The symptom of popping or snapping in the knee can be a sign of a few different problems. One of the key distinguishing factors is whether or not the popping or snapping causes or is associated with pain. Popping or snapping not associated with painful symptoms is often not a sign of a significant problem.
Patients may experience a symptom of popping called a "mechanical symptom." This may feel as though something is caught within the knee and is popping as the knee bends back and forth.
This type of mechanical symptom is often a sign of a meniscus tear or a loose piece of cartilage within the joint. The torn meniscus or loose cartilage may catch in the knee as it moves back and forth causing a popping sensation.
Crepitus is the word used to describe a crunching sensation as the knee bends back and forth. Crepitus can be seen in patients with cartilage irritation, as is the case in chondromalacia, or in patients with cartilage wear, such as knee arthritis. Unlike a mechanical popping where there is a sensation of something getting caught in the knee, the sensation of crepitus is a more constant problem.
Crepitus can often be felt more easily than heard. Sit on the edge of a table with your knee hanging down. Then gently bend your knee back and forth with your palm resting over the front of the knee. Crepitus can be felt as a crunching sensation under your hand.