Bunions are actually bone spurs, or osteophytes, that form along the medial (inside) aspect of the first metatarsal bone of the foot just below the base of the great toe. The presence of this spur is an indication of mechanical or alignment problems that exist between the great toe and the first metatarsal.
Bunions can be caused by several factors from genetics to lifestyle choices.
Mechanical Widening
There is a condition that involves an abnormal widening of one's foot. It is called splaying. Splaying occurs when the foot widens across the part of the foot that lies just before the toes, or in the distal metatarsal area.
As this widening increases, it places abnormal forces upon ligaments and other soft tissues that can cause the big toe to angle toward the second toe. The apex of this angulation occurs at the joint of the great toe and is where abnormal pressure from footwear is exerted. The greater the angle, the greater pressure and the more reaction in the bone. Consequently, the larger the bunion.
Fashion
Women's foot wear can be a significant causative factor in bunion formation, especially with the proliferation of narrow-toe high heel shoes. Unfortunately, while these shoe styles are good looking and extremely popular, they are also quite constrictive due to the very nature of their design. The great toe is typically forced into a valgus (pointing toward the little toe) angulation, which in turn increased the pressure placed upon the junction of the toe and the first metatarsal head.
That being said, in general any kind of restrictive or ill-fitting footwear can serve to be a significant factor for the formation of bunions.
Heredity
All extraneous factors aside, bunions can be caused by genetic tendencies toward this type of deformity. Family members over the course many generations can display various components of splay-foot (as mentioned above), for example, as well as anatomic variations that cause angulation of the great toe.
Diseases and Conditions
Cerebral palsy is a neurologic condition that can cause spasticity and muscular imbalances in the extremities, both upper and lower, and can predispose many children to bunions. Over-pronation of the feet secondary to fallen arches can also set the stage for bunion formation and angulation problems because of abnormal weight bearing mechanics.
