Bunions form when the bones in your big toe joint gradually shift out of alignment. The first long bone of your foot (the metatarsal) drifts inward toward your other foot, while the big toe angles outward toward your smaller toes. This creates the characteristic bony bump on the inside of your foot. The process is almost always gradual, driven by a combination of inherited foot structure, how your foot moves when you walk, and external pressures like tight shoes.
What Actually Happens Inside the Joint
A bunion isn’t new bone growth. It’s the head of your first metatarsal bone becoming more prominent as it shifts out of position. As the metatarsal drifts inward, the big toe rotates and angles toward the second toe. The soft tissues around the joint, including the capsule and ligaments that hold everything in place, stretch on one side and tighten on the other. Over time, the muscles that normally keep the toe straight lose their mechanical advantage, which locks the deformity in place and makes it progressively harder to reverse.
Small structures called sesamoid bones sit beneath the joint and normally track within grooves on the underside of the metatarsal head. As the metatarsal shifts, it slides off these sesamoids. The sesamoids stay anchored in place by a strong ligament, so it’s the bone above them that moves. This displacement is a key reason bunions tend to worsen rather than correct themselves.
Genetics and Foot Structure
The single biggest factor in whether you develop a bunion is the foot you were born with. Inherited traits like the shape of your metatarsal head, the length of your first metatarsal relative to your second, and how loose or tight your ligaments are all influence your risk. Flat feet are a well-established predisposition because a collapsed arch changes the angle of force through the big toe joint with every step.
Bunions often run in families, though what’s inherited isn’t the bunion itself but the foot mechanics that make one likely. If your parents or grandparents had bunions, your foot structure may distribute weight in a way that gradually pushes the joint out of alignment. This explains why some people wear narrow shoes for decades without developing a bunion, while others develop one despite wearing supportive footwear their whole lives.
How Your Foot Moves Matters
The way your foot rolls during walking and standing plays a major role. When the big toe and first metatarsal rotate inward more than they should (a movement called pronation), it increases the load on the inner side of the joint. Research using cadaver feet showed that when the first metatarsal was rotated inward by 30 degrees, it produced a nearly equal degree of big toe misalignment. That rotation weakens the ligaments on the inner side of the joint, and once those ligaments stretch out, the metatarsal head displaces medially while the toe drifts the other way.
This is why bunions are progressive. Each step applies force through a joint that’s already slightly off-center, which stretches the supporting tissues a little more, which makes the next step even less balanced. It’s a slow feedback loop that can play out over years or decades.
Shoes Don’t Cause Bunions, But They Speed Them Up
Narrow, pointed shoes and high heels do not create bunions on their own. They do, however, accelerate the process in feet that are already predisposed. A tight toe box forces the big toe laterally and holds it there for hours, placing sustained pressure on a joint that may already be slightly unstable. High heels compound the problem by shifting your body weight forward onto the ball of the foot, increasing the load through the first metatarsal joint.
People with a genetic predisposition or preexisting foot conditions are significantly more likely to develop bunions when they regularly wear these types of shoes. If your foot structure is naturally stable, tight shoes might cause discomfort but are unlikely to cause a lasting deformity. If your ligaments are looser or your arch is flatter, the same shoes can tip the balance toward progressive misalignment.
Inflammatory Conditions and Joint Disease
Rheumatoid arthritis is one of the clearest medical pathways to a bunion. The disease causes chronic inflammation of the joint lining, which swells and produces enzymes that erode cartilage and bone. Over time, the inflammation stretches and weakens the joint capsule and surrounding ligaments, destroying the structures that keep the toe aligned. The balance between the muscles controlling the toe breaks down, leading to subluxation (partial dislocation) and eventually full dislocation of the joint. At the big toe, this commonly results in a bunion.
Osteoarthritis can also contribute, though through a different mechanism. Rather than inflammatory destruction, it involves gradual cartilage breakdown that changes how the joint surfaces interact, altering the forces through the toe over time.
Who Gets Bunions Most Often
Bunions are roughly twice as common in women as in men. A large meta-analysis found a prevalence of about 24% in women compared to 11% in men. The disparity is likely a combination of differences in foot structure, ligament laxity, and footwear choices, though the relative contribution of each is still debated.
Age is the other major demographic factor. Bunions affect about 11% of people under 20 (these are called juvenile or adolescent bunions and are almost entirely genetic). The rate stays relatively stable through working age at around 12%, then climbs to nearly 23% in people over 60. That jump reflects the cumulative nature of the condition: decades of walking on a slightly misaligned joint eventually produce a visible deformity.
Occupational and Activity-Related Risks
Jobs that keep you on your feet for long stretches increase the stress on your forefoot with every hour. Nurses, for example, show elevated rates of bunions and other foot deformities, a pattern attributed to prolonged standing on hard surfaces in work shoes that often prioritize function over fit. Retail workers, teachers, and factory workers face similar occupational exposure.
Ballet dancers are another high-risk group. Dancing en pointe forces the full body weight through the tips of the toes, placing extreme and repetitive pressure on the first metatarsal joint. Combined with the hypermobility many dancers have (which is an asset for flexibility but a liability for joint stability), the conditions for bunion formation are nearly ideal.
Any repetitive activity that loads the forefoot, from running with poor mechanics to years of wearing steel-toed boots, can contribute if the underlying foot structure is susceptible.
Why Bunions Get Worse Over Time
Once the joint starts shifting, several forces conspire to keep it moving in the wrong direction. The tendons that flex and extend the big toe, which normally run straight along the toe’s axis, get pulled to the outside as the toe angles laterally. Instead of stabilizing the joint, they now act as bowstrings that pull the toe further out of alignment. The more the toe drifts, the stronger this pull becomes.
Meanwhile, the bursa (a small fluid-filled sac over the bony prominence) becomes irritated from rubbing against shoes, causing swelling and pain that can make the bump appear even larger. The joint capsule on the inner side stretches permanently, while the outer side contracts. Without intervention, mild bunions typically progress to moderate ones, and moderate bunions to severe ones, though the speed varies enormously from person to person. Some bunions take decades to become symptomatic. Others progress noticeably within a few years, particularly in people with rheumatoid arthritis or very flexible ligaments.