Osteoarthritis is the gradual breakdown of the material, called cartilage, that covers the joint. Cartilage is a tough elastic material that acts as a shock absorber. In osteoarthritis, the cartilage becomes thinner and may even disappear in some places. When this happens, bone rubs against bone, causing pain. Under this weakened cartilage, the bone tries to regenerate and forms growths, called spurs or osteophytes. The joint may become deformed, larger, and lose its mobility.

Osteoarthritis of the base of the big toe, or metatarsophalangeal osteoarthritis, may be caused by an injury, a problem with how the foot is shaped, or a mechanical problem when walking.

Normal	Diseased
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Osteoarthritis is the most common cause of hallux limitus (reduced mobility of the big toe) and hallux rigidus (stiffness of the big toe).

These two problems should not be confused with hallux valgus (a lateral deviation of the big toe) which may lead to osteoarthritis.

Hallux limitus and hallux rigidus
These problems are characterized by pain when pressure is put on the big toe when walking. The patient is unable to bear weight on the painful big toe with limited mobility, and resorts to putting weight on the outside of the foot. Reduced mobility of the toe may lead to immobility of the joint. When the joint moves, it may make a cracking noise, and you may be able to feel a bone spur or osteophyte at the base of the big toe, on top of the foot. This osteophyte may put pressure on the shoe and lead to local inflammation.

Normal toe	Hallux limitus	Hallux rigidus
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Hallux valgus (bunion)
With this problem, the big toe turns outward. The metatarsophalangeal joint forms a lump on the inside of the foot. Pressure from the shoe may cause inflammation. The second toe is eventually pushed up or down or may remain in a bent position (hammertoe). Osteoarthritis may develop in the metatarsophalangeal joint because of the abnormal pressures on the joint.