ulna vs elbow bone

ulna and elbow bone both are nouns.

Word	Noun	Adjective	Verb	Adverb
ulna	Yes	No	No	No
elbow bone	Yes	No	No	No

As nouns, ulna and elbow bone are synonyms defined as:

ulna and elbow bone: the inner and longer of the two bones of the human forearm

ulna (noun)	elbow bone (noun)
the inner and longer of the two bones of the human forearm	the inner and longer of the two bones of the human forearm