Patella: it's called kneecap also, which is the largest sesamoid bone in the body.    

Patella fracture: it typically results from a hard blow to the front of the knee or falling on the knee (direct) or rapid contracture of the quadriceps with a flexed knee (indirect).

The treatment depending on the type of fracture, displaced, or nondisplaced. 

The lateral view is the best to demonstrate the transverse fracture, while the skyline view is the best to show the vertical fracture.