Pain in the back of the knee when straightening the leg is called posterior knee pain. Finding the cause can be difficult as it can come from problems with the bones, muscles, tendons, ligaments, nerves, or vascular system.

Pain in the back of the knee, called the popliteal fossa, is common, but there is a wide range of causes, ranging from ligament injury to arthritis.

Some of these causes will get better with rest, while others will require surgery or will gradually worsen over time.

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The posterior cruciate ligament (PCL), located in the back of the knee, is responsible for holding the knee in place and providing stability to the joint. It helps keep the shinbone from moving too far backward.

Although injuries to the PCL are not as common as in other parts of the knee, a person can injure the PCL if they experience trauma to the the knee. This can also occur if a person lands or falls heavily on their knees.

Twisting and hyperextension can also cause injury to the posterior cruciate ligament.

A person may experience pain and swelling soon after an injury to the PCL has occurred. It may make the knee stiff, create difficulty walking, and give a feeling of instability in the knee.

Treatment

Treatment can include:

If a person’s symptoms do not improve after these treatments, a healthcare professional may recommend surgery to reconstruct the ligament with a tissue graft.

Tendinopathy is the name for a swollen tendon.

It can affect the hamstrings, gastrocnemius, and popliteus tendon, all of which are located at the back of the knee.

Tendinopathy occurs due to repetitive knee movements and regular strain on the joint. Regularly playing sports that require sprinting can cause tendinopathy in the back of the knee.

Symptoms of tendinopathy include pain and tenderness at the back of the knee and a reduction in flexibility and range of motion.

Treatment

Rest, anti-inflammatory painkillers, flexibility training, and physical therapy can be successful treatments for tendinopathy.

Biceps femoris tendinopathy affects the biceps femoris tendon in the hamstrings.

Sports that involve sprinting can lead to small tears in the tendon.

Biceps femoris tendinopathy causes pain in the outside of the back of the knee. A person can also experience bruising on the back of the leg, below the knee, for a few days.

A person may experience stiffness, swelling, and weakness in the joint, and it may hurt to straighten their leg.

Treatment

Physical therapy, cold packs, and resting the affected knee can all help to treat biceps femoris tendinopathy.

Chondromalacia patella is when the cartilage underneath the kneecap breaks down. This means that when a person uses the knee joint, there is no cushioning between the kneecap and the thigh bone, which causes friction.

Chondromalacia can occur due to a misaligned kneecap. It can also occur due to trauma to the area, such as a fracture or dislocation.

A person with chondromalacia may have trouble straightening their leg. However, most pain will occur at the front or the side of the knee.

Treatment

Treatment can include:

  • physical therapy
  • pain management with cold packs and nonsteroidal anti-inflammatory drugs (NSAIDs)
  • surgery to replace the cartilage or adjust existing cartilage in the joint

According to the American Academy of Orthopaedic Surgeons (AAOS), the most common types of arthritis are osteoarthritis and rheumatoid arthritis.

In osteoarthritis, the cartilage in the knee joint gradually wears away. This causes the bones in the knee to rub against each other, which can cause bone spurs. Pain from these bone spurs can worsen over time.

Rheumatoid arthritis causes swelling in the synovial membranes that cover the knee joint. A person with rheumatoid arthritis may experience pain and stiffness in their knees.

A person may experience pain in the knee when they straighten their leg because arthritis can cause the joints to swell, making them feel stiff and painful. This can also occur due to fluid building up in the joint and collecting in the back portion of the knee.

Learn more about arthritis in the knee.

Treatment

The AAOS notes that there is currently no cure for arthritis, but there are ways a person can manage their symptoms. This can include:

  • Switching to low impact exercise: Cycling and swimming can put less stress on the knee joints than running or tennis.
  • Physical therapy: This can help to increase a person’s range of motion, flexibility, and strength in the legs.
  • Pain medication: NSAIDs, such as ibuprofen and corticosteroids, can improve swelling and pain.
  • Maintaining a moderate weight: The United Kingdom’s National Health Service (NHS) notes that maintaining a moderate weight can also reduce stress on the knee joint.

Sometimes, surgery is necessary to improve a person’s symptoms if other treatments are not effective.

Deep vein thrombosis (DVT) is a blood clot that forms in one of the body’s deep veins found in the lower leg.

Blood clots can form in the popliteal vein that is found in the back of the knee.

According to the Centers for Disease Control and Prevention (CDC), DVT is an underdiagnosed condition.

The symptoms of DVT include:

  • swelling
  • pain
  • tenderness
  • flushed skin in the affected area

A person may feel more pain when they straighten their legs to stand up. However, the CDC states that approximately half of all people with DVT do not have any symptoms.

Learn more about deep vein thrombosis.

Treatment

Treatment for deep vein thrombosis includes blood-thinning medication and compression stockings to relieve pain and swelling.

If a person has severe DVT, a surgeon can remove a blood clot through a catheter or surgery.

A DVT is considered a medical emergency. If a person experiences any symptoms related to DVT, they should seek immediate medical attention.

A Baker’s cyst is a lump that forms on the back of the knee. It is filled with fluid and is soft to the touch. Healthcare professionals may also refer to it as a popliteal cyst.

A Baker’s cyst typically occurs due to inflammatory joint conditions, such as arthritis. However, it can also occur as a result of overuse and injury.

Symptoms include pain, tightness, and discomfort at the back of the knee. The swelling may be more noticeable when a person stands and extends the knee fully.

Learn more about Baker’s cysts.

Treatment

Treatment is not necessary if a cyst is not causing symptoms. However, a person can use NSAIDs to reduce inflammation and pain, as well as cold packs.

If necessary, a healthcare professional can drain the cyst. However, it is possible for a cyst to return after it is drained. If this occurs repeatedly and other nonsurgical treatments are ineffective, a healthcare professional may recommend surgery to remove the cyst.

To prevent pain in the back of the knee, a person can avoid putting too much stress or strain on the knee joint when possible.

To do this, a person can engage in low impact sports, such as swimming or cycling.

Consuming a healthy diet and staying active can help reduce a person’s risk of deep vein thrombosis.

Read more about eating a balanced diet.

A person should speak with a healthcare professional if:

  • pain is severe
  • the knee is swollen and inflamed
  • they develop a fever

If a person has a history of blood clots, they should also make an appointment to see a healthcare professional. If they experience any signs of a DVT, such as swelling, pain, or flushed skin, they should seek immediate medical attention.

If a person has symptoms of DVT, they should seek treatment quickly. This is because blood clots can travel to the lungs and cause a pulmonary embolism. A pulmonary embolism is when a blood clot blocks a blood vessel in the lungs.

Pain in the back of the knee can develop for many reasons. The back of the knee may hurt when a person straightens their leg because of a variety of issues, including blood clots, muscle or tendon injuries, arthritis, or cysts.

Physical therapy, rest, and pain medications are common treatments for many of these causes, but sometimes a person will need surgery treat the issue.