Before we progress, let's clarify that what people call 'knee cartilages' are in fact the shock absorbers of the knee known medically as 'menisci' (singular='meniscus').
The following is a list of the more common 'knee cartilage' (meniscus) problems:
The meniscus may be torn in an injury, usually a twisting injury. There may be severe pain at the time. Maybe some swelling due to bleeding inside the joint. May progress to a feeling of instability and joint line tenderness.
The meniscus wears out, becoming friable at its edges and less effective as a shock absorber.
Displaced tear with locking
A specific type of tear of the meniscus, where the fragment flips over and locks the joint. This is the common cause of a knee which cannot fully straighten after and injury.
Broken off fragment with loose body
If the torn fragment breaks off it may float in the joint as a 'loose body', and cause catching or locking.
A fluid-filled lump which you can feel at the joint line. Arises from the meniscus, often from a pre-existing horizontal cleavage tear (a hidden horizontal tear).