Bunions rarely have one single cause. They usually develop through a combination of inherited anatomy, joint mechanics, ligament balance, activity and footwear. This helps explain why one person can wear narrow shoes for years without developing a bunion while another develops one despite choosing sensible footwear.
Family history is an important factor. What may be inherited is not the bunion itself, but features such as the shape of the first metatarsal, joint orientation, ligament flexibility, arch structure and the way the foot functions during walking. These characteristics can make the big-toe joint more vulnerable to gradual displacement.
Foot mechanics can also contribute. If the first metatarsal is relatively mobile or the foot rolls in excessively during gait, the forces passing through the big-toe joint may become less balanced. Over time, the big toe can drift towards the lesser toes while the first metatarsal moves medially.
Footwear is best understood as an aggravating factor rather than the sole explanation. Shoes with a narrow toe box can compress the toes and place pressure over the bunion prominence. High heels move body weight towards the forefoot and may increase discomfort. In a person with a predisposed foot, these forces may accelerate symptoms or progression.
Bunions may also occur with inflammatory arthritis, neuromuscular conditions or marked ligament laxity. Adolescent bunions can have different structural features from those that develop later in adulthood and require careful evaluation before any treatment plan is recommended.
Because several factors may be involved, treatment should not focus only on the visible bump. A clinical assessment considers the entire foot, including the range of motion of the big-toe joint, the stability of the first ray, the position of the lesser toes, arch mechanics and areas of pressure beneath the forefoot.
Understanding the contributing factors does not mean that every bunion will worsen or require surgery. It does, however, help explain why symptoms vary and why treatment must be individualised.