Anatomy

The structures involved in a bunion consist primarily of five bones (figure 1). There are two bones in the big toe called phalanges. The next bone in line with the phalanges is the first metatarsal. The metatarsal and its closest phalanx together with ligament and tendon attachments make up the first metatarsal phalangeal joint-it is at this joint that the bunion forms. Underneath the first metatarsal lie two small bones called sesamoids (not visible in figure 1). When in proper alignment, these sesamoids provide stability to the first metatarsal phalangeal joint via their attachments to various tendons and ligaments.

As a bunion begins to form, the big toe drifts towards the smaller toes. Consequently, the joint and base of the big toe are pushed outward beyond the normal contour of the foot, forming a mild bony prominence at the arch side (medial aspect) of the foot. As the bunion worsens, the first and second metatarsals spread farther apart. The prominence of bone on the side of the first metatarsal becomes even more impressive. In severe cases, the metatarsal phalangeal joint can be partially or, even worse, totally dislocated. The big toe may eventually end up underlapping or overlapping the second toe. Stiffness in the big toe joint may be associated as the deformity progresses. Occasionally, a fluid filled cyst or bursa may develop between the skin and medial bony prominence of the first metatarsal. Therefore, the bunion that you see may be bone, bursa, or both.