This condition is also known as Runners Knee and is characterized by a dull pain that is behind or around the top of the kneecap. 

 

Typical aggravating motions include squatting, running (especially downhill), descending stairs, and prolonged sitting.

 

It is common to see swelling,  hear crepitus (crunchiness)  and feel a catching, popping, or grinding sensation when you are walking or when you are moving your knee.

 

You may also experience occasional knee buckling, in which the knee suddenly and unexpectedly gives way and does not support your body weight.

 

Please note - symptoms can be similar to a meniscus injury - if sudden onset from a twisting motion then likely meniscus damage.