There are a number of causes to a meniscus tear. The two most common ones would be your traumatic knee injuries or your traumatic versus your insidious onset or degenerative, it's called. They're two very different causes. The degenerative issue is more of a wear and tear process. The cartilage inside your knee, the meniscus, it's really susceptible to a lot of degeneration, too much activity, obesity, ageing, etc.
So you have your degenerative cause and what you'll have is the meniscus and cartilage begin to crack or fissure and eventually start to deform. It might flatten out, and as a result of this, the bones of your leg, being your femur and your tibia, may start to come closer in contact together, start to rub causing a little friction.
So that's your first cause of a meniscus tear, the cracking and the fissuring and deforming, eventually lead to an actual tear. The second cause is more traumatic, so if you're running or playing sports and you get hit or you're tackled or you twist your knee the meniscus can actually in one shot, just sort of tear. And in either case it's going to be quite painful and there'll be plenty of swelling to deal with.
So depending on how much pain and swelling, you might want to seek your physician out to get an X-ray or an MRI of the knee, but those are the two main causes of your meniscus tears. Your traumatic ones versus your degenerative ones. Very different, but unfortunately a very similar ending, a lot of pain and swelling.