The knee joints can lock if something get stuck between the 2 joint surfaces. The joint’s surfaces are normally very smooth and allow joint movements. They are covered with synovial cartilage. Various conditions (inflammations, degenerative processes and traumas) can damage the cartilage and make joint movements painful, restricted or even completely impossible. In the case of joint inflammation (arthritis) cartilage becomes damaged by inflammation. Then, the chronic inflammatory process creates connective tissue that makes “bridges” between the two surfaces and disables the movements.