Much like the god Atlas of Greek mythology, the job of the knee joint is to bear weight. As a weight bearer, the knee is subject to athletic, overuse and arthritic injury. Because of the joint's complex array of bone, muscle, cartilage and ligaments, pain or stiffness can be an indication of other maladies arising from vascular and cystic conditions or resulting from tumors. Any discomfort should be taken seriously and thoroughly investigated.
Response to Injury
Athletic injuries from overextension of the knee, as with martial arts kicks or landing awkwardly during jumping, can damage the posterior cruciate ligament, or PCL. The PCL attaches the femur -- the bone connecting the pelvis and knee -- to the tibia, the bone running from knee to ankle. It prevents the tibia from moving behind the femur. The PCL can also be injured from a car accident when a bent knee is forcefully hit, as on a dashboard. Symptoms include swelling and pain in the popliteal fossa -- the space behind the knee. Dislocations of the knee, though uncommon, run the risk of damaging the blood vessels behind the knee, particularly the popliteal artery.
Blood vessel damage in the knee is particularly dangerous, because irreversible ischemia -- poor blood flow -- to the knee can occur in as little as 6 hours. This is why injury to the popliteal artery, when left untreated, has an amputation rate of 30 percent. Other reasons for emergent surgical evaluation of the knee would be lack of a detectable pulse and unrelenting pain behind the knee, which could be a sign of an expanding clot of blood, or hematoma. Even without injury, the popliteal artery wall can weaken and expand, resulting in an aneurysm. Aneurysm may present with no symptoms or with pain in the knee that increases with prolonged standing.
An Arthritic Origin
Osteoarthritis is a degenerative joint disease resulting from aging. Knee osteoarthritis is a major cause of disability in those 65 and older and results in joint stiffening and pain. It is also commonly associated with Baker cysts. A Baker cyst is a herniation, or outpouching, of the fluid-filled cushion of the knee. Symptoms of Baker cysts depend on their size, but pain and swelling in the popliteal fossa are common. Baker cyst may also be an indication of an underlying tear in the meniscus -- the cartilage that cushions the knee. A burst Baker cyst can lead to infection and hemorrhage within the knee, so it requires medical attention.
When to See a Doctor
In any case of prolonged pain or stiffness in the knee joint, a doctor should be consulted. Missed meniscal or cruciate ligament tears can lead to complete dislocation of the knee joint. Vascular damage from trauma or aneurysms can lead to ischemia and amputation if untreated. Baker cysts do not usually result in long-term harm, but they can be painful and a sign of underlying joint damage. An evaluation by a doctor employing an ultrasound of the joint or magnetic resonance imaging will quickly identify the cause of the discomfort and appropriate treatments.