The joint behind the big toe is called the first metatarsophalangeal joint, and the following diseases commonly occur: first, degenerative osteoarthritis of the metatarsophalangeal joint, the first metatarsophalangeal joint can be complicated by osteoarthritis of the first metatarsophalangeal joint at the onset of a bunion, which clinically manifests as the first metatarsophalangeal joint bulges medially and raises the skin medially to form a callus and bunion, resulting in pain in the first metatarsophalangeal joint, which is significantly worse when walking under weight; second, sprain of the first metatarsophalangeal joint, which usually occurs when the first metatarsophalangeal joint is hyperflexed or hyperextended, after which the first metatarsophalangeal joint is locally painful, swollen and afraid to walk under weight.