metacarpophalangeal joint vs knuckle joint

metacarpophalangeal joint and knuckle joint both are nouns.

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

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

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

Other synonyms of metacarpophalangeal joint include knuckle.

metacarpophalangeal joint (noun)	knuckle joint (noun)
a joint of a finger when the fist is closed	a joint allowing movement in one plane only
	a joint of a finger when the fist is closed