. Ehlers-Danlos syndrome and Marfan syndrome both involve generalized ligament laxity, which leaves the first metatarsophalangeal joint vulnerable to gradual displacement. Neuromuscular conditions that alter the way muscles pull on the toes, such as cerebral palsy or stroke-related spasticity, can also drive the deformity by creating muscle imbalances across the forefoot.
Age, Sex, and Body Weight
Hallux valgus becomes dramatically more common with age. The ligaments and tendons supporting the big toe joint lose elasticity over decades, the cartilage thins, and the cumulative effect of millions of walking cycles takes a toll. By age 65 and older, prevalence rates climb well above what is seen in younger adults.
Women are affected roughly twice as often as men. Part of this is footwear-related, but hormonal factors also contribute. Estrogen influences collagen turnover in ligaments, and shifts during menopause may reduce the structural integrity of the tissues that stabilize the forefoot. Higher body weight adds to the equation by increasing the load the forefoot absorbs with each step, though its independent contribution appears smaller than genetics or foot type.
How Severity Is Measured
Doctors classify hallux valgus severity using the hallux valgus angle (HVA), measured on a weight-bearing X-ray between the long axis of the big toe and the first metatarsal. A normal HVA is under about 15 degrees. Mild hallux valgus falls below 30 degrees, moderate ranges from 30 to 40 degrees, and severe is anything above 40 degrees. A second measurement, the intermetatarsal angle (IMA) between the first and second metatarsals, tracks how much the metatarsal itself has splayed: under 13 degrees is mild, 13 to 20 is moderate, and over 20 is severe.
These numbers matter because they determine which treatments make sense. Mild deformities often respond to wider shoes, toe spacers, and orthotics that control pronation. Moderate and severe cases, especially those that are painful or progressing, typically require surgical correction. Knowing the angle also helps predict whether the deformity will continue to worsen, since higher angles tend to accelerate further drift as the mechanical advantage of the surrounding muscles changes.
Why Bunions Get Worse Over Time
Hallux valgus is a progressive condition. Once the big toe starts to drift, the forces acting on the joint shift in a way that promotes further deviation. The tendons that normally pull the toe straight begin to bowstring across the joint at an angle, actively pulling the toe further out of alignment. The inner ligament stretches, the outer soft tissues contract, and the joint capsule remodels. Without intervention to address the underlying mechanics, mild bunions commonly become moderate ones over a span of years. This self-reinforcing cycle is why early attention to footwear, arch support, and biomechanics matters, even when the bunion is small and painless.