You will feel pain at the top, the lower end and behind the knee cap. Pain behind the knee cap is mainly due to wear and tear of the inside surface of the knee cap. It can be treated with medication, knee supports and physical therapy.
Advanced damage to the knee cap needs surgical replacement which is usually done along with total knee replacement surgery.
Pain over the top and the bottom of the knee cap are due to inflammation at the site of tendon attachments to knee cap. This settles with medication, knee bracing and physical therapy.