Kneecap
Patella
noun
Your knee's head is the kneecap. It's also called the patella, if you want to get fancy and talk about bone names.
This word uses the kun'yomi readings for both kanji. You haven't learned this reading for 頭 yet, so here's a mnemonic to help you:
You're walking around in your mom's house, minding your own business, when you slam your kneecap right into that gargoyle she loves (がしら). It hurts so bad, you can't even think out that phrase and instead you shout out, "GA SHE RA" and fall to the floor, clutching your searing kneecap (which has probably broken in half).
膝頭の毛が気になって眠れることすらできない。
The hair on my kneecap bothers me so much that I can hardly even sleep.
コウイチが寝ている間に、膝頭に目を書いた。
While Koichi was sleeping, I drew eyes on his kneecaps.
今年アップル社は、膝頭認証の開発を始めるらしい。
Apparently, this year Apple is going to start developing kneecap recognition.