Bunion on big toe. X-ray showing a bunion on a patient's foot, before (left) and after surgery. A bunion is a swelling of the joint between the big toe (hallux) and the first metatarsal bone. This joint deformation is called hallux valgus, in which the joint at the base of the big toe projects outwards while the tip of the toe turns inwards. Severe cases may lead to the formation of a bursa, a small sac of fibrous tissue filled with synovial fluid. Corrective surgery (at right) removed an excess growth at the site of the joint, and straightened the big toe. Bunions are often caused by the wearing of ill-fitting shoes.