In the space just behind the bony cap of your knee (the patella), there are a pair of tough, gel-like disks that sit on the left and right sides of the top of the tibia. These are the meniscus, which act as smooth surfaces for the femur bone above to move on, and as shack absorbers for the weight of the body coming down on the tibia. Tears of the meniscus vary in size, and usually come from twisting of the knee while it bears weight. At first, many people can still walk on a torn meniscus if other structures are not torn as well. Over a couple of days, the knee swells and becomes painful to move; if you continue to walk on it, a piece of the meniscus can tear off, enter the joint, and directly lock it up.