The knees are the largest joints and contain many moving parts. They participate in endless daily activities: walking, climbing stairs, tying shoes, picking something up from the floor, or sitting cross-legged.
For all those purposes, it’s not surprising that these large, complex joints are prone to injury. Although knee pain is very common, many people associate pain with the front of the joint. Pain behind the knee, or pain in the back of the knee, also affects many people.
Knee joint structure
The knee joint is classified as a modified hinge joint and works together with three bones: the tibia, patella, and femur. One intricate network of cartilage disks, ligaments, tendons and muscles work together to provide structural support and facilitate movement.
The normal range of motion for knee joints includes extension and flexion, which are demonstrated when we bend and straighten our legs. Knee joints may also rotate slightly. Joint fluid, a clear, viscous fluid in the joint, acts as a lubricant and cushion during these movements.
The posterior cruciate ligament or PCL helps keep the bones in their proper position and allows the knee to move smoothly. This ligament is located behind the knee and connects the lower leg to the thigh. Anyone can experience a PCL injury, but it’s most common in athletes and skiers.
Potential causes of a PCL injury includes: hitting the knee against a hard surface, landing incorrectly after a jump, falling on a bent knee, dislocating the knee joint, or bending the knee too far back.
Biceps Femoris Tendinopathy
The biceps femoris tendon behind the knee is part of the hamstring muscle group and helps control movement of the lower leg near the knee. Tendinopathy occurs when vigorous or repeated use of the tendon results in small tears and inflammation. Causes include walking or running, sitting or driving for long periods, or a sudden increase in walking or running.
The popliteal fossa is a small muscle located behind the knee and patella runs from the thigh to the lower leg. This triangular muscle and its tendon help bend the knee.
Popliteal tendonitis is associated with athletic activities such as skiing or running, especially running on hills or uneven terrain. It can also occur if the knee is overstretched while climbing stairs or uphill. Pain tends to worsen during activities that bend or straighten the knee.
Sometimes pain in the back of the knee is actually referred pain that doesn’t come from the knee at all. Sciatica occurs when pain of a compressed nerve in the lower back – the sciatic nerve – radiates along its length.
People can experience sciatica pain in the back, buttocks, and hips, and down the backs of the legs and knees. Up to 40% of adults experience sciatica pain at some point in their lives, although it is most common after age 50.
Arthritis is a common cause of swelling, stiffness and pain behind the knees† Osteoarthritis slowly wears down the cartilage between bones in the knee joint and can cause bone spurs.
Post-traumatic arthritis occurs after an injury damages the knee or displaces cartilage. Rheumatoid arthritis is an autoimmune disease that causes an inflammatory response in joints that slowly wears down cartilage and causes swelling of the synovial membranes.
Gastrocnemius tendinopathy refers to a injury of the gastrocnemius tendon run from the back of the knee to the large calf muscle. The tendon can become inflamed or degeneration from overuse. Sudden onset of pain is likely due to inflammation, while gradually increasing pain and stiffness may indicate degeneration.
Contributing factors include high-intensity sprinting, muscle imbalances in the hip and knee joints, ill-fitting or worn-out footwear, or an abrupt increase in running intensity or distance.
Chondromalacia patella, also called runner’s knee, occurs when: cartilage under the kneecap softens and breaks.
The patella, or kneecap, is covered with cartilage that should slide smoothly. Cartilage deteriorates when parts of the knee joint do not move properly and cause the kneecap to rub against the bone. Causes include poor knee alignment, weak thigh muscles, a previous dislocation or injury, or repetitive stress from skiing, running, or jumping.
A Baker’s cyst, also called a popliteal cyst, occurs when the knee joint produces too much synovial fluid. This can happen when bone or cartilage around the knee is damaged by arthritis or an injury. The fluid-filled cyst may have swelling and a noticeable bobble, mee of a tight feeling behind the knee. The pain may worsen when the knee is extended.
Deep Vein Thrombosis
A deep vein thrombosis or DVT is a blood clot in a deep vein, usually in a calf or thigh muscle. Clots can form when a vein is damaged or blood flow in a vein stops or slows.
Symptoms include pain in the calf or thigh muscles, pain behind the knee, and swelling, warmth, and redness over the clot. Contact a doctor immediately if you suspect a DVT.