knee-jerk reflex vs knee jerk

knee-jerk reflex and knee jerk both are nouns.

Word	Noun	Adjective	Verb	Adverb
knee-jerk reflex	Yes	No	No	No
knee jerk	Yes	No	No	No

As nouns, knee-jerk reflex and knee jerk are synonyms defined as:

knee-jerk reflex and knee jerk: a reflex extension of the leg resulting from a sharp tap on the patellar tendon

Other synonyms of knee-jerk reflex include patellar reflex.

knee-jerk reflex (noun)	knee jerk (noun)
a reflex extension of the leg resulting from a sharp tap on the patellar tendon	a reflex extension of the leg resulting from a sharp tap on the patellar tendon