knee jerk vs knee-jerk reflex

knee jerk and knee-jerk reflex both are nouns.

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

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

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

Other synonyms of knee jerk include patellar reflex.

knee jerk (noun)	knee-jerk reflex (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