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).
The hair on my kneecap bothers me so much that I can hardly even sleep.
While Koichi was sleeping, I drew eyes on his kneecaps.
Apparently, Apple is going to start developing kneecap recognition.