This Primer explains why the little kneecap bone is central to knee function, and also to knee rehabilitation.

The patella (kneecap) is an important bone that forms a part of the muscle/tendon complex that stretches from hip to shinbone, and which acts to straighten the knee - anything affecting the structures here will also affect the patella, and may cause distressing symptoms. Hence it can be a complex issue to tease out the cause of pain or instability in this area.