膝の皿

Etymology
Compound of, from the shape of the bone.

Noun

 * 1)  the kneecap, the patella

Usage notes
Both and  are in use in modern Japanese, with  used more frequently. The term is somewhat more technical, a bit like English  compared to.

Idioms

 * : "fire comes out of one's kneecaps" → To be extremely down and out.
 * : "one's kneecaps catch on fire" → To be extremely down and out.