A bunion, known clinically as hallux valgus, is a bony prominence that forms at the base of the big toe where it meets the foot. It develops when the metatarsophalangeal (MTP) joint — the joint connecting the big toe to the first metatarsal bone — gradually shifts out of alignment. As the big toe angles towards the second toe, the joint protrudes outward, creating the visible bump commonly associated with a bunion. The bump itself is the repositioned joint rather than a new growth of tissue. Over time, the skin over the bump may thicken and become sore, bursitis (inflammation of the fluid sac over the joint) may develop, and, in some cases, arthritis can occur. Without appropriate management, bunions often progress gradually, although the rate of progression varies between individuals.
These are the most common type and usually develop gradually due to a combination of inherited foot structure and biomechanical factors affecting walking and weight-bearing.