long bone vs leg bone

long bone and leg bone both are nouns.

Word	Noun	Adjective	Verb	Adverb
long bone	Yes	No	No	No
leg bone	Yes	No	No	No

As nouns, leg bone is a hyponym of long bone; that is, leg bone is a word with a more specific, narrower meaning than long bone:

long bone: in limbs of vertebrate animals: a long cylindrical bone that contains marrow
leg bone: a bone of the leg

Other hyponyms of long bone include epiphysis, diaphysis, shaft, arm bone.

long bone (noun)	leg bone (noun)
in limbs of vertebrate animals: a long cylindrical bone that contains marrow	a bone of the leg