kneecap vs knee joint

kneecap and knee joint both are nouns.

Word	Noun	Adjective	Verb	Adverb
kneecap	Yes	No	Yes	No
knee joint	Yes	No	No	No

kneecap (noun)	knee joint (noun)
a small flat triangular bone in front of the knee that protects the knee joint	hinge joint in the human leg connecting the tibia and fibula with the femur and protected in front by the patella

kneecap (verb)	knee joint (verb)
shoot in the kneecap, often done by terrorist groups as a warning