The patella (kneecap) is a 'sesamoid bone'.
A sesamoid bone is a bone that develops in a muscle tendon, rather than being attached by ligaments to another bone. The tendon in which the patella develops is the tendon of the quadriceps muscle, the big muscle that makes up the bulk of your 'lap'. The whole structure is often referred to as the extensor mechanism.
The extensor mechanism
Quadriceps muscle group
The quadriceps muscle at its upper end is actually comprised of four heads- hence the name quadri-ceps, and attach to the hip and the femur bone. This illustration shows for simplicity just the one head of the quadriceps - the most superficial of the four muscles, and which is called the rectus femoris.
At the lower end of the muscle group the four heads fuse into a common tendon, a strong and fibrous structure that attaches to the tibia bone about three centimetres below the bottom of the patella. It is in this tendon - the quadriceps tendon - that the patella develops.
Quadriceps tendon & Patellar tendon
The bit of tendon above the sesamoid patella is known as the quadriceps tendon and the bit below the patella is known as the patellar tendon (or sometimes it is called the patellar ligament).
The place on the tibia bone where the tendon attaches is called the tibial tubercle or tibial tuberosity.
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