epidural vs extradural

epidural is a noun but extradural is not a noun.

Word	Noun	Adjective	Verb	Adverb
epidural	Yes	Yes	No	No
extradural	No	Yes	No	No

As adjectives, epidural and extradural are synonyms defined as:

epidural (noun)	extradural (noun)
regional anesthesia resulting from injection of an anesthetic into the epidural space of the spinal cord; sensation is lost in the abdominal and genital and pelvic areas; used in childbirth and gynecological surgery

epidural (adjective)	extradural (adjective)
on or outside the dura mater	on or outside the dura mater