The knee is the largest joint in the body and pain in this area is the number one reason people visited an orthopedist in 2003, according to the American Academy of Orthopedic Surgeons. The joint is complex being made up of three leg bones and the knee cap as well as numerous muscles, tendons ligaments and menisci. Pain that comes on suddenly along with swelling or a loss of movement in the joint must be evaluated and treated quickly.
Gout
While gout more commonly affects the first toe, it can affect anywhere in the foot, hands, elbow, knee or hip, according to the Merck Manual. Gout is caused by a buildup of uric acid crystals. The buildup can be caused by the kidneys not processing the uric acid as well or from eating foods which produce uric acid as a by-product of digestion or from certain medications. Due to the body's metabolism cycle gout will set on quickly and painfully at night. The joint will become hot and swollen and will be difficult to walk on.
Torn Cartilage
Cartilage is a firm yet spongy tissue which is a shock absorber for the joints. In the knee the cartilage is more specifically called meniscus. The tissue can tear due to trauma and cause the joint to lock, pop, catch and swell in addition to causing intense pain. The location of the pain will depend on what part of the meniscus is torn.
Joint Mouse
Pieces of bone or torn cartilage can break free and become free floating in the joint. Many people may have these present and not know until and x-ray is taken. Occasionally these joint mice can get pinned between bone and cause intense joint locking and pain. The Merck Manual states this condition is much like a pencil being caught in a door hinge, which does not allow for smooth and proper movement.
Tissue Injury
Ligaments and tendons in and around the knee joint work to keep the joint stable. Trauma or a rapid twist or turn, like in sports, can lead to a popping or tearing sound in the joint accompanied with pain, local tenderness, swelling and sometimes bruising. Either the tendons or the ligaments can tear partially or completely. A complete tear requires surgery to fix.
Fracture
A fracture can happen with trauma or severe osteoporosis, a thinning of the bones. In young athletes a stress fracture can develop with repetitive overuse. Stress fractures are when the shock of the overuse causes the bone to become stressed and creates a minute crack in the bone, this occurs most commonly in the lower legs and feet. A regular fracture will be visible on x-ray where as a stress fracture will need more advanced imaging like a CT or an MRI, according to the American Academy of Orthopedic Surgeons.
