sunken arch vs fallen arch

sunken arch and fallen arch both are nouns.

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

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

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

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