fallen arch vs sunken arch

fallen arch and sunken arch both are nouns.

Word	Noun	Adjective	Verb	Adverb
fallen arch	Yes	No	No	No
sunken arch	Yes	No	No	No

As nouns, fallen arch and sunken arch are synonyms defined as:

fallen arch and sunken arch: an instep flattened so the entire sole rests on the ground

fallen arch (noun)	sunken arch (noun)
an instep flattened so the entire sole rests on the ground	an instep flattened so the entire sole rests on the ground