What is a bunion?
A bunion is a bulge or bump that develops on the inner side of the foot, near the base of the first toe. It is caused by a poor alignment of the metatarsal-phalangeal joint of the big toe, or hallux. Physicians call this deformity "hallux abducto valgus," or HAV, a term referring to the hallux abducting or going away from the midline of the body. It also refers to a twisting of the toe so that its inside edge touches the ground and its outside edge turns upward. Essentially, it describes the big toe's tendency to deviate toward the outside of the foot. The condition tends to worsen over time leading to discomfort and skin problems, such as corns and lesions, and eventually difficulty in walking. Bunion
Click on the image to view a larger version.

Causes
Bunions are among the most common problems of the foot. They tend to run in families, suggesting genetic factors associated with the inherited shape of the foot in certain families. These genetic factors may predispose some people to develop bunions. Some doctors believe the pronated or flat foot, with its characteristic instability, is a common cause of HAV. This is because in walking, the body's weight is repeatedly transferred to the hallux. Normally, this isn't a problem, but in an unstable flat foot, this transfer of weight allows certain muscles to be stronger than others. Thisoverpowering of muscles causes the toe to bend and deform. Other doctors believe bunions are caused by excessively tight and pointy toe-boxes in shoes, high heels, and shoes that are too small. This premise is supported by the fact that women, whose shoe fashions run toward narrow widths and high heels, tend to get bunions much more often than men do. However, research suggests that such factors as improper shoes don't cause bunions as much as they exacerbate the underlying causative problem of flat, unstable feet.

Progression
The typical bunion starts off as a mild bump or outward bending of the big toe. This is only a cosmetic concern at first. However, beneath the surface, strong forces are at work. The forces imparted by the misaligned, outward-bending toe stretch the ligaments that connect the bones of the foot. This in turn pulls against the tendons, gradually drawing the big toe farther out of line. Over time, the hallux will continue to twist away from its original position until it no longer lines up properly with its corresponding metatarsal. The end of the metatarsal may become enlarged. Pressure from the first toe can lead
to deformity of the metatarsal-phalangeal joint in the second toe, pushing it toward the third toe. In some cases, the second toe may ride up and over, or down and under, the hallux. At this point, the range of motion in the big toe will decrease, a condition known as hallux limitus.
The condition begins to become painful at this stage. Because the bunion has changed the shape of the foot, the biomechanics of walking become altered. Normally, the hallux can bend at least 65 degrees, enabling it to be the last part of the foot to leave the ground during walking. However, with hallux limitus, the big toe cannot perform its function properly, and the weight of the body is transferred almost completely to the bunion. The body tries to compensate for this by changing. Typically, a person with uncomfortable bunions gradually begins walking in an exaggerated toe-turned-out fashion, somewhat like a duck, so the painful hallux does
ot have to bend so far. This is a vicious cycle, however, as the same forces causing the feet to turn outward steadily force the hallux even farther out. Inevitably this causes the bunion to worsen. Unless something is done to break this cycle, the deformity eventually becomes disabling.