Bunions: A bunion is a structural deformity involving the angulation between the metatarsal bones. The bunion is an enlargement of bone or tissue around the joint at the base of the big toe the metatarsophalangeal joint.The big toe may turn in toward the second toe and the tissues surrounding the joint may be swollen and tender.The deformity may grow so great that the big toe can either overlap or underlap the second toe resulting in contracture deformities of the remaining toes known as hammertoes. Today the term usually is used to refer to the pathological bump on the side of the great toe joint. The bump is the swollen bursal sac and/or an osseous (bony) deformity that has grown on the metatarsophalangeal joint (where the first metatarsal bone and hallux meet). If left untreated the joint can adapt irreversibly and be painful with every step.