metacarpophalangeal joint vs knuckle

metacarpophalangeal joint and knuckle both are nouns.

Word	Noun	Adjective	Verb	Adverb
metacarpophalangeal joint	Yes	No	No	No
knuckle	Yes	No	Yes	No

As nouns, metacarpophalangeal joint and knuckle are synonyms defined as:

metacarpophalangeal joint and knuckle: a joint of a finger when the fist is closed

Other synonyms of metacarpophalangeal joint include knuckle joint.

metacarpophalangeal joint (noun)	knuckle (noun)
a joint of a finger when the fist is closed	a joint of a finger when the fist is closed

metacarpophalangeal joint (verb)	knuckle (verb)
	press or rub with the knuckles
	shoot a marble while keeping one's knuckles on the ground