Diseases such as rheumatoid arthritis, degenerative arthritis, and hemophilia can cause inflammation of the synovium, which is the special tissue that surrounds the inside of the knee joint. Synovium inflammation causes swelling. Because the knee is enclosed, this swelling puts pressure on other structures within the knee, causing knee pain especially when the knee is bent.
Loose bodies inside the knee are often fragments of bone and cartilage that have broken off during an injury or have formed because of long standing arthritis. These loose bodies can float freely between the thighbone and shinbone, and may sometimes wedge into the joint. When this happens the knee may lock, causing pain or the feeling that it is going to give out.
A tear in the meniscus cartilage may result from twisting injuries or increasing age. Most meniscal tears occur in the back portion of the meniscus. When the knee bends, like when you are squatting, the thighbone pushes against this torn area, causing pain. Furthermore, a meniscal tear can cause symptoms of the knee giving out, clicking or catching and sometimes locking. This occurs because the torn piece of the meniscus can actually move around inside the joint. Because the space in the joint is very small, when the torn piece of the meniscus moves, it often becomes caught between the bones of the joint, the thighbone and shinbone.
The ACL can be injured or torn in a number of ways. The most common way is a sudden pivoting or cutting maneuver done during a sporting activity. At the time of the injury a "pop" or "snap" can sometimes be felt or heard.
When the ACL is torn, the shinbone is able to slide forward. This causes the knee to give out or be unstable in an abnormal position. Other structures in the knee can be damaged, especially when the knee bends. For example, your meniscus can tear, due to the additional force against it. If your meniscus tears or becomes damaged, the pressure between the shinbone and thighbone is increased. This increase in pressure will speed up the wear and tear on the articular cartilage and lead to arthritis.
These examples represent some of the more common problems that can occur within the knee joint.