thigh bone, hipbone, kneecap
髁 centers on large bones of the leg: the thigh bone, hipbone, and kneecap. These are all major structural bones that support the body.
髁 combines 骨 (bone) with 果, which likely contributes the sound. The character was created to name specific leg bones.
The left side 骨 means bone. The right side 果 looks like a fruit on a tree, but here think of the round, hard fruit as the round, hard bones of the hip and knee. Together they point to the big leg bones.
For カイ, imagine a thigh bone shaped like a kayak paddle: kayak -> カイ, and the bone helps you paddle through deep water.