It is then considered as being a mechanical function problem of the forefoot. Cause
The exact cause is unclear. It may be due to a combination of internal and external causes. Proposed factors include wearing overly tight shoes, high-heeled shoes, family history  and rheumatoid arthritis. The American College of Foot and Ankle Surgeons states that footwear only worsens a problem caused by genetics.Excessive pronation of the foot causes increased pressure on the inside of the big toe that can result in a deformation of the medial capsular structures of the joint, subsequently increasing the risk of developing bunions. Pathophysiology
The bump itself is partly due to the swollen bursal sac or an osseous (bony) anomaly on the metatarsophalangeal joint. The larger part of the bump is a normal part of the head of the first metatarsal bone that has tilted sideways to stick out at its distal (far) end (metatarsus primus varus). Bunions are commonly associated with a deviated position of the big toe toward the second toe, and the deviation in the angle between the first and second metatarsal bones of the foot. The small sesamoid bones found beneath the first metatarsal (which help the flexor tendon bend the big toe downwards) may also become deviated over time as the first metatarsal bone drifts away from its normal position. Osteoarthritis of the first metatarsophalangeal joint, diminished and/or altered range of motion, and discomfort with pressure applied to the bump or with motion of the joint, may all accompany bunion development.
Atop of the first metatarsal head either medially or dorso-medially, there can also arise a bursa that when inflamed (bursitis), can be the most painful aspect of the process. Diagnosis
Bunion can be diagnosed and analyzed with a  simple x-ray, which should be taken with the weight on the foot. The hallux valgus angle (HVA) is the angle between the long axes of the proximal phalanx and the first metatarsal bone of the big toe. It is considered abnormal if greater than 15–18°. The following HV angles can also be used to grade the severity of hallux valgus:
Mild: 15–20°
Moderate: 21–39°
Severe: ≥ 40°The intermetatarsal angle (IMA) is the angle between the longitudinal axes of the first and second metatarsal bones, and is normally less than 9°. The IM angle can also grade the severity of hallux valgus as:
Mild: 9–11°
Moderate: 12–17°
Severe: ≥ 18°

Treatment
Conservative treatment for bunions include changes in footwear, the use of orthotics (accommodative padding and shielding), rest, ice, and pain medications such as acetaminophen or nonsteroidal anti-inflammatory drugs. These treatments address symptoms but do not correct the actual deformity. If the discomfort persists and is severe or when aesthetic correction of the deformity is desired, surgical correction by an orthopedic surgeon or a podiatric surgeon may be necessary. Orthotics
Orthotics are splints or regulators while conservative measures include various footwear like toe spacers, valgus splints, and bunion shields. Toe spacers seem to be effective in reducing pain, but there is not evidence that any of these techniques reduces the physical deformity.